1. Introduction to the GPS Global Positioning System In 1973, the United States launched the first NAVSTAR satellite, beginning the construction of the world's first Global Positioning System (GPS). The final configuration of this positioning system consists of 24 NAVSTAR satellites, whose signals effectively cover the entire Earth. The GPS system transmits a signal to Earth once per second, containing time information with an accuracy of 1 μs. This signal can be received from anywhere in the world. To correctly receive this signal, the GPS receiver receives two parts. The first part is a 1PPS strobe pulse produced every second with a start time accuracy of 1 μs. The second part is a string of information, including the time (Universal Coordinating Time) in UTC, the date, and the receiver's own location. The GPS system uses a special signal modulation technique. After decoding the received signal, the receiver can align its own clock with the satellite clock, measure the distance between itself and the satellite, and calculate its own location (latitude and longitude). The receiver can compensate for the transmission delay of the signal between the satellite and the receiver, output a second pulse gating signal with an error of 1μs compared to International Standard Time (UTC), and output information such as international standard time, date, and location through the serial port. 2. Introduction to Satellite Synchronization Clocks Currently, microcomputer-based fault recording devices, event recording devices, automatic safety devices, and remote control devices are increasingly used in power grids. Strict requirements have been placed on clock synchronization, aiming for accuracy at the 1ms or even μs level. The emergence of the GPS system perfectly meets this requirement. GPS receivers can transmit very accurate time information, but this information is fixed. It must be converted to meet the synchronization requirements of various devices already in use or to be used within the system. Different manufacturers and users have different synchronization requirements. Some use pulse synchronization methods with different amplitudes, frequencies, and delays, while others use standard serial encoding methods, such as MSF or IRIG-B formats. Most users prefer to use local clock formats (such as Beijing time) rather than UTC clock formats. Therefore, a protocol converter has inevitably emerged. This protocol converter transforms the fixed information sent by the GPS receiver into various output formats to meet the requirements of different devices and users. This protocol converter is commonly known as a GPS synchronization clock, and its block diagram is shown in Figure 1. (1) GPS signal receiver: used to receive GPS satellite signals, output 1PPS pulse with a time accuracy of 1μs, and output UTC standard time, date and receiver location information via RS-232 port. The receiver antenna is installed in a plastic rod with a diameter of about 3cm and a height of about 8cm. The antenna should generally be installed on the roof to provide a wide field of view. (2) Pulse circuit: outputs second (1 PPS), minute (1 PPM), hour (1 PPH) synchronous pulse signals. The output format can be level output or static dry contact output. (3) Central processing unit: converts the UTC standard time information from GPS into local time, sends it to the LCD display, and outputs it via serial port according to a certain format. (4) RS-232/485 interface: outputs local time, date and other information once per second. The baud rate is selectable. (5) MSF, IRIG-B, BCD interface: output time and date codes according to their respective standard formats. [img=300,331]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/zjdl-3/23-1.jpg[/img] 3 Application of GPS Satellite Synchronization Clocks Due to the accuracy and openness of GPS positioning and timing systems, they are widely used in power systems. They can be used for fault location, fault recording, state determination, motor excitation and speed regulation, power angle measurement, etc. In terms of protection, they have been used for power system out-of-step protection, line current differential protection, etc., and also for the integrated automation system of the power grid and the synchronization and accurate timing of relay protection devices. 3.1 Standard Clock Synchronization Sources Using the same signal to synchronize all clocks in the power grid in real time or periodically can achieve the purpose of unifying the clock. Currently, there are roughly three ways to synchronize clocks: (1) The power grid central dispatching station synchronizes the clocks in the system through the communication channel; (2) Using the wireless time signal of the radio station, television station, and observatory; (3) Using the clock signal of the GPS global positioning system. The first synchronization method is the most common in current telemetry systems, but it requires channel time. Because the signal transmission delay varies between different plants and stations, it can only guarantee a time error within the millisecond range and places high demands on the channel. The second synchronization method is significantly affected by weather and