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User-side battery energy storage systems (UESSs) are a rapidly developing form of energy storage system; however, very little attention is being paid to their application in the power quality enhancement of premium power parks, and their coordination with existing voltage sag mitigation devices. The potential of UESSs has not been fully exploited. Given the above, this paper proposes a hierarchical power supply strategy for premium power parks (PPPs) based on the coordination of UESSs and dynamic voltage restorers (DVR). Firstly, the topology and control strategy of the UESS for voltage sag mitigation are devised. Further, we construct a prototype device, and experimental tests are conducted to verify its effectiveness in voltage sag mitigation. To avoid the negative effects caused by the UESS control mode switching between the voltage sag mitigation mode and the peak-shaving and valley-filling modes—namely, the long control mode switching time and low state of charge (SOC)—a hierarchical power supply strategy is proposed. The basic idea is to supply three levels of power quality service for loads with different sensitivities in PPPs by coordinating UESS with DVR. Several cases are explored to demonstrate the feasibility of the proposed strategy.

Modular multilevel converter (MMC) has been widely used in VSC-HVDC system for its smooth output waveform, low harmonic content and low switching loss. But compared with the conventional LCC-HVDC system, modular multilevel converter has a short development history, few engineering applications, high failure rate and lacks of very effective protection strategies, which limits its further development. Therefore, this paper firstly introduces topological structure of MMC and its working principle. Then the commonly used control policy of dual-terminal MMC-HVDC project is elaborated. Finally, the transient electrical characteristics of the typical faults are detailedly analyzed through the combination of theoretical analysis and simulation, such as three phase short circuit of converter valve, single phase grounding of converter valve, and the bridge arm short circuit. The conclusion provides an essential theory for the research of the converter protection strategy in the MMC-HVDC project.

Nowadays, cold ironing technology has been demonstrated to be an effective solution to deal with the environmental and social problems brought by port ship emissions and relevant effects. The working states of cold ironing equipment, especially the key components such as circuit breakers, transformers and frequency converters, have a significant effect on the safety and reliability of the whole system. However, due to the harsh working environment of cold ironing equipment, they are prone to a high risk of failure. In this respect, fault diagnosis methods can play a significant role in detecting potential failure in time and guarantee the safe and reliable operation of the cold ironing system. In recent years, research on the fault diagnosis of a cold ironing system has been rapidly growing, and this paper aims to present a comprehensive review of this literature, with an emphasis on the fault diagnosis technology applied to the key components in a cold ironing system. This review classifies the literature according to the type of key component, and, for each special type of component, the fault diagnosis methods are further categorized and analyzed. This paper provides useful references for professionals and researchers working on the fault diagnosis of a cold ironing system and points out valuable research directions in the future.

HVDC converter transformer oil-flow relay plays an important role in realizing non-power protection of converter transformer on-load tap changer, which can act accurately and directly affect the safe and stable operation of the whole HVDC program as well as the converter transformer itself. In this paper, the research revolves the hidden dangers of HVDC converter transformer and its identification. The paper begins with the analysis of the converter transformer operating principles of two German companies: MR and EMB. Then, the current application status and related inspection requirements of oil-flow relay are expounded. Finally, pertinent hidden danger identification and modification schemes are provided based on analysis of the danger in actual operation of oil-flow relay and its causes. The feasibility and effectiveness of the scheme are verified by the actual operation on site.