Uninterruptible power supply systems: types, characteristics, installation. Uninterruptible power supplies

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UPS stands for "uninterruptible power supply". Abbreviation in English - UPS (Uninterruptible Power Supply) , therefore the names UPS, YUPS, and oopsnik are also common.

The main function of an uninterruptible power supply is to ensure the supply of electricity to the equipment connected to it during outages in the main network. But, depending on the type of equipment, the parameters of such autonomous power supply may be required to be radically different. Accordingly, the UPS market offers different types of devices, which differ in a lot of parameters:

principle of operation: offline, linear-interactive, online;
type automatic adjustment tension;
quality of filtering network interference;
capacity (number of ampere-hours, or in other words - how long the battery life will last);
time to switch to batteries during a power outage;
possibility of connecting additional external batteries;
various additional functions (filtering sockets, sockets for telephone and network cables, LCD display, synchronization with a PC), etc.

How to choose a UPS with such a variety of models ? How to understand how they differ? In this article we will look at the main types of uninterruptible power supplies, their differences, and what additional functions manufacturers equip UPS with. In the next one - how to choose a UPS depending on the features of your equipment, how to calculate its required power, etc.

Three main types of UPS

Off-line (Back-UPS, backup, Standby) uninterruptible power supply

Example of a backup UPS: model .

The operating principle of this type of uninterruptible power supply is very simple:

As long as there is electricity in the network within the set values, the UPS supplies the connected devices with voltage directly from the network, while simultaneously recharging the battery. The power passing through the UPS is not regulated; pulses and noise are filtered at the simplest level, using passive filters. The signal shape corresponds to the network signal, i.e., a sinusoid.

As soon as the mains power is lost, the UPS switches to battery power. The inverter that converts direct current from the battery into alternating current output is one of the simplest installed in this type of UPS, so the waveform does not correspond to the correct sine wave. The maximum that manufacturers do is to bring it somewhat closer to a sinusoid, making it stepwise.

The UPS also switches to off-line autonomous power supply if the voltage level in the network falls below or rises above the threshold values, they can be different depending on the brand of uninterruptible power supply.

The switching time to batteries in various models ranges from 5 to 20 ms. This is relatively long, and for some equipment models such a long delay may adversely affect operation . The long-term operation of the relay is due to the fact that the device needs the phases of the mains and battery voltages to coincide when the autonomous power is turned on, and since they are not synchronized, this takes some time.

Scheme of operation of a backup uninterruptible power supply.

Pros of Standby UPS:

inexpensive price,
high efficiency,
silent operation.

Flaws:

long switchover to battery operation (from 5 to 20 ms);
the output signal shape is not a sinusoid;
filtering interference, noise and impulsesquite rough on the line;
there is no voltage and frequency adjustment when operating from the network.

Line-interactive UPS

Example of a line-interactive UPS: model

Buyers choose this type of uninterruptible power supply most often, as it optimally combines functionality and price.

IN schematic diagram operation of line-interactive UPS includes AVR - module for automatic regulation of incoming network voltage. That is, unlike a backup UPS, it not only passes power through itself, but also stabilizes it, although not smoothly, but in steps.

When operating from the mains at normal voltage levels, the line-interactive uninterruptible power supply passes the incoming signal through passive interference and noise filters, while the battery is charged.

When the voltage in the network increases or decreases, the line-interactive UPS makes its stepwise adjustment. When the voltage reaches a certain threshold, the AVR lowers or lowers it by a fixed amount (or percentage). Several such threshold-steps can be specified in the AVR operating scheme; also, for working with a lower and higher level, a different number of adjustment steps can be assigned (for example, 2 for an increase, and 1 for a decrease).

If the mains voltage drops or rises to values that lie outside the available input range of the uninterruptible power supply, the device switches to battery operation, just as in the case of a complete power outage. These minimums and maximums may vary depending on the load on the UPS. For example, if the UPS is 70% loaded and the voltmeter shows 160V in the network, the uninterruptible power supply switches to the batteries. And at 30% load and a voltage of 150V, it still makes adjustments using an AVR transformer.

Some linear-interactive models are no different in the shape of the output signal from backup-type uninterruptible power supplies: they have a stepped sine wave. Some manufacturers, especially with the growing demand for UPS for boilers, equip their uninterruptible power supply systems with inverters that produce the correct sine wave.

The switchover time to battery operation in a pure sine wave line-interactive UPS is faster than that of its standby counterparts. The reason is that in UPSs of this type, the voltage waveforms coincide (both from the network and from the battery, this is a sinusoid), which speeds up phase synchronization and, accordingly, the start of autonomous power supply.

Pros of line-interactive UPS:

reasonable price,
silent operation,
automatic regulation of incoming voltage,
in some models - pure sine wave at the output,
switching time is less than in backup ones (on average 4-8 ms, in some models 2-4 ms).

Flaws:

no frequency adjustment,
insufficiently complete filtering of interference, noise and network impulses,
voltage regulation is not smooth, but stepwise,
The efficiency is lower than in an off-line uninterruptible power supply.

Double conversion UPS (on-line)

Double conversion UPS example: model .

This is the most expensive, but also the most best view UPS. It is optimally suited for expensive, capricious equipment, for which not only constant voltage is important, but also frequency, as well as effective noise filtering, a signal in the form of a pure sine wave, and the absence of delays when switching to battery operation.

In fact, such an uninterruptible power supply operates constantly, stabilizing, filtering the incoming signal, equalizing the frequency and shape of the output signal.

In mains mode, the incoming AC voltage is stabilized and converted to DC by the rectifier and distributed between the battery (for recharging if necessary) and the inverter. The inverter converts direct current into alternating current, producing an output signal in the form of a pure sine wave, the correct frequency, the correct voltage. Interference and noise are completely absent - they simply do not remain after double conversion.

This constant “inclusion” of the uninterruptible power supply into the network provides one of its significant advantages: Instant switching to battery operation. Actually, it’s hard to even call it “switching”, since power passes through the rectifier, battery (during charging) and inverter constantly. When the network voltage drops below threshold values or there is a complete power outage, the inverter simply begins to take part of the energy from the battery, and not from the rectifier. It happens instantly.

Double conversion UPSs usually have another operating mode: bypass. This is a backup line that goes directly from the input to the output of the UPS, bypassing the rectifier, battery and inverter. It allows in critical moments for the UPS: overload (for example, with starting currents), failure of the inverter and others - to supply electricity directly to the connected devices, avoiding failure of the device elements.

Constant operation of the UPS has a certain disadvantage: increased heat generation, which requires effective cooling. Therefore, UPS online are most often equipped with fans, which makes their operation in residential areas not as comfortable as other types of silent uninterruptible power supplies.

Pros of online UPS:

constant voltage stabilization,
constant frequency stabilization,
pure sine wave at the output,
effective filtering of noise, impulses and interference,
Instant switching to batteries.

Flaws:

high price,
increased noise level,
the lowest efficiency among all types of UPS.

When choosing an uninterruptible power supply, you need to take into account that there are exceptions. Some line-interactive UPSs may cost more than online models from another manufacturer, the switching time to battery operation in a backup UPS may be no more, or even less, than in some line-interactive UPS, etc. Therefore, In any case, you need to read the specifications specific model.

Additional UPS functionality

In addition to determining the type of uninterruptible power supply you need, when choosing a UPS you should also pay attention to what functionality is included in it. UPS can have various additional functions and design features:

Synchronization with PC. This feature is not present in the cheapest models, but it is very convenient. Using special software, the UPS transmits data to real mode to the computer about the state of the power line, battery charge level. In addition to the purely informational component, there are also features such as, for example, autonomous shutdown of the computer while saving data in all applications during a power outage.

Cold start. An uninterruptible power supply equipped with this function can be turned on when there is no power in the network. For example, the lights went out, you saved the documents, turned off the computer and UPS, but after some time there was an urgent need to copy the document to a flash drive. A UPS with cold start support can be turned on, even if there is still no power, and get the job done.

Previously, connectors for connecting devices in a UPS looked basically like this:

This IEC 320 standard connector is ideal for connecting a variety of computer equipment. However, equipment with a regular power cord is the same WiFi router, you can’t connect it to it. For these purposes, you can use a surge protector with a similar connector, which is connected to the UPS, and then plug it into it. various equipment. But this is not always convenient.

Therefore, now many models have simply begun to be supplemented with Schuko-type sockets (in our country they are often called Euro sockets) so that the equipment can be turned on directly:

Sockets for filtering interference. A UPS may be equipped with an outlet or several for sensitive equipment that does not provide power support during a power outage but protects the connected equipment from utility power interference.

Sockets for telephone line, twisted pair. High-voltage pulses can be transmitted not only directly via an electric power cable, but also in the event of various accidents and breakdowns - both via a telephone cable and twisted pair cable. To protect telephone, network and computer equipment, some manufacturers provide special connectors (input/output) where you can connect a telephone or Internet line.

To be continued in the next article.

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Electrical vibrations and their characteristics

Classical electrical oscillations, occurring for example in oscillatory circuit or at the output of an alternator, are harmonic. This means that the dependence of the intensity of the oscillation (instantaneous value of voltage or current) on time can be represented graphically in the form of a sinusoid.

IN real life The appearance of the voltage or current waveform may deviate slightly from a pure sine wave. Let's see what parameters characterize the electrical oscillatory process.

Rice. 21. Electrical vibration parameters

The amplitude value or amplitude is the maximum deviation of an oscillating quantity from the zero level.

The effective value of current or voltage is numerically equal to such direct current or voltage direct current, which produces the same thermal effect in the conductor. The effective value of voltage or current is equal to the root mean square value of the corresponding value over the oscillation period.

The amplitude factor or crest factor is the ratio of the amplitude of the oscillation to its effective value. It is always greater than or equal to 1. For a harmonic oscillation (sinusoidal voltage or current), the amplitude coefficient is = 1.41 (more precisely, the root of two). The crest factor of non-sinusodal oscillations can differ greatly from this value.

The amplitude factor does not characterize a non-sinusoidal oscillation unambiguously. Oscillatory processes of different shapes can have the same amplitude coefficients.

In order to fully characterize a complex periodic oscillation, it is artificially represented as the sum of several harmonic oscillations of multiple frequencies (harmonics). So, for example, in order to describe a non-synsoidal process with a fundamental frequency (first harmonic) equal to 50 Hz, it is represented as a sum of oscillatory processes with frequencies of 50 Hz, 100 Hz (second harmonic), 150 Hz (third harmonic), etc. .d.

The analysis of a complex vibration carried out in this way is called harmonic analysis or Fourier analysis (named after the French mathematician and physicist). The result of harmonic analysis is the so-called spectrum of the oscillatory process - the dependence of the intensity of each harmonic on its number.

In Fig. Figure 22 shows an arbitrary oscillatory process and the beginning of its spectrum.

Rice. 22. Electrical vibration and its spectrum.

To accurately represent a complex vibration, at least several dozen harmonics must be taken into account.

As an integral characteristic of the degree of difference between the shape of an oscillatory process and a sinusoid, in Russia the harmonic distortion coefficient (harmonic factor) - Kg - is often used. It shows what proportion of energy is contained in the higher harmonics, compared to the energy contained in the first harmonic.

