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1 Introduction
The Xiaolangdi Water Control Project on the Yellow River is located at the exit of the last section of the canyon in the middle reaches of the Yellow River. The Huanghe Xiaolangdi Hydropower Plant has a total installed capacity of 6*300MW. It is the main power plant for peak and frequency regulation of Henan power grid and emergency backup. Xiaolangdi Power Plant is a national first-class hydroelectric power plant, fully applying cutting-edge automation products. In the modern automated production process, it is necessary to use various sensors to monitor and control the various parameters in the production process to make the equipment work in normal or optimal state. Therefore, good sensors are the basis of modern production.
2 Introduction to the sensor
A sensor is a device or device that can sense the prescribed measurement and convert it into a usable output signal according to a certain rule. The input signal (measured) is often non-electricity, and the output signal is often the amount of electricity that is easy to handle, such as voltage or current .
There are many types of sensors, with different classification standards and different names. Common ones include resistance sensors, inductive sensors, capacitive sensors, temperature sensors, piezoelectric sensors, Hall sensors, thermocouple sensors, photoelectric sensors, and digital sensors. Position sensors, etc. The sensors used in hydropower plants mainly include current sensors, voltage sensors, photoelectric encoders, pressure sensors, liquid level sensors, temperature sensors, flow sensors, etc., which are mainly used to detect position, linear displacement, pressure, temperature, flow, etc.
3 Requirements for sensors in power plants
Modern large-scale hydropower plants generally have the requirement of “no one on duty, few people on duty”. In order to improve the overall efficiency and reliability of equipment work, there are very high requirements for sensors:
(1) High reliability and strong anti-interference ability;
(2) Meet the requirements of accuracy and speed;
(3) Easy to use and maintain, suitable for the special environment of the power plant;
(4) Low cost.
4 Temperature detection
A temperature sensor is a device that converts the temperature into a resistance value or other electrical signals. Commonly used are thermal resistance sensors based on platinum and copper, thermistor sensors based on semiconductor materials, and thermocouple sensors.
PT100 platinum thermal resistance has been widely used in Xiaolangdi Hydropower Plant. The temperature sensor is not only used to detect the temperature of the upper guide, thrust, lower guide, water guide bearing and the oil temperature in the oil grooves of the four guide bearings, but also mainly Measure the temperature of the stator and rotor of the generator and the temperature of the water in the air cooler used to cool them.
In actual work, due to the harsh environment of the four guide bearings of the hydro-generator set, the platinum resistance wire is often broken. Therefore, special protection is required for the root wire of the platinum resistance to prevent the oil groove of the generator set during rotation. Impurities in it constantly impact. In addition, there is a considerable distance between the installation position of the panel and the position of the sensor, and a three-wire connection method is required to prevent the measurement error caused by the wire.
5 Pressure detection
A pressure sensor is a sensor that converts a pressure signal into an electrical signal. According to the working principle, it can be divided into piezoelectric sensor, piezoresistive sensor and capacitive sensor. It is the general term for detecting the force and energy between all substances such as gases, liquids, solids, etc. It also includes pressure gauges that measure above atmospheric pressure and vacuum gauges that measure below atmospheric pressure. The capacitance of a capacitive pressure sensor is determined by the electrode area and the distance between the two electrodes. It has recently developed rapidly due to its high sensitivity, good temperature stability, and large pressure range. In the Xiaolangdi Hydropower Plant, the pressure sensor is mainly used to detect the pressure of the pressure gas tank and the pressure oil tank, and the pressure in the water collection well to detect the water level. At this time, the pressure sensor converts the pressure signal into a 4-20mA electrical signal to calculate the corresponding pressure signal or water level signal.
Whether in the air compressor control system or oil pressure device control system, or in the power plant maintenance or leakage and drainage control system, it is necessary to install 2 sets of pressure sensors or even 3 sets of sensors for mutual backup, mainly to be able to detect safely It is measured to prevent the accidental shutdown of the unit due to the drastic reduction of the pressure of the pressure gas tank and the pressure oil tank, and to prevent the flooding of the factory building due to the excessively high water level in the collection well and the leakage of the factory building can not be discharged in time.
Xiaolangdi Power Plant is also equipped with some pressure switches. When the pressure is greater than or lower than a certain set value, the contact will act and send the signal to the corresponding control circuit.
In practical applications, the zero point of the pressure sensor is prone to drift, so it needs to be checked regularly. In addition, long-term use of the sensor is prone to fatigue resulting in inaccuracy of the measurement signal, and it needs to be replaced regularly.
