Abstract
Under weak grid conditions, the variation of the grid impedance will affect the steady-state and dynamic performance of the LCL-filtered grid-connected inverter and even make the inverter unstable. To ensure the system stability and further improve the dynamic performance in a weak grid, a control parameter design method with multi-constrains considering the system bandwidth for the current controller and active damping is proposed in this paper. First, based on the current controller and active damping with only grid current feedback, the effects of control parameters and grid impedance on the LCL resonant suppression and the performance of the inverter are analyzed. Moreover, the parameter constraints of the controllers are derived considering the grid impedance, including stability, resonance suppression, and margin constraints. Furthermore, as the system bandwidth affects the dynamic performance of the inverter, combined with the obtained multi-constraints, the optimal control parameters are determined by achieving the maximum bandwidth of the system against the impedance variation. Compared with other two methods, when the proposed method is applied, the system can operate with a better dynamic and steady-state performance. Finally, experiments are performed on a 2 kW three-phase grid-connected inverter in the weak grid, which verify the effectiveness of the parameter design method proposed in this paper.
Highlights
In order to maintain the grid-connected inverter, a good dynamic performance with the stability, resonance suppression, and gain margin and phase margin constraints in the weak grid, the bandwidth of the system should be set to the maximal value considering the variation of the grid impedance
To maintain the system a good dynamic performance in the weak grid, considering the maximal bandwidth of the system in the weak grid, the optimal Kp and kad are derived from the stability, resonance suppression, and gain margin and phase margin constraints
To verify the effectiveness of the parameter design method proposed in this paper under the weak grid, an experimental prototype of the three-phase LCL-filtered grid-connected inverter is built
Summary
Distributed power generation systems based on renewable energy have been widely used in power grids (Zhang et al, 2021; Li et al, 2021), and the three-phase grid-connected inverter with an LCL or L filter, as an important interface between the distributed generation system and the grid, plays an important role in it (Busada et al, 2005; Wu and Lehn, 2006; Malinowski and Bernet, 2008; Xin et al, 2017; Xing et al, 2018). In order to analyze the influence of the PR controller and the active damping loop on the resonance peak suppression of the LCL filter and the stability of the inverter under weak grid conditions, Figure 3 shows the Bode diagram of the openloop transfer function of the system. In order to ensure the stable operation of the system and improve the dynamic performance under the weak grid, a multi-constrain parameter design method for the active damping loop and current controller considering system bandwidth is proposed in Sections 3, 4. In order to ensure the effective operation of the LCL gridconnected inverter and make it have better stability and dynamic response performance, the parameters of the current controller and active damping loop for the grid-connected inverter need to meet certain constraints. When the grid inductance Lg is in the range [0, Lg_max], in order to ensure that Kp simultaneously satisfies the three constraints in (7, 13, 16), Kp needs to be less than or equal to the minimum among them as
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