Modulation method for inhibiting leakage current of H-bridge cascade inverters

Abstract

The invention discloses a modulation method for inhibiting leakage current of H-bridge cascade inverters. The modulation method can maintain the sum of parasitic capacitance voltages of all modules to be constant, and thus the leakage current is eliminated. The method comprises the following steps: dividing all modules into module groups in pairs according to rules, and dividing into a power frequency module group and a high frequency module group; obtaining the on-off state of the power frequency module group through comparison of modulating waves and fixed values; obtaining the on-off state of the high frequency module group through comparison of corrected modulating waves and two stacked triangle carriers; and generating corresponding PWM signals through the combination of the on-off states, and distributing to corresponding pins. The scheme can be used for multiple H-bridge cascade inverters, and extra hardware cost does not need to be added, so that the leakage current flowing into a power grid can be eliminated, meanwhile the switching loss is relatively low, and the implementation manner is simple.

Description

technical field [0001] The invention relates to a modulation method of a single-phase H-bridge cascaded inverter, in particular a modulation method capable of suppressing the leakage current of a non-isolated H-bridge cascaded grid-connected photovoltaic inverter; it is suitable for the field of photovoltaic grid-connected power generation. Background technique [0002] In order to ensure the safety of use, the VDE4105 standard has strict restrictions on the common mode current (leakage current) of photovoltaic (Photovoltaic, PV) grid-connected systems. The power frequency isolation transformer on the grid side can realize electrical isolation between PV and power grid, and suppress leakage current. However, power frequency transformers are large in size, heavy in weight, high in cost, and low in system efficiency. If a high-frequency transformer is used to realize the electrical isolation of PV and grid, the system volume, weight and cost can be reduced, but the power conve...

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Problems solved by technology

[0005] 1) Passive filters are mainly composed of inductors and capacitors, which will greatly increase the volume, weight and cost of the inverter when used in the circuit, and at the same time reduce the power conversion efficiency of the inverter;

[0006] 2) The leakage current harmonic range is wide, and it is affected by environmental factors, which makes the filter parameter design process more complicated;

[0007] 3) The actual operating frequency of the cascaded H-bridge inverter is low, and the main harmonic frequency of its leakage current is also low, which leads to the unsatisfactory suppression effect of the leakage current of the filter

[0009] 1) This modulation method cannot suppress the leakage current flowing into the grid, and cannot improve the quality of grid-connected current;

[0010] 2) This method uses two in-phase stacked carriers, but the carrier needs to be phase-shifted by 180° every half power frequency cycle, which undoubtedly increases the difficulty of hardware implementation

[0011] 3) This modulation method is only suitable for two-module cascaded inverters and is not universal

[0012] It can be seen that the existing technology cannot better solve the leakage current problem of bridge cascaded H-bridge inverters without increasing additional costs and without affecting the quality of grid-connected current.

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