How To Test A Transformer

July 11, 2020
insulation resistance test of transformer

Transformer testing is one of the most important lectures. We do transformer testing to collect some information about transformer efficiency, losses, and reliability record. These tests are for single-phase and three-phase transformers.

Similarly, we perform various tests on the transformer to ensure the transformer’s required technical specifications and analyze its performance.

Also, transformer testing is carried out in accordance with applicable IEC and ANSI specifications, unless otherwise specified in the contract documents. Where the test procedure is not specified, the requirements of IEC 60076 apply.

We can classify transformer testing according to:

  • Firstly Electrical type transformer testing
  • Secondly, Electrical routine transformer Testing.
  • Thirdly, Electrical special transformer tests.
  • Fourthly, Pre-commissioning, and commissioning tests.
  • Fifthly, Periodic maintenance transformer tests.
  • Lastly, Emergency tests.

From the previous classifications, we can deduce that the first, second, and third types of transformer testing are carried out by the factory manufacturing the transformer. However, for the fourth, fifth, and sixth types, they are carried out at the transformer’s worksite.

Classification of Transformer Testing

1- Electrical Type of Transformer Testing.

The manufacturer who manufactured the transformer does this test to ensure that the practical measurements are consistent with the simulations’ theoretical tests.

These tests are carried out by the manufacturer on a prototype only of that transformer’s production and not on all the transformers produced .. why? Because some tests (such as short circuits) affect the lifetime of the transformer, these tests are:

  • Transformer Turns ratio test.
  • Transformer winding resistance measurement.
  • Insulation resistance test of transformer.
  • Open circuit test of transformer.
  • Short circuit test of transformer.
  • Induced overvoltage test of transformer.
  • Separate-source voltage withstands the test.
  • Partial discharge test of transformer.
  • Lightning impulse test (Dielectric type test)
  • Temperature rise test.
  • Vector group test.
  • Measurements of dissipation factor.
  • Oil tests.

2- Electrical Routine Transformer Testing.

These tests are part of the electrical type transformer testing.

The manufacturer carries out electrical routine tests to verify the operation efficiency and the transformer’s lifetime, and these tests are carried out on all manufactured transformers.

These tests include the following:

  • Transformer Turns ratio test
  • Transformer winding resistance test
  • Insulation resistance test of transformer
  • Open circuit test of transformer
  • Short circuit test of transformer
  • Induced overvoltage test of transformer
  • Separate-source voltage withstand the test
  • Partial discharge test of transformer

3- Electrical Special Transformer Tests.

The manufacturer carries out these tests to obtain important information that assists the user during the transformer’s operation and maintenance.

Tests include the following:

  • Sound level measurement.
  • Measurement of zero sequence impedance.
  • Measurement of the harmonics of the no-load current.

4- Pre-commissioning and Commissioning Tests.

These tests are carried out at the worksite of the transformer. This ensures that the results of the tests carried out in the factory match with the results obtained at the site before the transformer’s operation.

For example, the transformer may pass the factory tests but may fail the test at the site due to the stresses during factory tests.

These tests are part of the routine tests.

5- Periodic Maintenance Transformer Tests.

We perform these tests on the site after the operation of the transformer. Also, they are carried out regularly. For instance, there is a weekly, monthly, and annual maintenance plan to ensure the transformer’s efficiency and performance.

So, the periodic maintenance plans vary according to the type of transformer. Moreover, these tests are performed to detect faults in their early stages, such as measuring insulation resistance. For example, if their value falls below normal, this indicates the beginning of problems in isolation.

6- Emergency Tests.

We carry out these tests at the site if the transformer has some problems during operation such as the high temperature of the windings, although the ventilators work very efficiently,

Then we can test the measurement of resistance to windings or analysis of oil used for cooling.

In conclusion: Therefore, we know that there are many types of transformer testing done at the factory or the transformer’s worksite, as mentioned above, and all revolve around the same tests.

So, we will show some of the tests in detail.

Various Transformer Test

1. Transformer Turns Ratio Test

Purpose of this Test:

We perform the Transformer turns ratio test to:

  • Determine the turn’s ratio between the primary and secondary windings.
  • Calculate the error ratio between the design value and the actual value; this value shall not exceed the limit given in relevant standards, normally 0.5% at no load.
  • Detect any short circuit between winding in a single side.
  • Verify that the transformer winding is connected to the correct vector group.

