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  • Key Technologies of Flexible Interconnected Distribution Network
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    • Distribution Network Reconfiguration with High Penetration of Renewable Energy Considering Demand Response and Soft Open Point

      2021, 45(8):86-94. DOI: 10.7500/AEPS20190930004

      Abstract (200) HTML (133) PDF 1.09 M (770) Comment (0) Favorites

      Abstract:This paper proposes a reconfiguration strategy for distribution networks with high penetration of renewable energy considering demand response (DR) and soft open point (SOP). Firstly, the models of DR and SOP for the reconfiguration strategy of distribution networks are constructed. Secondly, with the aim of maximizing social benefit, a multi-period reconfiguration model is built for distribution networks with high penetration of renewable energy considering the costs of network losses, curtailment of wind power and photovoltaic power, and switch operation. Thirdly, the model is converted into a mixed-integer second-order cone programming problem by using the Big-M method and the second-order cone relaxation, and YALMIP and CPLEX are utilized to solve the converted model. Finally, the modified IEEE 33-bus distribution network is utilized to verify the effectiveness of the proposed strategy. Sensitivity analysis and comparison of different reconfiguration strategies of the distribution network are also performed. The results show that the proposed reconfiguration strategy considering DR and SOP can effectively improve accommodation capability of distribution networks for renewable energy, and reduce the peak-valley difference of the loads, while enhancing the economy benefit for distribution network operation.

    • Optimal Allocation of Soft Open Point in Active Distribution Network Based on Improved Sensitivity Analysis

      2021, 45(8):129-137. DOI: 10.7500/AEPS20191021004

      Abstract (117) HTML (105) PDF 1.05 M (532) Comment (0) Favorites

      Abstract:As a large number of intermittent distributed generators (DGs) keep penetrating into distribution networks, the problems such as voltage violation and bidirectional power flow become more and more serious. Soft open point (SOP) is a flexible power electronic device that is installed in distribution networks to replace the traditional tie switch. It can realize fast voltage regulation and accurate power flow control, thus effectively solving the corresponding problems caused by DGs. However, the high investment cost of SOP makes the SOP siting and sizing problem particularly critical. An SOP optimal allocation method in active distribution networks based on improved sensitivity analysis is proposed. Firstly, combined with the timing variation characteristic of DG output, an improved sensitivity analysis method is used to optimize the SOP location from the perspective of improving the voltage distribution. Secondly, the second-order cone programming algorithm is used to solve the SOP siting and sizing optimization model under improved sensitivity calculation. Finally, the proposed optimization method is analyzed and verified on the modified IEEE 33-bus distribution system. The results of the case study show that the SOP optimal allocation method proposed in this paper can effectively save annual comprehensive operation cost of distribution networks and improve system voltage, which is beneficial to improve the economics and reliability of active distribution network operation.

    • Adaptive Voltage Control Strategy for Soft Normally Open Point in AC/DC Distribution Network

      2021, 45(8):68-76. DOI: 10.7500/AEPS20191124007

      Abstract (127) HTML (99) PDF 1.70 M (700) Comment (0) Favorites

      Abstract:In AC/DC hybrid distribution networks, the DC distribution network needs to provide power support due to the weak voltage regulation capability of AC weak distribution network at receving end. The DC distribution network is both affected by itself and the AC distribution network, resulting in large deviation of DC voltage and low inertia characteristics more obvious. In light of this, a DC adaptive voltage control strategy considering soft normally open points (SNOPs) is proposed in this paper. Firstly, the influence principle of voltage disturbance in the AC power grid on the dynamic response and inertia regulation process of DC voltage is analyzed, and the disturbance can be effectively suppressed by designing the voltage feedforward control method. Then, aiming at the problem of insufficient regulation capacity of weak AC distribution networks, an adaptive power compensation control method is introduced, which makes SNOP adaptively compensate power shortage according to the voltage variation at the operation point. Meanwhile, the influence of the power demand on the DC voltage deviation is analyzed. On this basis, for the DC system, an improved adaptive virtual inertia control method is devised by comprehensively considering the DC voltage deviation and the voltage change rate, and the influence of main control parameters on the DC system stability is also discussed, which improves the inertia of the DC system. Finally, a simulation model of AC/DC hybrid distribution network is built in MATLAB/Simulink, and the validity of the control strategy is verified.

