I. Introduction With social development and the improvement of people's living standards, the power quality of power systems has become an increasingly important topic of concern. As an important measure to improve power quality, how to improve the reliability of substation automation systems remains a crucial task. In this paper, we discuss the reliability design scheme of the ZXS10 substation automation system based on its actual development. [b]II. Introduction to ZXS10 Reliability[/b] The ZXS10 substation automation system is the first-generation power automation product independently developed by Shenzhen ZTE Corporation. It is used for centralized monitoring and unattended operation of 110kV and below substations. This system integrates data measurement and control, microcomputer protection, computer, communication, fault recording, and ranging functions. It can be used for newly built unattended substations as well as for the renovation of existing substations. The system mainly consists of I/O units (including the SL series line protection, ST series transformer protection, and SM series monitoring units), C700 communication units, and a local backend. The I/O unit collects telemetry, telesignaling, and telepulse information and exchanges information with the C700 communication unit to execute remote control and remote adjustment commands. The local backend receives data from the I/O unit through the C700 communication unit and performs calculations to complete various calculations and human-machine interface functions, such as voltage and reactive power control (VQC) fault analysis. The C700 communication unit is responsible for connecting various parts of the network, including communication processing between the remote dispatch unit and the microcomputer protection unit, as well as information reception and forwarding. Substation automation systems have high real-time requirements and operate under harsh conditions. Besides climate and mechanical environmental stress, the most significant factors affecting the system's reliability are spatial electromagnetic interference and conducted interference, as the power system itself is a strong source of interference. Furthermore, the control of important primary equipment such as medium- and high-voltage lines and transformers places high demands on system safety. Therefore, the reliability of substation automation systems is extremely important. Relevant IEC standards, national standards, and industry standards have very high requirements for the reliability of power automation systems and monitoring products, with some indicators reaching the most stringent levels for industrial field equipment. Based on relevant standards and market demands, the reliability indicators and requirements for the ZXS10 system are as follows. 2.1 Environmental Conditions • Ambient temperature: 0～45℃ (10～55℃ for distributed remote terminals and protection units); • Relative humidity: 5～95% (maximum absolute humidity 28g/m3); • Atmospheric pressure: 66～106kPa; • Mechanical vibration: 0.5g, 10～150Hz. 2.2 Electromagnetic Compatibility: • Electrical Interference (Muffled Damped Oscillating Waves): 2.5kV, 100KHz and 1MHz; • Electrostatic Discharge: 6kV; • Radiated Electromagnetic Field: 10V/m; • Fast Transient Burst: 2kV 2.3 Power Supply Influence: • Power Supply Voltage Variation: Rated voltage DC220V, -20% to +20%; • Power Supply Ripple Factor: 5%; 2.4 Safety Performance: • Insulation Resistance: 100MΩ; • Dielectric Strength: 2kV; • Impulse Voltage (Lightning Surge): 5kV; • Temperature Rise: Not exceeding 40℃. 2.5 Reliability Indicators • Main Control System: MTBF≥10000h, MTTR≤0.5h • I/O Unit, Protection Unit: MTBF≥16000h [b]III. Reliability Design Technical Measures[/b] Based on the reliability indicators and requirements of the ZXS10 system, and in accordance with relevant standards and enterprise reliability design specifications, reliability design was carried out during the system development phase. 3.1 Reliability Prediction and Analysis In the early stages of development, a bottom-up reliability prediction and analysis was conducted on the system to obtain relatively rough reliability prediction results for the system, units, and single boards, as well as weak links and key components affecting reliability. This served as the key basis for the specific application of reliability technical measures, eliminating some components with high failure rates and high power consumption, such as potentiometers and cement resistors. 3.2 Derating Design In accordance with the enterprise's derating design standards, derating design technology was widely applied to the components on each single board of the system. All microelectronic devices meet the enterprise's Class II standard for operating frequency and output load derating. Some microelectronic device power supplies have also been derating based on actual conditions. The discrete components used in the system mainly include resistors, capacitors, diodes, transformers, and relays, most of which are low-power devices, and the electrical parameter derating design is excellent. 