33kV zig-zag earthing transformer
33kV zig-zag earthing transformer
(OP)
My utility recently commissioned a zig-zag earthing transformer with the following rating: 33kV, 2000A for 10 seconds, 19.4 ohms on one of its 33kV network. Two days after commissioning, it was detected that oil was dripping from the transformer as a result of overheating. The grounding of the neutral of the transformer was checked and an earth resistance value of 0.5 ohms was obtained from measurement. Current measurements from the earth fault relay on the neutral of the transformer as well as direct measurement of the neutral current showed zero current. The transformer was taken out of circuit and after 3 days it was put back. It was then monitored for 2 days. The temperature started rising again even though no earth fault was detected in the network. What could be the cause of this overheating? Thanks for your responses in advance.






RE: 33kV zig-zag earthing transformer
The data you haven't mentioned is what the PHASE currents are? It is difficult for the transformer to overheat unless it is drawing current. I would check the phase currents into the unit ASAP.
I assume you've verified that the voltage applied is consistent with the nameplate ratings?
RE: 33kV zig-zag earthing transformer
Regards
Marmite
RE: 33kV zig-zag earthing transformer
I guess this "wrong connection" may cause a internal current circulation, overheating the transformer. Also, this current may generate a magnetic flux that leaks the core and goes throught the tank overheanting it (eddy currents).
Best regards,
H. Bronzeado
RE: 33kV zig-zag earthing transformer
RE: 33kV zig-zag earthing transformer
The insulation resistance measurement will not detect the way the windings are connected.
I suggest to ask the manufacturer to open the transformer to see if the connections are OK.
Please, send to us the final result of this problem.
H. Bronzeado
RE: 33kV zig-zag earthing transformer
Grab an oil sample to verify that overheating is really the problem.
Hope it helps.
RE: 33kV zig-zag earthing transformer
You need trafo mnf. person on the site ASAP!!!!!.
This is my strongly recommendation.
Is not games.
Regards.
Slava
RE: 33kV zig-zag earthing transformer
Regards
Marmite
RE: 33kV zig-zag earthing transformer
I don´t think a small voltage imbalance (normally below 2%) will overheating this zigzag earthing transformer.
Regarding the use of these transformers, it is not so unusual, as you said, but, also, it is not very common.
In brazilian electric network, few utilities use this kind of equipment. One of them is the transmission company I work for (Chesf, Recife, Brazil), which use them in the isolated 69kV system.
Regards,
H. Bronzeado
RE: 33kV zig-zag earthing transformer
My standard practice now for a grounding transformer is to use a ordinary wye-delta unit. It's much easier to get and much easier to replace.
It is interesting to hear that the zigzag grounding transformer is still commonly installed outside the US. Thanks for the feedback.
Cheers,
Dave
RE: 33kV zig-zag earthing transformer
Used 3 options: solidly grounding, via resistor and via Petersen coil.
Part of them installed as part of main trafo bay, part of them have dedicated CB.
Used at 22kV and 36kV levels.
Two local mnf. provided zig-zag trafos.
Regards.
Slava