Method and system for mid-point balance control of three-level inverter under low-voltage ride-through condition

Abstract

The present disclosure provides a method and a system for controlling the midpoint balance of a three-level inverter under low voltage ride-through conditions. The first vector selection is performed according to the relationship between the midpoint potential, the three-phase current direction and the set midpoint potential threshold; According to the relationship between the reference voltage and the first, second, third and zero vectors, the volt-second balance equation is established, and the mid-point potential balance equation is established according to the relationship between the mid-point potential deviation and the small vector. By solving the volt-second balance and the mid-point potential balance equation to obtain the duty ratio of each vector; rearrange the switching action sequence of each vector according to the principle of minimum switching loss to form a switching sequence; construct a value function, select a switching sequence that minimizes the value function, and drive the action of the switching tube; the present disclosure The obtained common-mode voltage of the three-level inverter is one-sixth of the DC side voltage, which reduces the leakage current and electromagnetic interference of the system, reduces the switching loss, and ensures that the mid-point potential is fast when the voltage shifts and oscillates. Recover, maintain balance.

Description

technical field [0001] The present disclosure relates to the technical field of inverter midpoint balance control, in particular to a method and system for midpoint balance control of a three-level inverter under low voltage ride through conditions. Background technique [0002] The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art. [0003] In recent years, inverters have played an increasingly important role in photovoltaics, UPS (uninterruptible power supplies), and power management. Compared with two-level technology, three-level technology has higher power levels and smaller harmonics. content has been extensively studied. T-type three-level has the advantages of small power loss, high conversion efficiency, and low hardware cost, and has become the first choice for energy conversion of low-voltage grid-connected systems such as photovoltaic power generation. [0004] With th...

AI Technical Summary

Problems solved by technology

[0006] The inventors of the present disclosure found that commonly used midpoint balance control methods include common-mode voltage injection method, hysteresis loop control method and midpoint voltage regulator method. When crossing such a low power coefficient, the inverter needs to transmit reactive power to the grid, it is difficult to achieve neutral point balance control, resulting in system instability

The virtual vector method can effectively suppress the midpoint potential oscillation of the DC side of the inverter under the condition of low power factor, but its switching loss is large and the system efficiency is low

In the prior art, there is a lack of effective control methods for suppressing neutral point balance and common-mode voltage and reducing switching losses when the non-isolated three-level inverter is operating under low-voltage ride-through conditions

[0007] At the same time, the non-isolated three-level inverter has the advantages of low cost and high power density and has been widely used. However, due to the lack of transformer isolation, the three-level inverter is connected to the grid without isolation, and there is an electrical gap between the photovoltaic array and the grid system. Connection, the existence of parasitic capacitance of the photovoltaic panel, caused the problem of leakage current to the ground, and even triggered the shutdown protection

Images