In other countries, this is usually done using the total harmonic distortion factor. THDF). It shows what proportion of energy is contained in the last harmonic, compared to the total energy of the vibration.

It is clear that for almost sinusoidal processes Kg and THDF are practically equal. But with significant distortions they differ. The table shows several points that characterize this difference.

Kg, %	THDF, %
0	0
10	10
20	20
30	29
40	37
50	45
60	51
70	57

The harmonic distortion coefficient of a purely sinusoidal oscillatory process is zero (all the energy is contained in the fundamental harmonic). It is usually considered that the oscillation is slightly different from a sinusoidal one if the harmonic distortion coefficient does not exceed 5%.

Linear and non-linear loads

If we connect a resistor to a DC voltage source and change the voltage value, the current flowing in the circuit will change in proportion to the voltage.

If we connect to a source of sinusoidal alternating voltage (for example, to a network or to UPS with a sinusoidal output voltage) resistor, the instantaneous value of the current in the circuit will be proportional to the instantaneous value of the voltage. Consequently, the current in the circuit will be sinusoidal, and in-phase with the voltage (i.e., the maximum current values will be observed at exactly the same times as the maximum voltage values.

Rice. 23a. Current consumption of a resistor in an AC circuit.

If we connect a capacitance, inductance, or any combination of them with resistors to a sinusoidal voltage source, the current in the circuit will still be sinusoidal (see Figure 23b).

Rice. 23b. Current consumption of a capacitive load in an alternating current circuit.

But in this case, the current maximums will advance the voltage maximums (as in the figure) or lag behind them. Depending on the predominance of capacitances or inductances in the circuit, such a load is called eco-load or inductive. And in total, all loads (electricity consumers) with a sinusoidal current consumption (at a sinusoidal voltage) are called linear.

A switching power supply (for example, a computer) is a nonlinear load. If the computer is connected to a sinusoidal voltage source, then the dependence of the current consumed by the computer on time will have the form shown in Fig. 23rd century

Rice. 23rd century Current consumption of a nonlinear load in an alternating current circuit.

The figure clearly shows that the computer consumes current only at moments when the voltage is close to its maximum, and does not consume current at low voltage.

The shape of the current consumed by a nonlinear load can be characterized by the same parameters as any oscillatory process.

Crest factor

The amplitude factor (peak factor) of the current consumption of switching power supplies is always much greater than unity. It is usually in the range of 2 to 3, but can be more than 5.

The uninterruptible power supply must be designed to handle such peak factors. Those. UPS must not only provide an effective current value corresponding to the maximum load, but also a maximum (amplitude) current value that significantly exceeds the amplitude of a sinusoidal current with the same effective value.

The magnitude of the crest factor is not a constant characteristic of the power supply. It is the product of the interaction between the power supply, its load (for example, a computer) and the current source to which it is connected. So, when powered from the network, it can be equal to 2 or 3. If the computer is powered from UPS with switching, which has an output voltage in the form of a meander with a pause, then the crest factor is reduced to 1.8-2. Connecting your computer to a ferroresonant transformer allows you to reduce the crest factor even more significantly. This reduces the load on the computer's power supply and increases its longevity.

On the other hand, if the computer’s power supply is left to work at idle or with a very small load (for example, take a 400 W power supply and put it in a simple personal computer with minimal power), then the current crest factor can be very large (for example, 5). If the same power supply is fully loaded (say, install it in file server With big disks, modems, etc.), then the amplitude factor will decrease (and amount to, for example, 2.5).

Harmonics

In Fig. Figure 24 shows an approximate view of the current spectrum of a switching power supply. It would be more accurate to say that this is the beginning of the spectrum. The full current spectrum of a switching power supply includes many dozens of harmonics.

Rice. 24. Beginning of the current spectrum of a switching power supply.

The current consumption of a switching power supply contains a set of odd harmonics, the amplitude of which decreases more or less monotonically with the harmonic number.

If computers are connected to an electrical network that includes other (and mostly linear) consumers of electricity, then the difference in the shape of the current consumed by the computer power supply from a sine wave does not affect either the computers themselves or other equipment connected to the same electrical network.

If the network includes mainly computers and their total power is comparable to the characteristic power of the electrical network, then the voltage in the network may no longer be sinusoidal. This is a sign of overload of the electrical network with non-linear loads, and can cause failures in the operation of sensitive equipment.

The first sign of network overload with computer loads is the manifestation of the most intense - the third harmonic. Its appearance can be determined even without a spectrum analyzer capable of constructing a beautiful picture, like the one shown in Fig. 24. For basic analysis, a simple oscilloscope is sufficient.

If a sinusoid has a flat top (as if “eaten out” by a large pulse current), this is the first sign: a third harmonic has appeared in the network, the network is slightly overloaded with nonlinear loads.

If the top of the sinusoid begins to sharpen, it means that in addition to the third, a fifth harmonic has appeared in the network: the network is heavily overloaded with nonlinear loads.

If waves appear on the sine wave, then the seventh harmonic is already visible to the naked eye: some measures need to be taken.

UPS output voltage waveform

An uninterruptible power supply is a temporary substitute for the electrical network for the equipment connected to it. The quality of this replacement greatly depends on the type and brand of UPS.

In an electrical network, the voltage has a sinusoidal shape or a shape close to a sinusoid.

All upscale UPS also have a sinusoidal output signal, i.e. provide a power supply that is practically no different from a regular network or even have a higher quality sine wave.

At the exit UPS(as on the network) the sine wave may not be entirely ideal.

Determining harmonic distortion usually requires special equipment. But you can approximately estimate the value of the total harmonic distortion coefficient simply from the voltage oscillogram. If you see slight distortion, the harmonic distortion factor is about 5%. If the distortion is very noticeable, the harmonic distortion factor is approximately 10%.

If the harmonic distortion coefficient is more than 20%, you will not be able to call the voltage waveform a sinusoid.

This method, like any simplification, has its limitations. In particular, the higher the harmonic number, the lower the harmonic distortion coefficient it is clearly visible.

All have sinusoidal output voltage UPS double conversion, ferroresonant UPS and the majority UPS, interacting with the network. For all these UPS Total harmonic distortion of the output voltage equal to 5% is limiting. If the output harmonic distortion factor UPS less than 5%, then UPS according to this parameter it can be considered “good”. If this value is greater than 5%, then the output signal shape UPS leaves much to be desired.

Typically, manufacturers indicate the degree of harmonic distortion in the general list of technical characteristics UPS. Almost always, only one value of the harmonic distortion coefficient is indicated, which relates to some average (if not ideal) conditions - for example, with a linear load. It should be borne in mind that the most significant distortion of the output voltage waveform can occur under various boundary conditions, as well as at parameters not typical for normal operation. UPS.

Such limiting conditions (their set or combination may be different for different models UPS) can be maximum load or idle (no load); marginal or prohibitive power factor (for example, less than 0.5), too high crest factor. The output voltage can also experience serious distortion during various transient processes (for example, during a step change in load).

On mains mode UPS with switching and interacting with the network, they supply their load with filtered mains voltage. That is, in this case they are not independent power sources. Such a source is the electrical network. This means that the harmonic distortion coefficient at the input of the computer power supply will be approximately the same as without UPS. This is so because the filters of these UPS are not designed to filter low-frequency harmonics, and allow them to pass freely. Accordingly, if there was strong harmonic distortion in the network before installation UPS(due to general network congestion or a large share of the power of non-linear loads), they will remain so. If these distortions did not exist, they will not appear.

The situation is different with ferroresonant UPS And UPS with double energy conversion. They are, in this sense, independent power sources. Therefore, everything said above regarding distortions in the mains voltage form must in this case be attributed to the output voltage UPS. If these UPS are heavily (almost up to the rated power) loaded with nonlinear loads, then fundamental harmonic distortion may appear at the input of these loads, which would not have existed without UPS. On the other hand, if harmonic distortion was observed when operating from the network, then it may disappear after installation UPS, If UPS underloaded

is more than two-thirds of its total power, then the output voltage UPS may be noticeably distorted. Voltage waveform distortion, which is not dangerous in itself for computers, is not a good sign that the load UPS too big. Better install UPS higher power or disconnect any equipment from it.

Some upscale UPS With double conversion are equipped with a special control circuit, the purpose of which is to adjust the shape of the output voltage even when working with nonlinear high-power loads. The output of these UPS voltage has no noticeable harmonic distortion, even if UPS supplies non-linear loads of significant power.

Of course, all computers and other equipment designed to be powered from an alternating current network are designed for sinusoidal voltage. It is unlikely that any manufacturer of this equipment is ready to guarantee normal work its equipment with highly non-sinusoidal voltage.

However, most electrical energy consumers can be powered by non-sinusoidal AC voltage. Moreover, for different equipment, different characteristics of the sinusoidal supply voltage are more important. For example, equipment equipped with switching power supplies (say, personal computers) consumes current only at times when the voltage is very close to the maximum. Therefore, to power such equipment, the correct amplitude voltage value is important. Equipment containing directly powered electric motors and heaters requires a rated effective value voltage. Sinusoidal voltage meets the requirements of any of these loads.

But almost all types of loads (equipment), including computers, can operate more or less normally with a voltage that is very different from the sinusoidal one. This circumstance is widely used by manufacturers UPS with switching.

Previously (very long ago) some UPS with switching had an output voltage in the form of a meander (rectangular pulses of different polarities).

Rice. 26. Meander

When we replace a sinusoidal voltage with one or another approximation, we must choose the parameters of this approximation such that they are closest to the parameters of the replaced sinusoid. But in a meander, the amplitude and effective voltage values are equal to each other (amplitude coefficient equal to one). Therefore, we cannot make the square wave voltage such that it can satisfy the requirements of different loads at the same time.

In an attempt to find a compromise, manufacturers of such UPS set the rectangular voltage equal to a certain value lying between the amplitude and effective. The result was that some loads (requiring the correct RMS voltage) could fail due to excess voltage, while other equipment (those that draw current at voltages close to the maximum) had too little voltage.

In order for the root mean square and peak value of the rectangular voltage to be equal to the corresponding values of the sinusoidal voltage, manufacturers of modern UPS with switching, they slightly changed the shape of the meander, introducing a pause between rectangular pulses of different polarities.

Rice. 27. Meander with a pause.

Voltage of this form manufacturers UPS called "stepped approximation to a sine wave". This shape of the curve allows, with correctly selected voltage amplitude and pause duration, to meet the requirements of different loads. For example, with a pause duration of about 3 ms (for a frequency of 50 Hz), the effective voltage value coincides with the effective value of a sinusoidal voltage of the same amplitude.

The output voltage of all the ones I came across UPS with switching, which is present on the Russian market, has the form of a stepwise approximation to a sinusoid.

Shown in Fig. 27 output voltage form is the ideal that UPS manufacturers should theoretically strive for. Actual output voltage shape UPS with switching, of course, it differs from the ideal.

Sometimes manufacturers UPS comply with the declared equality of the effective value of the output voltage UPS The effective value of the network voltage is very approximate. The duration of pauses and the amplitude of the rectangular voltage deviate noticeably from the calculated values.

These deviations apparently cannot serve as a basis for declaring this or that UPS bad. After all, they all work normally with personal computers, for which they are actually intended to work.

Actual output voltage shape UPS with switching is shown in Fig. 28.

