Function of Surge Arrester Surge Arrester is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current.It prevents continuous flow of follow current to ground and is capable of repeating these functions.Creepage distance 25 mm / kV.

Working principle of LARef FigureThe LA conducts at Impulse Spark over Voltage and surge is diverted to the Earth.The voltage across the LA remains at Residual Value B for a short time.

Power follow current (Follow current) :

The current which flows from connected power source through lightning arrester following the passage of discharge current.So the arrester has to act as a circuit breaker to quench this current. The time given for this function is half a cycle.

Rating of LAArrestor ratings ( IS 3070/ 1974 Part1)1) Voltage rating of LA= Line to line voltage 1.1 coefficient of earthing.(Assuming coefficient of earthing equals 0.8 for effectively earthed system ie, ELG/ ELL)Voltage rating of 110 KV, LA= 1.1 1100.8 = 96.8KV

Rating of LA2) Power frequency spark over voltage = 1.5 Voltage rating of LA.Power frequency spark over voltage = 1.5 96.8 = 145.2KV3) Normal Discharge current: It is the surge current which flows through the LA, it is expressed in crest value (peak value) for a specified wave shape 8/20 Second. Example 10, 5, 2.5, 1.5, 1kA rating.

General Classification of LAs 1) Distribution Class: For lighter duty up to 34.5 kV.2) Intermediate class: For moderate duty up to 138 kV.3) Station Class:For heavy duty applications from 2.4 KV to 800 KV.

Classification based on working principle.Gapless Arrester: Zinc Oxide Arrester.It offers extremely high impedance at normal voltage and very low impedance at high voltages.It will work like a Zener diode.High purity ZnO is a semiconductor. ZnO with additive materials such as Bi2O3, MnCO3, and Sb2O3 are added to obtain the required characteristics.

Zinc Oxide ArresterEach ZnO grain will be coated with Bi2O3 which is insulating in nature.This mix is compressed at high pressure and temperature (1200 0 C) to form hollow cylindrical discs.Out side diameter of light duty LA element is 50mm to 70mm and that of heavy duty is 90 mm.Each ZnO grain will be surrounded by thin barrier of insulating material which will not conduct at normal voltage.

Micro structure and equivalent circuit of Zn O Arrestor.

Characteristics of Zinc- oxide element.The electrical equivalent circuit of Zn O element.rRC

Comparison of ZnO Arrestor and SiC Spark Gap Arrestor

Comparison of ZnO Arrestor and SiC Spark Gap Arrestor Zn O ArresterSiC ArresterCharacteristics of Impulse responseNormal operating duty

Comparison of ZnO Arrestor and SiC Spark Gap ArrestorFrom the comparative study it may be seen that ZnO has practically no follow current.The current at operating voltage of SiC arrester is of the order of 10-2 Amps but it is only 10 4 in the case of ZnO.So SiC need a gap to isolate the system.

Comparison of ZnO Arrestor and SiC Spark Gap Arrestor

SiC arrester respond only to surges of large magnitude.But ZnO arrester will respond for all type of surges.Quick response during steep front surges.Discharge energy level = 5KJoules/KV.Better contamination and pollution performance.In order to reduce follow current in SiC arrester, its protective level is increased which will reduce the protective margin. So ZnO offers better protective margin

VOLTAGERegion 1Region 2Region 3Volt- ampere characteristics of Zn O element.Leakage regionNon linear conduction RegionHigh Current Region

In region 1 the metal oxide composition will appear to be a ceramic capacitor, with capacitive and resistive currents having a dissipation factor in the range 5 % to 20 %.Region -2 : is the nonlinear region. Variation of current with voltage is nonlinear.Region 3 : High current density region. Losses the nonlinearity due to the linear resistance of ZnO grains.Volt- ampere characteristics of Zn O element.

VOLTAGELeakageRegionNon linear conduction RegionHigh Current Region Volt- ampere characteristics of Zn O element.Discharge currentNormal operating voltageCOVMCOVSurge current 10 KA

Special characteristics of ZnO1) MCOV : Maximum continuous Operating voltage (MCOV) is the maximum rms value of power frequency voltage which can be applied continuously between the arrester terminals.

Special characteristics of ZnO2) COV : Cross over voltage or threshold voltage or critical voltage. It is the voltage at which the ZnO element go into conduction and ceases to conduction when voltage drops below this level.

VOLTAGELeakageRegionNon linear conduction RegionHigh Current Region Volt- ampere characteristics of Zn O element.Discharge currentNormal operating voltageCOVMCOV

Specification of Gapless arrester.

System voltage kV rmsArrester rating kV rms MCOV kVrms COV kV crest119 73330263866605183132120102184220198168290400360306490

Technical Specification as per IEC 99-4 for ZnO Arrestor

Max. System Voltage kV420245145System BIL kVp14251050650Arrester Rated Voltage kV390198120MCOV kV 306168102Normal Discharge Current kA101010Line Discharge Class433 or 2High Current withstand kA100100100

Technical Specification as per IEC 99-4 for ZnO Arrestor

Max. System Voltage kV420245145Creepage Distance mm 1050061253625Weight kg570230135Height mm426527551560