Research On Zero Current Switching DC/DC Converter Suitable For New Energy Medium Voltage

With the continuous rise in global energy consumption,renewable energy sources such as solar photovoltaic(PV)power and wind power are paid more and more attention.With large scale integration of renewable power generation,employing medium voltage DC(MVDC)grids as the power collection is beneficial over traditional medium voltage AC grids.Step-up DC/DC converters are the key interface between renewable energy sources and MVDC grids.There are two main kinds of topologies which are utilized as the step-up DC/DC converters,namely the isolated and the non-isolated high power DC/DC converters.For step-up DC/DC converters suitable for MVDC collection,the isolated DC/DC converter is preferred.A three-level(TL)zero-current-switching(ZCS)converter suitable for MVDC collection is proposed where the proposed converter adopts four IGBTs and two MOSFETs and all the six switches operation in discontinuous mode which can be obtained by adding a control circuit into the basic neutral point clamped(NPC)TL circuit with a fixed duty cycle.Pulse width modulation(PWM)is adopted for the control circuit to realize ZCS for the basic NPC TL circuit under full load range.As a result,the switching loss can be reduced significantly.A zero-voltage-zero-current-switching(ZVZCS)TL DC/DC converter based on a dual transformer with two output capacitive filters has been proposed.An auxiliary circuit is added to the conventional NPC TL circuit,which realizes the power transmission and voltage regulation of the whole converter,while the basic TL four switches operated with a fixed duty cycle.By reasonably designing the turn’s ratio of the transformer of the conventional NPC TL circuit,the conventional NPC TL circuit can transfer most of the power and can achieve ZCS under full load range with the switching losses remarkably reduced.The converter losses can be suppressed,and the conversion efficiency can be improved significantly by enhancing the turn’s ratio of the secondary transformer.Furthermore,by improving the auxiliary cells according to the detailed analysis and experimental results of the working principles of the converters in the Chapter 2,the auxiliary switches of the above two new ZCS converters can realize ZVS turn-on and turnoff.As mentioned in the first chapter,ZVS is more suitable for MOSFETs,so if the auxiliary switches uses MOSFETs,the realization of its ZVS is more conducive to reducing the switching losses of the converter,but the voltage stress of the MOSFET is not high,so you need to find a way to low the voltage stress of the auxiliary switch.Hence this chapter has improved the auxiliary circuit,where the voltage stress of the auxiliary switches is successfully reduced to half of the input voltage provides favourable conditions for using MOSFET as an auxiliary switch,and a low voltage stress for the auxiliary switch resonant ZVZCS full-bridge converter is proposed.