A Current-Regulated Voltage-Controlled Scheme for DC to AC Voltage-Source Static Power Supplies

Abstract

This thesis pn~~ents a cost-effective control strategy for fixed-switching frequency variable duty-cycle control of static voltage-source inverters witit output filter for uninterruptible power supplies (UPS) and. utility interactive systems (UIS). The pro posed control scheme is based on sensing the current in the capacitor of the load filter and using it in an inner feedback loop. An outer capacitor voltage ieedback loop is also incorporated to ensure well-regulated sinusoidal load voltage in UPS applications, and indirect regulation of the utility line current in UIS applications. The performance of the proposed control scheme is examined for voltage-source inverters with output filter. A general model of the power circuit which includes the inverter and a second-order load filter, and the load is established for each application. These models are discontinuous because of the switching nature of the power inverter, but they are employed to obtain computer simulation results of various waveforms in the inverter system. In order to examine the steady-state and dynamic behaviour of each system, ana lytical models are derived from the general model. Employing Fourier series analysis, a time-continuous model is derived from the system discontinuous model. For the three-phase systems, time-invariant models are obtained by transforming the time continuous models to the rotating frame of reference using the forward-backward transformation for UPS and 0-q-d transformation for the UIS. Perturbation and small signal approximations are applied to the time-invariant models to obtain steady-state and linearized small-signal models which are used to examine the steady-state and dynamic performance of the systems. Using the root-locus analysis technique and pole-zero maps, the incremental dy namics of the power circuit state variables are investigated. In particular, the transfer functions of the incremental dynamics of the power circuit state variables due to iu cremental changes in the control signal are examined. The results of the investigation are used to select appropriate feedback variables in the control circuit such that a stable and successful operation of the feedback control scheme is achieved. It is shown that choosing either the inverter output or capacitor current as the feedback •1ariable in the system produces stable pperation. Selection of the inverter output current as the feedback variable is overruled due to possible harmonic insta bility in the load circuit or the need for an extra load current sensor. Consequently, the capacitor current is chosen as the feedback variable. To ensure sinusoidal load voltage which faithfully tracks its reference signal, an outer feedback voltage loop is incorporated. The resultant control strategy applicable to the UPS and UIS is a current-regulated voltage-controlled scheme involving an inner feedback loop with the capacitor current and an outer loop with the capacitor voltage. Selection of the controllers of the feedback loops is carried out using Bode diagram and time-domain error analysis. !t is shown that proportional controllers in the feed forward path of both the inner and outer feedback loops are sufficient to produce a system with excellent dynamic performance, and well-regulated and nearly perfect sinusoidal load voltage or utility line current. As a result, the proposed control scheme is simple to implement. Experimental verification of the proposed scheme for single-phase and three-phase UPS, and single-phase UIS is presented in the thesis to demonstrate the feasibility of the proposed control strategy, and the validity of the analytical models.