DEVICE FOR INCREASING THE VOLTAGE RESERVE OF THREE-PHASE PULSE RECTIFIER SYSTEM WITH CONNECTING THE OUTPUT VOLTAGE CENTER TO A FICTIVE NETWORK POINT

摘要

The invention describes an apparatus for increasing the voltage utilization of a three-phase PWM rectifier system with a star connection of filter capacitors connected to the main phase terminals, where this star point is connected to the output voltage center point. The control of the input currents is done by a current controller in each phase that defines the gate signals for the bridge-leg change-over switches. The phase current reference value that is according to known technology formed by a supervising output voltage controller, is extended according to the invention by adding a phase current reference component that is equal for all three phases. To form this signal, the filter capacitor voltages measured positively against the output voltage center point are added and attenuated by a gain element by a factor of 3. A zero-voltage reference value function generator creates a third harmonic of the main voltages with defined amplitude and phase position according to the main voltages. The output signal of this function generator is compared to the output of the gain element by a subtracting element. The output of the subtracting element is dynamically weighted by a controller which results in the reference current zero-component. The potential of the positive output voltage bus, therefore, according to the time-behavior of the capacitor voltage zero-component reference value increases periodically in the vicinity of the positive maximum of the fundamentals of the rectifier input phase voltages referring to the main star points, while in the vicinity of the negative maximum the potential of the negative output voltage bus is decreased. This results in a virtually higher output voltage available for the formation of the rectifier input phase voltages. Therefore, the degree of modulation can be increased by a factor of 1.15 compared to a filter capacitor voltage-system without zero-component.