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Medium Voltage Transformer Testing
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Electrical transformers have continuously evolved since the advent of modern electrical systems. Therefore, the required conditions and standards for designing, testing, and operating transformers have also had to be updated. The IEEE Transformers Committee has defined the modern types and procedures of transformer tests. The standards this committee sets are published in the C57 series of standards. These standards define the procedure for designing, testing, repairing, installing, operating, and maintaining transformers. A transformer is generally tested for its electrical, mechanical, and thermal properties to determine its suitability to relevant electrical system requirements. It is, therefore, essential to know about the various tests. Such information is provided below, with a special focus on tests conducted by Meta Power Solutions to routinely evaluate their medium voltage transformers.


The following table shows key IEEE Standards for General Requirements and Testing Procedures for Liquid immersed and Dry Type Transformers. Further Descriptions of the tests can be found on the IEEE website.

Table 1: IEEE Standards

Transformer Type (Standarts)

Liquid-Immersed Distribution, Power, and Regulating Transformers

Dry-Type Distribution
and Power

Standard Test Code

IEEE C57.12.90

IEEE C57.12.91

General Requirements

IEEE C57.12.00

IEEE C57.12.01

Kinds of Tests

IEEE classifies different transformer tests under the following categories:

a. Design Tests

Transformers and their components must meet certain standards based on their applications, type, style, model, and assigned rating. The manufacturers must also consider the unit’s performance under normal and special system conditions and make design adjustments accordingly. The manufacturer conducts design tests to evaluate the design of their transformer units. Design tests are only performed on representative apparatus, and all units of the same design pass the test if the representative unit passes. Design tests are not used for the normal production of transformer units. Instead, an approved design may be adopted to implement a similar, newer design without repeating these tests unless there are considerable changes in the new design.

b. Routine Tests

The manufacturer conducts these tests to verify the conformity of the transformer to the required design specifications. Thus, these tests are sometimes called quality control tests. These tests are conducted not just on the individual transformer units but also on the parts and materials used to manufacture the transformer.

c. Conformance Tests

These tests are used to check the conformity of the transformer units to relevant standards.

d. Other Tests

Sometimes, manufacturers perform additional tests on their transformer units and use their test results to showcase the comparative advantage of their product. These tests are referred to as the Other Tests and are described within individual product standards alongside the design and routine tests. Other tests usually depend on the transformer’s size, rating, and application. Some examples of other tests include Gas Analysis and Radio Influence Voltage tests.

Table 2: Routine, Design, and Other Tests

Test Type


Other Tests

Zero Sequence Impedance, Insulation Power Factor1, Short Circuit, Radio Influence Voltage, Insulation Resistance, and Gas Analysis Tests.

Routine Tests

Turns Ratio, Polarity and Phase Relation, Resistance, No-Load Loss, Excitation Current Impedance, Load Loss, Quality Control Impulse, Applied Potential, Induced Potential, Partial Discharge (Cast only), Leak (liquid-filled Transformers only), Regulation2, and Efficiency2 Tests.

Design Tests

Audible Sound Level and Temperature Rise Tests

1 Classified as a Routine Test for liquid-filled substation transformers.

2 No extra tests are performed; these parameters are calculated from other test results.

Type and Purpose of Test

1. Design Tests

 a. Audible Sound Level Test

In this test, the transformer is energized at the rated voltage while the transformer’s secondary side is kept unloaded. Under these conditions, the transformer noise is measured to check for the average sound level generated by the transformer.

b. Temperature Rise Test

The Temperature Rise Test is used to verify the temperature rise characteristics of the transformer windings when the rated current is passed through them. The insulation of transformer windings is heavily dependent on these characteristics of the transformer windings. Thus, this test ensures that the transformer design produces an average winding temperature within the temperature tolerance values of the insulation system.

The procedure of this test is very simple. The temperature rise of the transformer windings is a function of the winding’s resistance. Therefore, the temperature average rise is found by comparing the cold start resistance value and the final stabilized winding resistance value when the rated current passes through the windings during operation.

