Abstract
This article proposes a control system of a three-phase grid-connected power converter, operating in the rectifier mode, which achieves the unity power factor target, i.e., unbalanced and distorted current instead of sinusoidal one for grid voltage imbalance and harmonics. Such a target is known from active power filters control, but it has not been used for power supply devices yet. Thanks to that, the three-phase converter is seen as resistive load by the utility grid, and active power is consumed with minimal possible rms current, keeping current asymmetry corresponding to the voltage asymmetry during unbalanced dips. Intentional introduction of current harmonics is not trivial from the point of view of both reference signals calculation and control system structure, because in the classical approach, it would require resonant terms in controller structure. Additionally, grid voltage asymmetry introduces another oscillating component, which imposes other resonant terms in the controller. The transformation presented in this article allows to achieve desirable current harmonics and asymmetry with the use of two proportional-integral controllers, one in each controlled axis. This article presents theoretical principles of the new transformation, control system structure as well as simulation and experimental tests of a three-phase converter utilizing this idea.
Highlights
THREE-PHASE grid-connected power converters are widely used for the purpose of current rectification combined with power quality improvement functions
This paper presents a control system of a three-phase gridconnected power converter operating in rectifier mode, under grid voltage harmonics and asymmetry, which allows to achieve the unity power factor, so the converter is seen as a balanced resistive load by the grid
Current components in the new d’q’ reference frame are slightly distorted, which is caused by some factors omitted in simulation like measuring noise, dead-time (2.5 μs) applied in the laboratory converter, imprecise grid voltage feedforward due to delays caused by signal processing (100 μs) or grid filter inductor resistance
Summary
THREE-PHASE grid-connected power converters are widely used for the purpose of current rectification combined with power quality improvement functions. This paper presents a control system of a three-phase gridconnected power converter (see Fig. 1) operating in rectifier mode, under grid voltage harmonics and asymmetry, which allows to achieve the unity power factor, so the converter is seen as a balanced resistive load by the grid. This target is achieved with two simple PI controllers, which is possible thanks to the new frame transformation. A derivative-invariant inverse transformation, providing an appropriate control signal, was proposed
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