Lightning Arrester Testers
Theory Lightning Arrester Test
Lightning Arrester – Theory
A Lightning Arrester is a device used on electrical power systems and telecommunications systems to protect the insulation.
and conductors of the system from the damaging effects of lightning. The typical Lightning Arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge) travels along the power line to the Arrester, the current from the surge is diverted through the Arrestor, in most cases to the earth.
If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilo Volts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices.
Lightning-produced extreme voltage spikes in incoming power lines can also damage electrical home appliances that’s why it is damn crucial to the integrity of Lightning Arrester.
Presently the monitoring of total leakage current (capacitive and resistive currents) is being used by many utilities. The Leakage Current Monitors are used to measure the Leakage Current of Surge Arrestors, and in case of high leakage current Surge Arrestors are replaced. However, it is felt that this method is not the fool proof method as the total leakage current, which is purely capacitive, does not signify precisely the health of the Surge Arrestors. There have been the cases when the Surge Arrestors have blasted even though total leakage current value was below the limit prescribed by the manufacturers.
Resistive current is 15-30% of total current and since capacitive and resistive currents are at 90 degree face shift even considerable change of resistive current results in very small increase in the total current. Hence monitoring total leakage current may not truly indicate the degradation of ZnO disc. Degradation of long linear ZnO disc generally leads to harmonics in the leakage current when system voltage of fundamental frequency is applied. Third harmonic resistive current measurement is based on filtering of third harmonic component from the total leakage current. Leakage current of the order of about 500 micro amps is generally considered to be safe.
The resistive part of the leakage current or the power loss can be determined by several methods given below:
Using a voltage signal as reference
Compensating the capacitive component by using a voltage signal
Capacitive compensation by combining the leakage current of the three phases
Third order harmonic analysis
Direct determination of the power losses
Third order harmonic analysis with compensation for harmonics in the voltage
Advance Monitoring System with “resistive current” component calculations.
The use of advance diagnostic methods greatly reduces the chances of failure & hence avoids loses of man and money. It is therefore desirable to check the condition of Surge Arresters at regular time intervals, by measuring the resistive component of the continuous leakage current in service without de-energising the Arrester. Reliable measurements are achieved by the instruments based on the principle of “Voltage Signal” as a reference .
Regular monitoring of LA has prevented many failures in 66 kV to 765 kV substations. The values of this current normally ranges from fractions of milli ampere to a few milli ampere, and are characterized by a resistive current variations whose value is an indicator of the deterioration of the Surge Arrester.
The resistive component of this leakage current may increase due to different stresses causing ageing and finally causing Arrester failures.