Abstract:
The transient response of the voltage source converter (VSC) interfaced DC system is significantly different from the AC counterpart. The rapid discharge current from the DC-link capacitors and the vulnerability of the freewheeling diodes during short-circuit in DC grid demands the transient detection algorithm to execute within few milliseconds. The rapidly rising fault current in DC grid is expected to have high frequency components which might be an effective indicator of the transient condition. This paper presents quantitative investigation of the high frequency components utilising Short-Time Fourier Transform (STFT) during transient conditions. Detailed operating principle with various factors affecting the STFT operation such as ripple content of the input DC signal and window type & length have been thoroughly investigated. STFT algorithm is able to detect low impedance faults within 1 ms and high impedance faults in 2 ms. Moreover, it is able to distinguish between short-circuit fault and less severe transient conditions such as sudden load change. The STFT algorithm is evaluated analytically and subsequently applied to MATLAB/Simulink based DC test system. It is further validated and substantiated with the real fault current data obtained from a scaled-down experimental testbed. Efficacy is substantiated by performing sensitivity analysis and comparison with the existing frequency domain based methods.