Precautions Before Turns Ratio Test:

  • Disconnect the transformer electrically.
  • Clean the terminals of the transformer.
  • Disconnect the voltage source before changing the voltage tap changer.

Steps for Transformer Ratio Testing:

We have two cases for this transformer testing, Firstly using measurement device:

Turns Ratio Test 1
  • On the device, adjust the test device by specifying the transformer’s vector group, for example, DY 11.
  • Also, determine the number of voltage tap changer points, for example, 7 points.
  • The amount of voltage step 2.5.
  • And determine the normal number of voltage tap changers, for example, point 4.
  • Determine the value of the voltage on the primary winding is 11 kV, and value Voltage on the secondary winding is 415 volts corresponding to the point of the normal voltage tap changer, which is No. 4
  • Then, through the test device, which has two voltage values ​​of 40 volts or 80 volts AC, select one of them.
  • Then starts injecting that voltage onto one of the high voltage windings of the transformer.
  • Measure the voltage on the two ends of the secondary windings, the ratio of which is the transformer ratio.
  • Disconnect the voltage source before changing the voltage tap changer.
  • Change the voltage tap changer mode and the test frequency.
  • Repeat the test at another tap changer point and compare the output with the transformer’s existing data.
Test Result:

Measure the voltage ratio for each tapping connection of the transformer. In the test report, the specified tapping voltage ratios are stated and the measured ratios. If the error rate exceeds 0.5%, the transformer has failed the test and is rejected.

We can also do this test with another method: the transformer turns ratio test can also be done using an external voltage source of 380 volts.

Turns Ratio Test 2
  • The voltage tap changer is set to position 1
  • Connect a three-phase voltage source 380 volts AC on the High voltage side.
  • Then measure the line voltage on the low voltage side as well as the phase voltage.
  • Divide the line voltage of the high voltage side on the line voltage of the low voltage side
  • And compare the result of the transformer ratio of point “1” to the voltage tap changer in the transformer nameplate with a measurement result where the error ratio should not exceed ±5.0 %.
  • Then we disconnect the voltage source from the transformer.
  • With the change of the voltage tap changer mode to mode number “2” and
  • Repeat the previous steps, disconnect the transformer, change the voltage tap changer mode to “3,” and repeat the previous steps.
  • Then disconnect the adapter and change the voltage switch mode to “4” and repeat the previous steps.

The previous steps should be equal to the measured transformer ratio with the existing nominal transformer ratio on the transformer’s nameplate. The error ratio should not exceed ± 5.0% of the nominal transformer ratio.

2. Insulation Resistance Test of Transformer

The transformer’s insulation resistance test is one of the most important transformer tests … we can perform this test at a factory or site … this test can be classified under type test, routine test, pre-commissioning and commissioning tests. Periodic maintenance transformer tests. 

Purpose of this Test:

Insulation resistance test of transformer is performed on the transformer to:

  • Verify insulation integrity due to the moisture and impurity contents of insulation.
  • Ensure that there are no leakage paths between phases or the transformer body.

Precautions Before this Test:

  • Disconnect the electrical current from the transformer and grounding it.
  • We should discharge any electrical static charge that may have been accumulated on the transformer utilizing the discharge stick or using a measuring device with the charges’ automatic discharge.
  • Clean the terminals of the transformer.

Steps of this Test:

Insulation resistance test of transformer is divided into three parts:

Step 1
  • Measure insulation resistance between low voltage windings and the main body of the transformer
  • Use the MEGGER test at a low voltage side.
  • Adjust MEGGER tester at 1000 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between another phase and transformer body as an example (a & TR body, b & TR body, c & TR body)

Insulation resistance of a new transformer should be greater than 1 Giga ohm, and the old transformer should greater than 300 Mega ohms.

Step 2
  • Measure insulation resistance between low voltage windings and high voltage windings.
  • Use MEGGER tester.
  • Adjust MEGGER tester at 2500 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between other phases as example (A & a , A & b , A & c , A & n , B & a , B & b , B & c , B & n , C & a , C & b , C & c , C & n ).

