Research On Impacts And Control Strategy Of Large-scale Pv Integration On Power System Low Frequency Oscillations

Facing the current social situation of increasing energy demand and worsening environmental pollution,changing the current energy structure and vigorously developing new energy has become the direction of future energy development.At present,the installed capacity of photovoltaic plants is gradually increasing,and the penetration rate is also getting higher and higher.However,the access of photovoltaic plants threatens the low-frequency oscillation stability of the power system.Based on the above background,this paper studies in detail the impacts of photovoltaic plants access on low-frequency oscillation,and proposes a reactive power damping control strategy to suppress the weak damping oscillation modes of the system.The specific research contents are as follows:(1)Firstly,a dynamic mathematical model of unipolar photovoltaic plants is built on the basis of considering the output characteristics of photovoltaic cells,the dynamic characteristics of photovoltaic inverter and its control system as well as the DC filter capacitors.Secondly,the time domain model of AVR and PSS is integrated to obtains the mathematical model of synchronous generator and its control system.Finally,according to the mathematical models of photovoltaic plants and synchronous generator,a linearized model of multi-machine power system with photovoltaic plants access is constructed.(2)Taking 4-generator 2-area system and 16-machine 68-bus system as research examples,the changes in the damping ratio of the key oscillation modes of the system before and after photovoltaic plants access are compared by eigenvalue analysis method and time domain simulation method,and the impacts of photovoltaic plants access on the damping characteristics of low-frequency oscillations is analyzed in detail.The results show that photovoltaic plants access will have different degrees of impacts on the damping characteristics of different oscillation modes,and these impacts are closely related to factors such as photovoltaic penetration,transmission distance,access location,and power distribution methods.(3)Aiming at the weak damping interval oscillation mode that still exists in photovoltaic grid-connected systems,an additional reactive damping control strategy for photovoltaic plants to suppress low-frequency oscillations is proposed.In the selection of feedback signal,the geometric measurement method is used to compare the joint controllability and observability of the signals to be selected so as to select the optimal feedback signal.In the establishment of system model,the system linearization model is completed identified by the frequency domain identification method based on pseudo-random signal.In terms of controller design,the traditional damping control design method based on the residue method and the multi-objective control design method based on the robust pole assignment method are introduced respectively,and implemented in the numerical example of four-machine two-area system is built in DIg SILENT/Power Factory.The simulation results show that the additional reactive damping controller of photovoltaic plants can effectively suppress the weak damping interval oscillation mode of the power system.Moreover,compared with the traditional residue method,the damping controller designed based on the robust pole assignment method can perform better oscillation suppression under various disturbance conditions.