METHOD FOR CONTROL OF THE START-UP REGULATING DEVICE FOR POWER TRANSFORMERS OF THE POWER SUPPLY SYSTEM

: Three-phase thyristor switches are designed for pulsed formation of inrush currents of electrical equipment with their subsequent shunting in steady state operation. In transformer substations, they perform a bumpless turning on of a power transformer by connecting its primary winding first to two phases of the network at the moment of zero crossing by the phase voltage of the network third phase, and then to the network third phase at the moment of zero crossing by the line voltage of the other two network phases. In this case, the starting currents of the transformer almost immediately enter the steady state without the appearance of constant components in the magnetization currents and voltage drop. To expand the functionality of thyristor switches, it is proposed, in addition to bumpless turning on of a power transformer, to disconnect it without arcing between the contacts of electrical equipment, as well as to carry out continuous voltage regulation for consumers when voltage in the network changes. The proposed method and structure for its implementation on the basis of two three-phase thyristor reactor keys and a capacitor bank make it possible while changing the network voltage to stabilize the generated reactive power at the input of the substation without creating the current distortions in the power transformer and power transmission simultaneously with stabilizing the substation output voltage. Modeling and research of the start-regulating device as part of a transformer substation was carried out in the MatLab environment. The results of numerical experiments in stationary and dynamic modes of the substation operation showed the feasibility of using the developed technical solutions for the industrial power supply system.


Introduction
The commercially available thyristor AC voltage regulators with natural switching (TRVN) have relatively small overall dimensions, high speed and operational reliability. They are used to start asynchronous motor, at the input of diode rectifiers, in heating installations, lighting systems and in other electrical technologies. According to individual projects, they are manufactured for a voltage of 35 kV and a current of 2 kA for static compensators of reactive power of electric steelmaking plants [1].
At the same time, experimental studies on the use of TRVN as start-regulating devices for power transformers (PT) of substations 6 (10)/0.4 kV did not find practical application. This is due to the fact that with an increase in the control angle of the thyristors, TRVN consumes additional reactive power and introduces distortions in the output voltage and input current of the substation [2][3][4][5][6][7][8][9][10][11].
Analytical study of appropriate literature, patents study, and survey of testing experience in industrial operation allowed creating new technical solutions for transformer substations (TS). This is a reactor-thyristor AC voltage regulator with natural switching (R-TRVN) [12, 13] specially designed for PT and its control method [14].
The proposed R-TRVN device is installed on the high side of the PT in the same manner as the mechanical on-load tap-changer, and under continuous regulation it provides stable voltage for consumers with voltage deviations from the nominal by ± 10%. At the same time, it provides a sinusoidal voltage at the input and output of PT for three values of voltage in the network (nominal value, increased and decreased relative to the nominal value by 10%). Between these sinusoidal levels, a continuous narrow-range regulation is carried out in two sub-bands: in the upper one it is with lowered network voltage, and in the lower one with increased network voltage. Regulation is performed with a harmonic composition of voltage, meeting the Russian State Standard requirements, and without distorting the shape and phase shift of the current network.
The R-TRVN control method, in addition to dual-band regulation, provides bumpless turning on PT under load, in which there are no shock electrodynamic forces on the transformer windings and voltage reduction, and shutdown is performed without arcing on the mechanical contacts of the high voltage switch [15].
The purpose and objectives of research The purpose of this work is to study the adjusting properties and energy parameters of the substation according to the P-TRVN -PT scheme using a mathematical model.
To achieve this purpose, the following tasks were set and solved in the work. 1. Development of a software package for substation research according to the P-TRVN -PT scheme.
2. Study of voltage at the PT input and at consumers when the voltage deviation in the network is ± 10% of the nominal.
3. Study of current shape in the network, reactor, thyristor switches and at the PT input during the process of voltage stabilization at consumers. 4. Study of the current network phase when voltage deviates from the input of the substation, which includes P-TRVN and a block of capacitors.

