Basic Principle, Installation and Debugging of Turbine Supervisory Instrument (TSI) System

A)     The working principle of TSI

For the most widely used eddy current sensor system, it consists of a probe, an extension cable and signal conditioner. The preamble has an electronic circuit, which can generate a low-power radio frequency signal (RF). This RF signal is sent to the coil inside the probe tip by an extension cable signal. If there is no conductor material within the range of this signal, the energy released into this range will return to the probe. If the surface of the conductor material is close to the top of the probe, the RF signal will form a small eddy current on the surface of the conductor. This eddy current causes energy loss in this RF signal. The size of this loss can be measured. The closer the conductor surface is to the top of the probe, the greater the energy loss in RF signal. The sensor system can use energy loss to generate output voltage that is proportional to the measured gap.

The signal conditioner is composed of high frequency oscillator, detector, filter, DC amplifier, linear network and output amplifier. Detector demodulates high-frequency signal into a DC voltage signal. This signal passes the low-pass filter to the high frequency after residual wave is removed, and then processed by DC amplifier, linear compensation circuit and output amplification, voltage signal proportional to the actual distance between the measured object and the sensor is obtained at output terminal. In general, the rated output voltage of the signal converter is -4V until -20V

The monitoring system is also called a frame. In general, frame consists of 2 parts: power supply and system monitor. The power supply provides the specified power supply for the monitoring table and corresponding sensor installed in the frame. The power supply is always placed in the first position of the frame; the system monitor checks the power supply level to ensure the normal operation of the system. At the same time, it also has control system “Ok” (normal operation). Indicates the system’s sensors and field wiring are performed at the specified level. The system monitor also controls the setting of alarm points and system reset. The system monitor is always placed in the second position of the frame.

1.      Turbine generator TSI Installation and debugging technology

1.1  Composition of TSI

The probe installed on the turbine shaft system are mainly divided into following categories:

a)      Displacement measurement of each part of the turbine; the axial displacement of the rotor relative expansion of the rotor and cylinder, thermal expansion of cylinder and eccentricity

b)     Steam turbine shaft state vibration: rotor shaft vibration (relative vibration), bearing vibration (absolute vibration) and phase angle vibration

c)      Speed measurement: zero speed and turbine speed

1.2  Vibration measurement, debugging and installation

Steam turbine mainly realize the energy conversion through the rotary motion to complete the predetermined function. The rotor is the core component of steam turbine. When the power of steam turbine increases and speed of rotation continues to increase, the stress, friction and excessive bending of rotating shaft caused by vibration will cause major accidents. The measurement and monitoring of vibration become more and more important

1.2.1        Vibration measurement principle

The displacement of the vibration is generally measured by non-contact eddy current sensor. The vibration displacement measure at this time is the change on the position of the rotor shaft surface relative to the eddy current sensor probe, so it is also called relative vibration measurement. The instrument including high frequency signal generator and filter circuit.

For vibration of the bearing housing or the cylinder, the change speed of the vibration can be measured by contact type inertial speed sensor, or the acceleration of the vibration change can be measured by a contact type of piezoelectric acceleration sensor. The measured vibration change speed and/or acceleration are measured relative to the fixed reference frame of the earth, so it also called absolute vibration measurement. People are used to transform the measured vibration velocity value and/or acceleration value into displacement through integral function.

1.2.2        Installation of vibration probe

Important task of eddy current sensor installation is to adjust the gap voltage, adjust distance between the front-end surface of the sensor and the surface of the measure surface, so that the gap is within the linear range. During the adjustment, the sensor probe needs to be connected to the signal pre-conditioner (pre-amplifier), proved 24VDC power supply to the pre-amplifier, then loosen the probe fastening nut, adjust the probe and measure the output voltage of the pre-amplifier at the sensor. It is generally adjusted to a gap voltage between -8V to -12V.

1.3  Displacement measurement, debugging and installation

1.3.1 Measurement and debugging

When the shaft is running, due to various factors, such as changes load, temperature, etc. The shaft moves in the axial direction. In this way, dynamic and static friction may occur before the rotor and the stator, so sensor is required to measure the change of the rotors axial position relative to the stator, that is, the gap of the shaft relative to the thrust bearing in the axial direction. Therefore, an eddy current probe is needed to detect the change of this gap. By detecting the DC gap voltage of the output signal of the sensor, relative position of the thrust plate in the bearing can be determined. Due to the importance of parameters, 3-4 probes are usually designed to detect an object at the same time. Axial displacement refers to change in the position on displacement of the rotor in the axial direction and thrust bearing; expansion difference in thermal expansion between the cylinder and rotor.

