GPS clock system (satellite clock synchronization) is the infrastructure of intelligent substation

Time:2020-10-12

GPS clock system (satellite clock synchronization) is the infrastructure of intelligent substation

GPS clock system (satellite clock synchronization) is the infrastructure of intelligent substation

Beijing Zhun electronic technology official micro enterprise — ahjzsz

1. Overview

In modern power grid, a unified time system is of great significance for fault analysis, monitoring and control and operation management of power system. The time synchronization of substation refers to the process in which the protection, measurement and monitoring equipment in the substation adopt corresponding time synchronization methods to realize the synchronization with the standard clock source time protection, so as to ensure the consistency of power system real-time data acquisition, provide accurate time basis for system fault analysis and processing, improve the efficiency and reliability of power grid operation, and improve the power grid The level of accident analysis and stability control can improve the accuracy of fault location, phasor and power angle dynamic monitoring, unit and power grid parameter calibration.

Conventional transformer is used in traditional substation, and the transmission delay of primary and secondary electrical quantities is very small, which can be ignored. As long as the secondary electrical quantities of relevant TA and TV are sampled at a certain time according to the sampling pulse of automatic devices such as relay protection, the simultaneity of data can be ensured. The data acquisition module of relay protection and other automation equipment in intelligent substation is moved forward to the merging unit. The primary electrical quantity of transformer needs to be collected by the front-end module and then processed by the merging unit. Because the acquisition and processing links of each interval transformer are independent, there is no unified coordination, and the transmission and transformation of primary and secondary electrical quantities are added with delay links, the output data of each interval electronic transformer does not have the simultaneity, and can not be directly used for the protection calculation which requires high data synchronization. Therefore, clock synchronization is the basis of network sampling synchronization. In the power system, the merging unit, synchronous phasor measuring device, fault recorder, electrical measurement and control unit, remote terminal, integrated protection and control device, microcomputer protection device, automatic safety device, electric energy acquisition device, master station of computer monitoring system, terminal equipment of distribution network and automation system of distribution network need time synchronization The requirements are shown in Table 1

1.1 basic concept of time

Time is a basic parameter of physics, and also one of the basic forms of material existence. It is the fourth dimension of the so-called space coordinate. Time represents the continuity of material movement and the sequence and duration of events. Its biggest feature is that it is impossible to keep constant.

Here are several different timing methods:

1. Universal time: ut / ut0 / UT1 / uT2

The mean solar time observed at Greenwich Observatory in England is called universal time, which is recorded as UT, and has been used up to now.

The world time directly obtained by observing stars is called ut0.

The earth’s rotation axis is not fixed, so it is necessary to modify ut0 by polar shift, and the world time obtained by polar shift correction is recorded as UT1, then UT1 = ut0 + Δ λ.

The earth’s rotation rate has irregular changes, and the rotation rate is slowing down. If UT1 is corrected for the periodic change of the earth’s rotation rate, uT2 is obtained. That is, uT2 = UT1 + Δ TS = ut0 + Δ λ + Δ ts.

2. Atomic time / International Atomic Time: Ta / Tai

The research of atomic physics and quantum physics tells us that the electrons around the nucleus will produce energy level transition, and the frequency standard is the frequency standard when the atom changes from high energy level to low energy level, that is, the so-called atomic frequency standard (atomic frequency standard). The time measurement system based on atomic frequency standard is called atomic time, abbreviated as Ta.

The atomic time established by the international time bureau is named Tai when it is designated as international atom by the international metrology conference.

3. Coordinated universal time: UTC

Coordinated universal time (UTC) is mainly used in China’s power system, which represents the combination of the two time scales of International Atomic Time (TAI) and universal time (UT1). UTC is defined as

UTC (T) – Tai (T) = N seconds (n is an integer)

|UTC(t)—UT1(t)|<0.9s

The specific implementation method of UTC is to cancel the frequency offset adjustment, so that the UTC second length is strictly equal to the Tai second length, and UTC is close to UT1 in time. In this way, the difference between UT1 and Tai caused by the nonuniformity of the earth’s rotation rate is compensated by adding 1s or subtracting 1s in UTC time. The time of leap second is set on June 30 or December 31, which means that the last minute of UTC on June 30 or December 31 is 61S or 59S. Due to the nonuniformity of the earth’s rotation speed, in the past 20 years, the world time is about 1 s slower than atomic time every year, and the difference between them has accumulated year by year, reaching 35 s by 2013.

