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Testing of transformers

Updated on November 5, 2014
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Osbert Joel is a Design Engineer in a leading LV Switchgear and Control System manufacturer in Oman.

Introduction

Transformer cannot be tested by connecting it directly to the load. This is because of two reasons. The first is large amount of energy has to wasted during such a test and the other is for large transformers it is impossible to arrange large loads enough for direct loading. Therefore simple tests called non loading tests are conducted on the transformer to determine the equivalent circuit parameters. From the equivalent circuit parameters the characteristics of a transformer can be computed. These tests consume very little power which is simply needed to satisfy the losses occurring in the transformer thus making the testing process more economical.

Two non loading tests conducted on transformers are the Open Circuit Test and Short circuit test. From these tests the necessary electrical parameters are measured, which corresponds to the appropriate equivalent circuit from which the characteristics of the transformer can be determined. Apart from these two tests another test to determine the temperature rise in the transformer called Sumner’s test or back to back is also conducted.

Tests done on transformers

Hence the tests conducted on a transformer are

1. Open circuit (OCC) test

2. Short circuit (SCC) test

3. Sumner’s or Back to back test

Circuit diagram for Open circuit test

Equivalent circuit

1. Open circuit (OCC) test.

Open circuit test is conducted on the transformer for the following purposes

  1. I. To determine the shunt parameters in the equivalent circuit
  2. II. To determine core loss
  3. III. To determine the magnetizing current (Im)

During this test, the rated voltage is supplied to one of the winding while the other winding is kept open. Normally LV side is provided with the rated voltage and the LV side is kept open. If the transformer is used at voltages other than the rated voltage, then the test should be conducted at that voltage. The meters are connected to the transformer as shown in the circuit diagram. After applying the voltage the meter readings are noted. The ammeter reading corresponds to the no load current Io and the watt meter reading corresponds to the core loss or iron loss Pi.

Pi = Po (Iron loss)

Shunt parameters in the equivalent circuit can be calculated from the following formula.

Yo = Gi - jBm

Yo = Io / Vi

Vi2 Gi = Po

Hence,

The conductance Gi = Po / Vi2

The susceptance Bm = √ (Yo2 - Gi2)

Circuit diagram for short circuit test

The reason for providing supply at the HV side can be clearly understood from the following example.

For a 200 kV, 440/ 6600 V transformer the voltage requirement for HV side is

6600 x 5/100 = 330V

And the current required is

200 x 1000/6600 = 30 A

If suppose the supply is provided on the LV side, then the voltage requirement is

440 x 5 / 100 = 22V

And the current requirement is

200 x 1000 / 440 = 445A

Equivalent circuit

Short circuit (SC) test

Short circuit test is conducted to determine the following

I. The full load cu- loss (Copper loss).
II. Leakage reactance and equivalent resistance.

In short circuit test supply arrangements are made at the HV side and the LV side is short circuited. The voltage needed for the short circuit test is 5 - 8% of rated voltage of the transformer.

Since the test on the HV side requires less current than that on the LV side supply is provided on the HV side. The supply voltage is gradually raised from zero till the transformer draws its full load current.

Voltage = Vs; current = Isc; Power input = Psc

The iron loss during the short circuit test is negligible due to very low excitation voltage. Therefore power drawn will be sufficient to satisfy the copper loss.

Hence the watt meter reading corresponds to the full toad copper loss.

Psc = Pcu (Copper loss)

Z = V sc / I sc


= √ (R2 + X2)

Equivalent resistance R = Pcu/ I sc2


Equivalent reactance X = √ (Z2 - R2)

Since the iron loss is neglected the shunt branch in the equivalent circuit can also be neglected.

Circuit diagram for sumpner's test

Sumpner’s test or back to back test

This test is conducted to determine the steady state temperature rise in if the transformer is full loaded continuously. This test is called Sumpner's after its inventers William Sumpner. In Short circuit and open circuit tests the power loss is due to either copper loss or core loss but never both. Sumpners test provides a way to find the steady rise in temperature of a fully loaded transformer without conducting an actual loading test. In this test two identical transformers required to determine the steady state temperature rise. But in case of very large sizes two identical transformers may not be available as these are custom built.

Principle

Sumpner's test is conducted by connecting to transformers in parallel across a single rated voltage supply. The two secondary winding are connected in series with opposing polarity. Since the transformers used are identical secondary voltages cancel each other. The two transformers acts like open circuited as their secondary are in phase opposition and no current flow though them. The current drawn from the primary source is twice the no load current and the power is twice the core loss. The secondary voltage source is adjusted to circulate full load current and the power fed is twice the full load copper loss. Thus by doing so full load core losses and full load copper losses are achieved in both the transformers without connecting actual load and the energy requires to fulfill the losses alone is drained from the sources.

A complete Electrical Engineering reference guide

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