Rice. 28. Oscillograms of voltage and current of a personal computer connected to UPS with switching.

The same oscillogram also shows the curve of the current consumed by the computer. This allows you to assess how hard it is for the computer being protected. UPS with switching. But, oddly enough, the strong pulse currents consumed by the computer at the beginning and end of the rectangular pulse do not affect the operation of the computer. They are completely suppressed by the computer's power supply, the output of which is a constant voltage with a normal ripple level.

We should also not forget that the computer being protected UPS with switching, powered by non-sinusoidal voltage only during operation UPS from the battery (i.e. very briefly). When working UPS from the network, the computer is powered by mains voltage, smoothed using built-in UPS noise and impulse filters.

Possibility of application UPS with switching to power other equipment (not computers) requires, generally speaking, verification in each such case. There are known cases when with such UPS Some printers refused to work. On the other hand, there is a known case of application UPS with switching to protect non-traditional loads such as telephone exchanges or cash registers with transformer power supplies.

For use UPS Switching to power appliances with transformer power supplies should be approached with caution. The fact is that the usual 5-10% losses for a transformer in the presence of harmonics increase in proportion to the square of the aharmonic number. Therefore, the resource of heavily loaded transformers when supplied with voltage in the form of a meander can decrease tens of times.

As with any power source, the output voltage waveform UPS with switching depends on the size and nature of the load. For UPS, produced by world-famous companies, this dependence is usually small.

However, some UPS have a strong dependence of the shape (and sometimes amplitude) of the output voltage on the load. Some of them cannot be used with light loads, since they have an output pulse voltage with an amplitude of up to 800 V. Others are tested by the manufacturer only when working with linear loads. Such UPS when working with a computer, they may be unstable during switching moments.

The above shows that you should not use UPS unfamiliar manufacturers or buy such UPS from non-specialized companies.

Voltage stabilization and regulation

According to the current standard in Russia, the voltage in the electrical network must be within +10% and -10% of the rated voltage. For 220 V voltage these limits are absolute values 198 V and 242 V. In this voltage range, all mains-powered equipment, from a light bulb to a computer, should operate normally.

Unfortunately, sometimes tension goes beyond the limits set for it by superiors. In some areas, such periods repeat with the regularity of sunrises. Owners of computers operating under these conditions are, of course, inclined to demand that UPS, protecting their computers, stabilized the voltage.

Two of the types of uninterruptible power supplies we examined stabilize voltages, so to speak, by definition. This on-line UPS: Double conversion and ferroresonant.

Accuracy of stabilization of alternating voltage at the output UPS with double conversion is usually about 1-3% with static (i.e. not changing over time) and balanced (uniformly distributed across phases for 3-phase UPS) load. In the case of a sudden change in load (for example, its complete or incomplete switching on or off), the stabilization error increases to approximately 10% for good UPS. Not all manufacturers UPS indicate this characteristic. In the case where it is not indicated, you need to be very careful if the work is important to you UPS under dynamic load.

With an unbalanced load (i.e. if the load is unevenly distributed among the phases 3-phase UPS) the stabilization error also increases. There are indeed three-phase UPSs with independent voltage regulation in each of the three phases. Load imbalance for such UPS doesn't matter.

The range of input voltages in which voltage stabilization occurs, for UPS double conversion always matches the acceptable input voltage range (i.e. the voltage range at which UPS works from the network). Thus UPS with double conversion can not help but stabilize the voltage. It produces either a stable output voltage (when operating from the mains or from a battery) or does not produce any. Input voltage range for different UPS varies greatly. A typical value is plus or minus 10-15% of the rated voltage. Some low power UPS can have an input voltage range from 100 to 280 volts and even wider (though often work UPS at minimum values voltage is provided only at part load).

If the user is not satisfied with the input voltage range, then for some models UPS it can be expanded through special tricks. Extending the input voltage range (in those rare cases where it is possible) should be done by a very qualified person who is very clear about what he is doing. You need to keep in mind that nothing comes for free, and in any case, you will have to pay something for expanding the input voltage range - for example, reliability UPS or the quality of the voltage supplied to the load.

For most Double conversion UPS The input voltage range depends on the load. With a lighter load, the input voltage range expands somewhat.

Ferroresonant UPS stabilizes voltage due to the properties of a ferroresonant transformer. The voltage stabilization error is 1-5% and depends on the load: with a lower load the error decreases.

The ferroresonant transformer is very resistant to any transient processes. Therefore, the stabilization error changes slightly under dynamic load.

The input voltage range of a ferroresonant UPS is highly dependent on the load. At light load it can start at 145 V.

UPS with switching do not have a voltage stabilization function.

UPS, interacting with the network, they can stepwise regulate the output voltage.

Step voltage regulation is implemented by switching the load to work from another winding of the autotransformer.

In the simplest case, there is only one voltage boost stage, which is triggered when the network voltage decreases. More modern ones interacting with the network UPS regulate the voltage even when it increases.

For example UPS Smart-UPS American Power Conversion switches the load to operation from the step-up winding of the autotransformer if the voltage drops below 196 V. The step-up winding allows the voltage to be increased by 12%. As the input voltage drops further, the output voltage drops linearly. When the input voltage reaches 176V ( factory setting) Smart-UPS

When the input voltage increases above 264 V, the load switches to operation from the autotransformer winding, which reduces the voltage by 12%. After the input voltage reaches 296 V, UPS switches to battery operation.

In the majority UPS interacting with the network, there is only one stage of voltage regulation (in each direction, if the regulation is two-way). But several UPS have two or more stages of stabilization in each direction.

Noise reduction

For protection against impulses in UPS Different types use different technologies. UPS with switching and interacting with the network suppress noise coming through the power network using R-C or L-C filters. In ferroresonant UPS The filter is a ferroresonant transformer. Noise reduction in UPS with double conversion is carried out in the process of two energy conversions. In addition, in the DC circuit of these UPS Usually there are special containers and chokes to smooth out charging current ripples. These L-C filters They also suppress noise penetrating through the rectifier very effectively.

Approximate noise suppression levels in the frequency range from 1 to 10 MHz for different types of UPS are shown in the table.

Pulse suppression

There are several standards around the world that describe the requirements for UPS, regarding impulse protection. Usually American UPS are tested to comply with ANSI/IEEE C62.41, which describes the surge parameters that a computer or equipment designed to protect against surges can withstand. The standard describes the voltage and pulse shape.

The standard provides two categories: A and B. Category A refers to typical office conditions and involves testing UPS by applying a 3000 V pulse to its input. Category B refers to more severe conditions (for example, for computers connected to the network close to the power input into the building) and requires a 6000 V pulse test.

Usually manufacturers UPS guarantee that their products comply with Category A of this standard or a similar standard. Some UPS also correspond to category B of the standard.

IN UPS Different types use different pulse suppression technologies. Varistor surge protection is used in UPS with switching and interacting with the network.

An obstacle to the path of the impulse through UPS with double energy conversion is double conversion itself, galvanic separation (in those models where it is) and a combination of capacitors and batteries in a DC circuit. However, in some models Double conversion UPS Varistor shunts are also installed.

In ferroresonant UPS the function of a pulse filter is performed by the ferroresonant transformer itself, although varistors are also present at the input UPS.

A very simple and effective varistor shunt can suppress pulses with currents of enormous amplitude (kiloamps for UPS with switching and up to tens of kiloamps for the best models UPS, interacting with the network).

For varistor protection, as already noted, there is a fundamental limitation on the pulse energy that the varistor shunt can withstand. Typically this energy is 80-500 J. When a higher energy pulse arrives at the varistor, it may fail. In this case, the varistor may be mechanically destroyed. This mainly limits the pulse duration, since the pulse amplitude can be quite large.

Another limitation of varistor protection is its resource. When suppressing pulses, the varistor gradually wears out and eventually fails.

The other two pulse protection technologies do not have a fundamental limitation on the resource and pulse energy. This, of course, does not indicate that they can work forever and effectively suppress impulses of any amplitude and duration.

The task UPS is not only to withstand the impulse, but also to reduce its amplitude to a value acceptable to the computer. The table shows the approximate values of the pulse suppression coefficient for different UPS. This coefficient is equal to the ratio of the pulse amplitudes without protection and when using protection. I am not aware of this value for a double conversion UPS.

Efficiency

Efficiency is not the most important characteristic UPS. If the computers being protected UPS really work, the electricity they consume costs significantly less than the data stored in them. Therefore, the efficiency factor itself cannot be considered as a parameter by which to choose UPS.

However, there are several important parameters related to efficiency that are worth discussing.

Efficiency is the ratio of the power consumed by the load UPS to total consumed UPS power. The greater the efficiency, the smaller the part passing through UPS power is released inside its body.

Additional heat generation inside the case UPS leads to a number of unpleasant consequences.

If not taken additional measures to remove heat from the case (for example fans), then the temperature inside UPS will rise. This will reduce battery life UPS(if it is installed inside). According to battery manufacturers, increasing the operating temperature of a battery by 10 degrees leads to a reduction in its service life by half.

Therefore everything Medium and high power UPS, which cannot be cooled by natural convection, are equipped with forced cooling.

Low power UPS, built according to a double energy conversion circuit, and ferroresonant ones, also have to be forcibly cooled, since they have the lowest efficiency compared to other types we have considered UPS.

Efficiency value for Double conversion UPS and ferroresonant UPS(according to manufacturers) is 85-94% at full power. If the load power is reduced to 70-80% of the rated efficiency of modern uninterruptible power supplies remains almost unchanged. It begins to drop noticeably only at even lower load power.

IN Lately appeared UPS with an efficiency of at least 70-80% even at powers of about 30% of the nominal.

Switching UPSs interacting with the network have approximately the same efficiency, since when operating from the network, the main power when operating these UPS is supplied to the load practically without conversion. Their efficiency when operating from the network is no less than 96% at full power and gradually decreases with decreasing load power.

Battery life

For most common office UPS low power, battery life at maximum load is 4-15 minutes.

less than the maximum, the battery life increases. Due to the non-linearity of the battery discharge curve, this increase is not proportional to the decrease in load. If the load is halved, then the operating time can increase by 2.5-5 times, if it is tripled, then the time increases by 4-9 times, etc.

Determine exactly how long it will work UPS at partial load, this can only be done experimentally or using the manufacturer’s data. The following figure shows a graph from which you can approximately estimate this value.

Rice. 29. Working hours UPS from the battery at a load less than rated

as a percentage of nominal. The y-axis is the number of times the battery operating time is greater than the battery operating time at rated load. The figure shows data from manufacturers for UPS more than 50 different power models from 250 to 18000 VA.

Using the chart is very simple. If your computer's power is 50% of the rated power of your UPS, then, having found the corresponding division on the abscissa axis (horizontal axis), rise vertically upward. At the intersection with the middle of the point cloud you will find the value you need: operating time UPS from the battery will increase approximately 3.5 times.

Battery life data is usually based on a new and fully charged battery. The characteristics for a worn out battery will be completely different. We can only say that the operating time from a worn out or not fully charged battery will be shorter.

UPS high power and some UPS low power units have the ability to increase battery life by replacing the battery with a larger capacity battery or installing an additional battery.

A larger capacity battery can be installed in the same case or can be installed additional housing for battery.

If the battery capacity UPS increases, but the power of its charger remains the same, then the battery charging time increases. When the battery capacity increases several times, the charging time increases approximately the same times.

