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

Abstract

The paper presents a new approach to positive and negative sequence current vector control of a grid connected three-phase three-wire power electronic converter operating under grid voltage imbalance conditions. The concept utilizes representation of unbalanced converter current in the new coordinates frame in which the current vector components are constant. The nonlinear trigonometric transformation of twodimensional current vector components from the stationary frame to the new frame is found on-line depending on the reference current asymmetry. The presented concept of new coordinates utilization allows implementation of proportionalintegral terms as current regulators without the use of resonant terms and without the use of the measured current symmetrical sequences decomposition. The paper presents the theoretical approach, simulation results, as well as laboratory tests results.

Amplitude-Phase Detection for Power Converters Tied to Unbalanced Grids with Large X/R Ratios

“…The accurate frequency detection should be delayed to the steady-state condition where the DDC component is negligible. Besides, ( 5) and (9) suggest that the voltages and currents during the grid transient encompass positive-, negative-, and zero-sequence components, as well as DDC components. Accordingly, the transient signal model for deriving the detection algorithm should also include the positive sequence component X + , negative sequence component X − , zero sequence component X 0 , and DDC component X dc .…”

“…Among them, fast and accurate amplitude-phase detection is the basis for the subsequent works [4], and is crucial for highperformance control of converters [5], [6]. Indeed, with the wide application of converters, the grid code requires grid-tied converters to equip with fault ride-through capabilities [7]- [9], making it necessary to study effective amplitude-phase detection methods when considering DDC components associated with large grid X/R ratios. However, the majority of existing detection methods for the application scenario of converter control and protection assume that the amplitude and phase of the grid voltage and current can change instantaneously during large disturbances, and that the transient process is negligible.…”