Frequency regulation in the power system

In electric power systems, at any given time, such amount of electricity must be generated as is necessary for consumption at a given time, since it is impossible to create reserves of electrical energy.

Frequency along with voltage is one of the main ones power quality indicators... Deviation of the frequency from the normal leads to disruption of the operation of power plants, which, as a rule, leads to burning of fuel. A decrease in the frequency in the system leads to a decrease in the productivity of mechanisms in industrial enterprises and to a decrease in the efficiency of the main units of power plants. An increase in frequency also leads to a decrease in the efficiency of the power plant units and to an increase in losses in the networks.

At present, the problem of automatic frequency regulation covers a wide range of issues of an economic and technical nature. The power system is currently performing automatic frequency regulation.

Effect of frequency on the operation of power plant equipment

All units performing rotary movement are calculated in such a way that their highest efficiency is realized three times from one very specific speed of rotation, namely at the nominal one. At the moment, the units performing rotary motion are for the most part connected to electric machines.

The production and consumption of electrical energy is carried out mainly on alternating current; therefore, the majority of blocks performing rotary motion are associated with the frequency of alternating current. Indeed, just as the frequency of the alternator generated by the alternator depends on the speed of the turbine, so the speed of the mechanism driven by the AC motor depends on the frequency.

Deviations of the alternating current frequency from the nominal value have a different effect on different types of units, as well as on different devices and apparatus on which the efficiency of the power system depends.

The steam turbine and its blades are designed in such a way that the maximum possible shaft power is provided at the rated speed (frequency) and seamless steam input. In this case, a decrease in the rotational speed leads to the occurrence of losses for the steam impingement on the blade with a simultaneous increase in the torque, and an increase in the rotational speed leads to a decrease in the torque and an increase in the impingement on the back side of the blade. The most economical turbine works at nominal frequency.

In addition, operation at a reduced frequency leads to accelerated wear of the turbine rotor blades and other parts.The change in frequency affects the operation of self-consumption mechanisms of the power plant.

Effect of frequency on the performance of electricity consumers

Mechanisms and units of electricity consumers can be divided into five groups according to the degree of their dependence on frequency.

First group. Users whose frequency change has no direct effect on the developed power. These include: lighting, electric arc furnaces, resistance leakage, rectifiers and loads powered by them.

Second group. Mechanisms whose power varies in proportion to the first power of the frequency. These mechanisms include: metal cutting machines, ball mills, compressors.

Third group. Mechanisms whose power is proportional to the square of the frequency. These are mechanisms whose moment of resistance is proportional to the frequency in the first degree. There are no mechanisms with this exact moment of resistance, but a number of special mechanisms have a moment approximating this.

Fourth group. Fan torque mechanisms whose power is proportional to the cube of the frequency. Such mechanisms include fans and pumps with no or negligible static head resistance.

Fifth group. Mechanisms whose power depends on the frequency to a higher degree. Such mechanisms include pumps with a large static resistance head (eg feed pumps of power plants).

The performance of the last four user groups decreases with decreasing frequency and increases with increasing frequency. At first glance, it seems that it is beneficial for users to work at an increased frequency, but this is far from the case.

In addition, as the frequency increases, the torque of the induction motor decreases, which can cause the device to stall and stop if the motor has no power reserves.

Automatic frequency control in the power system

The purpose of automatic frequency control in power systems is primarily to ensure economical operation of stations and power systems. The efficiency of the operation of the power system cannot be achieved without maintaining the normal frequency value and without the most favorable distribution of the load between the parallel working units and the power plants of the power system.

To regulate the frequency, the load is distributed among several parallel work units (stations). At the same time, the load is distributed among the units in such a way that with minor changes in the system load (up to 5-10%), the operating mode of the huge number of units and stations does not change.

With a variable nature of the load, the best mode will be one in which the main part of the blocks (stations) carries the load corresponding to the condition of equality of relative steps, and small and short fluctuations of the load are covered by changing the load of a small part from the units.

When they distribute the load between the units working in parallel, they try to ensure that they all work in the area of ​​the highest efficiency. In this case, minimum fuel consumption is ensured.

The units tasked with covering all unplanned load changes, i.e. frequency regulation in the system must meet the following requirements:

• have high efficiency;

• have a flat load efficiency curve, i.e. maintain high efficiency over a wide range of load variations.

In the case of a significant change in the load of the system (for example, its increase), when the entire system switches to a mode of operation with a larger value of the relative gain, the frequency control is transferred to such a station in which the magnitude of the relative gain is close to that of the system .

The frequency station has the largest control range within its installed power. The control conditions are easy to implement if frequency control can be assigned to a single station. An even simpler solution is obtained in cases where regulation can be assigned to a single unit.

The speed of the turbines determines the frequency in the power system, so the frequency is controlled by acting on the turbine speed governors. Turbines are usually equipped with centrifugal speed governors.

