Research on embedded merging unit based on IEC61850 and Windows CE ZHANG Xiao-yu, ZHAO Long-zhang, FANG Zhi (College of Automation, Nanjing University of Industry, Nanjing 210009, Jiangsu province, China)
ABSTRACT : Merging unit is an important part of the secondary protects equipment and electronic transducer interface which defined in the IEC60044. This paper has raised a plan of design merging unit based on ARM9 chips and IEC61850, discussed and analyzed the possibility of using a single process chip and introduce a real-time and multitask operation system: Windows CE, as the operation system of merging unit. We also divide the work of the merging unit, analysis how to realize the multi-thread and multi-priority programming on Windows CE. At the end, we discuss the advantage of using Windows CE and advanced some prospect of the application of integrate Windows CE and IEC61850.
Key word: ARM; Windows CE; real-time; multitask; embedded system; merging unit; IEC61850
Abstract : Merging units are an important component defined in IEC 60044 for the interface of electronic instrument transformers and secondary protection control equipment. This paper proposes a merging unit design based on an ARM chip and Windows CE. The feasibility of using a single chip in terms of processing power is theoretically analyzed. A Windows CE embedded operating system supporting real-time multitasking is introduced as a platform to divide the tasks to be completed by the merging unit. The paper analyzes how to implement multi-threaded processing with different priorities through programming. A comparison is made with a merging unit using FPGA+DSP. Finally, some prospects are proposed for the application of the combination of Windows CE and IEC 61850 in power systems and for the functional expansion of the merging unit.
Keywords : ARM; Windows CE; real-time multitasking; embedded systems; merging unit; IEC61850
1. Introduction
With the development and maturation of electronic instrument transformer technology, and the emergence and practical application of electronic instrument transformers using new design methods in recent years, my country has gradually begun to digitize the design and construction of substations. To address the problems of incompatibility between networks and communication protocols used by different manufacturers in digital substation equipment, cumbersome communication protocol conversion, low reliability, and high maintenance costs, the International Electrotechnical Commission (IEC) issued protocol standards such as IEC 60044-7/8 and IEC 61850-9-1. IEC 60044 first introduced the concept of a "merging unit" as an important component of the interface.
Currently, the FPGA+DSP architecture is widely used in merging units because FPGAs have a unique FIFO (First-In-First-Out) structure, which facilitates the sorting and preprocessing of multi-channel sampled data. After high-speed data reception and restoration by the FPGA-based data restoration module, the data is sent to the DSP for processing and filtering of a series of complex protocols. With the development of embedded technology, ARM-based processors are becoming more widely used and the technology is maturing. The advantages of ARM processors, such as multi-pipeline design, higher clock speed, and more functions, are becoming increasingly apparent. To meet the high requirements of power systems for system stability and real-time performance, as well as the networking requirements of intelligent devices in digital substations under the IEC 61850 standard, this paper introduces Windows CE as the operating system for the embedded merging unit.
The embedded merging unit design method proposed in this paper, based on the ARM9 chip and Windows CE, leverages the advantages of the ARM chip, such as its high speed, strong processing power, rich interfaces, and powerful networking capabilities, as well as the strong real-time multitasking capabilities of Windows CE. Considering that the Substation Configuration Language proposed in the IEC 61850 specification follows XML syntax, and that WinCE fully supports the latest W3C XML specifications, and that Microsoft provides a powerful XML class library in the .NET Compact Framework 2.0, the operating system can more conveniently and quickly process data streams using XML format.
2. Definition and functional division of merging units
The IEC 60044-7/8 standard first introduced the concept of a new physical unit, the "merging unit," primarily designed for electronic instrument transformers using digital outputs. Typically, the merging unit takes the signals (12 channels) from the primary side, pre-processes them, and sends them to secondary protection and control equipment according to the encoding format and protocol specified in the IEC 61850 standard. Since the digitization of substations needs to be implemented gradually, the initial design of merging units in digital substations included both digital and analog signal interfaces.
Figure (1) Definition of the merged unit
The communication between the merging unit and the digital output interface of the electronic instrument transformer has the following important characteristics:
(1) Many tasks are processed at the same time.
(2) High reliability and strong real-time performance.
(3) Large communication information flow.
(4) The communication speed is relatively high[2].
According to the data processing flow in the merging unit, it can be divided into 4 processing steps:
(1) Send synchronous sampling signals to the high-voltage side transformer and receive the sampling data transmitted from the acquisition system in real time, process it accordingly, and make it into a digital signal that can be accepted and recognized by the system.
(2) Extract useful data from the converted data packets;
Figure (2) Data processing flow of the merged unit
(3) Process the data, including digital filtering, phase shifting, repackaging, etc.;
(4) Send the data to the secondary device via Ethernet.
