Testing Power and Distribution Transformers

The typical practice for testing power and distribution transformers is to carry out a comprehensive set of tests at the manufacturer’s works. The number and nature of the tests depend on whether the transformer is the first of a new design or not, additionally a few relatively simple tests are done after installation at site to prove that the transformer is ready for service.

The works tests are can be classified into three types:

• Routine Tests.
• Type Tests
• Special Tests.

The first transformer of a particular design or contract is subjected to both type and routine tests while routine tests are only applied to later units. Special tests are only required at the specific request of the purchaser.

Routine Tests

Routine tests typically consist of:

• Checking voltage ratio, polarity and phase displacement.
• Winding resistance measurement.
• Insulation resistance measurement.
• Load loss and short-circuit impedance measurement.
• No-load loss and magnetising current measurement.
• Dielectric routine tests.
• Tests on on-load tap changers, where appropriate.

Voltage Ratio, Polarity and Phase Displacement Tests 

This involves the use of a ratiometer, which basically consists of a multi-ratio transformer from which tappings are taken to the coarse and fine adjusting switches. The figure below illustrates the type of circuit used for measuring ratio:

In reference to the above figure, the ratiometer and the transformer under test are connected in opposition. When the ratiometer is adjusted to give a ratio exactly equal to that of the transformer under test, no current will flow in the secondary circuit. The ratio can then be read directly from dial readings on the ratiometer.

Polarity and interphase connections are checked by measuring voltages between various terminals when the transformer is energized at a low voltage.

Winding Resistance Measurement

The dc resistance of each phase of each winding is measured separately by the voltammeter technique and is recorded together with the temperature of the winding at the time. This information is needed for use in connection with later measurements of the load loss and the temperature rise of the transformer under rated load.

Due to the inductive effect of the core, care must be taken to ensure that a steady dc value is reached before voltage and current readings are recorded.

Insulation Resistance Measurement

The insulation resistance between windings and from each winding to earth is measured by a special instrument such as the Megger.

The insulation resistance is commonly used as one of the criteria for establishing that the transformer has been properly dried out. It varies extensively and inversely with temperature, and care is necessary to ensure that the readings are correctly interpreted.

Load Loss and Short-Circuit Impedance Measurement

Load loss and impedance are measured by short-circuiting the terminals of one winding of the transformer and applying a low voltage to the other winding sufficient to cause rated full-load current to flow. Since the applied voltage, and thus, the magnetization of the core, is extremely low, the core loss can be reasonably be neglected and the measured input power represents the total load loss at rated load on the complete transformer.

A short-circuit is applied to the low-voltage winding, and the supply is connected to the high-voltage winding, at which side all the readings are taken. In principle, the same result would be obtained if the high-voltage winding were short-circuited and the supply connected to the low-voltage winding, but this involves measuring the heavier low-voltage rated currents, which may be too large for practicality.

The legacy 2-wattmeter method was used in the past to measure the load loss of smaller transformers, presently 3 wattmeters are required to make the measurement. Nonetheless, an electronic power analyser may be used to measure all the quantities on single equipment with higher accuracy.

In transformers with exceptionally high reactance the voltage that has to be applied to circulate rated full-load current through the windings, even under short-circuit, may be enough to magnetize the core to a level at which the loss therein may be significant. In such instances, the core loss during the short-circuit test may be established by removing short-circuit and measuring magnetising loss when the transformer is excited on open circuit, at the previously measured voltage required to circulate full-load current. The true load loss is then the difference between the two successive measurements.

It is important to keep in mind that the short-circuiting connections applied in such a way that the loss therein does not represent a significant fraction of the loss within the transformer. It is also important to note that, the temperature is measured at the time that load loss measurements are conducted, so that the necessary correction can be made to deduce the copper loss at the temperature.

No-Load Loss and Magnetizing Current Measurement

The basic principle of this test is that normal rated voltage is applied to one winding while the other is left open circuit. The current flowing in the winding to which the supply is connected is the magnetizing current and this is recorded as part of the test records. This magnetizing current is normally a small percentage of the full-load current and the I²R loss is negligible compared with the core loss. To prevent unnecessarily high voltages in the test circuit during the core loss test, it is normal practice to connect the supply to the lower voltage winding of the transformer.

