Analysis On Response Of DC Converters To Ac Faults And Operating Characteristics Of DC Differential Protection

With more and more HVDC transmission projects in recent years, an AC/DC hybrid system has been shaped in China and even the world. In particular, China Southern Power Grid has eight AC transmission projects and eight HVDC transmission projects, becoming the most complex AC/DC hybrid power system in the world. In such a system, the interaction between AC and DC system and its corresponding new constraints to the power grid are worth of attention. The response of DC converters to AC faults is one of the main forms of AC/DC interaction, which may cause troubles to the operation of DC system. This paper studies the response characteristics of DC converters to AC faults in detail, providing theoretical basis for the AC/DC hybrid power system to cope with the challenges and operate safely and stably.Firstly, the paper analyzes the response process of the rectifier and inverter under AC faults on AC connection wire and AC system comprehensively, and sum them up to two pathways, including the direct one and the indirect one. Then the paper studies the mechanism and influencing factors of commutation failure of the inverter caused by AC faults. Apart from this, the paper summarizes the common converter differential protections and their principles, and analyzes their response to AC faults on the rectifier and the inverter sides qualitatively. The study above provides a basis for the subsequent analysis.Secondly, the paper proposes a criterion of extinction angle which takes into account both the change of amplitude and phase of the converter commutation voltage, and points out that the assessment method of commutation failure based on this criterion considers all the affect factors comprehensively. Furtherly, by comparing with the simulation results of PSCAD/EMTDC and BPA software in both a simple AC/DC hybrid system and an actual AC/DC hybrid system, the accuracy and practicability of the proposed method are verified.Finally, the paper studies the malfunction operation of DC differential protection under faults on AC connection wire. The fault coverage and function orientation of DC differential protection are analyzed and its selectivity problem is revealed. Based on the parameters of an actual HVDC transmission project, the fault feature boundaries of the DC differential protection natural operating characteristics under different faults are studied by analyzing the valve commutation processes and current paths. And the impact of DC control on the fault features of DC differential protection is also analyzed. Furthermore, this paper proposes strategies to optimize the protection selectivity and points out that adding an auxiliary criterion with electric parameters on the AC side of converter can solve the problem in principle, which is verified by the simulation example of the actual HVDC transmission project above.