Ground Fault Protection

[timm333]

I have a 2MVA 13.8kV/480V delta-wye transformer. The NEC230.95 asks for ground fault relay on the service disconnects up to 600V. Butservice is 13.8kV primary of transformer which is more than 600V, so I guessthe ground fault relay should be installed on the secondary of the transformerbecause secondary of transformer is acting as main service feeder here.

Will it be possible to use the same over current relay(like Schweitzer 551) on both the primary and secondary side of the transformer?In this case on primary, the 50/51 functions can be enabled and 50G/51G disabled.And on secondary the 50/51 functions can be disabled and 50G/51G functionsenabled. Should zero sequence CT be used for the GF relay, or the residual CTis ok?

Does NEC require both 50G and 51G functions forservices up to 600V, or only 51G is enough? When we use ground fault relay(50G/51G), should it be plotted on the TCC chart together with other over currentprotection devices? Thanks

 

[templdl]

I have a 2MVA 13.8kV/480V delta-wye transformer. The NEC230.95 asks for ground fault relay on the service disconnects up to 600V. Butservice is 13.8kV primary of transformer which is more than 600V, so I guessthe ground fault relay should be installed on the secondary of the transformerbecause secondary of transformer is acting as main service feeder here.

Will it be possible to use the same over current relay(like Schweitzer 551) on both the primary and secondary side of the transformer?In this case on primary, the 50/51 functions can be enabled and 50G/51G disabled.And on secondary the 50/51 functions can be disabled and 50G/51G functionsenabled. Should zero sequence CT be used for the GF relay, or the residual CTis ok?

Does NEC require both 50G and 51G functions forservices up to 600V, or only 51G is enough? When we use ground fault relay(50G/51G), should it be plotted on the TCC chart together with other over currentprotection devices? Thanks

I don't have my code book handy but does it specifically state "ground fault relay" or just ground fault?
And at what amperes at up to 600v or is it over 240v?

 

[timm333]

NEC only says ground fault. It says ground fault isrequired up to 1000A on service disconnect for voltage levels starting from240V phase to phase (or 150V phase to neutral) up to 600V (phase to phase.)

What other ground fault equipment (other than ground fault relays) is availablein market?

 

[rcwilson]

Best protection would be 50/51, 50GS on 13.8 kV primary, plus 50/51 phase and 50/51G ground on 480V secondary winding. The 50 GS is a zero sequence CT with all three 13.8 kV cables going through it. The 480V 50/51G should be an element fed by a CT on the transformer neutral to ground connection.

The delta-wye windings will block any 480V ground fault current from being sensed by the 13.8 kV relays. Note that 13.8 kV protection only works if you have a breaker that can be tripped. If 13.8 kV protection is a fused switch, the relay won't help.

If there are no 277 V loads (3-wire system) a neutral CT will work for ground fault. If it is a 3-phase, 4-wire system with 277 loads, the CT has to be on the N-G bonding jumper or it will trip on load current or have to be set so high it will not provide the needed ground fault protection.

A single relay with two sets of 4 CT inputs ( 3phase + neutral/ground) multiple 50/51 elements and two trip outputs would work.

 

[templdl]

NEC only says ground fault. It says ground fault isrequired up to 1000A on service disconnect for voltage levels starting from240V phase to phase (or 150V phase to neutral) up to 600V (phase to phase.)

What other ground fault equipment (other than ground fault relays) is availablein market?

Then a secondary insulated case or power breaker that has integral ground fault protection would work for you. SHuld you have an exising breaker you could opt for an external GFR but would have to provide a means to trip the breaker with a shunt trip type device.

 

[timm333]

The transformer primary current is only 42A (at 13.8kV.) But the HV brealker size mimimum available is 600A. It is true that the function of the HV breaker is only to take signal from the relay. But is 600A too big for a current of 42A, or does it not matter?

 

[timm333]

Is it normal to use large Medium voltage 600A circuit breakers for small currents like 40A? For example if 13.8kV utility fuse is 40A and the downstream transformer primary current is 30A, then perhaps a 40A circuit breaker should be sufficient for 13.8kV transformer primary. But as the smallest medium voltage breaker available is 600A, so is it normal to put a 600A breaker on a 40A circuit?

 

[templdl]

Is it normal to use large Medium voltage 600A circuit breakers for small currents like 40A? For example if 13.8kV utility fuse is 40A and the downstream transformer primary current is 30A, then perhaps a 40A circuit breaker should be sufficient for 13.8kV transformer primary. But as the smallest medium voltage breaker available is 600A, so is it normal to put a 600A breaker on a 40A circuit?

In my attempt in trying to keep this very simplistic with MV breakers there is no "small" that you can go with MV breakers which is unlike LV breakers. The rating of MV breakers is determined by a max. ampcity of the device and then the actual ampacity of the application determined by a set of CTs and a relay such as a 'CO' relay which then controls the breaker. Basically a MV breaker is a dumb device by itself.

 

[abhishekbt]

It is not normal, but in most of the cases it has been seen that people are using this.