7 Common Transformer Testing Procedures Explained

Transformer Testing

What is a Power Transformer?

A power transformer is a type of electrical transformer used to transfer electrical energy between two or more circuits through electromagnetic induction. It is primarily used in high-voltage transmission networks to step up (increase) or step down (decrease) voltage levels for efficient power transmission over long distances. As there is no rotating or moving parts, Transformer operates on an AC supply. Transformers operate based on the principle of mutual induction.

 Uses of Power Transformer

1. Voltage Step-up: At power plants, it increases voltage for transmission to reduce power loss.
2. Voltage Step-down: At substations, it decreases voltage for safe distribution to consumers.
3. Electric Grid Efficiency: Maintains voltage levels across the grid for stable operation.
4. Interconnection of Systems: Connects different parts of the power system with varying voltage levels.

Generation of electrical power in low voltage level is very much cost effective. Theoretically, this low voltage level power can be transmitted to the receiving end. This low voltage power if transmitted results in greater line current which indeed causes more line losses.

But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in Ohmic or I² R losses in the system, reduction in cross-sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission.

This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. Electrical power transformer thus plays a vital role in power transmission.

Two winding transformers are generally used where ratio of high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2.

Again a single unit three phase transformer is more cost effective than a bank of three single phase transformers unit in a three phase system. But a single three phase transformer unit is a bit difficult to transport and have to be removed from service entirely if one of the phase winding breaks down

 

 Difference between Power Transformer and Distribution Transformer

Feature	Power Transformer	Distribution Transformer
Usage	Used in transmission networks	Used in distribution networks
Voltage Level	High voltage (33 kV to 400 kv)	Low to medium voltage (230 V to 33 kv)
Power Rating	High (above 200 MVA typically)	Low to medium (up to 200 MVA or less)
Operation	Operates at full load most of the time	Operates at partial load much of the time
Efficiency Focus	Focus on maximum efficiency at full load	Focus on efficiency at light loads
Size	Larger and heavier	Smaller and more compact
Location	Installed at generation and transmission points	Installed near end-users (homes, offices, etc.)
Cooling System	More advanced (oil cooling, fans, radiators)	Simpler cooling (oil or air-cooled)
Efficiency formula	Efficiency is measured as the ratio of output to the input power	Here All Day Efficiency is considered. It is the ratio of output in kilowatt hour (kWh) or watt hour (Wh) to the input in kWh or Wh of a transformer over 24 hours
Losses	Copper and iron losses take place throughout the day	Iron losses take place for 24 hours and copper losses are based on load cycle
Load fluctuation	In power transformer the load fluctuations are very less	Load fluctuations are very high
Operating condition	Always operated at full load	Operated at load less than full load as load cycle fluctuates
Considering time	It is independent of time	It is independent of time
Flux density	In power transformer flux density is higher	As compared to power transformer the flux density is lower in distribution transformer

Transformer Testing :

Transformer testing is the process of verifying the operational integrity, performance, and safety of a transformer. These tests are essential before commissioning, during maintenance and after repairs to ensure reliability, efficiency and compliance with standards like IEC, IEEE, ANSI.

For confirming the specifications and performances of an electrical power transformer it has to go through a number of testing procedures. Some tests are done at a transformer manufacturer premises before delivering the transformer.

Tests done at factory

1. Type tests
2. Routine tests
3. Special tests

Tests done at site

1. Pre-commissioning tests
2. Periodic/condition monitoring tests
3. Emergency tests

 Types of Transformer Testing:

Transformer tests are generally categorized into two types:

1. Routine Tests (as per standards done on every transformer) Performed to verify the operational performance of each transformer unit before delivery.
2. Type Tests (done on one unit of a design/type)

Routine Tests:

Test Name	Purpose
Insulation Resistance Test	Measures resistance between windings and ground to ensure insulation health.
Turns Ratio Test (TTR)	Verifies the correct transformation ratio of primary to secondary windings.
Polarity & Phase Relation	Ensures correct winding orientation and phase displacement.
Winding Resistance Test	Checks winding integrity and connection quality
No-load Loss & Current Test	Measures core loss and magnetizing current under no load.
Short-circuit (Load) Loss Test	Measures copper losses under load
Dielectric Tests	Verifies the dielectric strength of insulation (power frequency, induced over-voltage tests)

2. Type Tests (done on one unit of a design/type):

Type test of transformer confirms main and basic design criteria of a production lot.

Performed to prove the design and construction meets standard requirements.

Test Name	Purpose
Temperature Rise Test	Checks the transformer’s cooling performance under load.
Impulse Voltage Test	Ensures insulation can withstand high transient voltages (lightning surges).
Noise Level Measurement	Measures acoustic noise from the transformer.
Mechanical Strength Test**	Verifies structural integrity during transport and short circuits.

 

3. Special Tests (as per customer requirement):

• Partial Discharge Test: Detects internal insulation defects.
• Harmonic of the no-load current Analysis: Checks for harmonics generated by the transformer
• Oil Dielectric Strength Test: Measures the breakdown voltage of transformer oil.
• Tan Delta / Dissipation Factor Test: Assesses insulation aging and degradation
• Measurement of zero-sequence impedance of three-phase transformers
• Measurement of acoustic noise level
• Measurement of the power taken by the fans and oil pumps.
• Tests on bought out components / accessories such as buchhloz relay, temperature indicators, pressure relief devices, oil preservation system etc.

