Method for representing cavitation degree of water turbine

Abstract

The invention discloses a method for representing the cavitation degree of a water turbine. According to the variation trend of the water turbine underwater acoustic signal spectrum power distribution along with the frequency, a mathematical model for describing the overall energy variation trend of the underwater acoustic signal in different stages of the cavitation phenomenon by using the relative change rate of energy contained in the low-frequency region and the high-frequency region of the underwater acoustic signal frequency domain is constructed; and a method for quantitatively representing the cavitation degree of the water turbine is provided by taking the underwater acoustic signal of the water turbine under the working condition of no cavitation phenomenon as a comparison reference.

Description

Technical Field

[0001] The invention belongs to the field of water turbine cavitation detection. Background Art

[0002] As users' requirements for turbine stability gradually increase, ensuring the safe and stable operation of turbines, especially giant turbines, in a cavitation-free state has become an important indicator for assessing the operating status of the unit.

[0003] Cavitation is the process of formation, development and collapse of vapor or gas cavities (cavities) inside the liquid or on the liquid-solid interface when the local pressure inside the liquid is reduced. It is a phase change process between liquid and gas, including the occurrence, development and collapse of cavitation. The cavitation occurrence stage is accompanied by a substantial increase in noise radiation; the cavitation collapse stage can release huge pressure energy and heat energy, which is the main process causing damage to the interface material between liquid and solid; the cavitation development stage is between the initiation and collapse of cavitation, which mainly interferes with the movement of cavitated objects and changes the acting force, such as a decrease in thrust and an increase in the vibration swing of components.

[0004] Given the current state of technology, only when cavitation is severe enough to affect the external characteristics of the turbine, that is, when the vibration swing increases and the noise increases, can the probability of cavitation be estimated based on the signal information from external equipment and the experience of technicians. This method is not very accurate, and all existing detection equipment cannot detect cavitation in its initial stage, let alone evaluate the extent of cavitation.

[0005] Therefore, it is necessary to develop a method that can accurately detect whether cavitation occurs in a turbine in real time and evaluate the extent of cavitation. Summary of the invention

[0006] The object of the present invention is to provide a method for characterizing the degree of cavitation of a water turbine. The method is implemented by the following steps: Step 1: Collect the hydroacoustic signal when the turbine is in a non-transitional stable operating state; Step 2: Analyze the time-frequency characteristics of the above turbine hydroacoustic signal to obtain the trend of the spectral power distribution of the turbine hydroacoustic signal with frequency under this working condition: Step 3: using a piecewise linear function, the frequency domain of the hydraulic turbine hydroacoustic signal is divided into two regions, a left region (1) and a right region (2), according to the distribution trend of the spectral power distribution of the hydraulic turbine hydroacoustic signal frequency domain; Step 4: Calculate the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) in the frequency domain of the hydroacoustic signal of the turbine under the operating condition; Step 5: Change the operating conditions of the turbine and repeat steps 1 to 4; Step 6: Determine the turbine cavitation determination index value under each stable operating condition of the turbine according to the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) in the frequency domain of the hydroacoustic signal; Step 7: Determine whether cavitation occurs in the turbine according to the turbine cavitation determination criteria; Step 8: Determine the degree of cavitation of the turbine.

[0007] Furthermore, the time-frequency characteristics of the turbine hydroacoustic signal are analyzed, and the trend of the spectral power distribution of the turbine hydroacoustic signal with frequency under this working condition is as follows:

[0008] Where: :The changing trend of the spectrum power distribution of the turbine hydroacoustic signal with frequency; : The frequency sequence obtained by time-frequency analysis of the turbine hydroacoustic signal, ; : Rational number sequence; : The distribution sequence of the spectrum power distribution of the turbine hydroacoustic signal with the corresponding frequency, ; Furthermore, the frequency domain of the hydraulic turbine hydroacoustic signal is divided into two regions, a left region (1) and a right region (2), according to the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal using a piecewise linear function according to the following formula: In the frequency range of the turbine hydroacoustic signal Inside, there is ,in, , Calculate the frequency resolution for the spectral power distribution of the turbine hydroacoustic signal. by , then the difference between the frequency domain data of the turbine hydroacoustic signal and the predicted value of the regression equation constructed by the piecewise linear function is defined as the residual error, Indicates that , respectively construct piecewise linear functions, then,

