Transformer differential protection 5

[nraju]

We have a problem with one of our transformers. As soon as the load crosses 15% of the rated current, the transformer trips on differential protection. The transformer is Dyn11, 13.8/4.16 kv. Primary CTs are connected in star (1000/1A). The secondary CTs are also star connected (3000/1A). The primary CTs are wired to a Siemens 7UT513 differential relay, which inturn is wired to the secondary CTs through a 1:1 matching transformer which is connected as star-star.
We have done a complete check of the transformer itself, the wiring between the relays and CTs, polarity checks, secondary injection tests, etc., and all of them are okay. But the moment we put the transformer on load, it trips when the load crosses more than 15%.
Would appreciate if someone can help me.

 

[Shortstub]

If the distance between the HV and LV CT's is appreciable, you may have a "wiring burden" problem.

Have you checked actual CT secondary current flows with a clamp-on ammeter? For different trafo loads?

 

[DiscoP]

I think PERRO's suggestions are quite good.
I'm not too familiar with the Siemens relay either, but if you are able to read the phase, bias and differential currents being calculated by the relay then you should be able to find the problem.
Post the measurements on here, along with the differential current settings (Pickup, bias characteristic, vector/ratio correction) and one of us could probably work it out if you have troubles.

 

[nraju]

Thanks everyone for their valuable inputs. A small correction, the relay is 7UT512, not 7UT513 (it was a typing error). I am giving below the details of tests conducted.
The relay has the provision to do commissioning tests and can give out phase displacement and phase angle measurements of both windings. The phase displacement values are as follows:
HV to LV Line 1: 330 degrees
HV to LV Line 2: 210 degrees
HV to LV Line 3: 90 degrees

Phase angle values of HV currents are:

HV Line 1 to HV Line 2: 240 degrees
HV Line 1 to HV Line 3: 120 degrees

Phase angle values of LV currents are:

LV Line 1 to LV Line 2: 120 degrees
LV Line 1 to LV Line 3: 240 degrees

Shortstub - The transformer had been in service for a long time, hence the wiring burden problem as suggested by you seems unlikely . It was sent out of the company to a contractor for repairing the tap changer. When the transformer was brought back and we tried to load it, the problem started.

PERRO - Currently the trip is blocked, but the relay is operating on differential protection as stated above.

bigamp and seethi - It is difficult to do primary injection test due to local conditions at site as it is a running plant.

I am also giving below the set parameters for the transformer and the differential protection data.
Starpoint formation of both CT winding 1 and CT winding 2 is towards the transformer. Pickup value of differential current is 0.15 I/In. Pickup value of high set trip is 7.5 I/In. Slope 1 of tripping characteristic is 0.25 and base point 2 of slope 2 of tripping characteristic is 2.5 I/In. Minimum restraining current for blocking at CT saturation is 7.00 I/In .

Would appreciate your help in solving this problem.

 

[jbartos]

Suggestion to bigamp (Electrical) Mar 9, 2003:
I do not think that the relay manufacturers expect the type of procedures you are stating in your posting to be performed for an application of their relay.

 

[jbartos]

Suggestion: Visit

http://www.eci.siemens.com/marketplaces/servlet/index.jsp?sdc_p=pfs3ndcm27

uo1000000000003l&eci_command=CoeeSiemensCategoryDisplay&categoryIdentifier=
1757%2CK1424&catalogId=33&categoryIdentifierList=1757%2CPortfolio_ZZ_27_2540
%2C1757%2CK1421%2C1757%2CK1422&sdc_rh=com.siemens.eci.cda.presentation.
ECIRequestHook&sdc_sid=25828463759&sdc_m4r=
etc. for more info

 

[bigamp]

jbartos, I am not sure what exactly you mean in your first posting of 15 March. Anyway, relay manufacturers just make the relays. They do not have to make the relays work in an actual installation.

