Stability Analysis and Improvement of the Capacitor Current Active Damping of the LCL Filters in Grid-Connected Applications

Jinming Xu,Shaojun Xie,Binfeng Zhang 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.4

For grid-connected LCL-filtered inverters, dual-loop current control with an inner-loop active damping (AD) based on capacitor current feedback is generally used for the sake of current quality. However, existing studies on capacitor current feedback AD with a control delay do not reveal the mathematical relation among the dual-loop stability, capacitor current feedback factor, delay time and LCL parameters. The robustness was not investigated through mathematical derivations. Thus, this paper aims to provide a systematic study of dual-loop current control in a digitally-controlled inverter. At first, the stable region of the inner-loop AD is derived. Then, the dual-loop stability and robustness are analyzed by mathematical derivations when the inner-loop AD is stable and unstable. Robust design principles for the inner-loop AD feedback factor and the outer-loop current controller are derived. Most importantly, ensuring the stability of the inner-loop AD is critical for achieving high robustness against a large grid impedance. Then, several improved approaches are proposed and synthesized. The limitations and benefits of all of the approaches are identified to help engineers apply capacitor current feedback AD in practice.

Systematic Current Control Strategy with Pole Assignment for Grid-Connected LCL-FilteredInverters

Jinming Xu,Shaojun Xie,Ting Tang 전력전자학회 2013 JOURNAL OF POWER ELECTRONICS Vol.13 No.3

For grid-connected LCL-filtered inverters, resonance yields instability and low bandwidth. As a result, careful designs are required. This paper presents a systematic current control structure, where pole assignment consisting of one or more feedbacks is the inner loop, and the outer loop is the direct grid current control. Several other issues are discussed, such as the inner-loop feedback choices, pole-assignment algorithms, robustness and harmonic rejection. Generally, this kind of strategy has three different types according to the inner-loop feedback choices. Among them, a novel pole-assignment algorithm has been proposed, where the inner control maintains four freely-assigned poles which are just two pairs of conjugated poles located at the fundamental and resonance frequencies separately. It has been found that with the different types, the steady-state and dynamic performances are quite different. Finally, simulations and experiments have been provided to verify the control and design of the proposed methods.

Research on Stability of Grid-Connected LCL-Filtered Inverter with Capacitor Current Feedback Active Damping Control

Jinming Xu,Shaojun Xie,Jiarong Kan,Binfeng Zhang 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

For grid-connected LCL-filtered inverter, the inherent resonance of LCL filter endangers the stability so that the inverter is hard to be controlled. To solve the resonance, the capacitor-current-feedback active damping (AD) is welcomed. However, the digital control delay was not seriously considered in the previous studies. The research on the stability with such control considering the digital control delay is lacking. Thus, this paper aims to provide a deep analysis of the capacitor-current-feedback AD in a digitally-controlled inverter. A stable region is derived. It is proved that the stable region for the capacitor-current- feedback AD is closely related to the sampling and computation delay, the control frequency, the resonance frequency and the capacitor-current-feedback factor. Several suitable approaches are then proposed to improve the performance with the capacitor-current-feedback AD. The experimental results verify the presented research.

An Improved Inverter-Side Current Feedback Control for Grid-Connected Inverters with LCL Filters

Jinming Xu,Shaojun Xie,Jiarong Kan,Lin Ji 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6

The grid-connected LCL-filtered inverter is widely used in distributed power generation systems based on renewable energies. Single inverter-side current feedback control only needs to sample one current to realize both current control and inverter protection. However, with single inverter-side current feedback control, the closedloop system is under-damped or even unstable due to the digital control delay. In this paper, an active damping control based on only inverter-side current feedback is proposed. With the novel control, the resonance in the digitally-controlled LCL-filtered inverter is successfully solved while only inverter-side current is sampled. Comparisons of the proposed and typical single inverterside current feedback control have verified the improvement of the proposed control. Thus, the proposed control has good application prospect in grid-connected LCL-filtered inverters.

Current Harmonics Rejection and Improvement of Inverter-Side Current Control for the LCL Filters in Grid-Connected Applications

Jinming Xu,Shaojun Xie,Binfeng Zhang 전력전자학회 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.6

For grid-connected LCL-filtered inverters, the inverter-side current can be used as the control object with one current sensor for both LCL resonance damping and over-current protection, while the grid-voltage feedforward or harmonic resonant compensator is used for suppressing low-order grid current harmonics. However, it was found that the grid current harmonics were high and often beyond the standard limitations with this control. The limitations of the inverter-side current control in suppressing low-order grid current harmonics are analyzed through inverter output impedance modeling. No matter which compensator is used, the maximum magnitudes of the inverter output impedance at lower frequencies are closely related to the LCL parameters and are decreased by increasing the control delay. Then, to improve the grid current quality without complicating the control or design, this study proposes designing the filter capacitance considering the current harmonic constraint and using a PWM mode with a short control delay. Test results have confirmed the limitation and verified the performance of the improved approaches.

