On the Design of UPS Power Supply Intelligent Monitoring System

    introduction

　  UPS power supply system is an essential power supply for power, communication, banking and other industries. It has a history of several decades since its creation. In the process of continuous development and improvement of technology, its protection function is also constantly changing. UPS can be divided into: backup type, online type and online interactive type according to the working state of the inverter in the host. Their function is to filter and stabilize the mains power in order to provide a more stable voltage to the load. At the same time, the charger converts the electrical energy into chemical energy and stores it in the battery. Once the power is interrupted, the grid voltage or grid frequency exceeds the input range of the UPS, it can open its own reserve power supply in a very short time to supply power to the load.

　　The UPS intelligent monitoring system designed in this paper has the following links and functions: it can operate in various complex power grid environments; it will not cause additional interference to the mains during operation; the output power performance indicators should be comprehensive and high-quality, and can meet all requirements of the load; the UPS itself should have high efficiency and an output capacity close to the actual mains; it is a highly intelligent device with highly intelligent self-test functions, automatic display, alarm, status memory functions and communication functions.

　　1. Overall design

　　The design consists of main monitoring unit, AC detection unit, battery detection and inspection unit, feeder detection and voltage regulation unit, insulation monitoring and grounding line selection unit and other unit modules. These modules communicate with each other through internal RS485 to realize AC power distribution and battery charging of power cabinet.

Real-time monitoring, control and alarm processing of discharge process, battery status, voltage regulation status, busbar-to-ground resistance, and feeder switch status. The entire system communicates with the host computer via RS232 to query and count historical data.

　　2. Introduction to each unit

　　2.1 Main monitoring unit

　　The main monitoring unit dispatches the operation of the entire system. The main monitoring unit consists of a main monitoring board, a 320X240 dot matrix LCD display, a keyboard and indicator lights, etc. It completes the battery charge and discharge management, the setting and display of operation and control parameters, the storage and query of alarm records, communicates with the host computer through RS232, and controls each internal unit through RS485.

　　2.2 AC detection unit

　　The unit mainly completes the acquisition of three-phase AC voltage, current and frequency; it also has the alarm functions of AC power failure, phase loss, overvoltage, undervoltage, etc. When the alarm occurs, the relay alarm contact is closed. The three-phase AC voltage display value can be corrected by adjusting the potentiometer on the board.

　　2.3 Battery detection and inspection unit

　　This unit consists of a battery detection board and a battery inspection board (optional), which mainly completes the acquisition of battery pack voltage (combination voltage), charge and discharge current, ambient temperature and single cell voltage; battery fuse status detection; can control the voltage or current setting of other manufacturers' modules or phase-controlled power supply three-phase trigger boards by outputting analog voltage and current setting (negotiate with the manufacturer for specific conditions), improving the compatibility of the system; on-time metering; and completes the functions of combination voltage over-under voltage, battery overcharge, battery feeding and single cell failure alarm at the same time; by adjusting the potentiometers on the battery detection board and battery inspection board, the combination voltage and single cell voltage display values ​​can be calibrated respectively. As shown in Figure 2.

　　2.4 Feeder detection and silicon chain voltage regulation unit

　　It consists of a feeder detection CPU board and a switch input board, and detects the switch status of the feeder on and off in real time. Through the switch expansion port, 24 feeders can be detected. When the switch position is changed or the control bus voltage exceeds the limit, an alarm is issued and the control bus voltage is automatically adjusted through the silicon chain (up to 7 silicon chains for voltage regulation). The control bus voltage display value can be calibrated by adjusting the potentiometer on the feeder detection board.

　　2.5 Insulation monitoring and grounding line selection unit

　　It consists of an insulation monitoring detection board and a grounding line selection expansion board. Its main function is to monitor the busbar-to-ground resistance in real time and to self-locate the grounding branch. When the busbar-to-ground resistance is lower than the alarm setting value, the alarm relay closes; the grounding line selection is connected through the grounding line selection expansion port, supporting up to 24 lines of line selection.

　　3. Design of key circuit units

　　3.1 Current Detection Circuit

　　The size of the battery charge and discharge current is particularly critical. The circuit diagram is shown in Figure 1. Because both the charging current and the discharging current are detected, the voltage on the small resistor is in two directions. In the circuit detection, two channels are used for detection respectively, which is also convenient for signal conditioning respectively, and it is also convenient to measure with one input channel of the AD converter.

　　
Figure 1 Current detection circuit

　　3.2 Monitoring of combined mother voltage

　　The combined voltage monitoring circuit is shown in Figure 2. The combined voltage flows through resistors R16, R17, and R54, and is sampled on resistor R17, so resistor R17 should be a high-precision resistor. R16 and R54 are much larger than resistor R17 and appear on the denominator, so high-precision resistors are not necessary. The function of LL is to suppress common-mode interference. The size of the voltage to be monitored can be adjusted to meet the input voltage requirements of the AD converter. [page]

　　
Figure 2 Monitoring circuit for combined mother voltage

　　3.3AD Conversion

　　The AD conversion chip uses TLV1544. The main features of TLV1544 are: wide range single power supply, VCC can be 2V, 7V, 5V, 5V; the chip has a high conversion rate, the conversion time is less than 10s; the chip provides 4 external input channels, and one of the 4 input channels can be selected by programming different status word settings for the chip; the chip has 4 ports as synchronous serial interfaces, connected to the microprocessor through the SPI bus; 11-bit AD conversion is enough to meet the system requirements. As shown in Figure 3.    

　　
Figure 3 AD chip circuit

      Controls the start of sampling of the analog signal input from the selected channel. From high to low, it starts sampling the analog input signal; from low to high, it puts the sample and hold function in the hold state and starts analog-to-digital conversion. Independent of the input and output clock signals, it starts working when it is high. The duration of the low control controls the duration of the switched capacitor array sampling cycle. When not in use, it is connected to a high level. The pin (EOC) becomes a high level at the end of the A-to-D conversion to indicate that the conversion is complete. This unit determines whether the conversion is complete by querying the EOC level to read the data.

　　3.4 Communication Circuit Design

　　The whole system communicates internally via RS485. The specific circuit is shown in Figure 4. Because the control chip uses AT89C52, the CPU as the main monitoring unit has only one serial port, and its parallel port is not fully utilized, so the programmable serial interface chip 8250 is used to expand the serial port and use the parallel port to simulate the serial port.