Soft open points (SOPs) are power electronic devices that are usually placed at normally open points of electricity distribution networks to provide flexible power control to the networks. This paper gives a comprehensive overview of both academic research and industrial practice on SOPs in electricity distribution networks. The topologies of SOPs as multi-functional power electronic devices are identified and compared, which include back-to-back voltage source converters, multi-terminal voltage source converters, unified power flow controllers, and direct AC-to-AC modular multilevel converters. The academic research is reviewed in three aspects, i.e., benefit quantification, control, and optimal siting and sizing of SOPs. The benefit quantification indices are categorized into feeder load balancing, voltage profile improvement, power losses reduction, three-phase balancing and DG hosting capacity enhancement. The control of SOPs is summarized as a three-level control structure, where the system-level and converter-level control are further discussed. For optimal siting and sizing of SOPs, problem formulation and solution methods are analyzed. Besides the academic research, practical industrial projects of SOPs worldwide are also summarized. Finally, opportunities of research and industrial application of SOPs are discussed.

Bulky electrolytic capacitors, which are often needed in dc systems to filter out voltage ripples, considerably reduce power density and system reliability. In this paper, a ripple eliminator, which is a bidirectional buck-boost converter terminated with an auxiliary capacitor, is adopted to replace bulky capacitors in dc systems. The voltage ripples on the terminals (i.e., the dc bus) can be transferred to the auxiliary capacitor, and the ripples on the auxiliary capacitor can vary in a wide range. Moreover, the average voltage of the auxiliary capacitor can be controlled either lower or higher than the dc-bus voltage, which offers a wide operational range for the ripple eliminator and also the possibility of further reducing the auxiliary capacitance. Hence, the total capacitance required can be much smaller than the originally needed. After proposing a control strategy to transfer the voltage ripples to the auxiliary capacitor, three control strategies are proposed to regulate the auxiliary-capacitor voltage to maintain proper operation. Intensive experimental results are presented to demonstrate the performance.

Connecting converters in cascade is a basic configuration of dc distributed power systems (DPS). The impedance interaction between individually designed converters may make the cascaded system become unstable. The previous presented stabilization approaches not only need to know the information of the regulated converter, but also have to know the characteristics of the other converters in the system, which are contradictory to the modularization characteristic of dc DPS. This letter proposes an adaptive-input-impedance-regulation (AIIR) method, which connects an adaptive virtual impedance in parallel with the input impedance of the load converter, to stabilize the cascaded system. This virtual impedance can adaptively regulate its characteristic for different source converters. Therefore, with the AIIR method, all the load converters can be designed to a fixed standard module to stably adapt various source converters. In addition, at any cases, the AIIR approach only changes the load converter's input impedance in a very small frequency range to keep the load converter's original dynamic performance. The requirements on the AIIR method are derived and the control strategies to achieve the AIIR method are proposed. Finally, considering the worst stability problem that often occurs at the system whose source converter is an LC filter, a load converter cascaded with two different LC input filters is fabricated and tested to validate the effectiveness of the proposed AIIR control method.

It has been shown recently that a cascaded dc/dc converter system can be stabilized via amplitude compensation (SAC) or phase compensation (SPC) for the input impedance of the load converter. In this letter, it is shown that the cascaded system when adopting the SAC is unconditionally stable but conditionally stable when adopting the SPC, that is, SAC is more stable than SPC. Then, the comparison is carried out for the parallel-virtual-impedance (PVI) and series-virtual-impedance (SVI) control strategies that are adopted to implement the SAC, and it is found that only the SVI control strategy can achieve the SAC for the whole load and input voltage range of the load converter without limitation. Therefore, SVI is in general better than PVI when realizing SAC. Following on this, an adaptive mechanism is introduced to improve the traditional SVI control strategy so that the load converter can be stably connected to different source converters such as $LC$ input filters and traditional dc/dc converters. Finally, a load converter cascaded with three different source converters is fabricated to validate the effectiveness of the proposed adaptive SVI control strategy.

DC technology has gained considerable interest in the medium voltage applications due to the benefits over the AC counterpart. However, to utilize the full capacity of this development, the selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSC's), it is unclear which topology is suitable for multi-megawatt applications at medium voltage dc (MVdc) levels. To address this, the paper proposes a selection guideline based on reliability and optimum redundancy levels of VSCs for MVdc applications. This will be combined with other functional factors such as operational efficiency and return-on-investment. Three candidate multi-level topologies namely three-level neutral point clamped converter (3L-NPC), modular multi-level converter (MMC) and cascaded 3L-NPC (which is being used for the first MVdc link in the U.K.) have been evaluated over two-level-VSC from $\pm$ 10 kV to $\pm$ 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc voltage levels 3L-NPC is the prominent topology.