Multi-Mode Coordination Control of Virtual Synchronous Generator under Unbalanced Power Grid

Virtual synchronous generator control has been widely studied in renewable energy generation to provide equivalent inertia support for the power grid. However, current distortion and power oscillation occur severely when the grid voltage becomes unbalanced, significantly aggravating the power supply quality of renewable energy generation. The existing methods generally take the fundamental-frequency negative-sequence component as the VSG control object to solve the above issues. It is obvious that positive-negative sequence separation increases the calculation burden, and active and reactive power oscillations cannot be suppressed simultaneously. In addition, the mutual influence between optimization targets is rarely considered, so the realization of one optimization target may lead to the deterioration of the others. Therefore, this paper proposes a multi-mode coordination control of VSG and quantitatively analyzes the restrictive relation of VSG current distortion and power fluctuation under an unbalanced power grid. This paper first analyzes the VSG power oscillation mechanism under an unbalanced power grid and then further studies the harmonic components of VSG grid-connected current in constant-power mode. The results indicate that VSG grid-connected current contains low-order odd harmonic components, and the proportion determines double-line frequency active and reactive power oscillation. On this basis, the initial current reference is obtained from VSG virtual power calculation to maintain the operating characteristics of the synchronous machine. Then a multi-resonance current inner loop controls the fundamental frequency current and the harmonic components. It should be noted that the coordination coefficient is introduced to modify the current reference of the harmonic controller, thereby realizing the VSG multi-mode coordination control. Finally, the influence of the coordination coefficient on VSG current quality and power ripple is quantified, and the calculation method of selection range is given. In order to validate the effectiveness of the proposed method, a modular multilevel converter (MMC) is selected as the grid-connected AC-DC topology, and the simulation and experiment are respectively performed on Matlab/Simulink platform and a three-phase five-level MMC prototype. In the simulation, the power step performance of the proposed method is well verified under balanced-current and constant-power modes. After that, VSG multi-mode coordination control and the corresponding coordination coefficient selection method are simulated, which verifies the proposed method. As for the experiment, the verification is carried out when the b-phase voltage drops to 0.6(pu). The experiment waveforms and the components of VSG current under the two typical modes further verified the validity of the aforementioned theoretical analysis and the control strategy. In the process of VSG multi-mode coordination, the experimental results show that the trends of current total harmonic distortion and the power ripple are consistent with the theoretical calculation. The following conclusions can be drawn from this paper: (1) Under an unbalanced power grid, the VSG grid-connected current appears as low-order odd harmonic components in constant-power mode, and the amplitude is positively related to the voltage imbalance degree. (2) The proposed method can realize the multi-mode coordinated control between balanced-current and constant-power modes to flexibly regulate the VSG output current quality and power ripple. (3) This method simplifies the control and reduces the calculation burden without positive-negative sequence separation and double dq transformation. © 2023 Chinese Machine Press. All rights reserved.