Uniform control for bidirectional interlinking converters in islanded hybrid AC/DC microgrid

• Main Author: Wang, Junjun
• Other Authors: Wang Peng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Electrical and electronic engineering::Power electronics
• Online Access: https://hdl.handle.net/10356/161817
• Institution: Nanyang Technological University

Hybrid ac/dc microgrids (HMGs) have been gaining in popularity over the past years because of the comprehensive advantage combination of both ac and dc microgrids. Coupling two subgrids and achieving system-level coordination, especially in islanded HMGs, generally require flexible control strategies for bidirectional interlinking converters (BICs) interconnecting the ac and dc subgrids that often serve as crucial roles in the aspects of efficient power management, improved power quality, seamless mode transition, etc. In addition to the power management, the BICs are expected to provide ac or dc voltage support when ac or dc main voltage sources (MVSs) abruptly fail. Mode transitions for multimode BICs, which rely on prompt and accurate detection, pose challenges to coordination controller design in HMG. A uniform control strategy, which provides a generic solution for multimode BICs that can be easily modified and adopted for different configurations and applications in HMG, has been proposed, developed and improved in this thesis. To demonstrate the operation principle of the proposed uniform control, a design paradigm for single BIC in hierarchical controlled HMG is firstly presented. The uniform control unifies the control scheme for multimode BIC in terms of power dispatch mode, ac voltage regulation mode and dc voltage regulation mode. Bidirectional droop with inherent ac voltage regulation capacity is adopted to BIC. By shifting the droop curve with shifting item through equalizing the normalized ac frequency and dc voltage, power dispatch and dc voltage regulation can be achieved. With this feature, various detection mechanisms for BIC mode transitions are no longer necessary when responding to the relevant scenario changes in the power network. Therefore, the adverse consequences, such as overall system collapse due to delayed or false mode transitions can be avoided. Moreover, the uniform control strategy is applicable for the hierarchical control in HMG. The communication fault ride-through capability is enabled and the impact from communication faults can thus be reduced. For high-power application, parallel BICs are normally deployed to improve the reliability, significantly increase the entire power capacity of HMG, and relieve the burden on single power conversion component. To extend the uniform control to parallel BICs, a distributed uniform control strategy is then presented. The proposed distributed uniform control relies on low-bandwidth neighbourhood communication and the simplest communication topology is demonstrated in this thesis. The shifting items of all parallel BICs are kept identical by exchanging information with neighbouring converter(s), thus the proportional power sharing among all BICs is achieved while maintaining the complete functionalities of the uniform control. With the distributed uniform control, accurate global power sharing can be maintained in normal condition, while ac or dc voltage regulation can be achieved when ac or dc MVSs fail. Communication fault ride-through capability is enabled with less global power sharing accuracy. The system reliability and resilience of HMG under various fault conditions can thus be improved. In HMG, constant power loads (CPLs) in dc subgrid that introduce negative incremental impedance and nonlinearity to the system is the main cause of instability. Although the existing control strategies for the dc microgrids with CPLs can be adopted to stabilize the dc subgrid, such strategies cannot be integrated with the uniform controlled BIC system as the uniform control regulates dc bus voltage indirectly by shifting bidirectional droop that fundamentally is ac voltage controller. By using dc voltage square instead of dc voltage as the feedback, the nonlinearity caused by dc CPLs can be alleviated. To enable the BIC to support dc voltage independently in HMG with dc CPLs without sacrificing the functionalities of the uniform control, an improved uniform control strategy coordinating with P – V^2 droop in dc subgrids is proposed. With the improved uniform control, the dc bus voltage can be stabilized by BIC when the dc MVSs fail and the instability caused by dc CPLs can be significantly attenuated.