Research On Control-related Commutation Failure Of HVDC Transmission Connected To AC Grid With Weak Receiving Terminal

With the rapid development of HVDC transmission and renewable energy power generation in China,the grid strength of receiving end AC grid has reduced significantly because of the widespread application of power electronic equipment.A weak grid means the receiving end AC grid only has limited ability to maintain the original state while facing faults or disturbance,so frequent and greater fluctuations will appear at the ports of the inverter station.The HVDC control system will frequently act to respond to these fluctuations.The control error caused by the dynamic process of firing control system will be a significant impact to commutation process.Based on the background of the AC grid with weak receiving terminal,this thesis studies the problems of control-related commutation failure caused by the dynamic process of the control system,and mainly completes the following content.(1)The generation mechanism of the firing pulse is explained.The role and function of the phase information output by the phase locked loop and the firing angle order output by the controller in the process of generating the firing pulse are analyzed.(2)The mechanism of control-related commutation failure is analyzed.The phase error caused by the dynamic process of the phase locked loop and the order error caused by the dynamic process of the controller are the direct factor leading to control error when valve control system generates firing pulses.Once the actual firing pulse cannot meet the requirements of the commutation process,control-related commutation failure will occur.(3)The influence of the parameters and structure of the phase locked loop on the control-related commutation failure is analyzed.After introducing the principle and basic elements of the phase locked loop,the model of the phase locked loop in locked status and unlocked status is provided.Then the SRF-PLL was used as the object to simulate and analyze the influence of parameter differences on dynamic performance and control-related commutation failure in HVDC transmission.Meanwhile,the influence of PLL’s structure on dynamic performance and control-related commutation failure is researched by comparing SRF-PLL and DDSRF-PLL in simulation.(4)The influence of the grid strength decrease of the receiving end AC grid on controlrelated commutation failure is analyzed.First,the concept of grid strength and its index are introduced,and then the impact of renewable energy generation on grid strength is analyzed.Finally,the influence of different grid strength of the receiving end AC grid and difference capacity of renewable energy generation on the control-related commutation failure is analyzed through the simulation.