PROJECT TITLE: Enhancement of Power Quality in DC-grid connected Adjustable Speed Drive Systems
PhD period: 2018.12.01 – 2021.11.30.
Section: Power Electronic Systems
Research Programme: E-Mobility and Industrial Drives
Supervisor: Frede Blaabjerg,
Co-Supervisors: Haoran Wang, Pooya Davari, Dinesh Kumar (Danfoss A/S)
Collaborator: Danfoss A/S.
The increasing growth of global electricity consumption and the desire to reduce CO2 footprint have intensified the penetration of power electronics technologies into many applications. Electric motor-driven systems with more than 40% of all electrical energy consumption have identified as one of the major source of electricity use especially in industry sector to become more energy efficient -. Introducing adjustable speed drive (ASD) utilizing power electronics technology leads to more energy efficient motor drive systems. Thereby, in the past decade drivetrain systems of many applications such as marine, electric-aircraft and industrial plants are enhanced by employing ASD technology.
Recently, in order to further improve the drivetrain system efficiency, DC grid supply is adopted instead of conventional AC supply system -. As it is shown in Fig. 1, in such system multiple ASD units are connected to a common DC-bus supplied from a centralized rectifier. Due to reduction of conversion stages and number of conductors DC grid connected systems can be more efficient, compact, reliable and cost-effective comparing to their AC supplied counterpart. Notably, the centralized rectifier is sized according to the total load power demand and is connected to the AC supply.
However, when multiple of ASDs are adopted in a drivetrain system interaction among ASD units may impair the power integrity of the entire system leading to their undesirable and unpredictable operation. These interactions at DC-link as a result of excessive harmonic generation , , resonances , , results in unstable operation and possible failure of the fragile components such as capacitors ,  and consequently multiple system downtime and maintenance requirement. Moreover, as Fig. 2 shows, the front-end rectifier can be a conventional three-phase diode rectifier or more advanced type such as active front-end (AFE) . Thereby, the behavior of the rectifier from generated harmonics and its output impedance point of view under normal and abnormal grid conditions should be considered as well to maintain the overall system performance.
Taking into account the above discussion, there is a research gap in systematic analysis and design of DC grid connected drivetrain systems to obtain suitable power quality and reliability. This PhD project aims to fulfill power integrity of DC-fed drivetrain systems through ensuring the delivery of the voltage and current from source to its destination in a desirable manner. To fill this gap, this study will develop power converters frequency harmonic models to characterize their behavior under steady-state operation and identify adverse operating points. Mitigation methods in damping generated switching harmonics, interaction among converters and improving lifetime span of installed energy storage capacitors will be proposed. Finally, the developed models will be extended to a complete drivetrain system through aggregation techniques in order to provide a systematical methodology in characterizing and mitigation of power converters interactions. During the this project activity, a test setup capable of testing parallel connected converters will be prepared and utilized at different stages of the project to validate the developed models, analysis and mitigation techniques.
Publications in journals and conference papers may be found at VBN.