Analysis Of Dynamic Characteristics And Control For DC Microgrids

The ways of distributed generating with multi renewable energy have becoming an effective solution for relieving environmental pressure and energy shortage.However,connecting the distributed generation to the main grid directly can be adverse to the stability and power quality of power systems.Relevant practices and researches show that microgrids are the best way to use distributed generating.Meanwhile,compared with AC microgrids,the advantages of DC microgrids are much more,such as elimination of problems of phase,frequency and reactive power,and ealzier connection between distributed energy generation and DC load.The dynamic performance of DC microgrids is influential to the stability and power quality of DC microgrids.Thus,the research of this thesis mainly starts around dynamic characteristics of DC microgrids,and the superior control level is researched as well.Firstly,according to the scenario of the DC microgrid with one converter,since the dynamic performance of three-phase interleaved DC-DC converters(TIDC)is relatively good,in order to further analyze its dynamic characteristics and improve its dynamic performance,the prototype of three-phase interleaved DC-DC converter is designed.Thanks to this prototype,the control strategy of TIDC can be further researched.When the prototype is running,the biggish voltage swell or sag can be caused when big loads are put in.Thus,the influence factors of dynamic characteristics of TIDC are analyzed,and a feed-forward control method for improving its dynamic performance is proposed.The feed-forward control method is achieved by adaptive adjusting the current reference outputted by voltage loop and the time the feed-forward control lasts during the dynamic process.Thus,the response rate can be obviously improved,and the voltage swell and sag of DC bus can be suppressed effectively.Secondly,when a DC microgrid includes multi paralleled DC-DC converters,the droop control method can be used to realize DC voltage stable and current sharing.According to the input and output of droop characteristics,the droop control method can be divided into Virtual Impedance Based Droop Control(VIBDC)and Embedding Droop Control(EDC).According to the paralleled DC-DC converters based on droop control in DC microgrids,a large signal modeling method is proposed.The state space model for VIBDC and EDC can be built by using his modeling method to analyze their dynamic performances.The initial steady valuables can be eliminated,and all the state valuables in the model are dynamic valuables including DC voltage,output currents of converters and duty ratios of converters,and the input is the load current.Thus,this model can be used to analyze the dynamic process from one steady state to another one.Until now there are no reports dealing with the comparison of dynamic response performances between the different droop controls.To fill this gap,based on the proposed model,this thesis presents a comparison of dynamic response performances between two droop control methods by using the root locus method,showing that the dynamic response of the EDC is faster.Thus,the influence factors of EDC and the coupling of different converters' dynamic performance are researched.Based on the analysis result,a novel adaptive Proportional and Integral(PI)control strategy is proposed,which is achieved by rapidly reducing the errors by using large proportional terms with no fluctuations and then completely eliminated under small proportional terms.Thanks to the duty ratio compensator which is added into the adaptive PI control,the fluctuation of voltage and current caused by PI parameters returning can be eliminated.The proposed adaptive PI control strategy can obviously improve the response of all the DC-DC converters in DC microgrids and decrease the voltage swell and sag and restoration time during dynamic process,so the dynamic characteristics of DC microgrids can be improved.Thirdly,when the communication topology is forced to change in steady state,although the traditional dynamical consensus algorithm is able to tolerate the topology change and ensure the accurate averaging,the oscillation of average value in each node is inevitable.Thus,the dynamical consensus algorithm is improved.By using this algorithm,the steady-state is not affected by topology changes and the average values are kept constant while the rate and characteristics of convergence can be guaranteed.Finally,since the existed secondary and tertiary control methods are based on the VIBDC,and the hierarchical control method for the EDC based converters has not been studied so much in previous works,a novel superior control method based on embedding control is proposed.By using the proposed consensus algorithm,the average values of bus voltage and voltage deviation can be achieved in the secondary control level to realize the accurate current sharing and voltage restoration by adaptive droop characteristics adjusting.The tertiary control is proposed to optimize the gross conversion efficiency of paralleled converters by using the average voltage restoration value obtained in the secondary control level,instead of using the total load current,so that the communication traffic can be reduced,which is the main contribution in this paper.Compared with the traditional method,the response of the proposed tertiary control can be higher,and its real-time capability is better.Meanwhile,the gap of the superior control based on the EDC is filled.For the paralleled converters connected to batteries,the algorithm for State of Chasrge(SoC)management is considered in the tertiary control level as well.Thus,all the SoC can be maintained around the average point and the SoC balance can be guaranteed.