[0009] Where: : residual; : Coefficient term of piecewise linear function; : constant term of piecewise linear function; : The intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal; For each intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal ,when Sometimes, there are frequency value, and when Sometimes frequency value, then and Must satisfy . Residual The sum of squares It is expressed as follows: (3-5) make , , , to determine the corresponding , , and value, in: : right The partial derivative of : right The partial derivative of : right The partial derivative of : right The partial derivative of Determine all residuals The sum of squares Minimum value of , The corresponding intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal The frequency value represented This is the dividing point frequency of the left area (1) and the right area (2) of the turbine hydroacoustic signal frequency domain, which is the power distribution of the turbine hydroacoustic signal spectrum. exist The area enclosed by the interval and the horizontal axis of the coordinate axis is It is divided into two parts. The left side is the left zone (1), and the right side is the right zone (2). for The corresponding spectrum power distribution value of the turbine hydroacoustic signal.

[0010] Furthermore, the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) of the spectral power distribution of the hydroacoustic signal of the turbine is calculated according to the following formula: The dividing point frequency of the left area (1) and the right area (2) of the frequency domain of the turbine hydroacoustic signal The frequency sequence obtained by time-frequency analysis of the turbine hydroacoustic signal The frequency sequence of the left area (1) is Contains frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are: , while the frequency series of the right area (2) Contains frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are: ,and but:

[0011] Where: : The ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the spectrum power distribution of the turbine hydroacoustic signal; : The area of ​​the right zone (2) of the spectrum power distribution of the hydroacoustic signal of the turbine; : The area of ​​the left zone (1) of the spectrum power distribution of the turbine hydroacoustic signal.

[0012] Furthermore, the turbine cavitation determination index value is determined according to the following formula:

[0013] Where: : A natural number sequence representing the number of stable operating states of all non-transition conditions experienced by the turbine; : The first time the turbine experienced The cavitation determination index value under the non-transitional stable operation state; : The first time the turbine experienced The ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the hydroacoustic signal spectrum power distribution under a non-transitional stable operating state; : The minimum value of the ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the hydroacoustic signal spectrum power distribution under all non-transition conditions experienced by the turbine in stable operation.

[0014] Furthermore, whether cavitation occurs in the turbine is determined by the following formula:

[0015] Where: : coefficient, generally taken as .

[0016] Furthermore, the degree of cavitation of the turbine is calculated as follows:

[0017] Where: : Characterization coefficient of turbine cavitation degree.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The present invention compares the different distribution laws of the spectrum power distribution of the turbine hydroacoustic signal under different working conditions, and constructs a cavitation determination system based on comparing the spectrum power distribution of the turbine hydroacoustic signal with the non-cavitation state to determine whether the turbine cavitation occurs. In principle, the accuracy of the determination of the occurrence of turbine cavitation is guaranteed.

[0019] The present invention proposes a method for determining the occurrence of turbine cavitation by analyzing the relative proportion of the frequency domain energy distribution area of ​​the turbine hydroacoustic signal collected in real time and provides a specific criterion value.

[0020] The invention discloses a method for characterizing the occurrence degree of cavitation of a water turbine by utilizing the relative proportion value of the frequency domain energy distribution area of ​​a water turbine hydroacoustic signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The high and low frequency energy area distribution diagram of the water turbine hydroacoustic signal frequency domain provided by the present invention; Figure 2 The frequency domain energy distribution trend diagram of the hydroacoustic signal of the turbine under different cavitation states provided by the present invention.

[0022] Description of reference numerals: 1: The left area of ​​the spectral power distribution of the turbine hydroacoustic signal, 2: The right area of ​​the spectral power distribution of the turbine hydroacoustic signal, 3: The energy distribution of the frequency domain of the turbine hydroacoustic signal when no cavitation occurs, 4: The energy distribution of the frequency domain of the turbine hydroacoustic signal at the beginning of cavitation, 5: The energy distribution of the frequency domain of the turbine hydroacoustic signal when cavitation is severe. DETAILED DESCRIPTION

[0023] In order to make the objectives, technical solutions and advantages of the embodiments of the present invention more clearly understood, the spirit of the contents disclosed in the present invention will be clearly explained with the accompanying drawings and detailed descriptions below. After understanding the embodiments of the contents of the present invention, any technician in the relevant technical field can change and modify the techniques taught by the contents of the present invention without departing from the spirit and scope of the contents of the present invention.

