Control and Black Start Restoration of Islanded Microgrids

Electric power systems face the risk of outages due to extreme weather events, cyber-attack, human errors, and other unforeseen circumstances. A power grid that is resilient to these unfortunate events has become a consistent theme in literature and media. One approach to improve the grid’s resilience is by decentralizing the grid into smaller controllable units called microgrids. This dissertation focuses on the control and operation of islanded microgrids for black start restoration (BSR). Two operation modes were identified based on islanded microgrids’ primary control for BSR. These modes are the single master operation (SMO) and multi-master operation (MMO) modes. These two control approaches were used to develop two sequential multi-step BSR methods for islanded microgrids. The first part of this dissertation presents the BSR formulation for SMO microgrids. The restoration problem was formulated as a mixed-integer linear programming (MILP) problem. The frequency response of the isochronous synchronous-machine based generator (ISMG) was derived and validated through transient simulation. The frequency response was then used to characterize the ramp rate and the settling time of the ISMG. With these characterizations, the BSR can minimize total restoration time and maximize energy restored. The developed BSR method was studied on a modified IEEE 123 node test feeder. The second part of this dissertation provides studies of the use of MMO microgrids for BSR. The MMO microgrids were assumed to be operating with conventional droop control. Before developing the BSR method, two novel linear power flow (LPF) formulations for islanded droop-controlled microgrids were derived. These two LPF formulations were extended to develop optimal power flow (OPF) formulations as a quadratic programming (QP) and a mixed-integer quadratic programming (MIQP) problem to minimize cost. These formulations were followed by a MILP formulation to realize a novel BSR method for MMO microgrids with the derived LPF equations incorporated as the power flow constraint. Extensive case studies were used to validate and verify the developed BSR method. Because the MMO microgrid can have multiple master DGs per microgrid, they are expected to improve islanded microgrids’ resilience compared to SMO microgrids which have one master DG per microgrid

Single-Phase PQ Inverter Model and Application to Unbalance Compensation in Islanded Microgrids (PSCAD Simulation)

This is a simulation study of islanded microgrids for black start purpose. The study focuses on the voltage unbalance that results from serving unbalanced loads in the distribution system and how this could be mitigated by the use of three-phase stacked single-phase PQ inverters. Simulation is performed in PSCAD/EMTDC software and the files have been freely made available. Full model and the voltage-controlled source model of the single-phase PQ inverters were developed and incorporated into the case studies

Black Start Restoration of Islanded Droop-Controlled Microgrids

The electricity grid faces the possibility of outages due to extreme weather events, cyber-attack, and unexpected events. When these unwanted events occur, it is desired that electricity be restored as soon as possible to meet the power demands of critical loads. The microgrid approach to power restoration holds a lot of promise, since microgrids can operate in island mode. This paper presents a novel sequential restoration methodology for microgrid black start. The microgrid architecture considered is assumed to be operating in multi-master mode. The master distributed generators (DGs) are coordinated to operate together through droop control. Several operational constraints are formulated and linearized to realize a mixed-integer linear programming (MILP) problem. The method is studied on an islanded microgrid based on a modified IEEE 13 node test feeder. Detailed transient simulation in PSCAD was used to verify the accuracy of the restoration methodology. The developed restoration method can maximize the energy restored while ensuring good voltage and frequency regulation, and ensure that power scheduling mismatch is shared in the desired proportion

Islanded Microgrid Restoration Studies with Graph-Based Analysis

The need to restore and keep the grid running or fast restoration during emergencies such as extreme weather conditions is quite apparent given the reliance of other infrastructure on electricity. One promising approach to electricity restoration is the use of locally available energy resources to restore the system to form isolated microgrids. In this paper, we present a black start restoration method that forms islanded microgrids after a blackout. The master DGs in the formed microgrids are coordinated to work together through droop control. Several constraints, including incentive-based demand response (DR) with direct load control (DLC) and distributed generator (DG) operation constraints, were formulated and linearized to realize a mixed-integer linear programming (MILP) restoration model. To improve compactness and to ensure that the model is neither under-sized nor over-sized, a pre-processing graph analysis approach was introduced which helps to characterize the least number of restoration steps needed to optimally restore the microgrid. Studies were performed on a modified IEEE 123 node test feeder to evaluate the effects of demand response, non-dispatchable DGs, and choice of restoration steps on the quality of the restoration solution. When possible, the proposed method yields an interconnected multi-master microgrid with improved redundancy

PSCAD Implementation of IEEE 123 Node Test Feeder

The accompanying file is a PSCAD setup of the IEEE 123 node test feeder. In the PSCAD model, the substation transformer has been omitted. The switches default states have been set according to their default setting in the IEEE 123 node test feeder documents. All voltage regulators’ tap settings have been fixed to those obtained in the IEEE results though an option is provided to set each regulator to vary its tap settings with changing primary voltage. The loads are modeled as ZIP loads with values set to be the same with the IEEE test system. The results from the PSCAD simulation closely matches the published result in the original test system. This PSCAD model could potentially be used in such applications as restoration, reconfiguration, distributed generator interconnection and transient simulation studies

PSCAD Implementation of IEEE 123 Node Test Feeder

The accompanying file is a PSCAD setup of the IEEE 123 node test feeder. In the PSCAD model, the substation transformer has been omitted. The switches default states have been set according to their default setting in the IEEE 123 node test feeder documents. All voltage regulators’ tap settings have been fixed to those obtained in the IEEE results though an option is provided to set each regulator to vary its tap settings with changing primary voltage. The loads are modeled as ZIP loads with values set to be the same with the IEEE test system. The results from the PSCAD simulation closely matches the published result in the original test system. This PSCAD model could potentially be used in such applications as restoration, reconfiguration, distributed generator interconnection and transient simulation studies