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    • Guest Chief Editor's Note

      2019, 43(23):1-1. DOI: 10.7500/AEPS20191102003

      Abstract (851) HTML (0) PDF 953.81 K (1263) Comment (0) Favorites

      Abstract:当前,光伏等直流型分布式电源快速发展,电动汽车充电桩、数据中心等直流型负荷日益增多,使得配电系统中“源荷”的直流特性日趋明显。由于直流配电系统能有效减少变流环节、提高对分布式电源的接纳能力和能源利用效率,在国内外引起广泛关注。未来直流配电系统的高效可靠运行与推广应用,对系统规划、运行、控制与保护等技术,以及直流相关设备的研发等诸方面都提出了新要求和新挑战。 《电力系统自动化》针对这一研究热点策划了“直流配电系统关键技术”专辑,旨在剖析直流配电系统在分布式电源与负荷接入、电压序列、电气装备、控制保护等方面的新特点,分析未来直流配电网技术发展趋势,研讨直流配电系统规划设计、设备研发、控制保护以及工程应用等关键技术。本期专辑收录论文17篇。 直流配电系统总体方面安排3篇论文。针对中压直流配电网应用场景与系统设计问题,华北电力大学韩民晓等论述了其应用场景、特点及优势,并从工程实现的角度阐述了中压直流配电网设计所涉及的关键技术。针对当今社会能源变革现状及电力系统发展新趋势,山东大学马钊等指出了新一代低压直流供用电系统的主要特征、典型应用场景、关键技术及未来研究方向。针对直流配电系统的控制与保护问题,天津大学王守相等对直流配电系统的控制与保护协同关键技术进行了总结和展望。 直流配电系统规划方面安排3篇论文。针对中低压直流配电电压层级选择问题,中国科学院电工研究所金吉等建立了直流配电电压评估模型,提出了中低压直流配电电压层级的选取方法。针对直流配电系统的组网问题,华南理工大学张勇军等开展了直流配电系统的接入方式及其控制技术研究。针对配电网直流改造问题,中国农业大学许彪等提出了考虑高密度负荷与分布式电源接入下城市配电网直流改造经济性评估模型和方法。 直流配电装置研发方面安排1篇论文。针对在直流变压器中广泛应用的隔离型双有源桥直流变换器关键技术问题,华中科技大学胡燕等提出了双重移相隔离型双有源桥变换器的自然软开关电流应力优化方法。 直流配电系统运行控制方面安排4篇论文。针对交直流配电网状态估计问题,浙江大学马鑫等考虑相量测量单元及数据采集与监控系统混合量测,提出了采用原对偶内点法进行三相不平衡下交直流配电网状态估计的方法。以中低压直流配电系统为研究对象,东南大学潘鹏鹏等提出了一种基于直流母线电压信号的分散式统一控制策略。为实现系统扰动后不平衡功率的优化分配以及直流电压的稳定控制,东北电力大学成龙等提出了基于元胞自动机的多端柔性直流配电网自组织下垂控制方法。山东理工大学彭克等还对柔性直流配电系统稳定性及其控制关键问题开展了综述研究。 直流配电系统保护方面安排3篇论文。针对采用故障自清除换流器消除故障电流的直流配电系统的健全区失电问题,天津大学李斌等提出一种基于二极管的直流配电系统故障分区方案及保护策略。针对环状直流配电网的中压直流线路单极接地故障的保护问题,华北电力大学戴志辉等提出了基于模型相似度比较的环状柔直配电网线路保护方案。针对直流配电线路保护,北京交通大学李猛等提出了基于全电流方向特征的柔性直流配电网纵联保护方法。 直流配电系统工程应用方面安排3篇论文。南瑞集团有限公司吴通华等结合即将实施的苏州中低压直流配用电系统示范工程,研究了其保护分区及原则、保护配置方案和故障定位方法。国网江苏省电力有限公司电力科学研究院刘瑞煌等探讨了中压直流配电网中直流变压器的工程化应用问题。针对珠海唐家湾三端柔性直流配电网故障隔离需求,清华大学屈鲁等开展了三端口混合式直流断路器工程应用研究。 最后,衷心感谢各位作者的踊跃投稿,感谢各位评审专家和编辑部为本专辑从酝酿到推出所付出的辛勤汗水。期盼通过本专辑的引领,以小流汇成大海,促进在直流配电这一领域更多的开拓创新,并收获累累硕果。

    • Application Scenarios and System Design of Medium-voltage DC Distribution Network

      2019, 43(23):2-11. DOI: 10.7500/AEPS20190320004

      Abstract (963) HTML (0) PDF 1.55 M (1400) Comment (0) Favorites

      Abstract:The application scenarios, basic characteristics and system design of medium-voltage DC distribution network are lack of systematic discussion. Firstly, the system characteristics of power supply capacity, interconnection, mutual support and short-circuit current suppression are discussed to show the application scenarios, characteristics and advantages of medium-voltage DC distribution network. Then, key technologies of design for medium-voltage DC distribution network are also given in the view of engineering realization, including the determination of voltage level, grid design, wire model and grounding mode selection. Finally, the problems that should be considered in the development and application of medium-voltage DC distribution system are presented.

