Public defence in Electrical Power and Energy Engineering, M.Sc.(Tech.) Tuure Nurminen

The title of the thesis: Multifunctional control for robust grid-forming converters 

Thesis defender: Tuure Nurminen
Opponent: Prof. Mebtu Beza, Chalmers University of Technology, Sweden
Custos: Prof. Marko Hinkkanen, Aalto University School of Electrical Engineering

Renewable generation is being introduced to the electric grid at a rapid rate in an effort to decarbonize the grid. The majority of this generation will be interfaced by power electronic converters. It is apparent that these converters should also be able to support the stable operation of the grid, i.e., they should be grid-forming converters. Grid-forming converters have voltage-source characteristics, i.e. they aim to control voltage magnitude and frequency. In addition to voltage-source characteristics, a converter should still be capable of operating as a current source, e.g., for current limitation purposes. 

This thesis deals with control methods for grid-forming converters that are multifunctional. Multifunctional converters can switch between different control modes according to operational requirements, making them a flexible asset for the future grid. 

The thesis introduces a multifunctional control structure, utilizing feedback linearization and a disturbance observer, to provide a systematic approach to the design of the control methods. The thesis focuses on grid-forming control with control methods being proposed for different system models. Both L and LCL filter models are used. Control of grid-forming converters in unbalanced grid conditions is also considered. Additionally, the thesis proposes dual-voltage forming control for simultaneous AC- and DC-side voltage regulation. 

All the proposed methods incorporate a compatible current control mode for fault ride-through purposes. The control structure utilized in the thesis allows for the derivation of small-signal models with relatively simple closed-loop control dynamics. This makes it possible to relate the control gains to the pole locations with relative ease, which aids in gain selection. The disturbance observer incorporates synchronization and plays a pivotal role in making bumpless control mode switching possible. 

The developed control methods are experimentally validated using a 12.5-kVA converter. The experimental results show that the control methods achieve excellent dynamic performance irrespective of the grid inductance. They are also shown to be robust against grid inductance estimation errors, i.e. knowledge of the grid inductance is not required.

Key words: current control, current limitation, disturbance observer, feedback-linearization, grid-forming converter, LCL filter, L filter, multifunctional control, pole placement, state-feedback, voltage control, voltage-source converter

Thesis available for public display 7 days prior to the defence at Aaltodoc. 

Contact:
tuure.nurminen@aalto.fi