3.3 Thermal Design The ZXS10 system structure adopts a plug-in cabinet-style panel configuration. Each cabinet can hold four layers of plug-in cabinets, with an actual total power of less than 100W, therefore natural cooling is used for heat dissipation. Air intake vents are located on the lower side of the cabinet, and an exhaust vent is located in the center of the top. Each plug-in cabinet has densely packed 3mm diameter holes on its bottom and top. The size and location of the ventilation holes in the entire system cabinet and plug-in cabinet structure ensure that system heat is dissipated into the atmosphere primarily through convection. The design of each system board takes thermal design requirements into account. The selected components have low power consumption, with the vast majority less than 1W. For all components with power exceeding 1W (maximum 1.8W), sufficient heat dissipation space and channels are provided on the board, the leads are very short, and components such as crystal oscillators, integrated circuits, diodes, and precision resistors are kept away from proximity to avoid thermal shock. Analysis of system thermal test results shows that after the system is powered on and thermally stabilized, the internal air temperature rise is less than 6℃, the highest component surface temperature rise is 33℃ (C700 unit 386EXCPU), and the temperature rise of other components is less than 27℃. 3.4 Electromagnetic Compatibility Design The system and single-board design adhere to hardware design specifications and electromagnetic compatibility design principles. Specific measures include: each unit power supply uses a distributed, shielded high-frequency switching power supply with DC-DC module isolation output; the CPU board uses a four-layer printed circuit board to improve noise suppression capability; the relay coil circuit uses rectifier diodes to eliminate the influence of coil back electromotive force; the measurement signal circuit uses optocoupler isolation to eliminate high-frequency interference; decoupling capacitors are connected in parallel between the power supply and ground of each integrated circuit; power lines and signal lines are routed separately and a certain distance is maintained at the terminals; the system ground (signal ground) adopts a floating ground method, and attention is paid to trunk-type grounding between boards and between boxes; the shield ground (chassis ground) grounding wire is kept as short as possible, and a thick cable is used to connect to the ground busbar to ensure that the grounding resistance is less than 10Ω; the insulation resistance between the system ground and the chassis ground is not less than 100MΩ, reducing the impact of common-mode interference. 3.5 Vibration Resistance Design Vibration resistance design measures were considered in the design of each box unit. Including: The two heavier single-board power supply boards and AC sampling boards in the SL200 line protection unit are placed on both sides of the socket box to ensure the stability of the socket box's mechanical structure; the relays are installed in a horizontal position to avoid the contact movement direction being consistent with the main vibration direction; the cable routing inside the socket box of the SM200 monitoring unit and SL200 line protection unit is fixed with retaining rings and clamped near the connection end; the current input circuit of the SM210 monitoring unit also uses a connector with a self-locking screw to prevent current interruption. 3.6 Other reliability design measures a) Software anti-interference measures. A WDT watchdog circuit is used to prevent system shutdown or program entry into an infinite loop due to external interference, hardware failure, and program errors; a software filtering method is used to eliminate high-frequency pulse interference; software linear correction technology is applied to improve the accuracy of analog signal acquisition without increasing hardware and reducing reliability; the remote information communication of the SM200 monitoring unit uses Hamming code error correction technology with a Hamming distance of not less than 4. b) Maintainability design. The system adopts a modular design with clear unit function interfaces. Each unit adopts a plug-in design, making maintenance and replacement convenient and quick; each telemetry, telesignaling, remote control, and communication unit and board is equipped with a fault indicator light to ensure rapid and accurate fault location; each unit is equipped with an RS232 maintenance communication interface to realize on-site system maintenance. c) Dedicated remote control interlocking design. To ensure 100% remote control reliability, the output of remote control commands adopts multiple interlocking measures; the output of the remote control relay provides an independent power supply and interlocking circuit. 3.7 Safety Design In accordance with the requirements of the power equipment safety design standards, the input and output devices use current transformers and relays with high isolation withstand voltage; the high-frequency filter capacitors for suppressing common-mode interference require high power frequency withstand voltage and low leakage current; the creepage distance between the relay board and the AC sampling board without electrical connection is greater than 2mm; the printed circuit board is treated with a protective coating to improve the insulation capability in harsh environments such as high humidity. [b]IV. Summary[/b] Through the above reliability design technical measures, as well as repeated reliability tests and improvements, the reliability of the ZXS10 system has been guaranteed and improved. The system experiment went smoothly and was handed over to the bureau on time; the SL200 line protection unit passed the network access type test of the national quality inspection center on the first attempt.