    • Decentralized Unified Control Strategy for Medium- and Low-voltage DC Power Distribution System

      2019, 43(23):72-80. DOI: 10.7500/AEPS20190430005

      Abstract (486) HTML (0) PDF 1.61 M (1165) Comment (0) Favorites

      Abstract:The medium- and low-voltage DC power distribution system consists of medium-voltage DC bus, low-voltage DC bus, DC transformer and so on, with the characteristics of multiple voltage levels, DC buses and converters. The operation modes and coordination control strategies of the system are more complicated. A medium- and low-voltage DC power distribution system is taken as the research object, and a decentralized unified control strategy based on the DC bus voltage signal is proposed. The photovoltaic power generation unit, energy storage unit and fuel cell unit on the DC bus adopt decentralized control strategies to adjust their working modes according to the DC bus voltage. The DC transformer utilizes a unified control strategy which integrates the functions of power control and the medium- and low-voltage DC bus voltage regulation. The DC transformer can automatically adjust the magnitude and direction of the transmission power based on normalized values of medium-voltage and low-voltage DC bus voltage deviations, and realize the global power sharing of the system. Finally, the simulation model of medium- and low-voltage DC power distribution system is built in MATLAB/Simulink, and simulation analysis of various operating states and scenarios is carried out to verify the effectiveness of the proposed control strategy.

    • Key Issues of Stability and Control in Flexible DC Distribution System

      2019, 43(23):90-98. DOI: 10.7500/AEPS20190320009

      Abstract (587) HTML (0) PDF 2.00 M (1129) Comment (0) Favorites

      Abstract:Flexible DC distribution systems mostly use power electronic equipment to interconnect with AC systems and accommodate distributed generators and loads. It has characteristics of low inertia and weak damping, and is easy to induce oscillations and instability with different frequencies by the random fluctuations of distributed generators and loads. This paper analyzes the challenges faced by stability analysis and control of flexible DC distribution system, and points out that its multi-time scale and stochastic characteristics are the main technical problems. By summarizing the research status of modeling, oscillation mechanism, stability analysis and control, the key research issues are prospected, including multi-time scale modeling, oscillation mechanism, stochastic disturbance stability analysis and robust stability control in multiple scenarios. The illuminated general framework is expected to bring more insights for the stability research of flexible DC distribution system.

    • Diode Based Fault Zone Partition Scheme and Protection Strategy in DC Distribution System

      2019, 43(23):99-106. DOI: 10.7500/AEPS20190420005

      Abstract (510) HTML (0) PDF 1.39 M (1190) Comment (0) Favorites

      Abstract:Fault processing technology including fault current clearing, fault zone partition and fault location is the key issue of the widely engineering application of DC distribution system. In order to solve the problem of power loss in the sound zone, a diode based automatic fault zone partition method, a fault location method and an integrated protection scheme are proposed for the DC distribution system that uses the fault self-clearing converter to eliminate the fault current. Firstly, the basic principle of diode based fault zone partition method and its additional hardware requirements are introduced. Then, the overall protection scheme and fault location method are proposed considering the fault characteristics of diode based fault partition. Finally, a five-terminal DC distribution system is built on the PSCAD/EMTDC platform to verify the effectiveness of the proposed scheme.

    • Similarity Comparison Based Single-pole Grounding Fault Protection for Lines in Ring-shaped Flexible DC Distribution Network

      2019, 43(23):107-115. DOI: 10.7500/AEPS20190114001

      Abstract (458) HTML (0) PDF 1.40 M (1151) Comment (0) Favorites

      Abstract:The probability of single-pole grounding faults on lines of DC distribution network is high. It is difficult to identify the fault due to the large variation range of the transition resistance. With regard to the protection against single-pole grounding faults of medium-voltage DC lines in ring-shaped DC distribution networks, this paper analyzes the zero-mode network of the system after single-pole grounding fault. Based on the zero-mode network, the mathematical models of internal-fault and external-fault are established. Then the Hausdorff distance algorithm is employed to compare the similarity between the sampled zero-mode differential current and the calculated value obtained by the internal-fault and external-fault models, which achieves rapid identification and isolation of the fault. It is simple to set the relay threshold as well. Finally, the feasibility of the protection scheme is verified via a simulation model in PSCAD/EMTDC. The results show that the proposed scheme has high fault identification accuracy and tolerance to transition resistance.

