Lecture No. 17

Voltage regulation methods.

Voltage regulation devices

1. General Provisions.

2. Voltage regulation in power centers.

3. Counter regulation method.

4. Voltage regulation at power plants.

5. Voltage regulation at step-down substations.

5.1 On-load tap-changer device for a two-winding transformer.

5.2 Autotransformer on-load tap-changer device.

General provisions

The voltage at network nodes is constantly changing due to changes in load, operating mode of power supplies, and network diagram.

Voltage mode in electrical network must be such that the GOST requirements regarding permissible voltage deviations for electrical receivers that are powered from this network are met. Voltage deviations often exceed acceptable values ​​for the following reasons:

· large voltage losses in the network;

· incorrect choice cross-sections of current-carrying elements and power of power transformers;

· incorrect construction of the network diagram.

Very often these reasons arise during the development of the network, during its reconstruction. Therefore, in order to ensure the necessary voltage deviations on the buses, the electrical receiver should use voltage regulation.

Voltage regulation is the process of changing voltage at characteristic points of the network using special technical means.

Methods of voltage regulation arose with the emergence of electrical networks. Their development came from lower levels management to the highest. At first, voltage regulation was used in power centers distribution networks both directly from consumers and at power generating units of power plants. Now these methods of voltage regulation are called local. As networks developed and merged into large energy systems, the need arose to coordinate the work local methods. Coordination refers to higher levels voltage regulation.

Local regulation can be centralized or local. Centralized management performed in nutrition centers. Local regulation is carried out directly at consumers. Voltage regulation in power centers leads to a change in the voltage regime in the entire network that is powered from it. Local regulation changes the voltage regime in a limited part of the network.

Voltage regulation in power centers

Power centers (CPs) can be generator voltage buses power stations, low voltage regional substations or deep input substations.

Voltage regulation on power plant generators is carried out by changing the excitation current using an automatic excitation control device (AEC).

Voltage regulation on the low voltage buses of step-down substations is carried out using:

· transformers with built-in devices for regulating voltage under load (OLTC);

· synchronous compensators (SC);

· linear regulators (LR).

In this case, voltage regulation is carried out automatically within the available regulation range. Voltage regulation occurs simultaneously for all power lines of the network, which are powered from the buses of the power center.

Voltage quality is ensured only when homogeneous consumers are connected to the power center buses. For them, the load change graph is the same.

If power receivers have different load schedules, then group centralized control schemes are used in the power center. In this case, electrical receivers are divided into groups in accordance with the nature of their load. They try to connect the power lines that feed such groups of electrical receivers to different sections of the buses of the power center and regulate the voltage on each section separately.

If this is not possible, then at the power supply center regulation is carried out as for a group of homogeneous consumers. For those consumers for whom this voltage regulation was not enough, local voltage regulation is also performed.

Depending on the nature of electrical receivers, three subtypes of voltage regulation can be distinguished:

· voltage stabilization;

· two-stage voltage regulation;

· counter regulation.

Stabilization voltage is used for consumers with an almost constant load during the day (three-shift enterprises).

Two-stage regulation performed for electrical receivers with a pronounced two-stage nature of load changes. (single-shift enterprises). In this case, two voltage levels are maintained per day in accordance with the load schedule.

In case of variable daily load, it is carried out counter regulation. This subtype of voltage regulation is the most common.

Counter regulation method

The essence of the counter-regulation method is to change the voltage depending on the change in the load diagram of the electrical receiver.

According to the counter-regulation method, the voltage on the low-voltage buses of district substations during the period of maximum load should be maintained 5% higher than the rated voltage of the supply network. This figure is given in the PUE (Electrical Installation Rules). Operating experience shows that the voltage should be increased by 10% if the voltage deviation at nearby consumers does not exceed permissible value. During the period of minimum load ( R min ≤ R max) the voltage on the 6-10 kV busbars of the substation is reduced to the rated voltage.

Let's look at this method using an example next network(Fig. 18.1).