    • Analysis on Application Scenarios of Flexible Multi-state Switch

      2021, 45(8):13-21. DOI: 10.7500/AEPS20200117005

      Abstract (112) HTML (183) PDF 1.03 M (890) Comment (0) Favorites

      Abstract:With the rapid growth of distribution network load, distributed energy and electric vehicles, the traditional distribution network structure and regulation modes are facing huge challenges. Problems such as unbalanced feeder power, heavy overload of transformers, and difficulties in urban power transmission and distribution capacity expansion are increasingly prominent. The flexible multi-state switch technology of distribution networks brings the possibility to solve the above problems. Taking the typical application scenario of flexible interconnection in the distribution network area as the starting point, the paper analyzes the different application scenarios applicable to the flexible multi-state switch, and studies the application background and advantage of the flexible multi-state switch in different application scenarios.The functional requirements, technical support, demonstration application and research difficulties in typical application scenarios are analyzed. The key problems and challenges facing different application scenarios are given, and the future research areas of flexible multi-state switches are prospected.

    • Soft Open Point Planning for Active Distribution Network Considering Influence of Access Mode Between Feeder Loops and Reliability

      2021, 45(8):120-128. DOI: 10.7500/AEPS20200505003

      Abstract (84) HTML (49) PDF 1.11 M (605) Comment (0) Favorites

      Abstract:In view of the problem that the current soft open point (SOP) planning method does not involve the access mode between feeder loops and the influence of distribution network reliability, a method for the location selection and capacity allocation of SOP within feeder loops and between feeder loops considering the failure loss cost is proposed. Firstly, the benefits of two access modes of SOP to the operation of distribution networks are analyzed, and a hybrid SOP and a linkage control scheme between SOP and the tie switch are proposed for in-loop access. Secondly, a bi-level programming framework and model are constructed considering the benefits of normal and fault operation of distribution networks. In the model, the planning layer aims to minimize the annual comprehensive cost of the system, the normal operation layer optimizes the network state considering the three-phase power flow regulation function of SOP, and the fault operating layer calculates the fault cost based on sequential Monte Carlo simulation reliability evaluation method. Thirdly, facing the nonlinear characteristics of mixed integer between discrete and continuous variables of the model, a solution method based on improved particle swarm optimization and cone programming is proposed. Finally, an example is given to verify the effectiveness of the proposed method.

    • Risk Assessment of Cascading Failures in Flexible Interconnected Power Distribution System

      2021, 45(8):112-119. DOI: 10.7500/AEPS20200505010

      Abstract (89) HTML (80) PDF 1.04 M (628) Comment (0) Favorites

      Abstract:In order to analyze the cascading failure risk of flexible interconnected power distribution systems caused by hidden fault protection of flexible multi-state switch (FMS), this paper first introduces its physical structure. Then, the steady-state characteristic, the transient characteristic of AC feeder failure, and FMS control and protection strategies are analyzed. The uncertain factors that cause the protection malfunctions are proposed and modeled. On this basis, the mechanism of cascading failure formation is analyzed, and the cascading failure risk indicators of flexible interconnected power distribution networks are established. The probabilistic indicators based on Monte Carlo method and the calculation methods based on optimal load shedding failure consequences are also proposed. Taking the modified IEEE 33-bus system as an example, the risk of cascading failures in the system is evaluated, and the influence of the failure location and the penetration rate of distributed generators on the risk is analyzed.