2. Routine Tests

a. Turns Ratio Test

It is important to ensure that the transformer output voltage matches perfectly with the rest of the electrical system for a given voltage value at its primary terminals. The turns ratio test is performed on a transformer to verify its voltage step-up and step-down characteristics. The turn ratios determine the output voltage for all possible values of the primary voltage. Therefore, the rated voltage is applied on the primary side, and the output voltage is measured on the secondary terminals. These two voltages are compared for every winding and tap position, giving a corresponding turn ratio between primary and secondary windings. The output voltage must not deviate by more than 0.5% of the rated nameplate voltage.

b. Polarity and Phase Relation Tests

The Polarity and Phase Relation Tests check the polarity and phase relationship between the primary and secondary windings and voltages. These tests can reveal the polarity and phase of the output voltage. They are key parameters when transformers are connected in parallel to one another. A single-phase transformer can have either additive or subtractive polarity between its primary and secondary windings. When transformers with different polarities are connected in parallel, the line voltage is effectively short-circuited, causing fault currents to flow in the system. The Phase Relation Test is performed for three-phase transformers to reveal the phase sequence of the three phases and their phase displacement from the ideal 120o Phase difference between them.

c. Resistance Test

The DC winding resistance is used to find the resistance of the winding. A known direct current is passed through the winding and is compared to the measured voltage drop across the winding to find the DC resistance. The resistance is an important parameter as it can be used to calculate the winding’s copper losses (I2R losses) and the winding temperature during the temperature test.

d. No-Load Loss Test

The No-load Loss Test is used to find the No-load losses. To perform the test, the full rated voltage is applied to the primary terminals of the transformer, and the transformer’s secondary terminals are left unloaded/open. Thus, any losses incurred are due to the magnetic linkages between the two windings instead of the power requirements of external loads. No-load losses consist of the core’s excitation losses (iron losses), the dielectric losses of the insulation, and the winding losses caused by the flow of excitation current in the windings. The most significant contributor to No-load losses is iron losses, which is why the term is used synonymously for No-load losses in general.

e. Excitation Current Test

The Excitation Current Test measures the current required to excite the transformer’s core. Therefore, this test measures the current needed to create and maintain the magnetic linkages between the primary and secondary windings at full-rated voltage. The results are expressed in per unit notation or as a percentage of the rated current.

f. Impedance Test

The impedance test is used to find the percentage impedance of a given transformer. The percentage impedance, also known as the impedance voltage, is the percentage of the rated voltage needed to pass the rated current through a short-circuited secondary winding. Normally, the IEEE Standards allow a value deviation of ± 7.5% and ± 10% for two-winding transformers and three-winding transformers, respectively. This tolerance is allowed to cover any variations caused by manufacturing and material.

g. Load Loss Test

The Load Loss Test is performed to find the losses in a fully loaded transformer. The Load loss is the combination of the winding copper losses (I2R losses), winding stray losses, eddy losses, and the losses in different parts of the transformer like the transformer assembly parts, enclosure, tank, etc. The most significant contributor to load losses is the copper losses, which occur in the primary and secondary windings.

h. Quality Control Impulse Test

The Quality Control Impulse (QCI) Test is performed to verify the insulation system’s ability to withstand high voltage impulses – the transformer’s Basic Impulse Level (BIL) rating. The test consists of one reduced and one full wave. The voltage class and the transformer’s corresponding BIL specification determine the impulse’s wave shape and voltage level. For example, the reduced and full waves are applied alongside two chopped waves for ANSI-specified impulse tests. The full wave represents a disturbance that occurs away from the transformer but travels along the transmission line towards the transformer. The chopped wave represents a similar traveling wave that flashes to the ground close to the transformer terminals. The test is only performed on the high voltage side for distribution transformers that belong to voltage classes below 5 kV.

i. Applied Potential Test

The Applied Potential Test is also known as the High-Pot or Low-Frequency Test. This test is performed to verify the transformer’s insulation by passing high voltage transients through the test subject. High potential at 60Hz frequency is applied on the transformer terminals for one minute to illustrate all possible leakage currents through the target insulation. The insulation being tested could be the one between windings or between the winding and ground.

