GPS network time service system

GPS time service system

The GPS time service system is a high-tech product for the computer, control device, etc. in the automation system. The GPS time service product obtains the standard time signal from the GPS satellite and transmits the information to the automation system through various interface types. Equipment that requires time information (computers, protection devices, fault recorders, event sequence recording devices, safety automation devices, telecontrol RTUs) can achieve time synchronization of the entire system.

The GPS time service system developed by Beijing Zhongxinchuang can be applied to military command systems and dispatch systems; time stamp servers in the financial industry; telecommunication network management systems, billing systems, distributed database servers, and railway dispatch systems, etc. all require LAN time synchronization. And strongly urges that there is a standard network time source for the entire system.

Foreword

With the rapid development of computer and network communication technologies, the era of digitization and networking of thermal power plant automation systems has come. This aspect provides a better platform for data exchange, analysis, and application among various control and information systems. On the other hand, it also puts forward higher requirements for the accuracy of various real-time and historical data time tags.

The use of an inexpensive GPS clock to unify the clocks of various plant-wide systems is the standard practice currently used in the design of thermal power plants. The master clock of the unit distributed control system (DCS), auxiliary system programmable logic controller (PLC), plant-level monitoring information system (SIS), and power plant management information system (MIS) in the power plant is obtained through an appropriate GPS clock signal interface. Standard TOD (year, day, hour, minute, second) time, and then according to the respective clock synchronization mechanism, the slave clock deviation within the system is limited to a sufficiently small range, so as to achieve the whole plant clock synchronization.

First, GPS clock and output

1.1 GPS clock

The Global Positioning System (GPS) consists of a group of satellites launched by the US Department of Defense in 1978. A total of 24 satellites operate in the six geocentric orbital planes, which are visible on Earth according to time and location. The number of satellites has been constantly changing from 4 to 11 stars.

The GPS clock is a receiving device that receives a low-power radio signal transmitted by a GPS satellite and calculates the GPS time by calculation. In order to obtain accurate GPS time, the GPS clock must first receive signals from at least four GPS satellites and calculate its own three-dimensional position. After the specific position has been obtained, the accuracy of the clock can be guaranteed by the GPS clock receiving only one GPS satellite signal.

As a standard clock for thermal power plants, our basic requirements for GPS clocks are: At least 16 satellites can be tracked at the same time, with the shortest possible cold and warm start-up times, back-up batteries, and high-accuracy, flexibly configurable clock outputs. signal.

1.2 GPS clock signal output

At present, there are mainly three types of GPS clock output signals used by power plants:

1.2.1 1PPS/1PPM output

This format outputs one pulse per second or per minute. Obviously, the clock pulse output does not contain specific time information.

1.2.2 IRIG-B output

IRIG (the Inter-Range Instrumentation Group of the United States) shares coding standards A, B, D, E, G, and H (IRIG Standard 200-98). Among them, IRIG-B encoding is most widely used in clock synchronization applications, and there are formats such as bc level offset (DC code) and 1 kHz sinusoidal carrier amplitude modulation (AC code). IRIG-B signal output one frame per second (1fps), each frame is one second long. One frame has a total of 100 symbols (100pps), each symbol is 10ms wide, and the binary 0, 1 and position flag bits (P) are represented by symbols of different positive pulse widths, as shown in Figure 1.2.2-1.

For ease of understanding, Figure 1.2.2-2 shows an example of the output of an IRIG-B time frame. The seconds, minutes, hours, days (days since January 1 of that year) are represented by BCD codes, and the Control Functions (CF) and the Straight Binary Seconds Time of Day (SBS) are It is filled with a string of binary "0" (CF and SBS are optional; this example is not used).