    • Energy Management Strategy and Control Method of Hybrid Distribution Network Based on Soft Open Point

      2021, 45(8):60-67. DOI: 10.7500/AEPS20200519001

      Abstract (105) HTML (150) PDF 1.05 M (552) Comment (0) Favorites

      Abstract:The energy management and control method of the AC/DC hybrid distribution network based on the soft open point (SOP) is a key technical challenge to ensure its high efficiency, reliability, flexibility and controllability. In order to reduce the unnecessary switching power of SOP and maximize the use of renewable energy, a new energy management strategy is proposed. The strategy gives priority to the use of renewable energy output power, and is supplemented by the mutual support of the AC/DC sub network energy storage. The grid-connected operation of the AC/DC hybrid distribution network is divided into twenty operation modes. Based on the energy management objectives, the reference instructions of the AC/DC sub network energy storage units in different operation modes are generated. Aiming at the problems of traditional droop control voltage drop and circulating current of parallel converters with the common AC/DC bus, the AC bus voltage drop compensation method and circulating current suppression method are given. Finally, a simulation platform of the AC/DC hybrid distribution network based on SOP interconnection is established by simulation software. The simulation results verify the feasibility and effectiveness of the platform.

    • T-type Alternate Arm Multilevel Converter with Fault Ride-through Capability and Its Modulation Strategy

      2021, 45(8):41-50. DOI: 10.7500/AEPS20200629006

      Abstract (73) HTML (73) PDF 1.83 M (724) Comment (0) Favorites

      Abstract:The conventional modular multilevel converter (MMC) is difficult to balance the DC fault ride-through capability and the system economy. Aiming at this problem, this paper proposes a T-type alternate arm multilevel converter (AAMC) and its modulation strategy. With the proposed new arm phase-shift modulation strategy, the DC arm of T-AAMC can be constructed by half-bridge sub-modules, which reduces the construction cost and operation losses. Meanwhile, the DC fault ride-through capability is realized by the AC arm constructed with full-bridge submodules in series. First, the topology of T-AAMC are described. The arm phase-shift modulation strategy is determined according to the energy balance condition of each arm, and the operation parameters and hardware configuration of T-AAMC are analyzed. Then, the closed-loop control system is also designed to dynamically balance the energy in AC and DC arms. Furthermore, a comprehensive comparison between T-AAMC and the existing modular multilevel converters is demonstrated. It is shown that, compared with the hybrid MMC, the submodule number, power device number and submodule capacitance of T-AAMC are all reduced. Finally, the proposed topology, the modulation method and the control design are verified by the simulation in the MATLAB/Simulink environment. The proposed T-AAMC can realize the energy conversion with a wide operation range and high efficiency under the normal condition, and it has the DC fault ride-through capability to ensure continuous and reliable operation of the system.

    • Delta AC/AC Converter for Three-port Flexible Interconnection of Distribution Network

      2021, 45(8):32-40. DOI: 10.7500/AEPS20200702004

      Abstract (68) HTML (66) PDF 1.17 M (547) Comment (0) Favorites

      Abstract:A three-port delta AC/AC converter is proposed for the three-port flexible interconnection of distribution networks. Firstly, the topology of the three-port delta AC/AC converter is described. Compared with the three-port back-to-back modular multilevel converter (MMC), the number of bridge arms in the proposed topology is reduced by half. Secondly, the working principle of the three-port delta AC/AC converter is analyzed, and its mathematical model is established, while its operation characteristics are studied by phasor diagram method. Thirdly, the control strategy is designed according to the application requirements of flexible multi-state switch (FMSS). According to the current regulation characteristics of the third bridge arm, the flexible power distribution between the two input feeders is realized based on the power demand of the output feeder. Finally, the validity of the proposed topology of the three-port delta AC/AC converter and the mathematical model, as well as the effectiveness of the control strategy, are verified by the simulation.