In maximum load mode, the power center maintains voltage U 1 NB. On the higher voltage buses of the substation, the voltage is lower due to voltage losses in power lines1. Let's denote this voltage U 2 V..gif" width="33" height="29">. This is the voltage on the busbars of electrical receiver A. Its value satisfies the standards of the Electrical Code. Voltage on the busbars of electrical receiver B ( U B without reg.) less voltage on the buses of power receiver A by the amount of voltage loss in power line2. Its value does not meet the requirements of the PUE. When regulating voltage (), the voltage on the low voltage buses of the substation is maintained 5% higher than the rated network voltage. It is impossible to raise the voltage by 10% above the nominal value of the network voltage, because in this case the voltage on the buses of consumer A would not comply with the PUE standards. When regulating the voltage, the voltage on the buses of power receiver B enters the permissible range.

In the minimum load mode, the voltage in the power center is higher, and the voltage losses in the network elements are lower. Therefore, without voltage regulation, both the voltage at consumer A and the voltage at consumer B are higher than the recommended PUE. By changing the transformation ratio, the permissible voltage deviation on the buses of both consumers is ensured.

The largest voltage deviation is observed in emergency modes system operation. In this case, maintain the voltage of all consumers within the specified limits for normal operation without significant costs for special devices voltage regulation is not possible. Therefore, in emergency modes, a larger voltage deviation is allowed.

Voltage regulation in power plants

In power plants, voltage regulation is carried out on generators and step-up transformers.

Changing the generator voltage is possible by regulating the excitation current..gif" width="16 height=17" height="17">2 x 2.5%. Step-up transformers of higher power are produced without off-circuit switching devices.

Voltage regulation at step-down substations

To regulate voltage by substation transformers, it is possible to change the transformation ratio within 10 - 20%. According to their design, there are two types of switching devices:

· with regulation without excitation (PBV), that is, to change the transformation ratio, the transformer is disconnected from the network;

· with load voltage regulation (OLTC).

The on-load tap-changer device is more expensive than the tap-changer device. The cost of the device depends little on the power of the transformer. Therefore, the relative increase in cost of a transformer with on-load tap-changer will be significantly greater for transformers of lower power. In this regard, transformers with voltages of 6 - 20 kV are mostly made with tap changers, and transformers with voltages above 35 kV with on-load tap-changers.

The on-load tap-changer is usually installed on the high voltage winding for the following reasons:

· on the higher voltage side there are lower currents, so the device has smaller dimensions;

· the higher voltage winding has a larger number of turns, so the control accuracy is higher;

· according to its design, the high-voltage winding is external (magnetic core – low-voltage winding – high-voltage winding). Therefore, it is easier to inspect the on-load tap-changer;

· The on-load tap-changer is located in the neutral of the higher winding. The higher voltage windings are connected in a star, and the low voltage windings are connected in a delta. Three-phase regulation is easier to perform on star-connected windings.

For transformers with a voltage of 110 kV with a power of 2.5 MVA and a voltage of 150 kV with a power of 4 MVA, the on-load tap-changer is located on the low voltage winding.

Transformers have a different number of branches and different stages of regulation of the on-load tap-changer..gif" align="left" width="368" height="350 src=">The high-voltage winding of a transformer with on-load tap-changer consists of two parts: non-regulated adjustable or main (a) and adjustable (b).

On the adjustable part of the winding there are a number of branches to fixed contacts 1, 2, 0, -1, -2. Branches 1, 2 are connected according to the turns of the main winding. When branches 1, 2 are turned on, the transformation ratio increases. Branches –1, -2 correspond to the part of the turns that are connected counter to the turns of the main winding. Their inclusion leads to a decrease in the transformation ratio.

The main terminal of the high voltage winding is the zero terminal. The rated voltage is removed from it.

There is a switching device on the adjustable part of the winding. It consists of moving contacts V And G, contactors TO 1 and TO 2 and reactor R. The middle of the reactor winding is connected to the unregulated part of the high voltage winding of the transformer. IN normal mode operation (without switching), the load current of the high voltage winding flows through the reactor and is distributed equally between the halves of the reactor winding. Therefore, the magnetic flux is small and the voltage loss in the reactor is also small.

