Design of AC Sampling Power Network Voltage Intelligent Monitor
1 Introduction
The measurement and monitoring of grid voltage in the power system affects the regulation and automated management of the grid system. In order to monitor the grid voltage in real time, a digital measuring instrument controlled by a microprocessor is used. In the early days of digital measurement, most of the grid voltage measurement uses rectified DC, but its measurement accuracy is directly affected by the rectifier circuit; the adjustment of the rectifier circuit parameters is difficult, and it is greatly affected by the waveform factor; while AC sampling is based on certain rules The instantaneous value of the signal, and then use a certain numerical calculation method to obtain the measured value. AC sampling depends on measurement accuracy and measurement speed. Here we introduce a kind of hardware and software design of grid voltage intelligent monitoring based on AC sampling, which can intuitively and accurately reflect the power quality of the power system.
2 System hardware design
2.1 System hardware architecture
The system hardware circuit is composed of 3 parts: data acquisition, single-chip system and interface. The hardware block diagram is shown in Figure 1. 
The measured three-phase voltage is added to the input end of the sampling circuit, after the signal is proportionally converted, and then through the impedance matching network, a multi-channel analog switch is selected by 16, and the sample and hold circuit is added to the input end of the A / D conversion. The data after A / D conversion is latched and input into the MCU, and then the operation judges whether the measured voltage is qualified. At the same time, the measurement results can be included in the storage device. The MCU can display the time and measurement results on the VFD in real time through the operation of the clock, and adjust the clock through the keyboard. Because there are storage devices in the system, the historical data can be recalled and displayed on the VFD. The measuring instrument can be connected to the microcomputer through the PC interface, and the instrument can be centrally operated and monitored on the microcomputer.
2.2 System circuit design
The design measurement range of the instrument is 90 ~ 110 V, so the peak voltage is through the matching network, the peak voltage becomes so, the primary to secondary ratio of the coupling coil is selected to be 12: 1, and the output voltage of the matching network is -10 ~ + 10 V.
It adopts polling design, selects analog multi-channel switch device CD4067B, selects 3 voltages to be tested respectively, and measures 3 channels through the same measuring circuit. The input impedance of CD40-67B is 50 Ω, and a matching network must be added to its input. The device has a maximum input VP-P of 20 V and a maximum delay time of 60 ns. The sample-and-hold circuit uses LF398, and the device inputs VP-P with a maximum value of 36V to meet the measurement requirements. The A / D converter uses AD574A, the input voltage of the device is +10 V, and the number of sampling bits is 12 bits. Sampling data is selected with signed binary representation, the highest bit is the sign bit, the last 11 bits are data bits, and the sampling speed is up to 35μs. AD574A can adjust the reference voltage to improve the measurement accuracy. The data after A / D conversion is latched by 74LS374 and input into the MCU for calculation. MCU selects AT89C51, with 4KB on-chip ROM, and the clock selects 11.0592 MHz, which can meet the calculation needs.
Time parameters are recorded using the HI1380 serial clock. The device is a serial clock holding device with seconds, minutes, hours, months, and years. By operating the device through the MCU, the time parameters can be correctly obtained and used to count voltage information. The statistical information of the voltage is stored in the storage device, which is convenient for reading historical information. The instrument uses a 24C64 device to save information. The device completes the operation through the I2C bus. Its capacity is 64 KB, which can meet the needs of recording two months of historical information.
The display part uses 16T202DAJ type VFD module, which can be used for character operation and is suitable for instrument display. The data line selects the 4-bit operation mode, and the display time, voltage information and historical information are controlled by the MCU. By operating the MCU with 3 buttons, you can complete operations such as modifying time and calling historical information.
The interface is built with SP490 device, which is a full-duplex RS-485 level transceiver. It can be operated by a PC through connection with the serial port of the MCU, thereby achieving remote operation and centralized monitoring of the instrument.
2. 3 system line layout
Figure 2 is a schematic diagram of the system circuit layout. The PCB board is laid out according to the signal flow. The signal is input from the rear panel of the chassis. After the voltage sampling, analog switch, sample hold, and A / D conversion, the input analog signal is converted into a digital signal. The dotted line in Figure 2 is the analog circuit. 
The digital signal after A / D conversion is input to the MCU for processing. The MCU controls the clock, storage device, display module operation and interface circuit part as pure digital circuits. The interface between the instrument and the PC is on the rear panel of the chassis, while the display and keyboard operations are on the front panel of the chassis.
Pay special attention to the treatment of the power supply. The power supply of the digital circuit will interfere with the analog circuit, thereby increasing the measurement error. The analog power supply adds inductance and capacitance filtering, the signal ground and power ground are separated, and inductance filtering is used when connecting. Through the reasonable layout of the PCB board and the special treatment of the power supply circuit, the power supply and signal interference can be reduced and the measurement error can be reduced.
3 System software design
The entire system software design process is shown in Figure 3. 
It can be known from the discretization formula that the effective value of the voltage can be calculated according to the voltage sampling value and the number of sampling points at different times in a cycle. According to the period T, select the appropriate sampling times N to determine the sampling time interval. Because the main frequency of AT89C51 is 11.059 2MHz and the conversion speed of AD574 is 35μs, and taking into account the accuracy requirements of power parameters, the sampling period is set to 312.5μs, that is, 64 points are sampled in each period. In addition, the ratio of the input voltage to the output voltage of the impedance matching network is so the voltage at the output of the impedance matching network is: 
In the formula, un is the instantaneous sampling voltage at the nth time.
Then the measured voltage is: 
According to equation (3), the measured signal voltage can be calculated, so that the daily voltage qualification time can be counted.
4 Conclusion
The system is a power parameter monitoring instrument designed based on AC sampling. By simply changing and measuring grid parameters such as current and power, all results can be displayed on the VFD. The system has the advantages of simple structure and low cost. In terms of data processing and conversion, it has the characteristics of good real-time performance, strong anti-interference ability of the system, good scalability, etc. It is easy to popularize and use in similar Ding industry and civil measurement and control systems.
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