Testing method for torsional vibration characteristics of shaft system of pumped storage unit based on hydraulic pulsation

Abstract

This invention relates to a method for testing the torsional vibration characteristics of a pumped-storage turbine shaft system based on hydraulic pulsation. This method utilizes random excitation generated by fluid pulsation within the unstable operating range of a 25% load turbine to identify the natural frequency and damping of torsional vibration. This method enables testing of the natural frequency and damping of the shaft's torsional vibration without external electromagnetic excitation, resolving the technical challenges of testing the dynamic characteristics of shaft torsional vibration. The method eliminates the need for applying an excitation torque, reducing test complexity. The hydraulic pulsation has a certain amplitude, which can stimulate a relatively large torsional strain signal, resulting in relatively reliable test data.

Description

Technical Field

[0001] The present invention relates to the field of pumped storage units, and in particular to a method for testing the torsional vibration characteristics of a shaft system of a pumped storage unit based on hydraulic pulsation. Background Art

[0002] Pumped-storage hydropower units are integrated power generation equipment that combines pumping and power generation. During operation, these two modes frequently switch, leading to frequent load shedding. These switching and load shedding processes generate significant torque pulsation. These units generally utilize a high-speed, bidirectional design, resulting in large capacity and high head. The flow paths of the pump-turbine runners are relatively narrow and long. Pumped-storage units exhibit significant hydraulic instability, prone to S-shaped and hump-shaped regions, and experience greater pressure fluctuations than conventional turbines. Operation in this hydraulically unstable region is also prone to torque pulsation. Torque pulsation generates alternating torsional stresses on the shaft, significantly impacting its strength and fatigue life, and thus compromising the unit's safe and stable operation. If the torque pulsation frequency coincides with the shaft's natural frequency of torsional vibration, torsional resonance will occur in the shaft, significantly increasing the alternating torsional stresses on the shaft. When the shaft's torsional damping is low, the torsional stresses generated by torque pulsation decay more slowly, resulting in a higher frequency of alternating torsional stresses and a greater risk of shaft fatigue damage.

[0003] The existing calculation method uses a finite element model for shaft system dynamics analysis based on the unit shaft system drawings. This model takes into account factors such as the added inertia of the water flow to calculate the natural frequencies and vibration modes of the shaft's torsional vibration. When using the finite element method for modeling, some factors are difficult to accurately account for, such as the moment of inertia added by the water flow to the shaft, added damping, and the torsional stiffness and damping of the coupling, leading to errors in the calculation results. Field testing is the most effective method for determining the natural frequencies and damping of the unit's shaft system's torsional vibration. Conventional testing involves applying electromagnetic excitation forces to the shaft using methods such as negative-sequence current, single-phase grounding, and variable-frequency excitation. The excitation mode can be either steady-state or transient. The shaft's torsional vibration response under the applied external torque is measured, and the torsional vibration natural frequencies and damping are determined by analyzing the torsional vibration response signals. During testing, a set of torsional excitation torques must be artificially applied to the shaft using methods such as negative-sequence current, single-phase grounding, and variable-frequency excitation. This method significantly impacts the shaft system and has certain safety implications for the unit. The excitation torque must be both moderately large and sufficiently high to ensure unit safety during testing. On the other hand, it must be sufficiently low to adequately stimulate the shafting's torsional vibration response. Accurately determining the excitation torque amplitude is difficult. Furthermore, these excitation methods are often complex to implement, making field testing difficult.

[0004] When a pumped-storage unit operates in the 25% low-load range, the turbine deviates from optimal operating conditions, generating various vortices, flow separation, and cavitation in the flow path. This can induce hydraulic instabilities such as draft tube vortices, pressure pulsations, blade channel vortices, and Karman vortices. Under these operating conditions, the impeller is subject to significant pressure pulsations, which, when combined, generate random torque pulsations on the shaft. Determining the natural frequency and damping of the shaft's torsional vibration through testing and analysis can help prevent torsional resonance under these operating conditions and mitigate the harmful effects of torque pulsation. This is crucial for pumped-storage units, which face dual-mode operation, hydraulic instability, and frequent load shedding. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for testing the torsional vibration frequency and damping of the shaft system of a pumped storage unit based on hydraulic pulsation. The method utilizes the random excitation generated by fluid pulsation in the unstable operating range of a 25% load turbine to identify the natural frequency and damping of the torsional vibration, thereby realizing the testing of the natural frequency and damping of the torsional vibration of the rotating shaft without external electromagnetic excitation, and solving the technical difficulties in testing the dynamic characteristics of the torsional vibration of the rotating shaft.