    • Distributed Optimization of Flexible Interconnected AC/DC Distribution System Based on Soft Open Point

      2021, 45(8):95-103. DOI: 10.7500/AEPS20200713009

      Abstract (145) HTML (55) PDF 1.08 M (640) Comment (0) Favorites

      Abstract:The optimal dispatch problem of flexible interconnected AC/DC distribution systems is studied. By introducing flexible interconnected devices combining soft open point (SOP) and DC charging devices at the boundary of the distribution network, an improved multi-regional and multi-objective distributed optimization model of flexible interconnected distribution systems is established, which takes into account various distributed resources. Through the establishment of boundary and SOP decoupling models, the system optimization is divided into several sub-problems, including microgrid sub-region optimization, distribution network sub-region optimization, microgrid and distribution network partition boundary optimization, and flexible interconnected device optimization. For protecting the privacy of each region, the dispatch scheme in which each region, partition boundary and SOP participate in distributed optimization is proposed. Furthermore, the adaptive step size adjustment mechanism for the alternating direction method of multipliers is adopted to improve the computing efficiency. Finally, the effectiveness and expansibility of the algorithm and the economy of the flexible interconnected model are verified by simulation.

    • Bi-layer Load Balancing Method in Active Distribution Network Based on Flexible Multi-state Switch

      2021, 45(8):77-85. DOI: 10.7500/AEPS20200714006

      Abstract (57) HTML (75) PDF 1.07 M (585) Comment (0) Favorites

      Abstract:With the integration of a large number of distributed generators (DGs) into the network, the uncertainty and randomness of the output of the supply and load in the distribution network gradually increase. The application of flexible multi-state switches (FMSs) enable the interconnection between supply areas without restriction of voltage levels and the phases. Therefore, the joint operation optimization of multi-supply areas in active distribution network (ADN) will become a new normal situation, which provides the foundation for the improvement of the load unbalance condition and the voltage level. By using flexible interconnection characteristics of the FMS, taking the weighted minimum of the supply area and feeder load balance indices as the objective function, the bi-layer embedded load balancing model for ADN is established, and a hybrid optimization algorithm consisting of improved particle swarm optimization (PSO) and second-order cone programming is used to solve the problem of network reconfiguration topology and FMS output. First, the closed-loop distribution network is simplified and equivalent to decrease the particle scale during the network reconfiguration of PSO algorithm in the outer layer of the whole method to speed up the calculation. In the inner layer, the objective function is linearized using optimal equidistant piecewise linear approximation algorithm (OEPLAA), by which the problem of output optimization of FMS can be transformed into a second-order cone programming problem. Finally, the proposed double-layer load balancing method is analyzed and verified through an actual distribution network.

    • Coordinated Control Method for Medium-voltage DC Interconnected Distribution Network Based on Power Electronic Transformer

      2021, 45(8):51-59. DOI: 10.7500/AEPS20200714012

      Abstract (75) HTML (102) PDF 1.33 M (827) Comment (0) Favorites

      Abstract:Multi-port power electronic transformer (PET) is the key equipment for the flexible interconnected distribution network. Its decoupling control of ports and coordinated control between regions are the main technologies to ensure the safe and stable operation of the cross-regional interconnection system. This paper uses the isolated modular multilevel converter (I-M2C) based multi-port PET as the key equipment to propose a structure of flexible distribution network interconnected by medium-voltage DC (MVDC) feeders. First, the technical characteristics of the scheme for the cross-regional interconnected distribution network based on the MVDC feeders are analyzed. A mathematical model of ports in I-M2C based PET based on the dual-modulation freedom is established. Then, according to the mathematical model of ports and the requirements of different steady-state operation modes, a decoupling control method based on the dual-modulation freedom for each port is proposed. At the same time, the master-slave control strategy between regions in the cross-regional interconnection mode is introduced. Operation modes of the flexible distribution network under various working conditions are summarized. Finally, a 10 kV simulation model is built in MATLAB/Simulink,which verifies the effectiveness and feasibility of the coordinated control strategy for the MVDC interconnected flexible distribution network.

