Technical measures to improve the reliability of DC operating power supply system

　　The DC operating power supply (DC panel) equipment is the secondary power supply of power plants and substations, used for circuit breaker opening and closing and secondary circuit instrumentation, relay protection, control, emergency lighting, etc.

　　With the development of science and technology and the improvement of user requirements, in recent years, the main circuit of the power operation power supply has gradually developed from the original thyristor phase-controlled type to the high-frequency switch type, and the control part has developed from the original analog control method to the digital control method controlled by the microcomputer. The application of microcomputer control has greatly improved the degree of automation of the operation of the power operation power supply, and can realize the four-remote networking function to meet the needs of unattended power stations. However, the primary requirements of the power system are safety and reliability , so the reliability and anti-interference ability of the microcomputer controller are very important. In recent years, the author has been engaged in the development and research of the power operation power supply microcomputer controller and other microcomputer systems, and now he will explain this issue.

　　1. Microcomputer power supply

　　The power supply is the energy supply source of the microcomputer control system. The quality of the power supply directly affects the stability and reliability of the entire system. At the same time, the power supply is also the main way to introduce external interference into the system, so the quality of power supply processing is a prerequisite for the reliable operation of the system. The controller power supply adopts a DC input switching power supply with a wide voltage input, so that it can be directly powered by the battery to ensure the uninterrupted power supply of the system. In order to suppress various external interferences introduced into the system by the power supply, the following measures have been taken.

　　(1) TVP devices are mainly used to absorb instantaneous surge voltages in power supply networks. When the device is subjected to a high-energy surge voltage, its impedance immediately drops to a very low level, allowing large currents to pass through while clamping the voltage to a predetermined level. The peak value of the instantaneous surge current it can withstand can reach hundreds of amperes, and it can absorb surge power of up to several kilowatts.

　　(2) EMI filters are mainly used to suppress high-frequency interference in the system. For common-mode noise, the filter forms a path between the power line and the ground line, which can lead the noise current into the ground. For normal-mode noise, the filter forms a path between the lines, short-circuiting the noise current between the lines so that it does not affect the circuit operation.

　　(3) Ferrite bead filter Ferrite has almost no impedance to low-frequency current, but has a higher attenuation effect on higher-frequency current. High-frequency current is dissipated in it as heat.

　　(4) Resettable fuse The difference between a resettable fuse and a conventional fuse is that when the line current is too large, the impedance of the resettable fuse will be greatly increased, thereby realizing the line protection function. When the line current is normal, the impedance value will be restored to the normal value, thus having a good protection effect on the line overcurrent caused by transient interference.

　　2 analog channels

　　(1) Field analog quantity input For the power supply of electric operation, the field analog quantity mainly includes: the voltage and current of the AC incoming line and the charger, the DC bus voltage and current, the battery voltage, the temperature of each point and the insulation to the ground and other parameters. Due to the complexity of the industrial field environment, these signals are often superimposed with various interference components. If these signals are directly introduced into the system, it will have a serious impact on the system and even cause the system to fail to operate. Therefore, these signals are all passed through the isolation transmitter before entering the system. The isolation transmitter isolates various interference components from the system on the one hand, and on the other hand, it conditions these signals into standard signals for A/D conversion input. In addition, in order to prevent the transmitter from affecting the system power supply, the transmitter is selected as a front-end passive transmitter.

　　(2) The analog output directly controls the working parameters of the charger. Once a fault occurs, the system will not be able to operate normally. To ensure its reliability, an isolation amplifier output is used. At the same time, hardware protection measures are added to the D/A conversion write end to ensure that the D/A can be rewritten only when the CPU is operating normally. When the CPU fails, it will not affect the output, ensuring that the device output will not be confused.

　　3 switch channels

　　The on-site switch input and output are similar to analog quantities, and are also superimposed with various interference components. In order to eliminate the impact of these interferences on the system, isolation measures must be taken. Switch input and output isolation generally uses optoelectronic isolation devices to achieve good isolation effects.

　　4 communication ports

　　The communication port is the channel for the device to communicate with the host computer. It is the key to ensure information upload and host computer control, and is the guarantee for unattended operation. Generally, there is a certain distance between the device and the host computer at the work site. To ensure the correctness of the information, the hardware adopts an isolated anti-static standard interface (interface hardware protocol RS-485/RS-422), and the communication software protocol adopts the standard station control protocol commonly used in the current domestic power system, with complete verification measures and clear information format.

　　5Central Controller

　　The central controller is the core of the entire control system. All functions of the system are completed through the central controller and related peripheral circuits. Once a fault occurs, the entire controller will not work. Its reliability is the decisive factor in ensuring the reliability of the system. For this purpose, the following technical measures have been taken.

　　(1) Hardware watchdog technology: To prevent the CPU from being affected by strong external interference, causing the program to run away and causing the system to deadlock, a hardware watchdog circuit is specially designed for the CPU in the system. When the program runs normally, the watchdog circuit is reset periodically. When the program runs away and the software trap fails to capture and process it in time, causing the system to deadlock, the watchdog circuit cannot receive the reset signal. The watchdog circuit will generate a CPU hardware reset signal to reset the system and restart it to resume operation.

　　(2) Use PSD and ISP devices: The basic structure of the central system is composed of CPU, address latch, decoder, program memory, data memory, etc. Conventional hardware circuits are often composed of multiple TTL circuits and EPROM, RAM, with a large number of devices, which increases the system's failure points. PSD devices integrate address latches, decoders, program memory, and data memory into one chip, which reduces the number of devices by several times and increases the reliability of the system. Since the DC operating power supply controls the corresponding input and output channels, if conventional TTL or other conventional interface devices (such as 8155, 8255) are used, the number of devices will increase. For this reason, large-scale programmable logic devices (ISP) are used, which can greatly reduce the number of devices. One ISP (such as ISP2064) can contain 2000 gate circuits and 64 registers. Originally, 4 8D latches (74LS373) and 4 8-bit buffers were used to complete 32-way input and output functions, but now only one ISP2064 is needed. In addition, ISP is a programmable logic device, which is also convenient for circuit modification and debugging.

　　(3) Software trap: When the CPU runs out of control due to strong external interference, a software trap is set in each address area in the program area. When the program is not executed in the normal order, the software trap will capture the erroneous program instructions and automatically restore normal operation.

　　(4) Multi-address backup of important data: When an error occurs during program operation, the data storage device may be written by mistake. To prevent errors when the program is restarted, data that is critical to system operation has multiple backups in different areas of the data storage device. When the program is running, it will automatically compare the identity of each data. If an error is found, the correct data can be automatically retrieved for use.

　　(5) System self-recovery: For the operation of the power supply controller, the equipment operation must ensure its continuity. It is required that when the central controller is reset abnormally (such as hardware watchdog action, human error operation, software trap restart program, etc.), the system should maintain the original operating state after restarting. In response to this requirement, when the program is started, by judging the specific feature code in the data memory, it can be determined whether it is a normal reset or an abnormal reset. If it is an abnormal reset, the system operation can be restored through data backup to ensure the continuity of work.

　　(6) System self-diagnosis: For key components of the central controller, such as program memory, data memory, CPU, etc., their status is checked regularly during program operation, so that faults can be discovered in time and corresponding measures can be taken.

　　The above-mentioned measures to improve reliability have been successfully applied to our MC-3 DC operating power supply microcomputer controller. They have been in long-term operation in industrial sites and have achieved good results.