Fault Location in Double Wye Shunt Capacitor Banks

Abstract

Shunt capacitor banks (SCBs) are usually used for providing reactive power support to power systems. Reactive power support leads to power factor correction, voltage regulation and reduction of network losses. Outage of the SCBs may affect the power system. Thus, SCBs should be well protected for efficient operation of power system. SCBs consist of many capacitor elements connected in series and parallel combinations. Failure of capacitor elements leads to cascaded failures within the SCB if left undetected. Thus, early detection of internal failures in SCBs is crucial. Double wye SCB con figuration is commonly used for reactive power support in high voltage transmission systems. Unbalance protection methods are mostly used for protecting SCBs against internal faults. Among them, neutral current unbalance methods are sensitive and are commonly used to locate the internal faults in double wye SCBs. Locating the internal faults helps in speeding up the repair process and reducing the outage time. One of the limitations of existing neutral current unbalance methods is that they fail to detect simultaneous faults. Simultaneous faults are those faults which happen at the same time or happen between two consecutive protection passes. During some simultaneous fault conditions, existing neutral based methods may misinterpret the fault condition as a healthy condition. In some other cases, one type of fault may be misinterpreted as another type of fault. The severity of fault may also be misinterpreted during simultaneous faults. Misinterpretation of fault type and severity delays the repair process. There is a need for a fault location method which overcomes the drawbacks of the existing methods. In this thesis, a novel method is proposed to detect different types of internal faults in grounded and ungrounded double wye SCBs. The proposed method detects different types of single and consecutive faults in double wye SCBs. The proposed method can also locate simultaneous faults happening in any of the two legs of the bank. This method uses compen- sated negative sequence quantity and compensated neutral currents to locate the fault. In the proposed method, compensated quantities are obtained by subtracting the pre-fault quantities from the quantities during fault. This helps in cancelling the effect of pre-fault conditions. The advantage of the proposed method compared to the existing methods is that it can lo- cate simultaneous faults. The proposed method is analyzed under different practical scenarios such as system voltage unbalances, temperature effects, switching transients, external faults and manufacturing unbalances. Severity of fault is indicated by the number of failed elements. Proposed method can also calculate the number of failed elements accurately. Simulation studies have been performed on a test system developed in PSCAD software which validates the proposed method. Fuseless ungrounded bank and grounded fuse type bank con gurations have been simulated. It has been found that the proposed method performs satisfactorily under conditions like load switching, external faults and voltage unbalance. Per- formance of the proposed method during internal faults like single faults, simultaneous faults and consecutive faults has been tested. The proposed method performs effectively even during external fault and load switching conditions. A laboratory scale test setup consisting of fuseless ungrounded SCB has been developed. It has been found that proposed method locates different fault types like single and simultaneous faults accurately. Both software and hardware results validate the successful working of the proposed method