6 Speed detection
The speed sensor is a sensor that converts a speed signal into an electrical signal. It can detect linear speed as well as angular speed. Commonly used are tachogenerators, pulse encoders, and gearwheel speed measurement.
In the Xiaolangdi Hydropower Plant, a toothed disk is used to measure the speed of the generator. The gear plate is installed on the flange of the generator’s extension shaft, and the speed signal is directly obtained from the rotating shaft of the turbine, which is an ideal speed input signal. The gear plate speed signal is not limited by the residual voltage of the generator, nor is it interfered by non-periodic clutter. It has higher reliability than the PT signal. In addition, the gear plate speed measurement device also has a highly reliable zero speed (creep) detection. Features. The number of teeth on the gear plate of Xiaolangdi Power Plant is 56 teeth, the rated speed is 107.1RPM, the probe pulse frequency is 100Hz, and the generated pulse signal is directly input to the speed sensor module.
However, the speed measurement of the gear wheel also has its shortcomings. One is that it increases the cost of hardware and the complexity of the software; the other is that due to the limitation of machining accuracy, the distance between the teeth on the gear wheel cannot be completely equal, or the tooth pitch is different. It may be completely uniform. For a high-sensitivity speed sensor, it calculates the speed by scanning the time between the tooth and the tooth. Even if the actual speed of the unit does not change, the different interval between the tooth and the tooth will reflect the change in speed, thereby generating interference signals. ; Third, the proximity switch is easily damaged by collision.
7 Flow detection
A flow sensor is a sensor that converts a flow signal into an electrical signal. The most commonly used are electromagnetic flowmeters and flow switches.
Electromagnetic flowmeter is an instrument that measures the volume flow of conductive liquids made by Faraday’s law of electromagnetic induction. The measurement channel of the electromagnetic flowmeter is a smooth straight pipe with no obstructed flow detection part. It is not easy to be blocked and is suitable for measuring liquid-solid two-phase fluids containing solid particles or fibers. It does not produce pressure loss due to the detection flow rate, and the resistance of the instrument Only the resistance along the pipeline of the same length is most suitable for large diameter water supply pipelines that require low resistance loss. The measured volume flow is not significantly affected by changes in fluid density, viscosity, temperature, pressure, and conductivity (as long as it is above a certain threshold).
In the technical water supply system of Xiaolangdi Power Plant, a large number of electromagnetic flowmeters are used to detect the flow of the upper guide, thrust, lower guide, and water guide bearing cooling water pipes, and the detected flow signal is converted into a 4-20mA electrical signal for calculation Out the corresponding water flow. However, the flowmeter has certain requirements on installation, and it cannot measure the flow of liquids with very low conductivity, such as oil.
Xiaolangdi Power Plant is also equipped with some flow switches. When the flow is greater than or lower than a certain set value, the contact will act and send the signal to the corresponding control loop.
8 Detection of oil and water
The oil-mixed water sensor is a capacitive sensor used to measure oil-mixed water or oily water. It is used to detect the content of water mixed into the oil return tank or the leaking tank in the automatic control oil system of the hydro-generator set. It can also be widely used as the detection of oil mixed or accumulated water in the oil tank.
The oil-mixed water sensor is placed in the measured medium. When the oil is mixed with water, the medium value changes, and the sensor outputs a corresponding electrical signal. After the signal sent by the sensor is processed, the 0-100% water content in the oil is converted into the corresponding 4-20mA output.
The oil-mixed water sensor is installed in the oil groove of the four guide bearings of the upper guide, thrust, lower guide and water guide bearing of Xiaolangdi Hydropower Plant to detect the water content in the oil groove and prevent the occurrence of water flooding or the oil groove used to cool the oil. The water pipe broke. However, in actual applications, the accuracy of oil-mixed water needs to be further improved, and false alarms sometimes occur.
9 Detection of generator swing and vibration
The eddy current sensor is used to detect the swing of the upper guide, lower guide, water guide, and large shaft of the hydro-generator set. It is actually a displacement sensor that relies on the high-frequency electromagnetic field generated by the probe coil to induce eddy currents on the measured surface And the resulting change in coil impedance reflects the distance between the probe and the measured surface.
The sensor that detects the vibration of the upper frame, lower frame, stator core, stator frame, top cover and draft tube of the hydro-generator set is a speed sensor. The commonly used speed sensor is a magnetoelectric sensor with a spring-mass system, which measures the absolute movement of the measured object relative to the ground.
