Auto-Transfer Problem

[nightfox1925]

We have a LV double-ended switchgear with two NC incomers and one NO bus tie. When one of the personnel made a test on for Bus 1 to feed bus 2(that means, opening incomer B and closing bus tie with incomer A maintained at closed position), the E/F relay element of the incomer A PCB suddenly picked-up and tripped incomer A PCB. If we do this the other way (Bus B feeding Bus A with incomer A open then closing the bus tie), Incomer B does not trip. Both buses are operating properly with NO tie so there is no ground fault at Bus A. Incomer B is already energized when we close the bus tie (after opening bus A incomer) so I don't think its an in-rush problem either. What do you guys think is going on? Thanks.

 

[dpc]

Is this new equipment?

 

[davidbeach]

Ground partial diff is not wired correctly.

 

[nightfox1925]

The relay being used (surprisingly) on each incomer breakers are GE F650. I am still getting more details on this.

 

[dpc]

If this is new equipment being commissioned, I agree with David Beach that some type of miswiring in the grounding scheme or ground relaying is the obvious suspect.

If this started up after being in service for a while, then that's a bit a puzzle.

 

[nightfox1925]

Thank you, we will further investigate the wiring.

 

[nightfox1925]

Hi all. I just got a confirmation that that separate GE MIG relay is tripping and specifically the 87R function. The readings indicated in the display are:

Ia = 0.17
Ib = 0.10
Ic = 0.08
In = 0.22

87R (Idiff) = 0.1In (range is 0.01 to 0.3 In, step 0.01 In)
87R (K, slope) = 20 (range = 1.0 to 100 step 1)
87R (time) = 0.1 sec (range = 0.0 to 99.99s step 0.01s)

I also requested a copy of the single line diagram. At first, I am thinking that the relay is tripping due to transformer in-rush since the CT connections include the phase CTs on one end and the neutral CT on the other end.

However, since the trip only happens during the transfer operation: Bus 1 incomer opened and bus tie closed and Bus 2 incomer is already NC, hence there is no in-rush generated by the upstream transformer in Bus 2.

 

[slavag]

Hi.
"However, since the trip only happens during the transfer operation: Bus 1 incomer opened and bus tie closed and Bus 2 incomer is already NC, hence there is no in-rush generated by the upstream transformer in Bus 2."
Isn't correct, it's inrush current too.
87 protection must have 2-nd harmonic monitoring continusly.
It's small level of 2-nd harmonic but still enough for maloperation of 87 protection.
Please check wiring and setting, of course manual of relay.
Best Regards.
Slava.

 

[nightfox1925]

Thanks Slavag. If the Bus 2 incoming transformer is already been energized continuously before the bus tie breaker is closed (with bus 1 incomer breaker already opened), is there any in-rush still that the 87R relay will see?

 

[slavag]

OK. I understand your Q, as well.
But, what you feeded on the bus 1, also trafos and motors.
Breaker 2 see THIS inrush current.

 

[nightfox1925]

Thanks slavag. They have performed a functional test on the auto-transfer with the downstream feeder loads energized. I actually requested them to de-energise the feeders and perform the same auto-transfer test again. If it does not trip, then can I conclude that the tripping is caused by downstream in-rush currents or phase current unbalance on the three phases which causes a residual current high enough to trip the 87R.

If this happens, is it acceptable to decrease the sensitivity of the 87R to override these phase unbalance?

 

[slavag]

NO, NO.
Before decide something please send us SLD of current wiring.
First of all check current wiring.
Are possible connect both sourses in parallel?
If yes, connect tie in parallel and this connection will show what is a problem setting or wiring.
Good Luck
Slava

 

[NAZ55]

I agree with Slava without a one-line it's pretty much impossible to recommend anything.

What's the location of the CTs involved?

How about your CT ratios?

Are all inputs coming from same polarities?

 

[nightfox1925]

Still waiting for the drawings.

The switchgear turns out to be a 415V, 3phase, 4wire.

Bus 2 is feeding small pumps while Bus 1 is feeding a firewater pump, compressors and cathodic protection transformers.

The REF relay is part of a multifunction relay: GE MIG. It is connected to the 2500/1A neutral CT and to a residually connected 3x2500/1A phase CTs at the line side of the incomer A circuit breaker.

