LQR control of dual-active bridge DC-DC power electronic converters

Abstract

Recent trends in power system operation have been constructed to implement controlled and bi-directional power flows on the load side. Such power flows have facilitated the implementation of the load-follows-generation new strategy for operating power systems. In addition, the successful implementation of controlled and bi-directional power flows has supported the integration of different types of distributed generation and storage (DGS) units. These new generation assets are typically interconnected to distribution systems by stages of power electronic converters (PECs). The back-to-back, modular, and solid-state transformers are examples of PEC topologies used to interconnect DGS units. The major functions of PECs in DGS units, include converting, processing, and controlling the electric power to meet certain conditions imposed by the host grid. These conditions mandate the design of controllers to accurately and effectively operate stages of PECs in stable and reliable manners. Among the key PECs to implement controlled and bi-directional power flows are the conventional and resonant dual active bridge (DAB) dc PECs. These bi-directional dc PECs are widely used to construct active DC-links in many applications, such as voltage and reactive power compensation, motor drives, plasma generation units, etc. The employment of DAB dc PECs in power systems requires the design and implementation of accurate, fast, and reliable controllers. This thesis aims to design, implement, and test linear-quadratic regulator (LQR) controllers for the DAB and resonant DAB dc PECs. The design of an LQR controller is achieved by the development of linearized models for the DAB and resonant DAB dc PECs. These models are developed to accommodate the switching scheme, as well as the relationship between the duty cycle and reference voltages. Furthermore, the developed models for DAB and resonant DAB dc PECs are used to devise a tuning procedure for the designed LQR controllers. The performance of the designed LQR controllers is tested for the DAB and the resonant DAB dc PECs under different conditions. Tested conditions include step changes in the power flow, changes in the voltage on the input side of the PEC, and bi-directional power flows. The results of these tests show that designed LQR controllers can operate DAB and resonant DAB dc PECs to adjust input and output voltages during step changes in the power flow, changes in the direction of the power flow, and changes in the voltage. Observed performance features are also compared with other controllers under similar conditions. Test and comparison results demonstrate the efficacy of the designed LQR controllers to operate DAB and resonant DAB dc PECs under different loading and dynamic conditions.