Abstract: Microprocessor-based relay protection devices are responsible for diagnosing power system faults and providing protection action signals. They also need to provide information to operators and communicate with other equipment. Therefore, they must be high-performance and highly reliable computer-based measurement and control systems. This paper analyzes the system composition of a microprocessor-based relay protection device based on the STD bus, and focuses on the application of floating-point data acquisition technology in microprocessor-based relay protection devices and their reliability design. The microprocessor-based relay protection device designed according to the technology described in this paper has high accuracy and reliability and has been successfully applied in the Central China power system. Keywords: Relay protection, STD bus, floating-point data acquisition, reliability design Abstract: Computer protective relaying equipment plays the role of judging the failure in the electric power system. Simultaneously, it sends messages to personnel and communicates with other devices, thus requiring highly reliable measurement and control systems with high performance. This paper analyzes the composition of computer protective relaying equipment based on the standard bus and mainly introduces the floating-point data acquisition technique and its application to computer protective relaying equipment, as well as the reliability design of such equipment. The computer protective relaying equipment designed according to these techniques possesses high precision and reliability performance and has been successfully applied to the China Electric Power System. Keywords: protective relaying, STD bus, floating-point data acquisition, reliability design 1 System Functions and Characteristics Relay protection devices play a crucial role in ensuring the safe and reliable operation of the power system. When a fault occurs in the power system, to prevent the fault from escalating, the relay protection device must be able to quickly and reliably disconnect the faulty component from the power system, thereby limiting the impact of the fault on the power system to the smallest possible extent. Microcomputer-based relay protection devices are a type of high-performance computer measurement and control system. They have higher reliability and sensitivity than traditional transistor-type and integrated circuit-type relay protection devices [1]. This paper introduces a microcomputer-based relay protection device based on the STD bus. Its main features are: (1) It adopts a powerful 16-bit microprocessor 80C196 and creatively uses many new criteria and algorithms to ensure the advanced nature of the system; (2) It adopts the STD bus structure and takes many anti-interference measures in both hardware and software to enhance the reliability of the system. 2 Bus structure of microcomputer-based relay protection device For microcomputer-based relay protection devices, the adoption of a bus structure can improve the reliability and maintainability of the system. In the measurement and control system with a bus structure, the bus constitutes the physical framework of the system, and the various functional modules are connected as a whole through the system bus. The use of a bus structure in microcomputer-based relay protection devices has the following advantages[2]: (1) The hardware and software design of the relay protection device is simple and convenient; (2) The device structure is simplified, and each functional module component only needs to be "connected" to the system bus, and it is easy to realize hardware redundancy design; (3) The system's scalability and maintainability are enhanced, and when the system fails, the faulty module can be quickly identified and replaced. STD bus is a commonly used system bus in industrial automation systems. It supports a variety of microprocessors, has high reliability, and strong expansion capability. The structural block diagram of the microcomputer-based relay protection device based on STD bus is shown in Figure 1. [align=center] Figure 1 Structural block diagram of the microcomputer-based relay protection device based on STD bus[/align] 3 Floating-point data acquisition system In order to quickly and accurately determine the fault type and calculate relevant parameters from the transient signals after the fault when a fault occurs in the power system, a high-precision data acquisition system is indispensable for microcomputer-based relay protection devices. The national standard for microcomputer-based relay protection devices stipulates that the relative error of the measuring element of the protection device should be less than 5%, and the current measurement range is 0.5A to 100A[3]. To achieve the specified accuracy requirements within such a large range, it is difficult to achieve using only a 12-bit A/D converter. A/D converters with 16 bits or even higher bit numbers are not only expensive, but also have very demanding requirements for peripheral devices, which are difficult to meet in the harsh working environment of power plants. However, the floating-point data acquisition technology can effectively solve this problem. Floating-point data acquisition first amplifies the input signal appropriately through a precise numerically controlled variable gain circuit, and then samples the amplified signal. The numerically controlled variable gain circuit has different amplification factors for input signals of different amplitudes, which is generally achieved through a voltage comparison network. If the floating amplification factor is called the exponent and the A/D conversion result is called the tail code, then each sampled value is composed of "exponent + tail code", which is similar to the representation method of floating-point numbers, hence the name floating-point data acquisition. Figure 2 is a block