13-09-2012, 01:04 PM
ANALYSIS OF DIFFERENT FAULTS AND PROTECTION IN HVDC LINES
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High Voltage Direct Current (HVDC) transmission system is widely recognized as having technical, economical and environmental benefits since its first commercial project introduced in Sweden in 1950s. HVDC transmission system has technical advantages in producing an asynchronous interconnection and long-distance bulk power delivery. Increasing numbers of HVDC projects over the world shows that HVDC technology plays a more and more important role in the world's power transmission system.
An HVDC transmission behaves differently compared to an AC transmission if there is a ground fault or short circuit. The different types of faults which occur in HVDC lines are
DC overhead line faults:
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, 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 %).
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 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.
DC cable faults :
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.
AC network faults:
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.