Insulation Coordination in High Voltage Electrical Power Systems

Each component of the power system, whether it is a simple string insulator or a large power transformer, is continuously stressed by the system operating voltage at power frequency. Occasionally over-voltages occur, having a peak value exceeding the peak value of the system operating voltage. The overvoltages can be divided into three categories:

·      Overvoltages caused by lightning discharges,

·      Switching overvoltages, and

·      Sustained AC overvoltages.

The lightning discharge is a current injection either in the vicinity of a line or substation, in the transmission line tower, or directly on the line.The voltages developed across the power system components depend on the characteristic impedance of the components. The waveforms of the lightning-induced overvoltages are all different in amplitude and capricious in shape. For the sake of testing the dielectric impulse strength of power apparatus in the high-voltage laboratory, the impulse voltage wave has been standardised with the 1.2/50-microsecond waveform.

Switching overvoltages result from switching operations in the network. Switching of a short-circuit current, clearing a short-line fault, disconnecting unloaded transformers, disconnecting or connecting unloaded distribution cables or transmission lines, all result in damped oscillatory voltages, the so-called transient recovery voltage or TRV.

AC overvoltages occur when the 50- or 60-Hz operating voltage temporarily attains a high value at the receiving end of a transmission line after a sudden loss of load. The resistive and reactive voltage drop disappears and the overvoltage stresses the system until the operating voltage is restored. Another situation for AC overvoltages to occur is in the case of a single-phase-to-ground fault in an isolated neutral system.

The healthy phases rise from phase voltage until square root 3 times higher line voltage. Also, the capacitance of an unloaded distribution cable in combination with the inductance of a power transformer or generator can increase the system voltage resulting in a sustained AC overvoltage.The IEC defines insulation coordination as ‘the selection of the dielectric strength of equipment in relation to the voltages that can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available protective devices.’ This means that the insulation level of the power system components should be such that its tolerance level under transient voltages is higher than the level to which transient voltages will be limited by protective devices. Power system components have to withstand the influence of overvoltages for a lifetime of thirty to fifty years. For this reason, tests have been specified and standards have been written. Equipment is designed according to the specification of the standards, but it remains often, if not always, necessary to demonstrate the adequacy of the design of such equipment by testing in the high-voltage and in the high-power laboratory. The testing laboratory issues a type test certificate, a document indicating that adequate confidence is provided and that a duly identified product design complies with the requirements of a specific standard. A certificate contains a record of a series of type tests carried out strictly in accordance with a recognised standard and the equipment tested has fulfilled the requirements of this standard. The relevant ratings assigned by the manufacturer are endorsed by the testing laboratory. The certificate is applicable only to the equipment tested and contains the essential drawings and a description of the equipment tested.Standards for the testing of power system components are issued by IEC and ANSI/IEEE. They are based on extensive system studies carried out by the numerous CIGRE working groups of the different study committees. The independent high-power laboratories are united in the short-circuit testing liaison (STL). STL issues guides for short-circuit testing of high-voltage apparatus.