Auto-Transfer Problem
Auto-Transfer Problem
(OP)
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.






RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
If this started up after being in service for a while, then that's a bit a puzzle.
RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
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.
RE: Auto-Transfer Problem
"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.
RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
But, what you feeded on the bus 1, also trafos and motors.
Breaker 2 see THIS inrush current.
RE: Auto-Transfer Problem
If this happens, is it acceptable to decrease the sensitivity of the 87R to override these phase unbalance?
RE: Auto-Transfer Problem
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
RE: Auto-Transfer Problem
What's the location of the CTs involved?
How about your CT ratios?
Are all inputs coming from same polarities?
RE: Auto-Transfer Problem
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
RE: Auto-Transfer Problem
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
RE: Auto-Transfer Problem
I got some of the SLDs and 3lines on PDF.
RE: Auto-Transfer Problem
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.
RE: Auto-Transfer Problem
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
RE: Auto-Transfer Problem
After doing item 1. What do you mean by item 2?
Where did you get 0.01 = 25A and what is it for?
RE: Auto-Transfer Problem
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?
RE: Auto-Transfer Problem
Im not familiar with MIG ( only with MIG21
I suggest, REF is standard 87N function, meas diff current
between neutral CT and residual ( internal calculated) connection of set of phase CT's. It's same logic for all companies. That means, it must be insensetive to unbalnce current. For check this, I asked provide test 1, with shorted input of neutral CT. :
Reasons for first test:
1. Check source of Id.
2. REF operation
If it's no trip ,in the second test swap two wiring of neutral CT.
We don't know what is 13.xx in the MIG relay ( Id show).
CT is 2500/1A that mean 1In=2500A primary.
0.01In=25A.
or it's 13%, that means it's about 300A primary, it's realy bad, but in both relay.
Please check this points ASAP.
Best Regards.
Slava
RE: Auto-Transfer Problem
RE: Auto-Transfer Problem
It's something crazy.
Id=13., it's Id ot Value Id.
I read now manual of MIG.
Please check connection of neutral CT, chapter 3 of the manual. Please check chapter 9.18 too.
Slava
RE: Auto-Transfer Problem
All displayed currents are CT secondary values with +/-3% accuracy.
Yeah the special wiring conditions shows a generator whose neutral CT are connected to terminals C7 and C8 of the MIG relay. No ground connection is shown.
The 3 line diagram showed the neutral CT is jumpered to ground like in the case of the diagram shown in Chapter 3.2.1
RE: Auto-Transfer Problem
With both incomers closed and bus tie open
@MIG 1 Bus 1(reading taken at 40sec intervals)
Rdg Id(%)
1 20.87%
2 20.86%
3 8.32%
4 15.86%
5 26.94%
6 15.45%
7 11.35%
8 22.47%
9 27.53%
10 35.84%
@MIG 2: Bus 2 (reading taken at 40sec intervals)
Rdg Id(%)
1 30.95%
2 45.69%
3 38.57%
4 49.56%
5 49.73%
6 29.28%
7 31.85%
8 29.73%
9 40.41%
10 30.84%
It was also found out that the 87R at MIG2 had picked up but did not trip Incomer 2 since it immediately dropped out after 0.047 seconds.
The F650 reading at Bus 2 compared to MIG2 (with both incomers closed and bus tie open) at the same time are:
F650 1 2 3 4 MIG 2
Ia (kA) 0.195 0.192 0.207 0.217 0.08In
Ib (kA) 0.195 0.200 0.202 0.202 0.08In
Ic (kA) 0.205 0.207 0.205 0.205 0.08In
Ig (kA) 0 0 0 0
Is this a manifestation that the MIG2 relay is picking up due to normal current unbalance during normal operations.
We are still going to measure the current at the neutral CT and short it as per your test 1.
based from the contractor's relay calculations, the sensitivity setting of this REF is calculated by:
S = S87 + I*K
S = adjusted differential current setting
S87 = differential current setting
I= Maximum phase current
K= % slope
The REF should not operate for the highest difference or error current that can flow for an external fault. The sensitivity settings are calculated to verify the following equations:
0.2 = S87 + 0.5K
3.2 = S87 + 15.5K
From these equations S87 =0.1 and K=0.2 settings was derived.
RE: Auto-Transfer Problem
It's realy "good" result.
Same problem in both incomers, Incomer B more sensetive, it's all.
Canceleed please test 1, enough 2, and meas again Id.
Additional Q, F650 I0 metering is on the same neutral CT as MIG?
