Wavelet-fuzzy based protection scheme of EHV-AC transmission system and efficacy of discrete Fourier transform

摘要

Extra high voltage (EHV) transmission system is the vital links between the generating units and the transmission substations. Transmission of power to the utilities located over long distances through extra high voltage (EHV) transmission is efficient and cost effective. It has the advantage of allowing the bulk amount of energy transmission from generating units to the widely distributed load centers economically. However, detection of faults on EHV transmission system is critical and a challenging task for a large variation in fault operating condition such as high fault resistance involved, inception of fault at the voltage zero crossing situation and the variation in load angles and the system loading levels. Moreover, fault characteristics on EHV transmission lines differ to those on medium and high voltage (MV/HV) due to the presence of larger shunt capacitance, higher voltage level, longer transmission radius and weakly damped decaying dc current caused by the high system reactance to resistance ratio. A number of deterministic and intelligent models based protection schemes of transmission systems are endorsed by several authors where attempts have been made to detect, classify and to locate the fault and the faulted section of the line but they have their own limitations and complexities and the results are not satisfactory. Keeping in view of the aforesaid difficulties, this paper proposes a new approach for the protection of EHV-AC transmission system based on novel application of discrete wavelet transform (DWT) in combination with fuzzy logic using discrete Fourier transform (DFT) at the fundamental frequency phasor. The proposed method retains both fundamental and the high frequency components of the fault current signals to achieve high reliability and selectivity of the protection. The obtained results have been validated with the well established mathematical model of the given EHV-AC transmission system. It is found that the proposed method is accurate, computationally simpler, faster and valid for a wide range of fault operating conditions.