Transformer differential protection
Transformer differential protection
(OP)
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.
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.






RE: Transformer differential protection
RE: Transformer differential protection
RE: Transformer differential protection
RE: Transformer differential protection
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
RE: Transformer differential protection
What would be the purpose of the 1:1 star/star "matching" transformer on the secondary.
RE: Transformer differential protection
As said above the secondary should be delta. The matching CTs are known as interposing CTs. Who designed the protection scheme anyway?
RE: Transformer differential protection
RE: Transformer differential protection
RE: Transformer differential protection
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.
RE: Transformer differential protection
I would not question the relay reliability; I would contact the Siemens for the Relay literature including application notes and ask questions about it.
RE: Transformer differential protection
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.
RE: Transformer differential protection
if this is the problem you can change the settings and set the relay to DYn1
RE: Transformer differential protection
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.
RE: Transformer differential protection
RE: Transformer differential protection
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
RE: Transformer differential protection
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.
RE: Transformer differential protection
If you see only load curren the problem is in the relay set or wiring.
RE: Transformer differential protection
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
RE: Transformer differential protection
Raghunath
RE: Transformer differential protection
Have you checked actual CT secondary current flows with a clamp-on ammeter? For different trafo loads?
RE: Transformer differential protection
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.
RE: Transformer differential protection
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.
RE: Transformer differential protection
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.
RE: Transformer differential protection
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RE: Transformer differential protection
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
RE: Transformer differential protection
You have to inspect your tap changer again.
RE: Transformer differential protection
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.
RE: Transformer differential protection
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
RE: Transformer differential protection
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.
RE: Transformer differential protection
RE: Transformer differential protection
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