Implementation of Optimal Protection Coordination for Microgrids with Distributed Generations Using Quaternary Protection Scheme The flexible operation of microgrids, along with the availability of distributed generation (DG) units, causes a variety of changes in shortcircuit current levels, magnitudes, and directions, which has undesirable effects on the operation of protection systems. Conventional protection schemes use typical directional overcurrent relays (DOCRs) with limited operating capability, unable to respond to microgrid operations in the manner of shortcircuit current changes. In this paper, a quaternary protection scheme implemented with dualdirectional overcurrent relays (dualDOCR) and a protection control strategy is developed for protecting against faults in microgrids, taking into account the existence of DGs and connection and disconnection of DG units. The optimal dualDOCRs setting and coordination are formulated as an optimization problem solved by evolutionary programming to minimize the relay operating times. The proposed protection scheme is implemented with a centralized protection control system based on the smart grid concept to increase the adaptability of the dualDOCRs, which have multiple relay setting groups in accordance with system state changes. The simulation case studies are performed using the IEEE 14bus test system, which is modified as a meshed microgrid operation. Test scenarios, including possible operations of microgrids, DGs availabilities, and different fault events, are analyzed and discussed. The comparative studies and simulation results show that the proposed scheme provides efficient coordination between the primary and backup relays and increases the responsibility of the protection system, which can be observed from the significant reduction in the relay operating times, resulting in the enhancement of selectivity, sensitivity, and speed of microgrid protection systems.

Impact of InverterInterfaced Renewable Energy Generators on Distance Protection and an Improved Scheme With different structures and control strategies, inverterinterfaced renewable energy generators (IIREGs) have different fault characteristics from synchronous generators. This makes the conventional distance protection used in networks with synchronous generators not applicable for transmission lines emanating from IIREGs. Therefore, supported by the fault current analysis, operating performances of distance relays on both sides of the transmission line are unveiled. It reveals the problem that the conventional distance relay on the IIREG side has a high risk of malfunction or refusing to operate. To cope with this adaptability problem, an improved scheme based on time delay and zerosequence impedance is proposed. To validate the operating performances of the scheme, a detailed IIREG model is built in Real Time Digital Simulator (RTDS) and simulation tests are carried out. Apart from these, a field shortcircuit test is performed in a real wind power plant to examine the practical feasibility of the proposed scheme. Both the simulation results and the field test confirm the problem of the conventional distance protection and verify the reliability of the improved scheme.