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| Common-Mode Resonant Current Suppression for Back-Connected LCL Photovoltaic Inverter Using Adaptive Third Harmonic Injection |
| Zhang Hongliang1, Zhang Zicheng1, Chen Jie2, Xing Xiangyang1 |
1. School of Control Science and Engineering Shandong University Jinan 250061 China;
2. Huawei Digital Power Technologies Co. Ltd Shanghai 201206 China |
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Abstract Due to the parasitic capacitance between the photovoltaic array and the ground, the leakage current is easily generated in non-isolated photovoltaic grid-connected system, which will reduce the equipment life and endanger personal safety. Therefore, the modified LCL filter with neutral-point back connection is often adopted for non-isolated system to provide a low-impedance bypass path and suppress the leakage current. However, the LCL filter modification brings potential risk of common-mode (CM) resonance in CM loop which will cause inverter-side current oscillation, leakage current increment, and system instability. Passive and active damping methods have been proposed to overcome the drawback of CM current but will bring extra power losses or complex controller design tasks. To address the CM resonance issue, this paper proposes an adaptive third harmonic injection algorithm based on the analysis of different modulation strategy influences on the CM resonant current.
Firstly, the mathematical model and generation mechanism of CM resonant current are introduced briefly and the CM voltage spectrums of space vector pulse width modulation (SVPWM) method, saddle pulse width modulation (SAPWM) method and third harmonic injection pulse width modulation (THIPWM) method are compared, which shows the advantage of THIPWM method on CM resonant current suppression. Secondly, the exact available range of the third-harmonic injection coefficient is derived to ensure the correctness of modulation and determine the adjustable range of the coefficient. Thirdly, according to DC voltage and modulation index, an adaptive law for the injection coefficient is proposed to improve the performance of the grid-connected inverter. Fourthly, the third-harmonic injection implementation method for current closed-loop control is derived in order to facilitate engineering application and reduce the calculation burden of the controller.
To verify the effectiveness of the proposed method, simulation and experiment are performed on Matlab/Simulink platform and on a 20kW inverter prototype. In simulation, transient waveforms of adaptive is firstly given to verify the adaptive law of injection coefficient. Then, waveforms of the CM current and the bridge currents as well as their FFT analysis results are given. These waveforms and analysis data validate that the adaptive THIPWM method can efficiently suppress the CM resonant current and reduce low-frequency CM currents. As for experiments, the waveforms of the bridge output voltage, the CM current, the bridge currents and relevant FFT analysis results under different DC voltages and different power factors are given in detail. These results further show that the proposed method is effective under different conditions. Then, grid current waveforms of the adaptive THIPWM method under different DC voltages are provided. The total harmonic distortions (THD) of grid current under three modulation methods are also given to validate the performance improvement of the proposed method. Finally, the performance of neutral-point balance control is verified.
The following conclusions can be drawn from the simulation and experimental analyses: ①The CM resonant current is effectively suppressed and the low-frequency CM currents are reduced by the proposed method. ②Smooth transient performance of the adaptive THIPWM method can be achieved. ③Compared with SVPWM and SAPWM, the grid current quality is improved when applying the adaptive THIPWM method. ④The performance improvement including CM resonant current suppression and grid current THD reducing can be achieved under different operating conditions.
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Received: 11 May 2022
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2023, Vol. 38 
