| High speed development of digital computer opened a new field for power system protection. To made the protection of power system with microprocessor's strong ability. It will forward along with development of the hardware and software and data communication network etc.Base on SCM to make the protection for 10kV distribution line. It is required to mirror any conditions of distribution line. It will act on warning or tripping while the distribution line unusual.According to the requirements of protection in power systems and 10kV distribution line's characteristic, the system design research put the importance on the follow points:1. The architecture and attribute of the distribute network. In our country, neutral conductor doesn't grounding in distribution network. If a single-phase grounding, it's only capacity current passes through the grounding line and it's not so important to three-phase systems. So it may keep 1~2 hours. In this time, please to find the grounded line in time and clear it up.2. Unusual and breakdown of 10kV distribution lines. Phase-phase short circuit is the main form breakdown. So great reflex is required to the protection. Over voltage and lower voltage, overload and phase equilibrium's broken may damage motors. Electric cables'insulation aging will bring about grounding or phase-phase short circuit.3. Deploy the protection. Current measure is used to reflect phase-phase short current, Zero sequence current measure is used to reflect grounding, voltage measure is used to mirror over voltage or lower voltage. Different electric measure act different function to achieve common distribution line's protection at perfection. Every function has marked in program. These means could increase reliability of the protection.4. Hardware architecture. Hardware is required offer precise data to CPU. CPU must finish the work such as calculate data and compare data and judge in a sample time. Sample module used VFC technology to achieve sample in same time, go through photoelectric transformer, measurements are send to counters, changed simulation to data. Think about all things to design system's parameter, make the protection's performance to the best. Master control module is made of SCM and extended chips. I/O module is used of external orders and switch measures'input or output. Communication module keeps up communicate with system manage computer.5. Design software. System's program is consist of operation part and debug part. In software, design the function to check itself on line. While the system unusual then act warning and deadlock the system. Difference filter and discrete Fourier transform is the base calculation in this system. It is simple and accurate. Use shorter data window calculates to achieve the protection that is required instantaneous act; Use longer data window calculates to achieve the protection that is required delay act. Passed the experiment by emulation of power systems'breakdown has shown that the calculation is fulfill the required. Overload protection with current and time limit be in reverse ratio. It improves sensitivity. To choose the breakdown phases with phase current difference at power frequency variable. It is accurate. It deals promptly and rapidly with breakdown lines. To decrease errors due to frequency change adopts sample frequency automatic tracking and adjusting technique.6. To increase the system's reliability, the hardware and software disturb-free techniques are used in this system. Hardware's self-reset circuit prevents wrong tripping while the program is out of bounds. I/O module isolates with photoelectron. To keep watch on data sample system with minus two same electric measures that from different way. Tripping program is blocked by distinction marks and tripping settings, the system could trip while checked them no problems. Starter allows act while calculate data gather three time over doorsill. Exit parts are blocked by multiple logic. Difference filter is used in front of Fourier calculation to upgrade accuracy of calculation. The system's test indicate that the disturb-free techniques make the whole system more stable and satisfied the requirements of the distribution line protection.
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