Design of local meteorological monitoring device for power transmission lines based on DSP and CPLD

1 Overview

The state of the transmission line directly determines the safe and stable operation of the entire power grid. Real-time monitoring of the micro-meteorological parameters of the transmission line can provide necessary on-site information for the normal dispatching of the power grid, as well as the prediction and control of natural disasters. The transmission line is one of the key components of the power system. In order to operate safely and stably, the dispatching system often collects the electrical parameters and operating parameters of the transmission line (such as the model and arrangement of the transmission line, and the flow distribution information on it, etc.) and performs appropriate control. In the research results of the power system, more attention is paid to flow optimization, system failures and system stability issues, while the research on the impact of meteorological conditions on transmission lines is relatively insufficient.

my country is one of the countries with serious natural disasters on power transmission lines. The problem of line icing caused by rain, snow and freezing weather has not been well solved, and line icing has caused great damage to the power grid. To predict and control these natural disasters, only the electrical operating parameters of the transmission line are not enough. For example, to study the icing problem of transmission lines, it is necessary to collect local meteorological parameters around the line that are directly related to the icing mechanism.

In order to realize the local meteorological parameter collection of transmission lines, it is necessary to design an online monitoring device for local meteorological parameters to provide basic data for higher-level application decisions. This paper designs a real-time data collection and monitoring device based on "DSP+CPLD" that can realize local meteorological monitoring of transmission lines, which can measure parameters such as ambient temperature, humidity, atmospheric pressure, wind speed and wind direction.

2 System Hardware Design

The device includes a data acquisition device and peripheral measuring sensors and transmitters. The data acquisition device uses TI's DSP chip TMS320VC33 (abbreviated as VC33). It has a rich instruction system, Harvard bus structure, and high-speed data processing capabilities.

The address decoding and timing control circuits are implemented by CPLD, which is used to coordinate the work between the various parts of the hardware. Lattice's CPLD chip ispLSI2032A has in-system programmability and in-system diagnostic capabilities, which can realize the software online modification of hardware functions, simplify hardware design, and improve the stability of the hardware system. It realizes the address decoding function of all extended devices, the external memory access wait state signal, and the generation of access timing signals of other DSPs and peripherals, and expands several digital I/O ports. The control signal between VC33 and peripherals is realized by CPLD, which has the characteristics of strict timing, stability and reliability. In this design, the simulation scan interface of ispLS12032A is JTAG interface; its software is written in ABEL language, and completes the design input, design file processing, pre-wiring simulation, design adaptation, post-wiring simulation, program download and other processes in the Synario development environment.

2.1 External Memory

The principle of memory selection is: the access speed and bus level must match those of VC33. Set the working mode of VC33 to Microcomputer/Bootloader, and the memory space is allocated as follows:

① Program SRAM. The starting address is 810000h, the length is 64K words, and it stores the program code of VC33 during operation. It is composed of two 64K×16-bit high-speed CY7C1021V33-12VI chips from Cypress.

② Data SRAM. The starting address is 820000h, the length is 64K words, and the running variable space of VC33 is also composed of two CY7C1021V33-12VI.

③ Flash ROM. The starting address is 400000h, the length is 3FFFFh bytes, it stores the BOOT FABLE of VC33, and uses a fast Flash chip IS28F020 from ISSI.

④NVRAM. The starting address is C00000h, the length is 3FFh bytes, it stores important operating parameters, can be modified online and the data will not be lost after power failure. It uses a Dallas non-volatile RAM.

2.2 DSP and external communication interface circuit and human-machine interface

The communication function includes two parts: communication with the PC to upload the water source heat pump operation data, which can be used for further analysis; communication with the LCD (liquid crystal display) and forming a human-machine interface with the keyboard interface circuit. The serial communication interface circuit of VC33 and LCD chip SC16C750B is shown in Figure 1.

The communication between VC33 and PC and LCD complies with the serial communication protocol RS232. Its physical interface is expanded by EXAR's UART chip ST16C550 plus a Maxim's RS232 interface driver chip MAX3232. The working mode is query type.

Real-time data acquisition systems always need human-computer interaction during operation (including displaying the operating status on the LCD, setting operating parameters, etc.), so a keyboard interface must also be designed. This article uses a Toshiba 82C79 chip to complete the scanning of the 4×4 keyboard matrix. The working mode of 82C79 is set to the decoding scanning keyboard working mode, and occupies the INT2 interrupt of VC33. When there is a key action, 82C79 generates an interrupt signal to VC33, and VC33 calls the keyboard scanning program to read the code of the key pressed.