    • Directional Pilot Protection for Flexible DC Distribution Network Based on Directional Characteristics of Instantaneous Current

      2019, 43(23):116-122. DOI: 10.7500/AEPS20190319003

      Abstract (455) HTML (0) PDF 1.47 M (1116) Comment (0) Favorites

      Abstract:A primary line protection method is proposed for the safety and reliable operation of flexible DC distribution network. Based on the analysis of current direction characteristics of bipolar short-circuit faults in each stage, a new directional pilot protection based on instantaneous current direction characteristics is proposed. The action criterion and the setting method of setting value are given, and the influence of various factors on the proposed protection is analyzed. The protection method is not affected by the voltage source converter(VSC)blocking and the transient current of the line distributed capacitance. The physics experiment and simulation results verify the feasibility of the proposed protection method. It has fast action speed and good resistance to noise and transition resistance. The proposed protection could meet the requirements of the flexible DC distribution network for protection quickness and selectivity.

    • Protection Scheme and Device Development of Flexible DC Distribution Network

      2019, 43(23):123-130. DOI: 10.7500/AEPS20190428004

      Abstract (806) HTML (0) PDF 811.96 K (1312) Comment (0) Favorites

      Abstract:The protection partition and protection configuration are studied based on the typical flexible DC distribution network. According to the fault characteristics of DC lines, the protection scheme of DC pole-to-pole short-circuit fault, single-pole grounding fault and broken line fault are analyzed, and a comprehensive fault location method based on differential protection and multi-point direction information is proposed. Based on the proposed protection and fault location scheme, the protection device of DC distribution network which has high sample rate and multiple kinds of analog input interfaces is developed. The test and dynamic simulation on real-time digital simulation(RTDS)system prove the feasibility of the protection scheme and correctness of the fault location method, and the action behavior and performance index of the developed protection device meet all the safety operation requirements of DC distribution network.

    • Engineering Applications of DC Transformer in Medium-voltage DC Distribution Network

      2019, 43(23):131-140. DOI: 10.7500/AEPS20190429020

      Abstract (474) HTML (0) PDF 1.40 M (1231) Comment (0) Favorites

      Abstract:Focusing on the problems of engineering applications of DC transformer(DCT)in medium-voltage DC distribution network, this paper firstly summarizes the requirements of DCT for practical engineering applications, such as quick recovery form fault, high reliability for power supply and flexible control. An improved submodule topology with isolated half bridge is adopted, and the fault blocking, on-line redundancy, on-load voltage regulation and voltage sharing control of the improved submodule topology are analyzed. Then, several typical engineering problems are discussed, including the power supply mode for submodules, soft start-up strategy and insulation design. Various solutions for these problems are further presented and compared with the adaptability analysis. Furthermore, the energy efficiency improvement which is the key issue for engineering is mainly studied by the analysis of hardware and software. And the corresponding improvement strategies are proposed in several aspects, which include the type selection of materials, design of high frequency transformer, improvement of control strategy and optimization of system energy. Finally, the engineering prospects of miniaturized apparatus are explored. Several schemes for miniaturized apparatus are respectively proposed from the perspectives of specific scenario design, improvement of module topology and application of new materials, and the existing technical bottleneck and corresponding solutions are clarified.

    • Engineering Application of Three-terminal Hybrid DC Circuit Breaker

      2019, 43(23):141-146. DOI: 10.7500/AEPS20190509005

      Abstract (668) HTML (0) PDF 1.81 M (1220) Comment (0) Favorites

      Abstract:The project of Tangjiawan three-terminal flexible DC distribution network in China successfully applies the first ±10 kV three-terminal hybrid DC circuit breaker all over the world, which can realize the contact and fault isolation of multi-terminal converter stations, thus ensuring safe and reliable operation of the distribution network. In this paper, the topology structure, working principle and technical parameters of the three-terminal hybrid DC circuit breaker are proposed for the fault isolation requirements of the three-terminal flexible DC distribution network in Tangjiawan. Furthermore, the engineering prototype of the three-terminal DC circuit breaker is developed and the current breaking test is carried out. The test results show that the engineering prototype of the three-terminal hybrid DC circuit breaker can interrupt the 10 kA fault current within 2. 7 ms. Finally, the three-terminal DC circuit breaker is installed in the project of Tangjiawan three-terminal flexible DC distribution network, and three key tests are carried out, including online input, online quit and current breaking tests of three-terminal system. The test results show that the three-terminal DC circuit breaker is in good condition, and its function and performance can meet the requirements of engineering design and operation.

