Research Of Subsynchronous Oscillation In PV-thermal Bundled System

With the increase of PV capacity,it is necessary to improve the ability of PV delivery and consumption.The structure of large-scale grid-connected PV is similar to wind power.PV power station is connected to the grid through voltage source converters and uses a large number of power electronic devices.Therefore,the risk of subsynchronous oscillation exits in PV system.However,there is relatively little research on the subsynchronous oscillation of PV.Directly connecting to the grid and tying up with the thermal power are two methods for PV to be connected to the grid.When PV is connected to the grid with thermal power bundled,it is a viable and economical transporting mode through series compensation capacitor or HVDC.The subsynchronous oscillation of large-scale photovoltaic power plant is studied in this paper,when PV is sent out through weak grid and tied up with the thermal power.The main content of the paper is shown below.The mathematic model of PV-thermal bundled system sent out via series compensation capacitor is established,which contains thermal power system,PV station and AC system.Modeling of thermal power system consists of shaft,synchronous generator,steam turbine and governor,excitation and PSS.Modeling of PV station consists of photovoltaic cell,inverter and its control system,phase-locked loop and filter.AC system is substituted by RLC transmission line.Aiming at the subsynchronous oscillation of large-scale PV station sent out via weak grid,the equivalent model of large-scale PV station is verified by time domain simulation.Then the small signal model of PV system is established based on the equivalent model.Through the eigenvalue analysis,it is found that there are four oscillation modes in the system of PV sending out through weak grid.Two high frequency oscillation modes are dominated by LCL filter,another two subsynchronous oscillation modes are respectively dominated by line current and phase-locked loop.When power network is weak and the inverter parameters are not reasonable,the subsynchronous oscillation mode dominated by line current will have negative damping,which is the main reason for the subsynchronous oscillation.In order to study the subsynchronous oscillation of PV-thermal bundled system,a block modeling method suitable for complex systems is proposed.The small signal model of PV-thermal bundled system is established on MATLAB/Simulink.The influence of photovoltaic power station access and its operation characteristics on oscillation modes of thermal power units are studied by eigenvalue analysis method and time-domain simulation method.The results show that the damping in the frequency range below 25 Hz slightly increases after the access of the photovoltaic power station.And the damping in the frequency range above 25 Hz decreases in the same condition.PV station has greater influence on thermal damping,when PV capacity is larger and the impedance between PV and grid is smaller.Additional damping controller based on ADRC is designed to suppress the subsynchronous oscillation of thermal power system.The parameters of ADRC are optimized by fuzzy control algorithm,so as to remedy the defect that ADRC parameters are not easy to set up.The domain simulation results show that additional damping controller based on fuzzy ADRC has good performance when suppressing the subsynchronous oscillation of thermal power system.