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.

 

[RajT]

Can you please advise the KVA rating and Z% of the transf in question.

 

[RajT]

Please also advise the present setting on the 7UT513 relay.

 

[DanDel]

Typically, if you have a delta primary/wye secondary, the primary CTs for the differential relays should be connected in wye, and the secondary CTs should be connected in delta. If both sets are connected in wye, you have a 30° shift that may be responsible for your nuisance tripping.

 

[bigamp]

I am not familiar with the Siemens relay you describe, but in order for the connection that you describe to work, the relay must include ratio matching, a zero sequence filter and vector shift compensation. If it does not, or if ratio matching and vector shift compensation are not correctly set, the relay will not function correctly. If your relay does provide ratio matching, a zero sequence filter and vector shift compensation then I expect it would be OK to have primary and secondary CT's both in star. Check this from the relay technical data. Certainly, with an Alstom KBCH, the correct CT connection is star-star irrespective of transformer vector group.

If the relay does not have the above functions then DanDel is certainly correct, and the relay can never work properly.

The best test for a differential protection scheme is always primary injection, but often the actual testing is easier said than done. In the case of a transformer, you need to be able to isolate the transformer in such a way that you can apply voltage and circulate current through primary and secondary CT's. Isolate the transformer and apply a 3-phase short immediately downstream of the secondary CT's then energise the transformer at much reduced voltage to circulate current. Then you can see and measure and find out exactly what is going on. The reduced voltage to apply for the test can be less than the transformer impedance voltage. If you apply the impedance voltage to a shorted transformer you will circulate one per unit current but often you can see what is happening at well less than full load current.

In a recent installation the above test resulted in the transformer differential tripping out at about the same current as yours. We found the secondary side CT's were connected the wrong way around. After the connections were reversed we got zero differential current when the tests were repeated.

You could possibly have a dud relay but I doubt it.

Regards

 

[electricpete]

The relays I worked with were always wired as Dan described. Good catch Dan. I've never heard of the special features mentioned by bigamp that allow it to be connected otherwise, but there's a lot of stuff I've never heard of.

What would be the purpose of the 1:1 star/star "matching" transformer on the secondary.

 

[neps3]

nraju

As said above the secondary should be delta. The matching CTs are known as interposing CTs. Who designed the protection scheme anyway?

 

[jliu1]

Since matching transformer is connected as star-star and the ratio is 1:1, I don't see its necessity in the circuit.

 

[jbartos]

Suggestion/question: Have you contacted Siemens 7UT513 differential relay tech support?

 

[nraju]

Thanks everyone for the response.
RajT - The transformer is 25 MVA, %z = 12.38. All the parameters required by the Siemens relay have been entered properly. Which particular setting do you need?

DanDel - I agree with you, but this particular relay is not a conventional relay. It is microprocessor based "smart" relay and gives the freedom to connect both primary and secondary CTs as star, irrespective of the transformer connection type. The only condition is that the parameters have to be entered properly in the relay.

Bigamp - As mentioned above, the relay is a microprocessor based one with ratio matching, zero sequence filter and vector shift compensation included. Primary injection test is a bit difficult due to the site conditions as it is a running plant and one section is already down. Regarding your suggestion of applying a reduced voltage, is it by just bringing down the tap changer taps or is there any other way? Please clarify.

electripete, kantor and jiliu1 - I am not too sure about the reason for the matching transformer, but my presumption is this (correct me if I am wrong): The distance between the main CTs and the relay room is quite far away, so just to avoid the CT secondary from getting open circuited due to a possible cable fault, the matching transformer has been provided to give isolation.
jbartos - We haven't contacted Siemens technical support, because we believe (atleast until now) that the relay is okay and the problem may lie elsewhere. Maybe if everything else is fine, we will replace the relay with another one and test again, but this relay is self-monitoring and is known to be a very reliable one.
Thanks eveyrone for your valuable inputs.

 

[jbartos]

Suggestion to the previous posting: Nowadays, integrated multifunction protective relays are very sophisticated devices with essential (and "peculiar&quot inputs only. Therefore, the transformer winding connections may be "changed/processed" internally within the device/relay.
I would not question the relay reliability; I would contact the Siemens for the Relay literature including application notes and ask questions about it.

 

[bigamp]

nraju,

Your relay sounds to be of the same sort of thing as the Alstom KBCH with which I am familiar. As you say, it is really important to set the parameters correctly. Are you certain that the transformer is the vector group that you have programmed into the relay? (stranger things have happened).

For the primary test, you need to apply reduced voltage from a separate source of power. For a transformer of your size, one option would be to use a portable diesel genset.

