Transformer Type Tests Routine Tests and Commissioning Tests Procedure

Transformer testing is important for you, if you work with transformer as transformer manufacturer, Transformer supplier, transformer Service Company or transformer user as power consumer or transformer user as Utility Company. But, you no need to carry out all kind of transformer test; depending on your service category transformer testing procedure and transformer testing tools and equipment will be different.

Depending on the transformer test frequency, you may own transformer testing equipment or test bench and you can do test by your own arrangement. On the other hand you may take help from transformer testing companies, they have all transformer test set facilities with well-furnished testing lab. They will provide you transformer test certificate mentioning the test result after testing the transformer in their transformer test laboratory.

Why Transformer Test is required?

If you are a transformer manufacturer or transformer user, you must manufacture or purchase a predesigned and specified transformer to fulfil your client or your requirement where transformer quality test or transformer commissioning test is important. So confirming the specifications and performances of an electrical power transformer you have to go through a number of testing procedures. Transformer manufacturer will carry out some tests at a transformer manufacturer premises before delivering the transformer. Transformer user end also need some transformer test before commissioning.

Two main types of transformer testingtype test of transformer and routine test of transformer is performs in manufacturer test bench.

Type of Transformer Testing at Factory and User End

Generally factory carried out some routine test and type test carried out by third party like STL member testing company, End User normally carried out pre-commissioning and commissioning test after installation the transformer.

A. Transformer Tests done at factory

To ensure the client’s requirement transformer manufacturer need to carry out some type test, routine test and special test at factory end before delivery the transformer to client. Confirming the basic design expectation of transformer some tests are carried are done mainly in a prototype unit not in all manufactured units in a lot. Transformer Type Test confirms main and basic design criteria of a production lot. Transformer Routine Test is carried out on every unit manufactured that mainly for confirming the operational performance of the individual unit.

1.    Transformer Type tests

a)  Winding resistance test of transformer;

b)  Transformer ratio test;

c)  Transformer vector group test;

d)  Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test);

e)  Measurement of no-load loss and current (Open circuit test);

f)   Measurement of insulation resistance;

g)  Dielectric tests of transformer;

h)  Temperature rise test of transformer;

i)   Tests on on-load tap-changer;

j)   Vacuum tests on tank and radiators.

2.    Transformer Routine tests

a)  Winding resistance test of transformer;

b)  Transformer ratio test;

c)  Transformer vector group test;

d)  Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test);

e)  Measurement of no load loss and current (Open circuit test);

f)   Measurement of insulation resistance;

g)  Dielectric tests of transformer;

h)  Tests on on-load tap-changer;

i)   Oil pressure test on transformer to check against leakages past joints and gaskets.

3.    Transformer Special tests

a)   Dielectric tests;

b)   Measurement of zero-sequence impedance of three-phase transformers;

c)    Short-circuit test;

d)   Measurement of acoustic noise level;

e)    Measurement of the harmonics of the no-load current;

f)     Measurement of the power taken by the fans and oil pumps;

g)   Tests on bought out components / accessories such as buchhloz relay, temperature indicators, pressure relief devices, oil preservation system etc.


B. Transformer Tests done at site

Transformer Site Acceptance Test or SAT is test that carried out at site after installing the Transformer, actual commissioning of the transformer at the site. Before energization the transformer, pre-commissioning and commissioning test is required to confirm the performance.

1.    Pre-commissioning tests;

2.    Periodic/condition monitoring tests;

3.    Emergency tests.

Winding Resistance Test of Transformer

The resistance of each winding, the terminals between which it measured and the temperature of the windings shell be recorded. 

Direct current shell be used for the measurement. In all resistance measurements care shall be taken that self-inductive effects are minimized. 

During that cold resistance measurements, the time for measuring current to become steady should be noted and use for guidance when making hot resistance measurement following a temperature rise type test.

Transformer winding resistance measurement is carried out as a type test, routine test and also as a field test.

