Weak-Grid Tolerant Control Methods for Voltage-Source Converters

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School of Electrical Engineering | Doctoral thesis (article-based) | Defence date: 2021-06-22
Degree programme
66 + app. 68
Aalto University publication series DOCTORAL DISSERTATIONS, 71/2021
This thesis deals with weak-grid tolerant control methods for voltage-source converters. The converters suffer from stability problems when connected to weak grids. A grid is categorized as weak if it has a large impedance behind the point of common coupling. In this thesis, two different control methods, state-space current control and power-synchronization control, are studied focusing on weak grids. In the state-space current control, the converter is considered to be equipped with an LCL filter and connected to an inductive grid. The current controller is directly designed in the discrete-time domain. Either the converter or grid current is measured and other states needed by the current controller are estimated using an observer. The closed-form expressions of the discrete-time model are used to design and analyze the current controllers and state observers. The control design is based on the direct pole-placement method with the radial projection for the resonance damping of the LCL filter. The current controller provides good dynamic performance and robust operation against grid strength variations. Furthermore, the controller enables converter operation in a wide range of sampling frequencies. For unbalanced grid operations, the state-space controller is extended to double-frequency current control, during which both positive and negative sequences of the current can be simultaneously controlled. In the power-synchronization control, the converter is considered to be equipped with an L filter and the controller is designed in the continuous-time domain. The power-synchronization control allows robust stability irrespective of the grid strength and of the operating point. Robustness is obtained by analytic gain selections, whereby adequate stability margins are obtained. An enhancement of the power-synchronization control is realized by adding a reference feedforward of the active power, by which a fast dynamic performance can be achieved without step-response ringing and overshoot. Both the state-space current control and power-synchronization control are experimentally evaluated using a 12.5-kVA 50-Hz grid-connected converter. The control methods can be applied in industrial applications, for example, in solar PV and wind turbine systems, as well as the active front-ends of motor drives.
Defence is held on 22.6.2021 12:15 – 15:15 https://aalto.zoom.us/j/61283640166
Supervising professor
Hinkkanen, Marko, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland
Thesis advisor
Kukkola, Jarno, Dr., Aalto University, Department of Electrical Engineering and Automation, Finland
disturbance observer, double-frequency controller, full-order observer, grid-connected converter, integrator, L filter, LCL filter, power-synchronization control, reduced-order observer
Other note
  • [Publication 1]: F. M. M. Rahman, U. Riaz, J. Kukkola, M. Routimo, and M. Hinkkanen. Observer-based current control for converters with an LCL filter: Robust design for weak grids. In Proc. IEEE International Symposium on Sensorless Control for Electrical Drives (SLED), Helsinki, Finland, pp. 36-41, September 2018.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201812105964
    DOI: 10.1109/SLED.2018.8486047 View at publisher
  • [Publication 2]: F. M. M. Rahman, V. Pirsto, J. Kukkola, M. Routimo, and M. Hinkkanen. State-space control for LCL filters: Converter versus grid current measurement. IEEE Transactions on Industry Applications, vol. 56, no. 6, pp. 6608-6618, November-December 2020
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202008214808
    DOI: 10.1109/TIA.2020.3016915 View at publisher
  • [Publication 3]: F. M. M. Rahman, V. Pirsto, J. Kukkola, M. Hinkkanen, D. Pérez- Estévez, and J. Doval-Gandoy. Equivalence of the integrator-based and disturbance-observer-based state-space current controllers for grid converters. IEEE Transactions on Industrial Electronics, vol. 68, no. 6, pp. 4966-4976, June 2021.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202106037207
    DOI: 10.1109/TIE.2020.2988194 View at publisher
  • [Publication 4]: F. M. M. Rahman, J. Kukkola, V. Pirsto, M. Routimo, and M. Hinkkanen. Observers for discrete-time current control of converters equipped with an LCL filter. In Proc. IEEE Energy Conversion Congress and Exposition (ECCE), Detroit, MI, USA, pp. 2884-2891, October 2020.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202011066320
    DOI: 10.1109/ECCE44975.2020.9235777 View at publisher
  • [Publication 5]: L. Harnefors, M. Hinkkanen, U. Riaz, F. M. M. Rahman, and L. Zhang. Robust analytic design of power-synchronization control. IEEE Transactions on Industrial Electronics, vol. 66, no. 8, pp. 5810-5819, August.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-201812106189
    DOI: 10.1109/TIE.2018.2874584 View at publisher
  • [Publication 6]: L. Harnefors, F. M. M. Rahman, M. Hinkkanen, and M. Routimo. Reference-feedforward power-synchronization control. IEEE Transactions on Power Electronics, vol. 35, no. 9, pp. 8878-8881, September.
    Full text in Acris/Aaltodoc: http://urn.fi/URN:NBN:fi:aalto-202002282302
    DOI: 10.1109/TPEL.2020.2970991 View at publisher