• Energy Research

The connection mode of the direct current (DC) power distribution system and the alternating current (AC) system is the foundation of system design, and it is also one of key technologies of the DC power distribution network. Based on the topology structure, grounding method, main equipment parameters, load parameters and system control protection strategy of the DC power distribution system, this paper establishes the system simulation model in the case of configuring the connection transformer and not configuring the connection transformer. Simulation results show that, when no connecting transformer is installed, the interaction between AC and DC systems will be great when faults occur, and the cost of converter valves and DC reactors will be increased. When connecting transformers are installed, the interaction between AC and DC systems can be effectively isolated, and the operation reliability of the system will be greatly improved while the cost is saved. Therefore, it is recommended to configure an independent connection transformer in the DC distribution system.

The integrated energy system includes various energy forms, complex operation modes and tight coupling links, which bring challenges to its steady-state modeling and steady-state power flow calculation. In order to study the steady-state characteristics of the integrated energy system, the topological structure of the cold-thermal-electric integrated energy system is given firstly. Then, the steady-state model of the power subsystem, the thermal subsystem, the cold subsystem and the distributed energy station are established, the unified power flow model is established, and the Newton Raphson algorithm is used to solve the unified power flow model. Finally, the influence of the key technical parameters on the steady-state power flow of the integrated energy system is analyzed. Research results show that the photovoltaic power generation plays a supporting role in the voltage of each bus; with the increase of electric load power, the unit value of bus voltage decreases continuously; the water supply temperature of the source node has a greater impact on the steady-state flow in the pipeline and the water supply temperature of each node; the pipeline length of the heat network has a greater impact on the end temperature of the pipeline, the water supply temperature, and the return water temperature of each node. The analysis results can support the planning, design, and optimal operation of the integrated energy system.

The DC distribution system is an important development direction of the distribution system, which can improve the reliability and the quality of the power supply, and support the new energy, the energy storage, the electric vehicles, and the flexible access of AC and DC loads to grid. To realize the demonstration application of the DC distribution technology, China’s first demonstration project of the medium voltage DC distribution network will be built in Zhuhai, Guangdong Province to support the construction of the city energy internet. First, this paper analyzes the demand of the DC distribution network project, and puts forward the construction content and construction target. Then, it designs and analyzes the electrical connection mode, system operation mode, and startup and shutdown mode of the DC distribution network, and proposes the overall project construction plan. Finally, it conducts the specific project design and analysis, which mainly include the selection of equipment such as inverters, DC transformers and DC circuit breakers, the design and analysis of the DC control and protection system, the design and analysis of the over-voltage protection and the configuration scheme of the lightning arrester, and analysis of the system transient characteristics. The design and analysis of the engineering program is a combination of China’s distribution network engineering practice and technical characteristics, which lays a solid foundation for the advancement of the DC power distribution technology in China, and has reference value and demonstration effect for the design and construction of other projects.

Due to the coupling of different energy systems, optimization of different energy complementarities, and the realization of the highest overall energy utilization rate and environmental friendliness of the energy system, distributed energy system has become an important way to build a clean and low-carbon energy system. However, the complex topological structure of the system and too many coupling devices bring more uncertain factors to the system which the calculation of the interval power flow of distributed energy system becomes the key problem to be solved urgently. Affine power flow calculation is considered as an important solution to solve uncertain steady power flow problems. In this paper, the distributed energy system coupled with cold, heat, and electricity is taken as the research object, the influence of different uncertain factors such as photovoltaic and wind power output is comprehensively considered, and affine algorithm is adopted to calculate the system power flow of the distributed energy system under high and low load conditions. The results show that the system has larger operating space, more stable bus voltage and more flexible pipeline flow under low load condition than under high load condition. The calculation results of the interval power flow of distributed energy systems can provide theoretical basis and data support for the stability analysis and optimal operation of distributed energy systems.

A DC circuit breaker is piece of core equipment for DC grid construction and can achieve fast isolation of DC faults in the grid. In this paper, based on the fault characteristics and protection requirements of an AC/DC hybrid distribution network, the technical parameters and topology structure of an inductance and capacitance (LC) resonant commutation-type mechanical DC breaker are proposed. Furthermore, an engineering prototype of the mechanical DC circuit breaker is developed, then a small current breaking test and a large current breaking test are carried out. The test results show that a 10 kA fault current can be interrupted within 10 ms by the engineering prototype. Finally, the engineering prototype of the mechanical DC circuit breaker is installed in the AC/DC hybrid distribution network project, and a current breaking test is carried out. The test results show that the mechanical DC circuit breaker has good running conditions, and the function and performance meet the engineering design and operation requirements.

The AC/DC hybrid distribution network is composed of a medium-voltage DC bus, a low-voltage DC bus, and a power electronic transformer, and has the characteristics of multi-voltage level, multi-DC bus, and multi-converter, so its operation mode and optimal scheduling strategy are more complex. Firstly, this paper constructs the AC/DC hybrid distribution network using an power electronic transformer. Then, a two-layer control structure including a scheduling management layer and a bus control layer is proposed, which simplifies the control structure and gives full play to the role of “energy routing” function of the power electronic transformer. Moreover, the minimum operation cost of the AC/DC hybrid distribution network in the whole scheduling cycle is taken as the optimization objective, considering the characteristics of various distributed generations, the structure of AC/DC hybrid distribution network, and the interaction of “source–load–storage”. Finally, the optimal scheduling model of the AC/DC hybrid distribution network based on power electronic transformer is established, and the feasibility of the optimal scheduling strategy is verified by the open-source solver, which can realize the complete absorption of renewable energy and the optimal coordinated control of “source–load–storage”.