Transformer winding resistance measurement helps in determining the following:
  1. Calculation of the I2R losses.
  2. Calculation of winding temperature at the end of temperature rise test of transformer.
  3. As a benchmark for assessing possible damages in the field
Transformer Winding Resistance Test

Principles of Operation 
The basic idea is to inject a DC current through the winding to be measured, and then read the voltage drop across that winding. 

Electrical Testing Instruments applies the dc current through the winding and an internal standard current shunt. After both DC voltage drops are measured they are ratioed and the display is read as resistance on the front panel meter. This method allows for the lead resistance to be omitted since the reading is independent of the current. In addition, no multiplication factors will be needed when changing current ranges. 

The DC current source must be extremely stable. Refer to formula for DC voltage across a transformer below: 

v = I * R + (L di/dt) 
vdc = voltage across transformer winding 
I = DC current through transformer winding 
R = resistance of the transformer winding 
L = inductance of the transformer winding 
di/dt = changing value of current (ripple) 

Assume that the tester has a very stable current source (i.e., no ripple), then di/dt is zero and the term L di/dt becomes zero.

Transformer Ratio Test

First of all isolate all equipments, connect groundings to all incoming and outgoing cables and disconnect all incoming and outgoing cables from the transformer bushing terminals connections.

Connect the H designated three-phase test lead with the military style connector at one end to the mating connection on the test set marked with an H. Ensure that the connector’s index notch lines up properly.

Connect the X designated three-phase test of lead military style connector at one end to the mating connection on the test set marked with an X. Ensure that the connector’s index notch lines up properly.

Transformer Turn Ratio Test

Connect the H1, H2, H3 designated test lead to the corresponding H1, H2, H3 transformer terminal / bushing. Connect the H0 test lead if H0 terminal/bushing is present.

Connect the X1, X2, X3 designated test leads to the corresponding X1, X2,X3 transformer terminals / bushings. Connect the X0 test lead if X0 terminal/bushing is present.

Perform turns ratio measurements for all tap positions.
Confirm that the measured ratios is within 0.5% of the calculated ratios.

Transformer Vector Group Test

Connect neutral point of star connected winding with earth. Join 1U of HV and 2W of LV together. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral.

Transformer Vector Group Test

Let’s consider a YNd11 transformer.
1) Connect neutral point of star connected winding with earth. 
2) Join 1U of HV and 2W of LV together. 
3) Apply 415V, three phase supply to HV terminals. 
4) Measure voltages between terminals 2U – 1N, 2V – 1N, 2W – 1N, that means voltages between each LV terminal and HV neutral. 
5) Also measure voltages between terminals 2V – 1V, 2W – 1W and 2V – 1W. For YNd11 transformer, we will find, 2U – 1N > 2V – 1N > 2W – 1N 2V – 1W > 2V – 1V or 2W – 1W 

The vector group test of transformer for other group can also be done in similar way.

Transformer Short Circuit and Open Circuit Test

Transformer Short Circuit Test

Transformer short circuit and open circuit test is very much common to everyone who jobs to related transformer. Allmost every transformer repearing  workshop have transformer testing for short circuit and transformer testing for open circuit. 

The connection diagram for transformer short circuit test diagram on the transformer is shown in the common figure above. 

Required tools for transformer short circuit:
  1. A voltmeter, 
  2. wattmeter,
  3. an ammeter.
Above meters are connected in HV side of the transformer as shown in diagram. Around 5-10% low voltage is applied to that HV side using a variac (i.e. a variable ratio auto transformer). 

LV side of the transformer is shorted using a wire. Now from the variac applied voltage is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the HV side.

After reaching HV side rated value, record Voltmeter, Ammeter and Watt-meter readings. The ammeter reading gives the primary equivalent of full load current IL. As the voltage applied for full load current in a short circuit test on the transformer is quite small compared to the rated primary voltage of the transformer, the core losses is negligible here.

short circuit test example
Let’s consider, 
Ammeter reading IL, is short circuit current;
voltmeter reading is Vsc, is short circuit voltage;
wattmeter reading is Psc, input power during the test.

 Let us consider .
Transformer Short Circuit and Open Circuit Test Formula
Where, Re is equivalent resistance of transformer.

