What are insulators
Insulators are used to provide electrical insulation between a live conductor and earthed structure under the operating and overvoltage conditions. It acts as a mechanical link between the structure and the conductor, it keeps the mechanical integrity under normal operating and overload condition
What is insulation coordination
It is the process to determine the proper insulation level for various components in a power system, it is the selection of an insulation structure which could withstand the voltage stresses to which the system or the equipment will be subjected to
Why do we need insulation
- It could withstand normal and abnormal stresses
- Efficient discharge of overvoltages can be done
- Safety can be ensured for operating personal and public
- It could ensure the current flow only in the conductors
- It could prevent the equipment damage due to overvoltage
What are the types of overvoltages that need to be considered while doing an insulation coordination
Three types of overvoltages can occur in a plant they are
- Internal over voltages
- Switching surges
- External over voltages
Internal overvoltages
It could be short power frequency overvoltages or weakly damped oscillatory voltages major causes of these overvoltages are
- Load rejection
- Ferro resonance
- Ferranti effect
- Phase to earth faults: single line to ground, double line to ground, 3 phase to ground
Switching surges
They have a short duration which will be irregular or highly damped such overvoltages have great concern if the transmission voltage is greater than 300KV
Some causes of these voltages below 300KV
- Resonance effect while switching transformer feeders cables or overhead lines
- Line energization could cause switching surges mostly at the remote end of the line which is being energized
External overvoltages
Overvoltages from lightning are a major concern for power system which operates below 145KV
Insulation flashover will be depended on
- Magnitude of the stroke
- Rise time of voltage wave
- System insulation levels and electrical characteristics
- Local atmosphere or ambient conditions
How insulation breakdown takes place
- Chemical – oxidation, hydrolysis …
- Mechanical – cracks, channels, tracks…
- Thermal – overheating
What are the major steps to determine the insulation coordination
- Determination of live insulation
- Selection of BIL and insulation levels of other equipment
- Selection of lightning arrestors
Substation insulation coordination for lightning surges
Even if the substation is shielded properly lightning surges can enter the station indirectly. All stations need a connection to the rest of the system through incoming and outgoing overhead conductors, so that all the following pertains to overhead air-insulated substations only. If the lightning strikes either incoming or outgoing lines a surge will enter the stations on the conductors
Separation distance
It is very important for the protection of substations and insulation coordination of substation, protected insulated must be close to the arrester so that it can be protected. Before proper insulation coordination, the distance and location of the critical insulation need to be known
Substation insulation coordination for switching surges
It is only concerned only on systems which have 245KV and above, switching surges magnitude does not exceed 1.5pu for systems below 245KV and it is because of the line capacitance, voltage is not high enough to result in challenging surges. These surges can be reduced by using resistors or arrestors
Need for arrestors for insulation coordination
It is the major part for insulation coordination in substation it is mostly used to protect the non-self-restoring insulation of power transformer
Arresters used in substations characterized by three voltages relative to insulation coordination they are the arrester operating voltage, lightning impulse protective level, and switching impulse protective level