Abstract : Microcomputer-based relay protection utilizes software technology to implement characteristic curves, breaking free from many constraints of traditional protection implementations. This allows for the design of more complex, flexible, and better-compliant action characteristic curves that meet the four unified requirements of protection. However, this also presents significant challenges to developing general-purpose relay protection testing software. Therefore, a well-encapsulated, reusable, and highly scalable relay characteristic curve library is of great importance for the development of relay protection testing software. Design patterns are a summary of design experience in object-oriented software and can be directly used to guide the design of various object-oriented software. This paper proposes a solution combining the Facade and Strategy patterns to address the problems encountered in the development of relay characteristic curve libraries. This library has been successfully applied to relay protection testing software. Keywords : Design patterns; Relay; Characteristic curve; Object-oriented 0 Introduction With the continuous development of microcomputer-based relay protection technology, more and more high-performance relay protection devices are being applied in modern power systems. The requirements for digital simulation performance in relay protection testing are becoming increasingly stringent, and traditional relay protection testing devices and methods can no longer meet these requirements. Reference [1] proposes the working principle of a new type of relay protection tester. The relay protection test system generally consists of two layers: the upper layer is a PC, and the test software communicates with the lower layer tester through a serial (or USB interface) connection to transmit control commands and data files, and receive feedback information from the tester; the lower layer is the tester, which is internally processed by a DSP for high-speed digital signal processing. The tester is directly connected to the protection device, and it outputs the voltage, current and switching quantities of the simulated power system to the protection device, while monitoring the status of the relay protection. The test software on the PC must have a user-friendly interface, be easy to use and powerful, and be applicable to all types of relay protection devices. Since there are many types of relay protection devices and new devices are constantly emerging, the test software must have good scalability. In order to achieve the above goals, it is necessary to develop a general and scalable relay characteristic curve library to make it convenient to present the relay's operating characteristic curve to the user intuitively. 1. Design Requirements and Scheme for Relay Characteristic Curve Class Library Microcomputer-based relay protection utilizes software technology to implement characteristic curves, breaking free from many constraints of traditional protection implementations. This allows for the design of more complex, flexible, and effective operating characteristic curves that better meet the four unified requirements of protection. Therefore, characteristic curves are not only diverse but also require expansion based on needs, which presents significant challenges for outputting various characteristic curves. The main information in relay characteristic curves is a finite number of characteristic values. Table 1 provides detailed information on three common types of characteristic curves. As can be seen from the table, the developed class library must provide a simple and unified interface, displaying the relay characteristic curve after inputting all characteristic values. [b][align=center]For more details, please click: Application of Design Patterns in Relay Characteristic Curve Class Library Development[/align][/b]