Switching Surges due to External and Internal factors

• The above all were the protection systems against the direct lightning strokes which is purely natural and we are not having any control over it.
• Switching surges are the over-voltage stresses applied on the traction system, and are generally transient in nature.
  
• The surge voltage is defined as the sudden rising of voltage to a high peak in very short duration. These surges are transient in nature that means they exist for very short duration of time.
• The main cause of these voltage surges are due to switching impulses of the system.
• It may also cause due to insulation failure, arcing ‘ground or due to resonance etc.
• If the over voltage protection is not provided to the traction system, there may high chances of severe damage.
• There are mainly three types of methods mainly used for the protection against switching surges 

• It is generally used for electrical traction substation. In this arrangement a net of GI wire is mounted over the substation.
• The GI wire, used for earthing screen is properly grounded through different substation structures.
• This network of grounded GI wire over traction substation provides very low resistance path to the ground for switching surges.
• This method of high voltage protection is very simple and economical but the main drawback of the system is that it cannot protect the system from the travelling waves which may reach to the substation through different feeders.

• This method of providing high voltage is very much similar to the earthing screen.
• The only difference is, an earthing screen is a traction substation, whereas, placed over overhead earth wire is placed over the traction network.
• One or two GI wires of suitable cross-section are placed over the traction lines. Also, each traction pole or tower is properly grounded.
• These overhead ground wires or earth wires divert all the surges to the ground instead of spreading throughout the traction network.

(c) Surge Arresters

• The earlier methods i.e. earthing screen and overhead earth wires are very suitable for protecting and electrical traction systems.
• But these methods cannot provide any protection against high voltage travelling waves which may travel upto the locomotive through the overhead conductor
• The surge arrester or lightning arrester is a device which provides a very low impedance path to the ground for high voltage travelling wave.
• The concept of surge arrester is very simple. This like device behaves nonlinear electrical a resistance.
• The resistance decreases as voltage increases and vice-versa, after certain level of voltage.
• The function of surge arrester can be listed below.

    (ii) On occurrence of voltage surge in the system, these devices provide very low impedance path for the excess charge of the surge to the ground.

    (iii) After conducting the charges of surge, to the ground, the voltage becomes to its normal level. Then lightning arrester regains its insulation properly and prevents regains its insulation property and prevents further conduction of current, to the ground.