Design and Analysis of Robust Active Damping for LCL Filters Using Digital Notch Filters

Abstract

Resonant poles of LCL filters may challenge the entire system stability especially in digital-controlled Pulse Width Modulation (PWM) inverters. In order to tackle the resonance issues, many active damping solutions have been reported. For instance, a notch filter can be employed to damp the resonance, where the notch frequency should be aligned exactly to the resonant frequency of the LCL filter. However, parameter variations of the LCL filter as well as the time delay appearing in digital control systems will induce resonance drifting, and thus break this alignment, possibly deteriorating the original damping. In this paper, the effectiveness of the notch filter based active damping is firstly explored, considering the drifts of the resonant frequency. It is revealed that, when the resonant frequency drifts away from its nominal value, the phase lead or lag introduced by the notch filter may make itself fail to damp the resonance. Specifically, the phase lag can make the current control stable despite of the resonant frequency drifting, when the grid current is fed back. In contrast, in the case of an inverter current feedback control, the influence of the phase lead or lag on the active damping is dependent on the actual resonant frequency. Accordingly, in this paper, the notch frequency is designed away from the nominal resonant frequency to tolerate the resonance drifting, being the proposed robust active damping. Simulations and experiments performed on a 2.2-kW three-phase grid-connected PWM inverter verify the effectiveness of the proposed design for robust active damping using digital notch filters.

Implementation of a Smart Power Conditioning System for Energy Storage System with a Novel Seamless Transfer Strategy

“…Therefore, damping methods are needed to inhibit this occurrence [34][35][36][37]. The damping method can be classified into two types.…”

LCL-resonance damping strategies for grid-connected inverters with LCL filters: a comprehensive review

“…The notch filter damps the resonance through direct magnitude adjustment as long as it has an extremely low gain at x res . Filter-based AD has been broadly discussed in [11][12][13][14][15][16]. 4 Overview of resonance damping strategies with multi-state feedback 4.1 Multi-state feedback control based on zeroplacement…”

“…The study in [13] tried to design the notch frequency away from the default resonance frequency so that notchfilter AD worked stably when the grid impedance varied within a certain range. In order to maintain a high robustness in the case of widely varying grid impedance, [14] proposed to adjust the notch frequency by estimating the grid impedance on-or off-line.…”

“…Given that i C1 has an inherent relation with u C1 , i C1 also has low-order current harmonics related to the grid voltage harmonics. Furthermore, according to the second equation in (13), i L1 also relates closely to the grid voltage harmonics. A similar argument applies to u L1 .…”

“…Filter-based AD improves stability by adding a digital filter next to the current controller [11][12][13][14][15][16]. For feedbackbased AD, the state used for feedback can be the capacitor current [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32], the capacitor voltage [33][34][35][36], the inverterside current [37], the grid current [38][39][40][41] or a combination of multiple states [42][43][44][45][46][47][48][49][50].…”