Research On Control Parameter Analysis And DC Line Protection Of Hybrid Multi-Terminal HVDC Systems

Hybrid multi terminal HVDC transmission system combines the advantages of traditional HVDC transmission,such as large transmission capacity and low cost,and the advantages of flexible HVDC transmission,such as no commutation failure risk and independent control of active and reactive power,which has become the development trend of HVDC technology.The control and protection is the basis to ensure the safety and stability of HVDC system.The converter station controller is closely related to the stability and dynamic performance of the system,which is an important guarantee for the stable operation of the system.DC faults is frequent,and the DC line protection is complex compared with other protections.This paper studies the control and protection of hybrid multi terminal HVDC transmission system,and the research results provide guidances for the control parameter design and line protection scheme of Kunliulong projects.The main contributions are as follows:1)To clatify the impact of control parameters on the stability and dynamic performance of the system,based on transfer function matrix,a control parameter analysis method for hybrid multi terminal HVDC transmission system is proposed.To avoid complicated mathematical derivation,a small signal model of hybrid multi termina HVDC system is established,which can be flexibly adapted to different operation modes.The small signal model can be transformed into the transfer function matrix,thus the fesible region of the control parameters and the influence of the control parameters on the dynamic performance of the system can be obtained according to the classical control theory.Taking a three-terminal HVDC system as an example,the PSCAD simulation model is used to validate the proposed control parameter analysis method.The results show that the proposed method can complete the feasible region analysis and dynamic performance analysis of the control parameters on the system under different operation modes in a short time,which is helpful for engineers to design control parameters according to actual preferences.2)To improve the performance of the line protection under high fault resistance,a transient protection scheme for hybrid multi terminal HVDC transmission lines based on time domain voltage ratio is proposed.The fault-generated traveling wave characteristics of HVDC transmission lines are analyzed mathematically,then a fault identification criterion based on time-domain transient voltage ratio is proposed.Combined with the criterion,a transient protection scheme for hybrid multi terminal HVDC transmission lines is proposed via the coordination of the protection measurements at the head point of transmission lines.According to fault recording data of the kunliulong project model,the proposed transient protection scheme is validated under different operation conditions.The results show that the time-domain voltage ratio can effectively weaken the influence of the fault resistance.The physical concept is clear and the calculation method is simple.It does not need the complex time-frequency analysis method to extract the fault traveling wave characteristics,which is easy to implement in project.3)To analyze the frequency-domain DC line fault characteristics,a single-end protection scheme based on convolution power for hybrid multi terminal HVDC transmission lines is proposed.Firstly,a fault characteristic quantity,i.e.,convolution power,is constructed.And the convolution power characteristics on HVDC transmission lines in frequency domain are analyzed.On this basis,a single-end protection scheme of HVDC transmission lines based on convolution power is proposed to identify fault pole,fault direction and internal and external faults.Finally,according to the kunliulong project model,the protection scheme is validated.The results show that the convolution power combines the characteristics of traveling wave voltage and current,which not only has the same fault direction discrimination ability as traveling wave current,but also can make full use of the attenuation characteristics of boundaries to identify the internal and external faults,and has high reliability and sensitivity.