1.2.3 RS-232/RS-422/RS-485 Output

This clock output sends a series of date and time messages expressed in ASCII code via the EIA standard serial interface, once per second. Parity check, clock status, and diagnostic information can be inserted in the time message. This output currently has no standard format. The following figure shows an example of sending a standard time with 17 bytes:

1.3 Application of GPS Clock in Power Automation System

There are many systems or devices in the power automation system that need to be synchronized with the GPS clock, such as DCS, PLC, NCS, SIS, MIS, RTU, fault recorders, and microprocessor protection devices. The following points should be noted when determining the GPS clock:

(1) These systems belong to the thermal, electrical, and system disciplines. If it is decided that the GPS clocks provided by DCS vendors implement time synchronization (currently common practice), cooperation between specialists should be carried out before the DCS contract negotiations to determine the clocks. Signal interface requirements. (The GPS clock can generally be configured with different numbers and types of output modules. If the relevant requirements cannot be determined in advance, the corresponding contractual terms should leave room for adjustment.)

(2) Whether each system shares a set of GPS clock devices should be based on comprehensive consideration of the difficulty of system clock interface coordination and the geographical location of the system. If each profession has a large difference in GPS clock signal interface type or accuracy, GPS clocks can be configured separately, which can reduce mutual restraint between professionals, and can make clock synchronization schemes of various systems easier to implement. In addition, when the systems are far apart (for example, the chemical treatment plant and the desulphurization plant are far away from the centralized control building), in order to reduce the electromagnetic interference when the clock signal is transmitted over a long distance, the GPS clock can also be set on the spot. Dividing the GPS clock also helps reduce the impact of clock failures.

(3) IRIG-B code has high reliability and interface specification. If the clock synchronization interface is optional, it can be used preferentially. However, it should be noted that IRIG-B is only a generic term for Class B coding, and it is divided into multiple types (such as IRIG-B000, etc.) according to whether the coding is modulated or not, whether there is CF or SBS, etc. Therefore, the corresponding decoder card should be configured on the clock receiving side. , otherwise it is impossible to achieve accurate clock synchronization.

(4) 1PPS/1PPM pulse does not transmit TOD information, but its synchronization accuracy is high, so it is often used for clock synchronization of SOE modules. Although the RS-232 time output is used more often, but because there is no standard format, the design should pay special attention to confirm whether the clock signal is authorized and whether the format of the two clock messages can be agreed.

(5) Although the control and information systems in thermal power plants are interconnected, the clock synchronization protocol of each system may not be the same. Therefore, GPS clock signals must still be connected separately. Even if the bridge DCS and public DCS are connected via a bridge, if the clock synchronization signal has a large delay in the network, each should be considered to be synchronized with the GPS clock.

Second, Siemens TELEPERMXP clock synchronization

Take the Siemens TXP system as an example here to see how the internal DCS and the clock are synchronized.

TXP's power plant bus is a CSMA/CD-based Ethernet with two master clocks on the bus: a real-time transmitter (RTT) and an AS620 and CP1430 communications/clock card. Under normal circumstances, RTT as the main clock of the TXP system, after about 40s, CP1430 as backup clock will be automatically replaced (in fact, can be configured on the ES680 2 blocks) CP1430 as a backup master clock). See Figure 2-1.

The RTT is free running or can be synchronized with an external GPS clock via the TTY interface (20mA current loop). Synchronous with the GPS clock is a serial message (length 32 bytes, 9600 baud, 1 start bit, 8 data bits, 2 stop bits) and seconds/minute pulse two ways.

The RTT generates and sends a master clock pair message at the network layer and sends it to the plant bus every 10 seconds. The RTT sends a maximum of 1ms for sending the time message. If the message cannot be sent to the bus within 1ms, the sending of the time message is cancelled. If the message sending process is interrupted, a message of the current time is immediately generated. The clock message has a multicast address and a special frame header. The date is the number of days from 1984.01.01 to the current day, and the time is 00:00:00,000h from the current day to the current ms value with a resolution of 10ms.

The OM650 obtains time messages from the plant bus. In the OM650, the Unix function is used to transfer time to SU, OT, etc. on the terminal bus. Usually one PU is used as a time server, and other OM650 devices are registered as an offshore customer.

