Supervisor:
Zhe Chen

Co-Supervisor:
Yanbo Wang

Assessment Committee:
Sanjay Chaudhary (Chair)
Antonio J. Marques, University of Beira Interior
Hasan Komurcugil, Eastern Mediterranean University

Moderator:
Sanjay Chaudhary

Abstract:

As the population continually grows in many countries, the demand for urban rail transportation such as railway, metro and tram are also increasing. It has intensely attracted attention to the energy consumption reduction of urban rail transit. Since most urban rail transits adopt DC traction power systems (DCTPS), renewable energy sources (RES) and energy storage systems (ESS) can easily access and improve energy saving. However, there are also some challenges to integrating the RESs into the DCTPS. 1) The RESs and ESS are easily subject to voltage fluctuation caused by frequent tractive and braking operations of trains. Thus, the way of connecting RES, ESS and DCTPS, and their operation strategies should be investigated. 2) The stability analysis is extremely important due to the high penetration of power electronics devices when RES and ESS are integrated into DCTPS. 3) A new damping control approach should be proposed to tackle the instability issue of the whole system. Therefore, the objectives of this thesis are to investigate the operation, stability analysis and oscillation suppression methods of DCTPS integrating RES and ESS. To do this, analyzing current topologies and methods of DCTPS integrating RESs and ESS in state-of-the-art literature has been carried out. It is found that most RESs and ESS are allocated scattered along the rail line and the energy saving effect is limited. In this project, RESs and ESS are combined as a microgrid to integrate with DCTPS through a multi-port converter such as triple active bridge (TAB) converter. The operation strategies of the proposed system are analyzed considering the intermittent feature of RESs and the regenerative braking energy (RBE) of trains. Furthermore, the stability of two-port DC/DC converter is widely investigated by small signal methods or nonlinear analysis methods. Also, various passive damping methods and active damping methods have been put forward to tackle instability issues. But the stability of multi-port converter is not explored. This thesis develops the extra element theorem to obtain the input and output impedance of TAB converter based on its small signal model. The impact of different types of loads, power-flowing directions and control strategies on the output impedance of TAB converter is also analyzed. Then, impedance-based methods are adopted to analyze the stability of the TAB converter-based system, especially when it connects with constant power loads (CPLs). Finally, an active damping control is developed to reshape the output impedance of the TAB converter, thereby eliminating the oscillation problem. This active damping control contains four plans corresponding to four different positions of virtual impedance. The virtual impedance damping control is verified by simulation and laboratory platforms. The main contributions of this thesis are: 1) Propose a new structure and operation strategies of DCTPS connecting with RESs and ESS through a TAB converter. 2) The extra element theorem is used for deriving the terminal impedance of the TAB converter. The impedance-based method is used to explore the stability of the TAB converter-based system. 3) A new active damping control is developed to tackle the potential oscillation of the TAB converter with CPLs. Overall, the results of this thesis provide a promising insight into the solution of impedance derivation and active damping control of the multi-port converter-based systems.