Power System Over Voltages

Power System Over Voltages

Power System Over voltages

Over voltages: These are generated due to :

• Fault
• Load rejection
• Switching operations
• Resonance condition Lighting phenomena
• Loss of ground

Over Voltage Classification

1.Maximum Continuous (Power Frequency) Over Voltage:

It is generated from the system working under normal condition.

2.Temporary Over Voltages :

These are generated due to faults, switching operation due to load rejection, resonance condition etc.

3. Slow Front Over Voltages:

They are generated due to switching and lighting strikes on  over head lines.

4. Fast Front Over Voltages:

They are generated due to switching, lighting strikes or faults.

5.Very Fast Front Over Voltages:

They generated in gas insulated switchgear due to switching and fault over voltages 3, 4, and 5 are called as transient over voltages.

Load Rejection

Here large load suddenly disconnected from the system. If over voltage is generated due

to this, then it is called as over voltage due to load rejection.

Determination Of Line Insulation

While determining transmission line insulation we have to consider.

Flashover due to lighting strike.

Insulation distance (clearance) by having technical and economical balance.

Avoidness of flashover caused by switching surges or sustained lower frequency over voltages.

Consideration of atmospheric air characteristics as in some electrical applications atmospheric air is used for the purpose of cooling and insulation.

Design of line insulation also depends on the protective efforts taken for the reduction of flashover. 

Some of the protective measures are

1. Use of automatic reclosures.
2. Use of over head grounding wires.
3. Reduce the surge impedance of the towers and overhead grounding wires.
4. Use of lighting arcs to avoid travelling waves to get to the substation terminals.