Dynamic behavior analysis of backup protection for HVDC transmission line

The backup protection of HVDC transmission line consists of differential undervoltage protection and differential protection. In this paper, based on the actual parameters, the simulation model of differential undervoltage protection and differential protection for DC line is built. The response regularity of each criterion of the two protections under different fault conditions is studied. The basic functions of each criterion are defined, which provides an important basis for the setting calculation of DC line backup protection.


Introduction
DC line plays an important role in HVDC transmission system [1][2][3][4]. Differential undervoltage protection and differential protection are mainly used as backup protection for existing DC transmission lines, which cooperate with traveling wave protection (the main protection of DC line) to form a complete DC transmission line protection system, so as to further improve the reliability of DC line operation [5][6][7].
At present, the protection setting of traditional AC system has formed a relatively complete calculation method. Compared with it, it is difficult to study the dynamic transmission characteristics of DC line under fault condition by analytical method. Especially, as the backup protection of DC line, the change of electrical quantity is significantly affected by DC control, so the setting calculation of DC line backup protection has been lack of systematic and effective methods. The function of each criterion, the basis of parameter calculation and the factors affecting the performance of protection are not fully understood [8][9][10][11]. In order to ensure the reliability of DC line protection, it is necessary to analyze the dynamic behavior of DC line backup protection.
In this paper, PSCAD / EMTDC electromagnetic transient simulation model for differential undervoltage protection and differential protection of DC line is established. The dynamic behavior of each protection criterion in the two protections under different fault conditions is analyzed, and the function of each criterion is also studied. The research results of this paper not only help to understand the function of the two protections, but also provide the basis for the setting of the protection, which has important theoretical and practical value.

Protection range and related fault types of DC line backup protection
Differential undervoltage protection and differential protection are equipped on the rectifier side of the positive and the negative line in HVDC system, and the protection range is also the full length of the positive or the negative line.  Table 1 for specific description of the fault: The parameters of the actual HVDC system used in the study are as follows: the rated voltage is 500kV, the rated current is 1.8kA, the bipolar rated transmission power is 1800MW, and the length of the DC transmission line is 960km. (1) It can be seen from formula (1) that the protection mainly consists of two criteria: one is the voltage change rate du/dt, and the other is the low voltage criterion (the absolute value of the DC line voltage). The criterion du/dt is the same as that in the traveling wave protection, such as the principle, calculation method and setting value, etc., which can be seen in reference [12]. In addition, as a backup protection, the differential undervoltage protection also has protection delay. After the differential undervoltage protection acts, DC line fault recovery sequence (DFRs) will be started. If the number of recovery exceeds the preset number, the pole will be locked [4].

Dynamic behavior of du/dt.
In the differential undervoltage protection, the voltage change rate du/dt is the same as that in traveling wave protection of DC line. Therefore, the analysis of its action behavior can refer to the published literature [12], and only the relevant conclusions are given in this paper: the response of du/dt to the faults at d 1 and d 2 is significantly greater than that to the faults at d 3 , d 4 , d 5 and d 6 .  3.1.4. Function analysis of differential undervoltage protection. The voltage change rate du/dt can distinguish the faults at d 1 and d 2 from those at d 3 to d 6 reliably. Therefore, the main function of the voltage change rate du/dt is to determine whether the fault occurs on the transmission line [12].
The low voltage criterion dl u can distinguish the fault at d1 from that at d2. Therefore, the main function of the low voltage criterion dl u is to determine whether the fault occurs on the positive line or the negative line, that is, it has the function of fault line selection.

DC line differential protection and its dynamic behavior analysis
The DC line differential protection is mainly used to detect the high resistance ground fault.
The operation equation of the DC line differential protection is Where idl and idl_os are the current measured at the rectifier side and the converter side respectively. At the same time, as the backup protection of DC line traveling wave protection and differential undervoltage protection, the DC line differential protection has protection delay to cooperate with other protections.
The dynamic response of

Conclusions
In the DC line differential undervoltage protection, the main function of the voltage change rate du/dt is to determine whether the fault occurs on the transmission line.
The main function of the low voltage criterion dl u in the DC line differential undervoltage protection is to determine whether the fault occurs on the positive line or the negative line.
In the DC line differential protection, the criterion dl_os dl -i i can not only distinguish whether the fault occurs on the line, but also distinguish whether the fault occurs on the positive or the negative line.

Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (NO.51567003).