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.

 

[slavag]

Hi Nightfox.
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

 

[nightfox1925]

Ok Slavag I will. Thanks.

 

[slavag]

Nightfox.
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

 

[nightfox1925]

Hi slavag, the Id is the ground differential current in amps (see chapter 2.14 of the manual).

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

 

[nightfox1925]

Hi slavag, latest info from site:

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.

 

[slavag]

Thanks Nightfox.
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

 

[nightfox1925]

Hi slavag,

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

 

[nightfox1925]

Just for my curiousity, how did you find Id = 40.41% (MIG2) and Id = 35.84% (MIG1) acceptable?

What Id=XX% represents?

 

[slavag]

Hey. Now we are in the sinchronazing .

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

 

[nightfox1925]

Looking at the formula for the Idiff current setting, how will I relate %Id to it?

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?

 

[slavag]

Hi.
%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

 

[nightfox1925]

Hi slavag,

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.

 

[nightfox1925]

I am thinking %Id to be the adjusted differential current or

example %Id = 40%

Id = 0.40*In = 0.40*(1A)
= 0.40A

Is my understanding correct? This %Id is also confusing me.

 

[slavag]

Hi Nightfox.
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.

 

[nightfox1925]

Hi Slavag,

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.

 

[slavag]

Hi Nightfox.
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

 

[slavag]

Hey. Nightfox.
Where are you?
Please update us with this issue.
It's intresting.
Regards.
Slava

 

[nightfox1925]

Hi slavag, I am still going to get the results till tomorrow because they only managed to get a shutdown tomorrow.

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.