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Control of a High-speed PMSM/G for Flywheel Applications

par Zhang, Xiang
Président du jury Garone, Emanuele
Promoteur Gyselinck, Johan ;Yang, Jiaqiang
Publication Non publié, 2019-03-25

Thèse de doctorat

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Résumé :

Flywheel Energy Storage Systems (FESSs) are 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. They are very suitable for applications with numerous charge and discharge cycles (up to hundreds of thousands) and medium to high power (kW to MW) during short periods (seconds to minutes), which contributes to their wide application prospects in the fields of uninterruptible power supplies, micro-grid regulations, wind power plants, rail transits, hybrid vehicles etc. High-speed Permanent Magnet Synchronous Motors/Generators (PMSMs/Gs) are one of the most commonly used electric machines in an FESS thanks to their high power density, low operating loss and flexible bidirectional power flow. However, when operating periodically in fast charge/discharge cycles, the performance of the universally used proportional-integral (PI) control theory based methods cannot fully meet the requirements of an FESS at a high electrical frequency within a wide speed range. Therefore, this study is focused on the control strategy improvement of high-speed PMSM/G in an FESS by taking the PMSM/G and its converter system as a combined control object. The detailed contents include:The dynamic model of the combined system of PMSM/G and converter is established, and the specific operation requirements of the FESS are discussed. The major drawbacks of the current control techniques of high-speed PMSMs/Gs and three-phase PWM converters are concluded and the possible research focus is pointed out. The working principle of the high-speed PMSM/G is analyzed in charging and discharging mode, respectively. The available operation range of the FESS within voltage, current and load boundary conditions is derived through voltage and current limit circle and equipower curve analysis methods, based on which a FESS prototype with rated power of 2.5 kW and rated speed of 12000rpm is built.A robust DC-link voltage control strategy with load power and speed compensation is proposed for wide speed range operation of an FESS. Wide speed range operation in discharging 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 discharging period causes consecutive decreases in ac voltage frequency and amplitude. As a result, the operating point shift leads to performance deterioration of the conventional local linearization based DC-link voltage control strategies. Therefore, a robust control strategy that incorporates the speed variation to the DC-link voltage controller is proposed to realize a consistent performance within the entire available operation range of the 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.A fast single loop DC-link voltage direct control strategy for high-speed PMSM/G of an FESS is proposed to improve its dynamic performance. Instead of the conventional strategy with cascaded 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; a control law that incorporates proportional and differential feedback, as well as speed variation and total disturbance compensation is proposed. The inner dynamics of the linear ESO are analyzed through bode diagrams, and the tracking performance and anti-disturbance capability of the proposed strategy is hereby derived.The delay compensation and cross-coupling problem of a discrete current controller for high-speed PMSM/G is studied. The high-speed PMSM/G in a FESS is faced with higher cross-coupling voltages and lower switching-to-fundamental frequency ratios. High cross-coupling voltage leads to transient error in the current tracking. If the current controller design does not properly incorporate the delays in a digital control system, the lower switching-to-fundamental-frequency ratio may result in oscillatory or unstable behaviors. Instead of discretizing a continuous controller with approximate methods such as the Euler difference and the Tustin transformation, an accurate discrete current controller is proposed based on an accurate discrete model. The model takes the phase and magnitude errors produced during the sampling period into consideration, and an Extended State Observer (ESO) applied to estimate and compensate the back EMF error. The cross-coupling problem 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 dynamic performance and robustness of 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.