Weak-Grid Tolerant Control Methods for Voltage-Source Converters

F. M. Mahafugur Rahman

Research output: ThesisDoctoral ThesisCollection of Articles


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.
Translated title of the contributionWeak-Grid Tolerant Control Methods for Voltage-Source Converters
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Hinkkanen, Marko, Supervising Professor
  • Kukkola, Jarno, Thesis Advisor
Print ISBNs978-952-64-0386-1
Electronic ISBNs978-952-64-0387-8
Publication statusPublished - 2021
MoE publication typeG5 Doctoral dissertation (article)


  • disturbance observer
  • double-frequency controller
  • full-order observer
  • grid-connected converter
  • integrator
  • L filter
  • LCL filter
  • power-synchronization control
  • reduced-order observer


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