Transformer Startup - Primary Overcurrent Protection

[hidalgoe]

One of our transformers (ABB 15/20MVA, 67/12.47kV, 8.46% @ nominal tap, primary 33 step OLT) recently was taken off line manually because of unbalanced voltage condition accompanied by inrush like noise. We have no primary SCADA sensing, but our secondary voltage waveforms revealed a 20% unbalance (highest) While off line a testing company was called in to test the transformer (TTR, Doble tests, DGA, excitation, etc) Nothing was found. When we tried to energize the transformer the primary S&C TransRupters tripped on Phase A overcurrent (BASLER 50/51-218) We tried to energize the transformer a second time with the same result. The testing company was called in again to retest the transformer. The same results were obtained.

We have no way of monitoring our primary voltage (69kV). We contacted the utility to seek there help. They said that their system showed no anomalies for the period of interest.

We are baffled. We are thinking of contacting the manufacturer's services group for a second opinion.

Please help.

 

[dpc]

It's hard to say without knowing the relay settings. Inrush current can vary depending on remanence flux and the voltage phase angle. Sometimes the relay may trip on only 10% of the energizations. I assume it it tripping on phase instantaneous at the instant of energization?

As far as the voltage imbalance - any chance the OLTC mechanism has failed on one phase? Was the tap changer tested in all phases in the TTR test?



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[hidalgoe]

dpc:

The company tested the OLTC was tested thoroughly and given a clean bill of health. The tripping did occur on phase instantaneous (set at highest settings) and a .3 sec additional time delay added.

 

[rbulsara]

What is the setting of phase o/C? in amps and Inst?
As dpc asked, if the ratio tests were performed with OLTC in circuit?

Obviously there was some issue with the OLTC, perhaps it is not fixed or got worse?

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

How was the core noise ? One reason could be core dislocation at the yoke-limb core joints.