Analysis and mitigation of circulating currents for grid-forming DERs during parallel black-start in islanded systems

Following a large-scale power outage, grid-forming (GFM) distributed energy resources (DERs) can facilitate rapid system recovery using parallel black start technology. However, the DER parallel-operation systems (DERPS) may generate circulating currents that destabilize the system and damage equipment. This article first derives an explicit expression for circulating currents and examines the electrical parameters that influence them, focusing on three scenarios: imbalances in voltage amplitude, voltage phase, and output impedance. Then a variable step-size black start scheme is proposed for mitigating circulating currents, which relies solely on local information and without the need for communication. This scheme features three key control components: – control with a negative frequency feedback, variable step-size soft-start for voltage buildup, and – droop control for removing sensitivity to impedance. This control scheme effectively mitigates circulating currents in DERPS during black start and load pickup, even amid imbalances in voltage amplitude, voltage phase, and output impedance. Numerical experiments conducted on two-DER and five-DER islanded systems in real-time simulators, accounting for various topologies and operational disturbances, validate the effectiveness of the proposed mitigation control.