Research on High Voltage Static VAR Compensator

1 Introduction

At present, a long-term problem in the construction and operation of my country's power grid is the insufficient reactive compensation capacity and unreasonable configuration, especially the insufficient adjustable reactive capacity and fewer fast-response reactive regulation equipment. In recent years, with the continuous increase of high-power nonlinear loads, the reactive impact and harmonic pollution of the power grid have been on the rise. The lack of reactive regulation means has caused the bus voltage to vary greatly with the change of operation mode, resulting in increased line loss of the power grid and reduced voltage qualification rate. In addition, with the development of the power grid, the problem of system stability has become increasingly important. Dynamic reactive compensation technology is an economical and effective measure to improve voltage stability. In addition, there is also a great demand for dynamic reactive compensation technology in industrial fields such as metallurgy, electrified railways, and coal. Under the current circumstances, the use of TCR static dynamic reactive compensation device (SVC) is very effective in eliminating reactive impacts generated by symmetrical loads such as rolling mills and other large motors. The voltage fluctuation of the power grid has been significantly improved, and the power factor has been significantly improved. It is a new energy-saving device with high technical content and significant economic benefits.

2 Basic types and structures of svc

The compensation principle of SVC is to change the size of the SVC equivalent susceptance connected to the system by controlling the thyristor trigger angle, so that the SVC can achieve the purpose of regulating reactive power.

The first type is the static VAR compensator (saturated reactor - SR); the second type is the thyristor controlled reactor (thyristor control reactor - TCR) and the thyristor switch capacitor (thyristor switch capacitor - TSC). These two types of devices are collectively called SVC (static VAR compensator).

2.1 Compensator with saturable reactor

(sr)

Saturated reactors are divided into two types: self-saturated reactors and controllable saturated reactors, and the corresponding reactive power compensation devices are also divided into two types. The reactive power compensation device with a self-saturated reactor relies on the inherent ability of the reactor itself to stabilize the voltage. It uses the saturation characteristics of the iron core to control the size of the reactive power emitted or absorbed. The controllable saturated reactor controls the saturation degree of the iron core by changing the working current in the control winding, thereby changing the inductive reactance of the working winding and further controlling the size of the reactive current. The disadvantages of SR are: high cost, high loss, vibration and noise, long adjustment time, and slow dynamic compensation speed. Due to these disadvantages, static VAR compensators with saturated reactors are relatively rare.

2.2 Thyristor switched capacitor (TSC)

The principle of single-phase TSC is to use two anti-parallel thyristors to connect or disconnect the capacitor to the power grid, while the small series inductor can suppress the impact current that may be generated when the capacitor is put into the power grid. The key technology of TSC is the selection of the time of switching the capacitor. After years of analysis and experimental research, the best switching time is the moment when the voltage across the thyristor is zero, that is, the moment when the voltage across the capacitor is equal to the power supply voltage. At this time, the capacitor is switched and the impact current of the circuit is zero. In order to ensure better switching of the capacitor, this compensation device must pre-charge the capacitor and put the capacitor into operation after the charging is completed. The advantage of TSC is that it can compensate for the three-phase unbalanced load in phases and does not generate harmful overvoltage during operation. However, it is not enough to adjust the voltage flicker caused by the sudden change of the load by the change of the capacitance of the capacitor put into the power grid. Therefore, the TSC device is generally connected in parallel with the inductor, and the TCR and TSC are used in combination to form a hybrid compensator. This compensator uses capacitors for graded coarse adjustment and inductors for phase control fine adjustment. However, when the SVC device dynamically adjusts reactive power, it will inevitably generate a large number of harmonics, and it is necessary to connect fixed capacitors and inductors in series to form a harmonic filter to filter out the harmonics. Moreover, when the SVC is running, part of the capacity of the capacitor and the inductor offsets each other, which is not economical, and the discontinuous switching of capacitor groups will affect the regulation quality.