Switching is performed as follows. Let us assume that it is necessary to switch from branch 2 to branch 1. To do this, the contactor is turned off TO 1, moving contact is transferred V to branch 1 and the contactor turns on again TO 1. As a result of these actions, section 1 - 2 is closed to the reactor. The significant inductance of the reactor limits the equalizing current, which arises due to the presence of voltage in sections 1 - 2. Then the contactor is turned off TO 2, moving contact is transferred G to branch 1 and the contactor turns on TO 2.

The reactor and all moving and fixed contacts of the switching device are located in the transformer tank. Contactors are placed in a separate casing. It is filled with oil and placed outside the transformer tank. This makes it easier to inspect contacts and change oil.

Switches with reactors are designed for long-term load current flow. But the reactor is a heavy and bulky element. Therefore, switching devices for transformers with voltages of 220 kV and above are performed at active resistance X. To reduce energy losses in such devices, they are designed for short-term operation. The device turns out to be compact, but requires the use of powerful high-speed drives. Let us consider the operating principle of such devices using the example of autotransformers with voltages of 220 – 330 kV.

Autotransformer on-load tap-changer

The on-load tap-changer of the autotransformer is located at the linear end of the medium voltage winding (Fig. 18.4). With this arrangement of the on-load tap-changer, the transformation ratio between the high and medium voltage windings changes. The transformation ratio between the high and low voltage windings does not change. At first, the on-load tap-changer of autotransformers was built into the neutral, like in transformers. During regulation, the transformation ratio between all windings changed. With this implementation, it was difficult to harmonize the voltage regulation requirements of consumers on the low and medium voltage sides. When the on-load tap-changer is located at the linear end of the medium voltage winding, the low voltage winding becomes unregulated. If there is a need to regulate the low voltage winding of the autotransformer, turn on in series with the low voltage winding linear regulator. From an economic point of view, such a solution turns out to be more feasible than constructing an autotransformer with two on-load tap-changers.

Making branches on the neutral side makes it easier to insulate the on-load tap-changer and calculate it for the difference in the currents of the high and medium voltage windings ( I IN - I WITH). But regulation will be tied. When making taps at the line end of a medium voltage winding, the device must be rated for the full rated current and its insulation for the voltage of the medium voltage winding U C. But the regulation will be independent.

According to the figure, the operating current flows through closed contact 1 and auxiliary contact 2. Switching occurs in the following order. When moving from a stage A for a degree V first the operating contact 1 opens, then the auxiliary contact 2. The load current flows through the resistance R. Arc extinguishing contact 3' closes. A bridge is formed - equalizing current flows through both active resistances R And R’. Arc extinguishing contact 3 opens and transfers the load current to the right shoulder. Contacts 2' and 1' close. A new working position is created.

For a detailed consideration of counter voltage regulation, we use the equivalent circuit shown in Fig. 2, a, where the transformer is represented as two elements - the transformer resistance and the ideal transformer. In Fig. 2,a, the following notations are used:

Voltage on HV buses district subs U 2v =U 1 -U 12

The voltages on the HV and LV buses differ by the amount of voltage loss in the transformer U t, and, in addition, in an ideal transformer the voltage decreases in accordance with the transformation ratio, which must be taken into account when choosing a control branch.

Figure 2,b shows graphs of voltage changes for two modes: the lowest and highest loads. In this case, the ordinate axis shows the values ​​of voltage deviations in % of the nominal value. Percentage deviations are meant for all V and U in the field of this figure.

From Fig. 2, b (dashed lines) it is clear that if n T = 1, then in the lightest load mode the consumers' voltages will be higher, and in the heaviest load mode they will be lower than the permissible value (i.e. the deviations U are greater than permissible). In this case, power receivers connected to the LV network (for example, at points A and B) will operate in inaccessible conditions. By changing the tr-ra coefficient of the district ps n T, we change U 2n, i.e. adjust the voltage (solid line in Fig. 2,b).

In the lightest load mode, reduce the voltage U 2n to a value as close as possible to U nom. In this mode, select the largest standard value n T so that the following condition is met: U 2n.nm U nom.

In the heaviest load mode, increase the voltage U 2n to a value closest to 1.05 - 1.1U nom. In this mode, select the largest standard value n T so that the following condition is satisfied:

U 2n.nb (1.051.1)U nom.

