Application of various types of sensors in hydropower plants

1.preface

The Yellow River Xiaolangdi Water Control Project is located at the exit of the last section of the Yellow River in the middle reaches of the Yellow River. It is the key hub for controlling the water and sediment of the Yellow River. The total installed capacity of the Xiaolangdi Hydropower Plant of the Yellow River is 6*300MW, which is the main power plant for the peak frequency regulation and accident backup of Henan Electric Power Network. Xiaolangdi Power Plant is a state-of-the-art hydropower plant that fully utilizes cutting-edge automation products. In modern automated production processes, various sensors are needed to monitor and control the various parameters in the production process, so that the equipment works in a normal state or in an optimal state, so excellent sensors are the basis of modern production.

2. Introduction to the sensor

A sensor is a device or device that can sense a specified measurement and convert it into an available output signal according to a certain rule. The input signal (measured) is often non-electrical, and the output signal is often a power that is easy to handle, such as voltage or current. .

There are many types of sensors, different classification standards, and different methods. Commonly used are resistance sensors, inductive sensors, capacitive sensors, temperature sensors, piezoelectric sensors, Hall sensors, thermocouple sensors, photoelectric sensors, digital Position sensor, 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 and so on.

 3 Power plant requirements for sensors

Modern large-scale hydropower plants generally have the requirements of ¡°unmanned duty, few people on duty¡±. In order to improve the overall efficiency and reliability of equipment work, there are 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 signal. Commonly used are platinum, copper-based thermal resistance sensors, semiconductor-based thermistor sensors, 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 upper guide, thrust, lower guide, water guide bearing temperature of the hydro-generator set and the oil temperature in the four guide bearing oil grooves. The temperature of the generator stator and rotor and the temperature of the water in the air cooler used to cool them are measured.

In the actual work, due to the harsh environment of the four guide bearings of the hydro-generator unit, the platinum resistance wire often breaks. Therefore, the root wire of the platinum resistance needs special protection to prevent the generator set from rotating during the rotation. The impurities in it are constantly impacting. In addition, the mounting position of the panel is quite close to the position of the sensor. To prevent the measurement error caused by the wire, a three-wire connection is required.

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 a general term for detecting the energy of interaction between gases, liquids, solids, etc. It also includes pressure gauges above atmospheric pressure and vacuum gauges below atmospheric pressure. The capacitance of the 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 pressurized gas tank and the pressure oil tank and the pressure used to detect the water level in the collecting well. At this time, the pressure sensor converts the pressure signal into an electric signal of 4-20 mA to calculate a corresponding pressure signal or a water level signal.

Whether in the air compressor control system or the hydraulic device control system, or in the power plant maintenance or leakage drainage control system, it is necessary to install two sets of pressure sensors or even three sets of sensors that are mutually standby, mainly to be safely detected. It is measured to prevent the unit from being accidentally shut down due to the pressure drop of the pressure gas tank and the pressure oil tank. It also prevents the flooding plant from leaking water that cannot be discharged from the plant in time due to the high water level in the water collecting well.

Xiaolangdi Power Plant is also equipped with some pressure switches. When the pressure is greater than or lower than a certain set value, the contacts will act and send signals to the corresponding control loop.

In practical applications, the zero point of the pressure sensor is prone to drift, so periodic calibration is required. In addition, the long-term use of the sensor is prone to fatigue and the measurement signal is inaccurate and needs to be replaced periodically.

6 speed detection

The speed sensor is a kind of sensor that converts the speed signal into an electric signal. It can detect both the linear speed and the angular speed. The commonly used speed generators and pulse encoders and the gear speed measurement.

In the Xiaolangdi Hydropower Plant, the gear plate is used to measure the speed of the generator. The toothed disc is mounted on the flange of the generator extension shaft and the speed signal is taken directly from the rotating shaft of the turbine. It is an ideal speed input signal. The gear 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 speed measuring device also has high reliability zero speed (creep) detection. Features. The number of teeth of the small wave bottom 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 gear speed measurement also has its shortcomings. First, it increases the cost of hardware and the complexity of the software. Second, due to the limitation of machining accuracy, the distance between the teeth and the teeth on the toothed disc cannot be exactly equal, or the pitch is not May be completely uniform. For high-sensitivity speed sensors, it calculates the speed by scanning the time between the teeth and the teeth. Even if the actual speed of the unit does not change, the different intervals between the teeth and the teeth will reflect the change of speed, thus generating interference signals. The third is that the proximity switch collision is easily damaged.

7 Flow detection

A flow sensor is a sensor that converts a flow signal into an electrical signal. Commonly used are electromagnetic flowmeters and flow switches.

The electromagnetic flowmeter is a meter that measures the volumetric flow rate of a conductive liquid by using Faraday's law of electromagnetic induction. The measuring channel of the electromagnetic flowmeter is a smooth straight pipe with an unobstructed flow detecting piece. It is not easy to block and is suitable for measuring liquid-solid two-phase fluid containing solid particles or fibers. It does not produce pressure loss due to the detection flow rate, and the resistance of the instrument. It is only the resistance of the pipeline of the same length, which is most suitable for large diameter water supply pipelines requiring low resistance loss. The measured volumetric flow is unaffected by fluid density, viscosity, temperature, pressure, and electrical 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 applied to detect the flow of the upper guide, thrust, lower guide and water guide bearing cooling water pipeline, and the detected flow signal is converted into an electric signal of 4-20 mA to calculate The corresponding flow of water. However, the flowmeter has certain requirements for installation, and it is not possible to measure the flow of a liquid having a low conductivity, such as oil.

