Lightning Arrester (LA)/surge arrester is a protective device used for limiting the affect of surge voltages on the equipment / power distribution system. This arrester helps in discharging the surge current, thus protecting the system and equipment from dangerous over voltages and also disturbances.
Lightning Arrester (LA)/surge arrester is protect substation equipment from lightning strike voltage during rainy season.
Introduction
These lightning arrestors, age out during its period of services due to moisture ingress due to sealing problem, ageing of Zink Oxide variations, dust particles on external surface, cracks on porcelain surface, internal partial discharges.
To avoid the failure of LA, it is necessary to monitor its health because if a lightning arrester fails, it explodes with porcelain splinters and apart from creating a short circuit, it also mechanically damages the other surrounding equipments like CTs, PTs, transformer bushing etc. Thus it creates a total disruption of power circuit.
Constructional details of zinc oxide (zno) Lightning Arrester
Gapless zinc oxide (ZnO) lightening arresters are widely used. These are usually connected between the phase and ground terminals. LA limit the voltage level below the withstand voltage in power equipment such as transformers etc.
LA consists of an insulating housing which is made of porcelain material, and the inner active column, composed of the ZnO varistors.
Why monitor the resistive leakage current of lightning arresters
In normal service a metal oxide surge arrester is carrying a continuous, but small leakage current, typically in the range of 0.1 -2 mA. The leakage current is dominated by a capacitive current component whereas the resistive component may be in the range of 5 -25% of the capacitive component.
Metal Oxide Surge Arresters (MOSAs) are normally operated at a continuous voltage ranging 60% to 80% of the rated voltage.
The non linear characteristic of ZnO blocks entails that even a large increase in the Resistive leakage current Ir is barely noticeable in the total leakage current. The 3rd harmonic component if leakage current is measured health of LA.
Measurement of the resistive current component by the LCM kit is therefore the best way to obtain sensitive and reliable information about the arrester condition.
The lightening arresters with resistive leakage currents in between 350-500 μA should be closely monitored and beyond 500 μA should be replaced & jointly checked with manufacturer.
Principle of Leakage Current Measurement (LCM)
The non linear resistance of the ZnO blocks will introduce a 3rd harmonic resistive component in the leakage current when it is stressed by a sinusoidal voltage.
This current component is the best indicator for detection of changes in the non-linear characteristics of the surge arrester over time.
Harmonic content in the operating voltage will increase the harmonic content in the leakage current.
If we ignore the harmonic content in the leakage current, we will not be able to know if an apparent increase in the resistive current is really due to the faulty arrester or if it is a false increase due to varying harmonic content over time.
Thus it is necessary to take the effect of harmonics into account and compensate for it in the final measurement.
Benefits of LCM Testing
The LCM can monitor the condition of high voltage Surge Arresters in service. The LCM measures the resistive component of the leakage current of the Arresters. Increased resistive current indicates a higher risk of breakdown of the Arresters. The LCM offers the following benefits.
Confident knowledge to replace surge arresters in due time before an arrester failure occurs. The LCM is the most reliable system for diagnosis of metal oxide surge arresters (MOSA).
Capability to prevent costly arrester failure and possible damages to other major equipment in the neighborhoods. An important achievement is an increased safety for the maintenance staff.
No disturbance for the normal operation of the system. The condition of surge arresters is checked without any influence on the operation of the power system.
Assured knowledge of the condition of the surge arresters. This experience is even more important as the age of the arrester increases. It is very satisfactory also to know whether the condition of the components in the supply system is “perfect”, or not.
Systematic information about all measured surge arrester. It is included a comprehensive data base to give all necessary information like the status of each arrester, key data of each item, numbers of measurements, time for next measurement etc.
The instrument should be used as a part of the regular maintenance program to increase the reliability of the supply.
The LCM is a very reliable system for field measurements. The instrument is portable and gives fast results. The method for measurements is well accepted internationally. The LCM is based on harmonic analysis of the leakage current using a principle called third harmonic with compensation. System for measurements of third harmonic resistive leakage current only, is considered very unreliable.