Modular multilevel converter (MMC)-based unbalanced fault optimization method

Abstract

The invention relates to a modular multilevel converter (MMC)-based unbalanced fault optimization method. The method is implemented on the basis of an MMC circuit topological structure and comprises the steps of calculating reference values of an output current and a circulation current by a corresponding control algorithm according to an MMC average bridge arm model, and measuring a DC bus voltage; inputting the output current reference value to an output current controller, and generating an output voltage reference value; inputting the circulation current reference value to a novel circulation current controller having a zero-sequence control function, and generating a circulation voltage reference value; and obtaining intercalation coefficients of an MMC upper bridge arm and an MMC lower bridge arm according to the output voltage reference value, the circuit voltage reference value and the DC bus voltage, and inputting the intercalation coefficients to an MMC for completing control. The voltage ripple of a DC bus is suppressed by improving the zero-sequence circulation controller, and the capacitance-voltage ripple of the DC bus can be effectively suppressed.

Description

technical field [0001] The invention relates to the field of power grid technology, in particular to an unbalanced fault optimization method based on a modular multilevel converter. Background technique [0002] Modular multilevel converter (MMC) has the advantages of modular design, high quality AC and DC output waveforms, large transmission capacity, decoupling control of active and reactive power, ability to supply power to weak systems or passive networks, and high reliability. It is recognized as the next-generation high-voltage converter device, but its system structure is very complex. MMC-HVDC systems often include hundreds of identical sub-modules and a considerable number of detection and control links, so the control system of MMC is also more complex than ordinary converters. The controller control system is much more complex. [0003] When the MMC-HVDC system encounters a fault, such as an unbalanced fault in the power grid, voltage fluctuations will occur on t...

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Problems solved by technology

[0002] Modular multilevel converter (MMC) has the advantages of modular design, high quality AC and DC output waveforms, large transmission capacity, decoupling control of active and reactive power, ability to supply power to weak systems or passive networks, and high reliability. It is recognized as the next-generation high-voltage converter device, but its system structure is very complex. MMC-HVDC systems often include hundreds of identical sub-modules and a considerable number of detection and control links, so the control system of MMC is also more complex than ordinary converters. The control system of the device is much more complex

[0003] When the MMC-HVDC system encounters a fault, such as an unbalanced fault in the power grid, voltage fluctuations will occur on the DC bus. The improved zero-sequence circulating current controller can optimize the voltage ripple of the DC bus. The general zero-sequence circulating current controller can effectively Suppresses the DC bus ripple when the grid is unbalanced, but does not optimize the system with a zero-axis DC loop controller

[0004] The existing zero-sequence circulating current controller of modular multilevel converters in the event of an unbalanced fault can reduce the fluctuation of the DC bus voltage to a certain extent, but the effect is not ideal, which will lead to excessive ripple of the DC bus voltage. Module capacitors cause damage

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