Abstract: The clock synchronization of the central control station is one of the important factors affecting the stability and reliability of the power system. This paper first introduces in detail the principle and process of realizing the clock synchronization of the entire network of the central control station using GPS clock, then analyzes the shortcomings of the current clock synchronization system of the central control station under the jurisdiction of Tangshan Power Supply Company, and finally proposes an improvement scheme. In this scheme, the GPS clock is redundant and mutually redundant, which greatly improves the accuracy of the clock synchronization system. Keywords: Central control station clock synchronization GPS clock redundancy and mutual redundancy 1 Overview of the power system clock synchronization system The power system clock synchronization system uses the global positioning system GPS clock to unify the time of the computer monitoring system, measurement and control device, line microcomputer protection device, fault recording device, and power energy billing system of power plants and substations, so as to realize the complete unification of the clock of the entire power plant and substation. The lack of synchronization of the entire network clock will cause some special faults, such as data and information loss, SOE event information logic confusion, some workstation crashes or even system paralysis[1]. Therefore, clock synchronization is one of the important factors affecting the stability and reliability of the power system. 1.1 GPS time synchronization GPS is a new generation of satellite navigation, positioning and time synchronization system that was fully built and put into operation in the United States in 1993. As shown in Figure 1, the GPS system consists of a ground control part (monitoring master station), a space part (GPS satellites), and a user part (receiver). GPS time synchronization uses the high-precision atomic clock carried by GPS satellites to generate reference signals and time standards, and provides global time services with a time synchronization accuracy of up to one part per two billion seconds. The power system mainly utilizes the precise time synchronization feature of GPS. At any time, the GPS receiver can simultaneously receive signals from 4-8 satellites within its field of view. Its internal hardware circuit and processing software decode and process the received signals, extract and output two time signals: (1) a pulse signal PPS with a time interval of 1s, whose pulse leading edge has a synchronization error of no more than 1μs with the international standard time (Greenwich Mean Time); (2) the international standard time and date code corresponding to the PPS pulse leading edge output through the serial port. If the PPS signal is used as the standard clock source to synchronize the various clocks running in the power grid, the high-precision synchronous operation of the clocks of each plant can be guaranteed [2]. 1.2 Clock Synchronization Principle Modern power systems are equipped with various automated devices, such as measurement and control devices, RTUs, fault recorders, microcomputer protection devices, time-of-use energy meters, etc. These automated devices all have real-time clocks inside. Real-time clocks are actually electronic clocks. Electronic clocks inevitably have errors: (1) inaccurate initial value settings; (2) quartz crystal oscillation frequency errors and temperature drift and aging drift of the frequency oscillation; (3) changes in the capacitance of capacitors in the circuit, etc. As time goes by, the accumulated errors will become larger and larger. Therefore, it is necessary to calibrate the electronic clock regularly. The principle is the same as how we calibrate our watches every day, setting it once according to a certain time reference signal at certain time intervals. This process of realizing automatic clock calibration is called clock synchronization. At present, using GPS satellites to obtain time reference signals is a convenient and economical means. The GPS clock receives the precise time signal from the GPS satellite as the time reference signal and converts it into the time signal output required by various automated devices to achieve time unification of various automated devices. Figure 1 Composition of GPS system 2 Central control station clock synchronization system The central control station automation system is divided into two parts: the main station system and the sub-station system. The main station system refers to the central control station, and the substation system refers to the various substations controlled by the central control station. Usually, within the entire network of the central control station, the central control station receives the GPS time and then synchronizes the time with the monitoring system of each substation through the communication channel to ensure that the time is consistent throughout the entire network of the central control station. The realization of the network clock synchronization is generally divided into three steps: (1) Synchronize the time of each operator station, front-end machine, and server in the central control station. (2) The central control station front-end machine sends a synchronization message to synchronize the time with the monitoring system of each substation. (3) The substation monitoring system synchronizes the time with each bay protection, measurement and control device. 