[0006] To achieve the above object, the technical solution provided by the present invention is:

[0007] The method for testing the torsional vibration characteristics of the shaft system of a pumped storage unit based on hydraulic pulsation, which measures the torsional vibration characteristics of the pumped storage unit through hydraulic pulsation, includes the following steps:

[0008] Step (1): When the pumped storage unit is operating in the low load range, random torque pulsation will be generated on the shaft under the action of hydraulic pulsation, which will stimulate random torsional strain of the shaft;

[0009] Step (2): the random torsional strain signal of the rotating shaft is regarded as consisting of three parts: the amplitude impact response strain signal, the positive velocity and negative velocity impact response strain signals, and the random response strain signal;

[0010] Step (3): setting a trigger threshold for torsional strain signal analysis, and obtaining a sampling point when the response signal crosses the threshold. A sampling point and several subsequent length sampling points constitute a group of sample signals;

[0011] Step (4): After averaging enough sample signals, the random response part and the positive and negative velocity impulse response parts in the strain response signal are averaged out, and the remaining signal is the free decay signal caused by the amplitude excitation, that is, the amplitude impulse response signal;

[0012] Step (5): According to the amplitude impact response signal, the natural frequency and damping characteristics of the torsional vibration of the shaft are obtained through testing.

[0013] Furthermore, the test was conducted under the operating condition of the pumped storage unit in the 25% low load range.

[0014] As a preferred solution, a strain gauge for testing torsional strain is pasted on the vertical rotating shaft of the pumped storage unit, and the output signal of the torsional strain gauge is transmitted to the strain tester through wireless transmission and reception.

[0015] Furthermore, the standard deviation of the measured torsional strain signal x(t) is denoted as σ x , set the signal analysis threshold a:

[0016]

[0017] Select sampling points on the torsional strain signal x(t) The corresponding time point is t i,i＝1,2,... , so that it satisfies:

[0018]

[0019] The total number of sampling points selected is recorded as N;

[0020] At each moment t i,i＝1,2,... As the starting point, intercept a subsample of a certain length of time, recorded as: z(t i +t). Calculate the average value of each subsample signal and record it as y(t):

[0021]

[0022] The torsional strain response x(t) of the rotating shaft under arbitrary excitation is considered to be composed of three parts: amplitude impulse response signal, positive and negative velocity impulse response signals, and random response signal.

[0023] Furthermore, after processing with the multi-sample statistical averaging method, the random signal part in the signal tends to 0, the response signals excited by the initial positive and negative velocities are averaged out, and the remaining signal is the free decay signal caused by the initial certain amplitude excitation.

[0024] Furthermore, by selecting an appropriate threshold, a more accurate free decay signal can be obtained.

[0025] Furthermore, the extracted free decay signal is subjected to fast Fourier transform to obtain a signal spectrum. The frequency corresponding to the main peak point on the spectrum is the natural frequency of torsional vibration.

[0026] Furthermore, the damping characteristic is the damping coefficient, and the peak points and their corresponding time points in the free decay signal are extracted to obtain a series of sample points, which are recorded as: (t i ,y i ), using the exponential decay function y = Ae -σt Approximate this series of sample points and obtain the damping coefficient.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The present invention proposes a method for testing the torsional vibration frequency and damping of the shaft system of a pumped-storage unit. This method uses random excitation generated by fluid pulsation in the unstable operating range of a 25% load turbine to identify the natural frequency and damping of torsional vibration. This method can realize the testing of the natural frequency and damping of torsional vibration of the rotating shaft without external electromagnetic excitation, thus solving the technical difficulties in testing the dynamic characteristics of torsional vibration of the rotating shaft.

[0029] In the past, the torsional vibration characteristics of pumped-storage units were often measured by applying various torque excitations, such as asynchronous grid connection and negative-sequence current excitation. This invention measures the torsional vibration characteristics of pumped-storage units through hydraulic pulsation, eliminating the need for external torque excitation, representing a new technological breakthrough.

[0030] The method of the present invention does not require application of an excitation torque, thus reducing the complexity of the test; the hydraulic pulsation has a certain amplitude, which can excite a relatively large torsional strain signal, and the obtained test data is relatively reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 : Vertical rotating shaft with torsional strain gauge attached.

