Testing of Power Transformers

Testing of Power Transformers

Transformer is heart of power system. These are important and costly elements in power system. The transformers are provided with protection against faults due o lightening strokes, over voltages and short circuit In this topic only over voltages tests are discussed. These tests are tabulated in Table.

Item	Sr. No	Test to be performed
Power transformer	1.	Induced voltage withstand stage
	2.	Surge/ Lightening impulse test
	3.	Partial discharge test
	4.	Sudden short circuit test

Induced Voltage Withstand Test of Power Transformer

1. This test is routine test i.e. it is carried on all the transformers.
2. The test is started by increasing applied voltage not greater than 1/3 " of test voltage, the voltage is then increased rapidly to full test voltage value for 60 seconds.
3. The frequency of applied voltage should be twice the rated frequency of transformer.
4. Then the voltage is rapidly reduced to 1 / 31d of full test voltage before switching off.
5. The purpose of using double frequency is to avoid excess excitation current during test. The double frequency supply is obtained from a separate alternator.
6. If the test frequency exceeds twice the rated frequency of transformer the 120 x Rated frequency duration of the test shall be Test frequency sec but not less than 15 seconds. This test is also known as flash test.

Surge/Lightening Impulse Test of Power Transformer

1. In practice the transformer is subjected to lightening strokes and switching surges.
2. In such case ordinary high voltage tests are not useful.
3. The test voltage wave is generated by a special type of generator known as impulse or surge generator.
4. Due to expense and complexity of this test it is applied only as a type test. It consists of application of a limited number of unidirectional surge voltages to one or two phases of L.V. and H.V. windings of transformer.
5. The arrangement of transformer for impulse testing is shown in Figure A.
6. The capacitor C is previously charged to a particular d.c. voltage is discharged by triggering the gap ' S '. This discharge is through wave shaping network (Ry , Ry and R , , C , and other combination). The discharge voltage gives rise to exponential wave shape. The wave shape can be controlled varying resistance R.
7. The voltage impressed on the transformer suddenly rises to a very high value and then falls to zero value in a very short period of time. This is called impulse voltage. The test schedule is as under : 1. A full wave test followed by 2. Two chopped wave test followed by 3. One full wave test.
8. These wave shapes one shown in Figure B (a) and (b).
9. The test impulse voltage shall be a full lighting impulse of (1.2 + 30 %) (50 + 20 %) used as defined in (IS 2071, part II 1974). This standard wave means the value of the full test impulse voltage (peak) is adjusted to reach in max. 1.2 usec and fall of its half value (50 % of peak) in 50 usec.
10. The magnitude of voltage that should be applied depends on the class of insulation used for transformer. The test results are recorded on a cathode ray oscilloscope. The record of waveform of sound insulation is available. The test results are compared with this record and if any difference is found, it means failure of insulation has occurred.

Figure A

Figure B

Sudden Short Circuit Withstand Test of Power Transformer

1. This is performed to determine effect of sudden short circuit Large mechanical stress.
2. Larger thermal stresses are induced in the transformer by repeated short circuit currents.
3. When short circuit current passes through transformer windings radial magnetic force is produced on the windings.
4. This force produce hoop - stress on outer winding and compressive stress on inner winding.
5. If axial forces are produced the winding collapses, conductors tend to bend between spaces provided between windings. This suggests that the design engineer should take into account these effects during repeated short circuits and design such a winding which will not deform and have adequate supports and clamps should be provided to the winding.
6. The test is performed in the short circuit testing station. It falls in the citatory of “special tests".

Test procedure :

1. The secondary of transformer is short circuited. A low voltage is applied to primary for 2 seconds.
2. After some time the process of supply low voltage is repeated. The winding is examined.
3. Insulation of winding is tested. If nothing happens then winding is pass in the test.