MMC direct circular current inhibition method suitable for asymmetric AC power grid

Abstract

The invention discloses a Modular Multilevel Converter (MMC) direct circular current inhibition method suitable for an asymmetric AC power grid. The circular current inhibition method derives a capacitance voltage fluctuation expression of an asymmetric system, and analyzes components of an imbalance voltage reference value for the circular current inhibition. A control strategy is suitable for two different negative sequence control objects. A result shows that the active component of the reference value is a second harmonic component no matter what kind of condition the AC system is operated in. The imbalance voltage reference value for circular current inhibition is obtained by direct calculation without a Proportional Integral (PI) controller or a resonance controller. The positive sequence, the negative sequence and the zero sequence circular current components in an operation state of the asymmetric power grid are simultaneously inhibited. At the same time each phase has an independent structure. The MMC direct circulation inhibition method is suitable for the symmetric or asymmetric AC system. A double end MMC-HVDC (High-Voltage Direct Current) model is constructed on a Power Systems Computer Aided Design / Electromagnetic Transients including DC (PSCAD / EMTDC) time-domain simulation platform. The validity of the MMC direct circular current inhibition method under the conditions of the symmetric and asymmetric power grids provided by the invention is verified.

Description

technical field [0001] The invention relates to the technical field of power electronic equipment and its control, in particular to an MMC direct circulation suppression method suitable for an asymmetric AC power grid. Background technique [0002] With the development of power electronic equipment and its control technology, the voltage source converter (voltage source converter, VSC) using IGBT and other semiconductor devices has been widely used in the medium and high voltage field. Among them, the modular multilevel converter (MMC) with a large number of levels is cascaded through submodules (submodular, SM), which has the advantages of highly modular structure, common DC bus, and easy engineering implementation. In HVDC The direction has been vigorously promoted. [0003] The energy storage capacitor inside the MMC adopts a distributed suspension structure, and the unbalanced capacitor voltage among the sub-modules leads to the second harmonic circulation inside the MM...

AI Technical Summary

Problems solved by technology

For example, combining the nearest level modulation based on sub-module capacitor voltage estimation and direct circulation control, a compound circulation control strategy suitable for asymmetric AC systems is proposed, but the control system structure of this strategy is relatively complicated

In order to adapt to asymmetrical operation conditions, the existing foreign literature proposes to improve the control method based on three-phase circulating current decoupling, and adds a zero-sequence control loop to suppress the DC voltage fluctuation on the basis of the original decoupling system; but the proposed The method requires additional measurement of the upper and lower arm voltages, which increases the complexity and cost of the control system. In addition, the proposed suppression strategy based on the proportional resonant (PR) controller is only under the premise that the negative sequence component of the current is suppressed. Effective, and a circulating current suppression strategy based on proportional integral resonant (proportional integral resonant, PIR) controller is proposed, but this strategy is only suitable for negative sequence inner loop control systems that suppress active power fluctuations

[0007] To sum up, in order to be suitable for asymmetric AC systems, the existing circulating current suppression strategy generally needs to use an additional controller to suppress the zero-sequence circulating current or DC voltage fluctuation, and the additional control will make the circulating current suppressor more complicated; at the same time, the zero-sequence circulating current Suppressors or DC voltage fluctuation suppressors are generally designed based on a single negative-sequence suppression target. When the negative-sequence control target changes, the original control strategy will no longer be applicable

Therefore, the existing circulating current suppression strategies are insufficient in terms of simplicity and versatility. Most of them are based on proportional-integral controllers or proportional resonant controllers. Under asymmetrical grid conditions, such strategies require additional zero-sequence circulating current suppression or DC voltage fluctuation suppression to control the zero-sequence component in the circulating current, which makes the control system more complex

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