This paper proposes a two-layer global synchronous pulsewidth modulation (GSPWM) method for attenuating the circulating leakage current of a photovoltaic (PV) station with many inverters connected in parallel, and meanwhile allowing the simple two-level string inverters instead of the commonly used three-level string inverters to directly connect in parallel without the transformer isolation in order to save cost and increase efficiency. The first-layer GSPWM could calculate the optimal phase-shift angles of triangular carriers among the parallel-connected two-level inverters to effectively attenuate their circulating leakage current. The second-layer GSPWM collects the equivalent variable per group provided by the first-layer GSPWM and can still significantly attenuate the total current harmonics at the point of common coupling. Experimental results of a small-scale prototype are presented to verify the performance of the proposed two-layer GSPWM.

This letter proposes a new algorithm for the fast extraction of symmetrical components from unbalanced three-phase signals. Being different from the conventional methods for the decomposition of positive and negative sequence components commonly realized at the cost of introducing significant delay, the proposed algorithm assumes the non-nominal dq-transformation to first produce the special d-axis and q-axis components, which will become the pure high-frequency ac signals, and then uses the typical half-cycle delay method to remove the unwanted negative or positive sequence component. Finally, the proper amplitude and phase angle coefficients can be easily calculated to compensate the remaining high-frequency d-axis and q-axis components before transforming them to the expected d-axis and q-axis values of corresponding symmetrical components. The performance of the proposed algorithm is evaluated through simulation and experiment results.

The common shortage of the existing methods for extracting symmetrical components from unbalanced and distorted three-phase voltages is that they introduce significant time delay before reaching an accurate result. Therefore, this paper presents a new filtering method to fast extract the positive sequence component based on asynchronous coordinate transformation. Unlike the conventional methods that filter out the harmonics in sequence, resulting in cascading delay, the proposed method has two signal streams operating in parallel, which in particular refer to the negative sequence component extraction and harmonic filtering, respectively, and then the positive sequence component can be easily derived by skillfully combining the results of two signal streams. In theory, the proposed method only suffers the largest delay introduced by these two streams. By carefully designing the asynchronous coordinate transformation, the delay can be restricted, thus improving the overall performance. Simulation and experimental results verified the performance of the proposed method.

Virtual synchronous generators (VSGs) powered by renewable energy sources prefer to have energy storage attached to the DC link with DC–DC converters, emulating the inertia and smoothing the intermittency of renewable generations. This paper proposes a new VSG method for a two-stage converter to eliminate the circulating power among inverters while having external characteristics identical to those of a traditional VSG. In particular, the proposed VSG method uses the inverter, energy storage converter, and DC capacitor together to implement the swing equation. Meanwhile, the method can still achieve strong similarities with synchronous generators in terms of physical structure, control, and mathematical model. Therefore, the stable operation of the proposed VSG is easy to evaluate by the well-developed traditional power system theory. Moreover, the method is developed to be capable of eliminating circulating power among paralleled VSGs. Simulation and experimental results have also verified the effectiveness of the proposed method.

DC bus voltage stability is a critical performance metric for interconnected DC microgrids. However, under severe disturbances, it is prone to transient voltage fluctuations in the bus voltage. Existing control strategies for interconnected microgrids fail to adequately account for the differences in load priorities and voltage requirements among subgrids, making it difficult to ensure the voltage quality of critical subgrids. To address this, this paper proposes a distributed control method for interconnected DC microgrids based on weighted voltage control. With the interlinking converter (IC) as the control core, this method designs a weighted voltage control strategy to balance the transient dynamic characteristics among subgrids without affecting the steady-state control objectives. Finally, a simulation model is established on the PLECS platform, and the efficacy of the suggested strategy has been confirmed.