If, Ze is equivalent impedance of transformer.
Transformer Short Circuit and Open Circuit Test Formula

Therefore, if equivalent reactance of transformer is Xe.
Transformer Short Circuit and Open Circuit Test Formula

Though the test is conducted at high voltage end, so these values are referred to the HV end of the transformer. to get the valu of low voltage end just divide these values with the square of transformation ratio.

The short-circuit test of a transformer actually determine the copper losses in the transformer at full load that can use to find the parameters to approximate the equivalent circuit of a transformer.

Ttransformer Open Circuit Test 

Similar to short circuit test of transformer, open circuit testing of trasformer also important and very much common for transformer users. 

The connection diagram for transformer open circuit testing is shown in the common figure above. 

Testing tools for transformer open circuit tes
  1. A voltmeter; 
  2. A wattmeter; & 
  3. An ammeter.
The avove meeters  are connected in LV side of the transformer as connection diagram above. The voltage at rated frequency is applied to that LV side using a variac that a variable ratio auto transformer and HV side of the transformer is kept open. 

Now apply rated voltage of the LV side using variac and record Voltmeter, Ammeter and Wattmeter readings.

open circuit test
The ammeter reading gives the no load current Ie. 
Ie is quite small so voltage drops due to this current that can be taken as negligible.

Since voltmeter reading V1, is secondary induced voltage of the transformer, 

Wattmeter reading Po indicates during no load condition. 

Hence, can take the wattmeter reading as equal to the core losses in the transformer.

Let us consider wattmeter reading is Po.
Transformer Short Circuit and Open Circuit Test Formula

Where, Rm is shunt branch resistance of transformer.

If, Zm is shunt branch impedance of transformer.
Transformer Short Circuit and Open Circuit Test Formula

Therefore, if shunt branch reactance of transformer is Xm,
Transformer Short Circuit and Open Circuit Test Formula

These values are referred to the LV side of the transformer due to the tests being conducted on the LV side of transformer. These values could easily be referred to HV side by multiplying these values with square of transformation ratio.

Therefore it is seen that the open circuit test on transformer is used to determine core losses in transformer and parameters of the shunt branch of the equivalent circuit of the transformer.

Measurement of insulation resistance of Transformer
Power and Distribution Transformer Test Laboratory frequently use the transformer IR test or Insulation Resistance test. It is very simple as like as other electrical circuit or equipment's IR test.

Transformer insulation resistance test or IR test carry out simply using Insulation Resistance tester or Megger (Megha Ohm measuring tools).

Procedure of Insulation Resistance Test of Transformer

Measures the insulation resistance of HV & LV winding with respect to earth (body) and between LV & HV winding. Recommended Values are 2000 Mohms for HV and 500 Mohms for LV:

Transformer IR value calculating formula considering phase:
1 Phase TransformerIR Value (MΩ) = C X E / (√KVA)
3 Phase Transformer (Star)IR Value (MΩ) = C X E (P-n) / (√KVA)
3 Phase Transformer (Delta)IR Value (MΩ) = C X E (P-P) / (√KVA)
Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for Oil filled T/C without Oil Tank or Dry Type T/C.

Transformer IR value calculating formula considering rated voltage:
VoltageTest Voltage (DC)  LV sideTest  Voltage (DC) HV sideMin IR Value
Up to 6.6KV500V2.5KV200MΩ
6.6KV to 11KV500V2.5KV400MΩ
11KV to 33KV1000V5KV500MΩ
33KV to 66KV1000V5KV600MΩ
66KV to 132KV1000V5KV600MΩ
132KV to 220KV1000V5KV650MΩ
  1. First disconnect all the line and neutral terminals of the transformer;
  2. Megger leads to be connected to LV and HV bushing studs to measure insulation resistance IR value in between the LV and HV windings;

