Testing of transformer
A power transformer steps voltage up or down in electrical transmission and distribution networks, enabling efficient power transfer at different voltage levels.
Since transformers operate under continuous electrical, thermal, and mechanical stresses, systematic testing is essential to ensure their safety, reliability, and long-term performance.
Transformer testing involves a range of tests conducted at different stages of a transformer’s life cycle, including factory testing (routine and type tests), on-site testing during installation and commissioning, and periodic testing during operation to assess its condition.
Whether it is an 11/0.4 kV distribution transformer or a 220/66 kV power transformer, the basic principles of testing remain similar, although the test procedures, voltage levels, acceptance criteria, and applicable requirements vary with the transformer’s design and rating.
Proper testing of transformer helps identify insulation defects, winding problems, overheating risks, and other potential faults before they lead to equipment failure. It also verifies compliance with applicable technical specifications and standards, such as the IEC 60076 series.
Understanding the different types of transformer tests, their procedures, and their purposes is essential for ensuring reliable transformer operation throughout its service life.
Categories of Transformer Tests
Testing of transformer procedures fall into three categories to manage cost, time, and risk across the manufacturing and operational lifecycle.
Type tests
Type tests validate transformer design at the manufacturer’s premises. They are performed on a prototype unit or representative sample before series production.
Typical type tests include dielectric tests, temperature rise test, short circuit withstand test, and sometimes sound level measurement.
Routine tests
Routine tests are performed on every individual transformer to verify manufacturing quality and compliance with specified requirements. These include:
- Voltage ratio measurement and check of phase displacement and polarity or vector group
- No load current and core losses measurement
- Load loss and short circuit impedance measurement
- Winding resistance test
- Insulation resistance test
- Separate source voltage withstand and induced voltage test
Special tests
Special tests are performed when agreed upon between the manufacturer and the purchaser. They provide additional information about the transformer’s electrical, thermal, acoustic, or mechanical characteristics.
Examples include zero sequence impedance measurement, power factor testing, acoustic noise level, no load current harmonics, and mechanical short circuit withstand verification.
Tests on accessories like tap changer operation, pressure relief devices, and Buchholz relay function also fall here.
On-site and in-service tests
On-site tests are performed after transportation and installation, particularly during commissioning, to verify that the transformer has not sustained damage and is ready for service.
In-service tests and condition assessments are carried out periodically or when abnormal operating conditions are detected.
Acceptance tests cover insulation resistance, ratio, and oil tests. Periodic condition monitoring adds dissolved gas analysis (DGA), sweep frequency response analysis, and partial discharge test – all designed to catch deterioration before failure.
Pre‑Testing Safety and Preparation
Safety is non-negotiable when working around high voltage power transformers. Even a “de-energized” unit can hold lethal residual charges in bushings and capacitive components.
Essential preparation steps include:
- Obtain work permits, lock out all incoming and outgoing breakers/isolators
- Discharge trapped energy and confirm de-energization with a rated voltage detector
- Earth all neutral terminals and line terminals per local safety procedures
- Visual inspection to check for physical damage, oil leakage, and loose connections on bushings and terminals
- Record ambient temperature and oil temperature for correcting test results later
Following are the required equipment for common testing procedures: calibrated insulation resistance tester (5 kV megger test capability), transformer turns ratio meter, micro-ohmmeter for resistance test, high voltage test set for dielectric tests, and temperature sensors for thermal monitoring.
Electrical Measurements and Performance Tests
Electrical tests establish whether the construction of the transformer delivers the performance its nameplate promises. Here are the specific tests performed.
Voltage Ratio Testing
A suitable low AC test voltage is applied to one winding, and the corresponding induced voltage in the other winding is measured. Turns ratio testing ensures correct voltage transformation between windings.
For three phase transformers, the test also verifies the phase displacement and vector group (e.g., Dyn11, YNd1). It prevents dangerous circulating currents during parallel operation.
Winding resistance test
A DC current is passed through each transformer winding, and the voltage drop is measured to determine its resistance using a winding resistance meter or micro-ohmmeter.
The measured resistance is corrected to a specified reference temperature, commonly 75 °C, when required for comparison.
Significant deviations from previous measurements or between comparable phases may indicate loose connections, poor joints, or other winding defects.
Winding resistance testing is distinct from insulation resistance testing, which evaluates the electrical insulation between windings and between windings and earth.
Short-Circuit Test
In this test, one winding of the transformer is short-circuited, and a reduced AC voltage is applied to the other winding until the rated current flows through the windings.
The input power measured during the test represents the transformer’s load losses, which are predominantly copper losses at rated current.
The applied voltage is used to determine the short-circuit impedance, usually expressed as a percentage of the rated voltage.
This percentage impedance is essential for calculating fault currents and evaluating transformer voltage regulation under load.
For more detail, learn the article on open circuit and short circuit test of transformer.
No-Load Test
In this test, rated voltage at the rated frequency is applied to one winding while the other winding is kept open-circuited. The input power, no-load current, and applied voltage are measured.
Since the secondary winding draws no load current, the measured input power primarily represents the transformer’s core (iron) losses, while the no-load current indicates the current required to establish the magnetic flux in the core.
In large power transformers, the no-load current is generally a small percentage of the rated current, although its value varies with transformer design and rating.
The results are used to assess core performance and determine iron losses for transformer efficiency calculations.
Insulation and Dielectric Tests
Most catastrophic transformer failures originate from insulation breakdown. Insulation tests prevent dangerous breakdowns in transformers and are the most critical category of all testing procedures. Electrical safety testing helps prevent insulation breakdown flashovers and earth faults.
