Virtual oscillator control (VOC) is a nonlinear time domain controller that achieves significantly faster primary control response in islanded microgrids, compared to droop or virtual synchronous machine (VSM) control. Despite its superior performance, adoption of VOC is limited due to the lack of compatible secondary regulation or grid synchronization techniques. This is attributed to the nonlinear nature of VOC that complicates secondary control design, and the third-harmonic component in VOC output voltage that severely restricts grid-tied operation. To leverage the faster primary control response characteristics of VOC, we propose a compatible hierarchical control structure that enables operation and seamless transition between islanded and grid-connected modes. In the islanded mode, the controller achieves voltage and frequency regulation and grid synchronization; in the grid-tied mode, notch filters are used to suppress harmonic currents and tertiary level power reference tracking is achieved. The proposed controllers are validated through a series of real-time hardware-in-the-loop tests and hardware experiments using laboratory inverter prototype and state-of-the-art controls and communications hardware.

For the LCL-type grid-connected inverter, there are basically three current control schemes, namely the grid current control, the inverter-side inductor current control, and the weighted average current control. This paper builds a general mathematical model to describe the three current control schemes. In this model, the grid current is an equivalent target control variable, the capacitor current feedback serves as a damping solution, and the computation and pulse-width modulation delays are taken into account. Based on the general mathematical model, a comparative analysis of different control schemes is carried out in terms of the grid current stability. It reveals that when the inverter-side inductor current is controlled, the grid current shows the same stability as the inverter-side inductor current; but when the weighted average current is controlled, both the grid current and the inverter-side inductor current are critically stable even though the weighted average current can be easily stabilized. Moreover, the general mathematical model also provides a unified perspective to design different control schemes, which makes the controller parameter tuning more straightforward and effective. In this way, a set of controller parameters which yields high robustness against the grid-impedance variation can be selected for all the three current control schemes. Finally, a 6-kW prototype is built, and experiments are performed to verify the theoretical analysis.

With the increasing integration of power electronics interfaced distributed generators, transient stability assessment of grid-connected inverters subjected to large grid disturbances is of vital importance for the secure and resilient operation of the power grid. Dispatchable virtual oscillator control (dVOC) is an emerging approach to implement nonlinear control of grid-forming inverters. Through coordinate transformation, a simple first-order nonlinear power angle dynamic equation is uncovered from the complex oscillator dynamics. Furthermore, this article proposes a concise and straightforward graphical approach to assess transient stability of dVOC using vector field on the circle. To provide a more in-depth analysis, a complete large-signal model is derived and the impact of dVOC voltage amplitude dynamics is analyzed. For comparison, transient stability of the currently prevalent droop control is also assessed using phase portraits. Salient transient stability features of dVOC and droop control during grid faults are summarized and compared. The theoretical analysis is validated by controller hardware-in-the-loop testbed using industry-grade hardware.

Cascaded H-bridge topology has been used in grid-tied converter for battery energy storage system due to its modular structure. To fully utilize the converter's modularity, this paper propose a hierarchical distributed control architecture that consists of primary control, secondary control and battery state of charge (SOC) balancing control. Primary control ensures accurate current tracking while a distributed secondary control based on consensus algorithm is presented to regulate power sharing among modules and is proved to be stable theoretically. A distributed SOC balancing control is further introduced to improve energy efficiency of battery energy storage system. Finally, the hierarchical distributed control strategy is implemented using hardware controllers and a software platform. Besides, a carrier phase shift control is also implemented to achieve multilevel output voltage and harmonic reduction. The experimental results demonstrate the performance of the proposed control scheme effectively.

Virtual oscillator control (VOC) is a nonlinear grid-forming controller that simultaneously achieves the functionality of output voltage control and primary control layers in a power electronics interfaced distributed generation (DG) unit. Unlike conventional phasor-based primary control methods such as the droop control and virtual synchronous machine control, VOC is a time-domain controller that can guarantee almost global asymptotic synchronization. However, the high-frequency dynamics has largely been ignored in the analysis of VOC in prior art; as a result, VOC-based DGs fail to suppress harmonic current in the presence of harmonic distortion in the network-side voltage. In this work, we demonstrate that frequency-domain method can be used for oscillator-based converters in the high-frequency range, specifically, for harmonic mitigation in the converter output current. We propose a virtual impedance-based selective harmonic current suppression method, and demonstrate that it is better to use the network-side current feedback rather than that of the converter-side current for VOC implementation with virtual impedance control. Established harmonic rejection strategies for grid-following and conventional grid-forming converters are compared with the proposed method for VOC. Through impedance-based analysis, we demonstrate that the proposed method augments the passivity range of the converter terminal response with a much simpler implementation. The proposed harmonic suppression strategy is validated through hardware experiments using a single-phase inverter prototype.

In this work, a double synchronous unified virtual oscillator controller (dsUVOC) is proposed for grid-forming voltage source converters to achieve synchronization to the fundamental frequency positive- and negative-sequence components of unbalanced grid voltage without any phase-locked-loop (PLL). The proposed controller leverages a positive- and a negative-sequence virtual oscillator, a double-sequence current reference generator, and a double-sequence vector limiter. Under fault conditions, the controller enables to limit the converter output current below/at the maximum value allowable by the converter hardware while retaining synchronization regardless of the nature of grid faults. Consequently, symmetrical and asymmetrical fault ride-through can be achieved without the need for switching to a back-up controller. The paper presents the implementation and detailed analysis of the double-synchronous structure, which enables simultaneous synchronization to both sequences during current-unconstrained and -constrained operations. Validation of the proposed controller is provided through laboratory hardware experiments.