diagram of the floating-point data acquisition system[4]. In a floating-point data acquisition system, if the number of bits of the A/D converter is N and the number of amplification levels of the digitally controlled variable gain circuit is L (varying in powers of 2), then the dynamic range of the constructed floating-point data acquisition system (the ratio of the maximum allowable input signal amplitude to the minimum resolvable input signal amplitude, generally calculated in decibels) is 20lg2L+N; if a 12-bit A/D converter is used and the amplification level of the digitally controlled variable gain circuit is designed to be 8, then the dynamic range of the floating-point data acquisition system can reach 120.4dB, equivalent to the quantization effect of a 20-bit A/D converter. Furthermore, since the digitally controlled variable gain circuit ensures that the signal amplitude entering the A/D converter is almost always within the upper half-range, the signal-to-noise ratio of the quantization will remain approximately consistent [5,6]. [align=center] Figure 2 Block diagram of a floating-point data acquisition system[/align] 4 Reliability Design of Relay Protection Devices Since the reliability of relay protection devices directly affects the reliability of the power system, the reliability design of relay protection devices is particularly important. There is serious electromagnetic interference in the power station, which can lead to increased noise in the analog circuit of the relay protection device and unstable operation of the digital circuit. It can even cause data loss in the memory and prevent the relay protection device from working properly. In order to effectively suppress electromagnetic interference, the following measures are mainly adopted in the microcomputer-based relay protection device based on STD bus[7]: (1) Shielding: Combined with the standard cage structure of STD bus, double-layer shielding measures are adopted in the protection device; the metal cage of STD bus is used as the inner shielding layer, the metal chassis is used as the outer shielding layer, and it is grounded during installation. (2) Isolation: Analog quantities are isolated by voltage and current transmitters, switch quantities are isolated by optocouplers, and strong and weak power supplies are isolated. (3) Filtering and grounding: Adding filter capacitors between the power supply and ground of electronic components is a simple and effective way to suppress interference; arranging a ground wire network around each template so that the circuit can be easily grounded nearby can reduce the influence of ground current on the circuit. (4) Dynamic hardware redundancy design: For important functional components, such as data acquisition components, two templates with different addresses but identical functions are used. One is used as the working board and the other as the backup board. The CPU periodically commands the two A/D templates to sample the same standard signal and judge the result. If the working board's result is correct, it will still be used as the working board. Otherwise, if the backup board's result is correct, the backup board will automatically become the working board, the original working board will exit the operation and display the corresponding information. (5) Timeout reset and multiple verification: Both hardware and software self-reset timers (WDTs) can enable microcomputer-based relay protection devices to have automatic timeout reset capability. If the program runs out of control, the device will automatically return to normal. Since the interference is random and short-lived, if it is stipulated in advance that multiple conditions must be met before an exit command can be issued, the probability that multiple conditions can be met due to interference will be very small, thereby avoiding the protection device from malfunctioning due to interference. 5. Conclusion The microcomputer-based relay protection device based on the STD bus is a high-performance and highly reliable computer measurement and control system. The use of floating-point data acquisition technology effectively solves the accuracy problems in measurement and control, thereby ensuring the safe and reliable operation of the power system. The microcomputer-based relay protection device designed based on the above technology has been operating without failure for five years at a hydropower station in Henan Province, fully demonstrating the correctness of the design and the reliability of the system. The authors' innovations include: using an STD bus architecture to enhance the system's scalability and maintainability; using floating-point data acquisition technology to improve the dynamic range and accuracy of the data acquisition system; and employing multiple anti-interference measures to ensure the system's reliability. References [1] Chen Deshu. Computer Relay Protection Principles and Technology [M]. Beijing: China Electric Power Press, 1992 [2] You Yiming et al. Single-chip Microcomputer Bus Expansion Technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 1993 [3] National Standard of the People's Republic of China. General Technical Conditions for Microcomputer Line Protection Devices. GB/T15145-94. State Bureau of Technical Supervision, 1995 [4] Lin Jun, Bie Hongxia. Electronic Circuit Systems and Standards [M]: Best Design and Practice. Beijing: National Defense Industry Press, 1998 [5] Chen Dixiang et al. Floating-point Data Acquisition Technology and Its Application in Microcomputer Relay Protection Devices [J]. Electronic Measurement Technology, 2000, 117(4): 31-33 [6] Wang Yifeng, Wen Xidong. Design of Data Acquisition Module Based on CAN Bus [J]. Microcomputer Information, 2005, 11-2: 58-60 [7] Chen Dixiang et al. High-Reliability Computer Relay Protection Device Based on STD Bus [J]. Journal of National University of Defense Technology, 2000, 22(4): 116-118