Some manufacturers UPS provide the possibility of replacing the charger with a more powerful one. This allows you to maintain acceptable charging time when increasing battery capacity.

Three-phase UPS They usually have the ability to regulate the charging current depending on the capacity of the installed battery.

Low power UPS, specifically designed for long-term battery life, usually have a modular design. This means that the user himself selects the type of battery or the number of battery units of the same type corresponding to the required operating time.

Increasing the battery capacity of the same type of modules to a capacity corresponding to the battery life (at full load) of more than several hours leads to the appearance of a battery station with a huge number of batteries. Such a system has at least one drawback: a large number of contacts that are prone to oxidation. Therefore, servicing such UPS can be a problem, and troubleshooting a battery can take several days.

Typically, diesel or other electric generators are recommended to keep equipment running for longer than a few hours.

Some UPS have an indicator that lets you know how charged the battery is UPS and how long can it last? UPS from battery.

Charge measurement UPS batteries quite a difficult task and very few manufacturers UPS They really know how to solve it. Sometimes UPS intended for relatively long battery life (for example, to complete any calculations or data transfer). In this case, you usually need to know more or less accurately how much time is left until the battery is completely discharged. In this case, it is better not to trust the statements of sellers or manufacturers UPS about the function at your disposal, but check its capabilities yourself.

There can be three options for estimating the time remaining before the battery is discharged.

The simplest option. UPS measures the current flowing through it and, after switching to battery operation, begins to count the time remaining until the battery is discharged, using the data recorded in permanent memory information about the discharge cycle. The calculation is made based on the full charge of the battery. Therefore, if your battery is somewhat discharged or worn out (you may not know it), you may be unpleasantly surprised by being left without power at the most crucial moment.

Second option. UPS measures the voltage on the battery and, based on the information about the discharge cycle recorded in permanent memory, displays (on a digital or LED indicator) battery charge. In this case, you are given the opportunity to independently determine approximately the moment when UPS will turn off.

The third option is actually a combination of the first two. Based on data on the battery charge and current consumed by the load, a number corresponding to the remaining battery life is displayed on the digital display.

As already mentioned, these data may not be entirely accurate. The best (of the ones I know) UPS) this function is implemented in UPS Ferrups.

Power factor. Watts and volt-amps

One of the most popular questions asked by buyers UPS, is the question of how watts differ from volt-amperes.

In a DC circuit the situation is quite simple. Electric current, coming from a direct current source to the load, produces useful (or useless) work in it by moving charges in the direction of the electric field. Calculating the power in such a circuit is very simple: you need to multiply the current by the voltage drop across the load:

P[Watt] = I[Ampere] * U[Volt]

In the AC circuit that we have to deal with when considering the operation UPS, everything is a little different.

For alternating current, the concept of instantaneous power is introduced - this is the product of the instantaneous values of the variable voltage and current. Active power (time-average power released in the load) is equal to the period-average value of instantaneous power.

If the voltage has a sinusoidal shape, and the load in the circuit is active (or, in other words, ohmic - for example, incandescent lamps), then the active power is equal to the product of the effective values of voltage and current. Those. it is calculated in approximately the same way as power in a DC circuit:

P[Watt] = Uact *Iaction.

Rice. thirty . Instantaneous power in an alternating current circuit.

a) sinusoidal current in the active load;
b) sinusoidal current in the load with a reactive component;
c) non-sinusoidal current (non-linear load).

In Fig. 30a it can be seen that in this case the voltage and current always have the same sign (they become positive and negative at the same time). Therefore, instantaneous power is always positive. Physically, this means that at any given time, power is being released into the load. In other words, just as in a direct current circuit, charges always move in the direction of the electric field.

If the voltage and current are sinusoidal, but the load has a capacitive or inductive (reactive) component, then the current leads or lags the voltage in phase. In this case, the power released in the load is reduced.

Figure 30b shows that due to the phase shift, at some points in time, voltage and current have opposite signs. At this time, the instantaneous power turns out to be negative and reduces the average instantaneous power over the period. An electrical engineer will say that at these times current flows from the load to the current source. From a physicist’s point of view, at these moments in time the charges move by inertia against the forces of the electric field.

The formula for the average active power over a period for the case of a load with a reactive component changes slightly. It shows the power factor. For sinusoidal voltage and current, it is numerically equal to the “cosine phi” familiar from high school:

P[Watt] = Uact * Iaction * Cos(F).

Here Ф is the phase shift angle between voltage and current.

The product of the effective values of voltage and current is called the apparent power of an alternating current circuit and is measured in volt-amperes (VA). The apparent power is always greater than or equal to the active (distributed in the load) power.

If the load is a computer, then the situation is a little more complicated. The current consumed by the computer has a non-sinusoidal shape (see Fig. 30c). The power released in the load with this current form is also less than the product of the effective values of voltage and current.

In Fig. 30v it can be seen that at certain voltage values (when the voltage is low) the computer does not consume current. Instantaneous power at these moments of time is zero - the voltage is “wasted” without producing work.

The active (distributed in the load) power for the case of a nonlinear load is expressed by the formula.

P[Watt] = Uact *Iaction *TO,

where K is the power factor.

The current of the “computer” load is usually somewhat ahead of the voltage. But the phase shift is very small (10-20 degrees), so the power factor for a computer is not equal to the cosine of the phase shift angle, but much less.

According to American Power Conversion, the power factor is 0.6 for personal computers and 0.7 for mini computers. In fact, the power factor of a computer load is related to the current crest factor and, even for the same switching power supply, depends on how much the power supply uses its rated power. So, if the switching power supply is lightly loaded (few consumers are connected to it - disk drives, processors, etc.), then the amplitude factor increases, and the power factor decreases.

Know the power connected to UPS equipment is necessary in order not to exceed the maximum permissible load UPS. But the load (or overload) UPS is determined not only by how much power is released in the load, but also by how much current flows through UPS. Therefore, when specifying the limit for UPS loads usually indicate the maximum apparent power in volt-amperes and the maximum active power in watts.

Choose UPS it is necessary so that the maximum load power does not exceed the maximum power UPS.

The question arises: what power - total or active? Answer: both!

The apparent power of the load must be less than the rated apparent power UPS(you need to compare volt-amperes - VA). And the active power of the load should not exceed the rated active power UPS(you need to compare watts - W).

For different loads and different UPS the limitation can be either apparent or active power. Most often (for computer loads) the limitation is total power.

Typically, the power of a computer or peripheral device is indicated in volt-amperes. If it is indicated in watts, you should be prepared for the fact that the power in volt-amperes will be 20-40% more, and choose UPS appropriate power.

Features of three-phase uninterruptible power supplies

Why three-phase UPS allocated to a separate group? After all, the principle of operation of most of them (and all described in this book) - double energy conversion - is the same as that of many single-phase devices.

In addition to the obvious differences from single-phase devices, three-phase UPS have some useful features that are not too noticeable at first glance. Usually, three-phase UPS provide a new quality of protection simply due to the fact that UPS has a three-phase input.

Load distribution by phases

One of the problems when using single-phase UPS(or even just any consumers connected to the network) is the distribution of the load across phases.

If electricity consumers are unevenly distributed across the phases of the electrical network, then, with a significant network load, two effects occur:

one of the network phases is overloaded, while other phases are not using their full capabilities;

phase imbalance - inequality of phase voltages in different phases of the network (the voltage in the overloaded phase is less than the nominal one, and the voltage in the underloaded phases is more than the nominal one).

A consequence of uneven load distribution across phases is also overload of the neutral wire. Traditionally, in domestic electrical networks, the neutral wire has a cross-section 1.5-2 times smaller than the phase wires (after all, it is intended for the flow of compensation currents, which should be less than the currents in the linear wires).

Therefore, currents arising in the neutral due to phase imbalance can lead to overload of the neutral wire. This usually affects the efficiency of grounding and can lead to equipment malfunctions.

Three-phase UPS solve the problem of phase imbalance automatically. At the entrance UPS the load is always evenly distributed among the phases due to the fact that the rectifier and inverter UPS work independently.

Therefore, the neutral wire is less loaded (there are no compensation currents associated with phase imbalance). Grounding works as efficiently as possible and there is little interference with computer operation.

At the exit UPS The problem of uneven distribution of load across phases of course remains. With single phase three phase UPS with a power of 30 kVA you cannot remove more than 10 kVA. But even if you load one of the phases completely, and the others are underloaded, then good three phase UPS with independent voltage regulation in phases it will work normally, and the influence of uneven load distribution will only affect during the transient process that occurs when the load changes sharply.

Thus, unloading the neutral wire leads to a general “improvement” of the electrical network.

Harmonics in a three-phase electrical network

The three-phase electrical network was invented to use sinusoidal currents, and is ideal for them. The use of nonlinear consumers (for example, computers) in a three-phase electrical network (and all our electrical networks are like this) has very serious (and very unpleasant) features.

Let's imagine an oscillogram of currents in a three-phase electrical network (see Figure 31). Let there be only linear loads in the electrical network. Consequently, only sinusoidal currents flow in all wires. Let us also assume that these currents are approximately equal.

Rice. 31. Sinusoidal currents in a three-phase electrical network.

In this case, the load in the electrical network is distributed approximately evenly: the currents in each phase are approximately the same (the root mean square or effective value of the current varies from 70 to 85 A). A current flows in the neutral wire, which is the geometric (vector) sum of all currents in the linear wires. The currents partially compensate each other, and the resulting current in the neutral wire is much less than the current in each of the linear wires. In this case, the effective value of the current in the neutral wire is 12 A.

The neutral wire is needed to compensate for differences in the currents of the linear wires. In the case when equal currents flow in all linear wires, compensation is not required: the current in the neutral wire is zero.

The case when the entire network load is concentrated in one of the phases is the worst: the current in the neutral wire is equal to the current in the phase wire. But usually electricians monitor, if not the uniformity of load distribution across phases, then at least, ensuring that none of the phases is overloaded. Therefore, as a rule, the load in a three-phase network is distributed more or less evenly, and the current in the neutral wire is small.

When designing electrical networks, this convenient fact is widely used to save material. In domestic three-phase cables, one of the wires (neutral) often has a much smaller cross-section. For example, in a cable designed for a current of about 100 A (three-phase network power is about 70 kVA), the linear wires have a cross-sectional area of 35 or 25 square meters. mm, and the neutral wire is only 16 square meters. mm. With sinusoidal currents and an approximately uniform distribution of the load across the phases, this does not matter: the neutral wire is very far from overloading.

Let's now see how a three-phase electrical network behaves when non-sinusoidal currents flow through it, characteristic of “computer” loads equipped with switching power supplies.

Figure 32 shows the oscillogram of nonlinear load currents in a three-phase electrical network. All three phases of the network are equally loaded with a “computer” load with a significant harmonic distortion factor and a crest factor of 3.

Rice. 32. Nonlinear load in a three-phase electrical network.

The effective current value in each of the three phases is 85 A. It is approximately the same as the effective current values in Fig. 31.

Despite the completely symmetrical load, a very large current is observed in the neutral wire. Its effective value is 120 A. The amplitude value of the current is 226 A. This means that the neutral wire does not perform (or does not perform well) its function of compensating currents under nonlinear loads.

