Series compensation of long high-voltage and extra-high-voltage lines has become almost standard practice. The presence of series compensation affects the X0/X1 ratios of the system, with the reactance of the series capacitor appearing in all three sequence networks.

Therefore, temporary and transient overvoltages as a result of faults, as well as circuitbreaker recovery voltages and surge arrester operation, are different than those that would appear in the uncompensated system.

There have also been concerns about ferroresonant TOVs in series-compensation systems , but few if any cases of ferroresonance have been reported for operating transmission or subtransmission systems.

However, because of this concern, some utilities buying series capacitors have specified special subharmonic detection devices as part of the series capacitor bank. There are also concerns about subsynchronous resonance (SSR) of rotating machine mechanical systems with the series compensated electrical system.

Additional concerns have centered on fundamental-frequency resonance conditions during faults at critical locations in the transmission systems. But economical applications of series capacitors dictate that some means be supplied to limit the overvoltage appearing across the series capacitor during faults to voltages no higher than economical design levels.

Limiting this overvoltage virtually eliminates the possibility of high temporary fundamental resonant overvoltages.
The overvoltage protection for series capacitors applied to transmission systems has taken two forms. The earliest forms of overvoltage protection were spark-gap systems that limited voltage to the sparkover voltage of the gap setting, which was generally no more than 3.5 times the rated voltage across the series capacitor bank, but often less.

More recently, the protection has been achieved by metal-oxide varistors, somewhat similar to surge arresters but applied across the series capacitor and limiting the voltage to about two times the rated voltage across the series capacitor bank.

Both forms, when acting during a fault, can reduce temporary and transient overvoltages, the spark gap by electrically bypassing the capacitor during its arcing time, and the metaloxide varistors by limiting the overvoltage, inherently reducing the capacitive reactance, and inserting some value of equivalent resistance into the circuit until the fault is cleared.

The effect on temporary and transient overvoltages (and the possibility of SSR) as a result\ of using of series compensation with its overvoltage protection should be carefully studied.

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