Some control aspects of single-phase power converters with active power buffer

Abstract

Single-phase power converters with an active power buffer have become a hot research topic in the recent decade. This new generation of single-phase converters are promising candidates for achieving high power density, high power-conversion efficiency, and high reliability (H3), and thus termed as H3 single-phase converters in the thesis. Although existing research works on topology and circuit design of the H3 single-phase converters have been successful in pushing the power density and efficiency towards the next level, their control design is still very much lagging. Achievable experimental results reported in the literature generally exhibit poor transient performances with large overshoot/undershoot and long excursion periods at the DC port. As a key enabler of effective and reliable operation, the control design of H3 single-phase converters deserves a thorough investigation. This thesis is devoted to discussing some control aspects of H3 single-phase converters. The major contributions of the thesis include: 1. The characteristics of H3 single-phase converters are analyzed and compared with those of the conventional counterparts. The analysis shows that the H3 single-phase converter is a highly nonlinear and highly coupled system, which involves large-signal operation and has a vulnerable DC link to external disturbances. These distinctive characteristics render linear control being less effective for H3 single-phase converters than that for their conventional counterparts. 2. A general nonlinear control method (termed as feedback-linearization-based automatic-power-decoupling control, or FBL-APD control) is proposed for H3 single-phase converters. The proposed nonlinear controller achieves global stability, fast dynamics, and enhanced robustness against external disturbances. Additional functionalities can be provided by the proposed nonlinear controller including active output voltage holdup, bidirectional power flow, and reactive power/harmonics compensation. 3. We identify the problem of internal dynamics instability that is resulted from direct application of the FBL-APD control to some systems. The instability problem is solved by an evolved FBL-APD control, namely, the Lyapunov-based automatic-power-decoupling (LP-APD) control. The proposed LP-APD control incorporates the Lyapunov’s direct control method into the FBL-APD control. With the LP-APD control, the internal dynamics are stabilized and the virtuous features of the FBL-APD control are retained. As a complement to the FBL-APD control, the proposed LP-APD control can be useful in the cases where the resulting internal dynamics are unstable when the converter is controlled by an FBL-APD controller. 4. The problem of a high sensor count is identified for H3 single-phase converters. This problem adversely offsets the gained advantages of H3 single-phase converters in terms of power density and reliability. To solve this problem, a general theory for reducing the number of sensors is proposed using algebraic estimation. The proposed method not only effectively reduces the sensor count, but also retains the advantages of the original control. Simulation and experiments have been conducted to verify the above proposed controllers and observer. The simulation and experimental results demonstrate that the proposed FBL-APD control, LP-APD control, and sensor-count-reduction method are effective for the H3 single-phase converters.