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.
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
Not sure though.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Unbalance voltages during Low voltage excitation test
RE: Unbalance voltages during Low voltage excitation test
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