Design and implementation of power transmission line monitoring system

　　0 Introduction

　　In order to solve the three major problems of phase deviation observation, recording and calibration in manual adjustment of power transmission lines. This design realizes the monitoring, recording and storage of current, voltage, zero-sequence current, power factor, active power, reactive power, apparent power, fundamental wave and harmonic power in power transmission lines for a long time and uninterruptedly through the design of software and hardware. This design uses MSP430F135 microcontroller as the control core, combined with voltage and current transformers, DSP power chips, human-machine interfaces, sound and light alarm circuits and signal processing circuits to realize the monitoring of power transmission line parameters. The system can use the data collected for a long time stored in the U disk to perform curve analysis on the host computer, providing a scientific and reliable basis for the power department to adjust the line load.

　　1 System Design

　　1.1 Selection of DSP power chip

　　The design uses a DSP power chip, which has seven second-order 16-bit sigma-delta ADCs, reference voltage output, voltage, current sampling, fundamental wave, harmonics and energy frequency measurement signal processing circuits, as well as an SPI communication interface and supports full digital domain gain and phase correction. It can automatically calculate active power, reactive power, apparent load power and power factor. It has an internal voltage monitoring circuit to ensure normal operation during power-on and power-off.

　　The design can perform high-precision measurements with an input dynamic operating range of 1000:1, while ensuring that the nonlinear measurement error is within 0.1%. It supports gain and phase compensation, and small current nonlinear compensation. The voltage and current RMS accuracy is better than 0.5%.

　　1.2 MCU Selection

　　The MSP430 microcontroller is a low-power 16-bit microcontroller with typical SOC characteristics and a large number of integrated peripherals. In particular, its internal integrated baud rate trimmer allows the MCU to work at any crystal oscillator not less than 32 768 Hz (but not exceeding the upper limit of the MCU's crystal oscillator requirements). When selecting the communication baud rate, it is not restricted by the baud rate factor without decimals, that is, any frequency crystal oscillator can be used within the allowable range of the baud rate. In addition, since the MSP430 MCU has an integrated temperature sensor, it is relatively easy to compensate for the temperature of the pressure sensor used to measure the liquid level. Moreover, the MSP430 series microcontrollers are composed of various modules for different applications. Due to the low power consumption of the microcontroller used, it can work normally with ordinary batteries and achieve long-term uninterrupted normal use.

　　2 System overall structure and working principle

　　As shown in Figure 1, the whole system is mainly composed of single-chip host circuit, control circuit, signal measurement, sound and light alarm circuit, key circuit, LCD Chinese character display circuit, power circuit and communication circuit. P1 port is used as the data line of the system. The DSP power chip uses P3.0 P3.1 P3.2 ports to access the MCU to provide input data to the system. The working signal of the sound and light alarm module is output through P2.5 port to generate alarm sound. The keyboard module is connected to the MCU through P2.0, P2.1 and P2.2 ports to control the 6 keys of the system. The system controls the LCD display through P1 port and P5.0 and P5.1. The serial communication is controlled by P3.6 and P3.7 through the communication interface circuit to control the reading and writing of the U disk. The power module of the system generates 3.3 V and +5 V voltages to provide a stable working voltage for the system.

Figure 1 System overall composition

　　The system uses voltage and current transformers to perform differential conversion and then input them into the DSP power chip for signal analysis and processing, and outputs high-precision three-phase voltage values, three-phase current values, zero-sequence current values, active power, reactive power, apparent power, power factor, fundamental wave and harmonic power values. These data are read out through the SPI communication interface after the DSP power chip calculates the data, and after data conversion, they are sent to the LCD for display [4-5]. When the system time runs to the writing time specified by the USB flash drive, the system automatically records all the collected data. Users can set the collection time interval through keyboard input as needed. The device number in the system can be set arbitrarily. At the same time, the system can also display the temperature of the working environment and the system time in real time.

　　3. Main Hardware Circuit Design of the System

　　3.1 Signal differential input circuit design

　　The signal input adopts differential input mode. The input current and voltage sensor signals are sampled through the sampling resistor R17, and C5 and C6 are filtered to reduce interference. REFO provides a 16-bit AD standard reference voltage through a 10K current limiting resistor. The voltage and current signal input circuit diagram is shown in Figure 2. [page]

Figure 2 Voltage and current signal input circuit

　　3.2 Communication control circuit

　　The communication circuit function in this system completes the conversion between TTL level and CMOS level. Passive input does not consume power when not in communication stage, and only consumes power when communicating. This is very helpful for the whole system to achieve low power consumption, which is also a feature that the dedicated conversion chip on the market does not have. Its control circuit is shown in Figure 3.

Figure 3 Communication control circuit

　　3.3 Sound and light alarm circuit design

　　The sound and light alarm circuit is composed of transistors, light-emitting diodes, resistors, capacitors, buzzer components, etc. When the measured liquid level value is less than the set warning value, the microcontroller will send out an alarm signal. After receiving the alarm signal, the light-emitting diode is lit and the buzzer sounds, producing a sound and light alarm effect.

　　4 System Software Design

　　The system software adopts modular structure design and is divided into seven modules: system initialization module, LCD display module, key recognition and processing module, SPI analog communication module, serial communication module, U disk reading and writing module, time and working environment temperature detection module.

　　The main function of the initialization module is to set the display buffer, stack pointer, each working unit, operation flag and working register, each I/O port and CMU working clock module setting, set the system timer module, communication module, DSP power chip module initialization and system interrupt setting, etc.; the main function of the LCD display module is to display the collected data and system working parameters and other data; the keyboard module is responsible for key recognition and key processing, and when there is a key action, the corresponding key processing subroutine is called for processing. It can realize the setting of system time, storage interval and device number; the main function of the SPI module communication module is to complete the data exchange between MCU and DSP power chip. The serial communication module sends data to the U disk module to prepare for data storage; the function of the U disk read and write module is to write the data transmitted by the MCU into the U disk and store it; the function of the time and working environment temperature detection module is to complete the calculation of time and collect the temperature of the working environment in real time.

　　5 Conclusion

　　The instrument can be widely used in the fields of electricity, petroleum and chemical industry. It can detect and record various parameters in the power line in real time and store them in a USB flash drive in time, so that technicians can adjust the equipment according to the actual situation of the line. The instrument has the advantages of stable and reliable working performance, small size, low cost, accurate and sensitive measurement and control, easy installation and use, low power consumption, etc., and has high practical application value.