Studies On Travelling Wave Protection For AC/DC Hybrid Transmission System Based On Electronic Transformers

The largest AC/DC hybrid transmission system with highest voltage level in the world has been built in China,and it requires stable operation state.As known,the length of DC transmission lines could be up to several thousands of kilometers,and the environment where DC transmission lines are located is complicated.The actual operation data proves that DC transmission line faults account for 50%of total DC system faults.Meanwhile,the faults on AC transmission line could lead to commutation failure of inverter station,or even cause DC blocking.Therefore,the operation speed and reliability of AC and DC transmission line protection principles are directly related to the stability of AC/DC hybrid system.As known,the travelling wave protection principle has fastest operation speed which is suitable for AC/DC hybrid system.However,it requires wide bandwidth of transformer which is between several kHz to several hundreds of kHz.Hence,the paper proposes novel travelling wave protection principles of AC/DC transmission lines based on electronic transformers.The following are main contributions of this paper.(1)The paper proposes high frequency model of electronic transformer,and proves the travelling wave transfer ability and differential output characteristic of it by frequency-domain analysis,time-domain analysis and experiment analysis.Meanwhile,to obtain actual waveform of primary travelling wave,numerical integration algorithm and numerical high pass filter algorithm are designed.In addition,the implementation scheme of electronic transformer in actual power system is proposed.(2)The paper proposes a novel travelling wave directional pilot protection principle of DC transmission line based on electronic transformers' differential output characteristics.It includes fault direction discrimination criteria,faulty pole selection criteria,protection start criteria and protection implementation scheme.The simulation results prove that the novel protection principle has high reliability,fault direction discrimination ability and whole transmission line protection zone.(3)The paper analyzes the special travelling wave refraction and reflection characteristics of AC/DC hybrid system.And it is confirmed that traditional travelling wave protection principles which are based on voltage travelling wave are not suitable for AC transmission line near inverter station.Based on the secondary output signal of electronic current transformer,a novel travelling wave pilot protection of AC transmission line near inverter station is proposed.It includes fault zone discrimination criteria,faulty phase selection criteria,protection start criteria and protection implementation scheme.The simulation results prove that the proposed protection principle has high reliability and fast operation speed(<5ms)which could avoid the affection of commutation failure of inverter station.(4)The paper proposes a novel travelling wave directional pilot protection principle of AC transmission line away from inverter station based on electronic transformer.It includes fault direction discrimination criteria,faulty phase selection criteria,protection start criteria and protection implementation scheme.The proposed protection principle has high reliability and fast operation speed(<5ms)which could avoid the operation speed and reliability issues of traditional protection principles which are based on power-frequency signal.(5)Considering traditional protection test system is not suitable for the tests of novel travelling wave protection principle,the paper establishes a travelling wave protection experiment system.It could output actual travelling wave signal based on PSCAD simulation data and the secondary travelling wave signal could obtained using electronic transformer.Eventually,the novel travelling wave protection principle could be tested.