Power Transformers Testing

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Published: 07th August 2009
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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

      • After performing the tests the oil can classified as reusable; reusable with minor reconditioning; or disposable.

      • Transformer Polarity (Acceptance test) The polarity of a transformer is either additive or subtractive. In order to find out the polarity of a transformer, a voltage is applied between the primary bushings. If the resultant voltage between the secondary bushings is greater than the applied voltage that means that the transformer has additive polarity. If it is lower, the transformer has subtractive polarity. Polarity is not important for a single connected distribution transformer, but it is a vital concern if transformers are to be paralleled or bank connected. Three phase transformers are also checked for polarity by the same means.

      • Transformer Phase Relation (Acceptance test)A phase relation test is carried out for polyphase (for instance, three-phase) transformers to make sure that they have been connected in such a way that their phase relationship is correct. A phase relation test calculates the angular displacement and relative phase sequence of the transformer, and can be carried out in conjunction with ratio and polarity tests. The voltages of the phase of primary and secondary can be recorded and comparisons made to get the phase relation.

      • Visual Inspection (Periodic and Failure tests)This may reveal either present or potential problems that may not be picked up by diagnostic testing. For example, deteriorating gaskets, low oil level or chipped bushing skirts. A standard list of check points should be established for each unit and then a record of each inspection maintained.

      • Gas/ Gas-in-Oil Test (Failure test):A study of gases either dissolved in the oil or from the gas above the oil can also show abnormal conditions within the transformers, such as incipient faults.

      • Three considerations are very important:

        • The total percentage of combustible gas

        • The percentage of each gas component

        • The rate of change in combustible gas content

        • If the percentage of combustible gases is above 5%, then immediate action is required

      • Excitation Current Test (Periodic and Failure tests)The excitation current is the minimum amount of current needed to maintain the core in a state of magnetic excitation. It is measured at the rated voltage, and usually given as a percentage of the rated current.

        The test is performed with a single phase supply with, preferably, a voltage rated at approximately 10% of the phase voltage of the winding to which the supply is to be connected, although lower voltages can be used.

        There are 2 methods that can be used: the first is to connect a single-phase supply to any available winding with an ammeter in the circuit to monitor the exciting current. Three such single-phase tests are necessary for a three-phase transformer. The relationship between the single phase readings is important; it should be as follows:

        • The readings taken on phase A and C should be within 5% of each other.

        • The reading on phase B should be between 65 and 90% of the readings on phase A and C.

        Readings that fall outside of the relationships given above may be indicators of a winding fault. In the other method, the same voltage level and ammeter requirements apply except the following connections should be made:

      • Short one winding on phase C and apply voltage and read the exciting current on phase A.

      • Short one winding on phase A and apply voltage and read the exciting current on phase C.

      • Short one winding on phase B and apply voltage and read the exciting current on phase A or phase C.

    Other Transformer Tests:

    Other tests which can be performed are:

    • Core Loss Test

      Under no-load conditions, a transformer will continue to drain sources of electrical energy. The chief source of this drain is core loss, which occurs in the magnetic core through a combination of hysteresis and eddy current loss, among others. Core-loss is calculated by applying the rated voltage and frequency to a transformer under no-load conditions. The resultant current is then measured, from which the loss of energy can be extrapolated.

    • Load Loss Test

      Load loss is a combination of I2R losses, stray losses and eddy losses, all of which contribute to the loss of electrical energy that is seen as current transferred from one winding to another. Load loss changes with the magnitude of the load: that is to say, higher loads see higher rates of loss. The load loss is therefore generally calculated for the rated load, while the transformer is under full-load conditions. It can be measured by applying a voltage to one winding while the other winding is short-circuited. The voltage is adjusted until the current flowing through the circuit is the same as the rated current. The power loss measured at this time is the load loss.

    • Impedance Test

      Impedance is a measure of the resistance that leads to the loss of electrical energy in a transformer at full load, causing the ratio of the input and output voltages to differ from the Turns Ratio. It can be measured at the same time as load loss. Impedance is found by measuring the voltage required to pass the rated current through one winding of the transformer, while the other winding is short-circuited. This voltage is called the impedance voltage.

    • Applied Potential Test

      The applied potential test is used to see how well the transformer's insulation deals with voltages higher than the rated voltage, for given periods of time. The applied potential test checks the insulation between individual windings; and between windings and ground by applying voltages to each of these areas.

    • Induced Potential Test

      The induced potential test is used to test the quality of the transformer's insulation, as with the applied potential test above. It tests the insulation of the individual windings of the transformer by applying voltages between turns, between layers and between lines.

    • Quality control impulse test

      Quality control impulse tests are made on transformers in order to simulate lightning; to see how well they withstand such high bursts of voltage. The electric impulses applied here can include reduced full-wave tests, chopped-wave tests and front-of-wave tests, to simulate a range of extreme voltage situations.

    • Pressure Leak Test

      A transformer can be checked for pressure leaks by pressurizing the tank and then leaving it alone for several hours. If the pressure drops during the intervening time, or if there are signs of liquid leakage, than a leak is present. Otherwise, the transformer is leakage free.

      While learning about and overseeing the standard testing procedures of your transformer can be a laborious task, it definitely helps better your understanding of the transformer's operation, minimizes hazard to life and property, reduces downtime, minimizes the chance of sudden failure and thus allows optimum use of the transformer.

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