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Unbalance voltages during Low voltage excitation test

Unbalance voltages during Low voltage excitation test

Unbalance voltages during Low voltage excitation test

(OP)
In India, transformer integrity is checked by a low voltage test called Magnetic balance test.It consists of applying a single phase low voltage to one phase and measuring voltage across the other phases.When voltage is applied across middle phase,other phases will read half of the applied voltage.When side limb phases are excited,voltage at middle phase should be 2/3 and end phase voltage 1/3 the applied voltage.In case of an interturn fault in a phase,the voltage induced in that phase will be zero and other phase will read same  as applied voltage.But this ideal voltage distribution  is obtained only in case of distribution trfs where rated voltage of winding is near to applied single phase voltage.When 220 V is applied to a winding of rated votage 220KV, due to the very low flux generated, the voltages will not be 2/3 &1/3 ,but erratic as say 60-95 % and 40-5%.

In one 220/66  KV transformer,star /star connected, the voltage distribution pattern is as below with applied voltage of 244 V
Hv winding

  UN         VN        Wn

  244         236       19
  141         244      106
   23         235      244

LV winding;

  un         vn         wn
  244         256        40
  131         244       113
   38         258       244

You can see that  in case of LV winding, voltage on middle phase is more than applied voltage.What can be the reason for this? The transformer is perfectly healthy with normal excitation current and working perfectly on grid.

RE: Unbalance voltages during Low voltage excitation test

My first thought would be that you are measuring with a d'Arsonval type voltmeter and the distorted wave form caused by the non-linear excitation current is causing metering errors.
Not sure though.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Unbalance voltages during Low voltage excitation test

(OP)
Thank you waross.What  type of voltmeter you will recommend?I believe they are using normal multimeter for measurement

RE: Unbalance voltages during Low voltage excitation test

You need a true RMS meter.  A d'Arsonval meter responds to average DC values. This is the old basic, galvanometer based moving needle meter. The AC is rectified to DC and the meter responds to the average value. The calibration is adjusted by a "Form Factor" so that even though average values are being read, RMS values are indicated. These meters worked well except when reading distorted wave forms such as the voltage from an unloaded transformer. The harmonics distort the wave form and the ratio between average and RMS changes.
Wait for other responders with more transformer testing experience. It's been a long time since I have seen this. It doesn't take much load on the transformer to swamp the harmonics and then the reading becomes resonable. That may not help your test, but it explains why the effect is not seen more often.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Unbalance voltages during Low voltage excitation test

(OP)
Here we find this induced voltage more than applied one only in one unit, with  transformers in the same series are showing lesser values.Meters used are the same.

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