Please provide same metering in the MIG for the I1( positive seq), I2(negative seq) and In ( grounding current).
Crazu situation.
Regards.
Slava
RE: Auto-Transfer Problem
You mean forget shorting the neutral CT.
Swapping the two wires of the neutral CT of MIG 2 as per your test 2 and measure Id...shall be proceed with this test? What is the purpose of swapping them...to check polarity?
F650 Io metering is getting its input from the residual connection of separate 3 x 2500/1A phase CTs.
What do you mean by "provide same metering in the MIG for the I1( positive seq), I2(negative seq) and In ( grounding current)".
Thanks
RE: Auto-Transfer Problem
What Id=XX% represents?
RE: Auto-Transfer Problem
You mean forget shorting the neutral CT.
Yes
to check polarity
Yes and affect on teh Id meas.
F650 Io metering is getting its input from the residual connection of separate 3 x 2500/1A phase CTs.
Good,
What do you mean by "provide same metering in the MIG for the I1( positive seq), I2(negative seq) and In ( grounding current)".
This type of meas.
@MIG 1 Bus 1(reading taken at 40sec intervals)
Rdg Id(%)
1 20.87%
2 20.86%
3 8.32%
4 15.86%
5 26.94%
6 15.45%
7 11.35%
8 22.47%
9 27.53%
10 35.84%
All data, more and more show us---> problem in the neutral CT connection or installation
Regards.
Slava.
BTW, test 1 maybe will show small Id mes
RE: Auto-Transfer Problem
Does this mean that if everything works fine, there should be a negligible %Id indicated on the MIG current indication (in actual situations not possible to get %0 Idiff)?
If the REF system is to work properly, there should be no (or negligible) current flowing out from the neutral CT secondary right?
RE: Auto-Transfer Problem
%Id of In, but it's umpossible 30-40% of In.
Something wrong.
I suggest it's A. and primary, NOT SECONDARY.
Your setting is 25A minimum w/o slope.
WE MUST NOW WHAT IS A SOURSE OF THIS CURRENT.
1. 3I0 internal calculated ( Im not think so).
2. I0 from neutral CT.
3. Polyrity, Suggest, you have some small unbalance current and some current in I0.
Id= 3I0-I0, if you have mistake in the polirity you have
3I0+I0.
With short of CT's and swap of I0 wiring you can found reason of problem.
Regards.
Slava
RE: Auto-Transfer Problem
How about %Id = Idiff/Imaxphase = K = %slope.
Somebody suggested to increase the slope setting but I am not happy to do that unless I find out from where is this %Id is coming from.
Basic question, if I have a 4 wire system and and even with some small unbalance current, there should be no current flowing into the neutral cable that is connected solidly to ground right? Is the neutral current (In) going to flow through the neutral CT? I don't think so. If this is true, then the neutral CT will not have any current regardless whether the neutral CT is a having wrong plority or not? Kindly correct me if I'm wrong.
Now, if I have anyone with the phase CTs with wrong polarity, then a residual current will flow into the relay. and this current will flow into the REF circuit.
RE: Auto-Transfer Problem
example %Id = 40%
Id = 0.40*In = 0.40*(1A)
= 0.40A
Is my understanding correct? This %Id is also confusing me.
RE: Auto-Transfer Problem
Isn't important Id% secondary or primary.
0.4A secondary=1000A primary
and your load is only about 300A.
You must meas all. Please meas with clump connection of
neutral current in both incomers.
BTW, try ask GE Spain.
Regards.
Slava.
RE: Auto-Transfer Problem
if you look at page 8-3 of the MIG manual,
value (Id) = Measure in times the rated current of the
restricted ground differential current.
What will be the rated current of the restricted ground differential current? Is the Id setting or In?
Id setting = S87 + K*Iphasemax
S87 = Differential pickup setting = X*In
K = %Slope
Iphasemax = maximum phase current (per CT secondary)
Id setting = adjusted differential current setting.
We will conduct the current test measurements on Saturday.
RE: Auto-Transfer Problem
For my pinion:
"value (Id) = Measure in times the rated current of the
restricted ground differential current."
isn't correct translation.
Wait to result of current meas.
Good Luck.
Slava
RE: Auto-Transfer Problem
Where are you?
Please update us with this issue.
It's intresting.
Regards.
Slava
RE: Auto-Transfer Problem
I was also informed that the design office advised them to increase the slope setting to within 0.30~0.40 just to override the nuisant tripping. I advised the field to ensure make the necessary tests and measurements first before ending up with the last resort.
Regards.