The figure shows that the amplitude of the current in the neutral is even slightly less than the amplitude of the current in the linear wires. Why is the effective value so much greater? Taking a closer look at Fig. 32 (and comparing it with Fig. 31), you will see the answer - the frequency of the current in the neutral does not coincide with the frequency of the current in the linear wires. A current flows in the neutral with a frequency of 150 Hz.

Having opened a reference book on electrical engineering, we can easily find that the wheel has not been invented. When equal non-sinusoidal currents flow in the linear wires of a three-phase network, the effective value of the current in the neutral wire is the sum of harmonic currents, the number of which is a multiple of 3. The intensity of the ninth and subsequent harmonics in the current consumption of a switching power supply is not too high. But the third harmonic is the main (after the first) harmonic in the computer's current consumption - its intensity can reach 60%, and it is the neutral wire that is mainly responsible for the overload. (That's where 150 Hz comes from in neutral).

Why is this dangerous? Let's look at a simple example.

Let's take a small building to which a three-phase cable is connected. Let three of the wires have a cross-section of 25 square meters. mm, and the fourth (neutral, of course) wire is 16 sq. mm. A three-phase 100 A circuit breaker is installed at the entrance to the building, approximately corresponding to the maximum current of the linear wires. The current limit of the neutral wire is 80 A, but fuses are not installed on the neutral wire due to the danger of severe distortion of the three-phase AC system if the neutral wire breaks.

With a linear load equal to approximately 80% of the maximum (see Fig. 31), the linear wires are well loaded, but not overloaded. The neutral wire, designed for a current of up to 80 A, is practically in idle mode.

With a nonlinear load equal to 85% of the rated load (Fig. 32), the linear wires are loaded in the same way as when sinusoidal currents flow in the network. The current in the neutral wire exceeds the current in the linear wires by almost one and a half times. Let us remember: the neutral wire is designed for a current of no more than 80 A. A dangerous overload is obvious.

The worst thing about this situation is that no one will notice this overload. No protection device will respond to it. After all, measuring instruments are usually not installed on the neutral wire.

What to do? How to protect the network from nonlinear load?

There are two options: create a new electrical network with two or three times the power reserve, or install three phase UPS.

UPS with three-phase input has a rectifier as an input device. The rectifier is definitely a nonlinear load. But in the spectrum of the current consumed by a three-phase rectifier, there is no third harmonic and all higher harmonics, the number of which is a multiple of three.

What will happen if from the spectrum of currents shown in Fig. 32 exclude the third and ninth harmonics (and, albeit of low intensity, other harmonics with a number divisible by 3)? Almost a miracle will happen: the effective value of the current in the neutral wire will become equal to zero. The electrical network of our example home was saved from overload, and the house was saved from fire.

Six-pulse and twelve-pulse rectifiers

A conventional full-wave rectifier in a single-phase electrical network has an input current spectrum consisting of harmonics numbered 2±1 (i.e., many odd harmonics). The harmonic amplitude decreases more or less monotonically with increasing its number (see Fig. 24).

Traditionally in three-phase UPS 6-pulse (or six-half-wave) rectifiers are used. The name implies. that during the period of a three-phase network, 6 current pulses appear at the output of such a rectifier. The simplest circuit of such a rectifier is a three-phase bridge (see Fig. 33)

Rice. 33. Three-phase bridge

The spectrum of current harmonics of a 6-pulse rectifier includes (except for the first harmonic) harmonics with numbers 6±1 - see Fig. 34.

Fig.34. Current spectrum of a 6-pulse rectifier

In theory nth amplitude harmonic is equal to the amplitude of the first harmonic divided by n. Those. the amplitude of the 5th harmonic is 20%, and the amplitude of the 11th harmonic is about 9% of the amplitude of the first harmonic. Accordingly, the theoretical harmonic distortion coefficient of the input current of a six-pulse rectifier is approximately 30%.

To reduce harmonic distortion, 12-pulse rectifiers are used. The twelve-pulse rectifier consists of two three-phase bridges. One of them is supplied with voltage directly from a three-phase network, and the second bridge is powered by a special transformer that shifts the phase by 30 degrees.

Theoretically, the current spectrum of a 12-pulse rectifier includes (except for the first harmonic) only harmonics with numbers 12±1 - see fig. 35.

Rice. 35. Theoretical spectrum of the input current of a twelve-pulse rectifier

Accordingly, the theoretical harmonic distortion factor of the input current of a 12-pulse rectifier is approximately 14%.

In practice, due to the incomplete coincidence of the characteristics of the two rectifiers, harmonics with numbers 6±1 cannot be completely suppressed. Therefore, the harmonic distortion coefficient of a twelve-pulse rectifier may differ slightly from its theoretical value.

For even more significant suppression of current harmonics, 24-pulse rectifiers or (somewhat more often) harmonic filters are used (very rarely).

A 24-pulse rectifier has harmonics numbered 24±1 in its spectrum. The theoretical coefficient of harmonic distortion of the input current of such a rectifier is less than 7%.

Harmonic filters are most often resonant L-C chains, designed to filter certain harmonics. So, to work with a six-pulse rectifier, filters are used that almost completely absorb the 5th and 7th harmonics. In this case, the input current harmonic distortion factor is reduced to approximately 18%.

In recent years, with the advent of fast power semiconductors, a breakthrough has occurred in the noble cause of combating current harmonics. Now in some UPS The rectifier is built on insulated gate bipolar transistors (or, in English, IGBT). The input current of such a rectifier has a sinusoidal shape. Those. harmonic distortion factor is 0.

Harmonics and electrical generators

When creating an uninterruptible power supply system, it is sometimes necessary to install diesel generators to ensure long-term operation of high-power equipment. The generator in this case has a power comparable to the power of the equipment as a whole (and not much greater, as in the case of power from the electrical network or, ultimately, from the electric generator of the power plant).

With this ratio of parameters, the generator strongly interacts with current harmonics arising in an electrical network with nonlinear loads. Currents that are dangerous to its safety arise in the generator, which are absent when the generator operates on a linear load of the same power. These currents cause the generator to overheat and reduce its resource.

Therefore, when operating the generator for computer loads, a large power reserve is required (see Chapter 11). Application of three-phase UPS allows you to eliminate the third harmonic in the spectrum of the current consumed by the generator and significantly reduce the required power reserve.

To reduce the power reserve even more significantly, all the measures described above are used to combat harmonics, but most often - special harmonic filters and three-phase UPS with 12-pulse rectifier.

UPS Feature Summary

Some of what has already been said about the properties UPS You can try to summarize the different types in a table.

In the table on the next page, the presence of a particular property in the UPS is marked with asterisks. The more stars in the table cell, the more developed the quality in question. The presence of an asterisk in the column does not mean that this type of UPS has the specified property by definition, but refers to the best models UPS the specified group.

UPS type	Switching UPS	UPS, inter- valid howling with the net	Ferro- reso- nano UPS	Double Conversion UPS
UPS type	Switching UPS	UPS, inter- valid howling with the net	Ferro- reso- nano UPS	One- phase	Three- phase
No voltage drop when switching to battery operation	*	*	*****	*****	*****
**	**	****	*****	*****
Resistance to voltage surges		*	****	****	****
Can be used for long battery life		**	***	***	*****
Electromagnetic noise suppression	*	*	****	*****	*****
High Voltage Pulse Suppression	*	**	*****	****	****
Correcting the sinusoid shape			**	*****	*****
Unloading the neutral wire			****		*****
Voltage stabilization		**	****	*****	****
Hot standby and parallel operation					****
Reliability under ideal electrical network conditions	****	***	*****	****	****
Reliability in conditions of poor electrical network	**	*	*****	**	****
Level of protection of equipment in a good electrical network	**	****	****	*****	*****
Level of equipment protection in a poor electrical network	*	*	****	**	***

An uninterruptible power supply is a temporary substitute for the electrical network for the equipment connected to it. The quality of this replacement greatly depends on the type and brand of UPS.

In an electrical network, the voltage has a sinusoidal shape or a shape close to a sinusoid.

All high-end UPSs also have a sinusoidal output signal, i.e. provide a power supply that is practically no different from a regular network or even have a higher quality sine wave.

At the output of the UPS (as well as in the network), the sinusoid may not be entirely ideal.

If the harmonic distortion coefficient is more than 20%, you will not be able to call the voltage waveform a sinusoid.

This method, like any simplification, has its limitations. In particular, the higher the harmonic number, the lower the harmonic distortion coefficient it is clearly visible.

All double conversion UPSs, ferroresonant UPSs, and most line-connected UPSs have a sinusoidal output voltage. For all these UPSs, a total harmonic distortion of the output voltage of 5% is the limit. If the harmonic distortion coefficient at the UPS output is less than 5%, then the UPS can be considered “good” by this parameter. If this value is more than 5%, then the shape of the UPS output signal leaves much to be desired.

Manufacturers usually indicate the degree of harmonic distortion in the general list of UPS technical characteristics. Almost always, only one value of the harmonic distortion coefficient is indicated, which relates to some average (if not ideal) conditions - for example, with a linear load. It should be borne in mind that the most significant distortion of the output voltage waveform can occur under various boundary conditions, as well as under parameters not typical for normal UPS operation.

Such limiting conditions (their set or combination may be different for different UPS models) can be maximum load or idle (no load); marginal or prohibitive power factor (for example, less than 0.5), too high crest factor. The output voltage can also experience serious distortion during various transient processes (for example, during a step change in load).

In mains mode, the switching UPS and the mains-interacting UPS supply their loads with filtered mains voltage. That is, in this case they are not independent power sources. Such a source is the electrical network. This means that the harmonic distortion coefficient at the input of the computer power supply will be approximately the same as without a UPS. This is so because the filters of these UPSs are not designed to filter out low-frequency harmonics, and allow them to pass freely. Accordingly, if there were strong harmonic distortions in the network before installing the UPS (due to general network overload or a large share of the power of non-linear loads), they will remain so. If these distortions did not exist, they will not appear.

The situation is different with ferroresonant UPS and double conversion UPS. They are, in this sense, independent power sources. Therefore, everything said above regarding distortions in the mains voltage waveform must in this case be attributed to the output voltage of the UPS. If these UPSs are heavily loaded (almost up to the rated power) with non-linear loads, then fundamental harmonic distortion may appear at the input of these loads, which would not exist without the UPS. On the other hand, if harmonic distortion was observed when operating from the network, then it may disappear after installing the UPS if the UPS is underloaded.

If the non-linear load on-line of the UPS is more than two-thirds of its full power, then the voltage at the UPS output may be noticeably distorted. While not dangerous in itself for computers, voltage waveform distortion is not a good sign that the UPS load is too high. It is better to install a higher power UPS or disconnect any equipment from it.

Some high-end double conversion UPSs are equipped with a special control circuit whose purpose is to adjust the output voltage waveform even when operating high-power non-linear loads. The output voltage of these UPSs does not have noticeable harmonic distortion, even if the UPS supplies non-linear loads of significant power.

Of course, all computers and other equipment designed to be powered from an alternating current network are designed for sinusoidal voltage. It is unlikely that any manufacturer of this equipment is ready to guarantee the normal operation of its equipment with highly non-sinusoidal voltage.

