Unified Virtual Oscillator Control for Grid-Forming and Grid-Following Converters

• Published in: IEEE journal of emerging and selected topics in power electronics 2021-08, Vol.9 (4), p.4573-4586
• Main Authors: Awal, M. A., Husain, Iqbal
• Format: Article
• Language: English
• Subjects: Control systems design; Controllers; Converters; Data buses; Design parameters; Distributed generation; Electric potential; Fault ride-through; Flow control; grid-following (GFL) converter; grid-forming (GFM) converter; oscillator-based control; Oscillators; Overcurrent; Phase locked loops; Power flow; Switches; Synchronism; Synchronization; unified virtual oscillator control; unified virtual oscillator controller (uVOC); Voltage; Voltage control; weak grid
• ISSN: 2168-6777; 2168-6785
• DOI: 10.1109/JESTPE.2020.3025748

A unified virtual oscillator controller (uVOC) is proposed, which enables a unified analysis, design, and implementation framework for both grid-forming (GFM) and grid-following (GFL) voltage-source converters (VSCs). Oscillator-based GFM controllers, such as dispatchable virtual oscillator control (dVOC), offer a rigorous analytical framework with enhanced synchronization but lack effective fault handling capability, which severely limits practical application. The proposed uVOC facilitates synchronization with an arbitrarily low grid voltage and fast overcurrent limiting; this enables effective fault ride-through unlike existing GFM controllers which typically switch to a back-up controller during the fault. GFM operation with uVOC is achieved in both grid-connected and islanded modes with seamless transitions between the two. In GFL converters, bidirectional power flow control and dc bus voltage regulation are achieved with uVOC. No phase-locked loop (PLL) is required for either GFL or GFM operation circumventing the synchronization issues associated with PLLs in weak grid applications. Detail small-signal models for GFM and GFL operation have been developed, and the systematic design guidelines for controller parameters are provided. The proposed controller is validated through hardware experiments in a hybrid ac-dc microgrid.