DCS event sequence recording performance analysis and testing
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The Distributed Sequence Control System (DCS)'s Event Sequence Recording (SOE) system captures and records the state change information of the digital signals with millisecond resolution, providing clear and effective clues and evidence for the analysis of thermal and electrical equipment accidents. Operation status monitoring, recording, and accident analysis play an important role. Therefore, the understanding of the measurement principles and performance indicators of SOE and the mastery of test methods have important practical significance.
First, the SOE measurement principle The SOE of DCS obtains and records the status change information and the occurrence time of the digital signal through an open module with the SOE function, and uploads the controller to the DCS through the DCS internal bus, and then the controller via the ether. The network is passed to the operator station (or historical data station) and stored in the SOE database for viewing by operators and maintenance personnel.
The difference between the SOE switch quantity and the ordinary switch quantity is that the time resolution of the SOE switch quantity is not greater than 1 ms, and the time of the switch quantity jump is recorded. The time resolution of the ordinary switch quantity is not more than 100 ms and the time of the switch quantity jump is not recorded. The open modules of the DCS are generally intelligent modules. The CPU of the open module checks whether the status of the channel signal changes through high-speed cyclic scanning and comparison of the open channel, and whether the SOE is set according to the open channel setting. The time of the high-speed cyclic scan is the time resolution of the opening module. The controller of the DCS periodically sends the synchronization timing signal and the summoning command to the connected module, and the opening module sends the SOE information data to the controller according to the summoning command.
In addition to the SOE time resolution, SOE performance indicators include the system SOE transmission rate, data capacity, and packet loss rate. The SOE transmission rate refers to the number of transmission events per second in the system. The rate is related to the internal bus transmission rate of the DCS and the number of calls per second by the controller. Generally, it is more than 200 per second. The SOE data capacity refers to the operator station storage. The number of events is generally more than 5,000. Due to the limited number of SOEs, the actual system SOE transmission rate and data capacity can meet the application. The packet loss rate refers to the number of lost SOE events. Generally, the packet loss rate is low. It should be less than one ten-thousandth.
Second, the error analysis of SOE resolution in the "Coalition Control System Technical Specifications for Thermal Power Plants" (G-RK95-51) 3.6.3.3 [1], provides that the time resolution of SOE should not be greater than 1 ~ 2ms, in the new The time resolution in the revised discussion paper that will be changed to SOE should not be greater than lms. The factors that actually influence the size of the SOE time resolution of the DCS include the channel scan cycle of the open module, the reference clock error of different open modules and different controllers, and the clock synchronization error. SOE time resolution can be estimated according to equation (1).
SOE time resolution = ΔT1 + ΔT2 + ΔT3 (l)
In the formula: ΔT1 represents the scanning cycle of the opening module; ΔT2 represents the time difference between different opening modules; ΔT3 represents the time deviation between different controllers.
The scanning cycle of the opening module is the time interval for the CPU module to open the channel at equal intervals. It is determined by the running speed of the CPU and is generally less than 0.5 ms.
The time difference between different open modules is mainly caused by the module's reference clock error and clock synchronization error. Incoming module clock synchronization is accomplished by receiving the controller synchronization signal that is connected to it. The module and the controller synchronize the signal transmission in two ways: one is through the communication bus that the module is connected to the controller. The synchronization accuracy of these methods is in the range of tens of microseconds or more; the other is through dedicated switching pulse transmission. The synchronization accuracy of this method is in the microsecond range.
The time difference between different controllers is mainly caused by the controller's reference clock error and clock synchronization error. There are two ways of clock synchronization of the controller: one is to synchronize the clock of the controller through an operator station (computer) via Ethernet to synchronize the clock. In this way, the synchronization accuracy is generally in second and cannot be used as the SOE clock. Synchronization; the other is to send a pulse synchronization signal through a dedicated time standard device to synchronize the clock of the controller. The synchronization accuracy of this method can be in microseconds or more.
It can be seen from the above that the size of the SOE time resolution is mainly determined by the size of the scanning cycle of the intrusion module; if the controllers do not pass the dedicated time synchronization, the SOE deviation of the two controllers may be large.
