How to Test Transformer and Types of Electrical Transformer Testing


How to test transformer and types of electrical transformer testing.

There are many ways of the testing transformer. Types of transformer tests are including such as megger transformer tests, winding resistance transformer test, ratio transformer test, no load loss transformer test, full load transformer test, impedance transformer test, continuity transformer, insulation resistance transformer test, polarity transformer test.

The above equation shows that the eddy current loss depends on the flux density, frequency and thickness of the material and inversely proportional to the resistivity of the material.

the total load current is measured by the meter connected across their high voltage winding

A number of tests are required to physically determine the electrical characteristics of power and distribution transformers. Many of those tests are indicated below:

In “REB” workshop has done 9 tests for a transformer those are:

There are many Testing Transformer 
  • Megger test (Insulation resistance test)
  • Winding resistance test
  • Ratio test
  • No-load loss test
  • Full load loss test
  • Impedance test
  • Continuity test
  • Insulation resistance test
  • Polarity test

Megger Test (Insulation Resistance Test)

I use a Megger meter to measure the insulation resistance. Its range is 0 to 1000+. Megger meter has two terminals. At the starting of Megger testing, I connect two terminals to the body (short) to set the Megger pointer at “0” position. Then I connect the body to the HV side. 1000. Then I connect one terminal to the HT side another terminal to the LT side. Megger shows 1000. If any fault occurs, then in Megger meter not show 1000. 1000 means that insulation resistance property is Ok of the transformer. Then I record Megger readings on

Table: Megger readings

Measure from high to low voltage winding HL-G HT+GROUND   LT+EARTH     HT+LT     Remark
01 1000+ 1000+ 1000+ Ok

Impedance Test

For the impedance test, I need max voltage in HV and the minimum that voltage (HT) which can supply full load current on LT side. The 100 kVA transformer is rated at 11 kV. So, 11000 V is the maximum voltage. By that given bellow equation I can test impedance. So, the impedance is:

                                                  * 100 %

Full Load Loss Test

A full load loss check is employed to work out copper loss in the electrical devices at full load. Full load loss check is additionally called short check on the electrical device. The check is conducted on the high voltage (HV) facet of the electrical device wherever the low voltage (LV) facet or the secondary is the brief circuit. For this check, we want a watt meter, 3 section voltage offer, clamp meter and wire for brief LT. After shorted the LT terminal, we tend to connect the HT 3 terminals with three section offer than the faucet changer position is in three. when connecting HT terminal with three section offer, then we tend to activate the three-phase voltage offer regulator to provide 3 section voltage in HT terminal. the availability voltage needed to flow into rated current through the electrical device. I applied three section voltage 399 V (red), four hundred V (yellow), 393 V (blue) in the HT terminal for one hundred kVA electrical device. I actually have to live full load current in 3 terminals in the LT facet. AN engineer is measures full load current with a clamp meter. For one hundred kVA electrical device we tend to measured current ar 259 A, 260 A and 254 A in the LT terminal. The wattmeter is connected with the HT facet. The core losses are terribly tiny as a result of the applied voltage is barely many percentages of the nominal voltage and therefore may be neglected. Thus, the wattmeter reading measures solely the complete load copper loss. For one hundred kVA electrical device, full load loss is shown in 310 W in watt meter. The watt meter issue is two. So, I actually have to multiply the wattmeter reading with two. For one hundred kVA electrical device full load loss is 310/2 = a hundred and fifty-five W. when measuring full load loss, recorded in the check result type. Table 5.2: Full Load Loss Test

SL Transformer SI number kVA rating Full load loss Amp (Watt)
1 2017-10-48770 100 150
2 2017-10-48550 100 155
3 2017-10-48796 100 152
4 2017-10-49163 100 148

The purpose of the short check is to work out the copper loss occurs on the total load.

The equivalent resistance, impedance, and discharge electrical phenomenon are best-known by the short check.

in brief circuit check or full Load check, we’d short the secondary winding of the electrical device.

The copper loss is employed for locating the potency of the electrical device.

