Enhanced Current Limiting Strategy for Droop-Controlled Grid-Connected Inverters During LVRT

Renewable energies mostly need power electronic devices to convert and adapt the energy generated to be available for the loads. Electrical engineers’ major challenge in the energy transition is keeping high energy quality and standards without compromising the stability of electric power systems. Microgrids are seen as an affordable and efficient answer to integrating them with the energy mix. International grid codes and standards are evolving to regulate the connection and disconnection of inverter-based resources; this thesis proposes a microgrid design to mitigate power cuts and extend the energy supply in developed countries. A hybrid 100 kW commercial inverter droop control is studied and upgraded to allow the inverter to limit the current to ride through voltage drops. This way, the UPS inverter can seamlessly supply energy to the local loads when the grid collapses. Grid-connected inverters with LCL filters require active damping to mitigate system resonance, typically achieved via inductor or capacitor current feedback loops. While inductor current feedback is favoured for its inherent current-limiting capability, capacitor current feedback provides a more inductive output impedance, enabling droop control with smaller inductors, improved transient response, and reduced total harmonic distortion (THD). Limiting the current in grid-connected inverters during voltage drops using the capacitor current as active damping is the main topic of the thesis. A dynamic voltage feedforward using a high-pass filter in a current control mode helps to reduce the current overshoot in front of a voltage sag. Simulation results show a reduction in the inverter side current from 1.89 p.u. to 1.09 p.u. A similar strategy has been explored in a hybrid droop-controlled inverter that can work grid-connected and standalone, only by changing some controller parameters. The strategy introduces double voltage feedforward, ensuring a rapid response to grid voltage sags and effectively limiting overshoot. A small signal model is developed to analyse stability, and the controller is validated through detailed simulations and experimental testing. Results demonstrate significant improvements in transient response, from 99 ms to 21.7 ms in the active power rising time, which is 78.62% faster, and current limitation preventing premature disconnection when the voltage grid drops from 230 V to 30 V; from a overshoot of 3.8 p.u. to 1.35 p.u. on the inductor current, approaching well-suited for next-generation grid-forming and grid-following inverters. The inverter and a static switch were manufactured in collaboration with HiT Power. Experimental tests have been conducted in the power lab of the University of Exeter- Penryn campus to verify and validate the theoretical and simulation results. The equipment has been sent and commissioned in Johannesburg, South Africa. Field data from another microgrid project installed in Nepal using the same 100 kW commercial inverter was available before and after the software was updated with the current limitation strategy developed in this research, showing a significant impact on the number of trips in front of power outages and low voltage situations, from 475 trips on February to 8 in August.