HVDC Classic fault performance

When a fault (flash-over) occurs on a AC line, there are circuit breakers that disconnect the line. It is then normally automatically re-connected again.

There are no DC breakers in the HVDC converter stations, so when a fault occurs on a DC line another method must be applied. This is what happens:

The fault is detected by the DC line fault protection. This protection orders the rectifier into inverter mode and this discharges the line effectively. After some 80 - 100 ms, the line is charged again by the rectifier. If the fault was intermittent, due to e.g. a lightning strike, then normally the line can support the voltage and the power transmission continues. Full power is then resorted in about 200 ms after the fault. But if the fault was due to contaminated line insulators, there is a risk that re-charging of the line results in a second fault. Many HVDC transmissions are designed such that after a number of failed restart attempts, the following attempts are made with reduced voltage (80 %).

It should be pointed out that the DC line fault clearing does not involve any mechanical action and therefore is faster than for an AC line. The DC fault current is also lower ( the load current) than the AC fault current and therefore the dead time before the restart is shorter than for an AC line. The reduced voltage restart is also unique for HVDC.

It should also be pointed out that DC line faults on a bipolar line affect only one pole (if fallen line towers is disregarded). The bipolar DC line is equivalent to a double circuit AC line!

Cable faults are very rare. They are as a rule caused by mechanical damage. Therefore submarine DC cables are often buried to prevent damage from anchors and trawls. The same protection action occurs as for a DC line but without the restart attempt.
When a temporary fault occurs in the AC system connected to the rectifier, the HVDC transmission may suffer a power loss. Even in the case of close single-phase faults, the link may transmit up to 30 % of the pre-fault power. As soon as the fault is cleared, power is restored to the pre-fault value.

When a fault occurs in the AC system connected to the inverter, a commutation failure can occur interrupting power flow. A commutation failure is an unwanted, but natural process in a classic HVDC inverter that the system can handle. If the AC-fault is temporary the power is restored as soon as the fault is cleared.

A distant fault with little effect on the converter station voltage (< 10 percent) does not normally lead to a commutation failure. A CCC (Capacitor Commutated Converter) HVDC converter can tolerate about twice this voltage drop before there is a risk of commutation failure.
HVDC converter stations are provided with an elaborate protection system that is designed to detect fault conditions or other abnormal conditions that might expose equipment to hazard and/or cause unacceptable disturbances. The faulty equipment is taken out of service by the protection system.