2.3 Power supply and clock circuit

The power supply circuit of VC33 uses the dual power supply application chip TPS767D318 from TI. Its peripheral I/O operating voltage DVDD is 3.3 V, while its core operating voltage CVDD is 1.8 V. The power supply circuit is shown in Figure 2.

In addition to the stable working voltage during normal operation, VC33 also requires that the voltage at the CVDD terminal cannot exceed the voltage at the DVDD terminal by 0.6 V during the power-on process. The Schottky limit rectifier DL5817 is used to provide this safety guarantee. Diodes D1 and D2 play the role of clamping the two working voltages of CVDD and DVDD. A large-capacity capacitor is connected to the voltage output terminal of TPS767D318 to handle the very large transient current at the initial stage of voltage output to avoid burning out VC33.

VC33 strengthens the clock configuration function and can provide multiple clock working modes. The design uses an external active clock, the internal clock circuit is not started, and the internal multiplication factor is 1.

2.4 Data Acquisition System

The basic parameters that the system should monitor include: ambient temperature, humidity, atmospheric pressure, wind speed and wind direction. Except for wind direction, the rest are analog quantities. In order to convert continuous analog quantities into discrete digital quantities that can be processed by VC33, an A/D conversion circuit must be designed.

ADI's AD7874 is a 12-bit data acquisition A/D chip with built-in sampling/holding, 4-channel simultaneous sampling, and high precision. It is suitable for the same-phase acquisition of temperature and pressure at a certain point of the working fluid of the water source heat pump. Its input signal range is ±10 V, and the single-channel sampling frequency can reach 29 kHz. The multiplexing of the AD7874 input channel is achieved by selecting Phillips' 16-to-1 multi-channel analog conversion switch HEF4067. Since the optimal control of water source heat pump energy saving is dynamic real-time measurement and control, and the dynamic process of water source heat pump thermal engineering is relatively slow, the acquisition frequency of each analog quantity is set to 24 Hz, that is, 24 points per second.

2.4.1 Atmospheric pressure measurement

The conversion of the working pressure and water pressure of the atmospheric pressure measuring pump to voltage signals is realized by Siemens' QBE620-P16 pressure transmitter. QBE620-P16 is suitable for both gas and liquid, with a wide temperature operating range, suitable for the harsh operating sites of power transmission lines. The external working voltage range is DC 18-33 V, and the measuring pressure range is 0-232 psi. The measuring signal outputs DC 0-10 V, which is input to AD7874 after active filtering and anti-aliasing circuit, and the AD7874 completes the A/D conversion.

It is generally believed that the relationship between the measured pressure and the output DC signal of the pressure transmitter is a linear transformation, but in fact the linearity is not zero. When the pressure measurement range is large, the measurement system error caused by linearity cannot be ignored and should be calibrated according to test data.

2.4.2 Ambient temperature measurement

Transmission line icing is closely related to temperature. The temperature measurement accuracy directly determines the accuracy of icing prediction. In this design, the temperature sensor is a three-wire Pt100 platinum resistance temperature sensor, according to the international temperature standard ITS-90:

Among them, RPt is the resistance value of Pt100, and T is the temperature. The resistance signal must be converted into a DC voltage signal before A/D conversion can be performed. The three-wire thermal resistor is used because the three-wire system can eliminate the measurement error caused by the additional resistance caused by the long wire.

Figure 3 is a high-precision temperature measurement circuit. The resistance value of the thermal resistor must be converted into a voltage or current signal before it can be input into the A/D circuit. XTR103 is a high-sensitivity transmitter produced by BURR-BROWN that uses a Pt100 thermistor (or other types) as an excitation and outputs a 4-20 mA DC current. Its internally integrated second-order correction linearization circuit can realize the linear conversion of Pt100 resistance value to DC current, and is widely used in industrial process control, factory automation, SCADA and other fields. The precision current/voltage converter RCV420 realizes the conversion of DC current to DC voltage.

The functional relationship between the XTR103 output current signal Io and the Pt100 resistance RPt is:

Among them, Io is the output current signal, which is in the range of 4 to 20 mA; RG is the range resistor of XTR103; and Rz is the reference resistor. Considering the operating conditions of the heat pump, it is necessary to select appropriate range resistors and reference resistors to set a suitable temperature measurement range. Select RG = 150 Ω, Rz = 80 Ω. From equations (1) and (2), it can be determined that the temperature measurement range is -50.77 to 132.55 °C, which can meet the requirements of the ambient temperature measurement range of my country's power transmission lines.

