Unified power quality conditioner system based on MMC technology and control method

Abstract

The invention discloses a unified power quality conditioner system based on an MMC technology and a control method. The system comprises a series MMC, a parallel MMC and a control circuit; wherein theseries MMC and the parallel MMC are connected to a power grid side and a load side respectively, and a unified power quality conditioner UPQC topological structure based on the MMC is formed by sharing a direct current link. The control method comprises the steps that an MMC model under an abc static coordinate system is transformed to obtain a differential equation matrix model under a dq rotating coordinate system; small signal models of the series MMC and the parallel MMC of the MMC are acquired under the dq rotating coordinate system; a control strategy of the MMC in parallel is executed;a control strategy of the MMC in series is executed; a loop current control strategy of a loop current direct-current component and a fundamental frequency component is utilized, and a modulation strategy of sub-module capacitor voltage estimation is adopted. According to the invention, the maximum voltage drop compensation capability of the system can be effectively improved on the topological structure, the circulation second harmonic component can be reduced on the control method, the control of the total energy stored by the phase unit capacitor of the converter and the control of the capacitor voltage balance between the upper bridge arm and the lower bridge arm can be better realized, and various power quality problems can be comprehensively compensated.

Description

Technical field[0001]The invention relates to the technical field of flexible AC power transmission and distribution and power electronics, and in particular to a unified power quality regulator system and control method based on a modular multilevel converter.Background technique[0002]In modern industrial power grids, various disturbances such as the start-up of motors, the start and stop of large loads, and the nonlinearity of the load lead to industrial power grid voltage distortion, unbalanced voltage rise and fall, voltage flicker, load current imbalance, etc., causing industrial power grids The power quality has dropped significantly. With the widespread application of a large number of new technologies, various detection and control equipment and other sensitive loads account for an increasing proportion, and accordingly the requirements for power quality of the power grid are getting higher and higher. Then, in order to provide users with pure power, the research of power qu...

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Problems solved by technology

[0006] In addition, the current modulation methods commonly used in MMC include carrier phase shift modulation, nearest level approximation modulation, etc. These modulation methods do not consider the periodic change of the capacitor voltage in the calculation, and the sub-module capacitor voltage is considered to be fixed.

As a result, the circulating current contains a high 2nd harmonic component, which increases the effective value of the bridge arm current. In addition, the rated current value and switching loss of the sub-module also increase. The most important impact is the energy transfer between the bridge arms.

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