Static Var Compensator (SVC), also known as the "static phase shifter", is a core component of modern Flexible AC Transmission Systems (FACTS). It is a parallel reactive power compensation FACTS device based on converter technology. A typical static reactive power compensation device uses fixed capacitors and thyristors to control the reactance (FC and TCR), enabling it to absorb and emit reactive current, improve system power factor, and stabilize voltage source voltage. Its important feature is to change the reactive power required by the compensation device by controlling the triggering delay angle of TCR. TSC can only be grouped to switch capacitors, and when used in conjunction with TCR, it can continuously adjust the reactive power of the compensation device. The static reactive power compensation device with fixed capacitor and thyristor controlled reactor can continuously adjust the reactive power of the compensation device and has a fast response speed. Therefore, it is possible to quickly and dynamically compensate for reactive power.
1. The basic structure of SVC


There are many structures of SVC, but the basic components are thyristor controlled reactors and thyristor switched capacitors. The TCR branch consists of a reactor and two reverse parallel thyristors connected in series, while the TSC branch consists of a capacitor and two reverse parallel thyristors connected in series, all of which are controlled by thyristors. The equivalent fundamental reactance of the TCR branch is a function of the conduction angle β or trigger angle α of the thyristor. By adjusting β or α, the equivalent reactance connected in parallel in the system can be smoothly adjusted.
2. TCR type SVC composition
It consists of four main components: high impedance transformer (or step-down transformer), capacitor bank (also serving as a filter), thyristor valve, and regulator.
advantage:
(1) Continuous inductive and capacitive reactive power regulation can be performed. The standalone TCR can only absorb inductive reactive power, and when used in conjunction with parallel capacitors, the total reactive power is the net reactive power obtained by offsetting the reactive power of the TCR and parallel capacitors. Therefore, the overall reactive current of the compensator can be biased within the range that can absorb capacitive reactive power.
(2) The secondary winding of the step-down transformer can be connected in an "open star" shape with the midpoint separated. This is to make each phase load independent of the other two phases, so that the amplitude of the positive and negative sequences can be controlled separately and adjusted in phase, which can balance unbalanced loads.
(3) The parallel reactor has good harmonic absorption ability, and the small tuned reactor in series can also serve as a filter, which can effectively absorb the harmonic current generated by TCR.
(4) Less noise
(5) Relatively small loss
(6) Good control flexibility
There are multiple compensation schemes for TCR type SVC, one of which is to fix the total capacity of the capacitor bank to the power grid, and divide the total capacity of TCR into n small units on average. The number of small compensation units and their control angles are determined based on the balance requirements of the system's reactive power. In this way, TCR type SVC can be roughly adjusted by controlling the number of input groups and finely adjusted by controlling the control angle, achieving smooth and stepless compensation. Therefore, the entire control process is very flexible and the effect is also quite good.
(7) The dynamic response time is fast (about 10mS), making it capable of handling dynamic reactive power compensation for multiple types of loads.


Disadvantages:
(1) Has harmonic content generated by itself
The harmonics generated by TCR type SVC are mainly odd harmonics, with 3K harmonics in phase, which can only flow in the triangular winding of the compensation device and cannot enter the system. The harmonic components flowing into the system are only 6k ± 1. The harmonic current generated will cause distortion of the system voltage, resulting in a series of hazards to the system and equipment. Therefore, TCR and capacitors used to improve power factor are used as filters. However, this type of filter has a large volume, occupies a large area, and is expensive.
(2) Cannot be directly connected to ultra-high voltage
(3) Complex operation and maintenance