CAUSES OF OVER-VOLTAGE SURGES

PHENOMENACAUSESEFFECT1. LightningCoulds get charged during thunderstorms, the high potential gradient causes breakdown of insulation of air producing a lightning stroke, which hits the earth, and is attracted by overhead lines.Line insulators flashover or puncture.Travelling waves carries its effects to sub-stations and Power Station. Insulation of the system is stressed.

PHENOMENACAUSESEFFECT

2. SwitchingBreaking Inductive Circuits: energy stored in inductances gives rise to a potential rise across capacitors.

Switching-on capacitive line: charging currents gives rise to a travelling wave.Other switching operations.CB voltage stressedRestrike possibleWave travels to both sides of CB.Voltage Oscillation in sub-circuit.(a) Transmission line insulation stressed.(b) Terminal apparatus insulation stressed by travelling waves.

PHENOMENACAUSESEFFECT

Arcing Grounds

4 Sudden Changes In System ConditionLine to ground capacitive charge gets discharged through earth fault.Repeated charging and discharging gives rise to intermittent arcs.

Energy in inductances transferred to capacitances.Temporary fault grows into permanent fault.Voltage rises to 3 or 4 times of normal values.Voltage of healthy lines increase.Voltage Oscillations,Travelling waves.

PHENOMENACAUSESEFFECT

Resonance

6. Travelling WavesThe fault causing resonance between inductance and capacitance in a part of circuit.High voltage waves get reflected on reaching a junction or end.Rapid increase in over-voltages, insulation failure likely to occur.

Reflected wave gets superimposed on initial waves; Voltage rises to several times normal values,Insulation failure likely to occur.

Methods of Reducing Switching Overvoltages

Condition of switchingMethod of reducing switching overvoltage1. Energising an uncharged line.High voltage shunt reactors connected to line to reduce power frequency-overvoltages.

Condition of switchingMethod of reducing switching overvoltage2. Elimination of trapped charge on the line

Line shunting after opening 1

R

3. Reduction of current chopping Opening Resistors (Resistance Switching) R 2 1

Condition of switchingMethod of reducing switching overvoltageReducing the switching overvoltages due to closing

Single stage pre-closing resistors insertion R 1 2

Two stage pre-closing resistors insertion 2 R R

1 3

Condition of switchingMethod of reducing switching overvoltage

Closing resistors in line between circuit breaker and shunt reactor 1 R

2Reducing switching overvoltages by improved switching sequenceSynchronous switching of three poles

Condition of switchingMethod of reducing switching overvoltage

6. Use of lightning arrestersSimultaneous operation of circuit breakers at both ends of lines

While closing of line

Condition of switchingMethod of reducing switching overvoltage

While disconnecting reactor

Symbol: R

CB RESISTOR LIGHTNING REACTORS ARRESTER1, 2, 3 ETC. SEQUENCE OF OPERATION