Abstract: This paper provides a detailed overview of the current development status of static var compensation (SVC) technology in power systems. It analyzes the principles, advantages, disadvantages, and current applications of various SVC technologies in power systems, and proposes future development trends for SVC technology. Keywords: Static Var Compensation (SVC ASVG), Development Trends, Power System 1 Introduction The reactive power balance at each node of a power system determines the voltage level of that node. Currently, many users of power systems have equipment with frequently changing reactive power, such as rolling mills, electric arc furnaces, and electrified railways. Simultaneously, many users also have a large number of precision devices with high requirements for system voltage stability, such as computers and medical equipment. Therefore, there is an urgent need for reactive power compensation in the system. Traditional reactive power compensation devices include parallel capacitors, synchronous condensers, and synchronous generators. However, parallel capacitors have fixed impedance and cannot dynamically track changes in load reactive power; while synchronous condensers and synchronous generators are rotating equipment with high losses and noise, and are not suitable for large or small reactive power compensation. Therefore, these devices are increasingly unsuitable for the needs of power system development. Since the 1970s, with further research, a static var compensation (SVC) technology has emerged. This technology has undergone continuous innovation and improvement over more than 20 years. Static var compensation refers to the use of different static switches to switch capacitors or reactors, enabling them to absorb and generate reactive current. This is used to improve the power factor of the power system, stabilize system voltage, and suppress system oscillations. Currently, these static switches are mainly divided into two types: circuit breakers and power electronic switches. Because circuit breakers are used as contactors, their switching speed is relatively slow, approximately 10–30 seconds, making it impossible to quickly track changes in load reactive power. Furthermore, switching capacitors often causes severe inrush currents and operational overvoltages, which can easily lead to contact point welding and internal breakdown of the compensation capacitors, resulting in high stress and significant maintenance requirements. With the development of power electronics technology and its application in power systems, the emergence of AC contactless switches such as SCR, GTR, and GTO has increased the switching speed by 500 times (approximately 10μs). Reactive power compensation for any system parameter can be completed within one cycle, and single-phase adjustment is possible. Currently, the term "static var compensator" generally refers specifically to reactive power compensation equipment using thyristors, mainly of three types: First, saturated reactors (SR); second, thyristor-controlled reactors (TCR) and thyristor-switched capacitors (TSC), both collectively referred to as SVCs (Static Var Compensators); and third, advanced static var generators (ASVG) employing self-commutation converter technology. The following section introduces these three types of static var compensation technologies one by one, focusing on SVC and ASVG, and outlining future development trends. 2. Static Var Compensation Devices with Saturated Reactors (SR) Saturated reactors are divided into self-saturating reactors and controllable saturating reactors, and correspondingly, reactive power compensation devices are also divided into two types. Reactive power compensation devices with self-saturating reactors rely on the inherent capability of the reactor itself to stabilize voltage. They utilize the saturation characteristics of the iron core to control the amount of reactive power generated or absorbed. Controllable saturating reactors control the degree of saturation of the iron core by changing the operating current in the control winding, thereby changing the inductive reactance of the working winding and further controlling the magnitude of the reactive current. Static var compensation devices composed of these types of devices belong to the first batch of static compensators. As early as 1967, this device was manufactured in the UK. Later, General Electric (GE) in the United States also manufactured such a static var compensator [1]. However, due to the high cost of the saturated reactor in this device, which is about 4 times that of a general reactor, and the fact that the silicon steel sheets of the reactor are in a saturated state for a long time, the core loss is large, which is 2 to 3 times greater than that of the parallel reactor. In addition, this device also has vibration and noise, and the adjustment time is long and the dynamic compensation speed is slow. Due to these disadvantages, the static var compensator of the saturated reactor is rarely used at present, and is generally only used in ultra-high voltage transmission lines. 