Insulation Resistance Test
This test evaluates the condition of a transformer’s insulation using a megohmmeter (insulation resistance tester) that applies a specified DC test voltage.
The selected voltage, which may be 1 kV, 2.5 kV, or 5 kV, depends on the transformer’s voltage rating, insulation system, and the manufacturer’s recommendations.
Measurements are commonly taken between the high-voltage (HV) and low-voltage (LV) windings, between the HV winding and earth, and between the LV winding and earth, with appropriate isolation and connections.
Insulation resistance readings help identify moisture, contamination, and insulation deterioration. However, there is no single minimum resistance value that applies to every transformer.
The results should be evaluated against the manufacturer’s guidance, applicable standards, temperature, previous test records, and the transformer’s operating history.
IEEE C57.152 provides guidance on the diagnostic evaluation of liquid-immersed power transformers, including insulation resistance measurements.
Dielectric Tests
Dielectric tests assess a transformer’s insulation strength and its ability to withstand specified electrical stresses without breakdown.
In the separate-source AC withstand voltage test, a specified AC voltage is applied to one winding while the other windings, tank, and other designated parts are connected to earth in accordance with the test procedure.
In the induced AC voltage test, an AC voltage higher than the rated voltage is applied to one winding to test the insulation between turns, layers, and other relevant parts of the winding insulation system.
The supply frequency is increased, where necessary, to prevent excessive core flux and magnetic saturation. The test voltage and duration depend on the applicable standard and transformer specifications; a 60-second duration is typical for certain withstand tests, but it does not apply universally to all dielectric tests.
Oil Tests
Transformer oil testing evaluates the condition of the insulating liquid and its ability to provide effective electrical insulation and cooling.
Common tests include the breakdown voltage (BDV) test to assess dielectric strength, moisture content measurement using the Karl Fischer method, and dielectric dissipation factor measurement to evaluate the oil’s dielectric losses.
Low BDV values may indicate moisture, contamination, or other impurities, while excessive moisture can reduce the dielectric strength of the oil and adversely affect the solid insulation.
These test results help determine the condition of the insulating system and whether oil treatment or replacement is necessary.
Partial Discharge (PD) Test
This test detects localized electrical discharges in the transformer’s insulation system, which may indicate voids, defects, or insulation deterioration.
During the test, the transformer is subjected to a specified AC test voltage while partial discharge activity is monitored and, where applicable, measured in apparent charge (pico-coulombs, pC).
The test voltage, duration, and acceptance criteria depend on the applicable standards and test requirements.
Power Factor Test
Also known as the dielectric dissipation factor (tan δ) test, this test evaluates insulation condition by measuring dielectric losses under an applied AC voltage.
Higher values may indicate moisture, contamination, or insulation deterioration. It complements partial discharge testing by assessing a different aspect of insulation performance.
Dissolved Gas Analysis (DGA)
This condition-monitoring technique analyzes gases dissolved in transformer oil to identify potential internal faults, such as overheating, partial discharge, and electrical arcing. Monitoring gas concentrations and their trends over time helps detect developing faults and assess the transformer’s in-service condition.
Thermal and Mechanical Tests
In addition to electrical performance and insulation integrity, power transformers must withstand thermal stresses during continuous operation and mechanical forces during short-circuit events.
Thermal and mechanical tests verify that the transformer can operate safely under specified conditions and maintain its structural integrity throughout its service life.
Temperature-Rise Test
It determines the increase in oil and winding temperatures above ambient temperature when a transformer operates under specified loading conditions. Sumpner’s test or back to back test is used to determine the temperature rise of a transformer.
The test is generally conducted at rated losses or under other conditions prescribed by the applicable standard until the required thermal steady state is reached.
The temperature-rise test verifies that the transformer’s cooling system can dissipate heat effectively and that the oil and windings remain within their specified thermal limits.
The average winding temperature can be determined using the resistance method, which compares winding resistance measured before and after the test.
The resistance change is used to calculate the average winding temperature, taking into account the conductor material and its temperature-resistance relationship.
Short-Circuit Withstand Test
The short-circuit withstand test verifies a transformer’s ability to withstand the thermal and mechanical stresses produced by short-circuit currents.
During this test, specified short-circuit currents are established under controlled laboratory conditions to assess the strength of the windings, insulation supports, spacers, and clamping structures.
The test evaluates whether the transformer can withstand the electromagnetic forces and heating associated with the specified fault conditions without unacceptable damage or deformation.
It is generally a design verification test performed on selected transformers in specialized laboratories, in accordance with the applicable standard and agreed test requirements.
Cooling System and Protection Checks
Additional checks verify the correct operation of cooling equipment and temperature-monitoring systems.
These include checking oil pumps, cooling fans, radiators, temperature indicators, alarms, and trip circuits, where fitted. The relevant alarms and protection functions should operate at their specified settings.
Excessive winding temperature accelerates the ageing of transformer insulation and can reduce its expected service life.
A commonly used approximation for certain cellulose insulation systems is that an increase of about 6 °C in winding hot-spot temperature may approximately double the rate of thermal ageing.
However, this is a simplified rule of thumb, not a universal relationship for all insulation systems and operating conditions.
The results of thermal and mechanical assessments help engineers evaluate loading capability, cooling-system performance, and short-circuit withstand capability.
Together with routine condition monitoring, these assessments support effective maintenance planning, safe loading decisions, and reliable transformer operation throughout its service life.
