Torsional Vibration Test of Turbine Generator Shaft System Chen Yong (School of Mechanical and Electronic Engineering, Nanjing Forestry University, Nanjing 210037, China) Technical points. The test results show that the steady-state asymmetric short-circuit frequency excitation method can not only measure the low-order natural frequency of the shaft system, but also can measure some high-order frequencies.
In order to prevent torsional vibration damage of the steam turbine generator set, the most accurate analysis and calculation should be carried out from the design and manufacture stage to master and control the torsional vibration characteristics. However, due to the complexity of the structure of the steam turbine generator, the torsional vibration mode and the mechanism, any theoretical analysis and calculations have more or less errors. It is necessary to correct the main test shaft of the shaft torsional vibration test by simulation and real machine test. The natural frequency and mode shape of the system. The test is based on the measurement method to eliminate the influence of the average angular velocity during the operation of the shafting system, and measure the alternating arc length (or torsion angle) caused by the alternating angular velocity and its torsional vibration angular velocity (or frequency). The measured dynamic process may be the angular displacement variation law of the torsional vibration at the measuring point, or may be the law of the torsional strain (stress) change at the point.
Test Method 1.1 When the field test is excited, in order to measure the natural frequency according to the response, the shaft system must be artificially excited to make torsional vibration. Theoretical analysis and experimental research at home and abroad show that five kinds of mechanical or electrical excitation methods can be used: cranking and excitation; grid-connected excitation; shunt-string capacitor excitation; steady-state asymmetric short-circuit excitation ; excitation frequency excitation.
Steady-state asymmetric short-circuit variable-frequency excitation is to apply the excitation current far less than the rated value during the process of raising and lowering the steam turbine, and then make the generator asymmetrical short-circuit or load operation directly or through an external impedance, thereby generating a negative sequence. The current component, which produces a counter-rotating magnetic field that interacts with the rotor's magnetic field that is rotating in the forward direction, producing an alternating torque twice the fundamental frequency that excites the torsional vibration of the shaft. This is a steady state excitation.
The test using steady-state asymmetric short-circuit frequency excitation only involves the unit itself, and has nothing to do with the grid. Therefore, it can avoid the impact on the test when the machine and network link*: 2001-cooperation. Pay attention to the test: (1) The speed change during the speed increase and fall process should not be too fast. At least near the possible natural frequency (calculated value) corresponding to the rotational speed, the rotational speed changes slowly to produce an effective resonance phenomenon to facilitate measurement and analysis; (2) from a test point of view, it is desirable to produce a steady-state asymmetric short circuit. The negative sequence current is larger, and the larger negative sequence current generates a larger electromagnetic torque, which easily excites the shaft torsional vibration. However, for the safety consideration of the generator, the magnitude of the negative sequence current must be limited to a certain range. For a 200 MW unit, the standard value of the negative sequence current is usually less than 7% to 10%; (3) It can be known from the reverse of the electrical fault that there are various methods for generating a negative sequence magnetic field in the stator. In terms of line wiring, it is possible to single-phase ground (or add resistance, or inductive load) two-phase short circuit (or force resistance, or inductive load). The short-circuit point can be selected on the high-voltage side of the step-up transformer (main transformer), and can also be on the low-voltage side of the plant high-change; (4) There are three ways to provide excitation current, that is, the unit's own excitation system, spare excitation and external excitation power supply. .
1.2 Torsional vibration signal measurement Torsional vibration signal measurement methods are contact measurement and non-contact measurement. The advantage of contact measurement is that the stress of the shaft can be directly measured. The disadvantages are low signal-to-noise ratio, low measurement accuracy and limited service life. This method is generally not used in domestic torsional vibration measurements. The non-uniform pulse signal used in the non-contact measurement is measured by the demodulation of the secondary instrument to achieve the purpose of measuring the torsional vibration. This method is often used in the torsional vibration test. Its advantage is that it is simple and reliable, and can be used for long-term monitoring. However, the method measures the angular displacement and needs to be converted to obtain the torsional stress.