is highly dependent on the geographical location of the plants and stations. It is also susceptible to electromagnetic interference, leading to signal loss. The third synchronization method is currently the most ideal: GPS clock synchronization. The GPS system sends a signal once per second, with a time accuracy within 1μs. It can be reliably received anywhere in the world, making it an ideal clock source. GPS satellite synchronization clocks offer various interface output methods, such as pulse synchronization, serial port synchronization, and coded synchronization, fully meeting the synchronization requirements of various devices. Currently, in the East China Power Grid, synchronization and time setting have been implemented for microprocessor-based fault recorders (HATHWAY's DFR16/32, ABB's INDACTIC650, and METHA's THRANSCAN) and microprocessor-based line protection devices (LFP-900 series and WXB-11 series), and their operation is satisfactory. 3.2 Phase Measurement To ensure the stable operation of the power system, it is necessary to control the voltage phase difference between key points in the system. After unifying the system clock, the sampling pulses of the input signal can be synchronized, and the phase relationship of the voltage between various substations can be easily measured using software methods. To ensure the accuracy of phase measurement, the synchronization error of the sampling pulses must be very small, strictly controlled within a few μs. For a 50 Hz system, 1 degree corresponds to 55 μs. Using radio stations or similar methods for time setting is clearly unsuitable, as their time setting errors are in the millisecond range. 1 ms represents an 18-degree phase difference for a 50 Hz system, which is absolutely unacceptable. Only GPS satellite clocks can meet this requirement. Utilizing the 1 PPS pulse synchronization method of GPS satellite clocks, the sampling pulse time error of the entire system can be kept within a few μs, and the corresponding phase angle measurement error is no greater than 0.5 degrees, fully meeting the system requirements. 3.3 Fault Ranging The emergence of GPS satellite clocks has created favorable conditions for developing devices based on the principle of traveling wave ranging. After a line fault, the normal load current jumps to a short-circuit current. This generates a traveling wave surge of current moving from the fault point to both ends of the line. Assuming the total length of the line is L and the propagation speed of the traveling wave is V, the times at which the initial traveling wave surge is received at both ends M and N after the fault are Tm and Tn, respectively. After exchanging information through the communication network, the distances from the fault point to both ends M and N can be calculated as follows: Xm = L/2 + (Tm - Tn) × V/2 Xn = L/2 + (Tn - Tm) × V/2 The key to the traveling wave ranging principle is to accurately record the time when the initial traveling wave of the fault arrives at both ends of the line, and the error should be strictly controlled within a few μs. For overhead lines, a 1 μs time error corresponds to a ranging error of approximately 150 m. For power cables, a 1 ms time error corresponds to a ranging error of approximately 70–100 m. Using the 1 PPS pulse from a GPS satellite clock and serial port time information, the error requirements can be easily met. A traveling wave ranging device based on this principle has been tested in the Northeast China Power Grid. For conventional impedance-based ranging devices, after clock synchronization, the phase relationship of the fault power supply at both ends of the line can be accurately calculated, compensating for the calculation error caused by the transition resistance during a fault. 3.4 Improving the Relay Protection Test Device The longitudinal protection devices of the line are installed on both sides of the line. Conventional joint commissioning tests are time-consuming and labor-intensive, and can only be performed using a single-end method. Using a GPS satellite synchronized clock, the test devices at both ends can be started in a pre-agreed time sequence, and the fault quantity can be applied to the protection devices on both sides, allowing for a more comprehensive verification of the protection device's operation. Currently, DOUBLE's F2000 series, OMICRON's CMC series, and PRO-GRAMMA's FREJA300 series relay protection integrated testers all have GPS synchronization clock interfaces, enabling end-end test mode. During the commissioning of the 500 kV Tianping line, the F2253 relay protection integrated tester was used to implement end-end test mode on GEC's LFCB-102 phase-differential current protection device, ensuring the stable operation of the protection device. [b]4 Conclusion[/b] With the increasing demands on power system operation and the development of automation technology, the requirements for synchronization clocks are becoming increasingly stringent. GPS satellite signal synchronization clocks can well meet the synchronization time requirements of power systems, greatly facilitating the development of power system accident analysis, fault location, stability judgment, and control technologies.