High voltage motor testing
The tests to be conducted on high voltage motors are as below
- High Voltage DC Test
- Sustained low Frequency Tests
- High Frequency Test
- Surge or Impulse Test
High Voltage DC Test
- A DC voltage about two times of the normal rated voltage is applied across the motor under test.
- The voltage is applied for specified time duration It may be from 15 minutes to 1.5 hours.
- Although high voltage DC test is not complete substitute of high voltage AC test but still it is applicable where HVAC test is not at all possible.
Sustained Low Frequency Test
- This test is conducted at power frequency (50 Hz).
- This test i.e. sustained low frequency test are carried out on a specimen of insulating material used in high voltage motors.
- From this test dielectric strength, dielectric losses of the insulating material are determined.
Procedure
- High voltage of specified magnitude is applied across a specimen for a specific duration of time.
- A special high voltage transformer is used for applying high voltage.
- A resistance is connected with series with the transformer to limit the short circuit current in the if in case breakdown occurs in the insulation.
- This test determines the continuous high voltage withstanding capability of the device.
High Frequency Test
- The insulation system of high voltage motor may be subjected to breakdown or flash-over during high frequency disturbances.
- The high frequency disturbances in the HV system occurs due to switching operations, sudden interruption of line current, open circuit fault or any other external causes.
- The high frequency disturbance may cause failure of insulators eventhough the magnitude of voltage is low.
- This causes high dielectric loss and heating. Thus the insulation of high voltage motors should maintain high frequency voltage withstanding capacity.
- High frequency voltage is applied to motor through frequency generator for specific duration of time And high frequency voltage withstanding capacity.
Surge Test or Impulse Test
Surge or lighting on the transmission lines may pass on to equipments connected to the line.
Surge test or impulse tests are very high or extra high voltage tests, carried out for investing the influences of surges or lightning on the transmission equipment (eg . High voltage motors).
- In practice the transformer is subjected to lightening strokes and switching surges.
- In such case ordinary high voltage tests are not useful.
- The test voltage wave is generated by a special type of generator known as impulse or surge generator.
- 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.
- The arrangement of transformer for impulse testing is shown in Figure A.
- 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.
- 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.
- These wave shapes one shown in Figure B (a) and (b).
- 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.
- 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.