The most suitable for frequency control are condensing turbines with normal steam parameters. Back pressure turbines are completely inappropriate types of turbines for frequency control, since their electrical load is determined entirely by the steam user and is almost completely independent of the frequency in the system .

It is impractical to entrust the task of frequency regulation to turbines with large steam suctions, because, firstly, they have a (very small control range and, secondly, they are uneconomical for variable load operation.

To maintain the required control range, the power of the frequency control station should be at least 8 - 10% of the load in the system so that there is sufficient control range. The regulation range of the thermal power plant cannot be equal to the installed capacity. Therefore, the power of the CHP, which adjusts the frequency, depending on the types of boilers and turbines, should be two to three times higher than the required adjustment range.

The smallest installed power of the hydroelectric plant to create the necessary control range can be significantly less than the thermal one. For hydroelectric plants, the regulation range is usually equal to the installed capacity. When the frequency is controlled by a hydroelectric plant, there is no limit to the rate of increase of the load starting from the moment the turbine is started. However, frequency regulation of hydroelectric plants is associated with the well-known complication of control equipment.

In addition to the station type and equipment characteristics, the selection of the control station is influenced by its location in the electrical system, namely the electrical distance from the load center. If the station is located in the center of the electrical load and is connected to substations and other stations of the system through powerful power lines, then, as a rule, an increase in the load of the regulating station does not lead to a violation of static stability.

Conversely, when the control station is located far from the center of the system, there may be a risk of instability.In this case, frequency regulation must be accompanied by control of the divergence angle of the e vectors. etc. c. system and station for managing or controlling the transmitted power.

The main requirements for frequency control systems regulate:

• parameters and limits of adjustment,

• static and dynamic error,

• the rate of change in block load,

• ensuring stability of the regulatory process,

• the ability to regulate by a given method.

Regulators should be simple in design, reliable in operation and inexpensive.

Frequency control methods in the power system

The growth of power systems led to the need to regulate the frequency of several blocks of one station, and then several stations. For this purpose, a number of methods are used to ensure stable operation of the power system and high frequency quality.

The applied control method must not allow an increase in the frequency deviation limits due to errors occurring in auxiliary devices (active load distribution devices, telemetry channels, etc.).

The frequency regulation method is necessary to ensure that the frequency is maintained at a given level, regardless of the load on the frequency control units (unless, of course, their entire control range is used), the number of units and the frequency control stations, and the magnitude and duration of the frequency deviation.… The control method must also ensure the maintenance of a given load ratio of the control units and the simultaneous entry into the regulation process of all units that control the frequency.

Method of static characteristics

The simplest method is obtained by adjusting the frequency of all units in the system, when the latter are equipped with speed regulators with static characteristics. In parallel operation of blocks operating without shifting the control characteristics, the distribution of loads between the blocks can be found from the static characteristic equations and the power equations.

During operation, load changes significantly exceed the specified values, therefore the frequency cannot be maintained within the specified limits. With this method of regulation, it is necessary to have a large rotating reserve spread over all units of the system.

This method cannot ensure economical operation of power plants, since, on the one hand, it cannot use the full capacity of economical units, and on the other hand, the load on all units is constantly changing.

Method with an astatic characteristic

If all or part of the system units are equipped with frequency regulators with astatic characteristics, then theoretically the frequency in the system will remain unchanged for any changes in the load. However, this control method does not result in a fixed load ratio between the frequency controlled units.

This method can be successfully applied when frequency control is assigned to a single unit.In this case, the power of the device should be at least 8 — 10% of the system power. It does not matter whether the speed controller has an astatic characteristic or the device is equipped with a frequency regulator with an astatic characteristic.

All unplanned load changes are perceived by a unit with an astatic characteristic. Since the frequency in the system remains unchanged, the loads on the other units of the system remain unchanged. Single-unit frequency control in this method is perfect, but proves unacceptable when frequency control is assigned to multiple units. This method is used for regulation in low-power power systems.

Generator method

The master generator method can be used in cases where, according to the system conditions, it is necessary to adjust the frequency of several units at the same station.

A frequency regulator with an astatic characteristic is installed on one of the blocks, called the main one. Load regulators (equalizers) are installed on the remaining blocks, which are also charged with the task of frequency regulation. They are tasked with maintaining a given ratio between the load on the master unit and the other units that help regulate the frequency. All turbines in the system have static speed governors.

The method of imaginary statism

The imaginary static method is applicable to both single-station and multi-station regulation.In the second case, there must be two-way telemetry channels between the stations that adjust the frequency and the control room (transmission of the load indication from the station to the control room and the transmission of automatic order from the control room to the station).

A frequency regulator is installed on each device involved in regulation. This regulation is astatic with respect to maintaining the frequency in the system and static with respect to the distribution of loads among the generators. It ensures a stable distribution of loads between the modulating generators.