Dielectric Routine Tests

Dielectric routine tests consist of an applied high potential (separate source test), a short duration ac test or a long duration ac test in combination with a switching impulse test, dependent on the voltage rating and a lighting impulse test, dependent on the voltage rating.

Applied-High-Potential Tests

These tests are usually made, in turn, between each winding, and the core and all other windings connected to earth.

Figure 1.3 below shows the arrangement with the high-voltage winding under test and the low-voltage winding and core connected together and to earth. The magnitude of the applied potential test depends on the rated voltage of the winding in question and on whether the main insulation between it and earth is uniform or graded. For a uniformly insulated winding the applied voltage test provides the principal dielectric test of the main insulation. For graded insulation windings the applied voltage test is at a value appropriate to the insulation level at the neutral point and thus does not adequately prove the strength of the line-end insulation.

Short-Duration AC Test

This induced voltage test involves exciting the transformer on open circuit at a voltage higher than the normal for a short period. This test is a routine test on transformers of up to 170 kV rating and a special test for transformers of higher voltage.

For transformers with uniform insulation on which the applied high-potential test provides the main check on the strength of the main insulation, the purpose of the induced test is to prove the strength of the insulation between turns and between other parts of the transformer operating at different potentials. The magnitude of the test is normally twice rated voltage and to prevent over excitation of the core, frequency of supply also has to be increased to at least twice normal.

On transformers up to 170 kV rating, with graded insulation, the induced overvoltage test constitutes the main test of the main insulation. The magnitude of the test is fixed so that the potential to earth of each of the high-voltage terminals in turn is raised to the appropriate test voltage for the system on which the transformer is to operate. The magnitude of the test may be as high as 3.46 times normal rated voltage, and the inter-tune and other insulation is tested to this degree at the same time.

The duration of the test is 60 s for any test frequency up to and including twice rated frequency. When the test frequency exceeds twice rate frequency, the duration of the test for 6000 periods i.e. 1 min at 100 Hz or a minimum of 15 s, whichever is greater. (Check IEC 60076 for more details on this test).

Long-Duration AC Test

For transformers rated above 170 kV, the routine test is a combination of a long duration ac test, lasting 60 m and a switching impulse test. The standard wave shape for a switching impulse test on air insulated equipment is of the order 250/2500 μs, that is, a relatively slow rise of front followed by a tail of long duration.

Partial discharge measurements are made during the long duration ac test to give a reliable check on the capability of the insulation in normal service.

For full detail on this test and specified test levels, you can check IEC 60076-3.

Lighting Impulse Test

Even though this is a type test for transformers rated up to 72.5 kV, the lighting impulse test is a routine test for all transformers of higher voltage.

The lighting impulse test simulates the conditions that exist in service when a transformer is subjected to an incoming high-voltage surge due to lighting or other disturbances on the connected transmission line.

Typically, the lighting impulse test follows the short-duration ac test or the combined long-duration ac test and switching surge test. Details of the lighting impulse test levels are specified in IEC 60076 for transformers for various system voltages.

Tests on On-Load Tap Changers

With the tap changer fully assembled on the transformer the following test sequences are done:

1. With the transformer un-energized, eight complete cycles of operation are performed.
2. With the transformer un-energized and with the auxiliary voltage reduced to 85 % of its rated value, one complete cycle of operation is performed.
3. With the transformer energized at rated voltage and frequency, one complete cycle of operation is performed.
4. With one winding short-circuited, and rated current in the tapping winding, 10 tap change operations over two tap steps on either side of the middle tapping or where any reversing switch operates, is done.

Related article: Step by Step Guide: How to Check and Test a Power Transformer

Type Tests

Type tests include:

• Dielectric type tests.
• Temperature-rise test.

Dielectric Type Tests

The lighting impulse test is a type test for transformers rated below 72.5 kV.

