The correlation of the insulation of the various components in a power system to the insulation of the protective devices used for the protection of that equipment against overvoltage is known as the insulation Coordination.
In the power system, there are different equipment working at different voltage levels. The levels of withstanding the voltage of different equipment are different.
There are different protective devices to give protection against the overvoltage. It is necessary to make coordination of all these. This is called insulation coordination.
The following points are considered in insulation coordination:
- Study the possibility of overvoltage coming due to lightning or the other way.
- Make use of a suitable device to give protection against the over-voltage surge.
- Keep-the protective device such that the protected equipment does not get damaged.
- Achieve this, suitable co-ordination of the strength of insulation of line, switchgear, transformer, machine, and the characteristics of protective devices like rod gap, lightning arrester, etc. is made.
Thus, the insulation coordination is defined as the making suitable co-ordination of the capability of the equipment of withstanding the impulse voltage protection characteristic of the surge arresters.
The above figure says that, how equipment is given protection against overvoltage through the lightning arrester.
The above figure says that, the insulation characteristic of the equipment, and that of the protective device is given. The characteristic of the equipment is shown by curve P and the coordination curve of the protective device is shown by curve Q.
From the curves, it is seen that the insulation characteristic of the equipment is much higher than that of the protective device. So, when the over-voltage surge comes, the spark over will take place first in the lightning arrester.
Choice of Insulation Levels for Substation Equipment
Basic Insulation Level (BIL): The value of the voltage the insulation of equipment can withstand without being damaged is called the basic insulation level of equipment.
Basic Insulation Level’ is a term which includes the following characteristics of the equipment.
- Power frequency voltage withstands level.
- Lightning impulse voltage withstands level.
- Switching impulse voltage withstands level (applicable to transformer windings rated 220 kV and above).
This is expressed in two ways: The first is the impulse withstand level which is the capability of withstanding specific type of the impulse voltage in specified conditions (in kV).
Second is the capability of withstanding the R.M.S. value of the power frequency wave for one minute. For example, equipment is designed for 11 kV and its impulse voltage level is 75 kV and its capability of withstanding the r. m. s. The value of the power frequency voltage for one minute is 35 kV.
The figure shows the line diagram of a substation. This includes circuit breaker, transformer, isolator, bus bar insulator, line insulator, etc.
All these are used to protect against lightning so proper insulation coordination of all this equipment has to be made.
The insulation levels of the equipment from the curve as under:
- Lightning arrester: 525 kV
- Transformer: 700 kV
- Line insulator: 890 kV
- Circuit breaker insulator: 1090 kV
- Circuit breaker insulator: 1180 kV
Thus, when the surge wave comes, the breakdown of lightning arresters will occur at 525 kV, and all other equipment in substation will remain safe.
The following points should be kept in mind for the insulation coordination of the equipment in substation:
- The spark overvoltage of the lightning arrester should be the minimum.
- The capability to withstand the voltage of all other insulation should be sufficiently more than that of the lightning arrester.
- The insulation coordination should be maintained in the worst conditions such as moisture, rain, snowfall, etc.
- The lightning arrester should work for waves of both the polarities.
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