Development of a hybrid protection scheme for active distribution systems using polarities of current transients

Effective protection of active distribution systems under changing states of distributed generators that have little or no contributions to fault currents is a major challenge. In case of microgrids, protection need to be effective in both grid connected and islanded modes, and fast enough to preserve the stability of the generators connected to the active distribution network. Intentional time delays inherent in the coordination of traditional overcurrent and distance protection may cause the distributed generators to significantly deviate from the nominal operating conditions during disturbances. This results in poor power quality and even disconnection from the grid. This paper investigates a novel hybrid protection scheme that combines the speed of transient based protection with the security of traditional phasor based protection to overcome the above-mentioned issues in active distribution system protection. The proposed transient based protection discriminates the faults by comparing the polarity of fault generated transients measured at the zone boundaries. The challenges in measuring high frequency transients is overcome by using Rogowski coil sensors. The problem of security during transients originating from non-fault events is addressed by employing traditional phasor based protection to supervise the transient based protection. The efficacy of the proposed protection strategy is evaluated by simulating some typical distribution network configurations in an electromagnetic transient (EMT) type simulation program. Simulation results demonstrate the increased speed in the protection operations. As a result, deviations in frequency and voltage are reduced. Results presented in this paper also demonstrate the capability of the proposed method in fast identification and isolation of faults under challenging situations such as high fault resistances and varying fault inception angles.