  3. Megger leads to be connected to HV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the HV windings and earth;

  4. Megger leads to be connected to LV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the LV windings and earth.

Dielectric tests of power transformer

Transformer dielectric test is generally performed in two different steps:
  1. Separate source voltage withstand test;&
  2. Induced voltage withstand test. 
Separate Source Voltage Withstand Test of Transformer
This Transformer main insulation to earth and between winding is ensured to performing the separate source withstand test of transformer. The procedure of separate source voltage test is as below:
  1. To perform this test connect all of three line terminals of the winding together;
  2. Other nontesting winding terminals and tank of the transformer should be connected to earth;
  3. The transformer testing terminal winding is connected for around 60 second to a single-phase power frequency voltage that shape approximately sinusoidal;
  4. One by one perform the test for all windings;
  5. The test is successful if no break down.

Induced voltage test of transformer

You may check the inter turn and line end insulation as well as main insulation to earth and between windings using the method of insulated voltage test of a transformer. The procedure of induced voltage test of transformer is as bew:

  1. Keep the primary winding of transformer open;
  2. Three phase voltage havingtwice of rated voltage of secondary winding in magnitude and frequency should apply to the secondary winding;
  3. The duration of the test shall be 60 second;
  4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to desired value.

The value of transformer test voltage is as below:

Nominal system
voltage rating
for equipment
Highest system
voltage rating
for equipment
Rated short duration
power frequency 
withstand voltage
        415V  1.1 KV   3 KV
 11 KV12 KV28 KV
 33 KV36 KV  70 KV
132 KV 145 KV  230 / 275 KV
220 KV  245 KV360 / 395 KV 
400 KV 420 KV570 / 630 KV 

Temperature rise test of transformer

Transformer temperature rise test is normally included in type test of transformer. The voltage is applied to the HV winding of the transformer that power input is equal to no load losses plus load losses corrected to a reference temperature of 75oC.

Factors Affecting Winding Temperature Rise:

  1. Winding resistance measuring equipment;
  2. Ambient temperature determination;
  3. Inadequate calculation of the average oil temperature (leads to wrong factor);
  4. Accuracy of Load Loss measurement;
  5. Assumed total loss as sum of NL + LL;
  6. Effect of the Core temperature dynamics;
  7. Cold resistance measurement errors;
  8. Not reaching steady state before shutdown.
Transformer Test on ON-LOAD tap-changer
The purpose of a tap changer in power transformer is to regulate the output voltage. The working principle of a tap-changer just altering the number of turns transformer one winding and thereby changing the transformer turns ratio. 

Transformer tap changing actually two types: 
  1. an on-load tap changer in short OLTC;&
  2. a deenergised tap changer in short DETC.
A low (DC) test current through the on-load tap changer is used during dynamic resistance measurements. This method is in particular sensitive to maintenance errors, contact degradation and contact timing problems. In general, distinction can be made between defects that interrupt the current through the OLTC, defects that affect the timing of the OLTC contacts and contact degradation resulting in abnormal contact resistance.

Usually, winding resistance measurements are performed on offline power transformers. In case of on-load tap changer, which permits tap changing and hence regulating voltage without interrupting the load current, a dynamic resistance measurement can be performed during the operation of OLTC to find degradation and defects inside the OLTC.

Vacuum test on Tank and Radiators of Transformer

Transformer vacuum test actually done for ensuring the transformer oil performance. If we not doing vacuuming, it may create bubble in oil which is very dangerous for Transformer operation at high voltage. If bubbles form in oil then dielectric strength of transformer oil will be reduce. It results in partial breakdown of an insulating material which finally causes breakdown of whole oil insulation. Regarding transformer oil, you may read the separate article on What is Transformer Oil and Transformer Oil Insulating Properties.
Vacuuming Procedure:
  1. When transformer is to be filled or topped off in the field, they must be filled under vacuum according to specific instruction;
  2. Transformer should not be left under vacuum except during the vacuum filling operation;
  3. Leaks in the temporary piping and connections lead to danger of drawing moisture into the tank if it is under vacuum during periods of high humidity or during rain;
  4. It is recommended that the tank be under positive pressure during rain to prevent drawing moisture in to the tank.
Transformer Vacuume Type Test:
 3 ¢ Transformers Upto 200 kVA 250mm of Hg. for 30 minutes
 3 ¢ Transformers >200kVA &
upto 2500kVA
 500mm of Hg. For 30 minutes

Oil pressure test on transformer to check against leakages past joints and gaskets.

