Battery charging and discharging voltage measurement device technology

　　The operating power supply is very important for the safe operation of the substation. It is required to work reliably and independently. As an independent operating power supply, the battery has the advantage of high reliability and is widely used in substations and power plants. Based on this, the daily maintenance of the battery has become a very important regular work of the substation operation. The substation department of our bureau stipulates that in addition to conducting a verification charge and discharge test on the battery group once a year (completed by the relay protection team), the operating personnel are also required to conduct a shallow discharge measurement (discharge for 10 seconds) on the battery group every month to monitor the operating status of the battery group and ensure the normal and reliable operation of the battery group. 

　　1 The current situation of the operating personnel conducting monthly battery charge and discharge voltage measurement 

　　For 220kV substations, the general configuration of the DC system is "two groups of chargers and two groups of batteries". Take the 220kV Nanping substation as an example. The DC is 220V, one group of batteries has 108 single cells, and the two groups have a total of 216 batteries. 

　　(1) The traditional measurement method requires three people to complete:
　　(A) The first person records the voltage reading of the multimeter when the discharge time reaches 10s.
　　(C) The second person connects the discharge resistor in series to the two poles of a single battery.
　　(B) The third person uses a multimeter to measure the terminal voltage of the battery.

　　(2) The resistor used to measure the discharge voltage is an open-type resistance wire, which poses a risk of burns and electric shock to the measurement personnel. 

　　(3) The device has poor operability, and the measurement takes a long time, which can easily cause hand fatigue to the measurement personnel. 

　　To complete the charge and discharge test of the 220kV Nanping Station battery group, it takes an average of nearly 2 hours of work time for three operators. At present, the substation operation of our bureau adopts the management mode of the central station. A central station has 5 to 7 substations under its jurisdiction. The batteries of 5-7 substations must be measured every month, which consumes a lot of manpower and time, and increases the workload of operation and maintenance. 

　　2 Improvement of the battery charge and discharge voltage measurement device 

　　Based on the above reasons, the following improvement ideas were made for the original measurement device: the battery discharge resistor and voltmeter are built in the same device, and the switch is used to select whether to put in the discharge resistor or the voltmeter, or both at the same time. At the same time, the original manual data recording and complicated operation are improved. The principle of single-chip microcomputer is used to realize automatic measurement and storage of battery floating charge voltage and discharge voltage values, and eye-catching sound and light prompts are used during the measurement process. The data is stored in a flash memory that does not lose data when the power is off. When the measurement is completed, the serial port communication between the single-chip microcomputer and the PC is used to transfer the recorded data to the PC, realizing the full digitalization of battery measurement. 

　　2.1 Functional analysis 

　　(1) Measure the total voltage of the battery pack. 
　　Requirements: Equivalent to a voltmeter, with an LCD panel that can display the voltage reading, and the software automatically records the voltage value. 

　　(2) Measure the floating charge voltage of the battery: measure at both ends of each battery. 
　　Requirements: The software can automatically record the voltage value, and set the upper and lower limits of the floating charge voltage at the beginning. If the upper or lower limit is exceeded, there should be a sound or light alarm. 

　　(3) Measure the discharge voltage of the battery. 
　　Requirements: After each battery is discharged for 10 seconds, the software automatically records the voltage value, and there is a sound or light to remind the operator that the time is up and the next battery can be measured. 

　　(4) Printing. 
　　Requirements: The measurement panel is connected to the computer, and the data inside can be read and printed out in the specified format. At the same time, the battery number with unqualified voltage will be displayed with the F symbol. 

　　2.2 Design principle 

　　The design principle block diagram is shown in Figure 1.

Figure 1 Schematic diagram

　　The measured voltage value is loaded onto a 4-way electronic switch (channel 1-4) through a voltage circuit to be tested (a passive voltage circuit to be tested is formed by a measuring clip and leads, a discharge switch and a discharge resistor). The voltage to be tested is divided into 4 sections and sent to a 10-bit A/D converter built into the CPU through a 4066 4-way switch circuit. This ensures that the test device has a certain accuracy under a wide voltage input (because the voltage values ​​of individual batteries in substations of different voltage levels are different, multiple channels are used to accommodate batteries with different voltage values). The CPU uses an AVR microcontroller MEGA168 with a built-in 10-bit A/D converter, and the A/D reference source circuit provides a reference voltage to the 10-bit A/D converter built into the CPU, so that the A/D converter can work normally under the condition of meeting the accuracy requirements. The input voltage value is processed by the pre-written software (see the software flow chart below for details), and the data is stored in the flash memory that does not lose data when the power is off. Then the data is transmitted to the computer through the RS-232 serial communication circuit to realize the function of printing out the data. 

　　2.3 Software Flow 

　　The microcontroller is programmed in assembly language, and the software realizes the functions of the hardware: 
　　(1) Realize the selection of whether to put in the voltmeter or the discharge resistor, and whether the voltmeter and the discharge resistor are put in at the same time; 
　　(2) Realize the setting of the upper and lower limits of the voltage; 
　　(3) Realize the timing function; 
　　(4) Realize the recording function of the test data. 

　　According to the required functions, the flow chart is shown in Figure 2.

Figure 2 Software flow of battery charge and discharge voltage measurement device

　　As shown in Figure 2, the floating charge mode or discharge mode is pre-selected, and the upper and lower voltage limits are set. After the measured voltage value of the reader passes through this process, it can be stored in the flash memory, and then the same cycle is performed until all batteries are measured. Finally, the measurement results are input into the computer through the serial El circuit, and then the printer prints out the table for storage. 

　　2.4 Comparison test of new and old devices 
　　We tested the same group of batteries with new and old devices, and the results are shown in Table 1. 

　　Table 1 Measurement data results

　　As shown in Table 1, the total battery voltage, battery floating charge voltage, and battery discharge voltage measured by the new device are compared with the measurement results of the original device. The error is ≤0-2%. According to the provisions of 6.8.1 of the central station DC system operation regulations: the battery voltage measurement error should be ≤5%, which fully meets the accuracy requirements of regular measurement work. Moreover, from the time comparison table, it can be seen that the measurement time of the new device is almost reduced by more than half of the time of the original device. 

　　3 Conclusion 
　　Using the principle of single-chip microcomputer, the software realizes the functions that were originally realized by manpower and hardware circuits. By comparing the measurement data of the new and old devices, the improved battery charge and discharge voltage measurement device not only saves labor (reduced from three people to one person), improves work efficiency and economic benefits, but also effectively avoids the risks borne by operators during measurement.