0 Introduction Currently, fault location in high-voltage transmission lines is mainly based on impedance algorithms [1]. However, the accuracy of this algorithm in practical applications is typically no better than 3% for high-resistance grounding and multi-terminal power supply lines, and it is difficult to meet the line-finding requirements for long lines (>100 km). Modern traveling wave location utilizes voltage and current traveling waves that appear on the line after a fault occurs for precise fault location. Its measurement error is less than 1 km, and it is less affected by factors such as line type and grounding impedance [2]. Therefore, it has attracted increasing attention from scholars both domestically and internationally, and some products are already in operation in the field [3]. Traveling wave location can be divided into single-end location and double-end location. Single-end location calculates the fault location using the time difference between the first traveling wave propagating from the fault point to the busbar and the reflected traveling wave at the fault point [4]. Due to the reflection, refraction, and attenuation of the traveling wave at various primary equipment and line connections, the identification of the reflected traveling wave front at the fault point becomes complex. Dual-end positioning only uses the time when the first wavefront of the traveling wave arrives at both ends of the line for calculation. It only needs to capture the first wavefront of the traveling wave and does not need to consider the reflection and refraction of the traveling wave. The traveling wave amplitude is large and easy to identify. At the same time, due to the advent of the Global Positioning System (GPS) [5, 6], the accuracy of time measurement has been improved to the nanosecond level, thereby improving the accuracy of dual-end positioning (up to ±150 m). Therefore, GPS dual-end positioning system is widely used at home and abroad. However, in field operation, GPS dual-end positioning system also has some shortcomings: a. The sampling frequency is relatively high: 1 MHz to 5 MHz, and the fault information storage is too large. b. Due to the limitation of the sampling frequency, it is impossible to identify the traveling wave of a fault at close range [2]. From the characteristics of the traveling wave, we know that: where L is the distance from the fault to the busbar; v is the propagation speed of the traveling wave, which is close to the speed of light, v≈3.0×108 m/s; f is the highest traveling wave frequency that the positioning system can measure. Let f=150 kHz, then L=1 km, that is, the nearest fault point that can be identified is 1 km away from the busbar. c. Faults that ground near the zero-crossing point of the voltage cannot be detected [2]. d. Clock signal distortion caused by GPS short-term loss of synchronization, satellite signal adjustment, antenna interference, etc. [7] can lead to positioning failure. In view of the above defects of traditional devices, this paper proposes a new type of power grid traveling wave positioning system. It uses a special traveling wave sensor and a high-precision GPS clock with a clock-keeping function. If such a GPS traveling wave recorder is installed in each substation in the entire power grid, a GPS traveling wave measurement network can be formed to achieve accurate recording and positioning of various faults. [b]1 Traveling Wave Characteristics Analysis[/b] Taking a 500 kV line (as shown in Figure 1) as an example, various fault simulations were carried out using EMTP/ATP. The simulation results of the A-phase grounding fault at 1 ms at a distance of 306 km from the M end are shown in Figures 2 to 5. [img=313,111]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/8.gif[/img] Fig. 2 Travelling wave of voltage on the busbar M [img=230,146]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/9-1.gif[/img] Fig. 3 Travelling wave of voltage on the busbar N [img=237,146]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/9-2.gif[/img] Fig.4 Travelling wave of current on the busbar M [img=240,150]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/9-3.gif[/img] Fig.5 Travelling wave of current on the busbar N From Figs.2 to Fig.5, it can be clearly seen that: a. When a single-phase ground fault occurs in phase A, current traveling waves and voltage traveling waves are generated in each phase at both ends of the line. The amplitude of the traveling wave front is large and the frequency is high. b. The arrival time of the first wavehead of the traveling wave is relatively obvious, and the amplitude is significantly attenuated due to reflection and refraction at multiple points such as the busbar, transformer, and fault point. c. The voltage traveling wave has a relatively larger amplitude than the current traveling wave and is theoretically easier to measure. In actual operation, the high-frequency characteristics of the 500 kV line capacitive voltage transformer are poor and the cutoff frequency is too low, so a dedicated voltage traveling wave sensor needs to be made. d. Voltage traveling wave sensors are installed at the M, N, and B terminals respectively. When a fault occurs near the N terminal, the recording and location of the fault can be achieved using the M and B terminals. It has N-1 fault tolerance. [b]2 Overall Scheme Design[/b] As shown in Figure 6, a dedicated voltage traveling wave sensor is used to detect the traveling wave head. The hardware circuit is used to start and record the accurate arrival time of the traveling wave head. A high-precision crystal oscillator drives an accumulator to generate a high-precision clock signal. The time difference (TM-TN) between the arrival times of the first wavehead of the traveling wave at both ends of the line is used for fault location. The distance from the fault point to end M is LM: LM = 0.5 [v(TM-TN) + L] where L is the length of the transmission line. [img=353,201]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/9-5.gif[/img] Fig.6 Block diagram of GPS traveling wave fault location model 2.1 Extraction of traveling wave head Each substation only needs to install one dedicated voltage traveling wave sensor. Hardware detection of the voltage traveling wave head generates a start signal and directly records the start time. High-speed A/D acquisition and frequency multiplication circuits are not required, which simplifies the circuit structure, reduces storage requirements, and facilitates calculation and processing. 