Pre Commissioning Test of Transformer:

The transformer testing performed before commissioning the transformer at the site is called the pre-commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports.

Transformer Winding Resistance Measurement:

Transformer winding resistance measurement is carried out to calculate the I2R losses and to calculate winding temperature at the end of a temperature rise test. It is carried out as a type test as well as routine test. It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings etc.

There are different methods for measuring of the transformer winding, likewise:

• Current-voltage method of measurement of winding resistance.
• Bridge method of measurement of winding resistance.
• Kelvin bridge method of Measuring Winding Resistance.
• Measuring winding resistance by Automatic Winding Resistance Measurement Kit.

Transformer Testing  Note: Transformer winding resistance measurement shall be carried out at each tap.

Transformer Ratio Test:

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ratio test is an essential type test of transformer. This test also performed as a routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer one of the important tests.

The procedure of the transformer ratio test is simple. We just apply three phase 415 V supply to HV winding, with keeping LV winding open. We measure the induced voltages at HV and LV terminals of the transformer to find out actual voltage ratio of the transformer. We repeat the test for all tap position separately.

Magnetic Balance Test of Transformer:

Magnetic balance test of transformer is conducted only on three-phase transformers to check the imbalance in the magnetic circuit.

Procedure of Magnetic Balance Transformer Testing 

1. Keep the tap changer of transformer in normal position.
2. Now disconnect the transformer neutral from ground.
3. Then apply single phase 230 V AC supply across one of the HV winding terminals and
4. neutral terminal.
5. Measure the voltage in two other HV terminals in respect of neutral terminal.
6. Repeat the test for each of the three phases.

Magnetizing Current Test of Transformer Testing :

Magnetizing current test of transformer is performed to locate defects in the magnetic core structure, shifting of windings, failure in between turn insulation or problem in tap changers. These conditions change the effective reluctance of the magnetic circuit, thus affecting the current required to establish flux in the core.

Vector Group Test of Transformer Testing :

In a 3 phase transformer, it is essential to carry out a vector group test of transformer. Proper vector grouping in a transformer is an essential criteria for parallel operation of transformers.

So we have to choose a transformer suitable for parallel operation whose phase sequence and phase divergence are same. All the transformers with the same vector ground have same phase sequence and phase divergence between primary and secondary.

Before procuring an electrical power transformer, you should ensure the vector group of the transformer, whether it will be matched with his or her existing system or not. The vector group test of transformer confirms his or her requirements.

Insulation Resistance Test or Megger Test of Transformer Testing :

Insulation resistance test of transformer is essential type test. This test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer.

Procedure of Insulation Resistance Test of Transformer

1. 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 Transformer Testing :

Dielectric test of a transformer is one kind of insulation test. This test is performed to ensure the expected overall insulation strength of the transformer. There are several tests performed to ensure the required quality of transformer insulation; the dielectric test is one of them. Dielectric test of the transformer is performed in two different steps.

First one is called Separate Source Voltage Withstand Test of transformer, where a single phase power frequency voltage of prescribed level, is applied on transformer winding under test for 60 seconds while the other windings and tank are connected to the earth, and it is observed that whether any failure of insulation occurs or not during the test.

The second one is the induced voltage test of Transformer where, three-phase voltage, twice of rated secondary voltage is applied to the secondary winding for 60 seconds by keeping the primary of the transformer open circuited.

The frequency of the applied voltage should be double of power frequency too. Here also if no failure of insulation, the test is successful.

Induced Voltage Test of Transformer:

The induced voltage test of the transformer is intended to check the inter-turn and line end insulation as well as main insulation to earth and between windings-

1. Keep the primary winding of transformer open circuited.
2. Apply three-phase voltage to the secondary winding. The applied voltage should be twice of the rated voltage of secondary winding in magnitude and frequency.
3. The duration of the test shall be 60 seconds.
4. The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to the desired value.

The test is successful if no breakdown occurs at full test voltage during the test.

Temperature Rise Transformer Testing :

Temperature rise test of transformer is included in type test of transformer. In this test, we check whether the temperature-rising limit of the transformer winding and oil as per specification or not. In this type test of the transformer, we have to check oil temperature rise as well as winding temperature rise limits of an electrical transformer.

The full load test on a small transformer is very convenient, but on the large transformer, it is very difficult. The maximum temperature rise in a large transformer is determined by the full load test. This test is called, back-to-back test, regenerative test or Sumpner’s test

Common Transformer Testing Equipment

• Transformer Turns Ratio (TTR) Meter
• Insulation Resistance Tester (Megger)
• Power Analyzer
• Sweep Frequency Response Analyzer (SFRA)
• Oil BDV (Breakdown Voltage) Tester
• Omicron CPC/CMC testing kits (for advanced testing)

Importance of Transformer Testing

• Ensures safety and reliability
• Detects manufacturing defects
• Prevents unexpected breakdowns
• Validates performance specifications
• Supports preventive maintenance