Open Circuit test procedure of a transformer

Open Circuit test or no load test of transformer

Open Circuit test which is also called as no load test is an important method to determine the magnetizing reactance (Xm) and leakage resistance (Rc). The combination of both these quantities are called as no-load impedance and represented as shunt branch components in transformer equivalent circuit diagram as shown in figure below.

Why open circuit test done on low voltage side of transformer?

Normally in open circuit test the supply is given to LV side and HV side is left open because test voltage to be given on LV side is less compared to HV side. This reduces the use of high voltage test source and reduces the testing cost. Handling of low voltage source is less compared to handling of high voltage source.


Open Circuit test procedure

A watt meter and an ammeter is connected in series with LV winding to measure the open circuit power and excitation current of the transformer. A Voltmeter is connected in parallel with the voltage source to read the test voltage applied to it. The HV winding is kept open.

Since the no load  current is very small percentage of full load current which can be in the range of 0.2 to 2 % , The voltage drop in LV winding due to LV leakage resistance is negligible. So the equivalent circuit for open circuit test does not contain leakage resistance and only contains shunt components as shown in figure2.

The LV power measured by the watt meter consists of the core losses and primary winding ohmic losses. The primary winding losses are very less i.e 0.01% of the rated load loss at rated current. So the value of winding loss is negligible compared to core loss. Hence the entire watt-meter reading can be taken as core loss in the transformer.

I.e. Pcore = V1I0Cosθ0

So with the above equation and meter reading Pcore(Watt-meter reading) , V1(Volt meter reading), I0(Ammeter reading). The no-load power factor (Cos θ0) can e calculated as follows

Cos θ0 = Pcore / V1I0

Once the no load power factor is known then the core loss component current Icore and Magnetizing component Im can be calculated as depicted in the vector diagram

i.e.      Icore =   I0 Cos θ0

             Im =   I0 Sin θ0

With the above parameters the magnetizing branch parameters are calculated by using following formulas.

1) The shunt branch resistance or core leakage resistance can be calculated using core power loss (P core) reading

i.e. Rc = Pcore / I core2

       Rc= V12/Pcore

 2) The magnetizing reactance component is calculated as follows

      Xm = V1/Im

The found values are with reference to LV side, since the measuing instruments are placed in LV side. If the required the HV paramters can be found by multiplying each parameter with a factor k2 ( where K = N2/N1– Turns ratio). The value of magnetizing reactance Xm is very high compared to leakage resistance Rc so in some cases core leakage losses are negligible where as magnetizing losses are accountable.

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