[0032] Figure 2 : Torsional strain signal full-bridge connection method.

[0033] Figure 3 : Measured shaft torsional strain signals under different power generation load conditions.

[0034] Figure 4 : Partially enlarged view of the torsional strain signal in the 25% power generation load range.

[0035] Figure 5 : Original signal, extracted shock signal and shock signal spectrum.

[0036] Figure 6 : The identified damping coefficient changes with the trigger threshold. DETAILED DESCRIPTION

[0037] The above contents of the present invention are further described in detail below in the form of embodiments, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following embodiments. All technologies implemented based on the above contents of the present invention fall within the scope of the present invention.

[0038] This invention provides a method for testing the torsional vibration characteristics of a pumped-storage unit's shaft system based on hydraulic pulsation. When a pumped-storage unit operates in the 25% low-load range, the hydraulic pulsation generates significant random torque excitation. This torque generates a certain amplitude of random torsional strain on the shaft. This signal contains the shaft's torsional vibration characteristics.

[0039] The torsional strain signal is considered to be composed of three parts: amplitude impact response strain signal, positive velocity and negative velocity impact response strain signal, and random response strain signal.

[0040] Set the trigger threshold for torsional strain signal analysis. When the response signal crosses this threshold, a sampling point is generated. A sampling point and several subsequent length sampling points constitute a signal group. After averaging a sufficient number of sample signals, the random response component and the positive and negative velocity impulse response components in the strain response signal are averaged out. The remaining signal is the free decay signal caused by the amplitude excitation, i.e., the amplitude impulse response signal. Based on the amplitude impulse response signal, the natural frequency and damping characteristics of the shaft's torsional vibration can be determined.

[0041] In the embodiment, Figure 1 The strain gauge for testing torsional strain is pasted on the vertical rotating shaft of the pumped storage unit shown.

[0042] use Figure 2 The full-bridge connection method shown improves the accuracy of torsional strain testing and transmits the output signal of the torsional strain gauge to the strain tester through wireless transmission and reception.

[0043] Figure 3 The torsional strain signal x(t) of the pumped storage unit under different power generation load conditions is given. Figure 4 A partial enlarged image of the torsional strain signal within this range is given. It can be seen that the measured torsional strain signal under the 25% power generation load condition has a certain amplitude of random pulsation. This section of the torsional strain signal is analyzed;

[0044] The standard deviation of the measured torsional strain signal x(t) is σ x , set the signal analysis threshold a:

[0045]

[0046] Select sampling points on the torsional strain signal x(t) The corresponding time point is t i,i＝1,2,... , so that it satisfies:

[0047]

[0048] The total number of sampling points selected is N.

[0049] At each moment t i,i＝1,2,...As the starting point, intercept a subsample of a certain length of time, recorded as: z(t i +t). Calculate the average value of each subsample signal and record it as y(t):

[0050]

[0051] The torsional strain response x(t) of a rotating shaft under arbitrary excitation is considered to consist of three components: an amplitude impulse response signal, positive and negative velocity impulse response signals, and a random response signal. After processing using the multi-sample statistical averaging method, the random signal component of the signal approaches zero, the response signals due to the initial positive and negative velocity excitations are averaged out, and the remaining signal is the free decay signal induced by the initial amplitude excitation. By selecting an appropriate threshold, a relatively accurate free decay signal can be obtained.

[0052] Figure 4 A local enlarged view of the torsional strain signal in the 25% power generation load range.

[0053] Figure 5 The original signal and the free decay signal under amplitude excitation extracted by the above method are given.

[0054] Performing a fast Fourier transform on the extracted free decay signal yields a signal spectrum. The frequency corresponding to the main peak in the spectrum is the natural frequency of torsional vibration. In this example, the resonant frequency of torsional vibration is 12.7 Hz.

[0055] extract Figure 5 The peak points and their corresponding time points in the free decay signal shown above are used to obtain a series of sample points, which are recorded as: (t i ,y i ), using the exponential decay function y = Ae -σt By approximating this series of sample points, the damping coefficient σ can be obtained. In this example, the damping coefficient σ = 1.49.

[0056] Figure 6 The variation of the identified damping coefficient with the trigger threshold is given. Trigger threshold a>0.5σ x After the identification, the damping coefficient value does not change much. This method is not sensitive to the threshold setting and solves the problem that the torsional vibration damping coefficient is difficult to accurately test.