After the AP620 of the AS620 starts up, it can automatically synchronize the clock with the CP1430 by calling the "synchronize" function block. The CP1430 then aligns itself with the AP every 6 seconds.

The accuracy of the TXP clock is as follows:

It can be seen from the above-mentioned TXP clock synchronization mode and clock precision that the various clocks in the TXP system adopt the master-slave hierarchical synchronization mode, that is, the lower-level clock is synchronized with the upper-level clock, and the higher the precision of the upper-level clock.

Third, clock and clock synchronization error

3.1 Clock Error

As we all know, computer clocks generally use quartz crystal oscillators. The crystal oscillator continuously generates clock pulses of a certain frequency, and the counter accumulates these pulses to obtain the time value. Since the clock oscillator's pulse is affected by various instability factors such as ambient temperature, load capacitance, excitation level, and crystal aging, the clock itself is inevitably subject to errors. For example, for a clock with an accuracy of ±20ppm, the hourly error is: (1×60×60×1000ms)×(20/10.6)=72ms, and the cumulative error of one day can reach 1.73s; if the ambient temperature of its work is Changing from the rated 25°C to 45°C will also increase the additional error of ±25ppm. It can be seen that if the clock in the DCS is not regularly calibrated, the error after free running for a period of time can be unacceptable to the system application.

With the development of crystal manufacturing technology, a variety of high-stability crystal oscillators are currently available for applications requiring high-precision clocks, such as TCXO (temperature-compensated crystal), VCXO (voltage-controlled crystal), and OCXO (constant-temperature crystal) )Wait.

3.2 Clock Synchronization Error

If you analyze the clock synchronization method similar to TXP, it is not difficult to find that the absolute time error of the DCS generated by the clock in the top-down synchronization process can be composed of the following three parts:

3.2.1 UTC (Coordinated Universal Time) Errors of GPS Clocks and Satellite Launches

This part of the error is determined by the accuracy of the GPS clock. For the 1PPS output, with the leading edge of the pulse as a punctual edge, the accuracy is generally between tens of nanoseconds and 1μs. For the IRIG-B code and RS-232 serial output, for example, the product of the time clock of the National Time Service Center of the Chinese Academy of Sciences, it is synchronized. Accuracy is measured by the deviation of the leading edge or start of the reference symbol relative to the 1PPS front, which is 0.3 μs and 0.2 ms, respectively.

3.2.2 Synchronization Error between DCS Master Clock and GPS Clock

The master clock on the DCS network is synchronized with the GPS clock via the "hardwired" method. The standard time code and hardware of the GPS clock output are generally accepted by a clock synchronization card in a DCS site. For example, if the receiving end compensates for the transmission delay of the ASCII code bytes output by the RS-232, or decodes the symbol carrier cycle count or high frequency pin decoding for IRIG-B encoding, the master clock and the GPS clock Synchronization accuracy can reach very high precision.

3.2.3 Synchronization Errors of Master-Slave Clocks at DCS Sites

The master clock of the DCS synchronizes with the slave clocks of the stations through the network. There are transmission delays, propagation delays, and processing delays of the clock messages. The performance is as follows: (1) When the master clock generates and sends a time message, the kernel protocol processing, the overhead of the operating system to invoke the synchronization request, the time to send the time message to the network communication interface, etc.; (2) at the time Before surfing the Internet, it is necessary to wait for the network to be idle (for Ethernet) and retransmit in case of collision. (3) After the time packet is online, it takes a certain time to transfer from the master clock end to the child clock end through the DCS network medium (electromagnetic waves are The propagation speed in the optical fiber is 2/3 of the speed of light. For DCS LANs, the propagation delay is several hundred ns, which is negligible; (4) After the network communication interface from the clock side confirms that it is a time message, it accepts the message. It also takes time to record the arrival time of the message, issue an interrupt request, calculate and correct the slave clock, and so on. These delays cause more or less time synchronization errors between the master and slave clocks of the DCS and slave clocks.

