Transformer Routine Test – Measurement Of No-Load Loss And Current

Introduction to test

The no-load losses are very much related to the operational performance of a transformer. As long as the transformer is operated, these losses occur. For this reason, no load losses are very important for operational economy. No-load losses are also used in the heating test.
The no-load loss and current measurements of a transformer are made while one of the windings (usually the HV winding) is kept open and the other winding is supplied at the rated voltage and frequency.

 

During this test the no-load current (Io) and the no-load losses (Po) are measured.

The measured losses depend heavily on the applied voltage waveform and frequency. For this reason, the waveform of the voltage should be very sinusoidal and at rated frequency.
Normally, the measurements are made while the supply voltage is increased at equal intervals from 90% to 115% of the transformer rated voltage (Un) and this way the values at the rated voltage can also be found.

No-load losses and currents

The no-load losses of a transformer are grouped in three main topics:

1. Iron losses at the core of the transformer,
2. Dielectric losses at the insulating material and
3. The copper losses due to no-load current.

The last two of them are very small in value and can be ignored.
So, only the iron losses are considered in determining the no-load losses.

Measuring circuit and performing the measurement

In general according to the standards, if there is less than 3% difference between the effective (U) value and the average (U’) value of the supply voltage, the shape of the wave is considered as appropriate for measurements.

If the supply voltage is different than sinusoid, the measured no-load losses have to be corrected by a calculation. In this case, the effective (r.m.s.) value and the average (mean) value of the voltage are different. If the readings of both voltmeter are equal, there is no need for correction.

During measurements, the supply voltage U´ is supplied to the transformer by the average value voltmeter. In this way, the foreseen induction is formed and as a result of this, the hysteresis losses are measured correctly. The eddy-current losses should be corrected according to equation below.
P_m = P₀ · (P₁ + k · P₂)
P_m: Measured loss
P₀: No-load losses where the voltage is sinusoidal
Here: P₀ = P_h + P_E = k₁ · f + k₂ · f²
k = [ U / U’ ]²
P₁: The hysteresis loss ratio in total losses (P_h) = k₁ · f
P₂: The eddy-curent loss ratio in total losses (P_E) = k₂ · f²
At 50 Hz and 60 Hz, in cold oriented sheet steel, P₁ = P₂ = % 50. So, the P₀ no-load loss becomes:
P_o = P_m / (P₁ + k · P₂)   where P₁ = P₂ = 0,5
According to IEC 60076-1: P_m = P₀ · (1 + d)   where d = [ (U’ – U) / U’ ]

During no-load loss measurement, the effective value of the no-load current of the transformer is measured as well. In general, in three phase transformers, evaluation is made according to the average of the three phase currents.

Before the no-load measurements, the transformer might have been magnetised by direct current and it’s components (resistance measurement or impulse tests).
For this reason, the core has to be demagnetised. To do this, it has to be supplied by a voltage value (increasing and decreasing between the maximum and minimum voltage values for a few minutes) higher than the rated voltage for a certain time and then the measurements can be made.
The no-load currents are neither symmetrical nor of equal amplitude in three phase transformers. The phase angles between voltages and currents may be different for each of three phases.
For this reason, the wattmeter readings on each of the three phases may not be equal. Sometimes one of the wattmeter values can be 0 (zero) or negative (-).