In this Thesis, a high-performance control structure is designed, implemented, and applied to a three-phase series-parallel uninterruptible power supply (SP-UPS). Firstly, two control strategies for SP-UPS are described and compared. An SP-UPS consists of a parallel and a series converter, and most of the power required by load is supplied directly from ac input line during standby mode. Therefore, the overall efficiency is increased as compared to conventional UPS’s. The first control strategy proposed for SP-UPS was based on voltage control of series converter and current control of parallel converter. However, recently an opposite strategy has been proposed, i.e., voltage control of parallel converter and current control of series converter. This thesis compares these two control strategies. The comparison is made based on different performance criteria like seamless transition between backup and standby modes, dynamic performance, reference generation and the size of filters. It is shown that this strategy improves the system operation as compared to existing strategy, and specifically results in more smooth and seamless transition between UPS operating modes. The controller in the proposed strategy is based on combination of two control method with different characteristics. In the proposed structure, also called hybrid structure, a fast deadbeat control law is used to achieve the fast transient response required for output voltage control. On the other hand, as most disturbances imposed on a UPS are periodic, a Repetitive Controller (RC) is added to the deadbeat controller to improve the steady-state performance and achieves fast error convergence. The system formulation for the controller design is given, and the derivation of control law based on descritized state-space equations is presented. The stability of hybrid controller is discussed and design procedure for a typical converter is given. Both simulations and experimental results are shown for verification and validation of the theoretical analysis.