Simultaneously, the merging unit also needs to address the data synchronization issue. In digital substations, several merging units operate concurrently, and it is crucial to ensure that the data collected by different merging units is sampled at the same point in time to avoid phase and amplitude errors. Currently, a commonly used synchronization method involves the substation's GPS module or high-precision clock sending a synchronized second pulse signal (synchronization signal 1) to each merging unit. Internally, the merging unit divides the second pulse to obtain the required clock signal (synchronization signal 2) and synchronizes the clock signal with the second pulse provided by the GPS once per second to ensure synchronized sampling between them.
3. Design of the merging unit
3.1 Feasibility Discussion
Firstly, in terms of hardware, this paper primarily uses an ARM9 chip to implement all control and data processing functions of the merging unit. The ARM microprocessor is a high-performance, low-power 32-bit microprocessor widely used in embedded systems. The ARM9 represents ARM's mainstream processor, featuring a high clock frequency and a 5-pipeline design, and has been widely used in handheld phones, set-top boxes, digital cameras, GPS, personal digital assistants, and internet devices. During the operation of the merging unit, sensors are controlled to sample the waveforms of current and voltage in the power grid. Assuming 80 sampling points per cycle, with a 250µs interval between each sampling point, the 200MHz Samsung 2410 processor has a 5ns clock cycle. Due to its multi-pipeline design, it can execute an average of 1.1 instructions per clock cycle, meaning 55,000 32-bit instructions can be executed between every two sampling points. This is sufficient to handle sampling, input, calibration, filtering, packaging, output, and some operations required by the operating system. If a 2440 processor with a clock speed of 400MHz is used, the system's processing power will be doubled, allowing more CPU time to be devoted to processing protocols, services, and user interfaces, resulting in better performance and reliability.
Secondly, in terms of software, Windows CE was introduced as the operating system of the merging unit. Windows CE is different from Linux. It is a strict hard real-time multitasking operating system with 256 levels of thread priority, supports nested interrupts, and features such as bounded interrupt response latency. Its powerful interrupt and thread scheduling mechanism and kernel operating system services can meet the time and performance requirements of different devices for time-critical tasks. According to the test, in a reference system with a main frequency of 200MHz, the real-time performance of Windows CE can reach at least 40-60µs[12]. Since the design in this paper uses a CPU with a main frequency of up to 400MHz, the real-time performance of the system can theoretically be improved by nearly double compared with the test, reaching or approaching 20-30µs, which is sufficient to meet the demanding requirements in power system applications. The multitasking function of Windows CE enables the system to handle data sampling, data processing, clock synchronization, data transmission, touch screen display and user input response tasks simultaneously. The higher the priority of the thread, the faster its response speed.
3.2 Hardware Structure
Due to various limitations during the digitization of substations, it's impossible to achieve full electronic and digital transformation of all current and voltage transformers in one step. For a considerable period, electronic and conventional transformers will coexist. Therefore, the merging unit will have both digital and analog signal inputs. Consequently, the merging unit should retain A/D conversion functionality. Since the built-in A/D conversion of the ARM9 chip has low accuracy, an external A/D chip is required. The A/D module and O/E module can be directly connected to the system bus, allowing the CPU to directly address them and promptly read the sampled data into the memory buffer.
Figure (3) Hardware block diagram of the merging unit
After the merging unit sends a synchronization sampling signal, the A/D unit and the O/E unit process the data simultaneously. The A/D unit is responsible for converting the conventional instrument transformer sampling signal into a digital signal that the computer can recognize; the O/E unit is responsible for receiving the digital signal from the electronic instrument transformer, unpacking it, and separating the valid information. Since both share the system bus, reading 12 data channels simultaneously will inevitably cause conflicts. In the design, the data is read sequentially according to the processing order of the 12 data channels within the specified sampling time limit.
Although both the Samsung S3C2440 and Windows CE provide good support for touchscreens, the current application of the merged unit does not require human-machine interaction, and using a touchscreen would consume some system memory bandwidth and resources. Therefore, in order to ensure the robustness of the system, a touchscreen was not introduced into the design.
3.3 Software Platform Design
In this paper, Windows CE is introduced as the operating system of the merging unit. Windows CE is an open, customizable, 32-bit real-time embedded window operating system. It is designed as a highly modular operating system to adapt to different types and requirements of smart devices. Designers can customize their own embedded real-time operating system by selecting only the necessary modules or components in the modules according to the nature of the device. Windows CE is divided into several different modules, of which the kernel, graphical window event subsystem (GWES), file system (Filesys), and communication module are the four main modules. A minimal Windows CE system consists of at least the kernel and file system modules [10]. Since the system applied on the merging unit does not require a touch screen human-computer interaction interface, the GWES module can be omitted or a SHELL with command line can be used during customization. During the operation of the merging unit, some tasks run simultaneously and cannot wait for each other. For example, when a synchronization signal is received, the system may be processing data or waiting for sample values. To solve this problem, multiple threads must be used to execute multiple tasks simultaneously. Windows CE supports 256 levels of thread priority. A thread is an executable unit. When the operating system creates a process, it also creates at least one thread. Using threads can make the most efficient use of CPU time. A process can include multiple threads, and each thread shares all resources, including the process address space [10]. According to the tasks to be completed by the merged unit, they can be divided into the following four threads: synchronization, sampling, data processing, and sending. Among these four threads, the synchronization thread can be awakened by the synchronization signal 1 and has the highest priority. The other three threads have lower priorities than the synchronization thread and are all awakened by events.