However, most electrical energy consumers can be powered by non-sinusoidal AC voltage. Moreover, for different equipment, different characteristics of the sinusoidal supply voltage are more important. For example, equipment equipped with switching power supplies (say, personal computers) consumes current only at times when the voltage is very close to the maximum. Therefore, to power such equipment, the correct amplitude voltage value is important. Equipment containing directly powered electric motors and heaters requires rated rms voltage. Sinusoidal voltage meets the requirements of any of these loads.

Previously (a very long time ago), some switching UPSs had an output voltage in the form of a meander (rectangular pulses of different polarities).

Rice. 26. Meander

When we replace a sinusoidal voltage with one or another approximation, we must choose the parameters of this approximation such that they are closest to the parameters of the replaced sinusoid. But in a meander, the amplitude and effective voltage values are equal to each other (the amplitude coefficient is equal to unity). Therefore, we cannot make the square wave voltage such that it can satisfy the requirements of different loads at the same time.

In an attempt to find a compromise, manufacturers of such UPSs set the square-wave voltage equal to a certain value lying between the amplitude and rms. The result was that some loads (requiring the correct RMS voltage) could fail due to excess voltage, while other equipment (those that draw current at voltages close to the maximum) had too little voltage.

To ensure that the RMS and peak-to-peak values of the rectangular voltage are equal to the corresponding values of the sinusoidal voltage, manufacturers of modern switching UPSs have slightly changed the shape of the square wave by introducing a pause between rectangular pulses of different polarities.

Rice. 27. Meander with a pause.

UPS manufacturers call a voltage of this form “stepped approximation to a sine wave.” This shape of the curve allows, with correctly selected voltage amplitude and pause duration, to meet the requirements of different loads. For example, with a pause duration of about 3 ms (for a frequency of 50 Hz), the effective voltage value coincides with the effective value of a sinusoidal voltage of the same amplitude.

The output voltage of all switching UPSs that I have come across, which are present on the Russian market, has the form of a stepwise approximation to a sine wave.

Shown in Fig. 27 output voltage form is the ideal that UPS manufacturers should theoretically strive for. The actual shape of the output voltage of a switching UPS is of course different from the ideal.

Sometimes UPS manufacturers observe the declared equality of the effective value of the voltage at the UPS output to the effective value of the mains voltage very approximately. The duration of pauses and the amplitude of the rectangular voltage deviate noticeably from the calculated values.

These deviations apparently cannot serve as a basis for declaring a particular UPS bad. After all, they all work normally with personal computers, for which they are actually intended to work.

The actual output voltage waveform of a switching UPS is shown in Fig. 28.

Rice. 28. Voltage and current oscillograms of a personal computer connected to a switching UPS.

The same oscillogram also shows the curve of the current consumed by the computer. This allows you to evaluate how hard it is for a computer protected by a switching UPS. But, oddly enough, the strong pulse currents consumed by the computer at the beginning and end of the rectangular pulse do not affect the operation of the computer. They are completely suppressed by the computer's power supply, the output of which is a constant voltage with a normal ripple level.

We should also not forget that a computer protected by a switching UPS is powered by non-sinusoidal voltage only when the UPS is operating on battery power (i.e. very briefly). When the UPS operates from the network, the computer is powered by the mains voltage, smoothed using noise and pulse filters built into the UPS.

The possibility of using a switching UPS to power other equipment (not computers) requires, generally speaking, verification in each such case. There are known cases when some printers refused to work with such UPSs. On the other hand, there is a known case of using switching UPS to protect such non-traditional loads as telephone exchanges or cash registers with transformer power supplies.

The use of a switching UPS to power appliances with transformer power supplies should be approached with caution. The fact is that the usual 5-10% losses for a transformer in the presence of harmonics increase in proportion to the square of the aharmonic number. Therefore, the resource of heavily loaded transformers when supplied with voltage in the form of a meander can decrease tens of times.

As with any power source, the output voltage waveform of a switching UPS depends on the size and nature of the load. For UPS manufactured by world-famous companies, this dependence is usually small.

However, some UPSs have a strong dependence of the shape (and sometimes the amplitude) of the output voltage on the load. Some of them cannot be used with light loads, since they have an output pulse voltage with an amplitude of up to 800 V. Others are tested by the manufacturer only when working with linear loads. Such UPSs, when working with a computer, may be unstable during switching moments.

The above shows: you should not use UPS from unfamiliar manufacturers or buy such UPS from non-specialized companies.

Magazine "Electronic Components" No. 9, 2008

Valery Klimov, Ph.D., technical director, Ruselt

When comparing uninterruptible power supplies (UPS) from different manufacturers, you should first of all pay attention to their technical characteristics, which reflect consumer properties and qualities. The article discusses the important energy indicators of the UPS and its overload characteristics. Dynamic characteristics reflect the reliable operation of the UPS during load switching, mains voltage surges, overloads and other disturbances that occur in the “network - UPS - load” system. The results of an experimental study of the dynamic modes of single-phase UPSs with double conversion, discussed in part 1 (“EK” 6, 2008), are presented.

Classification of UPS electrical characteristics

Requirements for UPSs and the classification of electrical characteristics of modern UPSs are most fully presented in the new international standard. The standard previously in force in our country does not reflect the full requirements for modern UPS structures. List proposed by the author electrical parameters The UPS is supplemented with a number of energy indicators:

Input characteristics include: rated values of powers, voltages, currents and their permissible deviations, starting currents, input power factor, harmonic composition of the input current;

Input characteristics reflect: static and dynamic accuracy indicators, sinusoidal distortion factor, efficiency, output power factor, UPS overload capacity;

Transitional (system) indicators characterize: frequency synchronization, reserve time, battery charge restoration time, generalized energy coefficient;

The parameters of the DC circuit characterize requirements for nominal battery voltage values;

Operational requirements (environmental conditions) reflect the influence of temperature, humidity, altitude, etc. on the performance characteristics of the UPS.

Let's take a closer look at the main electrical characteristics UPS.

UPS input characteristics

Input Voltage Ratings , adopted in our country: for single-phase UPS - 220 V; for three-phase UPS – 220/380 V, 50 Hz.

*The first, second and third parts of the article were published in “EK” on 6, 8, 9, 2008.

Permissible deviations input voltage characterize the limits of change in input voltage at which the UPS continues to operate in network mode without switching to autonomous power supply mode from the battery. Modern booster UPS structures provide a range of +/-20% or more. It should be noted that for a number of single-phase UPS models, the lower limit of the input voltage expands as the load decreases.

Rated input apparent power (Sin.nom) – total power loading the network at 100% load factor and standard operating conditions. A distinction is made between the input power consumed when the battery is charged (Sin.min) and the power during forced battery charging (Sin.max), which exceeds the first value by 25 - 30%, depending on the size of the battery capacity and the degree of its discharge. For example, for a UPS with a rated output power of 30 kVA and an input power factor of 0.8, we have: Sin.min = 32.8 kVA and Sin.max = 41 kVA.

Rated input active power (Rvkh.nom) characterizes the energy consumption at the UPS input at rated load:

Rin.nom=RinxSin.nom

Input power factor (Крвх) characterizes the ratio of active input power to total power at rated input voltage and 100% load.

Krvh values for different UPS models and capacities can vary from 0.8 to 0.99. The higher the value of Krin, the lower the distortion of the input current sinusoidality. In this case, the input resistance of the UPS in relation to the network will be purely active. The highest value of Krvx = 0.99 was achieved in UPS structures with an input PWM converter based on IGBT transistors.

The components of the reactive power currents and distortion power in the input circuit of the converter (bridge circuit of a three-phase rectifier) will be closed in the input circuit of the system and depend on the parameters of the input filter, the reactive parameters of the DC link (as this affects the shape of the current consumed from the network) and the degree system load.

Maximum input current – parameter that determines the choice of an external UPS circuit breaker. The maximum current value is determined at 100% load factor, minimum input voltage in the forced battery charge mode:

Iin.max=Sin.max/Uin.min

Starting current value – characterizes the input current surge due to the charging of storage capacitors when the UPS is turned on. To limit the current surge, modern UPSs use starting circuits or a UPS soft start algorithm.

UPS output characteristics

Static accuracy The output voltage for single-phase low-power double conversion UPS is +/–2%, for medium power and three-phase UPS it reaches +/–1%, which allows parallel operation of 4–8 units for a total load. The dynamic accuracy of modern UPSs is +/-5% at 100% load surge.

The external characteristic of the UPS characterizes the degree of static accuracy of the output voltage. In the general case, the rigidity of the external characteristic is determined internal resistance power circuit, including a rectifier, power factor corrector (PFC), direct-voltage converter (DCC) and inverter. KKM - PPN have stabilizing properties. Thanks to this, the inverter supply voltage is also stable, so we can assume that the main parameter that determines the external characteristics of the UPS is the output resistance of the inverter. Modern inverters based on IGBT transistors with pulse-width modulation (PWM) of the output voltage have a low internal resistance. Compared to power transformers, the inverter has 5 times less internal resistance, which ensures not only high accuracy of output voltage stabilization (1 - 2)%, but also low values of the output voltage sinusoidal distortion coefficient (less than 3%) at currents in nonlinear loads with amplitude factor up to 3.

Rated Apparent Power Output (Sout.nom) – the maximum total power that the inverter can deliver to a linear load with a power factor (Krn) equal to the output power factor of the UPS (Krout) under standard operating conditions (temperature, humidity, altitude).

Output power factor (Curve), specified by the manufacturer, corresponds to the value of the load power factor at which maximum efficiency of electricity consumption from the UPS is ensured. Curve values for modern UPSs are 0.7...0.9.

Rated active output power (Rout.nom) – maximum active power supplied to the load:

Rin.nom=RinxSin.nom

Efficiency and heat losses

Efficiency characterizes the efficiency of using the UPS and represents the ratio of the output active power consumed by the load to the input active power consumed by the UPS from the network. Active power losses (heat losses) in a UPS are characterized by a number of components:

∆P=Pin Pout=∆Pxx+∆Psc+∆Padd

∆Pхх – constant component of losses (no-load losses of the UPS) does not depend on the load factor and is determined by the energy required to service the control system of power units, power supply to cooling fans and other auxiliary units. In UPS of low and medium power 1 - 10 kVA, no-load losses account for 20 - 30% of the total losses. As the UPS power increases, the relative share of no-load losses decreases.

∆Pсс – variable component of losses, which depends on the load factor

∆Psc = ∆P1+∆P2+∆P3+∆P4

∆P1 – losses in the rectifier power circuit;

∆P2 – losses in the power circuit of the power factor corrector;

∆P3 – losses in the power circuit of the DC-DC converter;

∆P4 – losses in the inverter power circuit.

The technical data of UPS manufacturers contains the efficiency values of individual UPS power units (mainly the rectifier and inverter) and the values of the overall (system) efficiency, amounting to 85 - 88% for low-power UPS and 90 - 94% for medium and high-power UPS;

∆Padd – additional losses per battery charge, which are variable over time and depend on the degree of discharge of the battery and its capacity. The greatest additional losses occur during forced battery charging. For example, losses at rated load in a 30 kVA UPS are: 2.8 kW - with forced battery charging mode and 2.2 kW - with a charged battery.

UPS load characteristic represents the nonlinear dependence of the total power transfer coefficient on the load power factor FORMULA

Let us introduce the concepts of total power transfer coefficient to the load and the load characteristics of the inverter.

