Modern uninterruptible power supplies. Uninterruptible power supplies (UPS), what they are and their operating principle
Off-Line uninterruptible power supplies
Uninterruptible power supplies of the Off-Line type are defined by the standard as passive, standby (UPS-PSO). In normal operating mode, the standard load power supply is the filtered voltage of the primary network with permissible deviations of the input voltage and frequency. In cases where the input voltage parameters are outside the configured ranges, the uninterruptible power supply inverter is turned on, ensuring continuity of power supply to the load. The inverter is powered by batteries.
These are the simplest UPSs (Figure 1), and therefore the cheapest. The uninterruptible power supply consists of two parallel branches:
. filter-load;
. rectifier-battery-inverter-load.
Fig.1. Stand-By type uninterruptible power supply circuits
Under normal network characteristics, voltage is supplied to the load through a filter that filters out all kinds of interference. This is usually a surge suppressor, although there may also be a line conditioner or a combination of both, as well as a static switch.
At the same time, the batteries are recharged through the rectifier. battery. If the input voltage is lost, overestimated or underestimated, the load power supply is switched to battery power via an electronic switch via an inverter (the inverter converts direct voltage into alternating voltage). The switch provides switching times from 2 to 15 ms. Note that the loss of power during this time does not have any noticeable effect on computer systems, which can easily tolerate a power outage of 10-20ms. Considering that almost all modern equipment has switching power supplies, the switching occurs unnoticed by the user. Uninterruptible power supplies of this type can support the operation personal computer within 5-10 minutes.
Main disadvantages of Off-Line UPS
The main disadvantages of off-line UPS are:
. poor performance of power supplies of this type in networks with low quality electrical networks: poor protection against voltage dips (sags), exceeding the permissible voltage value, changes in the frequency and shape of the input voltage;
. the impossibility of timely restoration of battery capacity due to frequent switching to battery power;
. non-sinusoidal output voltage when powered by a battery.
Line-Interactive uninterruptible power supplies
Uninterruptible power supplies of the linear-interactive type (Line-Interactive, sometimes Ferroresonant) combine the advantages of the On-line type with the reliability and efficiency of standby ones. In uninterruptible power supplies of this type, in contrast to Off-line technology, a stepped circuit is included in the direct circuit. automatic regulator voltage (booster), built on the basis of an autotransformer (transformer with switching windings). Some models use a network voltage stabilizer.
The inverter is connected to the load. During operation, it supplies the load in parallel with the stabilized (conditioned) AC mains voltage. The load is fully connected only when the input mains voltage is lost.
Fig.2. Line-Interactive type uninterruptible power supply circuits
Because of this interaction (“interaction”) with the input mains voltage, this architecture got its name. IN certain range changes in mains voltage, the output voltage is maintained at given boundaries by switching the windings of a transformer or stabilizer. The inverter typically operates at low voltage, adjusting the output voltage and charging the batteries until it needs to be turned on to fully power the load during a power outage. Linear-interactive uninterruptible power supplies have found the most widespread use in computer network protection systems.
The transformer, made using a special so-called ferro-technology, smoothes out voltage surges, while the uninterruptible power supply switches less frequently to operation from the battery, and therefore the battery life increases. Typically, these uninterruptible power supplies are equipped with advanced filters that provide protection against interference of various origins. Typical switching time to or from battery mode is 2 ms.
Structurally, the transformer does not have several additional taps in the secondary winding (this can be an autotransformer with a single winding); the switching of transformer taps when the input voltage changes is controlled by a controller (microprocessor), maintaining the output voltage in the required range. So, the Line-Interactive uninterruptible power supply operates on the principle of a controlled LATR and actually switches less frequently to battery power when the input voltage surges. In this circuit, the charger is structurally combined with the converter.
One of the advantages of this type of UPS is the wide range of permissible input voltages.
