Hybrid modulation strategy for improving electric energy quality of MMC-UPFC series connection side compensation voltage

Abstract

The present invention discloses a hybrid modulation strategy for improving the electric energy quality of MMC-UPFC series connection side compensation voltage and smooth switching. Based on nearest level modulation (NLM) and carrier disposition CD-SPWM-VLM modulation with the introduction of virtual cyclic mapping, a hybrid modulation strategy is proposed, combined with Nanjing UPFC actual engineering parameters, the relationship between grid voltage harmonic content and an inverter modulation ratio is analyzed, and thus a control threshold of the hybrid modulation strategy is determined to achieve smooth switching as much as possible. First of all, an average modulation ratio m of three periods of the series connection side compensation voltage is read and calculated, the NLM is employed when m is larger than 0.5, otherwise, the CD-SPWM-VLM is employed. In the operation process, when detected m is larger than or equal to 0.6, the NLM is employed, when the detected m is smaller than or equal to 0.4, the CD-SPWM-VLM is employed, while the harmonic characteristics of MMC output series connection compensation voltage is improved well, no excessive device switching loss is generated, and the validity of the control strategy is verified in a UPFC engineering simulation model.

Description

technical field [0001] The invention relates to a hybrid modulation strategy for improving the power quality of the power grid MMC-UPFC series compensation voltage. The method is based on the nearest level approximation and the carrier cascading modulation with virtual cycle mapping, and belongs to the technical field of UPFC power quality improvement research. Background technique [0002] Unified Power Flow Controller (UPFC) is currently the most powerful flexible AC transmission system (FACTS) device. In 1992, UPFC was proposed as a complete device concept. So far, a series of fruitful results have been achieved in the field of UPFC from theoretical research to modeling analysis to engineering applications. Integrating the functions of voltage regulation, impedance compensation, phase angle adjustment and power flow control, UPFC plays an important role in improving the stability of the power system. [0003] Modular multilevel converter (MMC) represents the development...

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Problems solved by technology

This method is theoretically feasible, but in actual engineering, if the DC side voltage drops too much, it may lead to insufficient reactive power compensation on the parallel side and increase the line voltage loss. The adjustment ability of this method is very limited

There are also literatures that can change the reactive power of the line to improve the modulation ratio of the MMC on the series side, but the control strategy obtained by this scheme will be more complicated

Some studies have proposed that specific harmonic elimination modulation technology (SHEPWM) and space vector pulse width modulation (SVPWM) are applied to the inverter control of UPFC, which can reduce the output voltage distortion rate and improve the utilization rate of DC voltage, but the two The modulation method still needs further theoretical research and engineering practice in the field of MMC

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