Modelling and fault ride-through control of grid supporting inverter-based microgrid
Authors:   ,   Durban University of Technology, Buraimoh Elutunji 
Published: September 2021

This thesis is focused on modeling and fault ride-through control, local load power delivery, and grid power exchange of power electronic interfaced Distributed Energy Resources (DERs) for grid supporting microgrids. Active and reactive power regulations are the requirements for a grid-supporting system operating as a current source, while frequency and voltage magnitude regulation in the grid-supporting system acting as a voltage source. Consequently, these are put into consideration as the primary control requirements for the inverter-based microgrid. To that end, two discrete-time models of a grid-feeding system and grid-forming system were developed to serve as controls for a single DER operating in grid-connected mode and islanded mode, respectively. Consequently, for the first set of mathematical models: grid feeding and grid forming were interfaced with a droop control to allow for parallel operation of additional DERs for power coordination within the microgrid for grid-connected and islanded operation. However, virtual impedance was incorporated into the grid-supporting system's droop control operating as a voltage source to emulate the link feeder's physical impedance to the main grid. Based on the developed grid supporting models, the microgrid primary control schemes effectively delivered power to the host grid and simultaneously contributed to the grid's frequency and voltage regulation. Furthermore, to ensure grid code compliance and ensure the microgrid provides ancillary services to the host grid, such as fault ride-through and reactive power compensation for voltage recovery, a novel technique is proposed in the microgrid's secondary control. The secondary control realizes the fault ride-through for the grid supporting system using a delay signal cancellation algorithm for negative sequence detection. The proposed control scheme actualizes grid code requirements by providing a secondary voltage control, which is active and more prominent in the transient period of faults without mode switching. The strategy's performance is further enhanced with an IGBT-Diodes switched AC reactor to improve the voltage and prevent the transient overcurrent in the microgrid during the grid fault. This ensures a continued supply of the microgrid's local sensitive load while meeting the grid code requirement. Similarly, the active power injection into the main grid is limited to maximize reactive power injection into the main network to support the grid voltage sag. The detection algorithm using the delayed signal cancellation algorithm is implemented to detect the instance of fault in 1.6% of the half-cycle under grid disturbance/fault to activate the proposed secondary control. This effectiveness and fault ride-through compliance of the developed control models were tested on an inverter-based microgrid system with an ideal voltage source DERs. Finally, to accommodate for the grid dynamics introduced to the DC link parameters of an ideal voltage source DER such as PV, the models were also implemented and assimilated for a solar PV sourced DER used with a grid supporting inverter-based microgrid. The injection of active power into the main grid is constrained by systematically shifting the MPPT operating point based on voltage sag depth to maximize reactive power injection to support the grid voltage sag. The strategy developed in the PV sourced system also ensured that the DC-link voltage and AC grid current raises are suppressed while meeting microgrid load requirements. The models' implementation, DER primary control, and proposed secondary control schemes are established through detailed time-domain simulation studies using MATLAB Simscape Electrical™ and Control System