Total power transfer coefficient to load – the ratio of the maximum permissible load power to the rated total power of the equipment: FORMULA The coefficient K5 is correlated with the concept of the power reduction factor Kd (derating factor), indicating the percentage of the value of the active component of the load power that can be connected to the inverter.

The power reduction factor depends on the nature of the load. Table 1 shows an example of the values of power reduction factors for an inverter output power factor of 0.8 and various values of load power factors.

Table 1. Dependence of the power reduction factor on the nature of the load.

The current of the output filter capacitor is summed with the current of the capacitive component of the load, which reduces the maximum permissible load at the inverter output. The reactive power component and high frequency harmonic power distortion components at the inverter output will be exchanged between the load, the inverter output filter and the DC link filter capacitance. Closing in the specified circuit of the power circuit of the converter, their values will depend on the load power factor. Moreover, the output power factor may differ from the load power factor. The value of the total power transfer coefficient to the load reaches 100% when the power factor of the linear inductive load is equal to the output power factor of the UPS. Figure 1a shows the load characteristics at various types linear load RL, RC and non-linear load RCD. With a nonlinear load, the power transfer coefficient decreases. The most common single-phase nonlinear loads of the RCD type are uncontrolled rectifiers with a capacitive filter. The current amplitude factor of such a load reaches 2.5 - 3 with a power factor of 0.7 - 0.6. In Figure 1b

The dependences of the power factor and the amplitude factor of the RCD load as a function of the duration of the current pulse during the half-cycle of the mains voltage are given. When operating the UPS on different types of loads, the equivalent nonlinear load is taken to be the sum of the loads: 50% – RL – linear load with Krn = 0.8 and 50% – RCD – load – uncontrolled rectifier with a filter capacitance of 2.5 µF/W. The power transfer coefficient to a nonlinear load at a current with a crest factor Ka = 3 does not exceed the value Ks = 70 - 80%.

Vector diagram of inverter power (see Fig. 2) clearly reflects the load capacity of the UPS, and has recently been included in the catalogs of a number of the world's leading UPS manufacturers. The upper quadrant of the diagram characterizes the power with an active-capacitive load (kVAr-C), the lower quadrant - with an active-inductive load (kVAr-L). The following notation is used here:

the horizontal axis corresponds to the relative values of active power P ;
O - the center of the circle of maximum total power with an inductive load;
OB is the vector of the relative maximum total power supplied to the inductive load ( S max) at rated active power;
O 1 - the center of the circle of maximum total power with a capacitive load;
ВС – value of rated active power at the output of the converter ( P nom) ;
OA – the limiting value of the relative total power supplied to the inductive load at reduced active power;
ОD is the limiting value of the relative total power supplied to the capacitive load at reduced active power.

The cosines of the rotation angles of the total power vectors relative to the actual coordinate axis will correspond to the power factors of the loads at the inverter output. The position of the vertical line of the rated output active power ( P nom ) is determined by the output power factor of the inverter K Pout = P nom/ S nom.

With the capacitive nature of the load, the center of maximum total power shifts O 1 down relative to the origin O and a decrease in the limit of the total power CD. Going beyond the indicated limits on the power vector diagram (A-B-C-D-O) means the inverter is overloaded. Modern inverter control systems in UPS analyze the values of the total and active component of power, recording excesses of limit values.

Reactive power factors of the inverter output filter

When choosing filter parameters, it is recommended to accept: Kc = Qc/Snom = 0.25 – 0.5; Kl = Ql/Snom = 0.07 – 0.2. Lower coefficient values can be accepted for lower inverter powers. An increase in the capacitive power factor leads to a decrease in the calculated power of the inverter, which provides rated operating modes in the safe region of the vector power diagram.

UPS overload characteristics and current short circuit inverter There are different overload capacities of the inverter and the bypass circuit. In case of significant and prolonged overloads, the UPS switches to automatic bypass mode, which is characterized by a large overload capacity. However, modern inverters based on IGBT transistors with PWM regulation are also distinguished by fairly high overload characteristics and short-circuit current values (Isc), reaching 200 - 300% of the rated output current. At overloads not exceeding 5–10% of the rated power, the UPS can operate in inverter mode for a long time without switching to bypass mode. Figure 3 shows typical UPS overload characteristics. Acceptable areas of UPS operation: 1 – inverter mode; 2 – automatic bypass mode; 3 – UPS shutdown area. It should be borne in mind that the quantitative indicators of the given current-time dependencies may differ for different UPS models. Knowledge of overload characteristics allows you to optimally select the required rated power of the UPS for loads with high starting currents, eliminating the low load factor of the UPS in static mode at rated load currents.

The issue of limiting the inverter current in overload mode is important for understanding the overload properties of the UPS. When the load current increases above the rated value, the inverter switches to current generator mode, limiting the maximum current value to a certain value Ilim. To ensure that the output voltage sinusoidal distortion does not exceed 5%, it is necessary to set the threshold for limiting the maximum (amplitude) value of the output current to 1.5 times the amplitude value of the rated current of the inverter with a linear load:
I limit = 1.5√2i output nom
Accordingly, the amplitude coefficient of the limiting current is:
Co.limit=Ilim

An inverter with PWM output voltage regulation is able to respond to changes in load current, limiting its amplitude. In this case, the duration of the current pulse increases during the half-cycle of the output voltage. For example, an inverter with a rated power of 5 kVA is capable of delivering 4 kW of active power to an RCD load with a distortion of the output voltage sinusoidality of no more than 5%. Thus, the output power factor of such an inverter is Krout = 0.8.

Table 2 shows typical overload characteristics of low- and medium-power UPSs.

Table 2. Typical overload characteristics of small and medium power UPS

UPS Transient Response

These characteristics are also called system or “input-output”. These include parameters such as energy coefficient, synchronization indicators, time characteristics of autonomous operation of the UPS and battery charge recovery.

The energy coefficient determines the ratio of total power - consumed by the UPS from the network and supplied by the UPS to the load:

FORMULA

If the condition Ke ≥ Krn is met, then the UPS consumes from the network a total power equal to or less than that which the UPS supplies to the load:

FORMULA

This provision applies to UPS with high input power factor when operating on non-linear loads with low power factor. This phenomenon is explained by the fact that with a nonlinear load, the reactive power current and high-frequency distortion power current harmonics are closed in the inverter-load circuit and do not appear in the UPS input circuit. It can be shown that for a given load power factor Krn and efficiency, the active power at the UPS input will be:

FORMULA 9

The apparent power at the UPS input will be determined by the input power factor:

FORMULA 10

Under the condition Uin = Uout, we have:

FORMULA 11

Let's consider an example of using a UPS with the following indicators: Krn = 0.95, efficiency = 90%, when operating on a non-linear load with a power factor Krn = 0.63.

From relation (11) we have: Iin = 0.74 Iout. A decrease in the effective value of the input current of the IDP relative to the output current leads to a decrease in the network load compared to when the load is directly connected to the network. Since power losses are proportional to the square of the current, the power losses in power lines using a UPS in our example will be 54% of the losses when powering the same load from the network without a UPS. This circumstance is especially important in the presence of so-called “soft” power lines. Thus, the generalized energy factor is one of the most important indicators that determine the feasibility of using a double conversion UPS not only to ensure uninterrupted power supply to the load in the event of a network failure or distortion, but also to optimize energy consumption for loads with a low power factor.

Temporary characteristics of autonomous operation of the UPS show the maximum operating times of the UPS from battery energy in the absence or unacceptable deviations of the network, depending on the load factor. A significant increase in reserve time is achieved by externally connecting additional battery modules. Attention should be paid to the nonlinear dependence of time characteristics on the value of the load factor.

Battery charge recovery time characterizes the ability of the UPS to operate in repeated autonomous modes and depends on the used battery capacity. The battery charging time from 20% to 90% capacity is on average 6 – 8 hours.

Synchronization metrics characterize the synchronous operation of the inverter and the bypass circuit, which must be maintained with frequency deviations within +/–8% of the nominal with a rate of frequency change within the range of 1 - 4 Hz/s. During autonomous operation, the inverter output frequency must be maintained with an accuracy of +/–0.1% of the rated one.

Characteristics of dynamic operating modes and spectral characteristics of the UPS

This section is devoted to the results of an experimental study of the dynamic modes and spectral characteristics of a double conversion UPS with a power of 1 - 3 kVA. These studies determined:

· dips and surges of instantaneous values of output voltage and current and the time of return to the steady state of operation of the UPS after load surges;

· UPS response to input voltage surges;

· overload and protective capabilities of the UPS;

· harmonic composition of the output voltage and current in steady-state processes with different types of loads and the shape of the input voltage.

Named list dynamic characteristics reflects the general requirements for UPS set forth in the standards. The results of the study of transient processes during load surges are shown in Figures 4 a, b. The analysis shows that when the linear load jumps to 100%, the output voltage decreases by 3.5% of the steady-state value and then recovers to its original level in 60 ms (see Fig. 4a). Note that the static accuracy of UPS stabilization is +/–2%. When a 100% linear load was abruptly reset, an increase in the output voltage by 4% was recorded and a return to the steady-state value within 100 ms (see Fig. 4b).

Figure 5a shows oscillograms of the output voltage and current when the motor load is turned on, the total power of which was 150% of the rated power of the UPS. Due to the overload, the UPS automatically switched to bypass mode, and then, upon completion of the UPS motor start-up mode, it again switched to double conversion mode. It is clear that the transition from double conversion mode to bypass and vice versa occurs instantly, without distortion of the voltage and current curves.

The process of switching to bypass and returning to double conversion mode was shown in Figure 5a. If the load exceeds 110%, the inverter continues to operate for 30 s, and then the UPS switches to bypass. If the load increases to 150%, the inverter continues to operate for 0.2 s before switching to bypass.

Figure 5b shows oscillograms of the output voltage and current of a 3 kVA UPS when a nonlinear load is turned on, the crest factor of which is 2.84, and the total power is 1.8 kVA. The initial current surge was 2.4 times the peak steady state current. In this case, the output voltage decreased by 9% of the steady-state value and then recovered to its original level within 40 ms.

When studying the behavior of the UPS during input voltage surges, it was noted that it provides an almost instantaneous response to disturbances, and the stability of the output voltage remains within the static accuracy of +/-2%. The effectiveness of the electronic protection of the inverter was tested during autonomous operation of the UPS by turning on the motor load in excess of 150% of the rated load (motor start). 0.22 s after the engine was turned on, the UPS was turned off by electronic overload protection (see Fig. 6). The experiment confirmed the passport data on the overload capacity of the inverter (200 ms) and the reliability of the UPS electronic protection.

A study of the harmonic composition of the output voltage and current under linear and nonlinear loads showed that the distortion coefficient of the sinusoidal output voltage does not exceed valid values for any type of load, both online and offline.

Table 3 shows the results of testing a 3 kVA UPS for the composition of higher harmonics in the output and input voltages and currents with a nonlinear load of 1.8 kVA.

