Protection of Microgrids: A Scalable and Topology Agnostic Scheme With Self-Healing Dynamic Reconfiguration

Momentum towards realizing the smart grid will continue to result in high penetration of renewable fed Distributed Energy Resources (DERs) in the Electric Power System (EPS). These DERs will most likely be Inverter Based Resources(IBRs) and will be an integral part of the distribution system in the near future. The drive towards resiliency with these IBRs will enable a modular topology where several microgrids are tied together, operating synchronously to form the future EPS at the distribution level. Since the microgrids can evolve from existing distribution feeders, they will be unbalanced in load, phases, and feeder impedances. A typical control strategy of a conventional inverter that follows the grid voltage and frequency while injecting positive-sequence current can lead to undesirable performance for the unbalanced systems, especially in the islanded mode of operation. So, the dissertation will first focus on the control aspect of IBRs in an unbalanced system. Acceptable operating conditions with stability against disturbances and faults are the primary focus. For the proper functioning of these microgrids, there is a need for grid-forming inverters that can enable acceptable performance and coexist with conventional grid-following inverters that supply only positive-sequence currents. In addition to the control objectives, limiting inverter output during faulted or overload conditions with a current limiter is essential. These control objectives can be implemented in both the synchronous reference frame ($dq$ coordinates) and the natural reference frame ($abc$ coordinates). Hence a comparison study is performed to understand the merit of each implementation related to this specific topology. As 100\% IBR-based microgrid becomes an integral part of the distribution system, the issues and challenges arising from its implementation should be addressed for successful operation. Designing reliable protection is one of the significant challenges for microgrids. Most microgrid protection schemes found in published literature suffer from a lack of generality. They work well for the assumed topology, including the type and placement of sources. Other generic protection schemes tend to be too complicated, expensive, or both. To overcome these drawbacks, a topology-agnostic, scalable, and cost-aware protection based on fundamental principles is developed that works in the presence of high penetration of inverter-based resources (IBRs). The protection system includes primary and backup. It also implements stable automatic reconfiguration of the healthy sections of the system after clearance of fault, thus increasing resilience by self-healing. The scheme is validated in PSCAD for primary and backup protection and reconfiguration on the IEEE 123-node feeder in grid-connected and islanded modes with 15 IBRs connected to the system. As the designed protection scheme requires communication between protective devices and the microgrid controller, the method must be validated in real-time with cyber-physical co-simulation for a successful demonstration. In this regard, a Hardware-In-the-Loop (HIL) platform between a simulated power system model using RTDS and physical protective devices is built. In the HIL platform, the primary protection of the scheme is programmed in SEL 421-7 relay, and backup protection is programmed in MATLAB on a generic computer acting as a microgrid controller. The IEC 61850 models are used to communicate between the SEL-421-7 relay and RTDS, whereas TCP/IP communication connects the microgrid controller to RTDS. The focus of the work is to demonstrate the co-simulation platform with communication links established using both protocols and validate the proposed scheme in real-time on the IEEE 123 node distribution feeder. The IEC 61850 and TCP/IP communications configuration are discussed as the interface requires proper hardware and software setup. The real-time performance indicates the Hardware In the Loop (HIL) framework as a competent testing environment for the developed protection scheme for microgrids. In summary, a scalable and topology agnostic protection scheme with self-healing dynamic reconfiguration is developed for microgrids. Clear guidelines for implementation of the proposed scheme on any microgrid topology are also described