Purpose of Transformer No Load Test Test Method
Transformer No Load Test is performed to determine the no-load impedance in the excitation arm of 10kv and above transformers.
The increase in the changing voltage no-load loss in the measurement is usually caused by a short circuit between the winding turns, a short circuit between the winding layers, a short circuit of the winding parallel branch, and the number of turns of each parallel branch, which causes magnetic potential unbalance and increases the no-load current. The no-load part is represented by the open circuit, which is shown on the right side of the figure as the “hole” or incomplete part of the circuit.
The open circuit test wiring diagram on the transformer is shown in the figures. A voltmeter, wattmeter and ammeter are connected to the LV side of the transformer as shown in the figures. The voltage at rated frequency is applied to this LV side with the aid of an autotransformer variac.
The HV side of the transformer is kept open. With the help of variac, the applied voltage is gradually increased until the voltmeter gives a value equal to the rated voltage of the LV side. After the LV side reaches the rated voltage, the reading on the voltmeter, ammeter and wattmeter is recorded.
The value read in the ammeter gives the no-load current I. Since the no-load current Ie is much smaller than the rated current of the transformer, the voltage drops negligibly.
Since the voltmeter reading V1 can be considered equal to the secondary induced voltage of the transformer, the wattmeter reading indicates the input power during the test. Since the transformer is open circuit there is no output, so the input power here consists of core losses in the transformer and copper loss in the transformer at no load. But as mentioned earlier, the no-load current in the transformer is quite small compared to the full load current, so we can neglect the copper loss due to the no-load current. Therefore, it can take the wattmeter reading equal to the core losses in the transformer.