2.2 GPS Precision Time Generation: A microcontroller receives the GPS serial clock and generates year, month, day, hour, minute, and second signals. A high-precision temperature-controlled crystal oscillator (5 MHz, error less than 10⁻⁹ s) drives a 24-bit accumulator. A pulse signal of one pulse per second generated by the GPS periodically clears the accumulator, producing a clock signal with 0.2 μs precision. When the GPS is out of sync or malfunctions, a pulse signal can be temporarily generated by the high-precision crystal oscillator to periodically clear the accumulator, ensuring an error of less than 3.6 μs within 1 hour, i.e., a positioning error of less than 1.18 km. 2.3 Signal Storage: Only the arrival time information of the traveling wave front needs to be stored, totaling 10 bytes. Thus, a single 28256 chip can record 3276 traveling wave time information events. Assuming 10 reflected and refracted traveling waves are recorded for each incident or operation, 327 incidents or operations can be recorded, thus fully capable of recording progressive faults. For faults occurring near the zero-crossing voltage, they are generally permanent faults and can be calculated using the traveling wave generated at the fault point after reclosing. 2.4 Formation of the Measurement Network For a power grid of a certain voltage level, a traveling wave recording device is installed in each substation. When any point in the power grid is faulted or struck by lightning, a traveling wave is generated throughout the entire power grid. Therefore, the recording device in each substation will have a traveling wave start-up record. As can be seen from equation (1), for a near-distance fault in a substation, a high-frequency traveling wave will be generated in that substation, making it difficult for the sensor to start recording. However, in other substations, the traveling wave frequency decreases, making it easier to record. In this way, near-distance faults can be calculated using measurement data from other substations. Similarly, in the entire traveling wave measurement network, if any device fails or fails to start, the fault location can be performed based on the records of other devices, thereby improving the reliability of fault location. [b]3 Traveling Wave Recorder Design[/b] 3.1 Sensor Connection Taking a 220 kV power grid as an example, the voltage traveling wave sensor is a voltage divider, which is connected in parallel to the surge arrester inside the voltage pumping device (as shown in Figure 7, connected between point A and ground). [img=244,180]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/10-1.gif[/img] Fig.7 220 kV Coupling Capacitor and Carrier Device Wiring When there is no fault in the line, the voltage divider output signal is the power frequency signal, reflecting the voltage at point A (effective value is 100 V); when a fault occurs in the line, the fault traveling wave enters from the line, the surge arrester 1 operates, and the voltage between point A and ground is the voltage across the surge arrester 1. Because the surge arrester has a low operating voltage (2 kV～4 kV) and a fast response speed, when the traveling wave front arrives, a rapidly rising or falling transition signal will be generated on the voltage divider. This transition signal can be used to start the traveling wave recorder for recording. 3.2 Hardware and Software Design The hardware structure of the traveling wave recorder is shown in Figure 8. [img=282,168]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9910/image/10-2.gif[/img] Figure 8 Block diagram of hardware a. The GPS receiver uses the Japanese-made KODEN GSU-25 model, with a timing accuracy of 0.5 μs. b. The logic device uses an ISP (Very Large Scale Programmable Device), which realizes online programmability, facilitates the modification of logic circuit functions, and basically achieves the "softening" of logic circuit hardware. c. The 24-bit accumulator is driven by a crystal oscillator. When the GPS clock is received accurately, the pulse signal generated by the GPS clears the accumulator to zero; when the GPS clock fails to be received or the clock is unstable, the pulse signal generated by the accumulator periodically clears the accumulator to zero, causing the accumulator to count cyclically within 1 second, and the CPU is started to perform second counting. d. The locking of the arrival time of the traveling wave head is completed by the start signal starting the latch, and the start signal interrupts the CPU to record the start time. e. Communication is started by the protection trip signal, and after reading the time recorded by other traveling wave recorders in the entire power grid, calculation processing is performed to display the fault location; the dispatcher can also read the recording time of each traveling wave recorder to perform fault calculation. [b]4 Conclusion[/b] In summary, the GPS traveling wave fault location system of this power transmission network, based on a dedicated voltage traveling wave sensor and a high-precision GPS clock with a clock-keeping function, has the following characteristics: a. It uses a dedicated sensor, only needs to record the arrival time of the traveling wave head, the amount of recorded information is small, the number of records is large, and it can record developing faults. b. High-precision crystal oscillator-synchronized GPS timing is used, ensuring high timing accuracy and maintaining clock accuracy even in the event of GPS clock distortion or failure. c. A traveling wave measurement network based on the entire transmission network is formed, enabling collaborative measurement and providing N-1 fault tolerance for near-field faults or equipment failures. d. Only one set of equipment is needed per substation, resulting in a simple network structure and low investment. The transmission network GPS traveling wave fault location system is currently under development and testing. The hardware circuits and calculation methods of each part have passed simulation experiments and analysis, showing good results. [b]References[/b] ［1］Ye Ping, Zhang Zhe, Chen Deshu. Theory and Application Experience of a New Algorithm for HV Transmission Line Fault Location. Power System Technology, 1995, 19（7） ［2］Xu Bingyin. 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