2.1 Synchronization within the central control station Synchronization of the time of each operator station, front-end machine, and server in the central control station is to ensure the consistency and integrity of the entire network when data is added, changed, or deleted. Inconsistency and incompleteness of data will cause the main and backup systems to switch or historical data storage to fail to correctly identify the consistency and integrity of the data, thereby causing the loss of information and data, and even leading to the paralysis of the system [3]. After prolonged operation, the quartz crystal oscillator chips in the operator station, front-end processor, and server may experience clock inaccuracies. Therefore, appropriate time synchronization methods are needed to achieve clock synchronization within the central control station. As shown in Figure 2: the operator station, historical server, SCADA server, and front-end processor are connected to network A and network B, respectively. The time signal output by the GPS clock is directly connected to network A and network B, and time synchronization commands are broadcast over the network to synchronize with each server, thereby achieving clock synchronization within the central control station. Figure 2 Clock Synchronization within the Central Control Station 2.2 Time Synchronization within Substations The purpose of clock synchronization within substations is to ensure the accuracy of system event sequence recording and accident reconstruction, providing a basis for accident investigation. After time synchronization between the front-end processor and GPS within the central control station, a precise time will be available. The front-end processor sends time synchronization messages to each substation at regular intervals, ensuring that the monitoring systems of each substation are consistent with the time of the central control station. The monitoring system broadcasts time synchronization commands over the network to synchronize with the protection, measurement, and control devices in each bay, thereby achieving time synchronization with the central control station. This achieves time unification for all equipment within the entire network of the central control station. Some substations have already installed GPS clock devices. Under normal circumstances, equipment within a substation synchronizes its time using the substation's GPS clock. Occasionally, the GPS clock may malfunction. In such cases, the equipment synchronizes its time based on synchronization messages sent by the central control station. 3. Current Status of the Clock Synchronization System at Tangshan Power Supply Company's Central Control Station Currently, most of Tangshan Power Supply Company's 220kV substations have installed GPS clocks. Most 110kV integrated automation substations have also installed GPS clocks; however, conventional 110kV substations do not. Therefore, many 110kV substations rely on synchronization messages sent by the central control station for clock synchronization. Furthermore, even in substations with GPS installed, the GPS clock may malfunction. For example, in August 2007, the GPS clock at the 110kV Longwangmiao substation failed, causing inconsistent protection action times and making it impossible to determine the sequence of events, resulting in significant inconvenience for operators. In March 2008, the GPS clock at the 220kV Jinyintan substation failed, causing time synchronization issues among the various monitoring and control devices within the substation. During periods of GPS clock malfunction, clock synchronization still relies on time messages sent from the central control station. Therefore, the accuracy of the central control station's clock is crucial. Currently, among the central control stations of Tangshan Power Supply Company: The Hedong central control station has a GPS clock synchronization system installed and is operating normally, although occasional satellite signal loss occurs. The Gudong central control station has a GPS clock synchronization system installed. In December 2007, the historical server GD4 crashed due to hardware problems. In March 2008, when the historical server GD4 resumed normal operation, the time error reached 19 hours, and historical data storage was abnormal. The Runbei central control station has a GPS clock synchronization system installed. In April 2007, the GPS antenna failed, resulting in a cumulative time error of 21 minutes. The urban central control station does not have a GPS clock synchronization system installed. There is an error of about 1 minute per month, requiring manual time adjustment. The Daodi central control station also does not have a GPS clock synchronization system installed. There is an error of about 1 minute per month, requiring manual time adjustment. It is evident that the time of the substations under the jurisdiction of Tangshan Power Supply Company is not very accurate, and the time synchronization system is not ideal, which will affect the stable and reliable operation of the system. 4. Design of the Central Control Station Clock Synchronization System A failure to receive a GPS antenna signal or damage to it will severely affect the accuracy of the GPS clock. To improve the accuracy of the time reference signal, we improved the original GPS clock synchronization system. (Source: http://tede.cn) Figure 3: GPS Clock Redundancy and Mutual Backup As shown in Figure 3, two GPS clocks are set up in the central control station. The antennas of the two clocks are kept as far apart as possible to ensure that at least one antenna can receive satellite signals at any given time. The time signal receiving units of Clock A and Clock B are connected via optical fiber, serving as backups for each other. The strong lightning protection performance of optical fiber reduces the probability of equipment damage caused by lightning strikes. The time signal output unit is connected to the network, and only one clock outputs a time signal at any given time. When the master clock time signal receiving unit malfunctions, such as failing to track satellites, antenna damage, or other issues, it will automatically switch to the backup clock signal to ensure the normal output of the reference time signal. References [1] Fang Kexing; Application of GPS and Relay Protection Precise Time Synchronization; Xuzhou Power Supply Bureau; [2] Chen Jing, Zhang Chengxue; Research on the Implementation Method of GPS-based Time Marking System; Electronic Technology Application; 2004, Issue 03 [3] Lu Liming; Requirements and Implementation Methods of Clock Synchronization in Power Grid Dispatch Integrated Automation System; Dispatch Center of Jiangmen Heshan Power Supply Branch of Broadcasting Group