Some line-interactive models have a shunt circuit between the primary power input and the load, such UPSs are called shunt line-interactive UPS (UPS -LIB, Reversible + Bypass). In shunt mode, the supplied load is not protected. When working with sources based on ferro-technology, you need to keep in mind:
Uninterruptible power supplies On-Line type
On-Line technology allows you to implement the most reliable type of uninterruptible power supply. From the rectifier (Figure 3), the mains voltage is supplied to the DC-DC converter high level to low PN1, and then to the DC-to-AC output voltage converter (PN2). Converter PN2 is an inverter, the power of which is supplied both from batteries and from the network through a rectifier-voltage converter PN1, connected in parallel:
.
at normal input alternating voltage, the PN2 inverter is powered by a rectifier;
. in case of deviations in the power supply network from the norm, the input voltage for PN2 is removed from the battery.
Fig.3. On-Line type uninterruptible power supply circuits
In most uninterruptible power supply systems with a power of up to 5 kVA, instead of a continuously connected battery, a backup DC-DC converter is connected, which turns on during network failures and duplicates the DC bus from the low-voltage battery.
Conclusion: even in cases of minor deviations of the input voltage parameters from the norm, On-Line devices provide a rated output voltage in the range of ±1-3%. The presence of a bypass circuit allows you to connect the load directly to the power network. The power quality and reliability of power supply provided by devices with this type of architecture are significantly higher than previous ones.
Disadvantages of On-line uninterruptible power supplies: low efficiency (85-90%) compared to previously discussed types due to double conversion (relative to Standby and Line-Interactive) and high price. However, the level of load protection and stability of the UPS output parameters is a reasonable compromise between safety, efficiency and price of the device. Losses in a 4000VA UPS do not exceed 380W and may be incommensurate with the task that such a power source solves.
New modifications of uninterruptible power supplies
There are now several new modifications of uninterruptible power supplies:
.
by-pass;
. triple-conversion;
. ferrups.
The first modification (by -pass), as in Figure 3, is additional channel transmission of electricity to the load, its presence allows for high reliability of the device. Switching to On-line mode is performed automatically when the parameters of the output network deviate from the norm or in emergency operating conditions. Thus, this mode helps to increase the reliability of the device. The second modification (triple-conversion) contains a power factor corrector. The third modification (ferrups) uses a ferroresonant transformer, which provides high reliability and a wide range of input voltages.
New approaches to the construction of uninterruptible power supplies are based on the use of systems with redundant power, which have higher reliability of the output network, so that the failure of one of the elements does not lead to failure of the entire system. Typically, these are modular systems designed either to increase load power, or to increase system reliability, or using both principles together. The simplest system has an auxiliary module in the structure of the uninterruptible power supply, “isolated in hot standby mode.” There are several options for technical solutions for such uninterruptible power supply systems.
The first option is to use an automatic switch (Figure 4). The inputs of one or more power supplies are connected to a single network, and are connected to the load via automatic switch. Information about the operating status of the installations and control commands are received via the communication channel connecting the UPS.
Fig.4. Parallel circuit using automatic switch
The second option contains a “load distributor” (Figure 5), which evenly distributes the load between the individual sources of the system.
Fig.5. Parallel circuit using automatic switch
The third embodiment of the parallel structure (Figure 6) uses the principle of a two-level system. In this method, one of the “master” modules controls load distribution among the other “slave” modules.
Fig.6. Parallel circuit based on a two-level Master-Slave system
The fourth option, with a redundant parallel architecture, looks the most promising. In such a scheme (Figure 7), not only the modules are redundant, but also the connections between them, and, if necessary, any module can act as a master. Only this scheme is characterized by an increase in power, the absence of shunt circuits, and continuous load protection with the help of a UPS is guaranteed.
Fig.7. Redundant parallel system diagram
Main technical characteristics of uninterruptible power supplies
Supply voltage form
This characteristic of an uninterruptible power supply is important for the load. In UPS operating mode, the load can receive an output alternating voltage close to a rectangular shape (meander) from the batteries, due to the smoothing properties of the filters, an approximate sinusoid and a pure sinusoid. The output voltage shape closest to a sine wave is obtained by using pulse width modulation. Receiving a sinusoid as a supply voltage is typical only for On-line UPSs and some Line-Interactive power supplies.
