Modulation method for npc type three-level inverter to suppress mid-point low-frequency oscillation on DC side

Abstract

The invention discloses a modulation method for suppressing direct current side midpoint low frequency oscillation of an NPC type three-level inverter. The modulation method is characterized by comprising the steps of mapping three items of reference voltages of the three-level inverter into a 60-degree coordinate system; based on six major sectors of space vectors, performing different modulationon odd number and even number sectors; in the odd number sectors, establishing a gh-axis coordinate system; in the even number sectors, establishing an hg-axis coordinate system; and by combination with a virtual space vector algorithm, eliminating the low frequency oscillation condition in the midpoint potential of a direct current side capacitor of the inverter through the virtual vector, thereby achieving a modulation effect for suppressing direct current side midpoint low frequency oscillation of the NPC type three-level inverter. By virtue of the modulation method, the low frequency oscillation in the midpoint of the direct current side capacitor can be eliminated; and compared with the conventional SVPWM, the voltage vector acting time and sequence can be directly obtained without needing a large amount of trigonometric function calculation and lookup judgment, so that the operation load of a controller is reduced, the three-level inverter space vector modulation algorithm is simplified, and popularization and application of the space vector modulation algorithm in the multi-level inverter can be facilitated.

Description

technical field [0001] The invention relates to a modulation method for inverter modulation, in particular to a modulation method based on an NPC-type three-level inverter for suppressing midpoint low-frequency oscillation on a DC side, and belongs to the technical field of inverter space vector modulation control. Background technique [0002] In order to ensure the long-term reliable and stable operation of the multilevel NPC inverter, it is necessary to ensure the neutral point voltage balance. At present, there are mainly the following types of control strategies for the neutral point voltage balance of NPC inverters: [0003] (1) Carrier sinusoidal PWM injected with zero-sequence components; [0004] (2) Based on the specific harmonic elimination PWM, by fine-tuning the specific harmonic elimination PWM switching angle to change the action time of the zero state to reach the control midpoint voltage; [0005] (3) Based on the space vector PWM (Space Vector Modulation,...

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Problems solved by technology

[0015] Although the above-mentioned documents also involve the midpoint voltage control of the NPC three-level inverter, and some control methods have been proposed, they are exploring a simple algorithm that can eliminate the low-frequency oscillation of the midpoint of the DC capacitor and facilitate the realization of the controller. There is no breakthrough yet, so the above-mentioned problems of complex algorithm, cumbersome implementation, and heavy computational workload of the controller have not been effectively solved, so further improvement is still needed

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