Research On Passivity-Based Control Strategy Of Virtual DC Machine Controlled DC/DC Converters In DC Microgrid

With the development of distributed generation and energy storage technology,DC/DC converter has gradually become the core of power conversion,power transmission and control in DC microgrid,and its performance determines the performance of DC microgrid.Meanwhile,it is easy to lead to the stability decline or even instability of DC microgrid when the DC microgrid is subjected to various large disturbances such as great sudden load changes or structural changes.Therefore,the requirements for the control strategy of DC/DC converter are higher and higher.The Virtual DC Machine(VDCM)control technology can provide additional inertia and damping for the system by emulating traditional DC machine,and can effectively suppress the DC bus voltage fluctuation of DC microgrid.Meanwhile,the passivity-based control(PBC)is widely used in nonlinear control and stability guarantee under large disturbance of DC/DC converter because of its advantages of simple control structure,good dynamic response and easy implementation.Therefore,VDCM and PBC can provide important support for the stability of DC microgrid.This dissertation mainly focuses on VDCM controlled DC/DC converter in DC microgrid.The control performance improvement,multi-converters parallel connection and PBC of VDCM controlled DC/DC converter are analyzed and discussed comprehensively,and the correctness of the theoretical research results is verified by simulations and experiments.The main research work is as follows.(1)Aiming at the problem that the traditional VDCM control with fixed parameters is difficult to achieve good dynamic performance while providing inertia and damping support,a parameter adaptive control strategy of VDCM is proposed.First,the control principle of VDCM is introduced and its small signal model is established.The effects of moment of inertia J and damping coefficient D on the stability and dynamic performance of DC/DC converter are analyzed,and the adjustment range of control parameters is given considering the stability and dynamic performance.On this basis,according to the dynamic response process of DC bus voltage under disturbances,the regulation laws of J and D are analyzed,and the parameter adaptive control strategy of VDCM for DC/DC converter is proposed.This strategy fully takes advantage of flexible adjustment of virtual parameters to improve the stability and dynamic performance of VDCM.At the same time,the adaptive adjustment principle of moment of inertia and damping coefficient is given,and the value selection method of adaptive adjustment parameters is given according to the adjustment range of parameters.The simulation and experimental results show that compared with the traditional VDCM control strategy,the proposed strategy not only provides inertia and damping support,but also has faster response and smaller overshoot.It can better suppress the fluctuation of DC bus voltage,and then improve the suppression ability of DC bus voltage fluctuation of DC microgrid.(2)Aiming at the problem that power distribution of the existing VDCM parallel control strategy depends on communication network,and the droop and VDCM combined strategy has many control links,a VDCM parallel coordinated control strategy with angular velocity and power droop is proposed.The strategy draws on the idea of primary frequency regulation of AC synchronous machine.It adjusts the mechanical power of VDCM according to the virtual angular velocity of DC machine in order to realize the power distribution among multiple VDCM controlled DC/DC converters according to the preset distribution coefficient.It can effectively reduce the control links and realize the organic combination of droop and VDCM.First,the power distribution principle of VDCM parallel system is studied,and the influence factors of power distribution are given.Second,the small signal model of VDCM parallel coordinated control strategy is established,and the influences of main parameters k_d,J and D on system stability and dynamic performance are analyzed,which provides the guide for the design of parameters.Simulation and experimental results show that the proposed strategy not only provides inertia and damping for the system,but also has good dynamic and steady-state power distribution performance,which can effectively suppress the fluctuation of DC bus voltage.(3)Aiming at the problem that the traditional linear control strategy is difficult to ensure the system stability and has slow dynamic response under large disturbances,the new PBC(NPBC)considering the dynamic and steady-state regulation characteristics of voltage and current based on Euler Lagrange(EL)and Port Controlled Hamiltonian with dissipation(PCHD)model is proposed respectively.First,the problems and limitations of the existing PBC strategies are analyzed.Second,based on the PBC theory and phase plane method,the influence of constant power load(CPL)on the passivity of DC/DC converter is analyzed.It is obtained that the negative impedance characteristic of CPL is the main non-passive factors of dynamic error of DC/DC converter.To address this issue,based on the dynamic error energy function of DC/DC converter,the EL-NPBC(ENPBC)and PCHD-NPBC(PNPBC)are studied by means of interconnection and damping injection.The strict passivity of DC/DC converter with CPL load under dynamic condition is achieved,and the stability of DC/DC converter at desired equilibrium point is ensured.The robustness of the proposed strategy is further improved by power observer.Combined with the analysis of the influence of parameters on stability and dynamic characteristics,the setting principles of main parameters are given.The simulation and experimental results show that the proposed ENPBC and PNPBC strategies can adjust the output voltage and inductance current error simultaneously and have faster response speed compared with the single variable regulation of the traditional PBC.The proposed strategy can effectively ensure the large disturbance stability of DC/DC converter and has good dynamic characteristics.(4)Aiming at the limitations of VDCM with small signal analysis method and PI controller under large disturbance and the problem of steady-state error in PBC under large disturbance,a VDCM-ENPBC control strategy combining VDCM outer loop and ENPBC inner loop is proposed.In this control strategy,ENPBC is used to replace the traditional VDCM current inner loop PI controller to improve the stability and dynamic performance of the inner loop controller under large disturbances,and the outer loop VDCM control provides current command for the inner loop ENPBC,which can provide additional inertia and damping for the DC/DC converter,and enhance the adaptability of ENPBC under large disturbances.It realizes the complementary advantages of VDCM and ENPBC.First,the analysis principle of system passivity and large disturbance stability of the multi-power converter system(MPCS)is given.On this basis,the VDCM-ENPBC control principle is given,and the influence of main parameters on system stability is analyzed based on phase plane method.Simulation and experimental results show that the proposed control strategy can achieve good steady-state and dynamic performance of the DC/DC converter,and can effectively suppress the fluctuation of DC bus voltage and steady-state error while realizing the stable operation of MPCS under large disturbance.