7/27/2019 Electrical Engineering Portal.com Using HighSpeed Grounding Switches

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Using High-Speed Grounding Switches

http://electrical-engineering-portal.com/using-high-speed-grounding-switches February 25, 2013

Using High-Speed Grounding Switches

Edvard

Automatic high-speed grounding switches are applied for protection of power transformerswhen the cost of supplying other prot ective equipment is deemed unjustifiable and the amountof 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 phaseshifted three-phase system by eff ectively removing one phase and causing the ot her twophases to become 180 phase shif ted relative to each other.

This system imbalance is remot ely detected by protective relaying equipment that operatesthe transmission line breakers at the remote end of the line supplying the power t ransformer,tripping the circuit open to clear the fault. This scheme also imposes a voltage interruption toall other loads connected between the remot e circuit breakers and the power transformer aswell as a transient spike to the protected power t ransformer, effectively shortening the

transformers useful life.

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

The rela s stems lo ic allows o eration of the motor operated disconnect switch when

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7/27/2019 Electrical Engineering Portal.com Using HighSpeed Grounding Switches

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there is no voltage on the transmission line to provide automatic isolation of the faulted powertransformer and to allow reclosing operations of the remote breakers to restore service to thetransmission line and to all other loads f ed by this line.

The grounding switch scheme is dependent on the ability of the source transmission linerelay protection scheme to recognize and clear the fault by opening the remote circuit breaker.Clearing t imes are necessarily longer since the fault levels are not normally within the levelsappropriate for an instantaneous trip response.

The lengthening of the trip time also imposes additional stress on t he equipment beingprotected and should be considered when selecting this method for powertransformer protection.

High-speed grounding switches are usually considered when relative fault levels are low so thatthe risk of signif icant damage to the power transformer due to the extended trip t imes ismitigated.

Resource: Electric Power Substations Engineering J. D. McDonald (Get it from Amazon)

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