SOME RESEARCH QUESTIONS OF REACTIVE ENERGY COMPENSATION

ABSTRACT

The article deals with the transient process in the power supply system of industrial enterprises with stepwise regulation of capacitor units. A mathematical model of the switching process is proposed, based on the results of its calculation, it is recommended to configure relay protection and automatic control devices with capacitor banks.

АННОТАЦИЯ

В статье рассматривается переходной процесс в системе электроснабжения промышленных предприятий при ступенчатом регулировании конденсаторных установок. Предложена математическая модель коммутационного процесса, по результатам ее расчета рекомендовано настраивать устройства релейной защиты и автоматического регулирования конденсаторными батареями.

Keywords: reactive power, automatic control, relay protection, capacitor banks.

Ключевые слова: реактивная мощность, автоматическое регулирование, релейная защита, конденсаторные батареи.

The operating conditions of modern power supply systems require the widespread use of reactive energy sources in industrial enterprises. Depending on the nature of the electrical equipment, the reactive load of industrial enterprises can be up to 130% compared to the active load. The most common type of such sources at present are capacitor banks (СB) [1].

At present, in world practice, one can observe two trends in the use of parallel CB compensation in distribution networks: a) as a means of compensating for reactive load currents; b) as the main means of voltage regulation. The first trend is inherent in the practice of most European countries and Japan, the second - the United States and partly France. The first trend is mainly present in the domestic practice of using CB. In general, the second direction is considered more promising.

The operating mode of the CB should ensure the maximum reduction in electricity losses in the networks while maintaining the permissible range of voltage deviations at the terminals of power receivers. However, at present, KBs often operate without autocontrol. In those cases when auto control is provided, then such control parameters are implied as: load power factor, magnitude and direction of reactive power. At the same time, it is obvious that the reactive power deficiency can first of all be identified by the effect of reducing the voltage in the network. In this case, there is a need for an additional inclusion of a reactive power source. When the voltage level rises, it is necessary to turn off part of the reactive power source [2,3]. Accordingly, capacitor units should consist of several sections: the main section (power is selected from the condition of an economically feasible degree of reactive power compensation) and additional sections (automatically switched on as the voltage in the network decreases). Thus, control over the voltage mode in the network with an impact on the CB is quite effective from the point of view of ensuring the balance of reactive power, which should take place with acceptable mode parameters and, above all, with acceptable voltage deviations.

However, turning on and off the CB affects the operation of the relay protection (RP) of the network elements due to current surges and its fluctuations in the network, and the operation of the CB auto device.

In order to determine the value of starting currents that are possible when additional sections of the CB are turned on, as well as the time delay of the controller, in order to prevent its operation during short-term changes in the controlled parameter, it is necessary to consider the electromechanical transient process in a particular network.

In a typical asynchronous load power supply circuit, presented in the form of an equivalent asynchronous electric motor (АM) connected to the voltage bus U₁, which are connected through a network with a resistance x_sv with a voltage bus U₂ of an infinite power supply, an electromechanical transient process when additional sections of the CB connected in parallel to asynchronous motor, can be represented by the following equations in the coordinates d and q.

Transient process equations for AM:

For a capacitor plant, the following equations are valid:

The mode of the external network connecting the load with tires of infinite power is determined by the equations:

For the load node we have:

As an example, an electromechanical transient process was considered in a typical asynchronous load power supply scheme, presented in the form of an equivalent asynchronous motor of the VDD-170/34-12 type. Capacitor units with additional sections are connected to the load busbars with voltage U₁ in parallel to the asynchronous motor. The main section of this capacitor bank is designed to compensate up to 100% of the load reactive power in normal operation. The load is removed from the busbars of the system with voltage U₂ on x_ex.=2x_tr, where x_tr is the inductive reactance of the transformer.

The integration of this system of differential equations was carried out by the fourth-order Runge-Kutta method.

The initial conditions were determined from the following system of equations for the steady state. For an asynchronous motor, the following equations are valid:

For capacitor installations, the equations will look like:

The external network mode is determined by the equations:

Current balance in the load node:

The system of equations for the steady state was solved by the Gauss method. The voltage U₁ on the load buses is monitored by an organ that reacts to a decrease in voltage U₁, for example, a voltage relay. The operation of the machine having a relay characteristic is described by the following expression:

if   then ;

if   then

where  is the main section of the capacitor unit;  − additional section.

It was assumed that U_1min = 0.95 U_1nom, and U_1max = 1.05U_1nom.

When analyzing electromechanical transients, it is necessary to take into account that the additional section does not turn on instantly, but with a certain delay, which is determined by the delay time due to the action of the switching equipment, and the exposure time necessary to detuning from short-term fluctuations in the network and from the transient caused by the inclusion of the previous sections of the condenser unit.

Conclusion

1. The use of capacitor units with additional sections will improve not only the voltage quality, but also, in case of voltage dips in the system, to increase the stable operation of asynchronous motors that set in motion critical mechanisms on which the entire technological process depends.
2. The solution of the proposed equations of the electromechanical transient process for a specific power supply system will make it possible to determine all the mode parameters when one or another power of the additional section of the capacitor unit is turned on and, accordingly, evaluate their effect on the operation of relay protection and select the response time of automatic devices of capacitor units.

References:

1. Zhezhelenko I.V., Saenko Yu.L. Kachestvo elektroenergii na promishlennix predpriyatiyax. –М.: Energoatomizdat, 2005. – 261 s.
2. Ivanov V.S. Sokolov V.I. Rejimi potrebleniya I kachestvo elektroenergii v sistemax elektrosnabjeniya promishlennix predpriyatiy. - M.: Energoatomizdat, 1987. - 336 s.
3. Karpov F.F. Kompensasiya reaktivnoy moshnosti v raspredelitelnix setax. M.: Energiya, 1975. - 184 s.