A Unified Active Damping for Grid and Converter Current Feedback in Active Front End Converters

Abstract

The Active Front End (AFE) is interconnected to the grid with an LCL filter to reduce the switching harmonics. Possible resonances induced by the LCL filter can be damped using Active Damping (AD). In AFE applications, the current sensor can be placed on either the grid-side or the converter-side, resulting in different frequency responses. This paper presents a novel active damping approach that is universally applicable to all active front end converters regardless of whether the current sensor is placed on the grid side or the converter side. At the heart of the proposed approach is a practically implementable fourth-order filter that dampens the resonance whilst being capable of mitigating the challenges associated with both grid current feedback and converter current feedback. These include amplifications associated with derivatives in active damping term with grid current feedback and high susceptibility to instability with converter current feedback due to digital delay. The proposed active damping is thoroughly investigated in terms of practical implementation issues such as different LCL filter parameters, different sampling methods, grid impedance, and digital delay. The approach is experimentally validated on motor drives consisting of 2 two-level three-phase converters interfaced with a dSPACE rapid control prototyping system across a wide range of resonant frequencies and PWM sampling methods. Stability analysis is performed on both current control methods to validate that active damping assures stable operation for a wide range of resonant frequencies and grid impedance.