1.2 common time service system

Clock source is used to provide standard clock signal. Time service system mainly includes wireless time service and wired time service. The wireless time service system includes GPS (Global Positioning System) navigation system of the United States, Galileo navigation system of Europe, Beidou navigation system of China and global navigation satellite system of Russia (glass), etc.; the wired time service system takes network or special line as carrier, such as communication network time service system. At present, the main clock sources used in substation are GPS satellite time service and Beidou time service technology.

(1) GPS satellite time service

GPS (Global Positioning System), or global positioning system, was developed in the United States since the 1970s. GPS system receives signals from satellites by special receivers, which can obtain position, time and other relevant information. GPS system sends signals once a second, and its time accuracy is within 100ns. Its time information includes year, month, day, hour, minute, second and 1PPS (standard second) signal, so it has high frequency accuracy and time accuracy. Using GPS satellite synchronous clock in the substation can realize the operation monitoring and fault analysis of all systems under the unified time benchmark.

(2) Beidou time service technology

Beidou satellite navigation system is a global satellite navigation system independently developed by China. It is similar to GPS in the United States and Galileo positioning system in Europe. It provides all-round global navigation and positioning services at sea, land and air. At present, it has developed to the second generation, and the timing accuracy can reach 20ns. At present, 13 Beidou navigation system networking satellites have been successfully put into the scheduled transfer orbit. It is expected that the “Beidou” satellite navigation and positioning system, which is composed of more than 30 satellites, covering the world, will be built in 2020. Beidou time system, referred to as BDT, is a continuous time system. The length of second is SI second, and the starting epoch is 0:00:00, January 1, 2006, coordinated universal time (UTC). The deviation between BDT and UTC is within 100ns.

Smart grid

1.3 intelligent substation time service technology and time synchronization system

The substation GPS time synchronization system is composed of master clock, extended clock and time synchronization signal transmission channel. The master clock and extended clock are composed of time signal receiving unit, time holding unit and time synchronization signal output unit.

Due to the high demand for time synchronization acquisition in intelligent substation, in order to ensure the consistency of real-time data acquisition time, the intelligent substation should be equipped with a set of time synchronization system common to the whole station, and the master clock should be dual configured. Clock synchronization accuracy and punctuality accuracy meet the time accuracy requirements of all equipment in the station, and the error prevention of abnormal clock information and transmission delay compensation of master-slave clock meet the synchronous sampling requirements of intelligent substation.

The smart substation should adopt the main standby time synchronization system, which is composed of two master clocks, multiple slave clocks and signal transmission medium, to synchronize the time for the equipment / system being served.

The master clock adopts dual configuration, supports Beidou time service system and GPS standard time service signal, and Beidou time service system is preferred. The master clock is used to time the slave clock, and the slave clock is the timing device / system to be timed. The time synchronization accuracy and time keeping accuracy meet the requirements of time synchronization accuracy of all equipment in the station. SNTP timing mode should be adopted for station control layer equipment, DC IRIG-B code timing mode should be adopted for Bay layer and process layer equipment, and IEEE1588 network timing mode can also be used when conditions permit. According to the needs and technical requirements, the master clock can be reserved with an interface to receive the wired time reference signal sent by the upper level time synchronization system.

In the intelligent substation, the technical characteristics and main indicators of time device are as follows:

(1) Multi clock signal source input seamless switching function. With signal input arbitration mechanism, 1PPS output is stable within 0.2 μ s during signal switching.

(2) Error proof function of abnormal input information. When the external input signal is disturbed, the time information can still be accurately output.

(3) High precision timing and punctuality. The time synchronization accuracy is better than 1 μ s, the second pulse jitter is less than 0.1 μ s, and the time keeping performance is better than 1 μ s / h.

(4) Slave clock delay compensation function. The effect of transmission medium on the delay of second pulse is compensated.

(5) Provide high precision and reliable IEEE 1588 clock source.

(6) Support DL / T860 modeling and MMS networking.

(7) Rich timing mode, flexible configuration. Support RS232, RS485, air contact, optical fiber, network and other time synchronization modes.