    • Key Technologies and Development Prospect of New Generation Low-voltage DC Power Supply and Utilization System

      2019, 43(23):12-22. DOI: 10.7500/AEPS20190905007

      Abstract (1017) HTML (0) PDF 1.78 M (1561) Comment (0) Favorites

      Abstract:Aiming at the current situation of energy revolution of the society and the development trend of power system, the characteristics, typical application scenarios, key technologies and development trend of new generation low-voltage direct-current(LVDC)power supply and utilization system are explored. Firstly, the characteristics of energy revolution and the development trend of power system in modern society are analyzed, the development demands of new generation LVDC power supply and utilization system containing power generation, energy storage and consumers are pointed out. The main characteristics and typical application scenarios of new generation LVDC power supply and utilization system are analyzed. Voltage levels, international standards and status of demonstration projects domestic and abroad of LVDC power supply and utilization system are introduced. The key technologies of future LVDC power supply and utilization system are arranged, and the development trend of LVDC power supply and utilization system is expected. Finally, the summary and suggestion of specific research proposals in the field of new generation LVDC power supply and utilization technology are given.

    • Optimization Method of Current Stress for Dual Active Bridge Converter Considering Different Soft Switching Modes

      2019, 43(23):58-64. DOI: 10.7500/AEPS20190429004

      Abstract (422) HTML (0) PDF 2.69 M (1048) Comment (0) Favorites

      Abstract:To comprehensively improve the overall performance of the dual active bridge(DAB)converter, this paper focuses on the necessity of ensuring zero voltage switching(ZVS)operation for all switches while minimizing current stress based on dual phase shift. Firstly, a linearized large signal model with transmitted power as input is established and the precise control of output voltage is realized. Then, the characteristics of soft switching and current stress in four power modes are analyzed based on the proposed model by directly regulating the transmitted power. Two current stress optimization methods with full ZVS operation and natural soft switching operation are implemented, respectively. Moreover, the improvement of efficiency using two optimization methods is analyzed and compared. The results show that ensuring all switches to realize ZVS greatly will increase the current stress and decrease the efficiency of DAB converter, when switches with similar turn-on loss and turn-off loss are adopted. Both theoretical analysis and experimental results show that the current stress optimization algorithm with natural soft switching can significantly minimize the current stress, reduce the loss, and improve the performance of the converter.

    • Self-organizing Droop Control of Multi-terminal DC Distribution Network Considering Power Margin and Voltage Deviation

      2019, 43(23):81-89. DOI: 10.7500/AEPS20190419005

      Abstract (449) HTML (0) PDF 1.63 M (1036) Comment (0) Favorites

      Abstract:In order to realize the optimal allocation of unbalanced power and the stable control of DC voltage after disturbance, this paper proposes a self-organizing droop characteristic optimization method considering power margin and additional constant DC voltage control based on the conventional voltage and power droop control of multi-terminal flexible DC distribution network. The proposed method breaks the inherent constraint that the unbalanced power of converter station is proportionally distributed according to its capacity in the conventional droop control, which realizes the rational transfer of unbalanced power and reduces the overload risk of smaller-capacity converter stations. By introducing the power deviation factor, the “dead zone” is appropriately set on the droop characteristic curve to relax the operation range of the converter station to a certain extent. By analogy to the concept of elasticity mechanics, this paper designs a self-organizing updating rule of operation state of the converter station based on the cellular automaton. The corresponding MATLAB simulation model of five-terminal DC distribution network is established, and the proposed model is compared with the electromagnetic transient model with traditional droop control. Finally, the simulation results verify the effectiveness and control effects of the proposed control method.

    • Key Technologies and Prospect for Coordinated Control and Protection in DC Distribution System

      2019, 43(23):23-30. DOI: 10.7500/AEPS20190506005

      Abstract (966) HTML (0) PDF 1.43 M (1409) Comment (0) Favorites

      Abstract:In DC distribution system, large number of power electronic devices increases the difficulty of DC protection for their vulnerability. However, based on the flexible and controllable power electronic devices, realizing quick recovery of fault by coordinated control and protection becomes an important idea in the research of DC distribution system. Firstly, the overall thought of coordinated control and protection in DC distribution system is summarized. Secondly, the influence of the structure and control strategy of sub-modules of converters on the fault characteristics in DC distribution system is analyzed. The protection principle of DC distribution system based on the idea of control and protection coordination is summarized, and the coordination process of control and protection in DC distribution system is analyzed. Finally, the research on the coordination of control and protection of the DC distribution system is summarized and prospected.

