Research On Fault Ride Through Control Strategy Of MTDC System

The multi-terminal flexible DC transmission system has the operating characteristics of multi-point power supply and multi-drop point power reception,and is usually suitable for application scenarios such as wind power transmission,island power supply,non-synchronous networking,and power supply in mega cities.In recent years,flexible DC transmission technology has been widely used in power systems.Considering the reliability and safety of power systems,the fault ride-through problem of MTDC systems has gradually become an important issue that cannot be ignored.The fault ride-through of the multi-terminal flexible DC transmission system means that during an AC or DC failure,the converter station will not leave the grid and maintain the power transmission between the AC system and the DC system.Compared with the double-terminal flexible DC transmission system,the AC or DC fault characteristics of the multi-terminal flexible DC transmission system are more complicated.During an AC fault,the output voltage of the AC grid will drop,and the input and output power of the converter station will lose balance.The unbalanced power will cause the DC voltage to shift,which may cause the converter station to trigger over-voltage protection and run off the grid;DC During the fault period,due to the low damping of the DC grid,the DC voltage drops rapidly and the fault current rises sharply.At this time,the DC fault needs to be quickly isolated to prevent the fault impact.Traditional fault isolation schemes usually need to lock the converter station,and then trip the AC circuit breaker to cut off the fault propagation path.The traditional scheme not only needs to lock the converter station,but also needs to cut off the power transmission between the AC system and the DC system.At this time,the multi-terminal flexible DC transmission system does not have the fault ride-through capability.In response to the above problems,this article analyzes the fault ride-through control strategy of the MTDC system.The main research work is as follows:1)Introduced in detail the working principle of the modular multilevel converter and the inter-station coordinated control strategy of the MTDC system.In addition,the real-time simulation technology of the MMC-MTDC system is briefly introduced,and an MMC acceleration model based on a controlled source is analyzed in detail,which lays the foundation for the follow-up research work of the thesis;2)Aiming at the AC fault ride-through problem of the MTDC system,a technical solution that combines software control and hardware equipment is proposed.which uses software to control additional reactive power,maintains a stable voltage at the public connection point,and uses hardware equipment to consume energy to compensate for software control.The limitations;3)Aiming at the DC fault ride-through problem of MTDC system,a DC fault ride-through control strategy based on DC circuit breaker and auxiliary circuit is designed.At the same time,an enhanced adaptive droop control strategy is proposed to improve the fault ride-through capability of droop-controlled converter stations during DC faults;4)Based on the real-time simulation system,combined with model segmentation and parallel computing technology,a real-time simulation model of the MMC-MTDC system is built,which lays the foundation for the establishment of a semi-physical simulation platform to facilitate future testing and verification of control strategies and hardware equipment.