Thus, the voltage at the terminals of consumers, both remote from the power center - at point B, and nearby - at point A, is brought within acceptable limits. With this regulation, in the modes of highest and lowest loads, the voltage increases and decreases accordingly. Therefore, such regulation is called counter.

Tasks for independent work:

1. Sources of reactive power in EPS.

2. Types, purpose, methods of connecting reactive power compensation devices, characteristics of their quality.

3. Influence of CP on the modes of electrical networks.

4. Selection of compensating devices.

Lecture 15. Determination of the rated voltage of the designed network. Features of selecting and checking sections in open-loop and simple closed networks.

Determination of the rated voltage of the designed network. Features of selecting and checking cross-sections in open-loop and simple closed networks for power lines and electrical networks in normal and post-emergency modes.

1. Calculation of power line modes in post-emergency conditions

The most severe are the failure and shutdown of sections 1-2 and 3-4 (closest to the power source). Let's analyze these modes and determine the greatest voltage loss U nb in the mode when section 4-3 figure e) is turned off. Let us denote the greatest voltage loss U 1-3 av.

In the mode when section 1-2 is disconnected (Figure g), the greatest voltage loss will be denoted by U 4-2 av.

It is necessary to compare U 1-3 av. and U 4-2 av. and determine the greatest voltage loss U av.nb If the line with double-sided power supply has branches ----- (Figure h))

This makes determining the greatest voltage loss more difficult.

So, in normal mode, it is necessary to determine the voltage losses U 1-3, U 4-3, U 1-2-5, compare them and determine U nb.

Currently, rural consumers are supplied with electricity mainly through radial electrical networks from regional transformer substations fed from powerful power systems. In this case, high and low voltage lines, as a rule, turn out to be extended and branched.

To ensure voltage quality, the value of which for rural electrical installations should not differ from the nominal value by more than ±7.5%, it is recommended to take measures to improve the voltage. Used as the main means counter voltage regulation at the regional distribution substation in combination with the selection of appropriate branches at consumer transformer substations.

Counter-voltage regulation refers to a forced increase in voltage in networks during periods of high loads and a decrease in voltage during periods of low loads. In cases where, with the help of counter voltage regulation at regional substations and selection of branches at transformers of consumer substations, it is still not possible to obtain permissible levels voltage, use group or local voltage regulation by other means.

Voltage booster transformers or longitudinal capacitive compensation devices are used as means of group voltage regulation. As a means of local regulation, transformers with a change in the transformation ratio under load (with on-load tap-changer) are used. To do this, switch the terminals of the turns of the primary winding of the transformer under load without breaking the circuit.

Currently, the most common are 10/0.4 kV transformers with manual switching of branch outputs when the load is removed and the voltage is turned off (with PCB). At the same time, branches are provided on the high-voltage winding of transformers, providing the following control stages: -5; -2.5; 0; + 2.5 and +5%.

When no-load step-down transformers of the rated regulation stage (0%) correspond to a constant voltage increase on the secondary side, equal to +5% In total, at each of the five regulation stages there will be the following voltage increases, respectively: 0; +2.5; +5; +7.5; +10%.

As step-up transformers, as a rule, ordinary step-down transformers are used, but connected in reverse, that is, the secondary winding of the step-down transformer for the step-up transformer becomes primary, and the switching taps are located on the secondary side of the step-up transformer. As a result, for a step-up transformer, a nominal step of 0% corresponds to a premium of -5%. the remaining voltage levels receive opposite signs. In total, at each of the five control stages there will be the following voltage increases, respectively: 0; -2.5; -5; -7.5 and 10%.

The selection of appropriate branches on transformers is carried out both during the design process and during the operation of rural electrical networks. The required branch, and therefore the corresponding surcharge, is selected based on the voltage level on the high-voltage buses of the substation in the minimum and maximum loads.

When designing rural distribution networks, when actual load schedules are difficult to establish, two conditional design modes are specified for selecting branches: maximum - 100% of the load and minimum - 25% of the load. For each mode, the voltage levels on the transformer buses are found and the appropriate increase (regulation stage) is selected that satisfies the condition of permissible voltage deviations (+ 7.5 ... -7.5%).

