Series compensation is a technology that connects reactive power compensation devices in series to the line for reactive power compensation. Its main application areas are rural power distribution, high-voltage long-distance transmission, and electrified railway power supply. Here we introduce the classification of series compensation devices in series resonance.
Using series compensation devices on transmission lines improves the stable transmission capacity of the system, improves line electrical parameters, and enables two lines to transmit the power of three lines, thereby increasing transmission power and saving investment. There are usually two types of capacitors used for series compensation: externally fused capacitors and internally fused capacitors.
1. Externally Fused Capacitors
Externally fused capacitors have the fuse device installed outside the capacitor unit. According to IEC standards, the fusing current of the external fuse should be at least 1.43 times the rated current of the protected capacitor, typically 1.5 times. For variable frequency series resonance withstand voltage test devices, capacitors used for series compensation also need to consider that the fuse should not blow during a short-circuit discharge across the capacitor bank; otherwise, when a system fault occurs and the series compensation capacitor bank is taken out of service, the bypass gap or shunt switch bypassing the capacitor bank may cause the external fuses of the capacitor bank to operate.
2. Internally Fused Capacitors
Internally fused capacitors have the fuse installed inside the capacitor. Each phase of the capacitor bank consists of 320 capacitor units. For variable frequency series resonance withstand voltage test devices, this capacitor is an oil-immersed all-film capacitor with an actual designed electric field strength of 170V/um. The protection level of the capacitor bank is 2.3pu, and the protection voltage is 230. Since the recovery voltage after the fuse of a capacitor unit is blown is relatively high, the manufacturing of the fuse is relatively difficult. When a capacitor element fails, only the fuse of that capacitor element blows, isolating the faulty element. After the faulty element is isolated, the capacitor unit can still operate normally. For variable frequency series resonance withstand voltage test devices, the capacitor capacity is relatively small. According to the design example of the capacitor bank, a single capacitor unit only loses 1/52 of its capacity.
