Network partition in complex power networks is essential for the var-voltage control. Traditional partition methods such as Ward method are applied in practical power networks, but they are unable to evaluate the quality of partition results. Moreover, they lack efficiency when dealing with large-scale networks. Since complex operation characteristics and topology have emerged in recent power systems, the power grid partition requires higher efficiency and quality. Therefore, this paper proposes a fast network partition method with a balanced-depth-based community detection algorithm. Its aim is to significantly improve the efficiency of partition while maintaining high quality of partition, with which the inter-zone coupling is minimized while the intra-zone coupling is maximized. In the meantime, a surrogate-optimization-based selection algorithm is proposed to select the zonal pilot bus, based on which the secondary voltage control method is used to evaluate the quality of partition. Results from four case studies conducted in various power networks with different sizes, as compared to other partition methods, validate the high efficiency and high quality of the proposed power grid partition approach.

This paper presents a new feed-forward strategy for suppressing low-order harmonic circulating currents in pulsewidth-modulated (PWM) based modular multilevel converters (MMCs). The approach is based on the new average model approach established in this paper, which identifies cross-coupling interactions between the system-level and submodule elements using decomposed dependent sources. As well as improving the damping of the system, the major advantage of this approach is that it maintains the natural DC bus balance property of a PWM modulated MMC by considering the upper and lower arms together (unlike existing feed-forward schemes which treat each MMC arm independently). Moreover, unlike existing direct control resonant strategies, which require careful gain tuning and are highly dependent on the ac system frequency, the proposed approach achieves wideband circulating current ripple suppression without requiring knowledge of the circulating current harmonic frequencies and needs only a simple proportional-integral (PI) controller to regulate the circulating current dc component for power balance. This makes it particularly suitable for use in multifrequency or variable-frequency ac systems. Extensive simulation and matching experimental results, including steady-state and transient responses compared against existing circulating current suppression approaches, validate the effectiveness and benefits of this new feed-forward compensation technique.

Transformer-less photovoltaic (PV) inverters are more widely adopted due to high efficiency, low cost and light weight, etc. Many novel topologies and their corresponding modulation methods have been proposed, verified and put into use, especially focus on active power injection only without leakage current issue. However, some new grid codes require PV inverters to have the ability of injecting reactive power into the utility for grid support. In order to map the challenge, an improved hybrid modulation method is proposed and evaluated for one non-isolated H6 topology as an example. With only simple modification for switching patterns and phase shift for current reference, variable reactive power generation ability and zero crossing distortion is achieved, while common mode voltage issue is also eliminated. The operation modes with the improved hybrid modulation approach are presented in detail as well. Some design considerations are also provided. Extensive results from a 4000VA prototype along with the SMPLIS simulation verify the proposed hybrid modulation method.

Modular multilevel converter (MMC) faces challenges ofcost and footprint in medium and high voltage grid applications. By adding a three-phase switching bridge on the basis of MMC, this article proposes a three-phase multiplexing arm modular multilevel converter (TPMA-MMC) with high power density and small volume. The multiplexing principle for TPMA-MMC is investigated, showing the multiplexing arm can be multiplexed for one-third fundamental period. After that, the subsection modulation scheme is further proposed for energy balance purpose. Compared with MMC, the number of submodules (SMs) and power devices are reduced by 22.8% and 13.83%, respectively, resulting in lower construction cost. Meanwhile, the SMs’ capacitance and the total capacitor energy storage of TPMA-MMC reduce greatly with the subsection modulation, which can greatly reduce the footprint and improve the power density. Moreover, the proposed TPMA-MMC outperforms the MMC in terms of efficiency. Finally, the topology and proposed modulation scheme are verified by simulation and experimental results.

This study presents a novel solid‐state transformer (SST) with four ports that can connect the medium‐voltage (MV) DC bus, the MV AC bus, the low‐voltage (LV) DC bus, and the LV AC bus, respectively. This SST is suitable for application in the hybrid AC/DC power grid that functions as an energy router between AC and DC systems. Furthermore, the SST submodule is enhanced with two types of dual‐half bridge converters, namely DHSM1 or DHSM2, depending on if an additional active switch is used to block the possible short circuit. Thanks to the designed structure of the proposed SST that combines DHSM1 and DHSM2 as the front‐end stage, it achieves reduced hardware complexity with the remaining capability to handle the DC short‐circuit fault that may occur at the medium‐ or high‐level voltage DC grid. In addition, an optimised overall control strategy that can reduce the measurement demand and control complexity is proposed, with the possible power flow path inside the SST fully analysed. Simulation and experimental results validate the feasibility of the proposed SST and the effectiveness of the overall control scheme with the DC fault handle capability.