3 Thyristor-controlled reactor (TCR) Two anti-parallel thyristors are connected in series with a reactor. Its single-phase schematic diagram is shown in Figure 1. Its three phases are mostly connected in a delta configuration. When such a circuit is connected to the power grid, it is equivalent to an AC voltage regulator circuit connected to an inductive load. The effective phase shift range of this circuit is 90° to 180°. When the firing angle α = 90°, the thyristor is fully conducting, and the conduction angle δ = 180°. At this point, the reactive current absorbed by the reactor is at its maximum. According to the relationship between the firing angle and the equivalent admittance of the compensator: BL = BLmax(δ - sinδ)/π and BLmax = 1/XL, increasing the firing angle increases the equivalent admittance of the compensator, thus reducing the fundamental component in the compensation current. Therefore, by adjusting the firing angle, the reactive component absorbed by the compensator can be changed, achieving the effect of adjusting reactive power. In practical engineering, the step-down transformer can be designed as a reactive transformer with a large leakage reactance, controlled by a thyristor. This eliminates the need for a separate transformer and circuit breakers. The primary winding of the reactive transformer is directly connected to the high-voltage line, while the secondary winding is connected to the thyristor valve via a smaller reactor. If an appropriate device circuit is selected for the third winding of the reactive transformer, such as adding a filter, the harmonics generated by reactive power compensation can be further reduced. The Swiss company Brønsted has manufactured such compensators for reactive power compensation in high-voltage transmission systems [2]. Since a standalone TCR can only absorb reactive power but cannot generate reactive power, in order to solve this problem, parallel capacitors can be used in conjunction with TCRs to form reactive power compensators. Depending on the components that switch capacitors, they can be divided into static var compensators (TCR+FC) used in conjunction with TCRs and fixed capacitors and static var compensators (TCR+MSC) used in conjunction with circuit breaker-switched capacitors. This type of compensator with TCR has a fast response speed and high flexibility, and is currently the most widely used in transmission systems and industrial enterprises. The static var compensator used in the Jiangmen substation in my country is the TCR+FC+MSC type SVC produced by Dumex BBC, with a control range of ±120Mvar [3]. Because the TCR+FC type compensation device with fixed capacitors requires the reactor capacity to be greater than the capacitor capacity when the compensation range extends from the inductive range to the capacitive range, and because when the compensator operates by absorbing a small reactive current, both its reactor and capacitor have already absorbed a large reactive current, only canceling each other out. The TSC+MSC type compensator overcomes this shortcoming to some extent by using group switching of capacitors, but frequent switching of circuit breakers should be avoided as much as possible to reduce the operating conditions of the circuit breakers. 4. Thyristor Switched Capacitor (TSC) To solve the problem of frequent switching of capacitor banks, the TSC device was developed. Its single-phase schematic diagram is shown in Figure 2. Two anti-parallel thyristors simply connect or disconnect the capacitor from the grid, and the small reactor in series is used to suppress the inrush current that may be generated when the capacitor is connected to the grid. TSC can be used in a three-phase grid in either a delta or star connection. Generally, symmetrical networks use a star connection, and unbalanced load networks use a delta connection. Regardless of whether it is a star or delta connection, group switching of capacitors is used. To achieve stepless regulation of reactive current, it is always desirable to have as many capacitor banks as possible. However, considering the complexity and economy of the system, a 2K-level capacitor bank is generally formed by K-1 capacitors with a capacitance of C and C/2 capacitors [4]. The key technical issue of TSC is the selection of the capacitor switching time. After years of analysis and experimental research, the optimal switching time is when the voltage across the thyristor is zero, that is, when the voltage across the capacitor is equal to the power supply voltage [5]. At this time, the inrush current of the circuit is zero when the capacitor is switched. In order to ensure better capacitor switching, this compensation device must precharge the capacitor and then put it into operation after the charging is completed. TSC compensators can effectively compensate for the reactive power required by the system. If the number of banks is sufficiently refined, stepless regulation can be basically achieved. Two 100t electric arc furnaces in a Swedish steel plant, after being equipped with 60Mvar TSCs, effectively kept the voltage of the 130kV power grid within a fluctuation range of 1.5%. Operational practice has proven that this