Control Of High-speed PMSM/G For Flywheel Energy Storage Systems

Flywheel energy storage system(FES S)is a rather competitive short-term energy storage candidate due to its long lifetime,high power density,high cycle efficiency,and more significantly,high reliability and fast dynamic response.It is very suitable for applications with numerous charge and discharge cycles(hundreds of thousands)and medium to high power(kW to MW)during short periods(seconds to minutes),which contributes to its wide application prospects in the fields of uninterruptible power supplies,micro-grid regulations,wind power plants,rail transits,hybrid vehicles and etc.High-speed permanent magnet synchronous motor/generator(PMSM/G)is one of the most commonly used electric machines in a FESS thanks to its high power density,low operating loss and flexible bidirectional power flow.However,when operating periodically in the fast charge/discharge cycles,the performance of the universally used proportional-integral(PI)control theory based methods cannot fully meet the requirements of a FESS at a high electrical frequency within wide speed range.Therefore,this study is focused on the control strategy im-provement of high-speed PMSM/G in a FESS by taking the PMSM/G and its converter system as a combined control object.The detailed contents include:1.The dynamic model of the combined system of PMSM/G and converter is established,and the specific operation requirement of the FESS is discussed.The major drawbacks on the current control techniques of the high-speed PMSM/G and three-phase PWM converter are con-cluded and the possible research focus is pointed out.The working principle of the high-speed PMSM/G is analyzed in charge and discharge mode,respectively.The available operation range of the FESS within voltage,current and load boundary conditions is derived through the voltage and current limit circle and equipower curve analysis methods,based on which a FESS prototype with rated power 2.5 kW and rated speed 12000rpm is built.2.A robust dc-link voltage control strategy with load power and speed compensation is pro-posed for wide speed range operation of FESS.Wide speed range operation in discharge mode is essential for ensuring discharge depth and energy storage capacity of a FESS.However,for a PMSM/G-based FESS,the wide-range speed variation in a short discharge period causes consec-utive decreases in ac voltage frequency and amplitude.As a result,operation point shift leads to performance deterioration of the conventional local linearization based dc-link voltage control strategies.Therefore,a robust dc-link voltage control strategy that incorporates the speed variation to the dc-link voltage controller is proposed to realize a consistent dc-link voltage control perfor-mance within the entire available operation range for FESS.The nonlinear dc-link voltage loop model is globally linearized by treating the square of dc-link voltage as the state variable,and lumping the nonlinear and uncertain terms proportional to the load power and parameter errors in the power balance equation as the total disturbance.A speed adaptive feedback control law is designed to ensure consistent dynamic performance within the entire available operation range.Finally,the dynamic performance and robustness discharge strategy is validated at different speeds on a high-speed FESS test bench.3.A fast single loop dc-link voltage direct control strategy for high-speed PMSM/G of FES S is proposed to accelerate its dynamic performance.Instead of the conventional strategy with cas-caded outer dc-link voltage loop and inner current loop,the proposed strategy is a single loop direct voltage control strategy without an intermediate current loop.The dc-link voltage loop and q-axis current loop are integrated together to a 3rd-order extended system.A linear extended state observer(ESO)is designed to observe the 3rd-order extended system and derive the differential of the square of dc-link voltage,and a control law that incorporates proportional and differential feedback,as well as speed variation and total disturbance compensation is proposed.The inner dynamic of the linear ESO is analyzed through bode diagrams,and the tracking performance and anti-disturbance capability of the proposed strategy is hereby derived.Finally,the proposed strat-egy is validated on a high-speed FESS test bench.4.The delay compensation and cross-coupling problem of a discrete current controller for high-speed PMSM/G is studied.High-speed PMSM/G in a FESS is faced with higher cross-cou-pling voltages and lower switching-to-fundamental frequency ratios.High cross-coupling voltage leads to transient error during dynamic process of the current loop.If the current controller design does not properly incorporate the delays in a digital control system,the lower switching-to-funda-mental-frequency ratio may result in oscillatory or unstable responses.Instead of discretizing a continuous-time domain designed controller with approximate methods such as the Euler forward difference and the Tustin transformation,an accurate discrete current controller for high speed-PMSM/G is proposed in this paper based on an accurate discrete model with the phase and mag-nitude errors produced during the sampling period taken into consideration,and an Extended State Observer(ESO)applied to estimate and compensate the back EMF error.The cross-coupling prob-lem between d-q axis current loops is well solved,and the dynamic performance of the current loop at lower switching-to-fundamental frequency ratios is improved.The three controllers proposed and designed in this thesis are validated by simulations and experiments on the 12000 rpm FESS prototype,results at different speeds prove the efficiency and improvement at wide speed range operation and low switching-to-fundamental-frequency ratios.