[0057] This method utilizes random excitation generated by fluid pulsation within the unstable operating range of a 25%-load turbine to identify the natural frequency and damping of torsional vibration. This method allows for testing the natural frequency and damping of shaft torsional vibration without external electromagnetic excitation, addressing the technical challenges of testing the dynamic characteristics of shaft torsional vibration. This method eliminates the need for applying an excitation torque, reducing test complexity. Hydraulic pulsation has a certain amplitude, which can induce a relatively large torsional strain signal. The resulting test data is relatively reliable, and the test results are insensitive to the trigger threshold setting.

[0058] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Any simple modification, equivalent replacement and improvement made by any technician familiar with the profession to the above embodiment without departing from the scope of the technical solution of the present invention and based on the technical essence of the present invention shall still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A method for testing the torsional vibration characteristics of a pumped storage unit shaft system based on hydraulic pulsation, characterized by: Measuring the torsional vibration characteristics of a pumped storage unit through hydraulic pulsation includes the following steps: Step (1): When the pumped storage unit is operating in the low load range, random torque pulsation will be generated on the shaft under the action of hydraulic pulsation, which will stimulate random torsional strain of the shaft; Step (2): The random torsional strain signal of the rotating shaft is regarded as consisting of three parts: the amplitude impact response strain signal, the positive velocity and negative velocity impact response strain signal, and the random response strain signal; Step (3): Set a trigger threshold for torsional strain signal analysis. When the response signal crosses the threshold, a sampling point is obtained. A sampling point and several subsequent length sampling points constitute a group of sample signals. Step (4): After averaging a sufficient number of sample signals, the random response part and the positive and negative velocity impulse response parts in the strain response signal are averaged out, and the remaining signal is the free decay signal caused by the amplitude excitation, that is, the amplitude impulse response signal; Step (5): According to the amplitude impact response signal, the natural frequency and damping characteristics of the torsional vibration of the shaft are obtained through testing; The test was conducted under the operating condition of 25% low load range of the pumped storage unit.

2. The method for testing the torsional vibration characteristics of the shaft system of a pumped storage unit based on hydraulic pulsation according to claim 1 is characterized in that: A strain gauge for testing torsional strain is pasted on the vertical rotating shaft of the pumped storage unit, and the output signal of the torsional strain gauge is transmitted to the strain tester through wireless transmission and reception.

3. The method for testing the shaft torsional vibration characteristics of a pumped storage unit based on hydraulic pulsation according to claim 1 is characterized in that: Record the measured torsional strain signal The standard deviation of , set the signal analysis threshold : ； The torsional strain signal Select sampling points , the corresponding time point is , i =1,2,…, so that it satisfies: ； The total number of sampling points selected is ; At each moment As a starting point, i =1,2,…, intercept the subsample of a certain time length, recorded as: ; Calculate the average value of each sub-sample signal, recorded as : ； The torsional strain response of the shaft under arbitrary excitation It is considered to consist of three parts: amplitude impulse response signal, positive and negative velocity impulse response signal, and random response signal.

4. The method for testing the shaft torsional vibration characteristics of a pumped storage unit based on hydraulic pulsation according to claim 2, characterized in that: After processing with the multi-sample statistical averaging method, the random signal part in the signal tends to 0, the response signals excited by the initial positive and negative velocities are averaged out, and the remaining signal is the free decay signal caused by the initial certain amplitude excitation.

5. The method for testing the shaft torsional vibration characteristics of a pumped storage unit based on hydraulic pulsation according to claim 3 is characterized in that: By selecting an appropriate threshold, an accurate free decay signal can be obtained.

6. The method for testing the shaft torsional vibration characteristics of a pumped storage unit based on hydraulic pulsation according to claim 4 is characterized in that: The extracted free decay signal is subjected to fast Fourier transform to obtain the signal spectrum. The frequency corresponding to the main peak point on the spectrum is the natural frequency of torsional vibration.

7. The method for testing the shaft torsional vibration characteristics of a pumped storage unit based on hydraulic pulsation according to claim 5, characterized in that: The damping characteristic is the damping coefficient. The peak points and their corresponding time points in the free decay signal are extracted to obtain a series of sample points, which are recorded as: , using an exponential decay function Approximate this series of sample points and obtain the damping coefficient.

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