Coupled Current Tracking Capability and Stability Analyses of 3Φ3W LCL Converter With Decoupled Control and Variable Filter Inductances

Abstract

In a three-phase three-wire ( 3Φ3W) converter, the average common-mode voltage between the neutral point of its ac source and the dc-source positive or negative rail over one switching cycle can be proved to be an arbitrary value. Therefore, 3Φ3W LCL (a filter that is composed of inductors, capacitors, and inductors) converters can be decoupled and controlled phase by phase in a natural frame, which results in equivalent three single-phase controls. With the single-phase control, inductance, dc-link voltage, and grid-voltage variations can be taken into account readily, yielding low-total harmonic distortion (THD) currents under distorted and unbalanced grid voltages. However, the 3Φ operations are still coupled each other and, moreover, the 3Φ filter inductances are varying with their currents simultaneously. This article presents system modeling of the decoupled digital controlled LCL converter from which coupled current tracking capability and stability of the system are investigated. Comparison between coupled and decoupled current tracking capability and stability has been verified with pole-zero plot and step response. The tracking performances of a 10 kVA with two types of feedback signals are compared and discussed. Moreover, simulated and experimental results of a 20-kVA 3Φ3W LCL grid-connected converter with wide filter inductance variation and distorted grid voltages have verified the analyses and discussions.