Transformer Oil Temperature Measurement and Fiber Optic Communication System Design

1 Introduction

The insulation aging of transformers is mainly caused by the influence of temperature, humidity, oxidation and degraded substances decomposed in oil. However, the aging speed is mainly determined by temperature. The higher the working temperature of the insulation, the faster the chemical reaction, the faster the mechanical and electrical strength of the insulation is lost, the faster the insulation aging speed, and the shorter the service life of the transformer. In fact, the winding temperature is affected by load fluctuations and temperature changes, and the range of variation is very large. Therefore, real-time collection of the transformer temperature to maintain it within a certain range is of great significance to the life of the transformer.

Online monitoring of transformer oil temperature is very important for early diagnosis of transformer faults. However, due to the complex structure of the transformer and the many factors that affect its safe operation, online monitoring is very difficult. In the past, oil temperature measurement generally adopted indirect analog measurement methods, which had poor accuracy and was not timely. In view of the above problems, this paper proposes to use platinum resistance as a sensor for transformer oil temperature measurement, MSP430F449 single-chip microcomputer as the core processing device to realize transformer oil temperature measurement, use optical fiber to realize communication with the host computer, run oil temperature monitoring software on the host computer, and monitor the transformer oil temperature in real time. The power transformer oil temperature measurement system replaces the direct input transformer temperature controller to control the power transformer cooling system, and has the characteristics of convenient real-time online detection and centralized control.

2 Analysis of transformer heat dissipation principle

The losses generated by the transformer during operation are dissipated in the form of heat through the oil, the tank wall and the radiator to the surrounding air. Heat is dissipated in three forms: heat conduction, convection and radiation. The heat from the inside of the winding and the core to its surface is mainly dissipated by heat conduction, and the heat from the winding and the core surface to the transformer oil is mainly dissipated by convection. The heat dissipated into the transformer oil causes the temperature of the transformer oil in the tank to rise, the density to decrease, and thermal buoyancy to be generated. Under the impetus of thermal buoyancy, the transformer oil enters the connecting oil pipe from the upper part of the tank and enters the radiator through the oil pipe. The transformer oil undergoes heat exchange with the outside air in the radiator, which reduces the temperature of the transformer oil in the radiator, enters the connecting oil pipe from the lower part of the tank, and re-enters the transformer tank through the oil pipe, forming a natural cycle. The heat dissipation of the transformer can be determined by formula (1):

Where, Ql is the unit heat load; Q is the loss of the transformer; F is the total heat dissipation area of ​​the transformer; C1 is a constant related to the transformer's own parameters; ty is the transformer temperature rise.

3 System Hardware Design

During the operation of power transformers, the measurement of oil temperature is the basis and key to maintaining the safe operation of power transformers. The activation and deactivation of the cooling system of power transformers and over-temperature alarm are all realized by the temperature controller installed on them.

The transformer oil temperature measurement system uses MSP430F449 as the main control device, which is a powerful 16-bit ultra-low power microcontroller produced by TI. The MSP430 microcontroller has multiple clock sources of high, medium and low speeds, which can be flexibly configured for use by each module and work in a variety of low-power modes, greatly reducing the power consumption of the control circuit and improving the overall efficiency. First, the oil temperature of the power transformer is collected and amplified by the sensor and signal conditioning circuit to a voltage value suitable for A/D conversion. Then, the A/D converter samples the analog signal and converts it into a digital signal, which is pre-processed by MSP430. The monitoring system uses optical fiber to achieve serial communication with the PC through the MAX3221 level conversion circuit, and the PC realizes further analysis of the temperature and humidity values ​​and control of the system. The optical fiber communication system built with the optical fiber transceiver module completes the remote transmission of data, and the human-computer interaction monitoring is completed with the help of the serial communication between the MSP430 microcontroller and the host (host computer). The system block diagram is shown in Figure 1.
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3.1 Platinum resistance and signal conditioning circuit

Platinum resistance thermometers have a series of advantages such as high accuracy, stable performance, good interchangeability, corrosion resistance and ease of use. They have always been an ideal temperature measuring element widely used in industrial measurement and control systems. For platinum resistance thermometers, the function of resistance R and temperature t is as shown in formula (2):

Rt=R0[1+at-bt2] (2)

Where Rt is the resistance value at t℃, R0 is the resistance value at 0℃, 100Ω; A is 3.908 02×10-3/℃; B is -5.802×10-7/℃2.

This process will directly affect the measurement accuracy of the system. Of course, the industrial environment with strong electromagnetic fields will also have a great impact on the measurement and control accuracy and stability of the system.

There is a nonlinear term bt2 between 0℃ and 650℃, so the resistance value of the platinum resistor and the temperature are not in a linear relationship. This requires that the problem of platinum resistor linearization correction should be considered when the platinum resistor is actually applied. For high-precision platinum resistor temperature measurement digital display instruments, the resistance temperature scale of the platinum resistor can be solidified in the memory EPROM with the output data of the A/D converter as the address, that is, in the EPROM, the temperature value corresponding to it is stored with the A/D conversion value as the unit address. When the EPROM is accessed with the output result of the A/D converter as the address, the temperature value stored in the unit is read and sent to the LCD for display.

3.2 MSP430 interface circuit

The output signal of the temperature sensor is converted into a standard voltage signal of 0V to 5V by the temperature transmitter module, and then enters the voltage/frequency (V/F) conversion module, and is converted into a frequency of 0kHz to 100kHz and output to the I/O port of the MSP430. After sampling and calculation by the MSP430, it is displayed on the liquid crystal module (LCD). The P2.4 port is used as the RXD to receive data, and the P2.5 port is used as the TXD to send data. The F449 peripheral interface circuit is shown in Figure 2.

