Failure Investigation of Series Capacitors on Transmission Lines and Novel Technique to Mitigate the Damage during Fire on the Platform

The diversification of generation, transmission and distribution, in addition to long transmission distances are resulting in an increased demand for economic and reliable operation of transmission systems. Further the rapid pace of renewable energy integration in Indian power sector and their concentration in few pockets of northern and western parts of India, necessitates the increased long distance power transfer capability of transmission lines. The increment in power transfer capability means either new transmission lines or an increase in capacity of the existing lines by re-conductoring or series compensation. In today’s world the increasing difficulty in obtaining right-of-way for new power lines coupled with the rising cost of new power lines, are making the use of a series capacitor bank a favourable option for utilities.

Series Compensation is proven method for improving power transfer capability of existing line with minimum outage of transmission system. The addition of series capacitor not only increase power transmission capability but also improves system stability. Many series capacitors on important long distance EHV transmission lines are installed and are under operation in POWERGRID since last more than fifteen years.

Series capacitor bank on the EHV transmission lines are used with combination of MOV (metal oxide varistor), spark gap, damping circuit and bypass breaker. All these components except by-pass breakers are installed on the platform on per phase basis.

In last few years POWERGRID has experienced multiple failures of series capacitor banks on the transmission lines. These failures mainly include sudden fire on capacitor units during internal as well as external faults and failure of MOVs. Since, major components of series capacitor banks are on the platform, during any fire incident the damage to the different components are very high. Mostly all the platform equipment like capacitor banks, MOVs, spark gap, signal column, damping circuit and control & protection measuring equipment needs replacement after any fire incident. These causes huge financial loss to the utility, also since the major equipment of the series capacitor banks are imported hence the restoration time after any failure is very high.

In this paper we will discuss about the root cause of different failures of series capacitor bank across POWERGRID. We will discuss in detail about the behaviour of MOV during line faults. The varistor role in the protection of capacitors in series capacitor applications is very simple and at the same time very unique. Simple, because the arresters are installed for one purpose only and that is to limit the voltage across the capacitors during a fault on the system and unique, because while limiting the voltage during a fault, they also conduct a large portion of the fault current. There are very few other applications of arresters where conduction of power frequency fault current is the responsibility of the arrester. We will discuss in this paper that how external faults are more prone to MOV failure as against the common understanding that internal faults lead to MOV failure.

This paper will also discuss in detail about the estimation of terminal voltage across the MOV using current through capacitor bank. Since, there is no provision for direct measurement of voltage across the MOV terminal, method of estimation of voltage discussed in this paper will help in understanding the behaviour of MOV during line faults.

In addition to above, this paper will discuss about the innovative methods adopted by POWERGRID to mitigate the risk of failure of series capacitor banks. In the end we will discuss about the innovative methods adopted to contain the fire on the platform in case of failure of capacitor units. We will discuss that, how using some innovative technique POWERGID has reduced the risk of damage of platform equipment during fire, reduced the downtime for restoration and reduced the financial loss.

The techniques discussed in this paper are unique and the analysis presented are from the real-world field events from the Indian power sector. We hope that the case studies presented in this paper will help the utilities and practicing engineers in better root cause analysis during failure of series capacitor banks, reduced downtime during breakdown and reduction in the financial loss during any fire incident on series capacitor bank.