Design of intelligent monitoring instrument for power grid voltage based on AC sampling

1 Introduction

The measurement and monitoring of grid voltage in power systems affect the regulation and automation management of power grid systems. 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, grid voltage measurement mostly uses rectified DC, but its measurement accuracy is directly affected by the rectifier circuit; the rectifier circuit parameters are difficult to adjust and are greatly affected by waveform factors; and AC sampling is to collect the instantaneous value of the measured signal according to a certain rule, and then use a certain numerical calculation method to obtain the measured value. AC sampling depends on the measurement accuracy and measurement speed. Here we introduce a hardware and software design of intelligent grid voltage 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 consists of three parts: data acquisition, single-chip microcomputer system and interface. The hardware block diagram is shown in Figure 1.

The three-phase voltage to be measured is added to the input end of the sampling circuit respectively. After the signal is converted in proportion, it is added to the input end of the A/D conversion through the impedance matching network, the 16-to-1 multi-way analog switch, and the sampling and holding circuit. The data after A/D conversion is latched and input to the MCU, and then the operation is used to determine whether the measured voltage is qualified. At the same time, the measurement results can be recorded in the storage device. The MCU can display the time and measurement results on the VFD in real time by operating the clock, and adjust the clock through the keyboard. Because there is a storage device in the system, the historical data can be called out 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 measurement range of this instrument is designed to be 90~110 V, so the peak voltage is through the matching network, and the peak voltage becomes Therefore, the primary and secondary ratio of the coupling coil is selected to be 12:1, and the output voltage of the matching network is -10~+10 V.

The polling method is adopted, and the analog multi-way switch device CD4067B is selected to select 3 voltages to be measured respectively, and the 3 voltages are measured respectively through the same measurement circuit. The input impedance of CD40-67B is 50 Ω, and a matching network must be added to its input end. The maximum input VP-P of the device is 20 V, and the maximum delay time is 60 ns. The sampling and holding circuit uses LF398, and the maximum input VP-P of the device is 36V, which meets the measurement requirements. The A/D converter uses AD574A, and the input voltage of the device is +10 V, and the sampling bit number is 12 bits. The sampled data is represented by signed binary, the highest bit is the sign bit, and the last 11 bits are the data bits, and the sampling speed is 35μs. AD574A can adjust the reference voltage to improve the measurement accuracy. The data after A/D conversion is latched by 74LS374 and input to MCU for calculation. The MCU uses AT89C51, which has 4KB on-chip ROM and the clock is 11.0592 MHz, which can meet the calculation requirements.

The time parameters are recorded by HI1380 serial clock. This device is a serial clock holding device with seconds, minutes, hours, months and years. By operating the device through MCU, the time parameters can be correctly obtained to count the voltage information. The statistical information of the voltage is stored in the storage device, which is convenient for reading historical information. The instrument uses 24C64 device to save information. The device completes the operation through 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 4-bit operation mode, and the display time, voltage information and historical information are controlled by MCU. Through 3 buttons to operate the MCU, operations such as modifying time and calling historical information can be completed.

The interface is constructed using SP490 device, which is a full-duplex RS-485 level transceiver. Through the serial port connection with MCU, it can be operated by PC, thereby realizing the remote operation and centralized monitoring of the instrument. [page]

2.3 System circuit 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 voltage sampling, analog switch, sample and hold, and A/D conversion, the input analog signal is converted into a digital signal. The dotted line part in Figure 2 is the analog circuit.

The digital signal after A/D conversion is input to MCU for processing. The MCU controls the clock, storage devices, display module operation and interface circuit part, which are pure digital circuits. The instrument and PC interface are on the rear panel of the chassis, while the display and keyboard operation are on the front panel of the chassis.

Pay special attention to the processing 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 has added inductance and capacitance filtering, the signal ground and the power ground are separated, and the inductance filter is used when connecting. Through the reasonable layout of the PCB board and the special processing 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.

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From the discretization formula, we can know 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 cycle T, select the appropriate number of sampling times N to determine the sampling time interval. Since the main frequency of AT89C51 is 11.059 2MHz and the conversion speed of AD574 is 35μs, and considering the accuracy requirements of power parameters, the sampling period is set to 312.5μs, that is, 64 points are sampled in each cycle. In addition, the ratio of the input voltage to the output voltage of the impedance matching network is, so the voltage at the output end of the impedance matching network is:

Where un is the instantaneous sampling voltage at the nth moment.

Then the measured voltage is:

According to formula (3), the voltage of the measured signal can be calculated, so that the qualified voltage time of each day can be counted.

4 Conclusion

This system is an electric parameter monitoring instrument designed based on AC sampling. Through simple changes, the current, power and other grid parameters are measured, and 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 system anti-interference ability, good scalability, etc., and is easy to promote and use in similar industrial and civilian measurement and control systems.