Development and experimental study of circuits of contactless device for automation of compensation of reactive power of capacitor batteries

. This article presents material based on the results of the analysis of literature sources on automatic devices for regulating reactive power in power supply systems. The operating mode of the compensating devices is determined taking into account the permissible voltage deviations at the terminals of the electric power receivers. Voltage, load current, reactive power directions, power factor, phase angle between the supply voltage and load current, and time can be used as control parameters. In addition, the article provides information regarding the performance of the developed circuits of contactless devices and verification of the experimental study of the operation of installations with their use. Also presented is material on the experimental study of a prototype of a contactless switching device for automatic power control of capacitor banks in various operating modes. Summarizing the results of experimental studies of a new device for automatic power control of capacitor banks, it can be stated that it fully meets the requirements for such devices and can ensure reliable operation of the power supply network with an appropriate power factor.


Introduction
Improving the reliability of power supply systems, developing and implementing energy-saving technologies and ways to reduce electricity losses is one of the topical issues of today. The reliability of the power supply system is associated with the safe implementation of switching in the system by turning on, off, transferring power to consumers. The efficiency of the power supply system is influenced by many factors, one of the main among them is the optimal start-up, regulation of the power of capacitor units [1][2][3][4].
Ensuring the economical operation of capacitor units is possible by regulating their power when changing the magnitude and nature of the loads, depending on the voltage at the point of connection of the capacitors [3][4][5].
Non-contact devices for switching and regulating the power of capacitor units are distinguished by their speed, long service life, quiet operation and other advantages [2][3][4][5][6].
In recent years, reactive power compensation devices with automatic control have been introduced in the power supply systems of large industrial and commercial consumers. The main reason for this application was the need to maintain the mains voltage at an acceptable level and compensate for reactive power to reduce losses in medium voltage distribution networks [4][5][6][7].
To ensure the most economical operating modes of power supply systems, power control of compensating devices is used. The regulation of the reactive power generated by the capacitors can be carried out in steps by dividing the batteries into parts The greater the number of such steps, the more perfect the regulation, but the greater the capital cost of installing switches and protective equipment [6][7][8].
The operating mode of the compensating devices is determined taking into account the permissible voltage deviations at the terminals of the electric power receivers [9]. Voltage, load current, reactive power directions, power factor, phase angle between the supply voltage and load current, and time can be used as control parameters [7][8][9][10].

Requirements for reactive power compensation in an industrial enterprise
The power control of the voltage compensating devices is carried out depending on the voltage deviation. In this case, the control of the compensating devices must be coordinated with the regulation of the voltage value by other means [1,[9][10][11][12].
The simplest way is time management. In this case, the operating mode of the power supply system must be studied in advance [11][12][13].
According to the daily schedule of reactive power consumption, it should be determined that one part of the capacitor banks is switched on continuously for 24 hours, and the second is switched on according to the load schedule only for a certain time t1 [10][11][12].
Stepwise regulation of the power of capacitor banks leads to a significant complication of control circuits, therefore, for voltages up to 1000 V, it is recommended to use complete capacitor units with a single power of 75, 100, 150, 200, 250, 300 kVar per unit, and at a voltage of 6-10 kV -complete capacitor units with a single power of 300, 450, 600, 750, 900, 1050, 1200 kVar with connection through a separate switch [13][14][15].
The requirement of consumers of industrial enterprises to ensure a minimum deviation of the operating voltage from the nominal can also be satisfied by adjusting the power of the capacitor bank depending on the voltage 1-4, 16. In this case, capacitor banks, along with the main function of increasing the reactive power factor of the enterprise, are also used to regulate the voltage [6][7][8]17].
During the hours of minimum load, when the voltage is high, the inclusion of capacitor banks leads to an excessive increase in voltage. Therefore, to maintain the rated voltage on the substation buses, the capacitor banks are disconnected when the voltage rises above the permissible value, and when the voltage decreases, they are turned on [6][7][8][9]18].
In power supply systems for reactive power compensation, automated low-voltage installations with switching capacitor bank stages with special electro-mechanical contactors are also used [18][19].
The time interval of the step switching delay (on average about 60 seconds), due to the requirements of the standard for the discharge level of capacitor banks before reswitching and detuning from short-term fluctuations of reactive power in the compensated network, limits the use of these capacitor units for large groups of technological equipment (welding, lifting and transport, forging and press and others) [19][20][21][22]. Therefore, with a very variable inductive load, the socalled "dynamic compensation of reactive power" -Dynamic Power Factor Correction is used, i.e. compensation of reactive power in real time, significantly expanding the functionality of using capacitor banks, but requiring the installation of special types of semi-conductor starters and reactive power controllers [23].
In addition, the above installations have a number of disadvantages associated with the appearance of an arc at each switching, wear and sticking of contacts, noise during operation, as well as power losses in the coils of the contactors.

