Method for setting the constant value of traveling wave protection based on the transmission characteristics of DC voltage divider

Author： GOZ Electric Time：2024-11-13 09:29:18 Read：10

1 DC voltage divider structure and its transfer characteristics

For two DC voltage dividers with different structures, first, their topological structures are introduced and equivalent circuit models are established; second, the transfer functions of the two DC voltage dividers with different structures are derived; finally, based on the derived transfer functions, the frequency response characteristics and step response characteristics of the two DC voltage dividers with different structures are analyzed.

1. 1 DC voltage divider structure

The DC voltage divider structure is shown in Figure 1. It consists of four parts: a DC voltage divider body, a balancing unit, a secondary voltage divider board, and an electronic isolation device. The 800 kV DC line voltage is reduced to 70V after passing through the DC voltage divider body; a gas discharge tube is installed in the balancing unit to protect the secondary system; in the subsequent module, the secondary voltage divider board further reduces the 70V voltage to 5V; the 5V voltage signal is sent to the UHV DC protection system after passing through the electronic isolation device.

1.1 Transfer function of DC voltage divider

Since the balancing unit only plays a protective role in the DC voltage divider and has no effect on the overall transfer characteristics of the DC voltage divider, only the transfer functions of the DC voltage divider body, the secondary voltage divider board and the electronic isolation device are derived in this section.

1.2 Influence of the transfer characteristics of the DC voltage divider on the performance of traveling wave protection

In order to study the influence of the transfer characteristics of the DC voltage divider on the performance of traveling wave protection, this chapter analyzes the influence of the transfer characteristics of the DC voltage divider on the fault characteristic quantity of traveling wave protection through the simulation of typical internal and external faults and disturbances. Combined with the judgment and logic of traveling wave protection, the influence of the transfer characteristics of the DC voltage divider on the output results of traveling wave protection is studied.

2.1 Principle of traveling wave protection

The basic principle of traveling wave protection is: first, by constructing an unbalanced voltage equation, polar waves and ground mode waves are generated; second, the polar waves and ground mode waves are differentially calculated to generate the fault characteristic quantity of traveling wave protection; finally, the generated fault characteristic quantity is compared with the fixed value to form the polar wave criterion and ground mode wave criterion; if the criterion meets the fixed value requirements of traveling wave protection, a comparison signal is output, which is output as a traveling wave protection action signal after passing through the traveling wave protection logic composed of delay, widening and other links.

2.2 Influence of DC voltage divider transmission characteristics on traveling wave protection

Based on the parameters of Xiangjiaba-Shanghai UHV DC project, the simulation model of ±800 kV bipolar UHV DC transmission system and its traveling wave protection is established using PSCAD/EMTDC software. The rated transmission capacity of the system is 6400 MW, the line model adopts the frequency-variable parameter model, and the total length of the line is 1935 km. The relevant fixed values of traveling wave protection are shown in the table (omitted).

This section simulates and analyzes the influence of the DC voltage divider transmission characteristics on the traveling wave protection by setting typical in- and out-of-area faults and system disturbances, combined with the traveling wave protection criteria and logic. Since the focus of this paper is the influence of the DC voltage divider on the protection performance of the UHV DC transmission system, the influence of the current transformer transmission characteristics on the traveling wave protection performance is not considered during the simulation.

3. 1 Protection setting principle

Before setting the set value of the traveling wave protection, the functions of each criterion in the traveling wave protection should be clarified first. Different setting principles should be adopted for criteria with different functions.

3. 2 Analysis of influencing factors and formulation of simulation conditions

The UHV DC transmission system has different operating conditions such as rated operation and step-down operation. Under different operating conditions, the traveling wave protection will automatically switch the set value size. Regardless of the operating conditions, the factors that affect the set value setting are the same, and the simulation conditions set when setting the set value are similar. Therefore, this paper takes the rated operating condition as an example to analyze the influencing factors and formulate simulation conditions.

1) Consider the influence of the DC voltage divider's transmission characteristics on the setting of the set value. The DC voltage divider's transmission characteristics affect the transmission of the characteristic quantity of the traveling wave protection fault, thereby affecting the protection performance. Therefore, the influence of the DC voltage divider's transmission characteristics cannot be ignored when setting the set value. When formulating the simulation conditions, this paper characterizes the transmission characteristics of two types of DC voltage dividers in the form of a transfer function. The line voltage first passes through the transfer function and then is sent to the protection system. This is used to measure the influence of the DC voltage divider's transmission characteristics on the traveling wave protection performance, and different traveling wave protection set values are proposed for different types of DC voltage dividers.

2) Consider the influence of the transition resistance on the setting of the set value. The larger the transition resistance, the smaller the traveling wave amplitude. Therefore, in order to make the traveling wave protection set value have a certain ability to resist transition resistance, the focus is on high-resistance faults when simulating faults within the zone, and the focus is on metallic grounding faults when simulating faults outside the zone. This paper refers to the on-site joint debugging data when setting the transition resistance. The transition resistance is set to 100 Ω when simulating within the zone, and the metallic grounding fault is set when simulating faults outside the zone.

3) Consider the influence of fault location on setting. In order to improve the reliability of the set value, the full length of the line is considered when simulating faults in the area, and the fault condition at the end of the line is focused on.

Conclusion

This paper analyzes the influence of the DC voltage divider transmission characteristics on the traveling wave protection, and proposes a method for setting the traveling wave protection setting taking into account the influence of the DC voltage divider transmission characteristics. The conclusions are as follows: 1) The transmission characteristics of different DC voltage dividers are different. The transfer function of the type I voltage divider will significantly decrease in amplitude in the high frequency band, and its step response has no overshoot, and the response time is long. The transfer function of the type II voltage divider increases first and then decreases in amplitude in the high frequency band, and its step response has a large overshoot, and the response time is much shorter than that of the type I voltage divider.

2) The DC voltage divider transmission characteristics will affect the performance of the traveling wave protection. When the type I voltage divider is used, the amplitude of the traveling wave protection fault characteristic quantity decreases; when the type II voltage divider is used, the amplitude of the traveling wave protection fault characteristic quantity increases.

3) This paper proposes a method for setting the constant value of traveling wave protection taking into account the transmission characteristics of the DC voltage divider. The proposed method enables the traveling wave protection to effectively identify faults inside and outside the zone, and improves the reliability of the traveling wave protection. This paper only analyzes the influence of the transmission characteristics of the DC voltage divider on the traveling wave protection and its setting value. Other protections involving voltage quantities in the UHV DC system are also affected by the transmission characteristics of the DC voltage divider. The influence of the transmission characteristics of the DC voltage divider on other protections involving voltage quantities can be studied in the future.