Research On The Parabolic PWM For Direct Current Control In Voltage Source Converters

Current control of PWM-based converters is one of the most important research subjects in nowadays modern power electronics. In many concrete applications with power converters, such as active power filters (APF), PWM rectifiers, power factor correction (PFC) converters as well as power inverters, accurate current control with preferable dynamic performance is always required.A parabolic PWM method is proposed for direct current control in voltage source converters (VSC), as falls within the principal research tasks of the thesis. This new developed method employs a pair of specified parabolic PWM carriers to directly control the peak current tracking error and the switching instants, which features, compared with the traditional hysteresis control and the synchronized on-off control strategies, a reasonably fast response, outstanding performance of average current tracking and also an almost constant switching frequency. The theoretical fundamentals of the proposed methodology are systematically established with implementation schemes being accordingly presented. Further, both qualitative and quantitative analyses specifically on convergence and stability behavior of the new method under different operating conditions are put forward.Several application-oriented fundamental research tasks have been carried out regarding the key impact factors and concrete control strategies, with a view to ameliorating the control performance of the proposed parabolic PWM methodology. Impacts from the input/output inductance bias, the equivalent series resistance (ESR) and the change of input/output voltage in converters are discussed and analyzed with a quantitative formula being deducted, which indicates the input/output inductance bias plays a critical part for the current control characteristics. Hence, an adaptive control scheme for parameter compensation is thereby proposed based on the switching frequency bias errors. With regard to the three-phase three-wire converter system, the phase dependency problem is studied in details and two decoupling schemes for the parabolic PWM current control are developed. One of the schemes is based on the superposition principle, in which a virtual three-phase four-wire system is established and can be controlled by three independent parabolic PWM current controllers. In the other scheme, the three-phase currents are virtually transformed into two-phase currents and therefore can be controlled independently by two parabolic PWM current controllers, which can greatly reduce the switching times and power loss by one third. The implementation of the two proposed decoupling control strategies are presented with both simulations and experiments followed for verifications.Loads of simulation and experimental studies have been done to verify the parabolic PWM methodology. Based on the proposed current controller, the mathematical models to fully account for voltage-current duo-loop control systems of four typical power converters, namely voltage-source inverter (VSI), PWM rectifier, power factor corrector (PFC) and active power filter (APF), are established respectively, and a unified prototype test rig rated at 220V/1.5kVA is specifically developed for three-phase APF and PWM converters. Both the simulation results and experimental studies demonstrate outstanding dynamic performance and control accuracy of the parabolic PWM methodology at different loads circumstances.Combined with concrete applications of the parabolic PWM methodology in APF and PWM converters and also based on the multipurpose test rig, further research is done regarding the control and compensation schemes of APF from a view angle of integrated control system. The influence of harmonic and reactive current detection on the compensation performance of a shunt APF is analyzed, and the inner relationship of the conventional control strategy and the direct AC main current control strategy is elucidated as to reveal the essential equivalence of the two strategies for current control, as well as the uniformity and generality of the APF direct AC main current control and the PWM rectifier control strategies. With establishment of the power gird models for harmonics propagation, two prevailing compensation schemes widely used in APF are comparatively studied, which further reveals that, from the harmonics suppression point of view, the nonlinear loads are better to be compensated finally as an equivalent linear resistance rather than to compensate the nonlinear feeder currents as to be a positive-sequence active current of fundamental frequency, and what more, the implementation of the APF control strategy is much easier. Subsequent simulations and experimental studies also verify validity of the conclusions drawn above.