Application of ARM Embedded System in GIS Online Monitoring

GIS (gas insulated switchgear) is a power equipment with extensive and important applications in the field of power transmission, distribution and transformation. The closed structure of GIS increases the difficulty of operation and maintenance, and its hidden faults are more difficult to find, which may cause greater losses. In order to be able to understand the GIS operation status in real time and accurately, and to find and eliminate hidden faults in time, it is particularly important to implement online monitoring of GIS.

In order to regulate the development of automation systems in the power industry, the International Electrotechnical Commission proposed the IEC61850 standard, which puts forward interoperability and stability requirements for substation automation systems and intelligent devices. Interoperability means that data and information can be directly and effectively shared between two or more systems. In addition, the system also requires that the automation system has long-term stability in communication functions and can adapt to the rapid development of communication technology over a long period of time.

In view of the specific requirements of the IEC61850 system for substation automation systems, this paper studies several major domestic substation online monitoring systems. In existing online monitoring systems, fieldbus CAN (controller area network) is usually used as the main communication mode of the system, and 16-bit microcontrollers are mostly used as the core of the device. This mode has shown limitations in operation. For example, CAN as the main communication mode limits the deep interoperability of this system with other systems; secondly, the chip

The external interface of the chip is relatively simple, which makes it difficult to ensure the free configuration of system functions. Local function adjustments often affect the entire system. Studies have shown that the embedded operating system uClinux running on ARM (advanced RISC machine) to achieve Ethernet communication can ensure the interoperability of the GIS online monitoring system and the long-term stability of the communication function, so the ARM embedded system is introduced into the field of GIS online monitoring research.

ARM-based embedded systems

ARM adopts the modular design of the advanced microcontroller bus architecture AMBA, which has the advantages of comprehensiveness, speed and high performance-price ratio. The ARM processor has an ICE-RT functional unit, which can be used to set breakpoints in any part of the code, even in ROM, which reduces the difficulty of device debugging and lays the foundation for the stability of the device.

In the ARM processor, 32b fixed-length instructions and three-stage pipeline instruction operations are used (as shown in Figure 1). Instruction execution is divided into three stages: instruction fetch, decoding, and execution. The pipeline allows several operations to be performed simultaneously. During the execution of the first instruction, the second instruction begins to be decoded, and the third instruction is taken out from the memory at the same time. Therefore, the three parts of instruction fetch, decoding, and execution can be performed simultaneously, which ensures the high-speed processing performance of the processor.

Figure 1 Three-stage pipeline operation

The improvement of microcontroller performance has laid the foundation for the introduction of operating systems. Traditional embedded systems generally do not use operating systems, so the introduction of operating systems gives embedded systems new connotations and becomes a distinct feature that distinguishes them from traditional embedded systems. Embedded operating systems will replace traditional manually compiled monitoring programs or schedulers and become important basic components, allowing programmers to develop applications only for the operating system. Embedded operating systems are becoming the basis for ARM application software.

Application of the system in GIS online monitoring

Based on the ARM embedded system, the design of the GIS online monitoring device mainly includes the internal framework of the device, hardware configuration and software configuration, etc. The substation automation system based on the IEC61850 standard (as shown in Figure 2) has a significant feature: in the automation system, Ethernet communication (based on TCP/IP) is adopted between layers, replacing various field bus communication methods in traditional systems (such as Lonworks, CAN and RS-485, etc.).

Figure 2 Block diagram of online monitoring system

Hardware framework based on ARM embedded system

After adopting the ARM embedded system, the hardware circuit schematic diagram (as shown in Figure 3) is mainly divided into the following modules according to function:

1) ARM CPU module This circuit is the core of the embedded system. This system uses AT91M40800, which is based on the ARM7TDMI core and is specially made for industrial control. All indicators can reach industrial level.

2) CPLD expansion circuit

The main function of this circuit is to expand the peripheral functions of the CPU chip. For circuits with complex functional requirements, the CPU+CPLD mode is generally used. There are two main reasons for using this mode: a) As the core of the system, the CPU cannot and does not need to meet the specific functional requirements of all users. It only needs to provide users with a standard external bus interface EB I (external bus interface) so that users can perform necessary functional expansion according to their needs; b) CPLD has the following advantages in circuit design: address decoding can be easily realized; the programming method is simple, and the functions of various logic devices can be easily realized through software programming; the clock delay can reach nanoseconds, which is particularly suitable for applications in the field of online monitoring and has high reliability.

3) Flash FLASH module

After the system is powered off, the flash memory chip can permanently save the original software settings and data required for the system to restart. This article uses the AT49BV1614 flash memory chip produced by ATMEL, which has a capacity of 2MB and stores the embedded operating system uClinux kernel, system startup program, and user programs.

Figure 3 Schematic diagram of embedded system based on ARM 4) Fieldbus CAN interface circuit

This part of the circuit is used for communication between the main board and other data acquisition boards inside the device. The communication speed of CAN can be as high as 1Mb/s (within 40m); CAN has no strict restrictions on the communication medium, which can be twisted pair, coaxial cable or optical fiber, etc., which is relatively flexible; among them, the most important thing is that CAN communication has high stability and cost performance, so using CAN inside the device to realize communication between the main board and the auxiliary board is the most popular method in industrial control devices. As a peripheral circuit of ARM, the CAN bus adopts a dual CAN mode, which enhances the stability of the internal communication of the device on the basis of fast communication. The CAN bus communication circuit in the device is shown in Figure 4. SJA1000 is an industrial-grade CAN communication controller, 82C250 is a CAN bus driver, and 4 optocouplers are used for photoelectric isolation to reduce communication interference, and at the same time, the communication board can be plugged and unplugged with power on.