Xiaolangdi Power Plant is also equipped with some flow switches. When the flow rate is greater than or lower than a certain set value, the contacts will act and send signals to the corresponding control loop.

8 Detection of oil mixed water

The oil mixed water sensor is a capacitive sensor for measuring oil mixed water or oil accumulated water. It is used to detect the content of water mixed into the return tank or the leak 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 stagnant water in the tank.

The oil mixed water sensor is placed in the measured medium. When the water 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% moisture content in the oil is converted into a corresponding 4-20 mA output.

The oil-mixed water sensor is installed in the oil tank 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 tank and prevent the flooding water guide or the oil tank from cooling the oil. The water pipe broke. However, in practical applications, the accuracy of oil-mixed water needs to be further improved, and false alarms occur.

9 Generator swing and vibration detection

The eddy current sensor is used to detect the upper and lower guides of the hydro-generator set, the water guide, and the swing of the large shaft. 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 surface to be measured. And the resulting change in coil impedance to reflect the distance of the probe from the surface being measured.

A sensor for detecting the vibration of the upper frame, the lower frame, the stator core, the stator base, and the top cover and the draft tube of the hydro-generator set is a speed sensor. A commonly used speed sensor is a magnetoelectric sensor with a spring-mass system that measures the absolute motion of the measured object relative to the ground.

The measurement of vibration and swing is fully applied in Xiaolangdi Hydropower Plant. The measured swing has the X and Y directions of the measured object, and the measured vibration has axial and radial vibration of the measured object. By collecting the data of the hydro-generator set swing and vibration, it can detect the pressure pulsation in the top cover and the draft tube, the mechanical imbalance of the rotating part of the unit, and whether the rotor center and the center of the unit are offset to cause electromagnetic imbalance. The health of the unit during the 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 an important part of the state maintenance of hydropower plants. Through the state maintenance of the unit, the efficiency of the unit is greatly improved.

10 displacement detection

The sensors for measuring displacement mainly include a pulse encoder and a magneto-displacement sensor.

A pulse encoder is an angular displacement sensor that converts a mechanical angle into an electrical pulse. In Xiaolangdi Power Plant, the encoder is applied to the detection of the displacement of the inlet gate and the tailwater anti-silting gate.

The displacement of the small wave bottom cylinder valve is measured by a magnetic displacement sensor. Due to the harsh installation environment of the cartridge valve and the high precision required, a magnetic displacement sensor is used. When the magneto-displacement sensor is in operation, a starting pulse is generated by the electronic circuit in the electronic compartment, and the starting pulse is transmitted in the waveguide wire, and a rotating magnetic field that advances in the direction of the waveguide wire is generated, when the magnetic field and the magnetic ring or the floating ball When the permanent magnetic field meets, a magnetostrictive effect is generated, and the waveguide wire is twisted. This twisting is sensed by the pick-up mechanism installed in the electronic compartment and converted into a corresponding current pulse, and two electric circuits are calculated. The time difference between the pulses can accurately measure the measured displacement and liquid level.

The output signal of the magneto-displacement sensor is a true absolute position output, not a proportional or re-amplified signal, so there is no signal drift or variable value, so it is not necessary to periodically re-mark like other liquid level sensors. And maintenance. However, magneto-displacement sensors require some hardware and software support.

11 position detection

A position sensor is used to detect a position and a switch that reflects a certain state. Common position sensors mainly include water float switches, magnetic float level gauges, and stroke switches.

The water float switch is a liquid level controller with a simple structure and convenient installation. It has been applied in the small wave bottom cover drainage control circuit. When the water level is higher than the installation position of the float, the float flips and the contact moves, sending a signal to the corresponding control loop. However, in practical applications, the sealing of the float is often poor, and the contact cannot be operated normally or cannot be returned after the action.

The oil level of the pressure tank in Xiaolangdi Power Plant is measured by a magnetic float level gauge, which uses a float ball as a measuring component to transmit the liquid level change through the magnetic rod on the connecting rod. The level gauge is equipped with a level alarm and a liquid level remote transmission. 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 4-20 mA current signal, and is applied to corrosion, toxic, flammable and the like due to the use of anticorrosive materials such as stainless steel.

The travel switch is a contact position sensor with simple structure, reliable operation and low price. When an object encounters a travel switch during motion, its internal contacts act to complete the control. In Xiaolangdi Hydropower Plant, the position of the generator damper and the position of the turbine guide vane lock are all used. In practical applications, the stroke switch is constantly subjected to an external force, and the whole displacement occurs so that the position of the object cannot be correctly reflected. This is to be noted.

12 high current, high voltage detection

The voltage transformer PT and the current transformer CT mainly convert the high voltage and the large current of the primary system into low voltage and small current, and are supplied to the measuring instrument and the protection device. The most common device for performing these transformation tasks is what we usually call a transformer. The transformer that performs voltage conversion for voltage conversion and current conversion is a 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 ratio of 15000A/1A.

It is worth noting that it is strictly forbidden to open the secondary side of the current transformer and to prohibit short-circuit or grounding on the secondary side of the voltage transformer.

Both voltage transformers and current transformers are used in the AC loop. In addition, there are shunts that convert large DC currents to millivolts.

13 Conclusion

The application of the above-mentioned sensors in Xiaolangdi Hydropower Plant is the current situation, but with the development of sensors and the improvement of science and technology, more and better sensors will continue to emerge, thus further improving the automation level of power plants.

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