Research On Voltage Class Series Of DC Distribution Network

With the rapid development of urbanization as well as standards of living, there emerges a demand for power supply and quality. Meanwhile, due to depletion of fossil fuels and low-carbon environmental concerns, the integration of distributed sources(such as wind farms, photovoltaic cells, etc.) will be widely occupied in utility grid. Within the low-voltage distribution system, frequency-variable devices which contain dc-link are taking over the market. In these regards, conventional AC distribution network has been challenged. On the other hand, thanks to the development on DC transmission and distribution technology, DC distribution network has emerged as an option of power distribution for its superiority shown in operation efficiency, power reliability and quality, flexible and secure integration of distributed generations. Similar to AC distribution system, DC distribution network should be a multi-level system in which each voltage cooperates with others. Therefore, formulation of DC voltage-class series is a key issue.With reference of voltage-class series in ac systems worldwide, essential principles are summarized. Furthermore, major constraints such as prospective load demand, related equipment manufacturing level, optimization of grid topology, transmission methods and transition of AC distribution network in formulating DC distribution voltage-class series are discussed.Based on power supply target focusing on prospective load demand, a set of DC voltage-class series in which high-voltage, medium-voltage, low-voltage and ultra low-voltage stages cooperate with each other is proposed via fuzzy comprehensive evaluation method. In the following sections, evaluation indexes and methods of economy and reliability analysis are ameliorated to fit in DC distribution network, including reasonable estimation about construction cost and operation loss of key devices such as DC transformers and DC breakers. On the other hand, reliability parameters of these devices are also predicted via parts count reliability prediction method.A case study is then carried out to evaluate economy and reliability of different DC voltage-class series. In the case study, detailed distribution schemes are developed according to related guidelines. In view of these schemes, economy and reliability of different voltage-class series is analyzed.Taking prospective load demand, integration of renewable sources and optimization of power distribution grid topology into account, the DC distribution voltage-class series is proposed as ±320k V,±150k V,±30k V,±10k V,±750V(±375V),400V(±200V),48 V. Through specific numerical case study, further conclusions are drawn as follows: the hoisting of medium-voltage level can effectively reduce operation loss, thus enhancing economic benefits of long-term operation. However, due to a surge in power electronic modules when hoisting voltage level, the reliability of the system is weakened. Compared with AC distribution network, only when the average efficiency of DC transformers reaches higher than 98.8%, can DC distribution network obtain better economic benefits. On the other hand, the reliability of DC distribution network can b promoted by enhancing withstand voltage of single module by further research on semiconductor materials such as Si C.In view of limited technical merit in medium and high voltage DC distribution devices, a mixed residential distribution system involving 750V(±375V)/48 V low voltage dc distribution and 10 k V medium ac distribution is explored. Compared with AC distribution and DC distribution, this mixed system has turned out to be more efficient and the feasibility of this system is verified.