Transformer Differential Protection Trip on Downtream Motor Starting

[ThePunisher]

Hi all,

We have experienced recently a strange percentage differential trip while starting a motor downstream. The GE Multilin T60 is installed to protect a 1 MVA, 7.2-0.60 kV power transformer. The differential CT inputs are 400:5A at the transformer's primary 7.2 kV bushing and 1200:5A secondary CT located load side of the downstream 600 V main circuit breaker of MCC. The differential P/U is 0.30 p.u., Slope 1 = 25%, Break Point 1 = 2 p.u., Break point 2 = 8 p.u., Slope 2 = 80%

The transformer was successfully energized and undergone a 24 hours no-load operation (Soaking).

After 2 days, a 30 HP, 575 V induction motor is started (bump-testing) at the 600 V MCC and it tripped the percentage differential at an Idiff event measured at 0.69 p.u.

This somewhat puzzles us as the current flow is unidirectional and if there are harmonics, the relay's harmonic inhibit was enabled and don't think it would go to a Idiff pickup as high as 0.69 p.u.

Did anyone had the same issues in the past? If yes, how did you mitigate it? If none, what do you think is the problem.

We checked CT polarities and system set-up in the relay ans they are all fine.

Regards,

 

[KHH1]

Hi,

Could you upload the fault records?

 

[ThePunisher]

Hi KHH1, the relay technician is currently downloading the fault records at site and will upload here very soon.

Regards,

 

[stevenal]

How did you check polarity? We've had CTs that were mis-labelled in this regard. Other problems might be cross-phasing, incorrect CT ratio, multiple CT secondary grounds, and shorted CTs. You say the relay settings were checked, but the phase shift compensation setting is one that can sometimes be tricky to get right.

 

[pwrengrds]

It's either wired wrong or programmed wrong. What type of TX is it and is it matched to the settings?

1 PU 600 volt side = 1 MVA/0.6/1.732 = 962 amps, 69% = 663 Amps.

If a 30 amp motor trips it on an inrush of 10x = 300 amps, my guess is a CT is backwards, either in the relay software or the field. (based on trip current being about 50% of expected inrush, I'm playing fast and loose with numbers, ...but based on information given it's a best guess).

 

[dpc]

Sounds like a classic case of CT phase and/or polarity error. Or relay configuration error. Another possibility is an incoming phase sequence that doesn't match your assumption.

You can look at the phasors via the T60 relay. A small amount of load will help make the problem more obvious when looking at the phasors.

Cheers,

Dave

 

[ThePunisher]

Hi all,

The problem was resolved. Yes it was a polarity problem.

Thank you for all your prompt and valuable inputs.