Model prediction control based leakage current suppression method of cascaded H-bridge photovoltaic inverter

Abstract

The invention discloses a model prediction control based leakage current suppression method of a cascaded H-bridge photovoltaic inverter. The method comprises the following steps of writing 256 switching states of four H-bridge cascaded inverters, and calculating a value of voltage sum Vc of a corresponding photovoltaic battery parasitic capacitor; selecting the switching states to form switching state combinations; calculating an output voltage Vk of an AC side of the inverter; detecting a voltage Vg(k) of a power grid and a power grid current i(k) at a moment k on line, and calculating a value i(k+1) of the power grid current at a moment (k+1); performing phase locking of the power grid voltage Vg(k) obtained through on-line detection to obtain a phase Theta, calculating a current given value i<*>(k+1) at the moment (k+1) according to the Theta, and substituting a current calculation value i(k+1) at the moment (k+1) and the current given value i<*>(k+1) to an estimation function h; and selecting and using the switching state combination corresponding to the minimum estimation function h as output quantity, and driving a switch tube by a driving circuit. By the method, the leakage current of a system can be effectively suppressed, extension is facilitated, and the method is suitably used for a single-phase non-isolated cascaded H-bridge photovoltaic grid-connected inverter with a plurality 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 PV Applications" ("Cascaded Multi-level Photovoltaic Inverter Leakage Current Analysis and Suppression" - 2013 IEEE Energy The modulation strategy proposed by Proceedings of the Plenary Meeting of the Chinese Academy of Sciences) makes the parasitic capacitor voltage change according to the power frequency step wave, but the modulation strategy is relatively complex 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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