Siting and sizing of converter interfaced distributed energy resources in medium voltage networks, ensuring small signal stability

The increasing penetration of converter interfaced Distributed Energy Resources (DER) poses new challenges to the planning and operation of power systems, as they have inherently different characteristics than traditional synchronous machines. Considering the hosting saturation of current MV networks observed in many countries, one of the critical planning decisions is the critical distance from the MV bus, that DER can be connected without causing stability problems. This is also affected by their size and their control parameters. In this work, a framework for determining the stability boundary of the distribution line length for specified DER characteristics and determining appropriate DER capacity/control parameters for an existing connection is presented and validated in time domain simulations. The critical modes for single and multiple inverters connected at MV lines and operating in either Grid-following (GFL) or Grid-forming (GFM) mode are calculated. The critical length for the MV voltage line of a single GFL inverter is calculated and compared to multi-inverter scenario. A sensitivity analysis of the different Phase-locked loop (PLL) bandwidths, grid strength and active power setpoints is conducted to investigate their influence on the critical length of the MV line. The impact of switching, the operating mode of the inverter from GFL to GFM in the single and multi-inverter scenario is also investigated.