Power
Total or output power.Denoted by the letter S, the unit of measurement is VA or Volt-Amperes. It is the geometric sum of active and reactive powers. The parameter is calculated as the product of the effective (rms) values of current and voltage. Its value is specified by the power source manufacturer.
Active power consumed by the load. Denoted by the letter P, the unit of measurement is watt (W). In cases where there is no reactive component in the network, it coincides with the full power. It is defined as the product of the total power and the cosine of the angle φ, where φ is the phase shift angle of the linear voltage and current vectors, i.e. P = S. cos(φ). A typical cos (φ) value for personal computers is about 0.6-0.7. This value is called power factor. Obviously, to select the required power for an uninterruptible power supply, you need to divide the load power in watts by the cos (φ) value.
Reactive - denoted by the letter Q and is calculated as the product of the total power S and the sine of the angle φ (Q = S. sin (φ)). The unit of measurement is reactive volt-ampere (var). Characterizes losses in supply wires due to the load on them reactive current. When cos (φ) = 1 there are no losses, all the power generated by the power source goes to the load. This is achieved through the use of passive compensating devices or active power factor correction.
Input voltage range
Input voltage range—defines the limits acceptable values network voltages at which the uninterruptible power supply is still able to maintain the output voltage without switching to battery power. For some models, this range depends on the load. For example, at 100% load the range of input voltages can be 15-20% of the nominal, at 50% load this range is 20-27% of the nominal, and at 30% load - 40% of the nominal. The service life of the batteries depends on this parameter; the wider the range, the longer the batteries will last, all other things being equal.
Input voltage frequency
Input frequency - characterizes the range of power supply frequency deviation. Under normal operating conditions, the frequency deviation from the nominal value usually does not exceed 1 Hz.
Output voltage waveform distortion factor
The distortion factor of the output voltage waveform (total harmonic distortion - THD) characterizes the deviation of the output voltage waveform from a sinusoid, measured as a percentage. Small values of the coefficient correspond to the output voltage shape approaching sinusoidal.
Mode switching time
The mode switching time (transfer time) characterizes the inertia of the uninterruptible power supply; for different sources it is approximately up to 2-15 ms.
load) characterizes the stability of the uninterruptible power supply during power overloads, measured as a percentage relative to the rated power. Determines the resistance of the UPS to non-stationary overloads.Battery life
Time battery life determined by battery capacity and load size. For typical low-power uninterruptible power supplies that power personal computers, it is 5-10 minutes. This time is designed to allow the user to close all running applications while saving information and turn off the PC in normal mode.
Crest factor
Crest factor is the ratio of the peak value of current consumption to the average current. The value depends on the form of the supply voltage.
Battery life
The service life of rechargeable batteries is 4-5 years, but the actual one highly depends on operating conditions: switching frequency in offline mode, charging conditions, environment.
Cold start available
The presence of a cold start is the ability to turn on an uninterruptible power supply in the absence of voltage in the supply network. This function is useful when it is necessary to urgently perform any action, regardless of the presence of voltage in the mains.
UPS batteries
General information
The source of energy used to power the load in critical operating modes is the battery. Uninterruptible power supplies with a power of up to 20 kW typically use sealed lead-calcium batteries with suspension-type electrolyte. In batteries of this type, the electrolyte is immobilized, either by silica gel or glass fiber, which makes them leak-proof. This property of the electrolyte allows the batteries to be used in any position; in addition, they do not require periodic replenishment of the electrolyte or other maintenance.
The electrodes are made of lead-calcium alloy, which ensures a long service life and a wide range of applications for batteries; the operating temperature range is from minus 20 to plus 50 ° C (for some types of batteries). The batteries do not suffer from the so-called “memory effect” and can be stored in a charged state for a long time (up to a year), while the self-discharge current is insignificant.
Battery design
The design of the batteries is traditional - the impact-resistant plastic case is divided into sections - “cans”. Sets of cathode and anode plates are separated by spacers - fiberglass separators. The active part of the electrolyte is sulfuric acid. The cover is hermetically connected to the body, without the ability to disassemble the battery. In the upper part of the cover there are valves (one for each section), which ensure the release of gas in the event of its excessive formation during operation, and plate terminals. The valves are closed with an additional removable cover.