Development of a specialized device for a transformer substation
Two schemes are proposed for connecting the P-TRVN device to the primary circuit of the PT substation. They are shown in fig. 1. In the first circuit ( Fig. 1, a), the device is connected between the network and the primary PT winding connected in a star, and its pulse-phase control system (PPCS) is synchronized with the voltage of the secondary PT winding. In the second scheme ( Fig. 1, b), the device is included in the cut of the star of the primary PT winding, and its pulse-phase control system is synchronized with the network. Both circuits perform regulation with identical physical processes and have their own advantages and disadvantages in the reconstruction of existing and newly designed TSs, taking into account the use of dry or oil PTs in them. At the same time, the advantage of the second circuit is that when a three-phase short circuit occurs in the primary winding of the PT, the thyristors are not exposed to emergency current. The circuits (Fig. 1) contain a three-phase network G, a power line W, an input highvoltage switch Q1, a high-voltage switch Q2 in the CB capacitor bank circuit, modules of the main VS-1 and additional VS-2 thyristor switches with a pulse-phase control system PPCS-1 and PPCS-2, AC contactor, main L1 and additional L2 reactors, power transformer PT and active-inductive load Z.
The device operates as follows.
The power transformer PT is turned on when the additional thyristor switches VS-2 are completely turned off. In this case, firstly two main thyristor switches VS-1 using an additional reactor L2 connect two phases of its primary winding to the corresponding phases of the network G at the moment of zero crossing by the phase voltage of the network third phase G, then the third main thyristor switch VS-1 connects the third phase of the primary winding of the power transformer PT to the network third phase at the moment the line voltage of the other two phases of the network is crossing zero. At the end of the process of turning on the PT power transformer while preparing the substation for voltage regulation, parallel to the fully open main thyristor switches VS-1, the main reactor L1 is connected via a three-phase contactor AC. Further, when the conductive state of the thyristors changes, the process of voltage regulation starts both up and down as relating to the network voltage. The upper limit is set by the transformation ratio of the power transformer, and the lower limit is set by the resistance of the reactor.
The proposed R-TRVN control method provides voltage regulation at the input of the power transformer relative to the network voltage and among consumers between the specified regulation limits: the maximum and minimum divided by the nominal level.
The maximum limit of voltage regulation at the power transformer load is set by the transformation coefficient of the power transformer PT with the main switches VS-1 completely turned off and the additional thyristor switches VS-2 fully turned on at a reduced voltage in the network G. The additional thyristor switches VS-2 at this moment bypass the main L1 and additional L2 reactors in the circuit primary winding of a power transformer PT.
The rated voltage level at the load is ensured when the main switches VS-1, which bypass the main reactor L1, are fully turned on, and the additional thyristor switches VS-2 are completely turned off at the rated mains voltage G and the rated load Z.
The minimum regulation limit on the load is set by the total resistance of the main L1 and additional L2 reactor with completely closed main VS-1 and additional VS-2 thyristor switches with increased voltage in the network G and rated load Z.
A change in the network voltage affects the voltage of consumers and leads to a change in the reactive power of the capacitor bank and in the network.
The inductance and voltage drop are regulated using R-TRVN at the TS input. Depending on the positive and negative voltage deviations at the substation input, it is necessary to individually select the resistance of the primary and secondary reactors. This contributes to the achievement of high quality load voltage.
It should be noted that the reactive power of capacitors depends on the change in the supply voltage and that during the process of voltage stabilization for consumers, the conductive state of thyristors and inductance of R-TRVN in the PT primary circuit are regulated using a thyristor switch. An increase in inductance simultaneously with an increase in the generated reactive power of capacitors eliminates the deviation of the reactive power (deviation of the current phase) of the network according to the principle of indirect compensation.
The known devices built on the principle of indirect compensation of reactive power with parallel connection to the network and capacitors and a thyristor-reactor device [19] create current distortions in the network and can operate either in the mode of reactive power compensation, or in the mode of voltage maintenance.
For the proposed method and device [13,14], the distinctive feature of the principle of indirect compensation of network reactive power with simultaneous compensation of voltage deviations at consumers without distorting the PT and network current is that the capacitor is connected to the network in parallel, and the reactor is connected in series between the network and TS .
An increase in the inductance in the PT circuit during the pulse-phase control of the alternating voltage at the reactors neutralizes the additional reactive power generated by the capacitors, providing a slight deviation of the network phase current from voltage and maintaining the maximum cosφ value. This is one of the great features of the proposed device.
Turning off the substation power transformer PT without the occurrence of an electric arc and switching overvoltages is conducted as follows.
Before turning off the power transformer, pulses are first removed from the additional thyristor switches VS-2. Then, the main thyristor switches VS-1 are transferred to the fully open state and the current through the contacts of the three-phase contactor AC and the main reactor L1 is set to zero. After this, the three-phase contactor AC is used to disconnect the de-energized main reactor L1 without arcing and overvoltage. At the final operation of the method, control pulses are removed from the main thyristor switches VS-1 with natural switching and they are turned off without switching losses when phase currents are crossing zero.

