A Capacitor Voltage Balancing Method in Uncontrollable Precharge Phase

Abstract

The capacitance voltage balancing method of the uncontrollable pre-charging stage of the modular multi-level converter of the present invention relates to the modular multi-level converter, and is especially suitable for the uncontrollable pre-charging stage of the MMC (modular multi-level converter) capacitive voltage balancing method. Improve the control circuit of the flyback energy harvesting power supply. The improved part of the control circuit structure includes a control unit, an optocoupler isolation unit, an AND gate unit, and a MOS field effect transistor. The two input terminals of the AND gate unit are respectively connected to the output of the control unit. terminal and the output terminal of the optocoupler isolation unit; the output terminal of the AND gate unit is connected to the gate terminal of the MOS field effect transistor through the resistor R2. By improving the control circuit of the traditional flyback energy harvesting power supply and the traditional sub-module capacitor voltage detection method, the improved voltage detection method reduces the frequency / voltage conversion circuit in the traditional method, and directly converts the frequency voltage that reflects the capacitor voltage amplitude The signal is fed into the FPGA.

Description

technical field [0001] The capacitance voltage balancing method of the uncontrollable pre-charging stage of the modular multi-level converter of the present invention relates to the modular multi-level converter, and is especially suitable for the uncontrollable pre-charging stage of the MMC (modular multi-level converter) capacitive voltage balancing method. Background technique [0002] Due to its high output waveform quality, low switching loss, fast fault recovery capability and high reliability, modular multilevel converters are more and more widely used in the field of HVDC transmission. At present, two MMC-HVDC (flexible direct current transmission based on modular multilevel converters) projects have been put into operation in my country, such as Guangdong Nan'ao Sanduan and Zhejiang Zhoushan Wuduan, and have achieved good operating results and economic benefits. However, since each bridge arm of a modular multilevel converter is composed of multiple sub-modules cas...

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Problems solved by technology

Aiming at the problem of capacitor voltage balancing in the uncontrollable pre-charging stage, the literature (A.Ajami, H. Shokri and A. Mokhberdoran, "Parallel switch-based chopper circuit for DC capacitor voltage balancing in diode-clamped multilevel inverter," IETpower Electron.) proposed A parallel switch chopper circuit is configured for the sub-module capacitors to achieve a better voltage balance effect, but this method increases the number of switching devices on the one hand, and on the other hand leads to an increase in the active power loss of the converter station; literature (Gum Tae Son, Hee-Jin Lee, Tae Sik Nam, Uk-Hwa Lee etc. Design and Control of a Modular Multilevel HVDC Converter With Redundant Power Modules for Noninterruptible Energy Transfer [J]) proposed that the sub-module capacitor voltage in the same bridge arm be realized by cascaded transformers Balanced, but the use of cascaded transformers will greatly increase the difficulty of designing MMC sub-modules. This method configures a unidirectional chopper and a single coupled inductor for each sub-module to form a balanced loop. Different balanced loops use carrier phase-shift modulation, effectively The balance of the capacitor voltage is achieved, but in the case of an MMC system with a large number of levels, the system hardware cost and control complexity will be greatly increased

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