Leakage Current Suppression Method of Cascaded H-bridge Photovoltaic Inverter Based on Model Predictive Control

Abstract

The invention discloses a method for suppressing the leakage current of cascaded H-bridge photovoltaic inverters based on model predictive control. The method includes: writing all 256 switch states of four H-bridge cascaded inverters and calculating the corresponding photovoltaic The value of the sum Vc of the battery parasitic capacitance voltage; select the switch state and form a switch state combination; calculate the output voltage Vk of the inverter AC side; detect the voltage Vg(k) and the grid current i(k) of the grid at time k online, and Calculate the value i(k+1) of the grid current at time k+1; phase-lock the grid voltage Vg(k) obtained by online detection to obtain the phase θ, and calculate the current given value i at time k+1 from θ *(k+1), and substitute the current calculation value i(k+1) at k+1 into the evaluation function h; select the switch state combination corresponding to the smallest evaluation function h, and drive the switch through the drive circuit as the output Tube. This method can not only effectively suppress the leakage current of the system, but also is easy to expand, and is suitable for single-phase non-isolated cascaded H-bridge photovoltaic grid-connected inverters with an even number of H-bridges.

Description

technical field [0001] The invention relates to a method for suppressing the leakage current of a cascaded H-bridge photovoltaic inverter based on model predictive control; the method is applicable to the field of non-isolated photovoltaic grid connection. Background technique [0002] Compared with the traditional inverter, the cascaded H-bridge multilevel inverter has the advantages of small grid current harmonics, low switching frequency, small filter size and easy modularization, so it has attracted more and more attention from scholars. . [0003] In addition, the DC side of each module of the cascaded H-bridge multi-level inverter can be independently powered by a photovoltaic panel, making it possible to make independent MPPT control, so the cascaded H-bridge multi-level topology is especially suitable for photovoltaic grid-connected inverters. Transformer. [0004] Due to the modular structure of cascaded H-bridge inverters, a certain number of cascaded inverters c...

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Problems solved by technology

Therefore, the method of suppressing the leakage current of a single H-bridge cannot be directly applied to the suppression of the leakage current of the cascaded H-bridge topology. There is a slight mismatch between suppression methods

[0008] For this reason, scholars have made a lot of efforts and attempts in the leakage current suppression of cascaded H-bridge inverters, such as the 2016 IEEE document "Single Phase Cascaded H5 Inverter with Leakage Current Elimination for Transformerless Photovoltaic System" ("Non-isolated cascaded Analysis of Common Mode Current Characteristics of H5 Photovoltaic Inverter"—Proceedings of the 2016 IEEE Energy Society Plenary Session) proposed a modulation strategy for cascaded H5 topology to suppress leakage current, although the leakage current was suppressed to a certain extent, but As the number of modules increases, the modulation strategy will be very complicated, which is not conducive to system expansion and modular design

In addition, the proposed cascaded H5 topology has increased costs and losses compared to the H4 topology

[0009] 2014 IEEE document "Analysis and Suppression of Leakage Current in Cascaded-Multilevel–Inverter-Based PV Systems," Y.Zhou and H.Li, "IEEE Trans. Power Electron.", 2014, 29(10), 5265–5277 (" Leakage Current Analysis and Suppression of Cascaded Multilevel Photovoltaic Inverters", "IEEE Journal-Journal of Power Electronics", 2014, Vol. 29, No. 10, pp. 5265-5277) proposed to add common mode on the DC side and the AC side respectively The filter suppresses the leakage current, but its switching frequency is set at 10kHz, which is not in line with the original intention of using a cascode topology to reduce the switching frequency

2013 IEEE document "A Modulation Strategy for Single-phase HB-CMI to Reduce Leakage Ground Current in Transformer-less PVApplications" ("Cascaded multi-level photovoltaic inverter leakage current analysis and suppression" - 2013 IEEE Energy Society The modulation strategy proposed by Proceedings of the Plenary Conference) makes the parasitic capacitor voltage change according to the power frequency step wave, but the modulation strategy is relatively complicated and not easy to expand the system

For example, adding common-mode filters on the DC side and AC side respectively, and adopting cascaded H5 or H6 topologies will undoubtedly increase the cost and loss of the system and reduce the power density of the inverter

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