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Transf ormer Rout ine Test - Measurement Of No-Load Loss And Current (on photo: Powertransf ormer - BEST)

http://electrical-engineering-portal.com/transformer-routine-test-measurement-of-no-load-loss-and-current April 14, 2013

Transformer Routine Test Measurement Of No-LoadLoss And Current

Edvard

Introduction to test

The no-load losses are very much related to t he operational performance of a transformer. As
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long as the transformer is operated, these losses occur. For this reason, no load losses arevery 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 thewindings (usually the HV winding) is kept open and the ot her winding is supplied at the ratedvoltage 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 thisreason, the waveform of the voltage should be very sinusoidaland at rated frequency.

Normally, the measurements are made while the supply voltage is increased at equal intervalsfrom 90% to 115% of the transformer rated voltage (Un) and this way the values at t he ratedvoltage can also be f ound.

No-load losses and currents

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

1. Iron losses at t he 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
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Connection diagram f or measuring no- loadlosses

In general according to the standards, if there isless than 3% dif ference between the effective (U)value and the average (U) value of the supplyvoltage, the shape of the wave is considered asappropriate for measurements.

If the supply voltage is dif ferent than sinusoid, the

measured no-load losses have to be corrected bya calculat ion. In this case, the effective (r.m.s.)value and the average (mean) value of thevoltage are different. If the readings of bot hvoltmeter are equal, there is no need forcorrection.

During measurements, the supply voltage U issupplied to the transformer by the average valuevoltmeter. In this way, the f oreseen induction isformed and as a result of this, the hysteresis

losses are measured correctly. The eddy-currentlosses should be corrected according to equationbelow.

Pm = P0 (P1 + k P2)

Pm: Measured loss

P0: No-load losses where the voltage is sinusoidal

Here: P0= Ph + PE= k1 f + k2 f2

k = [ U / U' ]2

P1: The hysteresis loss ratio in total losses (Ph) = k1 f

P2: The eddy-curent loss rat io in tot al losses (PE) = k2 f2

At 50 Hz and 60 Hz, in cold oriented sheet st eel, P1 = P2 = % 50. So, the P0no-load loss

becomes:

Po = Pm / (P1 + k P2) where P1 = P2 = 0,5

According to IEC 60076-1:Pm = P0 (1 + d) where d = [ (U' - U) / U' ]

During no-load loss measurement, t he eff ective value of the no-load current of the transformeris measured as well. In general, in three phase t ransformers, evaluat ion is made according tothe average of the three phase currents.

Before the no-load measurements, the transformer might have been magnetised by directcurrentand it s components (resistance measurement or impulse tests).

For this reason, the core has to be demagnetised. To do t his, it has to be supplied by a voltage

value (increasing and decreasing between the maximum and minimum voltage values for a fewminutes) higher than the rated voltage for a certain time and then the measurements can bemade.

The no-load currents are neither symmetrical nor o f equal amplitude in three phase
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transformers. The phase angles between voltages and currents may be different f or each ofthree phases.

For this reason, the wattmeter readings on each of the three phases may not be equal.Sometimes one of the watt meter values can be 0(zero) ornegative (-).

Resource: Transformer Tests BEST Transformers

electrical-engineering-portal.com-transformer routine test measurement of noload loss and current

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