A review of embedded web server technology and its application in power systems

(3) Information flow analysis based on HCPA interface form
This interface form is similar to SSI. First, a Web page with C source code is generated. The C source code is marked and can be easily and efficiently interfaced with the embedded software system. The processing process is shown in Figure 5. The preprocessor compresses the Web source page, extracts parameter tags, parses the HTML page, and finally generates C code. The C compiler connects and compiles the generated C code, the C code on the Web side, and the C code of the application system into an executable file. The executable file is stored in the memory of the EWS, and its information flow diagram is similar to the information flow diagram under the CGI interface.

The processing and parsing process of the preprocessor greatly reduces the running load of the EWS system, while the embedded C code and HTML page parsing method provide great flexibility for the development of embedded systems. It combines the advantages of efficient operation of the CGI method and the quick development of the SSI method, and is suitable for the development of small and efficient embedded application systems.
2.5 Key issues in implementing EWS
(1) Resource allocation issues
A good EWS design must be a system solution that reasonably allocates system resources. The main consideration here is the allocation of system hardware resources. Hardware resources mainly include CPU computing resources and system storage space resources.
Since the CPU of EWS also bears the computing and control tasks of the application system itself, the EWS process must use as little CPU computing resources as possible, especially for real-time monitoring devices. The operating system generally allocates low-level threads to EWS without affecting the performance of the system itself. At the same time, EWS adopts the HTTP 1.1 communication protocol and explicitly introduces a buffer control mechanism, which can effectively reduce the CPU load. Establishing a single continuous connection for multiple HTTP transactions instead of establishing a separate TCP connection for each HTTP request can also improve network and system performance.
The allocation of storage space mainly solves the problem of reducing the amount of software code. The HCPA interface solution analyzed in the previous section can use the generated executable file to reduce code redundancy and occupy as little storage space as possible. The simplified real-time operating system kernel and TCP/IP protocol stack can also reduce the amount of storage space required. The efficient TCP/IP protocol stack and Web server software only occupy about 48k ROM and 16k RAM storage space.
At the same time, for different application systems, the overall resource allocation of the EWS system must also be considered. EWS realizes network control of field devices, but it does not mean that every embedded device in the system must become a Web server. In areas where devices are relatively concentrated, it is possible to consider establishing an independent EWS system, communicating with a certain number of embedded devices through the field bus, and becoming a common Web server for these devices, so that a better combination design can be obtained between system cost and performance.
(2) Security issues
Due to the openness and standardization of Web technology, its advantages are sometimes also disadvantages, that is, visitors must be distinguished, otherwise the security of the system will be unpredictable. An EWS system without detailed security and confidentiality measures is an incomplete system. The possible security strategies currently available are as follows [4]:
​​1) Closed-loop network access strategy: Limit network access to the Intranet. Remote monitoring, diagnosis, and even remote control and parameter setting can be performed through a browser on any PC in the network. At the same time, the network is closed, and all possible attack points are limited to the network.
2) Callback strategy: When a user accesses device resources in the network by dialing a phone outside the Intranet, a high level of security is provided through an authorized call-back connection. "Call-back" ensures that only authorized phone connections can access resources within the Intranet. At the same time, the phone number must be kept confidential to prevent malicious calls from blocking the access channel and causing errors such as "denial of service".
3) Read-only access strategy: For critical and sensitive information, an important security measure is to mark it as read-only, especially for access requests from IP addresses outside the Intranet. In this way, users can view the status of the embedded system from anywhere and take timely measures when problems arise.
4) Password and firewall policy: If you want to enable users at any location to control and set parameters of the embedded system, you must introduce firewall technology into the Intranet gateway. If necessary, you must also integrate it into the EWS to filter out external data flows according to the rules. At the same time, all data transmission must be encrypted, and all points that may be attacked must be accessed with the correct username and password. [page]
(3) Real-time issues
In order to solve the shortcomings of traditional Web technology in real-time performance, EWS provides a software and hardware platform for real-time access. EWS uses a microprocessor of 16 bits or more, contains a large RAM storage space, supports multi-level interrupts, and has good real-time response performance. The software system with a real-time operating system (such as pSOS, QNX, VxWork, etc.) as the core provides TCP/IP protocol support, which can implement HTTP Server functions and fully meet the real-time requirements of EWS [8]. The main measure to improve the real-time performance of the system is to reduce the occupation of system resources. The relevant measures are analyzed in the resource allocation problem in this section.
