Insulation Coordination

Insulation coordination comprises the selection of the electric strength of equipment in relation to the voltages which can appear on the system for which the equipment is intended. The overall aim is to reduce to an economically and operationally acceptable level the cost and disturbance caused by insulation failure and resulting system outages.

To keep interruptions to a minimum, the insulation of the various parts of the systems must be so graded that flashover only occur at intended points. With increasing system voltage, the need to be reduces the amount of insulation in the system by proper coordination of the insulating levels become more critical.

Terminology Insulation Coordination

1. Nominal System Voltage: It is the r.m.s. phase-to-phase voltage by which a system is designated.

2. Maximum System Voltage: It is the maximum rise of the r.m.s. phase-to-phase system voltage.

3. Factor of Earthing: This is the ration of the highest r.m.s. phase-to-earth power frequency voltage on a sound phase during an earth fault to the r.m.s. phase-to-phase power frequency voltage which would be obtained at the selection location without the fault.

4. Effectively Earthed System: A system is said to be effectively earthed if the factor of earthing does not exceed 80% and non-effectively earthed if it does.

5. Insulation Level: For equipment rated at less than 300kV, it is a statement of the lighting impulse withstand voltage and the short duration power frequency withstand voltage. For equipment rated at greater than 300kV, it is statement of the switching impulse withstand voltage and the power frequency withstand voltage.

6. Conventional Impulse withstand Voltages: This is the peak value of the switching or lighting impulse test voltage at which insulation shall not show any disruptive discharge when subjected to a specified number of applications of this impulse under specified conditions.

7. Conventional Maximum Impulse Voltage: This is peak value of the switching and lighting overvoltage which is adopted as the maximum overvoltage in the conventional procedure of insulation coordination.

8. Statistical Impulse withstand Voltage: This is the peak value of a switching or lighting impulse test voltage at which insulation exhibits, under the specified conditions a 90% probability of withstand. In practice, there is no 100% probability of withstand voltage. Thus the value chosen is that which has a 10% probability of breakdown. As shows in figure 30 below

Figure 30 Statistical Impulse withstand Voltage

9. Statistical Impulse Voltage: This is the switching or lighting overvoltage applied to equipment as a result of an event of one specific type on the system (line energising, reclosing, fault occurrence, lighting discharge, etc), the peak value of which has a 2% probability of being exceeded. As shows in figure 31 below

Figure 31 Statistical Impulse Voltage

10. Rated Short Duration Power Frequency withstand Voltage: This is the prescribed r.m.s. value of sinusoidal power frequency voltage that the equipment shall withstand during tests made under specified conditions and for a specified time, usually not exceeding one minute.

11. Protective Level of Protective Device: These are the highest peak voltage value which should not be exceeded at the terminals of a protective device when switching impulses and lighting impulses of standard shape and rate values are applied under specified conditions.