Research On Frequency Regulation And Voltage Stability Of Renewable Energy Dominated Power Systems

Increasing the proportion of renewable energy in primary energy is of great significance for ensuring energy security,protecting ecological environment,and achieving sustainable development.In recent years,renewable energy has developed rapidly in China,accounting for an increasing proportion in power grids,and a large number of thermal power units have been replaced.Due to the water hammer effect of hydropower units,the primary frequency regulation(PFR)characteristics of hydropower units is poor compared with thermal power units.The new energy units represented by wind and photovoltaic are usually non-rotating power electronic devices,which are difficult to provide inertia support for power systems,and have very different AC voltage regulation characteristics from synchronous generators.As the proportion of renewable energy in the power system increases,many new frequency and voltage stability problems have emerged.Under this background,this thesis systematically studies the mechanism and suppression measures of the ultra-low frequency oscillation(ULFO)caused by hydropower units,two key technologies for new energy highproportion power systems to deal with power disturbances,voltage stability analysis of power systems with a large number non-synchronous machine sources connected,and the impact of different power supplies on dynamics of induction motors:(1)The generation mechanism of the ULFO caused by hydropower units is revealed,and an ULFO suppression measure is proposed.First,the PFR model of the multi-machine system is established,and the closed-loop characteristic equation is derived.Then,the influence of hydropower and thermal power units on the PFR stability is analyzed using the closed-loop characteristic equation,and the generation mechanism of the ULFO is revealed.The Nyquist criterion is used to analyze the influence of the DC modulation on the ULFO.Finally,a method for tuning the PI parameters is proposed,which can effectively ensure the system stability in the PFR process.The simulation results of the IEEE 9-buses system and the Yunnan power grid show that the proposed PI parameter tuning method can effectively suppress the ULFO.(2)Two key technologies for new energy high-proportion power systems to deal with power disturbances are studied,namely,the voltage source converter based multiterminal direct current(VSC-MTDC)system and the battery energy storage system(BESS).For asynchronous AC power grids connected by a VSC-MTDC,a VSCMTDC based joint PFR strategy of asynchronous grids is proposed.First,an improved VSC-MTDC linearization model is derived to calculate the reference value of the outer loop controller of each VSC in the joint PFR process.Then,to prevent overload of the converter station or instability of the power grid,an overload handling method based on the sequential search method is given to modify the calculation results of the improved linearization model.Finally,simulations based on PSS/E in a four-terminal VSC-MTDC system verifies the correctness of the proposed strategy.For the scenario where the BESS participates the PFR,a BESS power capacity and energy capacity estimation method considering frequency stability constraints is proposed.Assuming that the BESS adopts the power-frequency droop control,the total power capacity and total energy capacity of the BESS required by the system are derived based on the PFR model.Then,a mixed integer programming model is given to locate and allocate the BESS capacity.Finally,a modified IEEE 39-buses system is simulated to verify the correctness of the proposed method.(3)The influence of non-synchronous machine sources represented by LCCs and VSCs on the voltage stability of the power system is studied.First,based on the Jacobian matrix singularity theory,the influence of the non-synchronous machine source on the small-disturbance voltage stability is analyzed,and the maximum allowable output of the non-synchronous machine source under the small disturbance voltage stability constraint is studied.Then,based on a large-disturbance voltage stability criterion whether there is a reasonable voltage solution at the initial stage after fault clearing,the influence of the non-synchronous machine source on the largedisturbance voltage stability is studied.Finally,the simulation is carried out in the Shanghai power grid,and the theoretical analysis results are verified.(4)The influence of different power supply types on the dynamics of induction motors is studied.Considering three types of the power supply: the synchronous generator,the VSC in the active control mode combined with the synchronous generator,and the VSC in the passive control mode,the dynamic characteristics of the induction motor supplied by different power supply types are analyzed.Taking the motor stall as the criterion of the load instability,the influence of different power supply types on the stability boundary is studied.The correctness of the theoretical analysis is verified by simulations in PSS/E.