Control Methods for Reduction of DC Link Capacitor and Restarting at Instantaneous Power Failure in PWM Converter

Abstract

Rectifiers composed of diodes or thyristors have many harmonics in line currents. Recently, PWM converters have been actively studied to reduce the harmonics. In order to put them into practical use, not only PWM control method but also the following control methods must be established.
(1) Method for reducing the capacity of dc link capacitor.
(2) Restarting method from instantaneous power interruption.
For reducing the capacity of do link capacitor, it is necessary to suppress the fluctuation of dc link voltage at a sudden change of load. In this paper, the load is inverter for variable speed induction motor drives. The load current of converter, that is the inverter input current, is detected without delay time and ripple by multiplying the inverter output currents or motor currents and the inverter PWM signals. By adding the detected signal to do link voltage control loop as a compensating component, the fluctuation of dc link voltage at a sudden load change can be suppressed. As a result, the capacity of the dc link capacitor or can be reduced to smaller than one-fifth of that in the conventional diode rectifier. For restarting from instantaneous power interruption, the very simple control method is proposed. When the dc link voltage becomes low limitation value setted at a little bit lower than that obtained in diode rectifier in the period of power interruption, the PWM signals to converter are blocked. After recovery from power interruption, when the dc link voltage reaches the low limitation value, the PWM control signals are supplied to the converter again and it is restarted the operation. From the experimental results, it was confirmed that the PWM converter can be restarted from the instantaneous power interruption very smoothly.