3.3 V/F module design

The system uses a V/F conversion chip to convert 0V~5V voltage signals into frequency output to the single-chip microcomputer, which not only saves I/O ports, but also saves A/D conversion chips, reducing system costs. The voltage-controlled oscillator LM331 chip is used, and its outstanding feature is that it converts analog voltage into a pulse train with strong anti-interference ability, which can be transmitted over long distances and directly input into the single-chip microcomputer. [page]

3.4 CMI encoding and decoding principle

CMI code is an alphabetic balanced code. According to the coding requirements of CMI, the data is first judged. If the input data is "0", the output data is "01". If the input data is "1", the parity of "1" must be judged. When the first "1" comes, the output is "00", and when the second "1" comes, the output is "11", and it is constantly reversed to achieve the coding of 1. The function of the CMI decoding circuit is to decode and restore the electrical signal received by the photoelectric conversion circuit.

3.5 Fiber Optic Communication Circuit Design

At the optical transmitter, the signal is modulated and then converted into an optical signal. The signal is transmitted over long distances through optical cables. At the receiving end, the optical signal is restored to an electrical signal, which is then demodulated and amplified to restore the original signal. The transmitting end of the optical terminal contains a carrier light source, which converts the electrical signal into an optical signal and inputs it into the optical fiber for transmission to a distant destination. The receiving end of the optical terminal contains a photodetector, which restores the optical signal from the optical fiber into an electrical signal and inputs it into the receiving end of the optical terminal. The data can be transmitted in full duplex. The repeater amplifies and reshapes the optical signal that has been attenuated and distorted after long-distance transmission to generate an optical signal of a certain intensity for further transmission.
The system block diagram is shown in Figure 3.

The output signal after optical fiber transmission can be connected to the serial port of the PC for communication after RS-485/232 level conversion.

4 Software Design

The software flow chart is shown in Figure 4. The software of this system is written in C language under the IAR development environment and adopts modular programming. After the system is initialized and the boot screen is displayed, the basic timer is turned on to wake up the MSP430 to sample and display the real-time temperature. After the temperature is displayed, it enters the low power mode, waits for the external keyboard interrupt and the timer interrupt, and enters the low power mode again after the interrupt returns, thereby reducing power consumption.

In the transformer oil temperature monitoring system, MSP430 is used as the lower computer and PC is used as the upper computer. The lower computer and the upper computer communicate through the serial port. After running the monitoring software of the upper computer, select the start communication button to establish communication with the lower computer, and the status of the lower computer can be monitored. The remote monitoring software displays the numbers and curves of the transformer oil temperature value, saves the data of each temperature measurement point, and alarms for abnormal conditions during the actual operation process. [page]

5 Test Results and Analysis

The 160MVA three-phase transformer of Zhengzhou West Substation has a rated voltage of 220/121/10.5kV. The temperature measurement system was put into use in January 2007. The normal test temperature is set at 45℃～75℃. The debugging can make the measurement error of 60℃ 0, so that the error is small within the test range.
The measurement system is used to measure the top oil temperature of the transformer, and a set of experimental data on June 2 is extracted, as shown in Table 1. The standard temperature (actual temperature) is measured with a high-precision mercury thermometer. From the experimental data, it can be seen that the accuracy of the system measurement is 0.1℃, and the error is within ±0.5℃, which meets the design goal.

The experimental results show that the temperature measurement system works stably, is convenient to measure, and has high accuracy. The main reason for the hysteresis of the system is the thermal conductivity of the sensor probe. At the same time, the signal conditioning circuit is the key part of the device. The quality of the input signal processing has a decisive influence on the performance of the device and the accuracy of the measurement. In the actual measurement process, the temperature value sometimes deviates. After conversion by the temperature transmitter, the displayed temperature is incorrect. It is possible that the dust and looseness of the terminal block increase the resistance and cause the temperature error.

It is generally stipulated that the fan is put into operation when the top oil temperature of the transformer reaches 65℃, or when the load current reaches 70% of the rated value. To prevent the fan motor from starting frequently, the device should also be adjusted to put the fan into operation at 65℃, and the fan will be turned off only when the oil surface temperature drops to 55℃, or the fan will be cut off when the load current is less than 50% of the rated value. Therefore, accurate measurement of the transformer oil temperature can save more than 40% of the time that the transformer fan is doing useless work. Since the system was put into use in 2007, it has saved 42.3% of the fan power consumption.

6 Conclusion

Due to the different regions and operating environments of transformers, the determination of the input temperature value of the transformer cooling system must consider both safe operation and energy saving. The output power of the transformer must change accordingly, so real-time online monitoring of the transformer oil temperature is very necessary. This system can realize the long-distance optical fiber transmission of the transformer temperature signal, accurately measure the transformer oil temperature, and has ultra-low power consumption, which has a high cost performance.

The principle of optical fiber communication is to use the modulated optical fiber carrier to continuously reflect in the optical fiber to reach the other party, and then demodulate the original information to achieve information transmission. Its main features are large transmission capacity, high speed, long transmission distance, strong anti-interference, good insulation performance, etc. In particular, the two major features of anti-electromagnetic interference and good insulation performance can be applied to high-voltage and strong electromagnetic interference environments such as substations and high-voltage lines. It is a communication method that is gradually being widely used in power system communications.