With the development of power electronic technology and modern distribution system, the application requirement of VSC DC distribution is increasing. The DC distribution system can be used to solve the problem of high reliability power supply and distributed power grid connected to urban power grid. To bring a reference for guiding the construction of medium voltage DC power distribution system, based on DC power distribution system analysis, control protection, key equipment development and other technical research work, the technical scheme for the construction of the ±10 kV multi-terminal DC distribution demonstration project in Shenzhen Baolong industrial district is put forward, and the electrical main connection types, main equipment selection scheme, overvoltage and insulation coordination requirements, system operation modes, control protection scheme and other key technical contents of the demonstration project are clearly presented.

With the depletion of fossil energy, the rise of distributed energy such as wind power and photovoltaic(PV) and the rapid development of power electronics technology, the VSC-DC power distribution network has more potential compared with AC power distribution network.In this paper, firstly, typical operation modes of “hand in hand ” VSC-DC distribution power network are researched. Secondly, switching methods of system control modes is proposed for the cases of disconnection and controllable devices partially out of operation resulting in changes of operation mode, At last,hardware in loop (HIL) tests on RT-LAB including 4 typical operation modes of DC distribution network and smooth automatic switching of system control mode are carried out. The simulation results indicate the effectiveness of the proposed mode switching methods and these methods provide a significant reference for the future research of DC power distribution network.

Overvoltage and insulation coordination is one of the key technologies of DC distribution network, and the research results of the coordination will play an important role in guiding the selection of the key equipment in DC distribution system. Based on the research results of traditional DC projects and the MMC-HVDC transmission projects, two insulation coordination schemes applicable to the ±10 kV DC distribution demonstration project in Shenzhen based on modular multilevel converter(MMC) are proposed considering the chracteristics of the DC distribution system. One scheme is without arrester, and the insulation levels of equipment are obtained by multiplying a bigger insulation margin. The other one is to configure a few arresters, and the insulation levels are determined based on the protection levels of the arresters. By establishing the internal over-voltage simulation model of the systems in PSCAD, the overvoltage of critical locations under typical fault conditions for different schemes are simulated and calculated, and two optional schemes of insulation level for the key equipment of the DC distribution system are ultimately determined, which provides reference for the project design.

The study on protection devices of MMC based DC distribution network is widely based on digital simulation software such as PSCAD/EMTDC, which brings shortcomings such as the simulation response characteristics without authenticity, inconvenience of developing and extension. In view of that, a hardware-in-loop(HIL) simulation platform which combines RT-LAB with protection devices is built firstly for the typical four terminals MMC Based DC distribution network in Shenzhen. The plantform solves the real time simulation problem of the high frequency DC transformer by the particular eHS technology, enhances the simulation timeliness and proticability with a high degree of confidence. Three typical fault simulaton experiments are tested based on the HIL platform, and the experimental results show that the protection configuration is suitable for the MMC based DC power distribution system, and the action is quickly and accurately. The conclusions can be directly applied to dynamic simulation test and can provide reference for further engineering construction.

Full-bridge modular multilevel converter (FBMMC) has strong ability to block DC fault current. It is more suitable for overhead lines and multi-terminal voltage source converter based HVDC system. This paper introduces the differences of startuping behaviors between FBMMC and HBMMC (half bridge modular multilevel converter). A sectionalized startup strategy is proposed and analyzed for FBMMC connected to active and passive system respectively. The startup strategy of passive network for black start is mainly studied. The proposed strategy is able to ensure a safe and reliable startup of the converter stations. In addition, the non-fault end of the system could work stablely. The validity of the proposed strategy is proved by PSCAD/EMTDC simulations.

With the characters of high stability, good flexibility and power quality, DC distribition network becomes the development tendency in the future. DC current breaker plays an important role in the stability and safety of DC distribution network as a protection device. A solid hybrid DC circuit breaker scheme for DC distribution network is proposed in this article. The breaker is made up of three branches: the main current branch consisting of thyristors and a modular IGBT in series, the transfer branch consisting of stakpak IGBTs, the absorption branch consisting of arresters. Simulations of the DC breaker are carried out, and a prototype of the breaker and a testing platform are built. The prototype cuts off 2 kA and 5 kA DC currents under the voltages of 10 kV and 7.5 kV respectively. The efficiency of the prototype is higher than 99.94%.

