Multi-Mode Inverters: A Unified Control Design for Grid-Forming, Grid-Following, and Beyond

Abstract

We present a novel, integrated control framework designed to achieve seamless transitions among a spectrum of inverter operation modes. The operation spectrum includes grid-forming (GFM), grid-following (GFL), static synchronous compensator (STATCOM), energy storage system (ESS), and voltage source inverter (VSI). The proposed control architecture offers guarantees of stability, robustness, and performance regardless of the specific mode. The core concept involves establishing a unified algebraic structure for the feedback control system, where different modes are defined by the magnitude of closed-loop signals. As we demonstrate, this approach results in a two-dimensional continuum of operation modes and enables transition trajectories between operation modes by dynamically adjusting closed-loop variables towards corresponding setpoints. Stability, robustness, and fundamental limitation analyses are provided for the closed-loop system across any mode, as well as during transitions between modes. This design facilitates stable and enhanced on-grid integration, even during GFM operation and weak grid conditions. Ultimately, we demonstrate the key attributes of the proposed framework through simulations and experiments, showcasing its seamless transition in on-grid operation, functionality in islanded mode, and robustness to line impedance uncertainty.

Figures (19)

• Figure 1: Parallel operation of the collection of inverter units, assimilating distributed energy resource and storage units such as the battery, EV, and renewables into the grid.
• Figure 2: Inverter as a controlled voltage source $v_c$, connected to the Grid $v_g$ via an RL impedance.
• Figure 3: (a) Active power control (APC) and reactive power control (RPC) based on $P/f$ and $Q/v$ droop. (b) The $dq$ synchronous rotating frame.
• Figure 4: Proposed closed-loop with line dynamics $G_L$ in (\ref{['eq:line_model']}) as plant and $K=\widetilde{K}K_L$ as MIMO feedback controller. The $K_L$ is the full matrix part of $K$, while $\widetilde{K}=\text{diag}(K^d,K_1^q + K_2^q)$ is the diagonal part of the controller. The input disturbances $\{d^d, d^q\}$ capture the effect of the PCC on the closed-loop, while $\{u^d,u^q\}$ integrates the capacitor voltage and the $dq$ frame angle $\theta$ as closed-loop control effort.
• Figure 5: Inverter operating mode based on the shape and number of zeros in $\widetilde{S}$ at low-frequency. (a) GFM, (b) voltage support (STATCOM), (c) frequency support (ESS), and (d) GFL.

Theorems & Definitions (10)

• Remark 1
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• Remark 10