1.3.2 Installation

Bearing seat and the cylinder expand laterally and longitudinally along the horizon pin and vertical pin. The intersection of the 2 straight line passing through the horizontal pin and vertical pin is called the dead point of cylinder. The rotor expands back and forth along the axial direction with the thrust disk as the dead point. The differential expansion is an important parameter that must be monitored during start-stop and normal operation

For the current unit, in addition to measuring the axial vibration, the eddy current sensor can measure the axial displacement and the change of high pressure and low-pressure expansion difference according to the change of the gap voltage. Due to the positive and negative directions of axial displacement and the difference between high pressure and low-pressure expansion, must pay attention to installing probe

a)      Axial expansion of rotor is greater than value of cylinder, it called positive expansion difference. Generally, in cold start process, mainly shows a positive expansion difference. In hot start and shutdown process, mainly shows negative expansion difference

b)     Some products have a certain angle of the measured surface of the high-pressure expansion difference, so when installing in the cold state, pay attention to consult the installation data to confirm the inclination angle value of the measured surface, and adjust the initial value by calculation to be the same as the axial displacement .

c)      Before installing the axial displacement probe, the thrust clearance of the turbine must be determined, that is, K value; first push to the working surface, then to the non-working surface, measure the thrust clearance. At this time, the axial displacement value is set to half of the thrust clearance value. This is the "intermediate zero setting" method, that is, the K/2 position is used as the zero point of the axial displacement. Close to the work surface is positive, away from the work surface is negative. In addition, there are two ways to set the zero: when the cold state, the thrust plate of the rotor is pushed to the working pad of the thrust tile (generator side), and is close to the working surface, at this time, the axial displacement is set to zero; Or push it to a non-working surface, close to the zero position, which one you choose depends on the manufacturer's requirements.

At present, there are many speed probes installed in steam turbines of large-scale thermal power units. DEH systems generally have independent measurement loops for the speed control of the units, while the speed measurement of TSI systems is mostly used for electrical overspeed protection.

1.4   Speed measurement and debugging

At present, there are many speed probes installed in steam turbines of large thermal power units. DEH systems generally have independent measurement loops for the speed control of the units, while the speed measurement of TSI systems is mostly used for electrical overspeed protection.

In general, the magnetic induction sensor is used to measure the speed of the unit, that is, the actual speed value is obtained by using the number of pulse signals formed by the high and low voltage changes between the side speed gear wheel and the measuring probe. Zero speed is the rotation speed of the shaft set in advance. When the running machine needs to be stopped, the speed of the machine reaches the set point of zero speed. Rotate slowly to prevent the shaft from bending, to prevent damage to the machine due to the bending of the shaft during driving. When the probe of the turbine speed port is installed, a feeler gauge can be used for measurement. The formula for calculating the rotation speed is RPM = (Frequency Pulse/Number of teeth) x 60

1.5  Installation of absolute expansion probe

During the startup process of the steam turbine, its cylinder expands due to heat. If the expansion is not uniform enough, it will cause the cylinder to buckle or slant. Such deformation will cause huge stress between the foundation and the cylinder, so there will be misalignment., And this phenomenon is generally caused by the "stuck" phenomenon of the sliding pin system. Connect the cylinder of the LVDT sensor to the iron core. When the iron core expands, the iron core will move, and a proportional electrical signal will appear. Input the measurement board and perform linear processing, display and output the (4-20) mA signal. During installation, the two thermal expansion sensors should be tightly installed on the thermal expansion bracket under the cold state of the unit. The zero point should be the pointer on the sensor indicating the zero position.

1.6  Precautions and solutions for on-site installation and commissioning of TSI system probes

1.6.1 Precautions during probe installation

a) Corresponding to the installation of the shaft displacement and expansion sensor, the center of the probe should be corresponding to the center of the effective surface to be measured.

b) In order to prevent mechanical damage when the probe is installed, the probe lead should be separated from the extension cable. The sensor lead cannot be held to rotate, and the wrench platform on the probe should be clamped with a tool to tighten the probe lead.

c) After tightening the probe as required, connect the probe lead and extension cable connector, and then bundle and fix it in the box to prevent damage to the cable during operation.

d) Each sensor is required to be vertical perpendicular to the measured surface, and the maximum deviation angle is <1 degree.

1.6.2 Problems encountered during debugging and solutions

a) When abnormal vibration occurs, pay attention to changes in oil temperature and lubricating oil pressure. Check whether the rotating part of the turbine-generator unit has large shaft bending or dynamic and static friction. At the same time, pay attention to the change in hydrogen temperature inside the generator.

b) After the installation of the low-pressure differential expansion probe and the high-pressure differential expansion probe is completed, and it is found that the low-pressure differential expansion and the high-pressure differential expansion deviate from zero, you should go to the site to fully understand the heating situation of the low-pressure cylinder, and then analyze the reasons for the abnormal phenomenon: The probe is not installed securely and the large-axis zero movement or loosening phenomenon occurs, and then the corresponding solution measures are taken in time for the problem.

c) When it is found that the axial displacement is increasing and the axial displacement value gradually increases to ±0.5 mm (depend on design manufacture), the steam parameters, condenser vacuum, load, close monitoring of differential changes, thrust bearing metal temperature should be checked and carried out in time Adjustments should be made to test the meter, while listening to the unit for abnormal sounds and vibrations in the bearings. Pay attention to the exhaust temperature of the low-pressure cylinder, the temperature of the main steam and the reheated steam.