Power systems equipment must withstand not only the rated voltage (Vm), which corresponds to the highest voltage of a particular system, but also over voltages. Accordingly, it is necessary to test high voltages. Equipment during its development stage and prior to commissioning. The magnitude and type of test voltage varies with the rated voltage of a particular apparatus. The standard methods of measurement of high-voltage and the basic techniques for application to all types of apparatus for alternating voltages, direct voltages, switching impulse voltages and lightning impulse voltages are laid down in the relevant national and international standards.
The tests helped to isolate defective cables without further damaging good cable insulation. With the widespread use of extruded insulation cables of higher dielectric strength, the test voltage levels were increased to 5–8V0. In the 1970s premature failures of extruded dielectric cables factory tested under dc voltage at specified levels were noted. Hence on-site testing of cables under very low frequency (VLF) of approximately 0.1Hz has been adopted.
Testing with power frequency voltages
To assess the ability of the apparatus’s insulation withstand under the system’s power frequency voltage the apparatus is subjected to the 1-minute test under 50 Hz or 60 Hz depending upon the country. The test voltage is set at a level higher than the expected working voltage in order to be able to simulate the stresses likely to be encountered over the years of service. For indoor installations the equipment tests are carried out under dry conditions only. For outdoor equipment tests may be required under conditions of standard rain as prescribed in the appropriate standards.Testing with lightning impulse voltages
Lightning strokes terminating on transmission lines will induce steep rising voltages in the line and set up travelling waves along the line and may damage the system’s insulation. The magnitude of these over voltages may reach several thousand kilo volts, depending upon the insulation. Exhaustive measurements and long experience have shown that lightning over voltages are characterized by short front duration, ranging from a fraction of a microsecond to several tens of microseconds and then slowly decreasing to zero. The standard impulse voltage has been accepted as a periodic impulse that reaches its peak value in 1.2 /sec and then decreases slowly (in about 50 /sec) to half its peak value. In addition to testing equipment, impulse voltages are extensively used in research laboratories in the fundamental studies of electrical discharge mechanisms, notably when the time to breakdown is of interest.Testing with switching impulses
Transient over voltages accompanying sudden changes in the state of power systems, e.g. switching operations or faults, are known as switching impulse voltages. It has become generally recognized that switching impulse voltages are usually the dominant factor affecting the design of insulation in high voltage. power systems for rated voltages of about 300 kV and above. Accordingly, the various international standards recommend that equipment designed for voltages above 300 kV be tested for switching impulses. Although the wave shape of switching over voltages occurring in the system may vary widely, experience has shown that for flashover distances in atmospheric air of practical interest the lowest withstand values are obtained with surges with front times between 100 and 300 /sec. Hence, the recommended switching surge voltage has been designated to have a front time of about 250 /sec and half value time of 2500 /sec. For GIS (gas-insulated switchgear) on-site testing, oscillating switching impulse voltages are recommended for obtaining higher efficiency of the impulse voltage generator.D.C. voltages
In the past dc voltages have been chiefly used for purely scientific research work. Industrial applications were mainly limited to testing cables with relatively large capacitance, which take a very large current when tested with ac voltages, and in testing insulations in which internal discharges may lead to degradation of the insulation under testing conditions. In recent years, with the rapidly growing interest in HVDC transmission, an increasing number of industrial laboratories are being equipped with sources for producing dc high voltages. Because of the diversity in the application of dc high voltages, ranging from basic physics experiments to industrial applications, the requirements on the output voltage will vary accordingly.Testing with very low-frequency voltage
In the earlier years when electric power distribution systems used mainly paper-insulated lead covered cables (PILC) on-site testing specifications called for tests under dc voltages. Typically the tests were carried out at 4–4.5V0.The tests helped to isolate defective cables without further damaging good cable insulation. With the widespread use of extruded insulation cables of higher dielectric strength, the test voltage levels were increased to 5–8V0. In the 1970s premature failures of extruded dielectric cables factory tested under dc voltage at specified levels were noted. Hence on-site testing of cables under very low frequency (VLF) of approximately 0.1Hz has been adopted.
Comments