Digital simulation of series-compensated EHV (extra high voltage) transmission systems

The development of flexible AC transmission systems (FACTS) technology offers better opportunities to utilize existing transmission systems. One of the key FACTS devices is controllable series capacitor compensation (CSC). The authors discuss the digital simulation of CSC protection schemes. A parallel protective device is required to provide overvoltage limitation during system faults. There are two approaches adopted by utilities: one involves the combination of a traditional gap and a mechanical bypass switch; the other involves the use of varistor technology, in which the varistor is connected directly in parallel with the capacitor and provides fault overvoltage protection. With the incorporation of the components of a 500 kV series-compensated system into the Electromagnetic Transients Program (EMTP), the authors present some simulation results and the analysis of the system waveform patterns under various conditions. The studies show that under fault conditions, the operation of the capacitor gap or varistor does not conform to any clearly defined pattern