Excessive Neutral on 3 Phase 12.5kV

[JHanson717]

We are starting to have trouble getting the main to hold in due to a trip on the 50/51 relay on the neutral leg. After closing the main, it tripped when closing 2 out of 4 of the load switches. We d/c'd the load from the transformers supplied by the switches with no change. After several attempts it will stay closed. Then we had the same thing occur on the next scheduled PM outage. We had the utility check calibration of the 50/51s, and they are all good. Transformer testing is unremarkable, within normal limits all around.

I fear it is trying to tell us something, but am unsure what at this point.

Any ideas someone would be willing to share?

 

[rbulsara]

Improper settings? Miswiring? Transients due to inrush tricking the relay? Just bad device? Misapplication?

Have some experienced engineer review the situation and details.


Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

After several attempts it will stay closed

Not good practice. But anyway....

Is this a residual CT connection or a separate neutral CT?

Are the disconnect switches being closed in single phase or three-phase? Imbalance can cause a trip if the 51N settings are on the low side.

Sometimes energizing a long run of cable or picking up a large transformer can create transients that can cause nuisance tripping of neutral and ground relaying.

You may need to add some additional time delay on the neutral relay.

Without more actual data, it is all speculation - which can be fun, but generally not an efficient approach.

David Castor

http://www.cvoes.com

 

[JHanson717]

The relay belongs to the utility, and they are the ones that calibrate it to protect their system. How much leverage does one have getting them to modify their equipment? The device passed calibration, all within 2% of specified values, but it's their specification.

There is no separate neutral CT, so I presume it's derived.

It is an instantaneous trip, not TOC, so I presume inrush is what is tripping it, but I question why it should see excessive current on the neutral, when it should be symmetrical.

As far as cabling it's ~150' of 1/0 133% EPR w/Cu tape shield, the transformers are 1500 and 2500 kVA.

They are 3 phase switches, Square D gear put in ~12 years ago. The problem just appeared with last years scheduled PM outage, and recurred this year.

 

[dpc]

The transformer inrush will not be symmetrical, since the inrush is partly a function of the voltage phase angle. Also, breaker and switch contacts in each phase do not close exactly simultaneously. Also, transformer inrush current is rich in harmonics. The fact that sometimes the relay does not always trip indicates it could be related to the closing angle and transformer inrush.

Unless someone wants to go to the trouble of monitoring the inrush current, the solution would seem to be to increase the pickup setting of IOC element or switch to something with a brief time delay.





David Castor

http://www.cvoes.com

 

[marks1080]

Are the AC signal to the switches and the trip input (DC) to the relay in the same cable? Could have some AC coupling to the DC input. Its a stretch I know, but I've seen it before.

 

[rockman7892]

dpc

I assume you are referring to an aysymmetrical inrush on the transformers downstream of the switches that cause and unbalance and cause the main transformer secondary residual CT schematic to pickup interperting this as neutral current? This is very interesting.

Reading this post and a residual CT pickup I am troubleshooting in another post asks me to question weather there is a disadvantage to using a residual CT scheme as opposed to having a core CT on the neutral or ground connection? Possibly with sensitivity or other such reasons?

 

[jghrist]

Do you have single phase loads? If so, maybe there is too much unbalance with some load switches open.

 

[rockman7892]

I suppose that if there are long cable runs there could be capacitance charging current to ground that may be unbalanced due to physical layout of cable that may cause current to flow in the neutral on a 3 wire system.

Balanced charging current would cancel in the neutral however if unbalanced I suppose it could flow in the neutral and be picked up by CT's