j. Induced Potential Test

The Induced Potential Test is performed to verify the quality of the inter-turn, inter-layer, and inter-phase insulations. For this test, a potential of twice the value of the rated voltage and 400Hz frequency is applied to the transformer for 7200 cycles.

k. Partial Discharge

The Partial Discharge Test is performed to check for voids and air pockets in windings, which may have formed during the casting process. Although it is considered Other Test in most cases, it is a routine test for cast coil transformers.

l. Leak

A liquid-filled transformer has to be filled with oil that performs different tasks like cooling, arc isolation, insulation, etc. These oils are, therefore, vital to the operations of a transformer. Before the production cycle’s final assembly and shipment phases, transformers are tested for leaks by filling them with liquid under pressure. The pressure exposes the leak in two ways, visible liquid spillage and reduced applied pressure. In case of leaks, the transformer is swiftly repaired and shipped.

m. Regulation and Efficiency Tests

Regulation and Efficiency define how well the transformer output is transferred from the primary side to its secondary. Regulation is the percentage of change in secondary terminal voltage when the load on the transformer is increased. Regulation is expressed in the percentage of the rated secondary voltage. The Efficiency of a transformer describes the power transfer capability from the primary side to the secondary. It is the ratio of output power to input power, and it should ideally be as close to the value of 1 as possible. Efficiency is also expressed in percentage. Regulation and efficiency tests are not physical tests; they are parameters calculated using data from other tests, i.e., the impedance and loss tests.

3. Other Tests

Zero-Sequence Impedance Test

The Zero-Sequence Impedance Test measures the line to the neutral impedance of a transformer device with wye-connected windings. The measured value of the zero-sequence impedance is expressed as a percentage of the rated voltage.

Radio Influence Voltage (RIV) Test

The Radio Influence Voltage (RIV) Test, or the Telephone Influence Factor (TIF) Test, is used to verify the dielectric strength of the insulation in liquid-filled transformers. The RIV test is the equivalent of the Partial Discharge Test performed on cast-coil transformers for the same purpose. The test applies high-velocity ionization through an electric field and measures the resultant electrical discharges in the insulation.

Short Circuit Test

The Short Circuit Test is performed on transformers to check their short circuit tolerance against special high current conditions that arise during short circuit (through) faults. The short circuit current is passed through the transformer to study the thermal and mechanical integrity of the transformer coils, core, and other electrical components. These tests require significant power, which is impossible without appropriate safety considerations inside the high-power test laboratory.

Insulation Power Factor Test

The Insulation Power Factor Test is used to find the dryness condition of the insulation within the transformer. The test applies a sinusoidal voltage to the transformer terminals and takes the ratio of the dissipated power (in watts) inside the insulation to the effective power (PE = VEIE, in volt-amps). There are no standards to limit the power factor readings obtained in this way; instead, acceptable power factor readings are based on the experience and discretion of the manufacturer. Generally, a 1% and a 0.5% power factor value are acceptable for distribution and large power transformers, respectively.

Insulation Resistance Test

The Insulation Resistance Test is also used to find the dryness condition of the insulation within the transformer. Some factors affecting the insulation reading include the insulating parts’ cleanliness, temperature, and moisture content.  

Gas Analysis Test

Gases are naturally produced during transformer operations. The main source of gas production is the gradual deterioration of the insulating material caused by various events like faults, conductor losses, thermal events, etc. The Gas Analysis Test measures the levels of these gases by performing a chemical analysis of the transformer liquid. The evaluation is necessary to ensure that no harmful gases damage the transformer. However, some transformers operate their entire life even in the presence of many combustible gases. The best form of chemical analysis is the Dissolved Gas Analysis (DGA) if it is performed periodically throughout the service life of the transformer. The test data of an earlier composition analysis test can be used as a baseline for future comparisons to evaluate the age and value of the transformer unit.

Transformer Applications



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