Comparison of converter control schemes for weak grids
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Journal Title
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Sähkötekniikan korkeakoulu |
Master's thesis
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Authors
Date
2018-05-14
Department
Major/Subject
Electrical Power and Energy Engineering
Mcode
ELEC 3024
Degree programme
AEE - Master’s Programme in Automation and Electrical Engineering (TS2013)
Language
en
Pages
62+7
Series
Abstract
Voltage source converter is used in power systems for connecting a renewable energy source to an AC grid or as an active front-end rectifiers in AC motor drives. When the conventional control methods (vector control and power-angle control) are used to control a converter that is connected to a weak grid, the performance of the resulting system is degraded. High power demands degrade the quality of the transferred power and can even make the system unstable. A relatively new control scheme known as power-synchronization control is introduced to rectify the problem. The scheme keeps the system stable under weak-grid conditions but the damping for strong grids is compromised. The scheme needs to switch to a back-up vector control during fault conditions. This violates the initial motivation of developing a new control scheme. This thesis identifies the root causes of the stability problems in vector control when connected to weak grids. The state-of-the-art vector control scheme is used as a starting point. The role of the phase-locked loop and AC-voltage controller in the stability of the system is explored. Two novel modifications in the vector control scheme are presented. The converter-voltage reference or the measured converter voltage is used as a feedback to modify the active and reactive power references. The modifications ensure improved performance of vector control for weak grids. Better damping and faster dynamic response are obtained as compared to power-synchronization control and conventional vector control. The benchmark performance of the standard vector control for strong grids is not compromised by these modifications. The dynamic performance and robustness of the control schemes are tested and compared using time domain simulations.Description
Supervisor
Hinkkanen, MarkoThesis advisor
Rahman, F. M. MahafugurKeywords
weak grid, vector control, power-synchronization control, grid-connected converter, converter-voltage feedback, phase-locked loop