Over-voltages and protection against them

Over-voltage:
                      Every electrical equipment is designed to operate at maximum supply voltage. If the voltage in the equipment is increased beyond its maximum limit, it is known as over-voltage. There are various types of over-voltages based on duration of their lasting and even on their nature. They are as listed below.

 1.Temporary Over-voltages

                      These over-voltages are of short power frequency. The main causes of temporary over-voltages are (a) Load rejection

      (b)Zero Sequence

      (c)Phase to earth faults

2. Switching Surges

                     These over-voltages are of short duration. The effects of these over-voltages are very much pronounced when the voltage is greater than 300kV. The causes of these voltages are

(a) Resonance effects when switching transformer feeders

(b) Line energisation especially at the remote end of line that is energised

3.Lightning Over-voltages

                      These occur for a very short duration. The effects of this over-voltage are far more pronounced compared to the above two types. As, the name itself suggest the primary cause of this over-voltage is lightning strike.
Protection Schemes for
A.Temporary Over-voltages

1. Using over-voltage relays
2. Using fast acting static VAR source 

B.Switching Over-voltages

1. Using surge absorbers
2. Using restrike free circuit breakers

C.Lightning Over-voltages

1.  Using surge arresters and spark gaps   
2. With the help of neutral grounding
3. Using overhead earthed conductors