Corresponding to the real-time performance of EWS itself is the real-time performance problem of the communication system connected to EWS. The current solution is to use fieldbus technology to improve the real-time performance of communication. With the maturity of broadband IP network technology with differentiated services, the real-time performance of the communication system will not become a bottleneck affecting the real-time response performance of the EWS system.
3 Application of embedded Web server technology in power systems
The power system is a complex system that includes the production, transmission and use of electric energy. Various production equipment is widely distributed in various regions, with many types and huge numbers of equipment. The monitoring, diagnosis and maintenance of these equipment have become an important part of improving the system management level. After the equipment transformation, the hardware foundation and information network construction of the power system have been greatly improved, which has laid a good foundation for the application of EWS technology in the power system. The following are examples of the application of EWS technology in the power system:
(1) Application of network video monitoring based on EWS technology in the power system [9]
The principle of remote network video monitoring based on embedded Web server technology is to build an embedded Web server in the video server and use an embedded real-time operating system. The video signal transmitted by the camera is digitized and compressed by an efficient compression chip, and then transmitted to the built-in Web server through the internal bus. Users on the network can directly use the browser to view the camera image on the Web server. Authorized users can also control the movement of the camera pan/tilt lens or operate the system configuration. The system is mainly used in unmanned substations, integrated monitoring systems of power bureaus/power plants, on-site production dispatching and command systems, and emergency response to disasters and emergencies to ensure the reliable operation of equipment in the monitoring area and the safety of personnel. Compared with traditional monitoring systems, it has the advantages of cost saving, plug-and-play, high performance, independent network environment and flexible access to equipment.
(2) Application of remote monitoring and fault diagnosis based on EWS technology in power systems
There are three strategies for combining Web technology with monitoring and fault diagnosis technology: device-side embedded Web server system, service-based Web server system and comprehensive multi-Web server system. Among them, the device-side embedded Web server system places the Web server at the device and integrates it with the device. Users monitor the device status through a Web browser and can control the data collection method of the device through browser programming. By reasonably allocating computing tasks, the powerful computing power of the client can be fully utilized to reduce the computing load on the device. Properly combining Web technology with the monitoring and fault diagnosis technology of the device will break through the relatively closed conceptual framework of monitoring and fault diagnosis, which is an improvement of the equipment monitoring and fault diagnosis technology.
(3) Application of protection and control equipment based on EWS technology in substation automation systems [4, 10]
Applying EWS technology to the protection and control equipment of substations will lead to a change in the operation and maintenance mode of traditional substation automation. The real-time information of the system in EWS is obtained through a Web browser, thereby realizing remote real-time control, adjustment and maintenance. The functions that can be realized by the substation automation system based on EWS technology include: dynamic publishing
of real-time data and historical data, parameter setting function, remote implementation control function, file download and upload function, email alarm function, access level setting and authority authentication function, PPP dial-up Internet access and other functions. At present, the "REF542 plus" new generation of integrated protection and control relays developed by ABB has integrated EWS technology, which can realize real-time control based on browsers or local control devices. The "DF 3600" system developed by Dongfang Electronic Information Industry Co., Ltd. in China can also achieve similar functions.
Due to the importance of safe operation of power systems, the actual application of EWS technology in field monitoring and control equipment is gradually deepening and expanding. With the application of TCP/IP technology in substation communication systems, especially the application of broadband TCP/IP technology with differentiated services to the communication between the device layer and the interval layer, the real-time requirements can be met, and the advantages of EWS technology can be more fully utilized to improve the level of remote monitoring and control.
4 Conclusion
This paper analyzes and introduces the main contents of embedded Web server technology, and illustrates its application in power systems with examples. With the maturity and development of embedded Web server technology, its wide application will greatly improve the performance of measurement and control equipment and fundamentally change the operation and management of equipment. In the field of power systems, new monitoring and control systems based on embedded Web server technology will effectively reduce system operation and maintenance costs and improve system management level, and have broad application prospects in power systems.

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