Control Of Electrical Excited Synchronous Machine At Low Switching Frequency

The three-level ac variable frequency converter fed electrical excited synchronous machine is in increasing demand for numerous medium-voltage high power industrial applications such as mining hoist and locomotive traction. The operation of the PWM converter should be kept at low switching frequency so as to restrain the dynamic losses of the three-level converter and improve its output power. Problems due to low switching frequency are studied in this paper, with system modeling and control strategies for solving the problems.First, the feathers of the three-level variable frequency converter fed electrical excited synchronous machine are described. The pulse width modulation (PWM) methods at low switching frequency are introduced. The complex state signal flow graph is used for modeling the PWM conversion part. With the analysis of zero-pole graph of the current control inner loop, a novel complex state current regulator is proposed. It can effectively reduce the cross coupling of the current control loop caused by low switching frequency, while realize high dynamic and steady performance at the same time are proposed.Based on the analysis of the flux observation principles of the electrical excited synchronous machine,the transition method between the current model and voltage model is proposed. A simple resonant type observer to compensate current measurement errors for the pure-integration-based flux estimation is proposed. The technique further contains a residual error compensator to eliminate miscellaneous residual error of the integrator. A novel hybrid fundamental current observer at different coordinates for the electrically excited synchronous machine is designed based on the observer theory and the complex vector model. This kind of hybrid observer can extract the fundamental component of the stator current instantaneously as well as the perfect performance of dynamic tracking. Finally, a real-time switching angle adjustment strategy of optimal PWM based on stator flux trajectory control at low switching frequency is proposed. Problems caused by using the offline calculated optimal PWM patterns at the dynamic speed control, such as switching angle disorder and equipment overcurrent trip are solved by this method.There are 83 figures, 5 tables and 231 references in this dissertation.