Battery storage
The battery life is approximately 5 years. When using an uninterruptible power supply on a daily basis, its own charging capabilities guarantee operation during this period. If left unused for a long period of time, batteries will self-discharge. For YUASA batteries, the self-discharge rate is approximately 3% per month at an ambient temperature of about 20°C. If batteries are not charged for a long period of time, lead sulfates form on the negative plates of the battery. This phenomenon is known as "sulfation". Lead sulfate acts as an insulator, preventing the battery from accepting a charge. The deeper the sulfation of the plate occurs, the less charge the battery can accept.
To eliminate irreversible consequences during storage, the charge must be carried out within a period corresponding to the ambient temperature conditions. To ensure optimal service life, long-term storage batteries must be recharged periodically.
Methods for charging UPS batteries
Charging the batteries is the main component of its maintenance. The battery life depends on the efficiency of the chosen charging method. The following charging methods are available:
— charging at constant voltage;
— charging at constant current;
— two-stage charging at constant voltage.
The preferred method is constant voltage charging. In this case, the battery is connected to an energy source whose charging voltage is maintained constant throughout the charging process. During charging, the current decreases and becomes significantly less than when charging using the direct current method, and at the end of the charge it drops to almost zero. In this case, the battery is charged to 90-95% of its nominal capacity.
Selecting an uninterruptible power supply
The range of types of uninterruptible power supplies as a means of protecting equipment and computer systems is quite wide. The question of choosing the required power source is very difficult. To resolve the issue of choosing a particular UPS, you need to try to analyze the factors that influence the operating conditions of the power source.
First, we must try to assess the significance of the powered system. It is quite possible that for a home or office option an Off-line or Line-interactive type uninterruptible power supply will be sufficient. On-line UPS is more suitable for a server computer and other types of loads that have increased requirements for the quality and reliability of the power supply.
Secondly, it is necessary to assess the quality of the electrical network: the probability and frequency of power outages, the presence of voltage fluctuations and various interferences.
Thirdly, you need to evaluate the power of the uninterruptible power supply. In order to roughly imagine how much UPS power is required, it is necessary to determine the equipment being protected and calculate the total value of power consumption for it. Then, the resulting watts must be converted to VA, divided by the power factor. For computer equipment, the power factor is 0.5-0.6.
Manufacturers do not recommend loading the uninterruptible power supply more than 80% of the maximum load. It should be noted that laser printers It is not recommended to connect to an uninterruptible power supply due to the high power consumption of the heating element.
Electrical vibrations and their characteristics
Classical electrical oscillations, occurring for example in an oscillating circuit or at the output of an alternating current generator, 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 a 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 that direct current or direct current voltage that 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, installed in a file server with large disks, modems, etc.), then the amplitude factor will decrease (and will be, 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 regular work 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 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 pulse blocks power supplies (say, personal computers) consume current only at times when the voltage is very close to 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.
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 different technologies are used. UPS with switching and interacting with the network suppress noise coming through the power network with 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 best main 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 battery can be installed in the same housing, or an additional battery housing can be installed.
If the battery capacity UPS increases, and its power charger remains the same, 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 information about the discharge cycle recorded in permanent memory. The calculation is made based on full charge batteries. 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, indicates (on a digital or LED indicator) the 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. Electricity, 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 that are not too noticeable at first glance useful features. 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 ensure, if not the uniform distribution of the load across the phases, then at least 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 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 scheme 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
Theoretically, the amplitude of the nth 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 | |
| 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 an important element when building complex systems that require continuous operation and guarantee the safety of the equipment from possible problems in the electrical network. Nowadays the market offers a wide variety of products in different categories of price, quality and production geography. It’s difficult to decide, especially if you don’t have the necessary experience. Finances suggest that you should approach the issue of choice with an eye on your own budget. Therefore, before investing in an uninterruptible power supply, you should answer several important questions:
- How much critical equipment are you going to protect?
