[Students] high-pressure bus on-line test temperature? ã€Answer】 In the power system, high voltage switchgear, GIS (gas insulated substation) and other high voltage electrical equipment and current carrying bus and other electrical equipment in the load current is too large will appear temperature rise, the final temperature may make the adjacent insulation components Performance degradation, or even breakdown. According to statistics, a considerable part of the causes of electric power system accidents are related to overheating. Therefore, effective measures to monitor the temperature of busbars and electrical contact are problems that the power system needs to solve. Running current bus, high-voltage switch at high potential, the temperature measurement device has the following characteristics: a. in high voltage environment; b. Allow the system to overload in a short time, but it must give an alarm signal before the bus temperature endangers the operation safety; c. Since the temperature rise is caused by the load current, the temperature varies with the load (time) and therefore requires real-time on-line monitoring and recording at regular intervals; d. The busbar temperature is one of the state parameters of the power system. To comprehensively monitor the system status, the busbar temperature measurement device is required to digitally output for computer processing and can be used with other electrical parameters to become part of the on-line monitoring system. 1, high-pressure bus temperature measurement technology status quo Bus at a high potential, the current domestic dedicated to high voltage bus and electrical contact fever measurement of the instrument is still small. One of the main methods of temperature monitoring is to apply a layer of luminescent material that changes color with temperature on the electrical contact surface and roughly determine the temperature range by observing the color change. This method has low accuracy, poor reliability and can not be used for quantitative measurement. The method is to use infrared (infrared) radiation characteristics of the infrared thermometer, which can measure the temperature between 0 ℃ ~ 200 ℃, the basic error of ± (1% × t å 0.5%), higher accuracy, but due to the need Optical devices, used in high voltage switchgear and other special occasions is not convenient, but the price is relatively high, there are some difficulties in the promotion and application. 2, high-pressure bus temperature measurement solution According to the characteristics of temperature measurement under high-voltage working environment, the busbar and the electrical contact temperature measuring device adopt the method that the temperature is measured in situ and the data is transmitted to the ground and processed by a computer. The temperature sensor is powered by a floating rechargeable battery to reduce the electrical connection between high and low voltage. It adopts all-digital mode to work with strong anti-interference ability, high precision and small volume. The system comprehensive test, its work is stable and reliable, able to meet the requirements of high-pressure bus temperature measurement. In this device, the use of thermistor as a temperature sensor: Compared with metal materials, thermistors resistivity temperature coefficient of 10 times to 100 times the metal material or even higher, and depending on the choice of semiconductor materials, the resistance The temperature coefficient can have various values ​​in the range of -6% / ° C to + 60% / ° C, and since the resistivity of the semiconductor material is much higher than the metal, the size of the thermistor can be small. For example, the bead thermistor can be as small as 0.2mm diameter beads, so tiny thermal components not only thermal inertia, fast response, the test environment has little effect, and can be used to measure very narrow space Temperature, such as voids, high voltage contact gaps, and the like. Because there is no thermocouple similar to the use of thermocouple cold junction compensation issues, but also without considering the line lead resistance and wiring on the accuracy of temperature measurement, it is more convenient to use. Because NTC negative temperature coefficient thermistor with stable performance, high sensitivity, good dynamic performance, affordable and can meet the temperature measurement requirements of the bus under high voltage environment, so the NTC negative temperature coefficient thermistor as the preferred sensing element . In order to get the digitized temperature sensing signal, it is convenient for logic processing. In the device, the change of the thermistor resistance is converted into the pulse period change, and then the pulse counting can obtain the digital signal which has a certain relation with the temperature value. Get the temperature of the object under test. The temperature sensor circuit composed of a thermistor, a high-precision standard resistor and a capacitor, and a timing trigger 555 is shown in FIG. 1. Flip-flop 555 is a square wave pulse output: high pulse T1 = RlCln2, low pulse T2 = RtCln2, in order to reduce the impact of capacitance C on the measurement accuracy, take the ratio of Tl / T2 as the sensing signal, Rt = R1T2 / Tl, measured by the two pulses Tl, T2 and resistance R1 calculated corresponding to the measured resistance of the thermistor R, Rt-T from the relationship curve to calculate the measured temperature. The thermistor of the sensor leads from the circuit board and is applied to the surface of the busbar to be measured. The sensor is installed in the proper position adjacent to the measuring point. 2.2 Sensing signal logic processing The signals obtained in the temperature sensing circuit are the charge and discharge pulses T1 and T2. In order to obtain the ratio T1 / T2, T1 and T2 are counted and counted by the count clock to obtain two count values ​​T1 and T2, SCM to deal with. To meet the data transmission needs, the use of adjacent cycles T1, T2. Since a charge-discharge cycle of about tens of milliseconds, due to thermal inertia in this range, the bus temperature to be measured changes very small, regardless of the resulting error. In the sensing signal logic processing, the main consideration of the following points: a. To reduce power consumption, CMOSIC is used in digital temperature sensors. b. Error: Due to the randomness of the lowest bit count when the counter is working, in order to reduce the measurement error, the counter adopts "down-count" method to set the initial value to (FF). In the case of high temperature, although the corresponding T2 pulse width is small, but can get a larger count, can effectively reduce the measurement error. Although errors can be significant at lower temperatures, since high-pressure busbar temperature measurements make sense only at higher temperatures, the measurement error at high temperatures should primarily be considered. Therefore, the method of "counting down" is correct The above content is based on the actual problems encountered by the students finishing for reference, if you have any questions, please communicate and correct.
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