    • Sitting Optimization of Soft Open Point Considering Operation Economy and Reliability of Micro-distribution Network

      2021, 45(8):138-145. DOI: 10.7500/AEPS20200715006

      Abstract (82) HTML (94) PDF 1.32 M (698) Comment (0) Favorites

      Abstract:Aiming at the overall economy of the active distribution network containing multiple microgrids connected by soft open points (SOPs), the sitting research is conducted. Firstly, the constraint model of the active distribution network containing SOPs under normal operation is established, and the objective function is set after fully considering the cost of real-time electricity price and the costs of the operation and maintenance of SOPs and energy storage batteries. On this basis, by relaxing and linearizing the original non-convex nonlinear constraints, a second-order cone programming model is obtained, which realizes the efficient solution of the nonlinear programming problem. Finally, the objective function is solved under the constraint conditions. The daily operation economy and node voltage deviation indices of the distribution network with different schemes are comprehensively compared, and the optimal sitting scheme of the SOPs is obtained, which verifes the significant advantages of SOPs in improving the economy and reliability of the micro-distribution network.

    • Critical Load Restoration Method for Distribution Network with Soft Open Point

      2021, 45(8):104-111. DOI: 10.7500/AEPS20200730004

      Abstract (67) HTML (65) PDF 1.01 M (596) Comment (0) Favorites

      Abstract:In the large-area blackout scenario, aiming at the critical load restoration problem for the active distribution network with multiple soft open points (SOPs), this paper proposes a two-step load restoration method. Firstly, a mixed-integer second-order cone program (MISOCP) is solved to determine the optimal restoration strategy in each period. Secondly, according to the recovery operation rule proposed in the second step, the switch operation sequence and SOP control mode of operation state transition between adjacent periods are determined. Among them,the MISOCP model aims to maximize the weighted load power supply time,which considers the operation constraints of the distribution network, SOP and power supply, etc. The restoration operation rule aims to restore the power supply of critical loads as quickly as possible. According to the first-step optimal recovery strategy and the role of the SOP in the system, switch operation and SOP control mode switching are carried out. In the case study, a hand-in-hand distribution network is used for comparison and verification. The results show the effectiveness and superiority of the proposed two-step load restoration method.

    • Unified Power Flow Calculation Method of Flexible Interconnected Distribution Network Based on Modified Augmented Nodal Equation

      2021, 45(8):22-31. DOI: 10.7500/AEPS20210104005

      Abstract (108) HTML (109) PDF 1.08 M (745) Comment (0) Favorites

      Abstract:The DC links might be coupled with AC power grids in the form of a large number of dispersive converters in the future flexible distribution network. It will be more applicable to establish a unified and simultaneous power flow model for accurate and effective solution of the system. A power flow calculation method for flexible distribution networks based on modified augmented nodal equations is proposed. Firstly, the network models of AC and DC power grids and the power flow constraints of nodes with different types are established. Secondly, the electronic equipment models of multiple modes including converters and soft open points as coupling links are considered. Finally, the modified augmented nodal equation of power flow calculation in the flexible distribution network is formed, and the Newton-Raphson method is used to solve the model. The effectiveness of proposed method is verified on the modified IEEE 123-bus flexible distribution network. The proposed method is appropriate for the time-series power flow calculation in multiple operation modes.

    • Research Progress and Prospects of Key Technologies of Flexible Interconnected Smart Distribution Network

      2021, 45(8):2-12. DOI: 10.7500/AEPS20210104007

      Abstract (181) HTML (343) PDF 1.26 M (1061) Comment (0) Favorites

      Abstract:Using power electronic devices represented by soft open points, energy routers, and smart power/information exchange stations to realize the flexible interconnection of the distribution network, and better adapting to the integration of multi-source-storage-load devices, will be the development trends of the future smart distribution network. Starting from the concept of flexible interconnected smart distribution networks, the paper introduces in detail the topological structure and working principle of its core devices, i.e. flexible interconnected devices. Meanwhile, the research status and difficulties of key technologies of flexible interconnection smart distribution networks such as planning and design, operation control and fault self-healing are analyzed. Finally, the technical fields that need to be studied in the future and the engineering application prospects are discussed and prospected.