The measurement of vibration and swing has been fully applied in Xiaolangdi Hydropower Plant. The measurement of swing has the X direction and Y direction of the measured object, and the measured vibration has the axial and radial vibration of the measured object. By collecting data on the swing and vibration of the hydro-generator unit, it is possible to detect the pressure pulsation in the top cover and draft tube, the mechanical imbalance of the rotating part of the unit, and whether the rotor center and the unit center are offset to cause the electromagnetic force imbalance, etc. The health of the unit during rotation is fully reflected by the measurement of vibration and swing. With the development of science and technology, the detection of vibration and swing is now an important part of the state maintenance of hydropower plants. The use efficiency of the unit has been greatly improved through the condition maintenance of the unit.
10 Displacement detection
The sensors for measuring displacement mainly include pulse encoders and magneto-induced displacement sensors.
Pulse encoder is a kind of angular displacement sensor, which can turn mechanical rotation angle into electric pulse. In Xiaolangdi Power Plant, encoders are used to detect the displacement of intake gates and tailwater silt prevention gates.
The displacement measurement of Xiaolangdi cylinder valve adopts a magneto-induced displacement sensor. Due to the harsh installation environment of the cylinder valve and higher accuracy requirements, a magneto-induced displacement sensor is used. When the magnetic displacement sensor is working, an initial pulse is generated by the electronic circuit in the electronic warehouse. This initial pulse is transmitted in the waveguide wire and at the same time generates a rotating magnetic field that advances in the direction of the waveguide wire. When this magnetic field interacts with the magnetic ring or floating ball When the permanent magnetic fields meet, the magnetostrictive effect is generated, which causes the waveguide wire to twist. This twist is sensed by the pickup mechanism installed in the electronic warehouse and converted into a corresponding current pulse. The two are calculated by the electronic circuit. The time difference between the pulses can accurately measure the measured displacement and liquid level.
The output signal of the magnetic displacement sensor is a true absolute position output, not a proportional signal or a signal that needs to be re-amplified, so there is no signal drift or value change, so it does not need to be recalibrated regularly like other liquid level sensors. And maintenance. But the magnetic displacement sensor needs certain hardware and software support.
11 Location detection
The position sensor is used to detect the position and reflect a certain state of the switch. Common position sensors mainly include water float switches, magnetic float level gauges, and travel switches.
The water float switch is a liquid level controller with simple structure and easy installation. It has been applied in Xiaolangdi roof drainage control loop. When the water level is higher than the installation position of the float, the float flips, the contact will act, and the signal will be sent to the corresponding control circuit. However, in practical applications, often due to the poor sealing of the float, the contact cannot operate normally or cannot return after the operation.
The oil level of the pressure oil tank of Xiaolangdi Power Plant is measured by a magnetic float level gauge, which uses a float as the measuring element and transmits the level change through the magnetic rigidity on the connecting rod. The liquid level gauge is equipped with a liquid level alarm and a liquid level remote transmission device. The liquid level alarm can realize the upper and lower limit alarm and limit control of the liquid level. The liquid level remote transmission device can convert the liquid level change into a 4-20mA current signal. Due to the use of anticorrosive materials such as stainless steel, it is used in corrosive, toxic, flammable and other occasions.
The travel switch is a contact type position sensor with simple structure, reliable action and low price. When an object hits the limit switch during movement, its internal contact will act to complete the control. In the Xiaolangdi Hydropower Plant, the position of the generator damper and the position of the guide vane lock of the turbine adopts a travel switch. In practical applications, due to the constant external force acting on the limit switch, its whole body will be displaced and thus cannot accurately reflect the position of the object. This should be paid attention to.
12 Detection of high current and high voltage
The voltage transformer PT and the current transformer CT mainly convert the high voltage and large current of the primary system into low voltage and small current proportionally, and supply them to measuring instruments and protection devices. The most common equipment that performs these transformation tasks is what we usually call transformers. The voltage transformer is the voltage transformer and the current transformer is the current transformer.
The voltage transformer PT in Xiaolangdi Power Plant mainly converts 220KV, 35KV, 10KV and other high voltages into 100V; the generator end uses a current transformer with a transformation ratio of 15000A/1A.
It is worth noting that it is strictly forbidden to open the secondary side of the current transformer and short-circuit or ground the secondary side of the voltage transformer.
Both voltage transformers and current transformers are used in AC circuits. In addition, there are shunts that convert large DC currents into millivolts.
13 Conclusion
The application of the above-mentioned sensors in the Xiaolangdi Hydropower Plant is the current situation, but with the development of sensors and the improvement of the level of science and technology, more and better sensors will continue to appear, which will further improve the automation level of the power plant.
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