Overvoltages and Insulation Requirements
The insulation requirements for lines, cables and substations are of critical importance in the design of power systems. When a transient event such as a lightning strike, a fault or a switching operation occurs, the network components can be subjected to very high stresses from the excessive currents and voltages that result. The internal insulation of individual items of plant (for example, generators and transformers) is designed to withstand the voltage transients that are anticipated at the location of the equipment. In addition, the insulation of the overall system, for example the number of insulators per string, and clearances of overhead lines, is designed to withstand these transient voltages. In some cases, additional devices such as over-running earth wires, lightning arrestors or arc gaps are provided to protect items of plant.
In earlier years, lightning largely determined the insulation requirements of both the transmission and distribution system. With the much higher transmission voltages now in use and with large cable networks, the voltage transients or surges due to switching, that is the opening and closing of circuit breakers, have become the major consideration.
A factor of major importance is the contamination of insulator surfaces caused by atmospheric pollution. This modifies the performance of insulation considerably, which becomes difficult to assess precisely. The presence of dirt or salt on the insulator discs or bushing surfaces results in these surfaces becoming slightly conducting, and hence flashover occurs.
A few terms frequently used in high-voltage technology need definition. They are as follows:
1. Basic impulse insulation level or basic insulation level (BIL): This is the reference level expressed as the impulse crest (peak) voltage with a standard wave of
Electric Power Systems, Fifth Edition. B.M. Weedy, B.J. Cory, N. Jenkins, J.B. Ekanayake and G. Strbac. © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.
Figure 10.1 Basic impulse waveform; Shape of lightning and switching surges; the lightning impulse has a rise time of 1.2 ms and a fall time to half maximum value of 50 ms (hence 1.2/50 wave). Switching surges are much longer, the duration times varying with situation; a typical wave is 175/3000 ms
1.2 x 50 ms wave (see Figure 10.1). The insulation of power system apparatus, as demonstrated by suitable tests, shall be equal or greater than the BIL. The two standard tests are the power frequency and 1.2/50 impulse-wave voltage withstand tests. The withstand voltage is the level that the equipment will withstand for a given length of time or number of applications without disruptive discharge occurring, that is a failure of insulation resulting in a collapse of voltage and passage of current (sometimes termed 'sparkover' or 'flashover' when the discharge is on the external surface). Normally, several tests are performed and the number of flashovers noted. The BIL is usually expressed as a per unit of the peak (crest) value of the normal operating voltage to earth; for example for a maximum operating voltage of 362 kV,
so that a BIL of 2.7 p.u. = 810 kV.
- 2. Critical flashover voltage (CFO): This is the peak voltage with a 50% probability of flashover or disruptive discharge (sometimes denoted by V50).
- 3. Impulse ratio (for flashover or puncture of insulation): This is the impulse peak voltage to cause flashover or puncture divided by the crest value of power- frequency voltage.