During operation, transformer branches must be selected taking into account the fact that the voltage level of consumers should not differ from the nominal value by more than ±7.5%.

Voltage deviations among consumers from the nominal value are determined by the formula

Δ U p = ((U consum - U nom) / U nom) x 100

1) By time of day.

Used if the daily chart parameters remain stable from day to day. Figure 4.12.a) shows an example of a graph of the voltage on the CPU buses in the absence of voltage regulation. At the same time, there is a significant decrease in voltage during the daytime.

Rice. 4.12. Daily schedules: a) without regulation, b) with single-stage voltage regulation

Figure 4.12.b) shows an example of a graph obtained with single-stage voltage regulation. Switching taps is carried out twice a day - in the morning and in the evening, due to which the daytime voltage increases.

To automate regulation, you can use either an electric clock with contacts or a software time relay.

2) By voltage (voltage stabilization law).

With such a regulation law automatic regulator ensures, with a certain degree of accuracy, maintaining the voltage on the 6-10 kV buses of the CPU at a level determined by the set voltage Uset (set voltage).

An approximate graph of voltage deviations along the power transmission circuit for the case of voltage stabilization in the CPU is shown in Fig. 4.13, where

Minimum load mode (min mode);

Maximum load mode (max mode);

BAURPN - block automatic control voltage regulator under load;

ε - dead zone width (Zone);

δ - permissible control error, δ = ε / 2;

E – control level;

Δt - delay time for detuning from short-term voltage changes.

ES – power system;

Zline – power line resistance 110 kV,

Zl – resistance of power lines 6-10 kV;

D - voltage addition, depending on the position of the tap switch;

TN - measuring voltage transformer;

CT – measuring current transformer;

AD – high-voltage asynchronous motor;

TP – transformer substation.

The controlled voltage U is supplied through the VT to the input of the BAURPN, where the error is calculated: ОШ = U - Uset. Depending on the ratio of the actual (osh) and permissible (δ) error values, commands are sent from the block output to the tap switch:

OIII > δ → “Reduce voltage” command.

- |OIII|< |δ| → нет команды.

OIII< -δ → команда «Повысить напряжение».

Fig.4.13. Automatic voltage regulation in the CPU according to the stabilization law

The value of the regulation stage E depends on the design of the transformer (indicated in the passport for the transformer), usually lies within the range E = (1.2 - 1.8)%.

The insensitivity zone (dead zone) ε is a certain range of changes in the controlled voltage in which the control equipment does not operate. The value of the dead zone ε determines the control accuracy, which is denoted by ±δ where δ% is a value equal to half of the dead zone. The dead zone of the regulator should be greater than the control stage E by an amount of about 0.2-0.5%, because otherwise the regulator will operate unstably, i.e. There will be an oscillatory mode of operation of the regulator and switching device.

From the graph in Fig. 4.13 it is clear that despite the stable voltage level in the CPU, the voltage at the ED terminals changes depending on the change in load current. The higher the resistance of the power lines Zl and the greater the difference between the load currents in maximum and minimum modes, the higher the range of these deviations.

Fig. 4.14 The regulation process by switching transformer taps

The time delay in the regulators serves to prevent their operation during short-term voltage deviations from the set value. As the time delay increases, the total number of switchings decreases, but at the same time the quality of regulation decreases. As the time delay decreases, the quality of regulation increases, but at the same time the switching frequency and their total number increase. This worsens the operating conditions of switching devices. In practice, the time delay is selected within 1-3 minutes.

To assess the influence of these quantities on the accuracy of regulation, consider the regulation process shown in Fig. 4.14.

At the initial time 0, the regulated voltage was inside the controller dead zone (ε). Then, at time 1, the decreasing voltage triggered the sensitive organ of the regulator and the countdown began. After the time delay t1 has expired, at moment 2 a command is given to switch the tap and after time t2 (the operating time of the switching mechanism), the voltage jumps by an amount determined by the control stage (E) and again finds itself inside the ε zone. In time intervals 4, 5, 6, a similar switching process occurs with the only difference being that the switch returns to its previous position. In the period of time 7, 8 there was a short-term decrease in voltage, to which the regulator did not react, because its time delay t1 turned out to be longer than the duration of this voltage drop t3.

