The overall absolute stability analysis of grid-connected inverters can be achieved by adopting an open-loop synchronization scheme, but its robustness is limited by the grid impedance, injected power, and port voltage level. Hence, this paper aims to propose a robust open-loop synchronous control strategy suitable for high impedance and wide disturbance. The control strategy, which introduces grid current feedforward signal into the synchronizer input, is proposed, and the frequency-domain conditions of the inverter system to be absolutely stable are obtained by using the circle criterion. On this basis, the reason why the proposed control strategy can improve the robustness of inverter against grid voltage and power control variation is explained. Compared with the traditional open-loop synchronous control strategy, this method extends the range of the disturbance which the system can withstand, and improves the absolute stability of the system. The correctness of the theoretical analysis and effectiveness of the proposed method are verified by simulation test results.
Grid-forming (GFM) converters, such as virtual synchronous generators (VSGs), behave as voltage sources behind a reactance within a specified frequency range. Therefore, solutions to limit the rapid growth of the converter output current, which may cause over-current tripping during grid faults, are necessary. One effective solution is to use outer dual-loop voltage and current control so that current limitation can be realized by saturating the current reference. In the condition, electrical power (EP) of VSG can be calculated either with the converter output current or with the virtual current from the voltage loop, leading to different small-signal stability for the GFM converter. In this paper, small-signal stability of these two EP calculation methods are analysed and compared. Small-signal eigenvalues analysis shows that using virtual current from outer voltage loop for EP calculation contributes to better small-signal stability as compared to the conventional EP calculation approach, under both the weak grid and the strong grid conditions. Simulation results are provided to validate the effectiveness of the analysis.
The repetitive controller is widely applied in grid-connected inverters to suppress harmonics. For the RC, an array with length N is required to store history samples, which may occupy large memory in digital realization process. In this paper, three low N-value RCs are designed for the modified LCL (MLCL)-type grid-connected inverter by decreasing the sampling rate and increasing the fundamental harmonic frequency of the RC. Finally, experiments are conducted on a 228kW inverter to compare the performance of these three RCs.
The test method for sub-circuits in modular topology has been widely studied in recent years. Most of the existing methods need to build extra hardware circuit and are not flexible. To improve the flexibility, this paper proposes a control and power hardware-in-the-loop test bench, which is capable of power and control signals interaction simultaneously. It could provide a real operating environment for sub-circuits under test, which is as like working together with the other parts of whole modular topology.
Multiple power modules are interfaced in parallel via laminated busbar in high-capacity active neutral-point clamped (ANPC) converters. Consequently, the complex mutual coupling among parallel branches profoundly affects their equivalent stray inductance. This study analyzes mutual coupling between parallel branches on the same plane of the busbar and how it affects the equivalent stray inductance, which has not been thoroughly reported. A mutual inductance experimental extraction method is proposed and a mathematical model of stray inductance is established. Then, using the proposed method, stray inductance and mutual inductance of two parallel branches for an ANPC busbar are calculated. Experimental tests verify the accuracy of the proposed method.
