A Method for Restoring Unbalance of MMC-HVDC Bridge Arm Based on Adaptive Virtual Resistance

Abstract

The present application relates to an MMC‑HVDC bridge arm unbalance recovery method based on adaptive virtual resistance. The method includes: collecting the actual value of the bridge arm circulation output by the MMC converter in real time; comparing the bridge arm circulation actual value with the bridge arm circulation reference value, when the bridge arm circulation actual value and the bridge arm circulation reference value When the values ​​are inconsistent, determine the difference between the actual value of the bridge arm circulation and the reference value of the bridge arm circulation; input the difference into the adaptive virtual resistance and the rebalance controller for processing, and determine the auxiliary control signal; according to the The auxiliary control signal adjusts the conventional control signal output by the proportional resonant controller to restore the balance of the MMC-HVDC bridge arm comprising the MMC converter, which can effectively reduce the unbalance recovery time of the system under different transient conditions and improve different The imbalance recovery efficiency of the system under transient conditions.

Description

technical field [0001] The present application relates to the technical field of high-voltage direct current transmission, in particular to a method for restoring unbalance of an MMC-HVDC bridge arm based on an adaptive virtual resistance. Background technique [0002] In recent years, Modular Multilevel Converter (MMC) has gradually been widely used due to its own advantages such as high controllability, superior output performance and flexible modular structure. The Modular Multilevel Converter High Voltage Direct Current (MMC-HVDC) system based on modular multilevel converters is an important application of MMC in the field of HVDC transmission technology. , the integrated grid-connected transmission of renewable energy, and the interconnection of urban distribution networks have played an important role. Similar to the traditional two-level voltage source converter, the control system of MMC-HVDC includes system-level control, converter-level control and valve-level con...

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Problems solved by technology

However, MMC-HVDC contains hundreds of cascaded sub-module units, and its internal dynamic behavior is more complicated than that of two-level converters.

Since the equivalent impedance of the MMC converter is very small, if no additional damping is added, the unbalance of the upper and lower bridge arms of the MMC during the transient process will have a non-negligible impact on the stable operation of the system

[0003] Researchers have also carried out some research on the internal dynamic characteristics and improvement methods of MMC. Generally speaking, most of the existing research studies the stability of the MMC-HVDC system from the perspective of system damping characteristics and oscillation suppression. For the system transient During the process, the unbalance characteristics of the upper and lower bridge arms and the corresponding recovery control are rarely discussed. At present, the virtual impedance method is mainly used to study the unbalance characteristics of the upper and lower bridge arms and the corresponding recovery control during the transient process of the system. The virtual impedance method is mainly to obtain the optimal impedance parameters for the specific operating conditions, and restore the control through the impedance parameters, so as to ensure the shortest fault recovery time under the optimal value. Since the modulation ratio M is different under different operating conditions, the The optimality of the obtained impedance parameters is only for a specific working condition. When the operating state of the system changes, the impedance parameters cannot be adjusted accordingly, and the influence of system state changes cannot be eliminated, which reduces the system imbalance under different transient conditions. recovery efficiency

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