Oil pressure test on transformer to check against leakages past joints and gaskets. That means Routine tests of transformer include all the type tests except temperature rise and vacuum tests. The oil pressure test on transformer to check against leakages past joints and gaskets is included.
Oil Leakage Type test
  • Non-sealed & sealed Type Transformers: Pressure of Twice the normal head measured at base of tank for 8 hours 
  • Tank with corrugations: Pressure of 15kPa measured at top of the tank for 6 hours.
Transformer oil leaking is dangerous for equipment as well as envirnment getting the Leaking oil onto the ground and into groundwater.  It can be expensive to remedy, Transformer leaking oil can carry lead from transformer paint into the ground causing further contamination.

Measurement of zero-sequence impedance of three-phase transformer

The zero-sequence impedance is usually measured for all star-connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallell connected phase terminals and the neutral terminal.
zero-sequence impedance of three-phase transformer

1st connect all phases in parallel (see C B A), and connected to AC Generator(single phase GS or G1). 

2nd neutral of Transformer(T1) is connected to return or neutral path of the generator 

3rd place 1 CT(T3) in series with the parallel connected CBA line 

4th connect Ammeter(P3) at secondary of series CT(T3)

5th connect 1 PT(T2) across CBA and Neutral path of Transformer , and connect voltmeter(P2) in secondary of this PT

7th, winding not under test remains open (see 2C,2B,2A, and 2N)

8th Apply current with frequency , now the question is how much current we need to employee, for that purpose you must have a knowledge of neutral current, because you need to inject Io=3 In, but it is very dangerous to that amount of current in the transformer as the temperature of the excessive metallic part will increase, so what engineers will do, he just insert the percentage of current, and then calculate the value by extrapolation 

We can calculate impedance after taking reading from Ammeter and Voltmeter or our test kit process calculation of zero impedance by below mention series of formulae 



In we will found from Ammeter, and Vn will be found from Potential or voltmeter. From these values, we will found Vo and Io, and zero sequence impedance will be found by below formula, 

Vo =-Io*Zo


In star connection, Zo reading is in the percentage of around 30% to 60% of rated phase impedance. Test findings will be compared with data or rating as stated in above mention statement.

Measurement of acoustic noise level of a Transformer

The Transformer noise level is measured basically using two methods:

  1. sound pressure measurement;&
  2. sound intensity measurement.

There are three physical phenomena that produce noise in the magnetic core :

1. The movement of the 90-degree Bloch walls inside the magnetic domains, frequently called Magnetoacoustic
Emission (MAE); see Figure 1.
2. The rotation of the magnetic domains, that is responsible for the bulk magnetostriction; see Figure 2.
3. The Lorentz Force Acoustic Signal (LFAS) causing mechanical forces between laminations of the core; see Figure 3.

Measurement of acoustic noise level of a Transformer

Sound pressure method:

The intent of this test is to report the spatially averaged total A-weighted sound pressure level for each power option accompanied by a single spatially averaged frequency spectrum. The same test procedure applies, either a walkthrough or point-by-point procedure, for both background noise measurements and test measurements. A spatially averaged total background noise level and the corresponding frequency spectrum shall be recorded immediately before and after each test measurement sequence. If the background noise level is at least 10 dB lower than that of the test object, then the background noise can be measured at a single location on the prescribed contour and no background noise correction is necessary.

Mehod sound intensity:

The noise level is commonly measured in decibels (dB) by comparing the pressure generated by a noise source with some standard level. The noise level is basically measured by two methods, the sound pressure measurement and the sound intensity measurement. The noise level measurements in CPRI are carried out using the sound pressure method. The test methods and the acceptable conditions of the test environment are specified in IEC: 60076-10. The methods are applicable to transformers, reactors and their cooling devices as well. The sound pressure level is a scalar quantity and requires simple instrumentation. Sound intensity is a vector quantity and the method measures directional sound. Therefore, it is less affected by background noise. Therefore, the sound intensity method can provide more accurate measurements in the presence of background noise.

