An active unit protection scheme for islanded microgrids with inverter-interfaced distributed generation

This thesis presents a solution to overcome challenges associated with the protection of microgrids that operate in islanded mode, and that are dominated by inverter-interfaced distributed generators (IIDGs). The solution is based around a unit protection scheme that does not require dedicated communications between the protection relays to coordinate and identify the faulted section in the microgrid. The research is justified through a detailed investigation and quantification of the issues and challenges associated with stability, control and protection of microgrids, along with a critical study and literature review of several control and protection schemes for microgrids with IIDGs, which shows the gaps in and shortcomings of other work which the solution outlined in this thesis addresses. The design and operation of the proposed scheme is demonstrated, for a wide range of fault scenarios. A detailed model of a microgrid with a range of IIDGs (utilising a range of different controllers and control strategies to ensure the generic applicability of the solution) is implemented and extensive simulations conducted for an extensive range of conditions, using both Simulink and Hardwarein-the-Loop (HiL) simulation through real-time digital simulator (RTDS) and OPAL-RT. The behaviour of the microgrid systems under various scenarios is modelled, simulated and demonstrated, and this is used to both verify the problems and challenges that will exist in the near and longer-term future, as well as demonstrating and quantifying the capabilities of the developed protection solution. It is shown that the protection system is capable of correctly dealing with faults at all locations in the system, can successfully detect any fault within 100 ms of fault inception, and is coordinated properly for all types of balanced and unbalanced faults, including operating correctly in backup modes where the failure of an element of the overall protection system is presumed. Furthermore, the protection scheme is shown to operate successfully for faults with a fault impedance of up to 60 Ω (in addition to any impedance to the fault) for three-phase to earth faults and 20 Ω for single-phase to earth faults. Moreover, it has been demonstrated and validated through simulation results that location of the IIDGs in the network does not have any adverse impact on the operation of the protection solution. The thesis concludes with an overview of future work, which, in summary, should focus on the practical implementation of the solution as a prototype and validating its operation in a realistic microgrid model using actual inverter hardware to verify proposer operation and injection of harmonics for a variety of different fault situations.This thesis presents a solution to overcome challenges associated with the protection of microgrids that operate in islanded mode, and that are dominated by inverter-interfaced distributed generators (IIDGs). The solution is based around a unit protection scheme that does not require dedicated communications between the protection relays to coordinate and identify the faulted section in the microgrid. The research is justified through a detailed investigation and quantification of the issues and challenges associated with stability, control and protection of microgrids, along with a critical study and literature review of several control and protection schemes for microgrids with IIDGs, which shows the gaps in and shortcomings of other work which the solution outlined in this thesis addresses. The design and operation of the proposed scheme is demonstrated, for a wide range of fault scenarios. A detailed model of a microgrid with a range of IIDGs (utilising a range of different controllers and control strategies to ensure the generic applicability of the solution) is implemented and extensive simulations conducted for an extensive range of conditions, using both Simulink and Hardwarein-the-Loop (HiL) simulation through real-time digital simulator (RTDS) and OPAL-RT. The behaviour of the microgrid systems under various scenarios is modelled, simulated and demonstrated, and this is used to both verify the problems and challenges that will exist in the near and longer-term future, as well as demonstrating and quantifying the capabilities of the developed protection solution. It is shown that the protection system is capable of correctly dealing with faults at all locations in the system, can successfully detect any fault within 100 ms of fault inception, and is coordinated properly for all types of balanced and unbalanced faults, including operating correctly in backup modes where the failure of an element of the overall protection system is presumed. Furthermore, the protection scheme is shown to operate successfully for faults with a fault impedance of up to 60 Ω (in addition to any impedance to the fault) for three-phase to earth faults and 20 Ω for single-phase to earth faults. Moreover, it has been demonstrated and validated through simulation results that location of the IIDGs in the network does not have any adverse impact on the operation of the protection solution. The thesis concludes with an overview of future work, which, in summary, should focus on the practical implementation of the solution as a prototype and validating its operation in a realistic microgrid model using actual inverter hardware to verify proposer operation and injection of harmonics for a variety of different fault situations