Counter voltage regulation

With counter regulation, a stable voltage level is ensured not on the CPU buses, but at some point in the electrical network remote from the CPU. This is called a “control” or “dummy” point.

In this case, the automatic regulator provides, with a certain degree of accuracy, a voltage level on the CPU buses equal to the sum of the setting voltage Uset and the voltage loss from the CPU to the fictitious point ΔU:

Utsp =Uset +ΔU =Uset +IZl.

In other words, the voltage in the CPU depends on the load current, it increases with increasing load current.

Fig.4.15. Dependence of the voltage in the CPU on the load current during counter-regulation (I",Utsp",I"",Utsp"" – load current and voltage in the CPU in min and max modes)

To implement such a law, the regulator must model a section of the electrical network from the CPU to a fictitious point. This simulation is carried out using a special current compensation resistance through which the load current is passed. The magnitude of this resistance serves as the second (after Urear) parameter of the counter law and is approximately determined by the formula

To implement the counter law, voltage U (from the VT transformer) and load current I (from the CT transformer) are measured and supplied to the regulator input. As a result, the voltage on the CPU buses in maximum mode (day) will be higher than in minimum mode (at night) (see Figures 4.15 and 4.16).

Fig.4.16. Automatic voltage regulation in the CPU according to the counter law

An approximate graph of voltage deviations in the electrical system. network with counter voltage regulation in the CPU is shown in Fig. 4.16. Fictitious point in in this example selected on 6-10 kV RP buses.

For a detailed consideration of counter voltage regulation, we use the equivalent circuit shown in Fig. 2, a, where the transformer is represented as two elements - the transformer resistance and the ideal transformer. In Fig. 2,a, the following notations are used:

Voltage on HV buses district subs U 2v =U 1 -U 12

The voltages on the HV and LV buses differ by the amount of voltage loss in the transformer U t, and, in addition, in an ideal transformer the voltage decreases in accordance with the transformation ratio, which must be taken into account when choosing a control branch.

Figure 2,b shows graphs of voltage changes for two modes: the lowest and highest loads. In this case, the ordinate axis shows the values ​​of voltage deviations in % of the nominal value. Percentage deviations are meant for all V and U in the field of this figure.

From Fig. 2, b (dashed lines) it is clear that if n T = 1, then in the lightest load mode the consumers' voltages will be higher, and in the heaviest load mode they will be lower than the permissible value (i.e. the deviations U are greater than permissible). In this case, power receivers connected to the LV network (for example, at points A and B) will operate in inaccessible conditions. By changing the tr-ra coefficient of the district ps n T, we change U 2n, i.e. adjust the voltage (solid line in Fig. 2,b).

In the lightest load mode, reduce the voltage U 2n to a value as close as possible to U nom. In this mode, select the largest standard value n T so that the following condition is met: U 2n.nm U nom.

In the heaviest load mode, increase the voltage U 2n to a value closest to 1.05 - 1.1U nom. In this mode, select the largest standard value n T so that the following condition is satisfied:

U 2n.nb (1.051.1)U nom.

Thus, the voltage at the terminals of consumers, both remote from the power center - at point B, and nearby - at point A, is brought within acceptable limits. With this regulation, in the modes of highest and lowest loads, the voltage increases and decreases accordingly. Therefore, such regulation is called counter.

Active power balance and its relationship with frequency

The peculiarities of electric power systems are the almost instantaneous transfer of energy from sources to consumers and the impossibility of accumulating generated electricity in noticeable quantities. These properties determine the simultaneity of the process of energy production and consumption.

At each moment of time in the steady state of the system, its power plants must generate power equal to the power of consumers and cover losses in the network - the balance of generated and consumed power must be maintained: Р Г =Р П =Р Н +Р.

where Р Г – generated active power of the station (minus the power spent for its own needs);

 P – total active power consumption;

 N – total active load power of consumers;

 - total active power losses.

