Over Voltage / Voltage Surge

A sudden rise in voltage for a very short duration on the power system is known as voltage surge.The overvoltage on a power system may be broadly divided into two main categories.

Internal Causes of Overvoltage

When circuit conditions are changed suddenly changed,oscillations are setup on the power system. These oscillations develop over voltages (twice the normal voltage) that are not severe as lightning surges. Only normal system insulation is needed for protection against over voltages due to internal causes.

• Switching Surges – The over voltages produced on the power system due to switching operations (opening and closing of a line using CB) are known as switching surges.
• Insulation failure – The most common case of insulation failure in a power system is the grounding of conductor (i.e. insulation failure between line and earth) which may cause over voltages in the system.
• Arcing ground – The phenomenon of intermittent arc taking place in line-to-ground fault of a 3φ system with an ungrounded neutral is known as arcing ground. The arcing ground produces severe oscillations (transients) of three to four times the normal voltage. The transients produced due to arcing ground may damage the equipment in the power system by causing breakdown of insulation. Arcing ground can be prevented by earthing the neutral.
• Resonance – Resonance occurs when inductive reactance of the circuit becomes equal to capacitive reactance. Under resonance, the impedance of the circuit is equal to resistance of the circuit and the p.f. becomes unity. Resonance causes high voltages in the electrical system.

External Causes of Overvoltage

• Lightning : An electric discharge between cloud and earth, between clouds, between the charge centres of the same cloud is known as lightning. Surges due to lightning are very severe and may increase the system voltage to several times the normal value.

Types of Lightning Strokes

• Direct stroke
• Indirect stroke