DC breaker, which can cut off the short current immediately and avoid the whole DC systems shut down, while DC fault occurs, is a key device for DC grid for the voltage source converter-high voltage DC (VSC-HVDC). A scheme of solid state DC beaker for the VSC-HVDC is proposed, then a simulation model of DC beaker based on series-connected IGBTs is set up. The influence of voltage-sharing characteristics of series-connected IGBTs caused by the gate resistor, the tail current and the gate charge is studied. Simulation results show that the static and dynamic circuits can greatly improve the performance of voltage balance. At last a ten IGBT series-connected DC breaker and its test system are developed, and the breaker successfully turns off 5.1 kA current under 10 kV DC, which shows the feasibility of the solid-state DC circuit breaker based on IGBT series-connected technique.

DC circuit breaker is the key equipment for the safe operation and protection of DC distribution network. In this paper, the application of DC circuit breaker in low voltage DC distribution system is investigated, a kind of all-solid-state DC circuit breaker prototype is developed. Firstly, taking Shenzhen DC dynamic simulation platform system as an example, the parameter requirements of typical low-voltage DC distribution system for the circuit breaker is analyzed. Secondly, the function and installation position of the DC circuit breaker in the protection system of the dynamic simulation platform are determined. At the same time, based on the requirements of DC distribution system for the main parameters of DC circuit breaker, the all-solid-state switch topology is selected for the DC circuit breaker, and on this basis, the absorption branch and the communication module are designed, and the software operating process in response to main control system of the DC circuit breaker is established. Finally, a prototype of ±380 V low voltage distribution DC circuit breaker with practical value is developed, and several related tests are carried out to verify the correctness of the circuit breaker design and protection scheme.

Simulation and experiment studies focusing on the fast mechanical switch in hybrid HVDC circuit breaker as the critical component applied to ±160 kV Nan’ao VSC-MTDC project are carried out. The fast mechanical switch with two 80 kV fractures connected in series and with SF₆ as insulating media is studied. Then electric calculation of each 80 kV fracture is performed with Ansoft Maxwell. The result shows that each fracture can withstand 80 kV DC voltage with 15 mm opening stroke between the static and movable contact. In addition, the feasibility of the fast mechanical switch using the electromagnetic repulsion mechanism as fracture is studied and the experimental prototype is built up. It is verified by experiment that this fast mechanical switch can be used to drive from its closing state to opening state within approximately 3 ms.

An input-series-output-parallel project prototype of 200 kW DC 15 kV/DC 400 V transformer is designed and developed in this paper. Series of key technologies of the prototype are studied, including main circuit topology design, high-frequency isolation dual active bridge unit design and switch selection, high-frequency transformer design, voltage control and balance control, etc. Finally, a full load test platform is set up for the prototype testing, and the results show that the efficiency of the prototype is more than 96% under the rated condition.

DC Transformers are the key device in the development of DC distribution. For the multiple modular high frequency link DC transformer (DCT), the value design methodology of series inductor and DC capacity is presented in consideration of the switch safety and the output voltage ripple. Then the DCT under single phase shift is analysed,and the characters of operation and harmonic symptom are derived. The numerical model of DCT is presented, and the flow chart of DC transformer numerical simulation computation is given. Finally, a prototype is built based on the aforementioned method, and the comparison between prototype and numerical simulation is achieved. The results verify the correctness and effectiveness of the theory in this paper.

Aiming at the HVDC cross-linked polymeric (XLPE) cables, the applicability of equations describing electrical conductivity of insulating materials under DC voltage to the XLPE material is discussed and studied. Based on two conductivity expressions, the critical temperature difference for field reversal, the temperature difference across insulation, the product value of local electric field, the electrical conductivity and the associated distance from the cable centre, and the field distribution across insulation at steady state are deduced and proposed. Simulation results from the finite elements analysis and the calculation results according to equations are compared, and these equations are proved reliable. The values calculated from the conductivities are compared, as well as the comparison in corresponding thermal and electrical characteristics, both forms of equations describing conductivity are proved exchangeable for the HVDC XLPE cable.