2.3 Thyristor Controlled Reactor (TCR)

The principle of single-phase TCR is shown in Figure 1, which consists of two anti-parallel thyristors and a reactor in series. The three phases are mostly connected in a triangle. Such a circuit is equivalent to an AC voltage regulation circuit connected to an inductive load when it is connected to the power grid. The effective phase shift range of this circuit is 90° to 180°. According to the relationship between the conduction angle of the thyristor and the equivalent susceptance of the TCR, when the trigger angle is 90

When the trigger angle is between 90° and 180°, the thyristors are fully turned on, and the reactors in series with the thyristors are all connected to the power grid. At this time, the reactive current absorbed by the reactor is the largest. When the trigger angle is between 90° and 180°, part of the thyristor is turned on. Increasing the trigger angle can reduce the equivalent susceptance of the compensator, which will reduce the fundamental component in the compensation current, that is, reduce the absorbed reactive power. Therefore, by adjusting the trigger angle, the reactive component absorbed by the compensator can be changed to achieve the effect of adjusting the reactive power.

3 Basic control principle of SVC

The svc control system consists of four parts.

The first part is "calculation of TCR fundamental current (or reactance) reference value", that is, according to the reactive current (or power) demand of the device, the TCR fundamental current (or power, or reactance) reference value is calculated; if the reference input of the device is the reactive current demand, and the effective value of the current of the FC branch is measured in real time, then the reference value of the TCR branch current is the former minus the latter.

The second part is "trigger delay angle calculation", that is, the trigger delay angle of the thyristor is obtained according to the reference value of the reactive current or reactance of the TCR. There are several ways to achieve this:

(1) Analog circuit method: An analog function generator is constructed through analog circuits to transform the input signal (such as the current reference value of the TCR branch) into an output signal that is proportional to the trigger delay angle.

(2) Digital lookup table method: the functional relationship between the input reference value and the trigger angle is stored in a digital table. The "trigger delay angle generation" module obtains the corresponding trigger delay angle based on the input lookup table at regular intervals.

(3) Microprocessor based method: a single-chip microcomputer or computer is used to form a signal processing system, which calculates the trigger delay angle in real time based on the reference input.

The third part is "synchronous timing", which provides a reference signal for synchronization to the pulse control. It has the same frequency as the input AC voltage and a fixed phase relationship. The controller generates a thyristor trigger pulse based on the reference signal.

The fourth part is the thyristor "trigger pulse generation", that is, according to the trigger delay angle generated by the "trigger delay angle calculation" module, a thyristor gate trigger pulse is formed to turn on the thyristor at the appropriate time to make the TCR branch work.

4 Components of svc

The primary part of the high-voltage static VAR compensation device is mainly composed of capacitors, reactors, thyristors, circuit breakers, contactors, etc. The secondary part is mainly composed of data acquisition board (das), data processing board (dsp), power module, drive module, etc. The main function of the thyristor is to control the output current of the reactor.

After the device is started, the voltage, current and contactor position of the system are collected first to calculate the power factor of the system. A control signal is given according to the actual power factor, and the drive module drives the thyristor to keep the corresponding conduction angle, thereby controlling the reactive output of the entire device and making the power factor of the system meet the requirements.

5 SVC main circuit wiring scheme

Figure 2 is the primary wiring diagram of the SVC device. The SVC device is connected to a single busbar, in which the 3rd and 5th filters are two groups of branches with equal capacity, which are connected to the 10kV busbar together with the TCR branch.

The six-pulse TCR consists of three single-phase units connected in a triangle, where each unit consists of a thyristor valve and two split reactors in series. The thyristor valve consists of multiple thyristor pairs in series to obtain a 10kv rated voltage and withstand overvoltage conditions in normal operation. The two groups of thyristors are alternately turned on in the positive and negative half cycles to achieve switching and control of the AC current.

6 Conclusion

The SVC device has been put into operation on site and has achieved the expected compensation target, providing a feasible solution and important technical means to solve the pollution caused by large-scale impact, three-phase asymmetry, low power factor and nonlinear loads to the power grid.