Power Transformers Testing

Testing of transformer is done to determine their electrical, thermal and mechanical suitability for the system where they will be applied or used. Most of the tests performed on power transformers are defined in national standards created by IEEE, NEMA and ANSI, whose purpose is to define a uniform set of tests recognized by both the manufacturer and the user.

Transformer Test Details:

Field Testing. Field testing can be divided into three categories 

Acceptance tests
Periodic tests
Tests after failure
Acceptance tests should be performed immediately after the product arrives at the destination. A few tests can be carried out which are stated below:
Turns ratio
Insulation resistance(Winding and core)
Power factor
Resistance (winding)
Polarity and phase relation
Oil tests (DGA, moisture, dielectrics, etc.)
Visual inspection
Periodic tests are done after the product is installed in its permanent location. The main purpose of this test is to monitor the condition of the unit so that any potential trouble may be spotted early before a failure occurs. Some of these are listed below:
Turns ratio
Insulation resistance 
Power factor
Resistance (winding)
Oil tests (DGA, moisture, dielectrics, etc.)
Excitation current test
Visual inspection
An unscheduled outage and the potential of outright failure can be prevented by following a periodic test schedule.
Failure tests conducted on electric transformers are:

Turns ratio
Insulation  resistance 
Power factor
Resistance
Oil tests
Excitation current test
Combustible gas/ gas-in-oil analysis
Visual inspection (internal)
When a transformer fails, the time of failure tests will decide whether the unit can be repaired at the site or whether it needs to be returned to the manufacturer, or a specialized center for repair. By comparing the results of the tests with the established norms, a 'history' of the transformer can be compiled, and the reasons for failure can be diagnosed.Here is a quick overview of the above mentioned tests:
Transformer Turns Ratio Test (Common to all categories)The Transformer Turns Ratio test (TTR) is used to make sure that the Turns Ratio between the windings of the transformer is correct. This ratio decides what the output voltage of the transformer will be with respect to the input voltage. The ratio is calculated under no-load conditions, with ratios calculated at the tap positions for each winding and for the winding as a whole. A voltage is applied to one winding and the voltmeters connected to both low voltage and high voltage windings are read simultaneously. The transformer ratio is the ratio of the HV voltmeter and the LV voltmeter readings. When ratio tests are being made on three-phase transformers, the ratio is taken on one phase at a time, and the measured ratio should be compared with the ratio calculated using nameplate voltages. Any variation should be within  .5%.
Transformer Insulation Resistance Test (Common to all categories)The winding insulation resistance test (also known as the Meggar test) is a measure of quality of insulation within the transformer. It can vary due to moisture content, cleanliness and the temperature of the insulation parts. All measurements are corrected to 20'C for comparison purposes. It is recommended that tank and core are always grounded when this test is performed. Each winding should be short-circuited at the bushing terminals. Resistances  are then measured between each winding and all other windings and ground (for 2 winding transformer - H-LG, L-HG and HL-G and three winding transformer H-LTG, L-HTG, T-HLG, HL-TG, HT-LG, LT- HG and HLT-G ).
vPower Factor (Common to all categories)This test is made to monitor the dryness of transformer insulation. Power factor is defined as the ratio of the power dissipated divided by the input volt-ampere multiplied by 100. The measurement of power factor is made with a capacitance bridge and the connections are the same as for the insulation resistance tests.

Transformer Resistance (Common to all categories)The resistance of a transformer winding can be measured after current has not passed through the transformer for several hours, allowing it to reach the same temperature as its surroundings. Winding resistance is calculated by measuring the voltage and current simultaneously, with the current as close to the rated current as possible. Calculating the winding resistance can be helpful as it lets you calculate and compensate for I2R losses, a major component of load losses as a whole.  Winding resistance measurements can be made to determine if any changes have occurred in the current carrying path. The winding resistance measurements should be made with a Wheatstone bridge, Kelvin bridge or similar bridge capable of measuring fractional ohms accurately.  For Wye connected values, measurements should be made between each pair of bushings, then summed and multiplied by three-halves to get the comparison value.
Transformer Oil Test (Common to all categories)A sample of insulating oil from a transformer in service can reveal much information about what is taking place inside the transformer. There are three basic enemies to insulating oil - oxidation, contamination and excessive temperature. The following tests can be done:
Acid number
Dielectric breakdown
Power factor
Moisture content
Interfacial tension