Load sharing between the frequency controlled devices is achieved by means of an active load sharing device. The latter, summarizing the entire load of the control units, divides it between them in a certain predetermined ratio.

The method of imaginary statism also makes it possible to regulate the frequency in a system of several stations, and at the same time the given load ratio will be respected both between stations and between individual units.

Synchronous time method

This method uses the deviation of synchronous time from astronomical time as a criterion for frequency regulation in multi-station power systems without the use of telemechanics. This method is based on the static dependence of the deviation of the synchronous time from the astronomical time, starting from a certain moment in time.

At the normal synchronous speed of the rotors of the turbine generators of the system and the equality of the turning moments and moments of resistance, the rotor of the synchronous motor will rotate at the same speed. If an arrow is placed on the rotor axis of a synchronous motor, it will show the time on a certain scale. By placing a suitable gear between the shaft of the synchronous motor and the axis of the hand, it is possible to make the hand rotate at the speed of the hour, minute or second hand of the clock.

The time shown by this arrow is called synchronous time. Astronomical time is derived from accurate time sources or from electrical current frequency standards.

A method for simultaneous control of astatic and static characteristics

The essence of this method is as follows. There are two control stations in the power system, one of them works according to the astatic characteristic, and the second according to the static one with a small static coefficient. For small deviations of the actual load schedule from the control room, any load fluctuations will be perceived by a station with an astatic characteristic.

In this case, a control station with a static characteristic will participate in the regulation only in transient mode, avoiding large frequency deviations. When the adjustment range of the first station is exhausted, the second station enters adjustment. In this case, the new stationary frequency value will be different from the nominal one.

While the first station controls the frequency, the load on the base stations will remain unchanged. When adjusted by the second station, the load on the base stations will deviate from the economic one.The advantages and disadvantages of this method are obvious.

Power Lock Management Method

This method consists in the fact that each of the power systems included in the interconnection participates in frequency regulation only if the frequency deviation is caused by a change in the load in it. The method is based on the following property of interconnected energy systems.

If the load in any power system has increased, then a decrease in frequency in it is accompanied by a decrease in the given exchange power, while in other power systems, a decrease in frequency is accompanied by an increase in the given exchange power.

This is due to the fact that all devices that have static control characteristics, trying to maintain the frequency, increase the output power. Thus, for a power system where a load change has occurred, the sign of the frequency deviation and the sign of the exchange power deviation match, but in other power systems these signs are not the same.

Each power system has one control station where frequency regulators and an exchange power blocking relay are installed.

It is also possible to install in one of the systems a frequency regulator blocked by a power exchange relay, and in an adjacent power system - an exchange power regulator blocked by a frequency relay.

The second method has an advantage over the first if the AC power regulator can operate at rated frequency.

When the load in a power system changes, the signs of frequency deviations and exchange power coincide, the control circuit is not blocked, and under the action of the frequency regulator, the load on the blocks of this system increases or decreases. In other power systems, the signs of the frequency deviation and exchange power are different and therefore the control circuits are blocked.

Regulation by this method requires the presence of television channels between the substation from which the connecting line departs to another power system and the station that regulates the frequency or exchange flow. The blocking control method can be successfully applied in cases where the power systems are connected by only one connection to each other.

Frequency system method

In an interconnected system that includes several power systems, frequency control is sometimes assigned to one system while the others control the transmitted power.

Internal statism method

This method is a further development of the control blocking method. Blocking or strengthening the action of the frequency regulator is not carried out by means of special power relays, but by creating statism in the transmitted (exchange) power between the systems.

In each of the parallel operating energy systems, one regulating station is allocated, on which regulators are installed, which have statism in terms of exchange power. Regulators respond to both the absolute value of the frequency and the exchange power, while the latter is kept constant, and the frequency is equal to the nominal one.

In practice, in the power system during the day the load does not remain unchanged, but the changes according to the load schedule, the number and power of the generators in the system and the specified exchange power also do not remain unchanged. Therefore, the static coefficient of the system does not remain constant.

With a higher generating capacity in the system, it is smaller and with a lower power, on the contrary, the static coefficient of the system is higher. Therefore, the required condition of equality of statism coefficients will not always be fulfilled. This will result in the fact that when the load changes in one power system, the frequency converters in both power systems will come into action.

In a power system where a load deviation has occurred, the frequency converter will act all the time in one direction during the entire regulation process, trying to compensate for the resulting imbalance. In the second power system, the operation of the frequency regulator will be bidirectional.

If the stat coefficient of the regulator in relation to the exchange power is greater than the stat coefficient of the system, then at the beginning of the regulation process, the control station of this power system will reduce the load, thereby increasing the exchange power, and after this increase the load to restore the set value of the exchange power at the rated frequency.

When the stat coefficient of the regulator with respect to the exchange power is less than the stat coefficient of the system, the control sequence in the second power system will be reversed (first, the acceptance of the driving factor will increase, and then it will decrease).