The short check is performed on the high voltage winding of the electrical device.

the total load current is measured by the meter connected across their high voltage winding

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Figure Full Load Test or Short Circuit Test

Table Full Load Test Data for Distribution Transformer

Transformer kVA rating Full- load loss (W) Impedance Voltage (V)
10 90.7 163
10 92.1 168
10 90.3 165
10 91.4 166

No-Load Loss Test

No load check is additionally referred to as circuit check. No load loss check is employed to see core loss in the electrical devices. No load losses consisting of the physical phenomenon loss and also the eddy current loss. The voltage at fifty cycles per second frequency is applied thereto fifty-five aspect with the assistance of three-part voltage offer regulator. The HV aspect of the electrical device is unbroken open. For this check, we’d like a watt meter, 3-part voltage offer and clamp meter. For checking the no load test initially, we have a tendency to connect 3-part offer to LT terminals. At this check LT neutral terminal & HT terminals ar unbroken open. once the applied voltage is traditional voltage then traditional flux are found out. because the Iron loss may be a perform of applied voltage, the traditional iron loss can occur. therefore, the iron loss is most at rated voltage. Then we have a tendency to applied 403 V (red), 402 V (yellow), 395 V (blue) rated voltage in the LT terminal with the assistance of 3 part voltage for measurement the no load current, we have a tendency to hold the clamp meter within the three-phase wire in LT aspect. The holding method of the clamp meter has shown in figure six.1.9. For ten kVA we have a tendency to live the no load current in LT terminal is six.69 A, 6.51 A and 4.88 A. A wattmeter is connected with the LT aspect. This most iron loss is measured victimization the wattmeter. This check solely measures the combined iron losses consisting of the hysteric’s loss and also the eddy current loss. though the hysterics loss is a smaller amount than the eddy current loss, it’s not negligible. Since the HT terminals of the electrical device are open, the LT terminals attract solely no-load current, which is able to have some copper loss. This no-load current is extremely little and since the copper loss within the LT is proportional to the sq. of this current, it’s negligible. there’s no copper loss within the HT as a result of there’s no HT current. For one hundred kVA electrical device no load loss shown ten.5 W in watt meter. The watt meter issue is a pair of. So, we’ve got to multiply the wattmeter reading with a pair of. For one hundred kVA electrical device no load loss is a pair of *10.5 = twenty-one watt. Then no load check result’s recorded in check result kind. The recorded in check result kind for one hundred kVA electrical device check listed below:

                                                         Table: No Load Loss Test

SL Transformer serial no kVA rating No load loss test (watt)
1 2017-10-48770 100 21.70
2 2017-10-48550 100 22.20
3 2017-10-48796 100 22.50
4 2017-10-49163 100 22.60

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Figure : No Load Loss Test

Table:  No Load Test

High volt side (V) Line current (A) Three phase power (watt)
               403 6.69 A  
402 6.51 A 300*2*2
395 4.88 A  

 The purpose of the no load takes a look at is to work out the no load current and losses of the electrical device, that is verify their no-load parameter. within the no load take a look at or circuit take a look at we have a tendency to open the first sides of the electrical device. then we have a tendency to area unit offer 240 V within the secondary aspect. A meter, meter and a wattmeter area unit connected with the within the secondary aspect. The reading of the wattmeter solely represents the core and iron losses.

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Figure No Load Test

Table : No Load Test Data for Distribution Transformer

Transformer kVA rating No load current (A) No load loss (W)
10 0.13 12.3
10 0.17 11.3
10 0.14 12.2
10 0.16 11.9


Winding Resistance Test

For measure winding resistance take a look at solely, a multimeter is wanting. In winding resistance take a look at LT facet affiliation ought to be open. no voltage supply is needed. Winding resistance solely measured in HT facet. initially 2 terminals of multi connected to the HT A and B terminal. Then activate the multimeter and set it pointer in ohm for measure resistance. stay up for few second. Result can show in millimeters show. once measure resistance in HT, A, I live HT B and C terminal winding resistance. Then output results shown in multimeter.  Then we tend to live HT A and C terminal. And last once measure the winding resistance HT A and B, B and C, C and A terminal in a hundred kVA electrical device.

Table : Winding resistance measurement

Terminal-1 Terminal-2 Resistance (Ω)
A B 12.4 Ω
B C 12.4 Ω
C A 12.4 Ω

Ratio Test

The performance of a electrical device for the most part depends upon perfection of specific turns or voltage quantitative relation of electrical device. So, electrical device quantitative relation takes a look at is an important take a look at of electrical device. The voltage ought to be applied solely within the high voltage winding so as to avoid unsafe voltage. {to take a look at|to check} electrical device quantitative relation test we want three-part offer, multimeter and Clamp meter. three-part offer is provided in HT terminal. By multimeter we tend to live output voltage. And Clamp meter for measure current. First, the faucet changer of electrical device is unbroken within the position 3 (3) and cardinal terminals ar unbroken open. Then apply three-part offer on HV terminals that shown in figr1. live the voltages applied on every part (Phase-Phase) on HV and induced voltages at cardinal terminals at the same time. three-part voltage offers regulators. From that regulator we tend to offer three-part voltage to the HV terminal. I will increase or decrease voltage by regulator switch. Red, Yellow, blue color indicates three-part voltage. By regulator switch I increase voltage. In HT terminals three-part voltage ar 402 V (red), 404 V (yellow), and 411 V (blue).