Stability Analysis and Improvement of the Capacitor Current Active Damping of the LCL Filters in Grid-Connected Applications

Xu, Jinming,Xie, Shaojun,Zhang, Binfeng The Korean Institute of Power Electronics 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.4

For grid-connected LCL-filtered inverters, dual-loop current control with an inner-loop active damping (AD) based on capacitor current feedback is generally used for the sake of current quality. However, existing studies on capacitor current feedback AD with a control delay do not reveal the mathematical relation among the dual-loop stability, capacitor current feedback factor, delay time and LCL parameters. The robustness was not investigated through mathematical derivations. Thus, this paper aims to provide a systematic study of dual-loop current control in a digitally-controlled inverter. At first, the stable region of the inner-loop AD is derived. Then, the dual-loop stability and robustness are analyzed by mathematical derivations when the inner-loop AD is stable and unstable. Robust design principles for the inner-loop AD feedback factor and the outer-loop current controller are derived. Most importantly, ensuring the stability of the inner-loop AD is critical for achieving high robustness against a large grid impedance. Then, several improved approaches are proposed and synthesized. The limitations and benefits of all of the approaches are identified to help engineers apply capacitor current feedback AD in practice.

Suppression of Harmonics and Instabilities of Single-Phase Inverters Caused by Delay-based Phase-Locked Loop in the Weak Grid

Jinming Xu,Shenyiyang Bian,Qiang Qian,Jiarong Kan,Shaojun Xie 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

Phase-locked loops (PLLs) are the key technique for inverters to be synchronized with the PCC voltage. Typically, in the single-phase applications, the delaybased PLL is being widely used. However, for nowadays applications where the inverter may be connected to the PCC with large grid impedance, the inverter performance can be endangered, and harmonics or even instabilities are aroused. Although there are some relevant literatures, the solutions for solving the adverse impacts of delay-PLL on the system performance are still not well documented. Therefore, this paper aims to study the limitations of delay-based PLL and provide an improved approach. Through the inverter output impedance modelling, the reason why the delay-based PLL causes the instability in the weak grid case has been clearly explained. Then, the way to solve the instability caused by the PLL has been discussed, and an improved delay-based PLL has been proposed to improve the weak grid compatibility of inverters. The single-phase inverter system can then work well when the grid impedance varies widely.

Current Harmonics Rejection and Improvement of Inverter-Side Current Control for the LCL Filters in Grid-Connected Applications

Xu, Jinming,Xie, Shaojun,Zhang, Binfeng The Korean Institute of Power Electronics 2017 JOURNAL OF POWER ELECTRONICS Vol.17 No.6

For grid-connected LCL-filtered inverters, the inverter-side current can be used as the control object with one current sensor for both LCL resonance damping and over-current protection, while the grid-voltage feedforward or harmonic resonant compensator is used for suppressing low-order grid current harmonics. However, it was found that the grid current harmonics were high and often beyond the standard limitations with this control. The limitations of the inverter-side current control in suppressing low-order grid current harmonics are analyzed through inverter output impedance modeling. No matter which compensator is used, the maximum magnitudes of the inverter output impedance at lower frequencies are closely related to the LCL parameters and are decreased by increasing the control delay. Then, to improve the grid current quality without complicating the control or design, this study proposes designing the filter capacitance considering the current harmonic constraint and using a PWM mode with a short control delay. Test results have confirmed the limitation and verified the performance of the improved approaches.

Systematic Current Control Strategy with Pole Assignment for Grid-Connected LCL-Filtered Inverters

Xu, Jinming,Xie, Shaojun,Tang, Ting The Korean Institute of Power Electronics 2013 JOURNAL OF POWER ELECTRONICS Vol.13 No.3

For grid-connected LCL-filtered inverters, resonance yields instability and low bandwidth. As a result, careful designs are required. This paper presents a systematic current control structure, where pole assignment consisting of one or more feedbacks is the inner loop, and the outer loop is the direct grid current control. Several other issues are discussed, such as the inner-loop feedback choices, pole-assignment algorithms, robustness and harmonic rejection. Generally, this kind of strategy has three different types according to the inner-loop feedback choices. Among them, a novel pole-assignment algorithm has been proposed, where the inner control maintains four freely-assigned poles which are just two pairs of conjugated poles located at the fundamental and resonance frequencies separately. It has been found that with the different types, the steady-state and dynamic performances are quite different. Finally, simulations and experiments have been provided to verify the control and design of the proposed methods.

EMP3 Overexpression in Primary Breast Carcinomas is not Associated with Epigenetic Aberrations

Wei Zhou,Jinming Yu,Zheng Jiang,Xingang Li,Fenghua Xu,Yanbing Liu,Peie Wen,Li Kong,Ming Hou,Jinming Yu 대한의학회 2009 Journal of Korean medical science Vol.24 No.1

Epithelial membrane protein 3 (EMP3) is a trans-membrane signaling molecule with important roles in the regulation of apoptosis, differentiation and invasion of cancer cells, but the detailed is largely still unknown. We analyzed the mRNA levels and methylation statuses of EMP3 in 63 primary breast carcinomas and assessed their correlations with clinicopathologic variables. The expression of EMP3 mRNA in primary breast carcinomas was significantly higher than the expression of 20 normal breast tissues (p<10-7). EMP3 overexpression in breast carcinomas was significantly related to histological grade III (p=3.9×10-7), lymph node metastasis (p= 0.003), and strong Her-2 expression (p=3.3×10-6). Hypermethylation frequencies of EMP3 were detected in 36.5% of breast carcinomas by methylation-specific polymerase chain reaction. However, no significant correlations were found between methylation status of EMP3 and mRNA expression levels as well as other clinical parameters. In conclusion, EMP3 may be a novel marker of tumor aggressiveness. Overexpression of EMP3 in primary breast carcinoma is not associated with DNA methylation.