Study On Dc Transformer Applied In Medium Voltage DC Distribution Network

As the key equipment in medium and low voltage DC distribution system,DC transformer undertakes the functions of interconnection and isolation of different DC buses,flexible management of DC power flow,and its performance directly affects the economy,reliability and stability of medium voltage DC distribution network.The existing DC transformer widely adopt the input series output parallel(ISOP)structure.Due to the centralized capacitor problem of the medium voltage terminal,the tolerance of DC short circuit fault is poor.Secondly,the classic structure such as dual active bridge(DAB)converter is easy to lose soft switch in the situation of DC distribution network line voltage fluctuation,and the efficiency is significantly reduced.Therefore,aiming at these two problems,this paper studies the topology,power and soft switching characteristics,modulation and control strategy of modular multilevel DC transformer.The main parts of this paper are as follows:1.Firstly,a parallel modular multilevel DC transformer(PMMDCT)topology is proposed to solve the problem of centralized capacitor in ISOP structure.The multi modular structure is adopted to avoid the series of DC capacitor at medium voltage terminal.On the other hand,phase-shifting control is introduced to realize the bidirectional power transmission.Otherwise,a modulation strategy suitable for wide input voltage range is proposed to expand the voltage transmission range of DC transformer.In this paper,time domain analysis method is used to analyze the working principle of the proposed DC transformer,the key formulas of different working modes are derived,and the soft switching range of different modes is studied.Furthermore,in order to realize the smooth change of DC transformer transmission voltage ratio,a control strategy based on switch pulse width gradient is proposed.On this basis,sub module voltage sharing performance and voltage and current stress transient characteristics in the process of smooth change of voltage ratio are analyzed.Then,the effectiveness of the above pmmdct topology and modulation and control strategy are verified by simulation.2.Secondly,in order to reduce the DC current fluctuation of the medium voltage terminal,a voltage smoothing control strategy based on the idea of constant input and output of sub modules is proposed.In this paper,the working principle of DC transformer with constant input and output is analyzed,and the corresponding transmission power formula and soft switching range are derived.Then,the performance of DC transformer with smooth switching modulation strategy proposed in Chapter 2 and this chapter is compared,and the feasibility of smooth switching modulation and control strategy is verified by simulation.3.Finally,the overall design scheme of the experimental prototype with 20 k Hz working frequency and 2k W capacity is proposed.The hardware design and software design of the main circuit and control circuit are introduced in detail.Then,the feasibility of the proposed converter and control strategy is verified under the conditions of bidirectional power,wide input voltage and load mutation.