The procedure of transformer ratio test is simple. We just apply 240 AC supply to the primary winding and took the value from secondary winding with the help of voltmeter. But the ratio test machine is converting the voltage into turn ratio and show the ratio value the formula of turn ratio is


For a single-phase distribution transformer, the ideal ratio is 26.30 to 26.50. This ratio is considered as ideal for a single-phase distribution transformer where Primary voltage is 6350 V and secondary voltage is 240 V.

                                    Table Ratio test Data for Distribution Transformer

Transformer kVA rating Voltage Ratio
10 26.473
10 26.469
10 26.482
10 26.457
10 26.489

Transformer Oil Test

For transformer oil test I have a machine which called ASTM D877. The full from of ASTM is American Standard Testing Machine. The whole test took only 5 min itself. It gives five results and end of all it also gives an average result. In 220 V and 50 HZ the machine starts, it is the rated value for that machine. Inside the machine there is there is two cathode rods. The gap is this two-cathode rod is 2.5. If the gap is more or less than the rated gap, the result will not may come appropriately.  They picked a sample of oil and put inside the cathode rod. Start the machine for 5 mines. And in this five min it gives us five results. The result of oil test is 18.5,24.5. This oil was from an old transformer so expect result was above 20. As the result is 24.5, I can say that the oil is fine. If I test new transformer oil the result needs to come above 30

Table : Oil Test Measurement

Number of tests Result
1st 21.4 kV
2nd 23.5 kV
3rd 24.7 kV
4th 27.4 kV
5th 27.8 kV

Continuity Test of transformer

The purpose of this test is to Checking the continuity both primary windings and secondary windings of a transformer by megger. For this test, a megger is connected across with primary and secondary windings of the transformer. If the megger gives very low Mega Ohm resistance this, mean that the winding wire is perfect (or have continuity). Otherwise, it is not perfect (or has discontinuity).

Table Continuity Test Data for Distribution Transformer

Transformer kVA rating Winding Resistance

(HT-HT) M Ohm

Winding Resistance

(LT-LT) M Ohm

10 0 0
10 0 0

From the above data table, we can say that there is no breakdown in the transformer coil.


Insulation Resistance Test of transformer

It is commonly known as the Megger test. This test is ensuring the quality of all insulation systems within a transformer. This is the resistance measurement of the windings by applying megger between the primary winding and secondary winding.

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Figure Insulation Resistance Test 

The tank and core of a transformer should always be grounded when this test is performed. Resistances are then measured between each winding and all other windings and ground. If the megger gives the resistance 1000 plus Ohm then insulation resistance is perfect otherwise, they have to change the insulation.

Table Insulation Resistance Test Data for Distribution Transformer

Transformer kVA Rating Insulation Resistance test Measured Value

M Ohm

10 HT-LT 6000
10 HT-Body 6000

From the above data table, we can say that, the insulation of the transformer coil is satisfactory.

Polarity Test of transformer

This test is needed for identifying a transformer either additive or subtractive. When voltage is applied between the primary bushings and the resultant voltage between the secondary bushings is greater, and then it means that the transformer has additive polarity. When voltage is applied between the primary bushings and the resultant voltage between the secondary bushings is lesser and then it means that the transformer has subtractive polarity.

Table Polarity Test Data for Distribution Transformer

Transformer kVA rating Supply voltage Induced


Measuring voltage Polarity
10 236 V 8.9 V 245 V Additive
10 237 V 8.9 V 243 V Additive
10 235 V 8.9 V 227 V Subtractive
10 235 V 8.9 V 243 V Additive

 In PBS system for distribution transformer they make the transformers which have additive polarity and for power transformer they make it as subtractive polarity.

How can you tell if a transformer is bad?

If transformer test result show error or failed. That’s way we say that transformer is bad.

Why transformer testing is required?

Transformer testing is required because we make sure that transformer condition is good or bad that’s way transformer testing is required.

Can a transformer get weak?

In the transformer get weak because when oil and some material are not work and transformer test result show error, in those purpose transformers get weak.

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