- What is the optimal battery life of equipment in the event of a power failure?
To answer the questions posed above, it is necessary to delve in detail into the classes of uninterruptible power supplies on the market today. And also decide on the main criteria that need to be taken into account in order to make an informed choice.
UPS classes
The entire variety of modern uninterruptible power supplies presented on the market today can be divided into several classes that differ from each other in their circuitry, as well as in their behavior both in normal operation and in battery operation.
Highlight:
- Backup or (BackUp),
- Line-interactive UPS (),
- Double conversion UPS ( , double-conversion).
They are considered the simplest and most unpretentious. When the network is operating in normal mode, electricity enters the UPS input and, passing through it, is supplied to the main load. In the event of losses and voltage surges in the network, the uninterruptible power supply automatically switches to the battery. The main disadvantages of this scheme are that switching UPS power supply to batteries takes from 4 to 10 milliseconds. When operating in battery power mode, the output of the UPS produces not the sine usual for the network, but an approximated sine.
An uninterruptible power supply with built-in batteries will be the right solution when, in case of problems with the voltage in the network, all that is important is the correct shutdown of the equipment, which takes from 5 to 10 minutes.
If you need more operating time of the equipment, you need to calculate the required battery discharge current. You can do this as follows:
From all of the above, it becomes clear that when choosing an uninterruptible power supply, it is necessary to take into account many technical and purely physical nuances, determined both by the specific location of the UPS and the equipment connected to it, and by a number of other factors.
To facilitate calculations when choosing a UPS, the NAG company has a convenient tool - with which you can determine all the necessary parameters.
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.
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 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.
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 switching UPS manufacturers.
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). At UPS operation from the network, the computer is powered by 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.
Requirements for the quality of electricity are legally prescribed by state standards and fairly stringent regulations. Electricity supply organizations make a lot of efforts to comply with them, but they are not always implemented.
In our apartments, and in production, the following periodically arise:
complete power outages for an indefinite period of time;
aperiodic short-term (10÷100 ms) high-voltage (up to 6 kV) voltage pulses;
surges and drops in voltage with varying durations;
high-frequency noise overlays;
frequency drifts.
All these problems negatively affect the work of household and office electricity consumers. Microprocessor and computer devices, which not only fail, but can also completely lose their functionality.
Purpose and types of uninterruptible power supplies
To reduce the risks of malfunctions in the power supply network, we use backup devices, which are commonly called uninterruptible power supplies (UPS) or UPS (derived from the abbreviation of the English phrase “Uninterruptible Power Supply”).
They are manufactured with different designs to solve specific consumer problems. For example, powerful UPSs with gel batteries can maintain power supply to an entire cottage for several hours.
Their batteries receive a charge from a power line, wind generator, or other carriers of electricity through the inverter rectifier device. They also feed the electrical consumers of the cottage.
When external source turns off, the batteries are discharged to the load connected to their network. The larger the battery capacity and the lower their discharge current, the longer they work.
Medium power uninterruptible power supplies can back up indoor climate control systems and similar equipment.
At the same time the most simple models UPS are only capable of completing a computer shutdown program. At the same time, the duration of the entire process of their work will not exceed 9÷15 minutes.
Computer uninterruptible power supplies are:
built into the device body;
external.
The first designs are common in laptops, netbooks, tablets and similar mobile devices powered by a built-in battery, which is equipped with a power and load switching circuit.
Laptop battery with a built-in controller is an uninterruptible power supply. His diagram in automatic mode protects operating equipment from electrical faults.
External UPS Designs, intended for the normal completion of programs desktop computer, are manufactured as a separate block.
They are connected via a power adapter to electrical outlet. They power only those devices that are responsible for running programs:
system unit with a connected keyboard;
a monitor that displays ongoing processes.
Other peripheral devices: scanners, printers, speakers and other equipment are not powered by UPS. Otherwise, in case of emergency termination of programs, they will take over part of the energy accumulated in the batteries.
Options for constructing UPS operating diagrams
Computer and industrial UPS are manufactured in three main options:
power backup;
interactive diagram;
double conversion of electricity.