Development of contactless device circuits
Next, we analyze the operation of devices for automatic power control of compensating devices using contactless elements [1][2][3][24][25].
In the world, scientific research is being carried out to improve reactive power regulators and improve the operational characteristics of contactless switching devices. At the same time, the creation of contactless devices for automatic control of the power of capacitor banks and switching various consumers operating in a high-voltage mode with frequent switching is considered one of the important tasks [3,[25][26][27].
We propose a scheme for automatic power control of capacitor banks directly as a function of the value of the reactive power factor, that is, the angle between the vectors of the supply network voltage and the load current. This method makes it possible to reduce the number of switching operations, since this angle changes significantly less than the value of the load current and supply voltage, which leads to an increase in the quality of work [26].
A diagram of one-stage power control of a capacitor bank as a function of angle  is shown in Fig.1.
The device consists of parallel-oppositely connected thyristors T1 and T2, a voltage relay KV, a current transformer with two secondary windings, a resistor R, diodes D and additional resistances Rd.

Fig.1. Capacitor bank power control circuit
The primary winding of the current transformer is connected in series with the load, and the secondary windings are connected to the control electrodes of the thyristors through diodes and resistors connected in series. A change in the value of the angle between the mains voltage and the load currents causes a change in the firing phase of the thyristors. The voltage across the resistor depends on the angle of the thyristors. With a decrease in the angle , the switching angle of the thyristors decreases and the voltage across the resistor R increases and this causes the KV relay to operate and disconnect the capacitor banks from the network [5,12,27].
Signals are fed to the control electrodes of the thyristors in the power capacitor circuit through a step-down transformer and a contactless voltage relay. The switching of three-phase capacitor banks as a function of voltage is carried out using two such relays, configured for different operating voltages [3,[28][29].
The choice of the elements of the considered voltage relay circuit was carried out taking into account the load power, the voltage of the secondary winding of the transformer, the operating and control currents of the thyristors [30].
To test the performance of the developed circuits of noncontact devices and to experimentally study the installations, prototypes of non-contact devices for regulating the power of capacitor banks were created [2,6,9,[30][31].
Experimental studies of the new device have shown that capacitor banks are connected to a single-phase network at a voltage of 140 Volts, and at a voltage of 190 Volts they are disconnected from the network.
The use of this device for contactless switching on and off of capacitor banks contributes to the optimal use of the capacity of the capacitor banks and the reduction of the reactive power deficit. In this case, a certain amount of electricity is saved. Regulation of the capacity of capacitor banks in industrial enterprises makes it possible to unload the system from reactive load [5,9,[32][33].
Prototypes of such developments were tested in the laboratories of the Department of Power Supply of the Tashkent State Technical University.

Experimental study of non-contact device circuits
Automatic power regulators of capacitor banks, which depend on the value of the phase angle φ and at the same time on the voltage across the load, make it possible to increase the reliability and improve the quality of operation of electrical devices, to reduce the weight and dimensions [3,4,34]. Figures 2-6 show the characteristics of experimental studies of the device for automatic control of the power of capacitor banks in terms of the shift angle φ and in voltage, which was created on the basis of the circuit shown in Fig.1.
For registration and processing of experimental data, a software-controlled apparatus "Fluke" was used. Curves of changes in current and voltage with a predominance of active load are shown in Fig.2. In this case, at U=224 Volts, the values of the consumed active and reactive powers were, respectively, 138 W and 28 VAr, and the value of the total power was 141 VA. Figure 3 shows the curves of current and voltage changes for the above case. In this operating mode, the opening angle of the thyristors is 13 0 and the power factor is 0,98 [9]. Figures 3 and 4 show the curves of current and voltage changes, as well as the opening angle of the thyristors and the value of the power factor.
In this case, the power values P=61 W, Q=34 VAR and S=70 VA, the value of the shift angle between the voltage and load current was φ=300 and cosφ=0.87 [34]. With a decrease in the active load, the cosφ value decreeses accordingly to the device pickup value (cosφ<0,95). In this case, the contactless regulator automatically connects the capacitor banks to the network. Each capacitor bank has a capacity of 1 μF for a voltage of U=400 V.
Further, the curves of the current and voltage changes in each phase were investigated during the switching on of the capacitor banks [5,6,13,19,[35][36]. Fig.6 shows the changes in current and voltage in phase A when capacitor banks are switched on; in other phases, the curves are of a similar nature.

Conclusions
1. For the first time, a circuit has been developed that makes it possible to experimentally investigate the characteristics of the circuit and the device for automatic power control of capacitor banks by the angle .
2. Investigations of the prototype of the contactless regulation of the capacitor banks' power in terms of voltage and load angle "" showed that, while ensuring the full functioning of the prototype of the devices, it is distinguished by simplicity of control.
3. In parallel -the opposite switching on of thyristors, a change in the value of the angles between the mains voltage and the load current is caused by a change in the phase of firing of the thyristors. The voltage values on the control element depend on the angle of the thyristors.
4. The proposed method and circuit for automatic power control of capacitor banks, the function of the angle between the vectors of the mains voltage and the load current showed high reliability and ease of operation of the device. 5. A comprehensive study of structures, elements and automation circuits for regulating the power of capacitor banks made it possible to increase the possibilities of synthesizing new circuits, and also created conditions for improving the characteristics.
6. Experimental studies were carried out in laboratory and in production conditions of a prototype of a non-contact device for regulating the power of capacitor banks, and oscillograms of changes in voltage, current and other indicators were taken.