Discriminating Between Capacitor Bank Faults and External Faults for an Unbalanced Current Protection Relay Using DWT

Abstract

Capacitor faults are a common issue in modern-day power systems. Such systems employ a traditional mechanism that solely relies on unbalanced relays as an indicator of faults in capacitor banks; however, in the case of a relay trip, operators may find it difficult to identify the cause of these faults. To address this issue, this study aims to detect and discriminate between faults in the capacitor bank (i.e., internal faults) and those in the transmission line (i.e., external faults) by employing discrete wavelet transform (DWT). For this purpose, simulations were conducted using PSCAD software and modelled according to the 115-kV system from the Electricity Generating Authority of Thailand (EGAT). Furthermore, case studies involving faults with different phases, side and branch connections, row connections, inception angles, and blown fuses were considered. Moreover, the phase current, unbalanced current, and unbalanced voltage were analyzed to elucidate fault behaviors and the variation in the coefficient obtained via DWT; the unbalanced current and unbalanced voltage were used as design criteria for the proposed detection and relay operation algorithm. Real-world signals were employed to verify the accuracy of the proposed model. The results indicate that the proposed algorithm achieves satisfactory accuracy in terms of fault detection and the operation of relays in capacitor banks. The proposed algorithm is expected to enable an improvement in the performance of protection systems for capacitor banks in the near future.

Application of Magnitude and Phase Angle to Boundary Area-Based Algorithm for Unbalance Relay Protection Scheme in 115-kV Capacitor Bank

“…In the simulation, the capacitor bank was energized within 0.15 s; the fault occurred at 0.5 s. As explained in the result of our previous paper [1], the duration before fault occurrence (normal condition) was not affected by the fault, as shown in Fig. 3 and 4.…”

“…Generally, the capacitor bank is protected by an unbalance relay during fault occurrence. As the system followed the EGAT standard [1], the unbalance relay tripped when the four internal fuses "blew up." Therefore, the capacitance of the fault unit in this study was 22.286 µF as shown in Table I.…”

“…6. The operation of the algorithm consists of three procedures as follows: (1) Identifying the protection zone: The efficiency of this procedure is shown in our previous paper [1], wherein the fault in the transmission line (external fault) and fault in the capacitor bank (internal fault) were identified by the following equation:…”

“…The results in Table V show that the methods based on the EGAT standard and phasor diagram were efficient. However, the unbalance relay based on the EGAT standard also caused an error, wherein an alarm notification was received or the fuse tripped, despite the non-occurrence of the fault in the capacitor bank [1]. This problem was solved by the first procedure of the proposed algorithm.…”

“…Using PSCAD, a 115-kV power system was simulated, as recommended by the EGAT. Moreover, the IEEE Guide for the Protection of Shunt Capacitor Banks was referred to for verifying the efficiency of the PSCAD-based simulated power system [1]. 2.…”