With the first method backup scheme, denoted by the English terms “Standby” or “Off-Line”, the voltage is supplied from the network to the computer through a UPS, in which electromagnetic interference is eliminated by built-in filters. It is also installed here, the capacity of which is maintained by the charge current regulated by the controller.
When the external power supply disappears or goes beyond the established standards, the controller directs the battery energy to power consumers. A simple inverter is connected to convert direct current into alternating current.
Benefits of UPS Standby
Off-Line uninterruptible power supplies have high efficiency when the voltage is applied to them, operate quietly, emit little heat and are relatively cheap.
Flaws
UPS Standby stand out:
long transition to battery power 4÷13 ms;
distorted shape of the output signal produced by the inverter in the form of a meander rather than a harmonic sinusoid;
lack of voltage and frequency adjustment.
Such devices are most common on personal computers.
Interactive circuit UPS
They are designated by the English term “Line-Interactive”. They are carried out according to the previous, but more complicated scheme by including a voltage stabilizer using an autotransformer with step regulation.
This provides adjustments to the output voltage, but they are not able to control the signal frequency.
Filtering of interference in normal mode and switching to inverter power supply in case of emergency occurs according to UPS Standby algorithms.
The addition of a voltage stabilizer of various models with control techniques made it possible to create inverters with a signal shape not only of a square wave, but also of a sinusoid. However, a small number of control stages based on relay switching does not allow the implementation of full stabilization functions.
This is especially true for cheap models, which, when switching to battery power, not only increase the frequency above the nominal one, but also distort the shape of the sine wave. Interference is introduced by a built-in transformer, in the core of which hysteresis processes occur.
Expensive models use inverters based on semiconductor switches. UPS Line-Interactive have faster performance when switching to battery power than Off-Line UPS. It is ensured by the operation of synchronization algorithms between the incoming voltage and the output signals. But at the same time there is some underestimation of efficiency.
The Line-Interactive UPS cannot be used to power asynchronous motors, which are widely installed on all household appliances, including heating systems. They are used to operate devices with power where the power is filtered and rectified at the same time: computers and consumer electronics.
Double conversion UPS
This UPS scheme is named after the English phrase “On-line” and works on equipment that requires high-quality power. It produces double conversion of electricity, when the sinusoidal harmonics of the alternating current are constantly converted by the rectifier into a constant value, passed through the inverter to create a repeated sinusoid at the output.
Here the battery is permanently connected to the circuit, which eliminates the need for its switching. This method practically eliminates the period of preparation of the uninterruptible power supply for switching.
The operation of an On-line UPS based on the battery condition can be divided into three stages:
charge stage;
waiting state;
discharge for computer operation.
Charge period
The sine wave input and output circuits are interrupted by the internal UPS switch.
The battery connected to the rectifier receives charge energy until its capacity is restored to optimal values.
Readiness period
After the battery is charged, the automatic uninterruptible power supply closes the internal switch.
The battery maintains a state of readiness for operation in buffer mode.
Discharge period
The battery is automatically transferred to power the computer station.
Uninterruptible power supplies operating using the double conversion method have lower efficiency in line power mode than other models due to energy consumption for heat and noise. But in complex structures, techniques are used to increase efficiency.
UPS On-line is capable of correcting not only the voltage value, but also its oscillation frequency. This sets them apart from previous models and allows you to use it to power various complex devices with asynchronous motors. However, the cost of such devices is significantly higher than previous models.
UPS composition
Depending on the type of operating circuit, the uninterruptible power supply kit includes:
batteries for storing electricity;
Ensuring maintenance of battery performance;
inverter for generating a sine wave,
process control diagram;
software.
For remote access A local network can be used for the device, and the reliability of the circuit can be increased by using redundancy.
Some uninterruptible power supplies use the “Bypass” mode, when the load is powered by filtered mains voltage without operating the main circuit of the device.
The UPS part has a step voltage regulator “Booster”, controlled automatically.
Depending on the need to implement complex technical solutions, uninterruptible power supplies can be equipped with additional special functions.