device has a fast response speed (about 5-10ms), small size, light weight, and can compensate for three-phase unbalanced loads phase by phase. The operation process does not generate harmful overvoltage or overcurrent. However, for TSC to suppress voltage flicker caused by impact load, it is not enough to rely solely on the change in the capacitance of the capacitor connected to the grid. Therefore, TSC devices are generally connected in parallel with inductors. The typical device is the TSC+TCR compensator. This compensator adopts a delta connection, with capacitors for graded coarse adjustment and inductors for phase-controlled fine adjustment. The third harmonic cannot flow into the grid, and a fifth harmonic filter is also provided to greatly reduce harmonics. The reactive power compensation equipment imported by the Pingdingshan to Wuhan Fenghuangshan 500kV substation in China is the TSC+TCR type [6]. 5 New Static Var Generator (ASVG) With the further development of power electronics technology, especially since L. Gyugyi proposed the theory of using converters for reactive power compensation, static compensators that use converter technology for dynamic reactive power compensation have gradually emerged. It is achieved by directly connecting a self-commutated bridge circuit to the power grid or by connecting it to the power grid through a reactor. Based on the different energy storage elements used on the DC side—capacitors and inductors—ASVG can be divided into voltage-type and current-type types, as shown in Figure 3. The schematic diagram shown in Figure 3 is a voltage-type compensator. If the capacitor on the DC side is replaced with a reactor, and the series inductor on the AC side is replaced with a parallel capacitor, then it is a current-type ASVG. The inductor L and capacitor C connected on the AC side respectively prevent high-order harmonics from entering the power grid and absorb overvoltages generated during commutation. Whether voltage-type or current-type, the dynamic compensation mechanism of ASVG is the same. When the inverter pulse width is constant, adjusting the angle δ between the inverter output voltage and the system voltage can adjust the reactive power and the inverter DC-side capacitor voltage UC. Simultaneously adjusting the angle δ and the inverter pulse width can maintain a constant UC while generating or absorbing the required reactive power [7]. Based on this principle, since Japan developed the first 20Mvar forced self-commutation bridge ASVG in 1980, the capacity of ASVG has been continuously increasing over more than 10 years. In 1991 and 1994, Japan and the United States successively developed 80Mvar and 100Mvar ASVGs. In 1995, Tsinghua University and Henan Provincial Power Bureau jointly developed my country's first ASVG with a capacity of 300kvar, pioneering the development of ASVG compensation equipment in my country [8]. ASVG uses bridge circuit multiplexing technology, multi-level technology or PWM technology to eliminate lower-order harmonics and limit higher harmonics to a certain range. Since ASVG does not require energy storage elements to achieve the purpose of exchanging reactive power with the system, it actually uses DC capacitors to maintain a stable DC power supply voltage. Compared with the AC capacitors used by SVC, the DC capacitance is relatively small and the cost is lower. In addition, it can still output rated reactive current when the system voltage is very low, while the reactive current compensated by SVC decreases as the system voltage decreases. Because of these advantages, ASVG has unparalleled advantages over SVC in improving system voltage quality and stability, which also shows that ASVG is the future direction of static var compensation technology. In addition, with the development of power electronics technology, electronic active filters are also becoming more and more perfect. Since the active power filter does not resonate with the power system when filtering harmonics, many power system workers are currently dedicated to the research of combining active power filtering with ASVG to eliminate the resonance problem caused by the parallel passive filter in the traditional ASVG equipment. References: [1] A. C. MATHEB. Static Var Compensator for Ultra-High Voltage Transmission Lines [C]. Hubei: Hubei Electric Power Technology, 1982 [2] W. Herbst. Controllable Static Var Compensation for High Voltage Systems [C]. Hubei: Hubei Electric Power Technology, 1982 [3] Tian Guangqing. Introduction to Static Var Compensator in Jiangmen Substation [J]. Guangdong Electric Power, 1988, (4) [4] Miller. Reactive Power Control in Power Systems [J]. [5] Wang Qinglin. Discussion on the design of fast automatic reactive power compensation device [J]. Electric Capacitor, 1993(2) [6] Liang Zhiyong. Overview of the operation of static var compensation equipment [J]. Electric Capacitor, 1997(2) [7] Liu Wenhua. A new type of static var generator using GTO [J]. Automation of Electric Power Systems, 1997(3) [8] Jiang Qirong, et al. A new type of ±120kvar static var generator using GTO [J]. Journal of Tsinghua University, Natural Science Edition, 1997(7)