Insulation resistance at all mentioned phases should be greater than 1 GIGA ohm.

Step 3
  • Measure insulation resistance between high voltage windings and the main body of the transformer
  • Use the MEGGER test at a low voltage side.
  • Adjust MEGGER tester at 5000 V DC.
  • Record the value of insulation resistance measured by MEGGER tester after applying 1 min.
  • Repeat this test between another phase and transformer body as an example (A & TR body, B & TR body, C & TR body)

Insulation resistance of a new transformer should be greater than 1 Giga ohm, and the old transformer should greater than 300 Mega ohms.

In summary, if the transformer’s insulation resistance test between high voltage windings and the transformer body is less than 300 Mega Ohm indicates the weakness of insulation. In other words, that means contact between the transformer body and high voltage windings.

Also, measuring resistance insulation between the low voltage windings and the transformer body should have a lower resistor as 200 Mega ohms. In other words, less than that indicates the presence of contact between the transformer body and low voltage windings.

And when measuring insulation resistance between high voltage winding and low voltage winding, it should show us less resistance of 300 megaohms.

3. Induced Overvoltage Test of Transformer

An induced overvoltage test of the transformer is one of transformer testing carried out at the manufacturing factory. This test is classified under the routine test of the transformer and type test of the transformer.

Purpose of the Test:

  • To secure the insulation between the phase windings, turns, coils, tapping leads, and terminals.
  • For non-uniformity insulated windings of the insulation between these parts and earth.
  • Withstand the temporary overvoltages and switching overvoltages to which overvoltages and switching overvoltages are subject to the transformer during its lifetime.
  • Ensure that the transformer is not cut into one of the electrical supply lines. This causes the resonant effect to occur, causing high vibration voltage waves that may damage the transformer if there is weak isolation.
  • Discovering the air gaps between the layers of the windings to determine the insulation’s efficiency between these layers and other layers and the high voltage high-frequency we use will detect these air gaps.

Precautions Before this Test:

  • Disconnect the electrical current from the transformer.
  • Clean the terminals of the transformer.

Steps of this Test:

  • Apply the excitation voltage to the terminals of the low voltage winding.
  • Leave the other windings open-circuited.
  • Normally, we use a high-frequency voltage source (the test frequency is typical 180 HZ) to prevent magnetic flux saturation, and the test voltage is twice the rated voltage.
  • Select the tapping of the off-circuit tap changer.
  • So, in all windings, the voltage during the test is as near as possible to the rated test voltage.
  • We can calculate the duration of the test from this relation:
    T test = [rated frequency / test frequency] x 120 seconds = 40 s (min 15 sec, max 60 sec)
  • Repeat the test at each tap changer point of the transformer.

The test is successful if no collapse of the test occurs.

4. Transformer Winding Resistance Test

Purpose of this Test:

This test is performed on the transformer to:

  • Check the continuity of all internal connections.
  • Check loose connections of bushing or tap changer that may cause high contact resistance.

Precautions Before this Test:

  • Disconnect the transformer electrically.
  • Clean the terminals of the transformer.

Steps for this Test :

Firstly measure the resistance value of high voltage windings;

HV transformer winding resistance test

  • Measure resistance between each phase with another phase
  • Repeat this test at all tap changer points.

These values of resistances should be the same and identical with the nameplate value recorded on the transformer with an error permeability of 5% maximum. If it is higher than 5%, the transformer test fails.

Secondly, we measure the resistance value of Low voltage windings:

LV winding resistance measurement

  • Measure resistance between each phase and another phase.
  • No tap changer points at Low voltage side.

Measured values should be identical to the nameplate value of the transformer. But when measuring the resistance of low voltage winding, we do not get accurate results. This is because of the small value of resistance to low voltage winding.

In this case, we can use any resistance bridge law. For example, the famous Wheatstone Bridge, where a known R1 and R3 resistors are used to measure the resistance of an unknown Rx by changing the value of the R2 resistance until we reach the equilibrium point between the two branches.

