Testing Restricted Earth Fault 1

[EngAriz]

Hello,

In a 33kV/11kV substation, we use SIEMENS Protection relay 7UT612 for Transformer differential and restricted earth fault protection. How can we test the restricted earth fault to know that the relay is responding or not.
Any information regarding testing procedure for Transformers restricted earth fault will be higly appreciated.

Thanks,

 

[ScottyUK]

Are you trying to test the relay, or the overall scheme? If the former then just use a relay test set. If the latter then you can use a generator to inject current into the transformer if you short one side and inject on the other. You will need to calculate the required voltage based on the current you want to circulate and the transformer impedance.

I've attached a sketch of a test scheme - currents are theoretical maximums and didn't account for cable and bus duct impedances. The test rig was capable of circulating roughly 50% of rated current through the transformer, certainly there was enough current to find a reversed neutral CT and after correcting the CT wiring to prove scheme stability.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[odlanor]

ScottyUK,
I guess Earth switch should have only one pole closed to get a fault phase-to-ground.

 

[slavag]

Hi.
What Scotty saied, test of the REF is possible only with generator injection. Isn't a simple procedure :-(.

Best Regards.
Slava

 

[ScottyUK]

Hi odlanor,

If this was a delta-star transformer then yes, one earth switch would be closed and the others open to steer current through the neutral CT. I've done exactly this on a delta-star GSU transformer, injecting from the LV delta winding. As this is a star-star transformer energising only one winding inherently forces neutral current to flow. I was pretty confident that we had a reversed neutral CT: the test shown should have resulted in a stable condition with no spill current.

It's not a particularly difficult test to do if it is carefully planned and tightly controlled. It involves a significant amount of power and needs careful control to ensure those involved remain safe.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[EngAriz]

Hi,
The transformer is Dyn11 33/11kV. Are one earth switch of star side should be closed and the injection should be at Delat side and are energizing one winding is enough in this case.

Thanks

 

[ScottyUK]

Here's the schematic from the GSU transformer test - it shows the earth switch configuration and test generator connections. I required an isolated and earthed section of bar between our test equipment and the 275kV bus; you can't use the HV earth switch you are using for the test as a safety earth.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[EngAriz]

Thanks ScottyUK.
Is this procedure can be applied for three winding power transformers, for e.g. transformer YN/yn0/D11 132/33/11kV.
Knowing that the Nuteral CT for Restricted earth fault is on 33kV side.
Can I have the shcematic for Differential test also.

Thanks,

 

[odlanor]

EngAriz ,
I am not sure you are right!
restricted earth fault protection of 7UT612 requires 3 phase current of line terminal and 1 neutral current of neutral terminal. An algoritm compares angles between 3 phases to decide is a fault or not.

 

[ScottyUK]

EngAriz,

Those are a couple of examples of how I've tested a conventional REF relay in the past. I haven't got an example for a diff scheme but you should be able to design a test using similar principles to prove the stability of a diff scheme or to test a three-winding transformer. If you aren't able to do so then please get some help from someone who has done this kind of test: you will be using a significant amount of power and in a fairly unusual configuration. There are a lot of pitfalls for the unwary which could result in something being damaged or someone being hurt.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[EngAriz]

odlanor ,
You are right, there should be three CT on the phases and one CT on Neutral point.
Thanks

 

[slavag]

Hi.
EngAriz, you can use second Scotty's example, only short all three phases and not to the ground.
Check stability, after move this 3-ph shorting on the next winding.
Delta side will be your reference.

You need calculate max current that will be enough for check stability, usually few %-s of nominal current.

You need prepare procedure with all safety rules, actually you need three tests.
1. 1-ph SC for REF stability.
2. 3-ph SC for D/Yn 33kV diff stability
3. 3-ph SC for 3-rd winding.
All SC out of zone.
in 1 test. turn in soft angle to 180deg and check 87N operating.
in 2 and 3 tests, short each phase on the current terminals.

Good Luck.
Slava