Third, SOE time value error analysis Because DCS SOE time value is accurate to milliseconds, and the time indication accuracy is lms. If the event sequence time is 1.Oms and 1.9ms, the display value is lms; the event sequence time is 1.9ms and 2.Oms, and the display values ​​are lms and 2ms, respectively. It can be seen that, although the two events record a few milliseconds, the actual time indication may have an error of lms.
In the actual measurement, measured by the event sequence signal generator with the interval 1 ms, although the SOE resolution of the DCS is below 0.5 ms, the interval between the SOE measurement indications will appear in addition to lms, and occasionally the phenomenon of Oms and 2 ms will occur. Value accuracy lms caused.
Tests show that when the time resolution of SOE is 0.4ms, the probability of occurrence of Oms and 2ms in SOE measurement interval is about 20%; when the time resolution of SOE is 0.2ms, the probability of Oms and 2ms is 2%. Left and right; the smaller the time resolution of the SOE is, the smaller the probability that the SOE measurement shows the interval between the Oms and the 2ms.
It can be seen that although the resolution of the DCS's entry module can be very high (less than 0.2ms), since the SOE recording time indication accuracy is 1 ms, the SOE measurement error is still 1 ms, and only the recording time indication accuracy is improved to 0.lms. In order to reduce the SOE measurement error. In practical applications, mechanical motion response is in the range of tens of millimeters or more, and the accuracy of the lms indication is sufficient to meet practical application requirements.
IV. SOE test The SOE resolving power of thermal power plant DCS has been clearly specified in Item 6.7.4 of the “Declaration of Acceptance Test for Thermal Power Plant Distributed Control System†(DL/T659-998) [2].
The switch signal generator is a device dedicated to SOE measurement. It can send multiple (16 or 32) switching quantities that change sequentially at a set time interval. The interval between each path is equally spaced and the accuracy can be set. The time interval can be arbitrarily set in 0. lms - l, the time setting step is 0. lms, absolute error ≤ 0. O1ms. Connect the switch quantity of the switch signal generator to the specified SOE input channel of the SOE system under test, so as to test the performance index of the SOE input channel.
The test of the open module SOE resolution is to connect the output signal of the digital signal generator to multiple input terminals of the same module and change the interval time of the signal generator until the SOE cannot be resolved. The signal generator interval is the SOE resolution of the open module. Since the SOE time indication accuracy is 1 ms, when the generator time interval is less than 1 ms, the channel time record cannot be resolved, but the SOE of the input module can be estimated by whether the sequence order of the channel time record is consistent with the signal sequence. Resolving power. The problem to be paid attention to is to prevent the signal from being generated in the same order as the module query processing module channel and cause misjudgment. The sequence of the input channel of the signal access module can be changed to check whether the sequence of the time records of the incoming channel changes or not. The above problem.
The control system can be composed of a single controller or multiple controllers. The SOE resolution of a single controller system is the SOE resolution of the controller. The SOE resolution of multiple controller systems must consider the time error between multiple controllers.
The SOE resolution of the controller is formed by the synchronization error between the scanning cycle of the opening module that is connected to the controller and the opening module. The test method is to connect the output signal of the switch signal generator to the input terminals of different input modules that are connected to the same controller, change the interval time of the signal generator, and record whether or not the signal time coincides with the time of the passage. Judgment, the problem to be noted is that the order of the time records of different modules cannot be used as a criterion.
The SOE resolution test of multiple controllers must be performed under the condition of time synchronization between the controllers. The output signals of the switch signal generators are connected to the input terminals of the input modules connected by different controllers. Change the signal generator's interval time, and judge whether the channel time record is consistent with the signal generation time.
In practice, SOE performance tests generally pass several tests. The test results are random and incomplete. Only through hundreds of consecutive tests, can statistical analysis be conducted to fully and accurately understand the SOE performance of a control system. This requires that the digital signal generator can periodically issue multiple switching quantities that change sequentially at set time intervals, and the period is 0.5s, ls, and the like. Through the analysis of the SOE database, you can confirm whether the system's SOE resolution is stable, whether the incident records are correct and complete, and whether event records are lost.