LAUSR

Abstract Third zone of a distance relay is sensitive to system stressed conditions, such as stable and unstable power swing, voltage instability and load encroachment. Mal-operation of a distance relay under these conditions can cause power system blackout and should be prevented for power system security. An integrated approach based on two indices is introduced to improve the relay performance during system stressed conditions. The first index is based on estimated dc offset magnitude from the current signal using half cycle Discrete Fourier Transform and the second index is based on energy of first level detail coefficients obtained using discrete wavelet transform. It can be hard for a distance relay to discriminate between a three-phase fault in the third zone and stressed system events as both are balanced phenomena. Therefore, during the occurrence of a transient event if the conventional impedance calculation method detects it as a fault, it will be rightly detected as a three-phase fault only if both the indices lie above their respective thresholds. Using these two indices as a combined supervisory logic, three-phase fault in the third zone can be rapidly and reliably detected and discriminated from system stressed conditions, and thus avoid false operation of distance relay during stressed system conditions. Current based and wavelet techniques are, respectively, sensitive to noise and sampling frequency. However, with the proposition of adaptive threshold setting in the wavelet approach the robustness of this method against noise and sampling frequency is guaranteed. The proposed method is reliable during CT saturation condition as well. The proposed method is based on local end information so system condition can be rapidly scrutinized. Results for different power system events are analyzed using New England 39-bus, 10-machine test system modelled in EMTDC/PSCAD software. Comparative assessment with conventional approach confirms that this algorithm can meet the accuracy standards of the industrial relays due to better reliability.