Figure 4 Dual CAN communication interface circuit

5) Ethernet module

The circuit uses the CS8900A industrial-grade Ethernet chip, which can reach a network speed of 10Mb/s. The online monitoring device transmits and shares data with the system's upper layer (such as the host computer, etc.) and other monitoring devices through this interface circuit to achieve interoperability.

Software framework based on ARM embedded system

The system software mainly includes three aspects: uCbootloader, uClinux and application software. uCbootloader is used for hardware system initialization, uClinux is an embedded operating system, and the application is a program module developed by the user to implement specific functions according to specific requirements. After the hardware system is powered on, uCbootloader is run to initialize the hardware system configuration and transfer the detected hardware to the operating system for unified control; the user program is built on uClinux, and it is responsible for booting and starting.

1) Embedded operating system uClinux

This device uses uClinux as the embedded operating system mainly considering the following factors:

a) The selected AT9M40800 does not have a memory management unit;

b) The uClinux kernel obtained by tailoring Linux-X is very small (about 900 kB), but it still retains the advantages of the Linux system, such as stability, powerful network functions and excellent file system;

c) uClinux has a complete TCP/IP protocol stack, which enables GIS monitoring devices to achieve standard Ethernet communication functions. Considering the real-time requirements of the system, this system adopts the kernel loading method, placing the kernel's compressed file on FLASH, and reading the compressed file when the system starts, decompressing it in the memory, and then running.

2) User Program

All applications in this embedded system are written in standard C language, and the compilation process is carried out in the cross compiler established under the Linux system. According to the functions to be realized by the embedded system, the application programs are mainly divided into the following parts: Ethernet communication, CAN bus communication and serial communication programs, etc. The following is a detailed discussion of Ethernet communication and CAN bus communication:

a) Ethernet communication

The TCP/IP protocol is a mature and widely used high-level network protocol. Because of its good openness, it is widely used in substation communication systems. The communication program based on TCP/IP mainly consists of two parts: server and client. The server and client can run on the host computer and the monitoring device to collect data and send corresponding network messages. Taking the client program as an example, the client program mainly includes the following functional links: establishing a socket, requesting a connection to the server, encapsulating data, receiving data and sending data, and finally releasing the socket, as shown in the following program snippet.

main (int argc, char 3 argv[])
{ .
sockfd= socket (A F-IN ET, SOCK- STREAM, 0);
.
connect (sockfd, &their-addr, sizeof (st ructsockaddr));
.
send (sockfd ,buf,MAXDA TA S IZE,0);
.
recv (sockfd,buf,MAXDA TA S IZE,0);
.
clo se (sockfd);}

b) CAN bus communication

The CAN bus communication software mainly consists of three parts: initialization program, sending program and receiving program. The initialization program sets various registers in the CAN controller, including mode register, time division register and output controller. Since these registers can only be set during reset, the CAN controller must be initialized before the communication starts. The CAN sending program mainly sends the information frame to be sent to the sending buffer, and then starts the sending command (as shown in Figure 5); the receiving program is similar to the sending program, and only needs to take the data out of the receiving buffer.

When designing the CAN communication mode, a multi-master communication method is adopted, that is, each node on the CAN bus can arbitrarily send and receive messages to other nodes. The communication programs on each node are similar, so as long as the communication program of one node is adjusted, the communication programs on other nodes can follow suit.

Figure 5 CAN bus sending program flow chart System application test

1) System interoperability

Establish a device status database in the GIS monitoring device, set up a server and client, and update the status value of the database in real time. Then start the GIS online monitoring system, and conduct an interoperability test during system operation. The test results show that: a) the data transmission speed of Ethernet can reach 10Mb/s; b) the host computer of the monitoring system can send requests to and receive data from the monitoring device through Ethernet, realizing data sharing between the host computer and the monitoring device, that is, interoperability; c) two or more monitoring devices can communicate through Ethernet to achieve necessary data exchange.

2) Monitoring device stability

According to the IEC1000 standard, the GIS monitoring device was tested for stability using a 4-level test level. The test content mainly includes 9 tests such as power frequency magnetic field, high-frequency interference, electrostatic discharge interference and surge interference. The test results show that the monitoring device has some electromagnetic compatibility design defects. However, after local adjustments to the device structure, it can meet the IEC1000 4-level standard; in addition, the device was tested for temperature characteristics, mechanical characteristics and stability. The test results show that the GIS monitoring device can meet the requirements for the stability of secondary equipment in the power system, and its stability has been verified.

3) Openness and scalability of the system

After the GIS online monitoring system was put into operation, the openness and expandability of the system were tested. A workstation was added to the monitoring system through Ethernet, and the workstation was used to communicate with other systems. After the expansion, the monitoring system worked normally and was able to communicate with other systems using the expanded workstation.

Conclusion

Running the embedded operating system uClinux on the ARM processor can realize Ethernet communication function; applying the ARM embedded system to the GIS online monitoring system can meet the interoperability and stability requirements proposed by the IEC61850 standard; the GIS online monitoring system using Ethernet and CAN bus communication is open and expandable.