Mathematical modeling of the reactor thyristor device
To study the regulatory properties and energy parameters of the substation according to the R-TRVN -PT scheme and physical processes in static and dynamic modes, a software package was developed in the Matlab environment [16]. The substation model is built for the circuit of Fig.  1b, and is shown in Fig. 2. It contains the following elements and modules: three single-phase sources forming a three-phase network (Uа, Ub, Uc); power line W; input high-voltage switch Q1; high-voltage switch Q2 in the battery circuit of the capacitor bank CB; contactor AC; modules of the main VS-1 and additional VS-2 thyristor switches with a pulse-phase control system PPCS-1 and PPCS-2; main L1 and additional L2 reactors; power transformer PT; active-inductive load Z and other auxiliary elements.  Figure 3 shows detailed schemes of modules that have particular differential equations and, when joint, form a common system of differential equations of the object under study. Research results of a transformer substation with a start-regulating reactor-thyristor device When modeling, the following physical processes of substation according to the R-TRVN -PT scheme were studied.
1. Study of voltages at the PT input and consumers with voltage deviations in the upper and lower sub-bands of regulation.
2. Study of voltages at R-TRVN at the PT input with respect to the network. The obtained oscillograms are shown in Fig. 4.
3. Study of the network current shape at that at the input of PT in the reactor and through the thyristors during voltage stabilization at the consumer when working from dual-band R-TRVN.
Further we analyze the results of TS numerical studies in MatLab using the R-TRVN scheme. Figure 4 shows voltage oscillograms at the TS elements using the R-TRVN scheme. They are obtained at rated load and network voltage deviations of ± 5% of the nominal level and are shown for one phase. It should be noted that when the regulation is shifted from the middle in one direction or another, the voltage shape improves and tends to be sinusoidal. The device employs a narrow-range pulse-phase regulation of alternating voltage with a parallel connection of reactor and thyristor switch [17]. This three-phase device is included in the primary circuit of the PT connected to a star without a neutral wire. Such an inclusion creates an interphase voltage interaction [18], as a result of which the voltage modulation frequency increases three times with respect to the switching frequency of the phase thyristor switches (Fig. 4).
When the network voltage is increased by the deviation +∆U, the device, increasing the control angle α, reduces the conductive state of the thyristors and, increasing the inductance on the high side of TS, creates a voltage drop -∆U on it, while maintaining the required voltage at consumers. With a decrease -∆U, the device reduces the control angle α, increases the thyristors conductive state and, reducing the inductive resistance on the high side of TS, creates a voltage drop of +∆U on it.
Thus, during stabilisation of load voltage, the X L of reactor thyristor device is regulated, providing the principle of indirect compensation of reactive power [19].
The oscillograms in Fig Analysis of these oscillograms shows that the shape of the network current is distorted slightly, and its phase coincides with the network voltage, which determines high efficiency of electricity consumption of the transformer substation.
R-TRVN studies have revealed that voltage regulation on the TS high side does not adversely affect the shape of the network current. The results of this study are illustrated by the following figures. Oscillogram in Fig. 6b, illustrates the PT operation at its rated mode and characterizes that, at rated PT operation, the currents of network, additional reactor, and main thyristor switch are equal. It can be seen from the oscillogram (Fig. 6, a and c) that the thyristor and reactor currents are distorted, and their sum, being the network current and the current of the power transformer, retains a sinusoidal shape at any control angles. This is another remarkable property of the device, which does not create additional losses in the power transformer and in the network during regulation [20].

Conclusions
Studies of a mathematical model of dynamic and quasi-stationary processes of a dual-band reactor-thyristor AC voltage regulator with natural switching as part of a transformer substation allowed us to draw the following conclusions: 1. The use of a reactor-thyristor device of continuous operation allows one to release a mechanical switching device with a current-limiting reactor of on-load tap changing type, to simplify the PT design and the technology for production of complete transformer substations.
2. When the voltage deviation in the network is ± 10% of the nominal value, the device maintains the voltage at consumers at a given level with an accuracy of not more than ± 1%.
3. The device, together with a capacitor bank, simultaneously with voltage stabilization at the transformer substation output provides stabilization of the generated reactive power at the substation input. 4. During the process of continuous regulation of voltage at the input of substation under load, the reactor-thyristor AC voltage regulator with natural switching does not create current distortions in the power transformer and in the network.
5. When applying a special control that takes into account electromagnetic processes, the device produces bumpless turning on the power transformer under load without phase currents exceeding their established values and turning it off without arcing at the contacts of high-voltage switching devices.