New High Voltage Gain Bidirectional DC-DC Converter With Partially Parallel Structure

Over the past few years renewable energies have experienced strong developments to become alternatives for conventional energy resources due to the global awareness on limited fossil fuel resources and a sensibility towards the environmental impacts.However,large scale integration of renewable energies may degrade the stability and reliability of the grid because of their irregularities and mostly unpredictable production.Energy storage systems are therefore a potential solution to improve grid system reliability and stability when supplied by renewable energy sources.Power electronics converters,as the interface between the energy storage systems and the grid or the renewable energy resources,play an limportant role in the power flow of the future grid.A new partially parallel high voltage gain dc/dc converter using Gallium Nitriode(GaN)devices is proposed.The converter has several advantages such as large voltage step-up ratio,automatical current-sharing,simple topology,easy control,convenient modularization,etc.The high voltage gain is realized through two transformers.The high voltage side windings of the two transformers are connected in series and then to the active bridge connected to the high voltage dc bus;the low voltage side windings of the two transformers are both connected to an inductor respect:ively and then to an active bridge connected to the low voltage dc bus.The paralleled structure on low voltage side automatically distributes the current evenly because of the series connection of windings on the high voltage side.An ideal power transfer model of the proposed converter is derived,and the model is verified to have the same characteristics as those of the single phase dual active bridge converter with single phase shift modulation.Then considering the dead time between the switches in the same leg,the on-state resistance of the switch,the voltage loss of the switch in reverse freewheeling state,ac resistance of transformer windings,as resistance of inductor windings,an improved power transfer model of the proposed converter is derived.The improved model is verified to be reasonable after comparing the calculations and the simulations done in PLECS and Matlab,and is compared with the derived ideal model.Based on the improved model,the power plateau,where the transferred power nearly keeps unchanged or even decreases with an increasing phase shift angle,vice versa,:is discovered and investigated.The reason of the power plateau is determined to be the commutation during the dead time of the lagging bridge.The influences of several circuit parameters on the characteristics of the power plateau are also analysed.The anti-power phenomenon,where the transferred power no longer increasing(decreasing)with increasing(decreasing)dc bus voltages connected to the lagging bridge during specific phase shift ranges in the buck mode,is discovered.The reason for the anti-power phenomenon is also given.The design procedure of the prototype is also given.The procedure includes:the selection of the GaN switches;the modelling of the converter efficiency and power density,and the selection of the switching frequency based on the power density and efficiency models;optimization of transformer turns ratio,inductance and dead time based on the improved power transfer model;the modelling of the inductor current,and the optimization of the magnetics based on Ansoft Maxwell simulations and calculations;the design of GaN related schematics and PCB layouts;the thermal design and mechanical design of the prototype;the selection of digital controller and the design of other control circuits.The measured parameters of the magnetics are given.A PPDAB prototype is built.With the prototype,the models previously derived are proved to be reasonable,and the proposed analysis is verified as well.All switches used in the prototype are GaN devices The prototype switches at 300kHz to deliever power bidirectionally.A measured maximum power of 1.67kW and a maximum efficiency of 99.58%are achieved at one power flow direction,while 1.68kW and 98.41%at the other power flow direction.Some evaluations of the used GaN devices are given at last.