<1> Ideally, the merging unit receives a synchronization signal 1 provided by the substation via GPS or a high-precision clock every second, and then generates a synchronization signal 2 by frequency division according to the sampling requirements of each wave. The sampling signals sent by different merging units should be equally spaced. However, due to the existence of crystal clock errors, signal 2 cannot be completely equally spaced. Especially as time goes by, the error between signals 2 sent by different merging units will accumulate and increase, which is not allowed by differential protection. The real significance of introducing signal 1 lies in this: multiple merging units are forcibly synchronized once every second. Within 1 second, using a high-precision and high-stability crystal oscillator, the error of signal 2 sent by different merging units is very small. Different merging units should ensure sufficient synchronization accuracy when sending the first synchronization signal 2. Its sending time should be as close as possible to the rising edge of the signal 1 pulse, because this is the only reference that different merging units can follow together. Afterwards, signal 2 is sent by equal interval counting through the crystal oscillator of each merging unit [13]. The synchronization thread is responsible for receiving synchronization signal 1 from the substation and dividing it to generate synchronization signal 2. To ensure sufficient synchronization accuracy between synchronization signals 2 generated by different merging units, the synchronization thread must have the highest priority within the process. Upon receiving synchronization signal 1, it can immediately take over system control and divide the signal to generate synchronization signal 2. This ensures that the transmission time of synchronization signal 2 is as close as possible to the rising edge of synchronization signal 1. After transmitting synchronization signal 2, the synchronization thread will wake up the sampling thread to perform data acquisition.
Figure (4) Multithreaded programming flowchart
<2> Sampling Thread: After being awakened by the synchronization thread, the sampling thread will check within a set time whether the A/D module and O/E module have sent interrupt signals indicating that sampling/conversion has been completed. If so, it will read the sampled data into memory in sequence and arrange them in order. Then, it will wake up the data processing thread and pass the data buffer address pointer to it. If some channels fail to send sampled data within the specified time limit, all of them will be filled with 0 and an error flag will be set.
<3> Data Processing Thread The data processing module is mainly responsible for designing related digital filters for the received data signals. A digital filter is added to the merging unit to eliminate noise and high-frequency interference components introduced by the A/D converter. Then, the root mean square value and phase angle are calculated for the data. Since there are phase and amplitude errors between the output digital quantity and the actual current value, the amplitude error within the effective frequency band can be considered when designing the filter and determining parameters such as the transformation ratio coefficient. Therefore, the phase error has a greater impact than the amplitude error, and phase compensation data processing is necessary [1,14]. After data processing, the data is stored in memory. Then, the sending thread is woken up and the data area pointer is passed to the sending thread.
<4> The sending thread reads the data at the specified address after being awakened, and then sends the data to the secondary device via Ethernet in accordance with the communication frame format specified in the IEC61850 protocol.
When the sampling thread, data processing thread, or sending thread is running, if the synchronization thread receives a pulse of synchronization signal 1, it will be immediately awakened. At this time, the operating system will suspend the execution of other threads and allocate the processor time slice to the synchronization thread to process the synchronization signal. After the synchronization task is completed, the synchronization thread enters a waiting state again, and the operating system will continue to allocate the CPU time slice to the thread that did not complete its operation to continue its processing task.
Figure (5) Multi-threaded, multi-priority task processing
4. Conclusion
Compared to merging units using no operating system or those employing µC/OSⅡ FPGA+DSP architectures, the Windows CE operating system proposed in this paper offers superior multitasking capabilities and system function expansion capabilities. The Windows CE platform facilitates the development of IEC61850-based applications and functions. Furthermore, Windows CE provides excellent support for SQL databases, and the IDE interface or SD card read/write interface provided by the ARM9 chip allows for real-time backup of sampled data to a hard drive or expansion card. IEC61850 is a future-oriented open standard. With the increasing use of photocurrent and voltage transformers, the trend in modern power technology is to decentralize more and more bay-level functions to the process layer. Merging units using a combination of Windows CE and ARM facilitate easier function expansion and system upgrades.
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Manuscript received date:
Author Biography: Zhang Xiaoyu (1984-), male, from Bengbu, Anhui Province, holds a Master's degree and mainly engages in research on digital substations for power system automation. (Tel) 13776633800, (Email) [email protected]
