2012 Synthesis of Active Damping for Grid-Connected Inverters with an LCL filter

Looking at these literatures about AD strategies of higher-order filter, systematical synthesis method of AD has not been studied. The current study is scattered to propose some effective strategies, their relationships are isolated,
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which stayed in exploration and finding stage. Literature [18] introduces an analysis method of AD to unify AD strategies of literatures [9] and [10], and providing a good research idea. Unfortunately, the literature [18] only fixes feedback variable on the state of filter capacitor, thus limiting the breadth of the paper, so the novel AD structure was not discovered.
active damping (AD) is repeatedly reported in some literature [9-15], but the LCL filter with AD and grid-side inductor current control is very rare in practical applications. Therefore, in order to popularize LCL filter with AD in the large-scale applications, there is still a long way to go.
Synthesis of Active Damping for Grid-Connected Inverters with an LCL filter
Huafeng Xiao, Xiaohui Qu
School of Electrical Engineering , China Xiaohf@
I. INTRODUCTION
Voltage source inverter is an important interface with the grid for renewable energy, which is widely installed in distributed generation systems [1] (such as photovoltaic power generation systems, direct-drive wind power generation system, fuel cell system, etc.), AC micro-grid [2], and DC micro-grid [3], et al.
In order to reduce and limit their impact on the grid quality, international standards such as IEEE Std 929-2000, UL1741, etc., do a strict specification limits [4, 5], including total harmonic content and single harmonic content. Considering the realization technology of grid-connected inverter, the switching frequency harmonic current suppression is the most difficult targets, involving in-grid filter selection. In-grid filter structure has two kinds of topologies: L, LC, and LCL, and their filtering performance and control features are discussed in the literature [6]. At present, the L-filter is commonly used in the small power grid-connected inverter; the LC-type filter or LCL type filter with a damping resistor are used in middle and high power grid-connected inverter [7]; Although the LCL filter with
Shaojun Xie, Jinming Xu
College of Automation Engineering Nanjing University of Aeronautics and Astronautics
Nanjing, China
Abstract—LCL filter characterized with high harmonic current attenuating performance and low cost are suitable for medium and high power grid-connected inverters. But, the resonance poles endanger the safety of operation, and the active damping (AD) is an effective means to increase the system security. This paper focus on the theoretical exercise of possible strategies to damp the resonance of grid-connected inverter with LCL-filter, the systematical synthesis method for AD is presented, and a unified analytical model for AD is proposed. The AD structures are listed based on single-state variable and single-compensator, and the effective AD strategies are distinguished by root locus. And then, the published AD approaches can be involved in derived AD structures, and the novel AD approach is successfully proposed. The characteristics and performance of the proposed AD approach is analyzed, and its validity is verified by experiment results to demonstrate the success of the theoretical exercise of active damping strategies.
Compared to the L-type filter, LCL type filter bring one pair of resonance poles into the grid-connected inverter system, its damping factor as zero and the oscillation frequency is higher. Obviously, the pair of resonance poles will hurt grid-connected inverter control system stability [17]. AD is the idea that through increasing conjugate zeros to cancel the conjugate poles or adding an independent zero to attract the conjugate poles to stable region with a certain safety margin [14]. Considering the roles of damping resistance in system transfer function of the LCL-type gridconnected inverter, the actual resistance can be transferred to the controller by signal flow graph transform [8]. Further, the concept “virtual resistor” and four kinds of virtual resistor implementations are proposed by Dahono [8]. Another way to increase the system damping is by increasing the filter state variable feedback. An AD approach is proposed by using lead - lag module of capacitor voltage feedback of the LCL filter [9]; Literature [10] proposes an AD approach with proportional feedback of the filter capacitor current; In literature [11], the partial feedback of grid-side inductor current and capacitor current downs the third-order LCL to first-order L, the resonance poles are cancelled in theory; The simplified first-order system is also achieved by feedback converter-side inductor current with the admittance compensator plus quasi - resonance proportion controller [12]; The AD is achieved through feedback converter-side inductor voltage with integral module in literature [13]; In above described ways, the sensor is generally required, the literatures [14] and [15] study the sensorless algorithm of AD to reduce system cost and improve reliability.