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

Abstract

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.

Date of Award	25 Mar 2019
Original language	English