[b]1. Introduction[/b] Series capacitor compensation technology can improve the transmission capacity and system stability of ultra-high voltage long-distance transmission lines, and has a certain regulating effect on the power flow distribution on the transmission channel. Controllable series compensation can also suppress low-frequency power oscillations and optimize the power flow distribution of the system; however, the series capacitor compensation equipment added to the system changes the original electrical distance between systems, especially when the series compensation degree is high, which may cause a series of system problems. Therefore, this possibility should be carefully studied in the early research stage of series compensation project, and corresponding solutions and measures should be proposed to solve the problem. my country's Southern Power Grid is an inter-provincial (regional) power grid with Guizhou, Yunnan and Tianshengqiao power grids as the sending end, and three 500kV AC transmission lines and one 500kV DC transmission line from Tianshengqiao to Guangdong connected to the receiving end Guangdong power grid. In June 2003, the Guizhou-Guangdong double 500kV AC transmission line was completed and put into operation, and the Southern Power Grid formed three major west-to-east power transmission channels in the north, central and south with "five AC and one DC" at the sending end and "four AC and one DC" at the receiving end. With the further expansion of the western-to-east power transmission scale of the Southern Power Grid, in order to improve the transmission capacity of these transmission channels and the safety and stability level of the entire network, as well as to suppress low-frequency oscillations in the system, it was decided to install Controllable Series Compensation (TCSC) and Fixed Series Compensation (FSC) devices at the Pingguo and Hechi substations, respectively. Through systematic research on the Pingguo Controllable Series Compensation Project and the Hechi Fixed Series Compensation Project of the Southern Power Grid, the authors gained a relatively comprehensive understanding of the system problems that may be caused by the use of series capacitor compensation technology in ultra-high voltage long-distance transmission systems, and summarized the measures and solutions to these problems. The research results show that the system problems caused by the addition of series compensation to ultra-high voltage transmission lines mainly include overvoltage, inrush current, transient recovery voltage (TRV) of circuit breakers, and subsynchronous resonance (SSR). [b]2. Structure and Principle of Series Compensation Device[/b] Currently, the series capacitor compensation devices used in power systems can be divided into four types according to their overvoltage protection methods: single-gap protection, double-gap protection, metal oxide voltage limiter (MOV) protection, and MOV protection with parallel gap. The series compensation device with parallel gap MOV protection has advantages such as fast reconnection time, reduced MOV capacity and backup protection. It is more conducive to improving the transient stability level of the system. Therefore, it has been widely used in the series compensation project of the power system. (1) MOV is the main protection of series compensation capacitor. When a large fault current occurs on the line where the series compensation is located, a high overvoltage will appear on the series compensation capacitor. MOV can use its own strong nonlinear voltage-current characteristics to limit the capacitor voltage below the design value, thereby ensuring the safe operation of the capacitor. (2) Spark gap is the backup protection of MOV and series compensation capacitor. When the current shared by MOV exceeds its starting current setting value or the energy absorbed by MOV exceeds its starting energy consumption, the control system will trigger the gap and bypass MOV and series compensation capacitor. (3) Bypass circuit breaker is a necessary device for system maintenance and dispatch. When the series compensation station control system triggers the spark gap, it commands the bypass circuit breaker to close, providing the necessary conditions for the gap to extinguish arc and deionize. (4) Damping devices can limit the discharge current of capacitors and prevent damage to series compensation capacitors, gaps, and bypass circuit breakers during discharge. 