Your transformer has an impedance of 12.38% therefore if you were to apply 12.38% of 13.8kV (i.e. 1708V) to the primary of the transformer whilst the secondary was short circuited you would circulate full load current in both the primary and the secondary of the transformer. You do not need such a high voltage though. You would probably be able to circulate sufficient current by direct application of LV from your LV MCC. For similar and smaller capacity transformers I have used big 3-phase variacs as the source of supply. All you really need is to circulate enough current in the transformer to see what the differential is doing. In a healthy connection the relay should be seeing zero differential current. If the connections or settings are wrong, the relay starts to see differential current at quite low transformer actual currents. If there is a problem, you can then identify which phase (or phases) it is in.

The test can be difficult to do but is well worth it. Sometimes you need to get a little imaginative as to where and how you connect the test supply and the short circuit and it all depends on your system configuration. Possibly you can isolate the section of bus your transformer feeder breaker is on and then bolt the test supply directly to the switchboard busbars. On the secondary side, you may be able to isolate a section of bus that the transformer feeds and apply a 3-phase short circuit directly to the busbars or possibily apply a short by closing in the earth switch on the secondary side incomer breaker (check the rating of the earth switch and be sure it is on the breaker side of the CT's).

If it was a problem generating sufficient voltage to get a high enough circulating current you could short the primary and apply voltage to the secondary side if necessary (which is often how they do the transformer SC test at the transformer factory).

Is your installation totally new or are the relays a retro fit?

It really does sound as though your problem is either CT's connected the wrong way around or incorrect relay vector compensation settings. The last transformer for which I commissioned differential protection gave results very much the same as yours. The problem was the secondary CT's were connected the wrong way around. That transformer was Dy11. At the time I never tried, but I bet that if I had set the vector group compensation as though the transformer was Dy5 the differential would have worked OK with the CT's as installed (this is not suggested as a permanent fix, but only to illustrate a point).

I don't think your transformer is actually faulted because if it was, it would trip out on being energised and not wait until the current was about 15% sa you say.

Hope this helps.

 

[jairo]

Are you sure that The transformer is energized phase R - U ; S -V and phase T to W because under other circunstances the transformer is DYn1 and you have problems
if this is the problem you can change the settings and set the relay to DYn1

 

[AFD]

nraju,

Dear friend, what I know from experience is that your CT connection should acount for the power transformer connection vectors (DY1, DYn11 etc..) causing a + or - 30deg phase shift.
ex. Delta/Y transformer CT shall be connected in Y on the primary and delta on the secondary. I also know that new microprocessor based relays can acount for all this bby merly inputing the correct information.

To make the long story short, check you vectors (CTs and relay input info.) and make sure that the currents through the relay are in fact canceling out completly.

 

[wowski]

From experience with the KBCH it is not always obvious the vector corrections needed for different transformer windings. I am not familiar with the Siemens relay range, but I know with the KBCH you can read the actual differential values the relay is seeing while on load. If this is possible then loading the transformer at 10% and checking these values may show where the problem is. What values are the trips set at?

 

[james001]

It happened to me on a project in SD. We set the relay and everything seems working fine until the customer increased the load, once they switched their line and increased the load, the 87T operated. We checked and checked and concluded that the phase input into the relay were wrong. Sometimes the 87T works fine with low load current (becuase there was no load current to test the relay) and once you crank the current up, it'll operate. My recommendation is to check the phase into the relay, print out the report, phase diagram etc to compare high side phase vs low side phase.

Also I believe the relay mentioned above should be capable to accept any CT connection regardless of the transformer winding, but not sure if you need to program it that or not.

Hope this helps

jtt

 

[PERRO]

The 7UT513 relay have a tool for comisiioning, if you are sure about the transformer is OK, try this.

1. Change the setting 1601 to "Trip bloked".

2. Put the transformer on load.

3. Whit the transformer in service, verify Idiff and Ibias.

Many problems in this relay in my company are because the secundary polarity is wrong, but you can change the vectorial group since the Idiff will be short and Ibias very high whit the relay in this condition.

We use this procedure when we can't put the transformer out of service to test.

Excuse my english.

 

[PERRO]

What you see in the oscilographic register next to trip?

If you see only load curren the problem is in the relay set or wiring.

 

[bitsian68]

nRaju,

Is this the first time that the transformer and the relay are commisioned that you are having problem, or have they been in service for a while and the problem started only now? If this is the first time for this transformer and the relay, the problem has to be in the wiring and/or the setup. We have used Siemens differential relay in similar applications without any problem. Siemens should be able to help you detect the source of the problem from your three line diagram and the relay setup parameters. Good luck

 

[RRaghunath]

I agree with PERRO. Further, you can download the relay literature from

http://www.siprotec.com

and benefit from the details available in it. The problem appears to be in the settings. Raghunath