Method for balancing three-level direct current midpoint voltage by using time factor

Abstract

The invention discloses a method for balancing a three-level direct current midpoint voltage by using a time factor, which comprises the following steps of: 1, sampling a midpoint voltage and a midpoint current, and judging the midpoint voltage and the midpoint current; 2, executing a step 3 when the midpoint voltage is not zero, and jumping to a step 5 when the midpoint voltage is zero; 3, determining acting time of positive and negative small vectors according to the directions of the midpoint voltage and the midpoint current; 4, balancing the midpoint voltage after the positive and negative small vectors are counteracted; and 5, finishing the program. The value range of the time factor is experimental data and is adjusted according to the voltage value and the current value detected byan active filter and the running directions of the voltage and the current. The method has the advantages of prolonging service life of the capacitor, along with simple and convenient operation, low voltage fluctuation on a direct-current side capacitor, few sampling data and low requirement on software and hardware.

Description

technical field [0001] The invention relates to a balancing method applied to a three-level system of an active power filter, in particular to a method for balancing a three-level DC midpoint voltage by using a time factor. Background technique: [0002] The three-level voltage balance problem of the DC side of the active power filter is one of the key technologies of the active power filter. When the midpoint of the DC side of the active power filter is unbalanced, the following hazards will be brought about: First, the output voltage waveform of the converter is distorted, and the harmonic compensation current input to the grid has low harmonics. As a result, the active power filter cannot track the harmonic current of the load well; second, the voltage borne by the switching devices is unbalanced, and some devices in each phase are subjected to high voltages, which will affect normal operation in severe cases; third, DC Voltage fluctuations on the side capacitors reduce ...

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Problems solved by technology

The methods for controlling the neutral point voltage balance in the prior art mainly include: 1. Open-loop control, by alternately selecting the positive and negative small vectors, there is no need to detect the DC bus voltage and the output terminal current, and the implementation is simple, but because the neutral point of the vector cannot be compensated The influence of the voltage, so the symmetry of the vector itself can only be used to complete the midpoint voltage balance in a complete output voltage cycle, resulting in a large low-frequency ripple in the midpoint voltage; 2. Detect the DC side capacitor voltage , and then according to the deviation direction of the midpoint voltage, reasonably arrange the action time of the PWM (abbreviation of Pulse Width Modulation, pulse width modulation) pulse of positive and negative small vectors, and perform midpoint voltage compensation, but the algorithm of this method is very complicated; 3. Detect the DC side capacitor voltage and the three-phase AC output current. This method obtains the direction function by detecting the direction of the three-phase AC output current in real time, and uses this function to select the positive and negative voltage vector action time in a PWM cycle to control the direction of the midpoint current. Voltage balance, in theory, this method is very good, it can calculate the specific time of compensation, but this method needs to sample a lot of data, the amount of calculation is relatively large, and the requirements for software and hardware are relatively high

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