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Anonymous Poster #1

### Transformer Winding Resistance

07/04/2012 8:54 AM

Dear Friends, I have some questions regarding 3 phase distribution transformer with a rating of 1600 kva, 13.8/0.4 kv. It has an unbalance output voltage of 400/240/236volts. when we measured the transformer winding resistance, the reading were:

Secondary Winding: U2V2 - U2W2 - V2W2 = 0.2 Ohms (Balance)

Primary: U1/V1 - 2.2 Ohms, U1/W1 = 4.7, U1W1 = 3.3 Ohms

Primary Voltages (supply) 13.8 KV (balance)

My question is, is the winding resistance in the primary which is unbalance cause the the unbalance voltage output? is their standard if how much is the allowable winding resistance unbalance in percentage for 3 phase transformer?

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Guru

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#1

### Re: Transformer winding resistance

07/04/2012 10:25 AM

How do those readings compare with the design readings, and the ones recorded at the Factory Acceptance Test? If they are the same, why was the thing shipped in that condition instead of being sorted out. And if they have changed since the thing was installed, with what event does the change in readings coincide?

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#2

### Re: Transformer winding resistance

07/04/2012 11:09 AM

the FAT is 3.3 ohms balance for 3 phase. the transformer is a hermitically sealed and been in operation for 7 years at 40 -70 % of its rated load. This thing happened during the Utility company voltage fluctuation. the transformer become hot and later trip. when we checked it we discover that the voltage at the secondary is 388 in one phase and 230/235 in the other 2 phases. but the supply is balance 13800 volts

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#3

### Re: Transformer winding resistance

07/04/2012 11:12 AM

Is it possible to gain access to the secondary windings and discover whether or not one of them is open-circuit to the neutral star point?

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#5

### Re: Transformer winding resistance

07/04/2012 1:58 PM

I think it is possible because the transformer was replaced already and ready for delivery to the manufacturer. I only want to know if the unbalance winding resistance in primary have something to do with the unbalance voltage in secondary (output). Because the difference is more than 100%. 2.2 ohms in 1 phase and 4.7 in the other phase

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#6

### Re: Transformer winding resistance

07/05/2012 3:24 AM

At the moment it sounds like it needs replacing a second time. That installation also needs some surge suppression on the input too, so as to prevent recurrence.

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#4

### Re: Transformer Winding Resistance

07/04/2012 12:49 PM

The result of the fluctuation is the short-circuited coils in the 2 primary windings (that induce now the lowered voltage into the 2 secondaries).

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Dalton (2); PWSlack (3); Yuri B. (1)

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