The significance of DC voltage divider's transmission characteristics to UHV DC transmission

Author： GOZ Electric Time：2024-11-11 09:27:21 Read：23

As the main protection of the UHV DC transmission system, the accuracy of its action is directly related to the safe and stable operation of the DC transmission system. The generation of fault characteristic quantities in each criterion of traveling wave protection is based on the measurement value of the DC voltage divider. However, it is difficult for the DC voltage divider to accurately transmit signals in each frequency band, which will lead to a decrease in the reliability of traveling wave protection. First, the transfer function of the DC voltage divider is derived based on the equivalent circuit model of the DC voltage divider, and the transmission characteristics of the DC voltage divider to the time domain signal and the frequency domain signal are analyzed. Secondly, combined with the criterion and logic of traveling wave protection, the influence of the transmission characteristics of the DC voltage divider on the performance of traveling wave protection is studied. Then, the transmission characteristics of the DC voltage divider are included in the setting process as a factor affecting the setting of the traveling wave protection, and a traveling wave protection setting method taking into account the transmission characteristics of the DC voltage divider is proposed. Finally, a simulation model is built with reference to the actual engineering parameters to verify the effectiveness and reliability of the proposed method. The results show that the proposed method can effectively improve the reliability of the setting of traveling wave protection.

Ultra-high voltage direct current transmission has the advantages of large transmission capacity, long transmission distance and high stability, and has been widely used in China's power grid construction. As the main protection of the transmission line in the existing ultra-high voltage direct current system, the reliability of the DC line traveling wave protection plays a vital role in the stable operation of the entire DC system. The DC line traveling wave protection mainly uses the high-frequency change signals of voltage and current for fault diagnosis, and the generation of fault characteristic quantities in each criterion is based on the measurement value of the DC voltage divider. However, in addition to the main body, the DC voltage divider also includes a secondary system composed of a secondary voltage divider board, an electronic isolation device, etc. Affected by its secondary system, the DC voltage divider is difficult to accurately transmit the signals of various frequency bands on the primary side, especially the high-frequency voltage signal. According to the test of the secondary system of the backup DC voltage divider of a DC project, the amplitude-frequency characteristics of the DC voltage divider secondary system have an amplification effect at high frequencies, and its broadband transmission characteristics are not ideal. In a subsequent annual maintenance of the DC voltage divider, it was found that the zero-position voltage of some secondary voltage divider modules was too large, and the linearity and consistency were poor, which affected the overall transmission characteristics of the DC voltage divider. Therefore, if the influence of the DC voltage divider transmission characteristics on the traveling wave protection performance is not considered when setting the traveling wave protection, it is bound to cause unreasonable selection of the traveling wave protection setting, resulting in reduced protection reliability.

At present, due to the complex fault characteristics of the UHV DC transmission system, the voltage and current fault boundary values used in the protection setting method are difficult to obtain by analytical methods. Therefore, the protection setting of the DC transmission system is mainly determined by simulation results. However, the current process of setting the protection setting of the DC transmission system is not clear, and the factors considered in the setting process are not comprehensive. In actual projects, the situation of protection malfunction due to unreasonable setting of the setting still occurs from time to time. In response to this problem, the relevant literature sorted out the characteristics of the existing high-voltage DC transmission protection, and proposed a setting process based on the fault characteristic quantity according to the characteristics of different protections. The relevant literature analyzed the influence of factors such as transition resistance and fault location on the setting of protection setting for two types of UHV DC line protection, and proposed setting processes for two types of UHV DC line protection respectively. However, the above studies ignored the influence of the DC voltage divider's transmission characteristics on the protection performance when setting the fixed value, and did not include the DC voltage divider's transmission characteristics as a factor affecting the protection setting in the setting process, which is bound to cause unreasonable protection setting selection, thereby affecting the protection performance.

At present, there are few studies on the DC voltage divider's transmission characteristics and its influence on the performance of the DC transmission protection system, which is one of the reasons why the DC voltage divider's transmission characteristics have not been included in the protection setting setting. The relevant literature analyzes the influence of the DC voltage divider's secondary system on the DC voltage divider's frequency response and discusses the feasibility of using the DC voltage divider for DC harmonic measurement. The relevant literature analyzes the transient process after the DC voltage divider's secondary side discharge circuit is broken down, and studies the mechanism of the DC voltage divider's transient characteristics causing UHV DC false lockout. On this basis, the relevant literature proposes to use the discharge tube series varistor scheme as the secondary side protection of the DC voltage divider in the converter station to avoid bipolar lockout accidents caused by protection misoperation. However, none of the above-mentioned related literatures conducts a specific analysis on the influence of the DC voltage divider's transmission characteristics on the DC protection system.

In view of the problems of unclear setting process and incomplete consideration of factors in the current DC line traveling wave protection setting value setting, this paper proposes a traveling wave protection setting value setting method taking into account the transfer characteristics of the DC voltage divider. Starting from the transfer characteristics of the DC voltage divider, an equivalent circuit model of the DC voltage divider based on actual engineering parameters is constructed, and the transfer function of the DC voltage divider is derived. Based on the frequency response characteristics and step response characteristics of two types of DC voltage dividers, the specific influence of the transfer characteristics of the two types of DC voltage dividers on the performance of traveling wave protection is studied, and the transfer characteristics of the DC voltage divider are included in the setting process as a factor affecting the protection setting value setting. Finally, a simulation model is built based on PSACD/EMTDC to verify the effectiveness and reliability of the proposed setting value setting method.