Then the current in the Galvanometer is zero, and here we apply the following rule:

Wheatstone Bridge

We must consider the surrounding medium’s temperature for more accurate testing as the resistance value depends on the temperature. Therefore, we must correct the measured value during the test according to the following equations.

5. Short Circuit Test of Transformer

This test essential and classified under type test, routine test, and emergency test of transformer.

Purpose of the Test:

  • Determine the load losses (Pcu), copper losses, and the impedance voltage of the transformer.
  • Verify guarantees, design calculations, and manufacturing quality.
  • Determine the resistance and reactance of windings (R1, R2, X1, X2) of the transformer at primary and secondary sides.
  • Determine percentage impedance (Z%) of the transformer used at short circuit calculation at the electrical network and select the proper circuit breaker that protects the transformer.
  • Detection of deformation defects in windings due to shipping and transportation or due to internal short or faulty ground connections.
  • And it also serves in the case of an unjustified rise in temperature in the transformer where the defective phase is known. If there is a higher current than the other two phases, then the transformer is stopped and checked for the reasons.

Precautions Before this Test:

  • Disconnect the electrical current from the transformer.
  • Clean the terminals of the transformer.

Steps for this Test:

  • Inject voltage on the primary winding.
  • Control this voltage from zero until we reach the secondary winding current to equal the transformer’s full load current.
  • Take the voltmeter readings from the primary winding.
  • Divide voltmeter reading on the applied voltage and multiply by 100. We get the percentage impedance,

For example, when testing a 33/11 kV, 5 Mega volt-ampere transformer, we inject voltage at the primary side and increase this voltage.

The applied voltages reached 2280 volts to reach the rated current at another side by dividing 2280/33000 = 0.069. If the result were multiplied by 100, the output would be 6.9%, called the transformer’s percentage impedance (Z%).

We can neglect the value of core resistance, reactance, and core losses as the current that passes through the core are minimal.

So, we can represent the equivalent circuit of the transformer in this case as follows:

load losses test of transformer 3 Such as:

 

 

Acceptable error at all parameters is 10% of nameplate values, which records on the transformer nameplate.

6. Open Circuit Test of Transformer

This test is crucial, and it is considered the backbone of transformer testing. Through this test, we will calculate no-load losses of the transformer at no-load condition. This test is classified under type test and routine test. 

Purpose of this Test:

  • Determine the no-load iron losses (P0)
  • Determine the no-load current (I0) of the transformer at rated voltage and frequency.
  • Verify guaranteed values to prove the quality of the core steel and the quality of core staking.
  • Determine core resistance and reactance.

Precautions Before this Test:

  • Disconnect the electrical current from the transformer.
  • Clean the terminals of the transformer.

Steps of this Test:

No-load test of transformer 2

No-load test of transformer 3

  • Connect secondary winding with AC source 380 Volt.
  • Keep primary winding without any load (open circuit).
  • Measure voltage, current, and power at the secondary side with a voltmeter, ammeter, and watt-meter
  • The current drawn in this case (I0) should not exceed 1% of the transformer’s rated current as the transformer is opened at primary. So, we can neglect power loss on the primary side.
  • We can also neglect voltage drop that means the measured power, in this case, represent iron loss (P0) only.
  • For more accuracy, we can repeat this test 5 times at 90%, 95%, 100%, 105%, 110% of rated voltage, and at every time, we can measure voltage, current, and power at the secondary side.

If the measured values differ significantly from the nameplate values, it means that we have an iron core problem. This means there may be a short in lamination, or there are gaps between the iron core layers or protrusions in the iron core sections.

This test is also useful to see if there is an unsymmetrical structure of the iron core or not. If it is observed that the current drawn in one of the phases is very different from the other two phases, then this means there is an unsymmetrical structure of the iron core.

7. Partial Discharge Test of Transformer

It helps to detect any defect in transformer insulation. Partial discharge occurs due to the presence of moisture in liquid insulators or the presence of a cavity in insulation due to defects in the manufacture of solid insulators.

Partial discharge is, in general, a consequence of local electrical stress concentrations in the insulation or on the surface of the insulation. Generally, such discharges appear as pulses having a duration of much less than 1 micro sec.