In order to improve the temperature measurement accuracy, in addition to using the high-precision signal conversion circuit shown in Figure 3, the following two problems must be solved:

① Calibration of the resistance-temperature characteristics of platinum resistors. Formula (1) gives the rated characteristics. The actual platinum resistors have statistical dispersion, so the resistance-temperature characteristics of each platinum resistor must be calibrated.

②Thermal lag problem of platinum resistance element. The thermal lag problem is caused by the heat transfer resistance between the element and the environment and the heat capacity of the element itself. Platinum resistance element consists of platinum resistance wire and stainless steel packaging shell, and ceramic packaging is used in high-temperature applications. Since the platinum resistance wire is very thin and has a small mass, its thermal inertia can be ignored.

2.4.3 Wind speed and direction measurement

The key to wind speed measurement is to convert wind speed parameters into electrical signals that can be processed by A/D circuits, which is accomplished by wind speed/direction sensors. This article uses the EA-V200 wind speed/direction sensor, which has a wind speed measurement range of 0 to 50 m/s and an output signal of 4 to 20 mA DC current with strong anti-interference ability. This DC signal is converted into a digital signal by the A/D circuit and then processed by VC33.

The wind direction measurement is provided by EA-V200 with 8 switch inputs to indicate different wind directions.

3. System software design

Set Microcomputer/Bootloader to VC33 operation mode. Before running, the program is stored in the Flash with low access speed. After the system is reset, the Bootloader fixed on the DSP chip moves the program to the high-speed SRAM and runs at full speed. This article only briefly introduces the functions of the software. The program is structurally divided into two parts: the main program and the interrupt service program.

The main program includes:

① System initialization program. Set parameters such as external memory interface, serial port, timer, interrupt, interrupt vector table, keyboard interface, etc., and determine the system operation mode.

②Data processing program: Convert the discretized data after A/D conversion into actual temperature, pressure, and working fluid mass flow, remove bad data, filter the high-frequency noise of the collected data, and finally obtain the state quantity of the actual working condition of the reaction system.

The interrupt service routine includes:

①A/D acquisition program. Complete the 12-bit acquisition of all analog quantities. According to the characteristics of the thermal sensor, the sampling frequency is set to 24 Hz for each channel. The A/D acquisition program occupies the INT0 interrupt of VC33.

②Keyboard scanning program. When a key is pressed, the key code is read and the INT2 interrupt of VC33 is occupied.

③Control quantity driver. Drive digital output or analog output. The timer interrupt of TIMER0 of VC33 provides the cycle of these control drivers. The specific control strategy should be determined according to the specific application.

④Communication program. Realizes the functions of LCD display and communication with PC. Occupies the timer interrupt of VC33.

4 Conclusion

The DSP-based local weather online monitoring device for power transmission lines can be dispersedly installed on the towers along the power transmission lines to measure meteorological parameters in real time, including ambient temperature and humidity, atmospheric pressure, wind speed, etc. These parameters are closely related to the prediction of ice coverage, de-icing jump, wind dance and control of the transmission lines, and can provide detailed information on the line site to the dispatching center. The design of this device involves multiple disciplines, and some experiences in the design are summarized as follows:

① Select appropriate sensors or transmitters according to the actual operating conditions of the transmission line. Pay attention to coordinating with the data acquisition system and reasonably arrange the number and measurement points of sensors or transmitters.

② Timing control issues between VC33 and peripherals. For slow external expansion devices, it is not enough to simply design a suitable access waiting state (for example, for A/D chips, the data bus needs to be blocked for a relatively long time after the chip select signal is invalid). You must also carefully study the access timing of the peripherals and design the corresponding blocking circuit to avoid bus conflicts that make the system inoperable.

③ The anti-electromagnetic interference problem of the system. The system based on VC33 is a high-speed system, and the electromagnetic interference problem is particularly serious. In particular, the clock line must be well shielded. For high-frequency signal lines, attention should be paid to transmission line distance, matching resistor design, printed circuit board wiring shape and other issues. Using a multi-layer wiring board is beneficial to resist electromagnetic interference, but the cost will increase.

The DSP-based local meteorological online monitoring device for transmission lines gives full play to the advantages of the "DSP+CPLD" system, and can realize multi-channel acquisition, data processing, natural disaster warning and other functions of parameters such as ambient temperature, atmospheric pressure, humidity, wind speed and wind direction, which has important practical significance for improving the safety and reliability of transmission lines and even the entire power grid.