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

Optimal Protection Coordination of Active Distribution Networks Powered by Synchronverters

The integration of distributed generators (DGs) into distribution networks leads to the emergence of active distribution networks (ADNs). These networks have advantages, such as deferring the network upgrade, lower power losses, reduced power generation cost, and lower greenhouse gas emission, DGs are classified due to their interface with the network as inverter-interfaced or synchronous-interfaced. However, DGs integration results in bidirectional power flow, higher fault current levels, deterioration of the protection coordination of the directional overcurrent relays (DOCRs) which are used in ADNs, reduced system stability due to the inverters’ lack of damping. The stability can be enhanced by controlling the inverters to behave as synchronous generators, which are known as synchronverters. In this thesis, a two-stage optimal protection coordination (OPC) scheme is proposed to guarantee reliable protection of ADNs while protecting synchronverters from overcurrent using virtual impedance fault current limiters (VI-FCLs). VI-FCLs provide a cost-effective way to protect synchronverters from overcurrent. The first stage integrates the fault current calculations of synchronverters in the fault analysis to find the parameters of VI-FCLs used to limit the synchronverter’s fault current. In the second stage, the fault current calculations, along with the designed VI-FCLs from the first stage, are employed to determine the optimal relays’ settings to minimize the total operating times for all the DOCR. It is found that fixed VI-FCLs can limit synchronverters’ fault currents but may make the OPC problem infeasible to solve. Thus, an adaptive VI-FCL is proposed to ensure a feasible OPC under various fault conditions, i.e., locations and resistances

Decentralized and Coordinated Vf Control for Islanded Microgrids Considering DER Inadequacy and Demand Control

This paper proposes a decentralized and coordinated voltage and frequency (Vf) control framework for islanded microgrids, with full consideration of the limited capacity of distributed energy resources (DERs) and Vf dependent load. First, the concept of DER inadequacy is illustrated with the challenges it poses. Then, a decentralized and coordinated control framework is proposed to regulate the output of inverter based generations and reallocate limited DER capacity for Vf control. The control framework is composed of a power regulator and a Vf regulator, which generates the supplementary signals for the primary controller. The power regulator regulates the output of grid forming inverters according to the real time capacity constraints of DERs, while the Vf regulator improves the Vf deviation by leveraging the load sensitivity to Vf. Next, the static feasibility and small signal stability of the proposed method are rigorously proven through mathematical formulation and eigenvalue analysis. Finally, a MATLAB Simulink simulation demonstrates the functionalities of the control framework. A few goals are fulfilled within the decentralized and coordinated framework, such as making the best use of limited DERs capacity, enhancing the DC side stability of inverter based generations, and reducing involuntary load shedding

Analysis and Mitigation of Temporary Over-Voltage (TOV) Phenomenon in Unintentionally Islanded Grid-Connected Photovoltaic (PV) Inverters

Grid-connected photovoltaic (PV) solar systems, like other inverter-based distributed generators, can cause temporary over-voltages (TOVs), especially subsequent to faults and unintentional islanding incidents, and can damage equipment and customers within the host distribution network. Thus, this thesis aims to study the phenomenon and propose corrective measures for it. Thus, the thesis first presents detailed models for a conventional single-stage PV system and a modified single-stage PV system. The conventional system uses a Δ/YG isolation transformer, whereas the modified system, proposed in the literature, assumes a Y/YG isolation transformer that is effectively grounded by an additional half-bridge leg energized by the dc-link of the voltage-sourced inverter (VSI) of the PV system. Moreover, the thesis proposes two TOV mitigation schemes that augment the basic controls of the conventional and modified single-stage PV systems, respectively. Further, the thesis models a two-stage PV system that adopts the same TOV mitigation scheme as that proposed for the conventional single-stage system. Then, the TOV caused by the two-stage system is evaluated, with and without the TOV mitigation scheme. It is shown that the proposed TOV mitigation schemes are effective. The thesis also compares the TOVs caused by the three aforementioned PV systems, with and without the TOV mitigation schemes, and concludes that a two-stage PV system without a TOV mitigation scheme produces smaller TOVs than its single-stage counterparts without TOV mitigation schemes. Similarly, a two-stage PV system with its TOV mitigation scheme produces smaller TOVs than its single-stage counterparts with their respective TOV mitigation schemes

Real Time Testing and Validation of a Novel Short Circuit Current (SCC) Controller for a Photovoltaic Inverter

About 45% applications from PV solar farm developers seeking connections to the distribution grids in Ontario were denied in 2011-13 as the short circuit current (SCC) capacity of several distribution substations had already been reached. PV solar system inverters typically contribute 1.2 p.u. to 1.8 p.u. fault current which was not considered acceptable by utility companies due to the need for very expensive protective breaker upgrades. Since then, this cause has become a major impediment in the growth of PV based renewable systems in Ontario. A novel predictive technique has been patented in our research group for management of short circuit current contribution from PV inverters to ensure effective deployment of solar farms. This thesis deals with the real time testing and validation of a short circuit current (SCC) controller based on the above technique. With this SCC controller, the PV inverter can be shut off within 1-2 milliseconds from the initiation of any fault in the grid that can cause the short circuit current to exceed the rated current of the inverter. Therefore, the power system does not see any short circuit current contribution from the PV inverter and no expensive upgrades in protective breakers are required in the system. The performance of the PV solar system with the short circuit current controller is simulated and tested using (i) industry grade electromagnetic transients software PSCAD/EMTDC (ii) real time simulation studies on the Real Time Digital Simulator (RTDS) (iii) physical implementation on dSPACE board to generate firing pulses for the inverter. The validation of controller is done on dSPACE board with actual PV inverter short circuit waveforms obtained from Southern California Edison Short Circuit Testing Lab. This novel technology is planned to be showcased on a physical 10 kW PV solar system in Bluewater Power Distribution Corporation, Sarnia, Ontario. This proposed technology is expected to remove the technical hurdles which caused the denials of connectivity to several PV solar farms, and effectively lead to greater connections of PV solar farms in Ontario and in similar jurisdictions, worldwide

A reliable micro-grid with seamless transition between grid connected and islanded mode for residential community with enhanced power quality

This paper presents a reliable micro-grid for residential community with modified control techniques to achieve enhanced operation during grid connected, islanded and resynchronization mode. The proposed micro-grid is a combination of solar photo-voltaic (PV), battery storage system and locally distributed DG systems with residential local loads. A modified power control technique is developed such that, local load reactive power demand, harmonic currents and load unbalance is compensated by respective residential local DG. However, active power demand of all local residential load is shared between the micro-grid and respective local DG. This control technique also achieves constant active power loading on the micro-grid by supporting additional active power local load demand of respective residential DG. Hence, proposed modified power control technique achieves transient free operation of the micro-grid during residential load disturbances. An additional modified control technique is also developed to achieve seamless transition of micro-grid between grid connected mode and islanded mode. The dynamic performance of this micro-grid during grid connected, islanded and re-synchronization mode under linear and non-linear load variations is verified using real time simulator (RTS)