1. Introduction With the promotion and popularization of AutoCAD, computer-aided design has been widely applied in many fields, from engineering surveying, scientific computing, and auxiliary drafting for optimization design to water conservancy and hydropower engineering design, industrial product styling design, and civil building design, gradually replacing manual labor. According to statistics from relevant departments, over 95% of the calculation work, 75% of the schematic design, and 50% of the drafting work in large design institutes in China are completed using CAD systems. CAD has driven a design revolution in almost all fields. It can reduce civil engineering design costs by 10% to 20%, shorten the time from product design to production by 20% to 50%, reduce scrap rates by 70% to 80%, and increase equipment utilization by 2 to 3 times. Therefore, we developed primary and secondary electrical CAD for hydropower stations using POR-TEL drawing software and MATLAB programming language. [img=550,177]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/xbslfd/2002-2/49-1.jpg[/img][img=550,165]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/xbslfd/2002-2/49-5.jpg[/img][align=left][b]2 Design Ideas and the Structure and Functions of CAD[/b] 2.1 Electrical Primary and Secondary Design Contents The electrical primary and secondary design includes the following contents, as shown in Figure 1. 2.2 Functions of PROTEL System Software PROTEL is a printed circuit board software developed by PROTELTECHNOL-OGY in Australia. It is an improved version of TAN-GO software. PROTEL schematic drawing software SCHEMATIC is used to generate high-quality circuit schematics for various purposes. Its features are as follows: • It adopts a drop-down window menu, providing an excellent human-machine interface; it supports EMS (extended memory); and it has powerful keyboard macro commands and macro command editing functions. • It offers 10 paper sizes (A0, A1, A2, A3, A4, A, B, C, D, E), supporting up to 99 sheets; it supports four line types: thin line, thick line, bus line, and dashed line; its comprehensive block operation functions allow defining, moving, deleting, and copying graphic blocks, and performing translation, rotation, and mirroring operations on entities. • The component library includes various common electrical components and can be expanded; it can simultaneously load three current component libraries, which can be changed at any time; the component library has forward and reverse compilation functions, and features display, addition, deletion, naming, and editing functions. • The input window uses a line editing method; the display window has zoom in, zoom out, and centering functions; the output supports various printers, plotters, and photoplotters. • It can suspend the current work and execute WINDOWS commands; the connection netlist generated from the schematic diagram can be converted into other CAD software formats [1, 2]. 2.3 Construction of Electrical Primary and Secondary CAD (1) Drawing part: The circuit schematic diagram is drawn using PROTEL-SCHEDIT software. First, the PLACE command calls the required entities (including components, connections, annotations, etc.) from the LIBRARY library file; then the EDIT command is used to edit the design drawing; finally, the CURRENT command is used to modify and organize the drawing and output the drawing. For the convenience of drawing, we also used the schematic diagram component library editing tool SLM to build the electrical primary component equipment library HVC.LIB and the electrical secondary component equipment library LHC.LIB, which include generators, transformers, circuit breakers, disconnect switches, surge arresters and various secondary components. (2) Calculation part: Since the MATLAB programming language has powerful calculation functions, has the characteristics of interface with C and FORTRAN language programs APL function group, and can be used in conjunction with WINDOWS [3], we use MATLAB to perform calculations and tabulations under the WINDOWS interface. The electrical primary and secondary calculation programs adopt the main and subroutines modular design, and its structure is shown in Figure 2. [/align][img=300,280]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/xbslfd/2002-2/50-1.jpg[/img][align=left] 3 Features of Electrical Primary and Secondary CAD (1) It adopts a human-computer dialogue format, which is easy to master, practical, and efficient. Its menu settings are reasonable, hierarchical, and easy to understand. Various editing, modification, and setting commands are designed very flexibly, giving designers a great degree of freedom. (2) It has moderate requirements for the configuration of microcomputer hardware and software. It requires 64MB of memory, a hard disk of 8.3G or more, a WINDOWS98 operating system, and a 14-color monitor to run. (3) It has Chinese character annotation function, which is convenient for domestic designers and facilitates the promotion and exchange of electrical primary and secondary design drawings. (4) It uses MATLAB to perform modular program design and complex array tabulation under the WINDOWS interface, which has good universality, readability, and portability. References: [1] Hu Wanhai, Zhang Fan, Long Lei, et al. Design and Examples of Multilayer Automatic Routing Printed Plates - Protel [M]. Beijing: Beijing Hope Computer Company, 1992. [2] Ning Yun, Li Jianchang. Practical Explanation of PROTEL [M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 1998. [3] Lou Shuntian, Chen Shengtan, Lei Humin. MATLAB Programming Language [M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 2000.