1. Arrangement of measuring points Arrangement of measuring points is an important part of real machine testing. Whether the position of measuring points is properly selected or not is directly related to the accuracy of test results. According to the vibration analysis, in the torsional vibration system of the unit shaft, the torsional vibration amplitude of each shaft section is different, the torsional vibration angle displacement near the junction is small, and the torsional strain (stress) is large. When using non-contact measuring gear measurement, it is often desirable that the gear-probe pick-up device is placed at a larger angular displacement amplitude; and when the strain gauge is used to directly measure the axial strain, the strain gauge should be attached to a large torsional strain. On the shaft segment. Therefore, it is better to calculate the torsional vibration characteristics of the unit shafting before the actual machine test, and select the measuring points according to the vibration mode to make the torsional vibration measurement get better results. However, the actual test point arrangement is limited by factors such as the shafting structure of the unit, and the above requirements cannot be fully met. Therefore, when the measuring point is arranged, it should be based on the actual situation of the unit shafting system, combined with the theoretical analysis of the torsional vibration characteristics of the shafting system to comprehensively consider various aspects. 1.4 Torsional vibration testing system test system mainly consists of sensors or strain gauges, torsional vibration analyzers, records It consists of a meter and an oscilloscope. When the torsional vibration angle is measured by the non-contact tooth measurement method, a magnetoelectric, eddy current or photoelectric sensor is selected, and a resistive strain gauge is used in the torsional strain (stress) test. The torsional vibration analyzer displays and outputs the amplitude of the torsional angular displacement or the torsional strain after shaping, amplifying, filtering, etc. the weak torsional vibration signal output from the sensor or the strain gauge. In the test, a tape recorder is commonly used to record the torsional vibration signal.
The actual machine test D power plant 1 is the N200 low-pressure rotor, generator rotor and exciter rotor produced by the Dongfang Steam Turbine Works in 1984. See the schematic diagram of the shafting system.
The test uses steady-state asymmetric short-circuit frequency excitation. During implementation, the generator is disconnected from the grid, and the B and C phases are short-circuited artificially on the high-voltage side of the main transformer. Then, the rotational speed is changed to obtain excitation of different frequencies, and the torsional vibration response of the recording shaft is measured to determine the natural frequency and vibration mode of the torsional vibration.
Axial position measuring point characteristic sensor type tachometer gear magnetoresistive sensor disk gear gear magneto-resistive sensor long shaft 4 watt side reflective belt photoelectric sensor 5.6 watt shaft segment reflective belt photoelectric sensor main exciter front wheel reflection belt Photoelectric sensor main exciter rear axle gear magnetoresistive sensor test uses two-channel torsional vibration recording analyzer, the frequency range is 0.2~2500Hz; the tape recorder is 21 channels, the frequency range is 0~5kHz, and the test speed is 19.0cm/s. The arrangement of 6 measuring points on the shafting is shown in Table 1. The test and calculation results are shown in Table 2. It can be seen from the test results that the test not only successfully measured the low-order natural frequency, but also measured some high-order. Natural frequency. Among them, the fourth-order natural frequency 50.70 Hz is within the frequency range that is required to avoid; the natural frequency does not exist within 97-104 Hz, and the torsional vibration frequency of the shaft system is qualified in the 2 octave interval.
Under the condition of applying 2 times frequency excitation to the generator rotor, not only the test and calculation results of Table 2 are easy to arouse the low-order mode, but the 16th order test value calculated value 119.17 excitation method can stimulate more. Higher order mode.
However, for safety reasons, the negative sequence current can only be controlled to a small value, and higher order modes are not easily provoked. Further analysis shows that the torsional vibrational response of the various axes of the shafting system can be all excited without any form of excitation, just like the lateral vibration. Nanjing: Southeast University Press, 1994: Liu Yingzhe. Turbine generator set torsional vibration. Nanjing: Southeast University Press, 1997. Lu Yiyuan. Turbogenerator vibration. Nanjing: Southeast University Press, 1998. Harbin Turbine Works. The structure of a 200,000 kW steam turbine. Beijing: Water Power Press, 1992 Chen Yong. Turbine generator shaft shaft system unbalanced vibration response. Journal of Nanjing Forestry University, 2000 Newsletter (Responsible Editor Li Yanwen) The Key Laboratory of Forest Genetics and Genetic Engineering is included in the Jiangsu Provincial Key Construction Plan. According to the requirements of the construction of key laboratories in Jiangsu Province, Jiangsu Provincial Department of Education on October 26, 2001 An expert demonstration was given to the Key Laboratory of Forest Genetics and Genetic Engineering of our school.
After expert argumentation and field investigation, it is believed that the laboratory's construction goals and research directions are clear, based on the frontier of the discipline, the laboratory layout is reasonable, the research strength is strong, and it has advanced research methods and a good working foundation. It is recommended to include the construction plan of key laboratories in Jiangsu Province.
Recently, the Jiangsu Provincial Department of Education officially approved the “Key Laboratory of Forest Genetics and Genetic Engineering†of our school to be included in the provincial key laboratory construction plan during the “10th Five-Year Plan†period.
(Technology Department)
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