• Location of Series Capacitor

    Location of series capacitor (i.e., compensation point) should be as close as possible to the load point for better effect of voltage compensation. Series capacitor banks are generally located at either end of the transmission line section at one of the existing substations. Generally a shunt reactor will be installed along with the series compensation facility to keep voltages down during periods of low flow. The reactor can either be on the bus side of the series compensation or on the line side.



    Results in lower short circuit current through the series capacitor equipment.

    Improves the effectiveness of series capacitors.

    Lower the rating of bypass equipments.


    Access issues: access to capacitor banks and equipment for maintenance and operations is not easy.

    The addition of series compensation installations along existing right of ways is not always possible;


    Protection of Series Capacitor

    Today, with the increasing difficulty in obtaining right-of-ways for new power lines coupled with the rising cost of building lines, the use of a series capacitor bank is increasingly seen as a favourable option. Moreover, for long transmission lines, where source and load are separated by great distance, the use of fixed series compensation (or FSC) has now become almost a necessity.

             The capacitor bank is usually rated to withstand the line current for normal power flow conditions and power swing conditions. It is not economical to design the capacitors to withstand the currents and voltages associated with faults. Under these conditions, capacitors are protected by a metal oxide varistor (MOV) bank. The MOV has a highly nonlinear resistive characteristic and conducts negligible current until the voltage across it reaches the protective level. For internal faults, which are defined as faults within the line section in which the series capacitor bank is located, fault currents can be very high. Under these conditions, both the capacitor bank and MOV will be bypassed by the "triggered spark gap." The typical protective system for series capacitor consists of a metal oxide varistor, bypass gap, damping reactor and bypass circuit breaker.


    Description of Main Components

    Capacitors: The capacitor bank for each phase consists of several capacitor units in series-parallel arrangement, to make up the required voltage, current, and MVAr rating of the bank.


    Metal Oxide Varistor: Metal oxide varistors, which are connected in parallel with the capacitors, provide overvoltage protection of the capacitors under fault condition and thus are conducting a large part of the fault current. The MOV is usually rated to withstand energy discharged for fault in the system external to the line section in which the series capacitor bank is located. The energy discharged through the MOV is continuously monitored and if it exceeds the rated value, the MOV will be protected by the firing of a triggered air gap, which will bypass the MOV.

    A metal oxide varistor (MOV) is built from zinc oxide disks in series and parallel arrangement to achieve the required protective level and energy requirement. The number of parallel zinc oxide disk columns required depends on the amount of energy to be discharged through the MOV during the worst-case design scenario.


    Triggered Air Gap (or Spark gap): The triggered air gap provides a fast means of bypassing the series capacitor bank and the MOV system when the trigger signal is issued under certain fault conditions (e.g., internal faults) or when the energy discharged through the MOV exceeds the rated value. It typically consists of a gap assembly of two large electrodes with an air gap between them.


    Damping Reactor: The damping reactor limits the capacitor discharge current resulting from gap sparkover or bypass breaker closure and damps the oscillations caused by spark gap operation or when the bypass breaker is closed. The amplitude, frequency of oscillation, and rate of damping of the capacitor discharge current will be determined by the circuit parameters, C (series capacitor), L (damping inductor), and resistance in the circuit.

    The damping circuit consists of an air-core reactor with a parallel-connected damping resistor. In series with damping resistor there is a small spark gap which connects the resistor to the circuit only during capacitor bank discharge and thus, minimizes the losses when the bank is bypassed.


    Bypass Breaker: The bypass switch is part of the protective system of series capacitor banks, together with metal oxide varistors (MOV)  and (if applicable) protective spark gaps, and is placed in parallel with the series capacitor bank. The main purpose of the by-pass switch is for deliberately by-passing and inserting the series capacitor (planned operation)  and for protective by-passing of the series capacitor in case of faults. While the bypass breaker closes automatically in the case of prolonged gap conduction or other platform contingencies.

    The bypass breaker is usually a standard line circuit breaker with a rated voltage based on voltage across the capacitor bank. In most of the installations, the bypass breaker is located separate from the capacitor bank platform and outside the safety fence. This makes maintenance easy.


    There are basically two types of faults of concern for series capacitor installations. If the fault occurs within the section of transmission line where the bank is located, it is referred to as an internal fault while if a fault occur on another section of line, it is considered an external fault. The difference between the two is only the magnitude of the fault.

    A nearby internal fault will result in significant fault current and significant duty placed on the varistors. A distant fault on an external section can result in a current only a few percent higher than the maximum rated for the capacitor bank. These two faults are therefore handled differently by the control system and the MOVs. If the control system senses a high current internal fault, it sends a signal to the triggered gap to bypass immediately. This pulls the MOVs out of the circuit before any significant accumulation of energy can take place. When an external fault occurs, the protection controls are all that the MOVs have between operation and overload. If the control system fails, it is likely that the MOVs will too. To achieve a high energy absorption capability, MOV columns are mounted in parallel.

    Prior to the development of the high-energy zinc oxide varistor in the 1970s, a silicon carbide nonlinear resistor was used for overvoltage protection. Since zinc oxide varistor has better (in comparison with silicon carbide) nonlinear resistive characteristics, provides better protection, and has become the standard protection system for series capacitor banks.


    Capacitor Bank

    Single protection of series capacitor bank



    Capacitor Bank

    ZnO protection of series capacitor bank



    Use of available space, no access issues when in substation.

    Easier access to capacitor banks and equipment for maintenance and operations.

    No additional site to acquire.


    Lower effectiveness meaning more compensation for the same impact.

    Increased short circuit-circuit currents require higher bypass equipment ratings.


    Sometimes, series compensation either consist of a single capacitor bank located halfway along the transmission line or two capacitors each located one-third of the length between the  line ends. in order to reduce the worst-case potential fault current.