Of course, different network types of DCS, different clock communication protocols and synchronization algorithms can make network synchronization accuracy different from each other. The above analysis is based on general principles. In fact, with the unremitting research on network clock synchronization technology, a variety of complex but efficient and accurate clock synchronization protocols and algorithms have emerged and been applied in practice. For example, the Network Time Protocol (NTP), which is widely used on the Internet, can provide ±1ms time accuracy (such as GE's ICS decentralized control system) on a DCS LAN, and a standard precision time protocol based on IEEE1588 ( The Standard Precision Time Protocol (PTP) enables sub-microsecond synchronization of master and slave clocks on real-time control Ethernet networks.

Fourth, clock accuracy and SOE design

Although DCS's conventional switching rate has reached 1ms, in order to meet the requirements of SOE resolution ≤ 1ms, people have been following this design method for a long period of time, that is, all SOE points are placed under a controller. The reason that the event-triggered switching signal is hard-wired into the SOE module is due to the fact that there are some errors in the clocks of different controllers. In this regard, Siemens described in the actual situation of the FUN B module distributed configuration of its TXP system, due to the clock can not be synchronized and can not achieve 1ms SOE resolution, even due to the clock difference of nearly 100 ms, resulting in SOE events Reverse the record order.

Then, how to meet the requirements of the SOE decentralized design of the project (for example, if the utility DCS is set, the unit SOE and the utility SOE should be separated, or the trip signals of the MFT and ETS that wish to enter the controller do not need to be output and return to the SOE module. Can be used for SOE, etc., but not too much to reduce the SOE resolution? Through the analysis of DCS products is not difficult to find, the usual approach is to synchronize the controller or SOE module clock directly with the external GPS clock signal. For example, in ABB Symphony, the punctual main module (INTKM01) of the SOE ServerNode (generally located on a public DCS network) receives IRIG-B time coding and links the generated RS-485 clock synchronization signal to each controller (HCU ) SOE time synchronization module (LPD250A), its on-board hardware timer clock can be connected with 1PPM synchronization pulse and cleared automatically once per minute; for example, the MAX1000+PLUS's distributed processing unit (DPU 4E) can be synchronized with IRIG-B. The DPU's DI point can be used as SOE at the same time. Due to the use of 1PPM or RS-485, IRIG-B hard-wired clock "out-of-sync" to avoid the problem that the current accuracy of the DCS clock via the network synchronization is still poor, so that each The deviation between the controlled clocks is kept within a small range, so the SOE point decentralized design is feasible.

It can be seen that the design of SOE should be determined in the engineering design with the characteristics of DCS adopted. It is not possible to equate a 1 ms switching rate or 1 ms controller (or SOE module) clock relative error with a 1 ms SOE resolution, thereby simply spreading the SOE points throughout the system. At the same time, it should also be noted that SOE points are “dispersed” compared to “centralized”, although the resolution is reduced, but as long as the relative error of the clock is small (such as an order of magnitude with 1ms), it can fully meet the actual needs of power plant accident analysis. of.

V. Conclusion

5.1 At present, the control systems of thermal power plants are no longer independent islands of information. A large amount of real-time data needs to be time-stamped in different places and sent to SIS and MIS for various applications. Therefore, the clock synchronization scheme of various systems and the accuracy of the clock synchronization to be achieved should be carefully considered in the design.

5.2 In DCS design, not only must pay attention to the absolute accuracy of the master and slave clocks of the system, but also should pay attention to the relative error between the clocks. Because if you want to decentralize the design of SOE points while not excessively reducing the resolution of events, the key is that the deviation of each clock should be as small as possible.

5.3 There is every reason to believe that with the continuous development of network clock synchronization technology, it will become very common to synchronize the system clocks through the network with high precision. In the future, the accuracy of each system of the power plant will be greatly improved. Applications based on high-precision clocks, such as the decentralized SOE design, will continue to emerge.

Beijing Zhongxinchuang Technology Co., Ltd.

Technical exchanges: Mr. Sun QQ46416346

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