July 11, 2020

Time Synchronization Solution of GPS Synchronous Clock in Power System

1. GPS Synchronous Clock and Output 1.1 GPS Synchronous Clock The Global Positioning System (GPS) consists of a set of satellites launched by the US Department of Defense in 1978 and has a total of 24 satellites operating in 6 geocentric orbital planes. Internally, the number of satellites visible on Earth has varied from 4 to 11 depending on time and place.

The GPS synchronization 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 synchronization 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 GPS synchronization clock can ensure the accuracy of the clock travel time as long as it receives one GPS satellite signal.

As the standard clock of thermal power plants, our basic requirements for GPS synchronous clocks are: At least 8 satellites can be tracked at the same time, with the shortest possible cold and warm start-up time, backup battery, high-accuracy and flexible configuration clocks. output signal.

1.2 GPS synchronous clock signal output At present, the GPS synchronous clock output signal used by the power plant mainly has the following three types:

1.2.11 PPS/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) has several coding standards A, B, D, E, G, and H (IRIG Standard 200-98). Among them, IRIG-B coding is most used in clock synchronization applications, and there are bc level shift (DC code), 1 kHz sinusoidal carrier amplitude modulation (AC code) and other formats. 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.

The seconds, minutes, hours, and days (days since January 1 of the current year) are represented by BCD codes, and the Control Functions (CF) and the Straight Binary Seconds Time of Day (SBS) are Serial binary "0" padding (CF and SBS optional, not used in this example).

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 through the EIA standard serial interface and is output 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 Synchronous Clock in Power Automation System There are numerous systems or devices in the power automation system that need to synchronize with the GPS synchronous clock, such as DCS, PLC, NCS, SIS, MIS, RTU, fault recorder, and microcomputer protection device. When determining the GPS synchronization clock, the following points should be noted: (1) These systems belong to the thermal control, electrical, and system disciplines. If it is decided that the GPS synchronization clock provided by the DCS manufacturer achieves time synchronization (currently common practice), then the DCS contract Before negotiations, cooperation between professionals should be carried out to determine the requirements of the clock signal interface. (GPS synchronization clock can generally be configured with different number and type of output modules. If the relevant requirements cannot be determined in advance, the corresponding contract clauses should leave room for adjustment.)

(2) Whether each system shares a set of GPS synchronization 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 major has a large difference in the type or accuracy requirements of the interface of the GPS synchronous clock signal, the GPS synchronization clock can be configured separately. This can reduce mutual restraint between professionals, and secondly, the system clock synchronization scheme can be more easily realized. In addition, when the systems are far away from each other (for example, the water 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, a GPS synchronization clock may also be set up locally. Separate GPS synchronization clock also helps to 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 the thermal power plant are interconnected, the clock synchronization protocol of each system may not be the same, so it is still necessary to separately access the GPS synchronous clock signal. 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, synchronization with the GPS synchronization clock should be considered separately.

Second, the new DNTS clock synchronization method

The new DNF4533 series of time and frequency products manufactured by China Shinsei Co., Ltd. using advanced algorithms and highly reliable devices can meet current and future demand for time and frequency.

DNF4533 uses a high-precision timing GPS receiver and a low-phase noise, low-drift dual thermostat high stability crystal oscillator, patented frequency measurement and control technology, precision measurement and calibration of the crystal oscillator output frequency, so that the GPS Tamed crystal output frequency Precise synchronization The DNF4533 is used as a reference clock source (PRS) on a GPS system. It provides highly stable primary clock synchronization signals that provide self-monitoring. Outputs 2048kb/s, 2048kHz, 1PPS, and IRIG-B signals. A Class 1 reference clock source that meets the requirements of ITU-T G.811.

Used as a retime device. The retiming function combines the device's tracking GPS (or terrestrial reference) good timing reference signal with the service code stream signal so that the service code stream can pass the timing reference signal well if the device itself degrades or powers down. , The device automatically starts through mode.

As a CDMA clock source. It can provide clock source for CDMA base stations. It can provide 19.6608Mhz square wave signal, PP2S signal, 10M sine signal or square wave signal. Customizable signals: 16.384MHz, 14.4MHz. (for digital cluster applications)

DNF4533GPS synchronous clock source products have been widely used in telecommunications, mobile communications, power and transportation, networking, digital broadcasting, metrology testing, astronomical observations, aerospace monitoring and control, defense and military and other departments.

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, the clock and clock synchronization error 3.1 clock error As we all know, the computer's clock is generally used quartz crystal oscillator. 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 for a day can reach 1.73s; if it is working at ambient temperature 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, there are various high-stability crystal oscillators available for applications requiring high-accuracy clocks, such as TCXO (temperature-compensated crystal), VCXO (voltage-controlled crystal), and OCXO (temperature-controlled 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 GPS Synchronization Clock and UTC (Coordinated Universal Time) Errors Transmitted by Satellite This part of the error is determined by the accuracy of the GPS synchronization 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 synchronizes the time. 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 Synchronization Clock The master clock on the DCS network and the GPS synchronization clock are synchronized through the "hardwired" method. The standard time code and hardware of the GPS synchronous 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 byte output from the RS-232, or decodes the card with the symbol carrier cycle count or the high frequency pin for the IRIG-B encoding, the master clock and the GPS synchronization clock The accuracy of synchronization 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 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 some 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 have been applied in practice. For example, Network Time Protocol (NTP), which is widely used on the Internet, can provide ±1 ms time accuracy on a DCS LAN (such as GE's ICS decentralized control system), and the IEEE1588-based standard precision time protocol (Standard Precision). Time Protocol (PTP) enables real-time control of master and slave clocks on Ethernet for submicrosecond synchronization.

Fourth, the normal digital scanning rate clock accuracy SOE design <br> <br> While DCS reached 1ms, but ≤1ms SOE resolution to meet the requirements of a long period of time, people are always follows the same design The method, that is, place all the SOE points under one controller, and the event triggering the switch signal to hardwire the SOE module, the reason is that there are some errors in the clocks of different controllers. In this regard, Siemens described in the practical situation of its FUPN module distributed configuration of the TXP system, due to the clock can not be synchronized and can not achieve 1msSOE resolution, even more because of the clock difference of nearly 100ms, resulting in SOE event recording sequence Reversed.

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 utility SOE should be separated, or the trip signals of the MFT and ETS that want 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 commonly used method is to synchronize the controller or SOE module clock directly with the external GPS synchronization clock signal . For example, in ABB Symphony, the punctual master module (INTKM01) of the SOE ServerNode (generally located on the public DCS network) receives the IRIG-B time code and links the generated RS-485 clock synchronization signal to each controller (HCU). The SOE time synchronization module (LPD250A), its onboard 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 (DPU4E) can be synchronized with IRIG-B, making 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 DCS clock synchronization via the network is still less accurate, so that each control The deviation between the 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

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