nraju, I am not familiar with the siemens relay but there seems to be an inconsistancey on the HV side? With Dy11, shouldn't HV1-HV2 be 120 deg not 240 deg and shouldn't HV1-HV3 be 240 deg not 120 deg? This would then match up with the LV side where 1-2 is 120 deg and 1-3 is 240 deg. Also, I can not understand the HV-LV phase displacements. Surely they should all be the same? i.e. displacement between HV1 and LV1 should be the same as displacement between HV2 and LV2 which should be the same as that between HV3 and LV3? Shouldn't they all be 30 degrees, LV leading HV for Dy11? Its looking indicative of horribly transposed CT's and/or main phase connections.

Where are the differential CT's located? Are either (or both) the primary and secondary CT's located at the transformer? Any chance of an incorrect re-assembly whilst the OLTC was being worked on? If at least one set is at the transformer, were they correctly re-connected when the transformer returned?

Any chance that connections were transposed on either (or both) the 3.8kV side or the 4.16kV side when the transformer returned?

You can get the strangest things happening sometimes.

Regards

 

[SMB1]

As long as there was no problem before in your Diff relay system before repairing the Xfmr. I think the problem is still in the tap changer may be there is lose connection in the tap changer which cousing fault when the load current passes through that connection.

You have to inspect your tap changer again.

 

[nraju]

bigamp - You were absolutely right. We did a thorough check of the CT wiring and discovered that the complete wiring had been chaotically changed by the contractor. It took us a lot of time and effort to correct the same, after which we checked the phase angle displacement. The phase angle displacement of currents of the LV with respect to HV is 30 degrees lagging in all the 3 phases. We are now suspecting that the HV and LV CT secondary wiring may have been interchanged. Is our presumption right? We loaded the transformer to 425A and found that the differential current is very less, around 0.7%.
Now, the problem is this. The relay is configured for Dy11 whereas the CT current displacement is Dy1 (due to the 30 degree lag). I would like to know whether it is safe to fully load the transformer.
Thanks once again bigamp, for your valuable inputs.

 

[bigamp]

nraju,

Good to see that you have made progress with the problem.

Because I am not familiar with how the relay works I can not really say if it is indicative that the primary and secondary CT's are interchanged. Do the primary and secondary amps as read on the relay display look correct? It should be easy to tell from that if the CT's are interchanged. Another way to check for sure would be to inhibit the relay from operating and then short out the CT's one at a time and see from the relay current display what the relay is seeing. If you short out what you know to be a secondary CT and you see that the relay loses a primary current this will be indicative of CT interchange.

The external CT connections and the relay settings all need to be correct for the relay to function correctly. Is it safe to fully load the transformer with the relay as it is...only you can decide. Provided there is no transformer fault then it is safe to load the transformer as such(presume there is back-up protection in service). The thing is, if the relay is not connected or set correctly, it may not provide effective protection and it may also mal-operate (i.e. operate when it should not).

Regards

 

[rowbotth]

Check the polarity of the wiring into your differential relay.

It sounds like you have reverse polarity at a CT. Generally, 15% of rating is where these relays will pick up.

"Differential relays do not mis-operate. The CT wiring into them is wrong, if there is not a fault."
This was taught to me years ago, and I've never seen it proven wrong yet.

HR.

 

[PERRO]

What about the fault record, what you see?

 

[PowerSim]

I would like to add few points,
If CT wiring is interchanged you can check it by calculating the load, Secondary side Cts Indicate High current with Respect to Primary as per the Transformation.
You can check with Phase angle Meter, If the signal of Any side of PT Voltage is available nearby.
Angle shift from DYn11 to DYn1 indicates that CT Polarity is ok, otherwise you will get 180 deg shift at any of CT.
330 Deg rotation indicates that Phase sequence is changed some where, either at primary/secondary CTs, Primary secondary Main connections, Transformer Delta Formation, Tap changer connections.

Hope this will help