The long-duration ac test is a special test for transformers rated above 72.5 kV but below 170 kV; however, more often than not, it is requested as a type test on new designs.

Temperature Test

Every single new design of transformer has to be subjected to a test to determine that the temperature rise at rated load will not exceed the guaranteed values.

The back-to-back connection or direct loading should be used when making temperature tests on dry-type transformers, because the heat transfers from the core and windings to the cooling medium (air) are largely independent. The test conditions ought to represent the actual conditions in service when the core is heated by the magnetisation loss and the windings are heated by the load current.

In oil filled transformers, the heat generated in both core and windings is transferred to the oil, and the total heat then has to be dissipated from the oil to the cooling medium. Since the loss in the core is normally less than loss in the windings, it is possible to use the short-circuit method to test.

Under short circuit the loss in the core is almost negligible and the current in the windings is adjusted to a value slightly above the rated figure, so that the total loss in the windings is equal to the sum total of the separately measured load and core losses. The procedure during a short-circuit temperature test is to load the transformer with this increased current until such time as the observed oil temperature rise becomes sensibly constant or is not rising at more the 1° C/hr. This segment of the test proves that the cooling equipment of the transformer is adequate to dissipate the total losses under normal full-load conditions and the oil temperature rise is recorded correspondingly.

The increased currently certainly results in the temperature gradient between windings and oil being higher than when the current is at the rated value. The thermal inertia of the windings is relatively low; nonetheless, any change in current is swiftly followed by a corresponding change in the temperature gradient between the windings and oil. After the oil temperature rise has been recorded, current is reduced from the increased to the normal rated value maintained at this level for 1 hr. The supply is then disconnected and the dc resistance of the windings measured in a way similar to that used prior to the loss measurement tests when the transformer was cold. By taking temperature measurements over a period of 10 m, plotting a graph against time from shut-down and extrapolating back to the time of shut-down, the resistance of the windings at the instant of shut-down can be established. The winding temperature rise is then calculated by comparing the cold and hot resistances, with an allowance being made for any fall in oil temperature during the last half-hour at rated current. For detailed description of this test, check IEC 60076.

Related article: How Power Transformers Differs From Distribution Transformers

Special Tests

Special tests include:

• Dielectric special tests.
• Determination of capacitances between windings and earth and between windings.
• Determination of transient voltage transfer characteristics.
• Measurement of zero-sequence impedances on three-phase transformers.
• Short circuit withstand test.
• Determination of sound levels.
• Measurement of harmonics in the no-load current.
• Measurement of the power taken by the fan and oil pump motors.
• Measurement of insulation resistances to earth of the windings and measurement of the loss angle (tan δ) of the insulation system capacitances.

It is important to note that, special tests are usually carried out by agreement between the buyer of the transformer and the manufacturer.

Commissioning Tests at Site

Commissioning tests vary significantly with the size and the importance of the installation as described in the following sections:  

For small or medium-sized distribution transformers the minimum requirements would be a visual examination for transport damage and an insulation test with a portable instrument. If possible, there should be a check of the ratio (by applying a medium voltage to the high-voltage terminals), and measuring the induced voltage at the low-voltage terminals), and on the oil level and condition to confirm the ingress of moisture has not taken place.

For large units which are usually despatched either without oil or are only partially filled, checks must be made on the filling procedure and of the condition of the oil prior to filling, in addition to ensuring that the insulation hasn’t become wet during transport. After filling, a sample of oil may be taken for dissolved gas analysis so as to provide a basis for comparison with similar samples taken as part of a routine maintenance procedure when the transformer is in service.

Auxiliary equipment such as on-load tap changing gear and any protecting relays and current transformers linked to the main transformer must also be checked for correct operation.

Generally, a repetition of high-voltage tests performed at the works is not considered as a requirement. Where the transformed is subjected to retesting on site at high voltage, the test voltage level is typically restricted to 75 % of that applied during tests at the works.

Related articles:

• Features and Applications of Instrument Transformers
• Ideal Transformers
• Transformers
• DC Transmission and Distribution
• Overhead vs. Underground Power Distribution Systems
• AC Transmission and Distribution Systems