With a constant composition of system loads, the power consumption or power is related to the frequency alternating current. If the original balance is disturbed, the frequency takes on a new value. A decrease in generated active power leads to a decrease in frequency; its increase causes an increase in frequency. In other words, with  Г  П the frequency decreases, with  Г  П the frequency increases. This will become clear if we imagine a system consisting of one generator and an engine rotating at the same frequency. As soon as the generator power begins to decrease, the frequency will decrease. The opposite is also true, similarly in the electrical system, for example, at  G  P, the turbines begin to accelerate and rotate faster, f increases.

The reasons for power imbalance may be:

a) emergency shutdown of the generator;

b) unexpected (unplanned, not included in calculations) increase in power consumption, for example, an increase in power consumption by electric heaters as a result of a strong decrease in temperature;

c) emergency shutdown of lines of communication transformers.

To explain the last reason, consider a system of two parts connected by a communication line. When both parts work together, the power balance is maintained:  G1 + G2  P1 + P2

However, in the first part of the system, generation is greater than consumption:  G1  P1, and in the second, on the contrary,  G2  P 2. If the communication line fails, both parts of the system will work in isolation and the balance of P in each of them will be disrupted. In the first the frequency will increase, in the second it will decrease.

The frequency in the system is assessed by the frequency deviation indicator (GOST 13109 - 99).

Frequency deviationf is the difference between its actual value f and the nominal f nom in this moment time expressed in hertz or percentage:

f=f-f nom; f%=

Frequency deviation is allowed:

nominal – within 0.2Hz and maximum – within 0.4Hz.

The given norms of frequency deviations relate to the nominal operating mode of the power system and do not apply to post-emergency modes.

In post-emergency operating modes of the electrical network, a frequency deviation from plus 0.5 Hz to minus 1 Hz is allowed for a total duration of no more than 90 hours per year.

Increased requirements are placed on maintaining frequency in electrical systems, because large deviations can result in failure of station equipment, decreased engine performance, disruption of the technological process and defective products.

The excess of  G over  P, leading to an increase in frequency, can be eliminated by reducing the power of generators or turning off some of them, thereby ensuring frequency regulation in the power system . A decrease in frequency due to the excess of  P over  G requires the mobilization of power reserves or automatic frequency unloading (AFD). Otherwise, a decrease in frequency can lead not only to defective products from consumers, but also to damage to station equipment and system collapse.

In all modes there must be a certain power reserve, which can be realized with a corresponding increase in loads. The reserve can be hot (generators are loaded to less than rated power and very quickly gain load when there is a sudden imbalance in P) and cold, which requires a long period of time to enter.

The total required power reserve of the power system consists of the following types of reserve: load, repair, emergency and national economic. The load reserve serves to cover random fluctuations and unexpected increases in load above the regular maximum load taken into account in the balance. The repair reserve must ensure the possibility of carrying out the necessary planned preventive (routine and major) repairs of power plant equipment. The emergency reserve is intended to replace units that are out of service as a result of an accident. The national economic reserve serves to cover possible excess of electricity consumption against the planned level.

In addition to the power reserve, the system's electrical stations require an energy reserve. Thermal power plants must be provided with an appropriate supply of fuel, and hydroelectric power plants must be provided with a supply of water. If the reserve of stations is exhausted and the frequency in the system has not reached the nominal value, then AFC devices, which are designed for quick recovery power balance when there is a shortage of power by turning off some of the less responsible consumers. All consumers of electrical energy are divided into three main categories based on the reliability of their power supply. First of all, the AChR turns off power supply interruptions for the time necessary to repair or replace a damaged network element, but not more than one day. The most critical consumers are disconnected last.

AChR – discrete system regulation, disconnecting consumers in stages (or queues). When the frequency decreases by an amount f, the frequency relay, which is part of the AFR device, is activated and disconnects some consumers with a power of .

The AFR system consists of automation kits installed at energy facilities. Each set of frequency relays has its own frequency setting at which it operates and disconnects part of the line supplying consumers; The AFR disconnects consumers so that the frequency does not fall below the maximum permissible under the operating conditions of technological equipment of power plants of 46 Hz.