Hybrid voltage sag mitigation for renewable integrated weak distribution systems using a load management and robust inverter control approach

Abstract

The world is rapidly transitioning towards renewable energy sources, with Inverter- Based Resources (IBRs) emerging as key power sources for future distribution systems. The electrification of rural and remote areas is progressing rapidly, bringing power to previously underserved locations. However, remote distribution networks face challenges, with one significant issue being lower fault levels. Weak distribution networks in these remote areas give rise to several problems, with voltage sag being a primary concern. Voltage sag, often caused by sudden changes in load or faults on the grid, poses a particular threat to process controllers, PLCs, variable speed drives, robots, data processing and control equipment, in particular, are highly sensitive to voltage sags, risking potential data loss. The simultaneous operation of large machines can further exacerbate the challenges associated with low fault levels. In the context of IBR-based distribution systems, vulnerability to unbalanced voltage sag is a specific concern. The Synchronous Reference Frame-Phase Locked Loop (SRF-PLL) may incorrectly estimate the positive sequence voltage component, resulting in a loss of controllability for the three-phase inverter. A grid-connected power converter, in particular, is sensitive to voltage disturbances, as its control system may lose controllability under such distorted operating conditions. This may lead to damage the inverter, as it is unsuitable for injecting unbalanced currents and lacks a specific control loop for negative-sequence current components. The abrupt disconnection of inverters can result in voltage collapse, frequency instability, and synchronous angle instability. This research has addressed these challenges by developing a voltage sag mitigation technique for weak distribution systems. A robust inverter control system capable of maintaining connectivity during voltage sag events and a pre-assigned priority load management scheme to effectively mitigate voltage sag were designed. Performance evaluation revealed the success in mitigating voltage sag by applying the proposed techniques when integrating IBR to weak grids.