3. Overvoltage problems caused by series compensation devices Although series compensation devices can improve the transmission capacity of lines, they also affect the voltage characteristics of the system and the transmission lines along which series compensation devices are installed. If the reactive component of the line current is inductive, the current will generate a certain voltage drop on the line inductor and a certain voltage rise on the capacitor; if the reactive component of the line current is capacitive, the current will generate a certain voltage rise on the line inductor and a certain voltage drop on the capacitor. Under normal circumstances, capacitors can improve the voltage distribution characteristics of the system; however, when the series compensation degree is high and the line load is heavy, the voltage along the line may exceed the rated allowable value. When the relative positions of the line high reactance and the series compensation in the Hechi and Pingguo series compensation projects are different, the operating voltage at some locations of the transmission line may exceed the operating requirements. For example, if the high-voltage reactor is installed on the line side of the series compensation when the Huihe Line or Tianping Line is faulty, the voltage on the line side of the series compensation can reach 561kV or 560kV or more [2], which exceeds the long-term operating voltage allowed by the high-voltage reactor. Therefore, it is recommended to install the high-voltage reactor on the bus side of the series compensation in both projects to avoid the problem of the system operating voltage exceeding the standard. After the series capacitor compensation device is installed on the transmission line, when the circuit breaker of the line is not fully operational, the voltage of the energized phase will be coupled to the disconnected phase through the phase-to-phase capacitor. Parallel reactors have been installed on the Hui (Shui)-He (Chi) and Tian (Shengqiao)-Ping (Guo) lines in the Hechi FSC and Pingguo TCSC projects. If the parameters of the newly added capacitor reactance and the inductive reactance of the installed high-voltage parallel reactor are not properly matched, electrical resonance may be triggered, resulting in a high power frequency resonance overvoltage on the disconnected phase [3]. Therefore, in the system research work of these two projects, multiple schemes of series capacitor parameters were compared to avoid the generation of power frequency resonance overvoltage. The overvoltage study of these two series compensation projects shows that, since both sides of the Huihe Line and Tianping Line are connected to large systems, regardless of whether the Huihe Line or Tianping Line has series compensation, the power frequency overvoltage on the Hechi and Pingguo bus sides is basically the same when the line experiences a load shedding fault; only when a single-phase grounding load shedding fault occurs, the addition of series capacitor compensation increases the single-phase grounding coefficient, thereby slightly increasing the power frequency overvoltage on the line side, but none of them exceed the allowable value of the regulations, and will not affect the safe and stable operation of the power grid. [b]3. The influence of series compensation device on the inrush current[/b] When a single-phase grounding fault occurs on the line, after the circuit breakers of the faulty phases at both ends of the line trip one after another, due to the electrostatic coupling and electromagnetic coupling of the healthy phase, a certain induced current (i.e., inrush current) will still flow in the arc channel[4]. If this current is too large, it will be difficult to extinguish itself, thereby affecting the automatic reclosing of the circuit breaker. After installing a series capacitor compensation device on an ultra-high voltage transmission line, if the bypass circuit breaker and spark gap in the series compensation device do not operate during a single-phase ground fault, the residual charge on the capacitor may discharge through the circuit composed of the short circuit point and the high reactance, thereby superimposing a considerable transient component on the steady-state inrush current. The transient component decays slowly and may affect the self-extinguishing of the inrush current, which is not conducive to single-phase reclosing; after the single-phase instantaneous fault disappears, the residual voltage of the capacitor will also be superimposed on the recovery voltage, and the recovery voltage will increase, affecting the success of single-phase reclosing. According to the study on the series compensation project in Hechi: when a single-phase ground fault occurs on the Huishui side of the Huihe line, the waveform of the inrush current is a low-frequency (f≈7Hz) decaying discharge current with a current amplitude as high as 250-390A[5] (see Figure 2). 0.5s after the circuit breaker trips, the current amplitude can still reach 200-300A, which will make it difficult to extinguish the lingering arc. If the bypass switch operates to short-circuit the series capacitor after a single-phase grounding, there will be no low-frequency discharge transient component in the lingering current. [b]4. Subsynchronous resonance problem caused by series compensation device[/b] After the series capacitor compensation technology is adopted in the ultra-high voltage long-distance power transmission system, especially when large steam turbine generator sets are connected to the system through series compensation (especially when the compensation degree is high), under certain operating mode or compensation degree, resonance may occur between the mechanical and electrical systems. Its oscillation frequency is lower than the rated frequency of the power grid, which is called subsynchronous resonance. It can be transmitted through a single unit with a series capacitor compensation device to an infinitely large line transmission system.