TESTING of EXTRA HIGH VOLTGE LIGHTNING ARRESTOR
- For finding out faulty LA & decision making on replacement.
SUDHIR KUMAR SRIVASTAV
AGM-RAPDRP
NTPC LIMITED, NEW DELHI
Abstract -This paper gives an insight of, how to test Extra High Voltage Lightning Arrestor, to come to a conclusion that whether we may continue with existing LA or there is a need to replace the LA with new one.
Keywords: LA (Lightning Arrestor)
RLA (Residual life assessment)
AC (Alternating Current)
DC (Direct Current)
EHV (Extra High Voltage)
I. INTRODUCTION
A lightning arrester is a device used on electrical power systems to protect the insulation and conductors of the system from the damaging effects of lightning. When a lightning surge travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth. Their purpose is to limit the rise in voltage when a power line is struck by lightning or is near to a lightning strike. If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices.
Lightning arresters built for EHV substation use are impressive devices, consisting of a porcelain tube several feet long and several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder. Lightning arresters are rated by the peak current they can withstand, the amount of energy they can absorb, and the breakover voltage that they require to begin conduction.
With the passage of time & with striking of multiple lightning, the conditions of LAs get deteriorated. But in Extra High Voltage (EHV) system, it is very difficult to come to a conclusion on replacement of old Lightning Arrestor.
In early days, only break down maintenance were in process, where electrical equipment were replaced with new one on its damage. Break down of equipments develops some disastrous situation, recovery of which is very time & resource consuming. Also there is every chance of damage of nearby equipments on break down of any equipment. So to avoid this situation, preventive maintenance have been started. With this practice we replace the equipments after a predefined time, without going into the detail of its healthiness.
In late years predictive maintenance / condition based monitoring / residual life assessment were started to optimize the cost & running time of any equipment.
In case of LA it is very difficult to do RLA & also there is no proven method to test it for taking any concrete decision on healthiness of equipment. Even leakage current meters of most of LAs are not giving correct picture of resistive current & healthiness of equipment.
With the experience & studies on the system we may say that, in addition to third harmonic resistive current measurement of LA, following more tests can be done to decide about the healthiness of LA:
1. DC component of leakage current.
2. Thermal scanning of LA
3. Tan∂ measurement of different stakes of LA.
II. DIFFERENT TYPE OF TEST / MEASUREMENT ON LA
To measure DC component of Leakage curren
Take a new / healthy LA connected with charged EHV system.
- Connect a full wave AC to DC bridge coveters in parallel to leakage current meters, as shown in figure.
- Disconnect the leakage current meter from circuit so that full amount of leakage current flows through bridge circuit.
- Measure the DC component of leakage current (I1) from the bridge as shown in figure.
Infrared Imaging of LA
Infrared Imaging of the LA shall be done after sun set to avoid any error in temperature measurement. If measured temperature through thermal scanning of any LA is 3°C more than the temperature of new / healthy LA, we may short it for other testing. This thermal scanning shall be done when LAs are connected to charged EHV system.
Tan∂ measurement of different stakes of LA
This test is done for disconnected LA from system. Tan∂ test of any equipment is done for resistive / leakage current measurement. These tests are normally done to check the healthiness of insulating material. Though LA is not an insulator but a non linear resistance element, we may treat it as insulator for test voltage and Tan∂ value of different stakes can be measured. If tan∂ value of any stake of LA is more than two times than that of new / healthy stake, we may short it out for other test.
III. Conclusion
Any LA whose DC component of leakage current is more than two times than that of new / healthy one, measured temperature through Infrared Imaging of any LA is 3°C more than the temperature of new / healthy LA. And tan∂ value of any stake of LA is more than two times than that of new / healthy stake we may conclude that the LA is defective and there is an urgent need to replace the LA with new / healthy one.
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