Can two Grounding transformers put to service at a time?

[tsknan]

we have 132/33kv transformers with star-delta vector group running parallel,33kv side is delta and to provide earth fault path we uses zig-zag transformers.in this common 33kv system can we put two grounding transformers in service at a time,recently i have read this is not reccomonded,if so any body clarify why?

 

[slavag]

Hi.
Good Q.
Only one "problem", a more ground fault (twice) in case of both zig-zag trafo's on the common bus.
It's all.
Best Regards.
Slava

 

[tsknan]

recently i have tested the the system with two gr.Tr ,when both are in service i found higher harmonics currents are found flowing through ground terminals of both ,but when only one in service the current disappeared v,caN ANY BODY EXPLAIN

 

[DanDel]

This is probably circulating currents due to the two ground paths. I would imagine it was high in 2nd harmonic current, correct?

 

[slavag]

Yep.
It's situation.
DanDel is right, it's zero seq current circulation between two grounding buses, but with some correction it's 3-rd harmonic current 150Hz ( ok 180Hz ).
I don't worried for this, you need only check what is a level of current and accordantly adjest. your GF protection.
Best Regards.
Slava

 

[Bronzeado]

tsknan,

We have similar system (star-delta, but 230/69kV) running with two zig-zag transformers. Have you got any evidence of problem? So far, we have none.

Regards,

Herivelto Bronzeado

 

[tsknan]

yes we have measured the harmonics of the current flowing through the grounding tr NCTs,which are used for E/f protection

 

[Bronzeado]

tsknan,

Is this really a problem?
Which order are the harmonics measured?
A zig-zag transformer works, also, as a sequence zero filter.

You maybe are measuring indirectly the voltage imbalance in your system.

Best regards,

Herivelto

 

[SMB1]

Thank you everyone,

We usually grounded at two points in case of two transformer working in parallel. What different does it make in case of two Zigzag?

another question, if your 33KV bus is common, why did you provide to Zigzag Xfmrs?

 

[arnabanindita]

Dear All,
We are also going to have exactly same situation of two zig zag transformer at 33 kv bus working in parallel to increase the ground fault level. I would like to know explanation of higher harmonic levels. So far I know, one of the purposes of putting zig zag is to limit the harmonic content also.
Another question,
We are using directional earth fault protection to differentiate between faulty and parallel unfaulty feeder. Now putting two zig zag in service will the angle between V0 and I0 change?

 

[slavag]

Hi.
Your second Q,
As far, as I understood, up today you worked with ungrounded system on the 33kV and must used directional E/F.
In case of additionals zig-zag trafos you have two choices:
1. Disconnect directional E/F 67N protection and used simple E/F 50/51N protection , ofcourse if you have enough current for provide selectivity.
2. Used directional E/F protection, but you need rotate angle to 90deg from capacitive to resistance area.
In ABB words from sin(phi) to cos(phi) in the VAMP word from CAP to RES. In case of basic angle from -90deg to 0deg.
Best Regards.
Slava