HIGH VOLTAGE TESTING OF ELECTRICAL POWER APPARATUS
Man has been power hungry since time-immemorial. In modern times the world has seen phenomenal increase in demand for energy, of which an important component is that of electrical energy. The production of electrical energy in big plants under the most economic condition makes it necessary that more and more energy be transported over longer and longer distances. Therefore, transmission at extra high voltages and the erection of systems which may extend over whole continents has become the most urgent problems to be solved in the near future. The very fast development of systems is followed by studies of equipment and the service conditions they have to fulfill. These conditions will also determine the values for testing at alternating, impulse and d.c. voltages under specific conditions. As we go for higher and higher operating voltages (say above 1000 kV) certain problems are associated with the testing techniques. Some of these are:
· Dimension of high voltage test laboratories.
· Characteristics of equipment for such laboratories.
· Some special aspects of the test techniques at extra high voltages.
The dimensions of laboratories for test equipments of 750 kV and above are fixed by the following main considerations:
· Figures (values) of test voltages under different conditions.
· Sizes of the test of equipments in a.c., d.c. and impulse voltages.
Distances between the objects under high voltage during the test period and the earthed surroundings such as floors, walls and roofs of the buildings. The problems associated with the characteristics of the equipments used for testing are summarized here. In alternating voltage system, a careful choice of the characteristics of the testing transformer is essential. It is known that the flash over voltage of the insulator in air or in any insulating fluid depends upon the capacitance of the supply system. This is due to the fact that a voltage drop may not maintain preliminary discharges or breakdown. It is, therefore, suggested that a capacitance of at least 1000 Pf must be connected across the insulator to obtain the correct flash over or puncture voltage and also under breakdown condition (a virtual short circuit) the supply system should be able to supply at least1 amp for clean and 5 amp for polluted insulators at the test voltage. There are some difficult problems with impulse testing equipments also especially when testing large power transformers or large reactors or large cables operating at very high voltages. The equivalent capacitance of the impulse generator is usually about 40 nano farads independent of the operating voltage which gives a stored energy of about 1/2 × 40 10–9 × 36 × 109 = 720 KJ for 6 MV generators which is required for testing equipments operating at 150 kV. It is not at all difficult to pile up a large number of capacitances to charge them in parallel and then discharge in series to obtain a desired impulse wave.
But the difficulty exists in reducing the internal reactance of the circuit so that a shortwave front with minimum oscillation can be obtained. For example for a 4 MV circuit the inductance of the circuit is about 140 μH and it is impossible to test an equipment with a capacitance of 5000 pF with a front time of 1.2 μ sec. and less than 5% overshoot on the wave front. Cascaded rectifiers are used for high voltage d.c. testing. A careful consideration is necessary when test on polluted insulation is to be performed which requires currents of 50 to 200 mA but extremely predischarge streamer of 0.5 to 1 amp during milliseconds occur. The generator must have an internal reactance in order to maintain the test voltage without too high a voltage drop.