Research On Multi-scale Instability Mechanism And Robust Passivity Control Method For Isolated Microgrids

The strategic goal of "Carbon Dioxide Peaking and Carbon Neutrality" has accelerated the process of energy reform,and the transition from fossil energy to renewable energy is urgent.With the deepening of "electricity electronization" of the core equipment of "source-grid-load",the penetration rate of power electronic equipment increases year by year,and the development trend of "high-proportion renewable energy" and "high-proportion power electronic equipment" is increasingly significant.As a small power generation and distribution system,the isolated microgrid has obvious "double height" characteristics,and there are multi-time scale dynamic interactions in the system.Besides,the microgrid system does not have the inertia support provided by a large number of synchronous generators in the traditional power grid,which makes the problem of multi-scale instability particularly acute for isolated microgrid systems with distributed generators as the main energy source.This dissertation focuses on the multi-scale instability mechanism and robust passivity control strategy for isolated microgrids,the main work is as follows:1.The modeling and control methods of wind power generation and photovoltaic power generation are studied.For wind power generation system,the phasor model of doubly-fed wind power generation system based on vector control strategy is established.The advantage of this model is that the modeling process is simple and has wide applicability,and more accurate signal tracking can be achieved without establishing accurate back-to-back converter models.Meanwhile,the calculation time is shortened and the calculation efficiency is improved.For the PV power generation system,the impedance expressions of the front-stage boost circuit DC output port,the rear-stage inverter circuit DC input port and AC port are derived separately,and the influence of system parameters on the port impedance characteristics and system stability is deeply analyzed.In addition,a harmonic suppression strategy based on resonant controller is proposed,which can effectively reduce the harmonic content in the system.A detailed simulation model is built in PSCAD,and the correctness of the theoretical analysis is verified by comparing the time domain simulation results with the impedance characteristic analysis.2.The multi-time scale transient instability mechanism and model order reduction method of isolated microgrid system are studied.Firstly,the state space model of isolated island microgrid system is established,and then the state space model is linearized to obtain the complete small signal model of isolated island microgrid system.Based on this model,the eigenvalue trajectory of the system is analyzed,and the influence of parameter changes on the stability of the system is discussed.It is shown that the stability of the system is greatly affected by the active power droop coefficient.When a suitable active power droop coefficient is selected,the system can be guaranteed to have good stability.If the active power droop coefficient is too large,the system will easily become unstable.To further analyze the multi-scale transient instability mechanism of microgrid system and clarify the relationship among state variables,eigenvalues and system modes,the analysis of the influence weight of state variables on system modes is carried out.On this basis,a high-precision order reduction model considering the influence weight of state variables on the system modal is proposed.The time domain simulation results show that the proposed model can accurately reflect the dynamic characteristics of the system while reducing the difficulty of model calculation.3.The transient stability study is carried out for isolated AC microgrid system.A large signal model of microgrid is established,which takes into account the DC side dynamics of inverter.The influence of DC side dynamics and different types of loads on the stability of large signal system is analyzed.The transient stability margin of isolated microgrid system is evaluated by the asymptotic stability region evaluation method based on Takagi-Sugeno multiple model.The results show that the increase of transmission line length,DC power input voltage,DC bus capacitance,DC line equivalent resistance and constant current load will increase the transient stability margin of the system,while the increase of DC line equivalent inductance,constant power load active power and constant impedance load resistance will lead to the decrease of the transient stability margin of the system.The time domain simulation is carried out in MATLAB for the operation scenarios of voltage fluctuation of DC input power supply and active power fluctuation of constant power load of the inverter.The simulation results are consistent with the divergence trajectory,which effectively verifies the accuracy of the evaluation results of the asymptotic stability domain of the system.4.A robust passivity control strategy for isolated AC microgrid is proposed,and its parameter design method is analyzed,especially the steady-state error,dynamic performance and parameter deviation of the system are studied deeply.The upper layer of the proposed control strategy adopts Virtual Synchronous Generator(VSG)to improve the frequency stability of the system.The passivity control is used in the bottom layer to improve the robustness of the system.Based on the Euler-Lagrange model,the passivity of the system is analyzed,and the two damping gains of the passivity controller are designed.Considering the deviation of filter model parameters and the constraints of stability,dynamic performance and cascade control,the design principle of damping gain and the method of designing double damping gain parameters by steps are proposed.This method makes full use of the feedforward control advantage of damping gain,and can significantly enhance the stability and robustness of the system.Even with large parameter offset,the system still has good dynamic performance and control bandwidth.Simulation and experimental results fully verify the effectiveness of the proposed control strategy and its parameter design method.