Wye-Wye Transformer with ungrounded primary and grounded secondary

[kellyo3]

I am looking for some insight this configuration we are experiencing problems with: I have a distribution transformer that is configured wye-wye, 4160-480/277, 3W, 3P. No neutral is brought out from my switchgear to this transformer, only an equipment ground. The transformer secondary has the X0 connection bonded to the transformer casing, effectively grounded. The load is 3-phase motors with electronic soft-starters. My starters keep shutting down and so far I have not been able to get the alarm code off of the them before the electrician resets them. But in doing so, they start right back up, run for a while then one may shut off again. We are measuring about a 6% voltage drop at the transformer secondary (250-volts) on b-phase only, while a and c phase are slightly above 277 phase to ground. Line to line secondary voltage seems fine. My thought is that the voltages are unstable due to the ungrounded primary neutral, but the motor load on the secondary sees only line-to-line voltage which seems okay. Could these starters be shutting down due to harmonics and interference due to the transformer connections??? If so, any recommendations on how to mitigate? Replacing with a delta-wye transformer is not feasable. Any input would be much appreciated! Thanks

 

[cranky108]

It does sound like a grounding issue, but with the information given I can't say exactly what.
The H0 should be grounded, unless there is a developed delta in the transformer. And this is possible.
But the issue might also be with the grounding of the motors and soft starters themselves.

 

[kellyo3]

The H0 connection was originally grounded, but was removed due to tripping of a 50G that was on the feeder breaker. The relay was tripping during magnetization of the transformer...zero-sequence currents. I've since replace the 50G with a 51G (time delay). I think I can skin this cat by re-grounding the H0 connection on the primary. If the motor starters continue to inadvertainly shut off, then its either noise interference, harmonics or possibly grounding at the starter as crank suggested. Its not the same starter everytime however. Its fairly random amoung the 6 starters on that transformer.

 

[magoo2]

I believe grounding the primary is the right thing to do.

With a floating-wye grounded-wye connection, the secondary phase-to-neutral voltages will be unstable with either secondary phase-to-neutral loading or ground faults. Also, you'll get distortion on the secondary phase-to-neutral voltages due to the suppression of the third harmonic exciting current.

Sounds like you have a problem on the secondary to contend with, but reconnecting the H0 to the grounded neutral should be done.

 

[NCTHAI]

What you have here is secondary neutral bonded together with transformer casing - a protective earth and power supply neutral are same - not a desirable / acceptable conditions.

You must have protective earth and power supply earth separate.

NC

 

[slavag]

Hi.
From my point of view all posts above are very correct.
H0 sould be grounded, 50N/50G isn't issue, it's only time setting correction.
About grounding of trafo neutral, your solution isn't seems good. you need use grounding systems like to TN-S or TN-C-S.
Best Regards.
Slava