Research On Cascaded Three-Phase AC Power System: Stability Critetion And Solution

Distributed Power System(DPS) has been widely used in communication systems, electric ships and renewable energy generation systems. Cascaded system is a typical configuration in DPS, which is to add multistage power converter modules between source and load. In cascaded systems, the interaction between the individually designed source module and load module may cause overall system instability. This thesis is dedicated to the stability criterion and strategy for improving the stability of three-phase AC cascaded systems.Since three-phase AC cascaded system is a multiple-input-multiple-output(MIMO) system, the General Nyquist Stability Criterion(GNSC) is needed to analyze the system stability. This thesis first introduces the cascaded system under discussion, which is comprised of a three-phase LC filter and a three-phase voltage source PWM rectifier. The small-signal model of three-phase AC cascaded system under dq frame is built, and based on which, the equivalent MIMO model is derived. Based on GNSC, a simplified stability criterion for three-phase AC cascaded systems is derived, which reduces a MIMO system to two single-input-single-output(SISO) systems. Hence the impedance-ratio-based stability analysis method for DC cascaded systems can be applied to three-phase AC cascaded systems. The proposed simplified stability criterion provides a simple and straightforward method to analyze the stability of three-phase AC cascaded systems.To improve the stability of the three-phase AC cascaded systems, this thesis proposes a virtual impedance-based control strategy to regulate the input impedance of the voltage source PWM rectifier. The regulation strategy is to ensure that the regulated input impedance and the output impedance of the LC filter satisfy the proposed stability criterion, thus the cascaded system can be stabilized. Then a design example of load input impedance regulator and its simulation results are given to validate the theoretical analysis.Finally, a 20 kW prototype of three-phase AC cascaded system is built in the laboratory, and the experimental results confirm the correctness of the proposed stability criterion and the effectiveness of input impedance regulation strategy.