Load power compensations for stabilized DC-link voltage of the cascade controlled rectifier/inverter-motor drive system

The necessity of the compactness of the converters in many applications imposes the reduction of the size of their different components when it is possible. In this paper, the cascade of a grid connected voltage controlled rectifier and an inverter supplying a motor is considered and the effect of diminution of DC-link capacitor (C₀ on Fig.1) on the stability of the DC-link voltage is investigated using small-signal linearization and impedance criterion. The output impedance of the controlled rectifier is studied in both cases of DC-link voltage control (DC-VC) and DC-link energy control (DC-EC). The input impedance of the inverter-motor stage, for which the control is based on the classical field orientation, is also explained. In ideal case, if the power delivered by the controlled rectifier can track the load power, the DC-link voltage remains almost constant even in transitory state. Under this condition a low value of C₀ could ensure the stability of the DC-link voltage. Then, three methods of power load compensation, i.e. a decoupling matrix, a nonlinear feedback compensation and the feedback linearization technique, are proposed and studied for stabilizing the DC-link voltage. The results are illustrated by means of digital computer simulations of complete induction motor and controlled rectifier models with full order linear closed loop flux observer.