    • Economic Evaluation Method for Multi-scenario DC Transformation of Active Distribution Network

      2019, 43(23):50-57. DOI: 10.7500/AEPS20190513003

      Abstract (530) HTML (0) PDF 1.70 M (1217) Comment (0) Favorites

      Abstract:DC distribution mode has the advantages of high power supply capacity, low loss and flexible control. With the continuous maturity of power electronic technology, AC/DC hybrid distribution network gradually becomes a new research hotspot and development trend. An economic evaluation model for DC transformation of urban distribution network considering high-density load and distributed generator(DG)supply is proposed. The calculation model of minimum installation capacity and its optimal control strategy are used to optimize investment cost and operation cost. In the model, the impairment income caused by the change of DG output curtailment, load shedding and operation cost is considered. In multiple scenarios, the technique of DC transformation is compared with other transformation methods. The influence of DG penetration and DC load ratio on investment economy of DC transformation is also analyzed. The simulation results verify the effectiveness of the proposed economic evaluation method. It also shows that the proposed DC transformation scheme has technical and economic feasibility when the power supply capacity and DG consumption capacity of network are highly required.

    • Networking Technology and Its Application of DC Distribution System

      2019, 43(23):39-49. DOI: 10.7500/AEPS20190529001

      Abstract (591) HTML (0) PDF 1.35 M (1409) Comment (0) Favorites

      Abstract:With the transformation of energy cleanliness and the construction of smart energy, the DC distribution system(DCS)has been paid close attention for its advantages of facilitating the reception of the distributed clean energy and easy control of power. This paper reviews the research status and development trends of the networking technology and application research of DCS. Firstly, the main performance features of the DCS are summarized. Secondly, a comprehensive analysis on the typical topologies and power supply patterns in the DCS is carried out, and the control modes of the modular multilevel converter(MMC), DC transformer and low-voltage converter are studied when these converters are connected to the DCS. Finally, combined with the analysis on the application scenarios and development trends of smart energy such as AC/DC hybrid distribution network, DC energy router, energy internet and ubiquitous power internet of things, the main research directions of the key technology of flexible DC power distribution technology are summarized and forecasted.

    • State Estimation for Three-phase Unbalanced AC/DC Distribution Network

      2019, 43(23):65-71. DOI: 10.7500/AEPS20190424006

      Abstract (494) HTML (0) PDF 1.36 M (1184) Comment (0) Favorites

      Abstract:Flexible DC technology based on voltage source converter(VSC)is increasingly applied in distribution networks. Accurate static state estimation of the three-phase unbalanced AC/DC distribution network with VSC is needed. An effective way to achieve accurate state estimation of distribution network is to configure measurement data of phasor measurement unit(PMU). Firstly, the state estimation model of AC/DC distribution network considering the three-phase unbalanced characteristics of distribution network is proposed. The weighted least absolute value method is used for the state estimation. Secondly, the linear description of PMU measurement is given. In addition, the hybrid measurement equations of PMU and supervisory control and data acquisition(SCADA)with various AC/DC devices are uniformly given. Finally, the effectiveness and practicality of the proposed method are verified on the three-phase unbalanced IEEE 33-bus AC/DC distribution network.

    • Voltage Hierarchy Configuration Method for Medium- and Low-voltage DC Distribution

      2019, 43(23):31-38. DOI: 10.7500/AEPS20190115009

      Abstract (533) HTML (0) PDF 1.97 M (1117) Comment (0) Favorites

      Abstract:With the development of DC distribution system and determination of the medium- and low-voltage DC voltage level, how to reasonably select a voltage sequence and determine the DC voltage hierarchy is the basic issue for the early development of DC distribution. Firstly, a voltage hierarchy method is proposed for medium- and low-voltage DC power distribution systems. The key factors affecting the DC distribution voltage level are analyzed based on the principle of minimizing comprehensive cost and the evaluation model of voltage of DC distribution is established. Then, the comprehensive cost of voltage at each level is compared by the proposed model to select the complete voltage sequence. Finally, a case of a power supply area with uniform load density is taken, which verifies the effectiveness of the proposed method and provides a reference for the establishment of DC power distribution systems.