T & D - 18) Using High-Speed Grounding Switches

Using High-Speed Grounding Switches

Automatic high-speed grounding switches are applied for protection of power transformers when the cost of supplying other protective equipment is deemed unjustifiable and the amount of system disturbance that the high-speed grounding switch creates is judged acceptable.

The switches are generally actuated by discharging a spring mechanism to provide the ‘‘high-speed’’ operation.

The grounding switch operates to provide a deliberate ground fault on one phase of the high-voltage bus supplying the power transformer, disrupting the normally balanced 120° phase shifted three-phase system by effectively removing one phase and causing the other two phases to become 180º phase shifted relative to each other.

This system imbalance is remotely detected by protective relaying equipment that operates thetransmission line breakers at the remote end of the line supplying the power transformer,tripping the circuit open to clear the fault.

This scheme also imposes a voltage interruption to all other loads connected between the remote circuit breakers and the power transformer as well as a transient spike to the protected power transformer, effectively shortening the transformer’s useful life.

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Frequently, a system utilizing a high-speed ground switch also includes the use of a motor operated disconnect switch and a relay system to sense bus voltage.

The relay system’s logic allows operation of the motor operated disconnect switch when there is no voltage on the transmission line to provide automatic isolation of the faulted power transformer and to allow reclosing operations of the remote breakers to restore service to the transmission line and to all other loads fed by this line.

The grounding switch scheme is dependent on the ability of the source transmission line relay protection scheme to recognize and clear the fault by opening the remote circuit breaker.

Clearing times are necessarily longer since the fault levels are not normally within the levels appropriate for an instantaneous trip response.

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The lengthening of the trip time also imposes additional stress on the equipment being protected and should be considered when selecting this method for power transformer protection.

High-speed grounding switches are usually considered when relative fault levels are low so that the risk of significant damage to the power transformer due to the extended trip times is mitigated.