Main transformer gap protection and system zero sequence protection mismatch problem

A power supply line exists a phenomenon occurs when the single-phase ground fault transformer neutral point gap protection while the power line protection operation, causing trouble to stop power transformer accident. The main reason is that the zero- sequence over-voltage of the single-phase ground fault of the system causes the neutral point gap of the transformer to be broken. The zero-sequence current flowing through the neutral point of the transformer is greater than the action value of the zero-sequence current protection of the gap, thus making the transformer gap protection. each side of the transformer action trip the circuit breaker. Therefore, a reasonable choice of the operating mode of the transformer neutral point, reducing the transformer neutral point is subjected zero-sequence overvoltage protection is the key zero sequence mismatches gap transformer protection system solution.

Failure phenomenon:

When the upper power line transformer ground fault occurs, the power line zero sequence â…¡ 0.5 s operation section protection circuit breaker to trip the power supply, while the main transformer neutral point gap breakdown, the gap current protection action, but also through the primary hop 0.5 s each side of the transformer circuit breaker; the transformer protection and zero gap loss protection system with the main transformer gap protection and zero-sequence while protecting operation, leading to loss of line with the main transformer when the same power, then even if the line is a transient fault, coincidence The gate action restores the power supply to the line instantaneously, but the transformer trips due to the gap protection action to cause power failure, and will not resume power transmission in time due to the line being charged .

Cause Analysis:

Three-phase single-phase ground fault causes asymmetric operation, since no transformer neutral point voltage offset ground running, would have a overvoltage. If the single-phase ground fault in the power supply bus line 110 kV substation terminal or terminals, 110 kV transformer neutral point suffered the highest zero-sequence voltage, zero-sequence reactance equivalent greatest; in this case the transformer neutral point gap fired Wear, cause the power line ground fault to trip, and the gap zero current protection action of the transformer.

solution:

Solving the gap 110 kV transformer protection and protection system zero sequence mismatch problem, the need to increase the ground point 110 kV transformer system local area.

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