High Voltage Test Bushings:
High Voltage Test Bushings Consisting of Three tests, namely
Power Frequency Tests
Impulse Voltage Tests
Power Frequency Tests:
Power Factor—Voltage Test: In this test, the bushing is set up as in service or immersed in oil. It is connected such that the line conductor side of the high voltage Schering bridge. Voltage is applied up to the line value in increasing steps and then reduced. The capacitance and power factor (or tan δ) are recorded at each step. The characteristic of power factor or tan δ versus applied voltage is drawn. This is a normal routine test but sometimes may be conducted on percentage basis.
Internal or Partial Discharge Test: This test is intended to find the deterioration or failure due to internal discharges caused in the composite insulation of the bushing. This is done by using internal or partial discharge arrangement. The voltage versus discharge magnitude as well as the quadratic rate, gives an excellent record of the performance of the bushing in service. This is now a routine test for High Voltage Test Bushings.
Momentary Withstand Test at Power Frequency: This is done as per the Indian Standard specifications IS: 2099 applied to bushings. The test voltage is specified in the specifications. The bushing has to withstand without flashover or puncture for a minimum time ( ~ 30s) to measure the voltage. At present this test is replaced by the impulse withstand test.
One Minute Wet Withstand Test at Power Frequency: The most common and routine tests used for all electrical apparatuses are the one minute wet, and dry voltage withstand tests. In wet test, voltage specified is applied to the bushing mounted as in service with the rain arrangement as described A properly designed bushing has to withstand the voltage without flashover for one minute. This test really does not give any information for its satisfactory performance in service, while impulse and partial discharge tests give more information.
Visible Discharge Test at Power Frequency: This test is intended for determining whether the bushing is likely to give radio interference in service, when the voltage specified in IS: 2099 is applied. No discharge other than that from the arcing horns or grading rings should be visible to the observers in a dark room. The test arrangement is the same as that of the withstand test, but the test is conducted in a dark room.
Impulse Voltage Tests:
Full Wave Withstand Test: The bushing is tested for either polarity voltages as per the specifications. Five consecutive full waves of standard waveform are applied, and, if two of them cause flashover the bushing is said to have failed in the test. If only one flashover occurs, ten additional applications are done. The bushing is considered to have Passed the test if no flashover occurs in subsequent applications.
Chopped Wave Withstand and Switching Surge Tests: The chopped wave test is sometimes done for High Voltage Test Bushings (220 kV and 400 kV and above). Switching surge flashover test of specified value is nowadays included for High Voltage Test Bushings. The tests are carried out similar to full wave withstand test.
Temperature Rise and Thermal Stability Tests: The purpose of these tests is to ensure that the bushing in service for long does not have an excessive temperature rise and also does not go into the “thermal runaway” condition of the insulation used.
Temperature rise test is carried out in free air with an ambient temperature below 40°C at a rated power frequency (50 Hz) a.c. current. The steady temperature rise above the ambient air temperature at any part of the bushing should not exceed 45°C. The test is carried out for such a long time till the temperature is substantially constant, i.e. the increase in temperature rate is less than I °C/hr. Sometimes, the bushings have to be operated along with transformers; of which the temperature reached may exceed 80°C. This temperature is high enough to produce large dielectric losses and thermal instability. For High Voltage Test Bushings this is particularly important, and hence the thermal stability test is done for bushings rated for 132 kV and above.
The test is carried out with the bushing immersed in oil at a maximum temperature as in service, and the voltage applied is 86% of the nominal system voltage. This is approximately √2 times the working voltage of the bushing and hence the dielectric losses are about double the normal value. The additional losses account for the conductor ohmic losses. It has been considered unnecessary to specify the thermal stability test for oil-impregnated paper bushings of low ratings; but for the large High Voltage Test Bushings (1600 A, 400 kV transformer bushings, etc.), the losses in the conductor may by high enough to outweigh the dielectric losses.
It may be pointed out here, that the thermal stability tests are type tests. But in the case of large sized High Voltage Test Bushings, it may be necessary to make them routine tests.