Table 3. Spectral composition of currents and voltages under nonlinear load

As follows from the analysis of the harmonic composition of the output voltage when using a double conversion UPS, we have an insignificant sinusoidal distortion coefficient Ki = 3.8% with a significantly nonlinear load and with the permissible content of higher harmonics of the inverter output voltage not exceeding 10%. With a significantly non-sinusoidal input voltage shape corresponding to a sinusoidal distortion coefficient of 36 - 41% (rectangular voltage with a significant third harmonic coefficient), the UPS output voltage has a sinusoidal shape Ki out = (0.6 - 1)%. This circumstance is especially important when UPS power supply from a low-power diesel generator set (DGS), when the DGS voltage has significant distortions from the sinusoidal shape.

Literature:
1. Klimov V. Modern sources uninterruptible power supply: classification and structures of single-phase IDPs. Part 1//Electronic components, No. 6, 2008.
2. Klimov V. Structures of power circuits of three-phase UPSs. Part 2//Electronic components, No. 8, 2008.
3. International Standard IEC 62040-3.1999, Uninterruptible Power Systems (UPS), part 3: Method of Specifying the Performance and Test Requirements.
4. GOST 27699-88. Uninterruptible power supply systems for AC receivers. General technical conditions.
5. Jean N. Fiorina Inverters and Harmonics, MGE UPS Systems, MGE 159, 1993
6. Klimov V., Moskalev A. Power factor and load characteristic of a PWM inverter in uninterruptible power supply systems // Power Electronics, No. 3, 2007.
7. Klimov V., Smirnov V. Power factor of a single-phase transformerless switching power supply // Practical power electronics, issue 5, 2002.
8. Klimov V., Klimova S. Energy indicators of uninterruptible AC power supplies, Electronic components, No. 4, 2004.
9. Klimov V. et al. Single-phase uninterruptible power supplies of the DPK series: dynamic and spectral characteristics // Power Electronics, No. 2, 2007.
10. Klimov V. Multimodular UPS structures and organization of parallel operation of monomodular UPSs. Part 3//Electronic components, No. 9, 2008.
11. GOST 13109-97. Standards for the quality of electrical energy in general-purpose power supply systems.

Domestic electricity supply is characterized by low reliability and unsatisfactory This is due to outdated electrical networks, wear and tear of equipment, low performance of energy converters, transient processes at sources and users of electricity, natural and climatic factors. In such conditions, uninterruptible power supply systems are extremely necessary to ensure the operation of consumers of both the first and other categories.

For apartment and house owners, stable operation of the electrical network is also important. Termination of work household appliances- this is not the biggest of troubles. Much more important is the trouble-free functioning of life support systems, in particular the heating system, if it directly depends on the power supply. Uninterruptible power supply UPS (UPS) comes to the rescue - a device that protects electrical receivers from shutdowns by storing electricity in rechargeable batteries (AB) and guarantees the required quality of energy (QE) in autonomous and network operating modes.

Before outlining an approach to creating power supply to loads without failures, you should find out what failures can be expected from domestic power grids.

Power failures in electrical networks

Low voltage is a common occurrence in the power supply. But it is not particularly dominant over the elevated one, which is also common. At night, the voltage is stable, during the day it decreases, and in the evening, when most of the loads are turned off, it increases.

Unstable frequency is also a failure, although quite rare. When the network is heavily loaded, it can drop to 45 Hz, which leads to significant signal distortion that negatively affects the operation of the UPS. Some devices perceive a decrease in frequency as an emergency, and the battery can quickly drain.

A complete power outage is not such a rare occurrence. Electricians are not very careful with the operation of electronics and can unexpectedly cut off power to a building. A momentary power outage is enough to cause loss of information on a computer. When networks are overloaded, power outages can occur. Therefore, it is important how reliably the UPS system supplies uninterruptible power.

UPS classification

They are divided into three groups:

Low-power UPS for connection through electrical outlets. The design can be tabletop or floor-standing, and the power ranges from 0.25 to 3 kW.
Medium power devices - from 3 to 30 kW - contain a block of sockets built inside, or are also connected through groups of sockets in the consumer power supply network from the control panel. The devices are manufactured for placement both in offices and in separate equipped rooms.
High power UPS - from 10 to 800 kW. Located in electrical machine rooms. They are collected in groups and created high-power energy systems - up to several thousand kW.

UPS types

There are currently 4 types of UPS (UPS). The properties common to all are:

filtering from impulses and noise;
elimination of signal shape distortions;
voltage stabilization (not available for all models);
keeping the battery charged;
When the UPS battery is low, it will first signal and then shut down the consumer.

Off-line UPS

The principle of operation of devices of this modification is to power the consumer from the existing network and instantly switch to an autonomous backup power supply in emergency situations (4-12 ms). They are simpler and cheaper than other types.

The UPS usually switches to operation from the built-in battery.

When operating from the network, the device suppresses noise with pulses and maintains the voltage at a given level. Part of the energy is spent on recharging the battery. If the network operates in a non-standard mode, the consumer switches to battery operation. Each UPS model determines in its own way the need to switch to this mode. The operating time of the battery depends on its characteristics and the power consumed by the load. If the backup power supply is discharged, a command is given to turn off the consumer. If the mains voltage reaches normal levels, the UPS goes into normal operation. network mode operation, battery charging begins.

Linear interactive

Line interactive ups models are equipped with stabilizers that operate continuously and ensure that batteries are rarely connected.

The device interacts with the network, controlling the amplitude and shape of the network voltage.

When the voltage decreases or increases, the unit adjusts its value by switching the taps of the autotransformer. In this way its nominal value is maintained. If the parameter exceeds the permissible limits and the switching range is no longer sufficient, the UPS switches to battery backup power. The unit can be disconnected from the main power when a distorted signal is received. There are models that adjust the voltage waveform without switching to battery operation.

Ferroresonant UPS

The device contains a ferroresonant transformer, which works as a voltage stabilizer. Its advantage is the accumulation of energy in the magnetic field, which is released during switching within 8-16 ms. This period of time is sufficient for the UPS to reach new mode work.

The transformer performs additional function noise filter. Input voltage distortion does not affect the output waveform, which remains sinusoidal.

Double Conversion UPS

The double energy conversion device works on the principle of rectifying the mains voltage, and then again converting it into stabilized alternating voltage. Here a more powerful rectifier is used, which not only recharges the battery, but also supplies the inverter with a stabilized constant voltage.

From the output of the device, a stabilized alternating voltage is supplied to the load.

When double conversion is not enough to adjust the grid voltage, additional charge is supplied from the battery to the inverter. There are no switches, but the mode is already different.

If the inverter fails, it switches to operation from the mains via a bypass. Choosing a double conversion UPS for private use is irrational due to high energy losses. This type protection is used by organizations where high equipment reliability is required.

Types of systems

Uninterruptible power supply systems can be centralized or distributed. In the first case, one UPS operates for the entire building or a separate floor, which copes with all loads.

Uninterruptible power supplies include several protection devices, each of which operates on a single computer or other piece of equipment. They are quite effective.

The advantages of a distributed system are as follows:

The UPS is selected specifically for separate device, which is the most important or operates under difficult conditions.
The system can be gradually expanded, starting with server protection and moving to workstations.
Failed UPSs can be replaced with others from less important system elements.
The low-power UPS does not require installation and maintenance by special personnel.
Possibility of connecting to a regular power supply via sockets.
UPSs are used independently of each other.

Centralized uninterruptible power systems include high-level UPSs that better protect equipment. Despite their high cost, overall cost savings are achieved since one device is cheaper than several. But for simple computers, the system will cost more, since its maintenance requires highly qualified personnel or the services of specialized companies that install uninterruptible power supply systems and service them.

It is necessary in the following cases:

computers are the main load on the network;
some organizations need very reliable systems, such as banks;
consumers vary significantly in power: computer system, communications, security system.

What to look for when choosing a UPS?

When choosing an uninterruptible power supply system, there are several important factors to consider. Let's list the main ones.

What is the equipment protected from?

First of all, it is necessary to measure the voltage in the electrical network. The minimum cycle duration will be a day. It best reflects the operation of the electrical network. If you have to work on weekends, you need to get information on a weekly cycle, during the day and night.

It is important to determine the maximum and minimum voltage, as well as the power and frequency of pulses in the network. The device can be a recorder.

The simplest way for the user is to measure the voltage, during which, in his opinion, the voltage reaches a maximum and minimum. Don't ignore weekends.

If the owner of the apartment has powerful equipment, it is necessary to measure the voltage in the home network when it is turned on and off. You should find out how often the power supply at home is cut off and for what reasons. It is important to have a grounding wire in the apartment. In this case, you should find out how reliably it is connected to the floor panel bus.

Type of protected equipment

A list of equipment for which it is necessary is compiled UPS application. In this case, you need to know what is consumed by each. It is enough to determine its nominal value, which is in the technical specifications. Some equipment sometimes consumes maximum energy that is several times its rated value. A power reserve should be set for it.

Battery life

Here it is important to determine for what period it is possible to safely save data or complete the necessary technological operations (transferring information, saving files, receiving messages).

Required Personnel

Depending on the complexity of the system, a certain staff of specialists is required to operate it. This must be clarified in order to correctly calculate all costs. The price of the protection system should not exceed 10% of the cost of the main equipment.

UPS for home

For an average cottage, a UPS (UPS) uninterruptible power supply system with a power of about 15 kW is convenient. To ensure autonomous operation for 2-3 hours, you need 4 batteries with a total capacity of 2000 Ah. They allow you to accumulate electricity of about 7 kWh.

The most important things in a house are the heating system and Appliances with a computer. The cost of a UPS depends on the power, number of batteries and manufacturer. For a boiler, you can purchase a source with a power of 360 W at a price of 7 thousand. For the whole house you will need a UPS power of up to 15 kW, the price of which is more than 70 thousand rubles.

In addition to converters, batteries are needed, which need to be changed periodically. A UPS for a home costs a lot of money. Battery uninterruptible power supply systems are especially expensive.

Despite this, you can save on repairing other equipment. In addition, there are alternative options using generators. Sometimes you can get by with installing voltage stabilizers, which cope with many tasks, including correctly turning off equipment.

Modern UPSs are equipped with a clear interface. Using the display, you can monitor the operation of the system, where the main parameters are input and output voltage, power consumption, operating circuit, battery charge.

Which UPS to choose depends on the user's needs. A home computer may have enough power for the duration of its shutdown. For uninterrupted operation of the boiler for 8-9 hours, you will need a 1 kW protective device with three 65 A/h batteries.

Conclusion

The systems are designed to ensure autonomous operation of electrical appliances and electronic equipment for a short time. The main indicator is the power of the UPS and the capacity of the battery. It is advisable to choose equipment that contains a voltage stabilizer.

The operating time of the battery depends on its characteristics and the power consumed by the load. If the backup power supply is discharged, a command is given to turn off the consumer. If the mains voltage reaches a normal level, the UPS switches to normal mains operation mode and the battery begins charging.

Uninterruptible power supply systems: types, characteristics, installation. Uninterruptible power supplies

Three main types of UPS

Off-line (Back-UPS, backup, Standby) uninterruptible power supply

Line-interactive UPS

Double conversion UPS (on-line)

Additional UPS functionality

Power failures in electrical networks

UPS classification

UPS types

Off-line UPS

Linear interactive

Ferroresonant UPS

Double Conversion UPS

Types of systems

What to look for when choosing a UPS?

What is the equipment protected from?

Type of protected equipment

Battery life

Required Personnel

UPS for home

Conclusion

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