| The medium voltage direct current (MVDC) distribution network has great application prospects. It not only can provide distribution network interface for DC transmission to power distribution, or isolate voltage drop of AC transmission and distribution system through AC/DC conversion , and compensates reactive power for AC grid, but also can provide interconnection interface for large energy storage system, photovoltaic system, and low voltage direct current(LVDC) distribution network, reduce voltage transformation links, and improve system reliability. However, it has complex structure, multiple ports, multiple voltage grades and many kinds of operation modes, which make voltage control difficult. How to determine appropriate voltage control strategy, and ensure the stability of LVDC voltage in a variety of operation modes of medium voltage DC power distribution network is the urgent problem to be solved.In the paper, the voltage control of flexible medium voltage DC distribution network is studied with the two-terminal flexible medium voltage DC distribution network as the research object . Medium voltage DC bus is connected with AC network through the VSC on both sides,which outputs three DC solid state transformers (DCSST) of input-series-output-parallel (ISOP)high-frequency isolated dual active bridge (DAB) converter, namely, PV systems, energy storage system and LVDC distribution network interface respectively.Firstly, it studies the operation mode of a double-terminal DC distribution network and controlmode for each interface device under different operation mode. Besides, it designs the power and voltage controller of VSC, MPPT controller of photovoltaic system, constant power charge-discharge controller and constant voltage controller of energy storage system, double closed-loop of voltage and current control as well as high voltage series side voltage-sharing controller of DCSST. Secondly, since the voltage controller of conventional MVDC distribution network based on the multiport DC transmission flexibility cannot adapt to the change in the multi-operation modes of flexible MVDC distribution network, this research puts forward a modified voltage margin control strategy, which includes the control of master station VSC ,slave station VSC and sub-slave station DCS ST. The master station VSC adopts constant DC voltage control, and the slave station VSC, as the backup station, adopts the voltage margin control. It normally operates under the constant power control mode, when the master station VSC exits operation, the slave station VSC replaces master station to control medium voltage side voltage of the system. Moreover, the sub-slave station DCSST, as a backup station, adopts voltage margin control, normally operates under the constant power control mode, and can stabilize medium voltage DC voltage when voltage is out of control, while its action priority level is below VSC of slave station. Finally, under the PSCAD / EMTDC, the simulation model of the two-terminal flexible MVDC distribution network with the double-end power supply,single-end power supply, double-end isolation and island operation is constructed. The result shows that the voltage of the DC distribution network under the voltage margin control can be well maintained and reduce the communication dependence of the system in the process of operation mode switching. |
PDF Full Text Request