Transformer and switchgear grounding.

[AmazingTrans]

I have a 300kva autotransformer that step down from 480 to 400vac that is wye-wye. Currently, our switchgear are set to a 200amps gfc pickup for 1 1sec. Whenever the transformer center tap neutral is grounded, the switchgear tripped during power up.

What would the reasons be? Would the current leakage be high during the transformer power up that it exceeds the 200amps?

AT

 

[kevin]

I have a 300kva autotransformer that step down from 480 to 400vac that is wye-wye. Currently, our switchgear are set to a 200amps gfc pickup for 1 1sec. Whenever the transformer center tap neutral is grounded, the switchgear tripped during power up.

What would the reasons be? Would the current leakage be high during the transformer power up that it exceeds the 200amps?

AT

Have you run this question past the autotransformer manufacturer?

 

[AmazingTrans]

Have you run this question past the autotransformer manufacturer?

Yeap, they are not sure why it would trip. I guess I would just want the theoretical part as to what possibilities there would be.

The current solution is to not ground the neutral as long as we have balance load. If unbalanced load is required, a neutral would be needed.

 

[petersonra]

I have a 300kva autotransformer that step down from 480 to 400vac that is wye-wye. Currently, our switchgear are set to a 200amps gfc pickup for 1 1sec. Whenever the transformer center tap neutral is grounded, the switchgear tripped during power up.

What would the reasons be? Would the current leakage be high during the transformer power up that it exceeds the 200amps?

AT

Do you know that it is the GFc that is tripping and not the IT side of the breaker?

Might be the breaker just can't take the inrush from the xfmr.

It seems unlikely that it is an autotransformer setup as the neutral relative to the secondary would not be at zero volts.

In any case if it really is an autotransformer you would not be allowed to ground it.

 

[big john]

Can you post a nameplate or manufacturer's specs on this transformer?

If it truly is an autotransformer then it is not a separately derived system. If everything were de-energized you would actually be able to measure continuity from the autotransformer wye all the way back to your main bonding jumper.

This means by grounding the wye you are actually double-bonding your system and causing neutral current to flow over the equipment ground. This will trip GFP.

 

[AmazingTrans]

Do you know that it is the GFc that is tripping and not the IT side of the breaker?

Might be the breaker just can't take the inrush from the xfmr.

It seems unlikely that it is an autotransformer setup as the neutral relative to the secondary would not be at zero volts.

In any case if it really is an autotransformer you would not be allowed to ground it.

Petersonra, yes the gfc is tripping because by not bonding the center tap, everything works.

 

[Bugman1400]

A lot of the inrush current depends on the available fault current at the POCO connection and the impedance of the xfmr. If its high then, 200A of ground current for inrush for that size xfmr longer than 1 sec is not uncommon.

 

[GoldDigger]

A lot of the inrush current depends on the available fault current at the POCO connection and the impedance of the xfmr. If its high then, 200A of ground current for inrush for that size xfmr longer than 1 sec is not uncommon.

And just why would you want to both ground the wye center point and connect it to the neutral in the first place?

The wye point should be connected to the grounded conductor (neutral) and the case should be connected to the grounding conductor (EGC). This is not a separately derived system and adding a neutral to EGC bond at the equipment is just wrong.