Dual-dVOC Based Controlled Negative Sequence Current Injection for Grid-Forming Inverters under Asymmetrical Grid Conditions

The conventional synchronous machine-based power system, during any unbalanced condition, including asymmetrical faults, injects a negative sequence current which is sensed by the sequence-based relays to ensure the requisite protection. Similarly, grid-forming (GFM) inverters, acting as a controlled voltage source, are also expected to inject a fully controlled negative sequence current under similar asymmetrical conditions. The dispatchable Virtual Oscillator Control (dVOC) has emerged as a suitable decentralized control scheme for GFM inverters, that deals with the instantaneous time domain signals. Owing to its benefits, a sequence-based control strategy using a dual-dVOC loop is proposed for control of GFM inverters. The positive and negative sequence components are controlled in a fully decoupled way and finally added up to generate the final PWM signal. The dual-dVOC-based control architecture is validated with detailed PLECS simulations under asymmetrical grid conditions. The results illustrate that the proposed controller provides the flexibility to inject programmable negative sequence currents and extends the advantages of dVOC in terms of protection of GFM inverter-interfaced distributed generation system.