The switchgear is operated with a NC incomers A and B and a NO bus tie. There is no auto-transfer, everything is done manually and there is an electrical interlock between the incomers A and B circuit breakers and bus tie to prevent closing of both incomers A and B if the bus tie is closed.

Inital condition: Bus A incomer closed, Bus B incomer
closed, Bus Tie open. No tripping

1 case: Incomer A circuit breaker closed, then incomer B
circuit breaker is opened. Then, bus tie is closed.
No trip.

2 case: Incomer B circuit breaker is closed, then incomer A
circuit breaker is opened. Then bus tie is closed.
REF relay and 51G tripped.

51G pickup setting = 0.5
51G time setting = 0.08

87R(REF) Idiff = 0.1*In
87R(REF) K, slope = 20%
87R(REF)time = 0.1sec

Is it possible that the REF relay is tripping due to some current unbalance on the residually connected phase CTs?

What are the possible causes? thanks

 

[slavag]

Hi Nightfox.
Somthing not seems in your case.
Lets go:
1. open-close of incomers A or B---> no trip
2. close bus-tie with incoumer A closed and incomer B opened----> no trip.
3. close bus-tie with incoumer B closed and incomer A opened----> trip.
Only one possibilites for this, some current wiring error between bus-tie and incomer B.
Maybe someone design partial ground bus protection?
It seems as not correct polarities.
of course possible trip due the inrush current (in this case you have unbalance), but protection must include 2nd harmonic monitoring and must be insensetive to unbalance situation. BTW, in two first situations, you haven't any problem. I stronly recommend verify current wiring between bus-tie and incomer B
Regards.
Slava

 

[nightfox1925]

Thanks slavag. We are presently confirming the current wirings.

I got some of the SLDs and 3lines on PDF.

 

[nightfox1925]

Hi Slavag, this is absurd. With both incomers closed and the bus tie at open position, the phase current readings at the F650 relay's metering function indicates:

Current Reading - Bus 2 incomer(via GE F650 relay):
Ia = 0.237
Ib = 0.240
Ic = 0.257
Ig = 0.000

Consequently current reading at GE MIG 2 (incomer 2):
Ia = 0.10
Ib = 0.10
Ic = 0.11
In = 0.00
ID = 13.79

Current reading - Bus 1 incomer 1(via GE F650 relay):
Ia = 0.297
Ib = 0.302
Ic = 0.305
Ig = 0.000

Consequently current reading at GE MIG 1 (incomer 1):
Ia = 0.08
Ib = 0.09
Ic = 0.09
In = 0.00
ID = 13.41

It seems that the normal phase currents are balanced. But there is a substantial differential current (Id). They checked the CT wiring and polarities and they are all ok.

The MIG relay's 87R (REF) is connected to a 1 x 2500/1A neutral CT at the 1600kVA, 11-0.433kV Tx and the other CT is a residual connection of 3 x 2500/1A phase CTs at the line side of 415V incomer circuit breakers. Same set-up applies to the other REF relay on the other bus.

We will perform further tests on the CTs and the MIGs including second inspection on the wiring. We will be grateful for any comments and observation that would help us find the cause of this nuisance. Thank you.

 

[slavag]

Yeah.
Start from small things.
See together
Ia= 0.237 (F650). 0.10(MIG)---> Infeed 1
Ia= 0.297 (F650). 0.08(MIG)---> Infeed 2
Im sure it's some mistake.
I suggest 0.237kA and 0.1In,

What is a point of neutral CT installation? On the cable or on the neutral of trafo?

Please meas. with clump current on the neutral wiring and on the phasse currents. Maybe problem with relay ( but not with both of them).
What is 13.xx Id, it's A or%, % is unpossible. 13A primary
is possible, 0.01=25A. 5% some unbalance is nothing.

Only one thing I can recommend on this moment.
1. Please short neutral CT input in the MIG of the infeed B
and closed bus-tie.
2. After this, swap wiring on this input and again close bus-tie.
Best Regards.
Slava

 

[nightfox1925]

"2. After this, swap wiring on this input and again close bus-tie."

After doing item 1. What do you mean by item 2?

Where did you get 0.01 = 25A and what is it for?

 

[nightfox1925]

Hi again Slavag,

I noticed in their three line diagrams, the single 2500/1A neutral CT connected to the star point of the power transformer is having its return leads (non-polarity lead)connected to ground. Is this correct?