33kV zig-zag earthing transformer

[Jorgy1]

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.

 

[dpc]

If its a new transformer, I would seek help from the transformer supplier, since it should still be under warranty. Since the zigzag autotransformer is an unusual beast these days, is it possible the transformer was not properly connected at the factory?

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?

 

[Marmite]

In addition to DPC's suggestion to check phase currents, earthing transformers quite often have an auxiliary star winding to provide substation LV supplies. If there is an auxiliary winding, is it giving normal volts?

Regards
Marmite

 

[Bronzeado]

As DPC said, it seems the transformer was not properly connected.

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

 

[Jorgy1]

I thank all of you for your valuable contributions. The transformer is a new one and we will be complaining very soon to the manufacturer. It hasn't got an auxiliary winding for station supply. We will put it back into service and measure the phase currents. If the windings were wrongly connected could it be detected from insulation resistance measurement. Because of the nature of the zig-zag connection, I am not too sure what to expect from insulation resistance measurement. The supply voltage is the same as the name plate voltage.

 

[Bronzeado]

Dear Jorgy1,

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

 

[unclebob]

If the core ground is readily available, check the insulation resistance of the core. It is possible that multiple grounds exist.

Grab an oil sample to verify that overheating is really the problem.

Hope it helps.

 

[slavag]

Hello Jorgi.
You need trafo mnf. person on the site ASAP!!!!!.
This is my strongly recommendation.
Is not games.
Regards.
Slava

 

[Marmite]

Another thing to check for is voltage imbalance on the 33kV system. If there is imbalance the earthing transformer may be acting to try and correct it. There's no harm in getting the manufacturer to check, but I'm sceptical that the connections would be wrong internally. I don't agree that these are unusual. In fact most if not all 132/33kV transformers installed in the UK distribution system have interconnected star earthing transformers connected to the 33kV side. The power transformer and earthing transformer are usually purchased as a pair from the manufacturer.
Regards
Marmite

 

[Bronzeado]

Marmite,

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

 

[dpc]

I can say that zigzag grounding transformers are virtually extinct in the US. There are still some in service, but very few new ones are installed and finding a supplier for a new one is a challenge.

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

 

[slavag]

Dave. In my area zig-zag trafo is common practic at utilities and at industry.
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