Research On The PWM Direct Current Control In Voltage Source Converters

Externally pressurized gas bearing has been widely used in the field of aviation, semiconductor, weave, and measurement apparatus because of its advantage of high accuracy, little friction, low heat distortion, long life-span, and no pollution. In this thesis, based on the domestic and overseas researching.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. Slop PWM Current Control and Corresponding Current Tracking methods is proposed for direct current control in voltage source converters (VSC), as falls within the principal research tasks of the thesis. For example in slope PWM current control current ripple transformed is compared with a slope wave to make the actual current track the reference current continually each cycle, and maintain the switching frequency to be nearly constant for a voltage-fed PWM converter in one switch period. As another example parabolic PWM current control technique employs a pair of specified parabolic PWM carriers to directly control the current ripple 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 probable implementation schemes being accordingly presented. Further, both qualitative and quantitative analyses specifically on convergence and stability behavior of the new methods under different operating conditions are put forward.Several application-oriented fundamental research tasks have been carried out regarding the 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 equivalent series resistance (ESR) and the White Noise in converters are discussed and analyzed with a quantitative formula being deducted. With regard to the three-phase three-wire converter system, the phase dependency problem is studied in details and one decoupling scheme for the Slop PWM current control are developed. The scheme 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. The implementation of the proposed decoupling control strategie is presented with simulations followed for verifications.