Non-Cartesian Frame Transformation-Based Control of a Three-Phase Power Converter During Unbalanced Voltage Dip – Part II: Simulation And Experiment

Abstract

The paper presents a control method for the three-phase power converter operating under unbalanced grid voltage conditions. The method uses a new transformation to the non-Cartesian frame, which makes the controlled current vector components balanced in this frame even if originally the three-phase current is referenced as unbalanced. Furthermore, Park’s transformation makes the controlled variables constant, which allows to apply proportional–integral terms as current controllers independent of the required control target. Several control targets known from literature have been analyzed with regard to the required new transformation parameters, and the transformation parameters for all targets have been found. Simulation results are shown to prove the theoretical analysis, and the experimental test results are presented as practical validation of the proposed use of the non-Cartesian frame in control.

Control of a Three-Phase Power Converter Connected to Unbalanced Power Grid in a Non-Cartesian Oblique Frame

“…Park's transformation [23] allows representation of vector containing only a positive sequence component by constant values in a rotating frame. However, for the vector containing positive and negative sequence, and components of the vector, besides dc components, contain also double grid frequency oscillatory components, which require more sophisticated controllers than proportional-integral terms.…”

“…The presented theoretical tool converts the two-phase signal from the Cartesian frame to the other non-Cartesian frame, in which axes are mutually obliquely oriented. The detailed properties of the transformation and detailed relations between axes of the old and new frames as well as detailed relations between vectors components and their lengths are shown in [23]. The transformation factors were calculated using alpha/beta voltage components and a limited number of current targets can be obtained, whereas in this paper the transformation factors are calculated based on the reference current positive and negative sequence components to obtain almost any state of current (excluding a very narrow range of dead-zone).…”