Study On Frequency Stability Control And Fault Ride-through Strategy Of Sending Of Hybrid Direct Current Transmission System

Incorporating the advantages of modular multilevel converters(MMC)with commutated line converters(LCC),the hybrid DC transmission system has a bright future.The rapid penetration of novel energy sources into the AC grid has led to a significant reduction in the inertia of the transmitting end of the hybrid DC transmission system compared with a few decades previously.The frequency steadiness problem is growing worse.Furthermore,the receiving end will possess a higher DC voltage than the sending end if the sending end has an AC fault.It will stop power transmission and make the system less able to function securely and firmly.It is critical to examine the stability improvement control technique of the hybrid DC system with a considerable quantity of new energy to deal with the issues of frequency reliability of the AC system at the transmitting end and fault ride-through of the AC network at the transmitting end.Based on examining the system frequency regulation and fault ride-through demand,this paper proposes a multi-timescale coordinated control approach and an AC fault ride-through strategy for the DC system to support the reliability of the sending end actively.It is due to the complex issues faced by the DC transmission system,which contains an increased amount of new energy.Firstly,mathematical modeling and analysis of the working principle of the send-side AC system and converter stations of the DC outgoing system are carried out.On this foundation,the DC system’s operation characteristics are explored with an emphasis on the frequency regulation needs and fault characteristics of the send-side AC system,laying the groundwork for further study and developing an appropriate strategy.The frequency response traits of the DC system and the traditional synchronous generator are thoroughly considered to address the issue of stable frequency at the sending end,and a multi-timescale frequency regulation control technique is suggested so that it is suited to maintaining inertia while regulating primary frequency.After combined with the maximum spare capacity of the synchronous machine for primary frequency regulation,the dead zone value of the frequency modulation control approach is developed.The dead zone value of the frequency regulation control strategy for the DC system is realized under different scenarios.On this basis,the dynamic response characteristics of the coordination control strategy under different scenarios are analyzed,further highlighting its superiority over existing control strategies.By simulation,the benefit of the suggested approach was confirmed.Finally,a fault ride-through control approach based on the converter’s working principle and a passive network’s response characteristics is proposed to resolve the interruption of DC power transmission caused by an AC side failure.The flexible regulation of the MMC DC voltage is achieved by using the operating characteristics of the full-bridge sub-module to quickly restore the DC power transmission while reducing the DC inrush current after the fault is cleared.In order to prevent cutting the load and achieve fault ride-through simultaneously,the frequency of the passive network is adjusted to match the DC voltage change on the inverter side.It allows the unbalanced power of the system to be actively dissipated by using the load’s frequency regulation characteristics.The simulation’s outcomes reveal that the proposed frequency stabilization control and fault ride-through strategy for the hybrid DC system can effectively improve the stability of the high percentage of new energy sources fed through the hybrid DC system.