More continuous forms can, however, occur, such as the so-called pulse-less discharges in gaseous dielectrics. This kind of discharge will normally be detected by the measurement methods described in this test.

Purpose of this Test:

  • Detect the presence of any partial discharge in the transformer’s insulators, as this could turn into a complete breakdown.
  • Verify that the transformer doesn’t exhibit a partial discharge greater than a specified magnitude.
  • Determine the voltage amplitude at which partial discharges of a specified low magnitude commence with increasing and crease with decreasing voltage.
  • To determine the magnitude of the specified discharge quantity at a specified voltage.
  • Detect the presence of any partial discharge in the cooling oil transformer as this is difficult to detect except by conducting that test.

Precautions Before this Test:

  • Disconnect the electrical current from the transformer.
  • Clean the terminals of the transformer.

Steps of this Test:

Assume that we have a transformer with some defects in its insulation. These defects, such as a gap in bushing insulation, are due to manufacturing or insulation degradation defects. The following is helpful;

  • Apply the excitation voltage to the terminals of the low voltage winding. Line-to-earth pre-stress voltage of 1.8Vm/√3 shall be induced for 30 sec).
  • Increase the voltage on the transformer gradually until the partial discharge begins to occur. Follow without interruption by the line-to-earth voltage of (1.3Vm/√3) for 3 minutes. In this case (the case of partial discharge), we call the voltage value inception voltage.
  • We will notice an increase in the value of the leakage current.
  • At this value of the inception voltage, the electric field will break the insulation value. If there is a bridge over the gap, then the total insulation value will decrease, and the current value will increase.
  • We begin to gradually reduce the voltage value until the partial discharge stops, which is called extinction.

If the extinction voltage is less than the transformer’s operating voltage, there is a real danger to the transformer, which means that the partial discharge will never stop if it happens to the transformer.

However, if the extinction voltage is higher than the transformer’s operating voltage, the partial discharge is not dangerous.

Phasor Diagram of the Transformer

This involves a transformer at no load condition and on load condition.

Let’s start with the phasor diagram of the transformer at no-load condition.

1. No-load Condition :

We already that the theory of transformer and its operation depend on mutual induction that “when the transformer is connected to the AC source, an electrical current passes in the primary winding and this current called No-load current (Io).”

This current causes production of variable magnetic flux. This magnetic flux cuts both the primary and the secondary winding and generates in each of them an opposite Electro-Motive Force (E.M.F) proportional to the number of turns and the rate of change in the flux of time.

And the No-load current divides into two compounds Ia, Im

Transformer equivalent circuit

     

Such that :

 

 

Power of the Transformer at no Load Condition:

Because the primary winding has physical resistance (R1) and inductive reactance (X1), the current of the load causes a voltage drop at the terminals of the primary winding shown in the following relationship:

Phasor diagram of transformer 1

At no-load condition, the output power of the transformer is equal to “Zero.” Therefore, the power is withdrawn from the source (input power) and is consumed in the loss of iron and copper. So, we can neglect the losses of copper because of the small primary current and the absence of current in the secondary.

  • The input power of the transformer is almost equal to the iron loss and calculated from the following relationship :

 

  • This iron loss is consumed in the resistance of the magnetic circuit Ro and shown at the following relationship :

  • Also, we can calculate reactance Xo from the following relationship :

So, we notice that the current (Io) is passed in the case of “on load ” or “without load,” and also, the iron loss is fixed as long as the transformer is connected to the rated operating voltage.

Phasor Diagram at no Load Condition:

If we assume that the voltage wave V1 is a sine wave, the current (Im) causing the magnetic flux is 90 degrees lagging, and therefore the flux is also lagging at the same angle due to the flow of current in the inductive reactance.

So,

Start with (Im & Φ) as reference

  • phasor diagram of transformer 1

The angle between Im and Ia is 90 (as Im lagging)

  • phasor diagram of transformer 2

Induced EMF at the same angle of Ia

2. On-load Condition:

When a load connects to the terminals of the secondary winding (Z2), an electrical current passes in the secondary side called the secondary winding current (I2) due to the load’s impedance. This current causes a magnetic flux in the iron core. This value of magnetic flux depends on the current of I2.

So, another counter flux must oppose this flux in the primary winding.

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