Measurement of the harmonics of  a Transformer at no-load current

Traditionally, transformer losses are divided into no load losses and load losses as described earlier. The same loss grouping is retained when considering the influence of power system harmonics.

Measurement of the power taken by the fans and oil pumps of a Transformer

Large power transformers have auxiliary loads such as cooling fans, oil pumps, motorized tap-changers, cabinet heaters, and lights, etc. The amount of power of these auxiliaries. The manufacturer measures the loads consumed at the factory and is provided by the transformer test report.

The aux. Loads can also be measured while the transformer is in service using simple hand-held power meters. For each aux. load, measure its voltage and current while operating, and then calculate the apparent power of that load.

Transformer Oil pumps

If the transformer has oil pumps, check the flow indicators and the pump.
isolation valves to ensure oil is circulating properly. Pump motors can also have reverse rotation, and flow indicators can shows that the oil is flowing. To ensure that the motors are working proper direction, use an ammeter to check the motor current. Compare results at full load current indicated on the motor nameplate. 

If the motor is inverted, the current will be much less than the nameplate full charge current. Check the oil pumps with a vibration analyzer if develop unusual noises Have the DGA lab check for dissolved metals into the oil and run a metal particle count for metals if the bearings are suspect. This should be done as soon as a bearing becomes suspect; bad oil pump bearings can put enough metal particles in the oil to threaten the insulation of the transformer and cause electric shock within the tank, resulting in explosive catastrophic transformer failure tank.

Transformer Fans and Radiators

Inspect all isolation valves on top and bottom of radiators for make sure they are open. Inspect cooling fans and radiators for cleaning and fans for proper rotation. Check if it is dirty or damaged partially blocked fan blades or radiators. Fans are much more efficient if the blades are clean and turning in cold air. Normally fans blow cold air through radiators; They should not be going through the air. Check if the fans are electrically reversed (i.e. pulling the air first) through the radiators and then through the fan blades). This means that the blades rotate in hot air after passing through the radiator, which is much less efficient. Place one hand on the radiator. in front of the fans; air should come out of the radiator against your hand. Watch the blades as they rotate slowly when they are starting or stop to determine which direction they should be turning and correct
Rotation if necessary.

Tests on bought out components / accessories such as buchhloz relay, temperature indicators, pressure relief devices, oil preservation system etc.

Visually check all others component and devices after installation transformer. you may read the separate article about buchhloz relay on Buchholz Relay for Power Transformer Protection Device

Pre-commissioning tests of Transformer

After installation on site, the transformer also goes through other tests, performed on it, before actual commissioning of the transformer on site. The transformer test performed before the transformer is commissioned on site is called the transformer pre-commissioning test. These tests are performed to assess the condition of the transformer after installation and to compare the results of all low voltage tests with the factory test reports.

Record of observations from general physical inspection/check:

re-commissioning tests of Transformer

Periodic/condition monitoring tests of Transformer

The test conducted for analysis of the transformer insulation condition by examining the transformer oil is called the DGA Test. DGA test must be conducted periodically to ensure the healthiness of the transformer for trouble-free operation of the transformer.

Condition monitoring shows the existing condition and the remaining life of the transformer. Analysis of dissolved gases provides a clearer picture of the health of transformer oil and paper insulation. The test performed to analyze the transformer insulation condition when examining the transformer oil is called DGA Test.

The DGA test should be performed periodically to ensure the health of the transformer for trouble free operation of the transformer. The DGA results indicate the healthiness of the transformer insulation system.

Transformer Emergency tests or Online Predictive Maintenance

Online predictive maintenance services can be performed while your system remains energized, eliminating costly shutdowns. By performing the Visual Inspections, Infrared Testing, Ultrasonic Detection can help you identify when a failure may occur so you can plan an outage instead of dealing with an emergency.

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