Composition and driver design of a new power grid voltage monitor

0 Introduction
The voltage monitoring system is an intelligent instrument system that monitors and automatically records the quality of power grid voltage, providing correct data for statistical voltage qualification rate and other parameters, and reflecting the management of voltage quality. The minimum combination of the voltage monitoring system is an intelligent voltage monitor. Through the use of the instrument, various voltage parameters of the voltage monitoring point can be measured and recorded, and the calculation results such as voltage qualification rate, high voltage failure rate, and low voltage failure rate can be given at the same time. Another combination of the voltage monitoring system is composed of a voltage monitor plus supporting equipment such as an electric card, a communication machine, and a host computer. The voltage monitor records a lot of data, and it is very time-consuming and labor-intensive to query and copy through the instrument keyboard, and there are many disadvantages and inconveniences to print directly with a small printer. This system can realize centralized monitoring of power grid voltage, and has a series of functions such as query, statistical report, voltage over-limit alarm, typical working day setting, and adjustable system ratio and voltage value error coefficient.

1 System Design
The acquisition module is based on the high-performance microprocessor STCl2C5A60S2, and its periphery consists of module circuits such as signal conversion, real-time clock, serial communication and large-capacity serial memory. The entire system structure is shown in Figure 1.

STCl2C5A60S2 is a new processor launched by Hongjing Technology this year, which is fully compatible with the traditional 51 series. STCl2C5A60S2 is a clock/and its cycle, enhanced 51 core, 8 to 12 times faster than the ordinary 8051. The operating voltage is relatively wide, 3.3 to 5.5 V. A second reset function pin is added, and an external power-off detection circuit is provided, which can save data into E2PROM in time when power is off. There is 1 280 B RAM data memory inside. The chip has E2PROM function inside, and the number of erase and write times is more than 100,000 times. It has ISP/IAP function, 8-channel 10-bit high-speed ADC, the speed can reach 2.5×105 times/s, and 2-way PWM can also be used as 2-way D/A. An independent baud rate generator has been integrated inside, and the serial communication rate of this series of microcontrollers can be determined by the overflow rate of the internal timer T1, so that T1 can realize the timing or counting function. This series of microcontrollers also has the function of dual serial ports. One serial port can be used by the system, and the remaining serial port can be used to output system program debugging information, avoiding the trouble of only using the I/O port to simulate the UART timing when one serial port is occupied.
The monitoring system is a weak current system, and the grid voltage is generally about 220 V AC, which cannot be directly measured by the system. The measured voltage must be reduced to a small AC signal of 3 to 4 V through a step-down converter with a buffer.
The voltage signal after the step-down is a small AC signal, which is then converted into a DC signal with equal effective value through the true RMS conversion chip AD536 and output. The
DC signal converted and output by AD536 is output through the V/F conversion chip LM331 to output the digital pulse of the frequency corresponding to the DC signal for the single-chip microcomputer to collect, so that the single-chip microcomputer calculates the size of the DC signal. In this design, the traditional A/D converter is not used to convert the analog signal into a digital signal, but LM331 is used for V/F conversion.
The circuit diagram of V/F conversion is shown in Figure 2.

LM331 has a wide dynamic range of up to 100 dB; good linearity, with the maximum nonlinear distortion less than 0.01%, and good linearity even when the operating frequency is as low as 0.1 Hz; high conversion accuracy, with a digital resolution of up to 12 b; simple peripheral circuits, and only a few external components are needed to easily form a V/F conversion circuit, and it is easy to ensure conversion accuracy. Based on the above circuit and data sheet, the calculation formula for the LM331 output frequency can be obtained:

The digital pulse output by LM331 is isolated by the optocoupler TLP521 and then sent to the timer T1 of the microcontroller for counting. Timer T0 is used for timing. Every 1 second, the microcontroller reads the value of the T1 counter in the timer interrupt service program of T0. Because the interval is 1 second, the read value is the magnitude of the LM331 output pulse frequency. The above formula can be used to calculate the effective value Vb of the AD536 output, and then the magnitude of the measured grid voltage at this time can be deduced.
According to the requirements, this system needs to record the historical voltage. Here, the ferroelectric memory FM24C512 is selected. It is a non-volatile memory with a capacity of up to 512 KB and uses advanced ferroelectric processing technology. The schematic diagram is shown in Figure 3.

FM24C512 writes at bus speed without delay. The next bus cycle can start immediately without data polling, and the maximum bus frequency is up to 1 MHz. In addition, FM24C512 has a much higher number of write operations than E2PROM. Moreover, because the write operation does not require the internal boost of the power supply voltage of the write circuit, FRAM consumes much less power than E2PROM during the write operation.
FM24C512 uses the I2C communication protocol to simplify the interface circuit with the microcontroller. It uses very few pins and occupies very little board space. It only uses two pins to communicate with the processor, one is the clock line SCL and the other is the data line SDA. Because the main processor used does not have a related I2C communication interface, only two I/O ports can be used to simulate the I2C timing. The timing diagram is shown in Figure 4.

When recording the historical voltage, the time generated by the real-time time chip DS1302 is used as a reference for recording the voltage. DS1302 is a high-performance, low-power real-time clock circuit with RAM launched by DALLAS Corporation of the United States. It can time the year, month, day, weekday, hour, minute, and second, and has a leap year compensation function. The operating voltage is 2.5-5.5 V. It uses a three-wire interface to communicate synchronously with the CPU, and can use burst mode to transmit multiple bytes of clock signals or RAM data at a time. There is a 31×8 RAM register inside the DS1302 for temporary storage of data. DS-1302 is an upgraded product of DS1202, which is compatible with DS1302, but adds dual power supply pins for main power supply/backup power supply, and provides the ability to charge the backup power supply with trickle current. Its connection with the microcontroller is shown in Figure 5. The connection between
DS1302 and the CPU only requires three lines, namely the clock line (SCLK), the data line (IO), and the reset line (RST). The corresponding timing diagram is shown in Figure 6.

According to Figure 6, the corresponding subroutine for reading the DS1302 time can be compiled.

In order to enhance the human-computer interaction performance, the OCMJ12232 LCD display module of Jinpeng Electronics is selected as the display part of the system. The display mode of OCMJ12232 is dot matrix graphics, which can display any graphics or Chinese characters. It has 8,192 Chinese dot matrices and has two interface modes: parallel and serial. This design uses the serial interface mode, which has the characteristics of occupying less I/O resources and simple programming.

2 Process Design
The system program consists of a display module, a key scanning module, a frequency counting module, a recording and statistics module, a system menu module, etc. In the software writing process, the recording and statistics module is relatively complex, and its algorithm is as follows: the program determines whether it is the next second, if not, it continues to collect voltage and then judge, otherwise it processes the relevant voltage second record; then it determines whether it is one minute, if not, it exits, otherwise it processes the relevant voltage minute record; the next step is to determine whether it is one hour, if not, it exits, otherwise it processes the relevant voltage hour record; and so on, it is also necessary to determine whether it is a full day or a month. Among them, it is also necessary to determine whether it is a typical working day. Three typical working days can be set each month, and the records of each hour in the typical working day are saved for users to view. The flow chart is shown in Figure 7.

3 Conclusion
This paper introduces in detail the voltage monitoring system based on STCl2C5A32AD, which is suitable for monitoring 0-480 V AC voltage. This design has been completed and debugged, and has passed the customer's acceptance and successfully operated in the monitoring hall of the power grid company.