[0024] The exemplary embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention. In addition, elements / components with the same or similar reference numerals used in the drawings and embodiments are used to represent the same or similar parts.

[0025] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] The present invention discloses a method for characterizing the cavitation degree of a water turbine, which is achieved by the following steps: Step 1: Collect the hydroacoustic signal of the turbine in a non-transitional stable operating state. The transitional condition is the process of starting and stopping the turbine. Compared with the stable operating condition, the time is very short and it is not easy to cause serious cavitation damage to the unit. In addition, the hydroacoustic signal of the turbine in the transitional condition changes dramatically and has poor regularity. Therefore, the hydroacoustic signal of the turbine in the stable operating condition is used as the basis for judging whether turbine cavitation occurs and the degree of cavitation.

[0027] Step 2: Analyze the time-frequency characteristics of the above turbine hydroacoustic signal to obtain the trend of the spectral power distribution of the turbine hydroacoustic signal with frequency under this working condition. The turbine cavitation process is essentially a cyclic process of cavitation on the surface of the turbine base material, development and collapse. During this period, the cavitation radiates energy to the surroundings, and 70-80% of this energy is vibration / sound energy. The more severe the cavitation, the more cavitation there are, and the stronger the total energy radiated outward. The frequency of the above vibration / sound energy is closely related to the geometric size of the cavitation. The smaller the cavitation size, the higher the frequency of the vibration / sound energy radiated outward. Conversely, the lower the frequency of the vibration / sound energy radiated outward. The vibration / sound energy caused by turbine cavitation is obvious in a very wide frequency domain.

[0028] Step 3: Using a piecewise linear function, the frequency domain of the turbine hydroacoustic signal is divided into two areas, left area 1 and right area 2, according to the distribution trend of the spectral power distribution of the turbine hydroacoustic signal frequency domain. The relatively low-frequency hydroacoustic signals such as the flow noise of the turbine itself represented by the left area 1 have no obvious correlation with whether cavitation occurs, while the vibration / sound energy caused by turbine cavitation represented by the right area 2 is obvious in the relatively high-frequency area.

[0029] Step 4: Calculate the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 in the frequency domain of the hydroacoustic signal of the turbine under this working condition. The ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 not only indicates the increase or decrease of the vibration / sound energy caused by the cavitation of the turbine relative to the energy of the flow noise of the turbine itself, but also provides an intermediate calculation parameter for the calculation and analysis of the subsequent steps.

[0030] Step 5: Change the turbine operating conditions and repeat steps 1 to 4. Cavitation may occur in turbines only under very few extreme conditions. Collect the turbine hydroacoustic signals under different turbine operating conditions and analyze the corresponding characteristics to provide data accumulation for the next step of determining whether turbine cavitation occurs.

[0031] Step 6: Determine the turbine cavitation judgment index value under each stable operation condition of the turbine according to the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 in the frequency domain of the hydroacoustic signal. The method disclosed in the present invention is used to express the variation law of the spectral power distribution of the hydroacoustic signal of the turbine under stable working conditions with different outputs with the frequency in a dimensionless manner of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 in the frequency domain of the hydroacoustic signal, as a characteristic index value for judging the cavitation of the turbine.

[0032] Step 7: Determine whether cavitation occurs in the turbine according to the turbine cavitation determination criteria. Determine whether cavitation occurs in the turbine by comparing the ratio of the right area 2 to the left area 1 of the turbine hydroacoustic signal spectrum power distribution under different working conditions.

[0033] Step 8: Determine the cavitation degree of the turbine. According to the determination result of step 7, when the turbine cavitation occurs, the spectral power distribution of the hydroacoustic signal of the turbine shows a significant increase in energy in the high-frequency region. The dimensionless value obtained by comparing the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine under the cavitation condition with the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine under the condition where cavitation does not occur is used to characterize the cavitation degree of the turbine. The larger the dimensionless value, the higher the cavitation degree of the turbine; otherwise, the lower the cavitation degree of the turbine.

[0034] Furthermore, in step 2, the time-frequency characteristics of the water turbine hydroacoustic signal are analyzed to obtain the following trend of the spectrum power distribution of the water turbine hydroacoustic signal with frequency under the working condition:

[0035] Where: :The changing trend of the spectrum power distribution of the turbine hydroacoustic signal with frequency; : The frequency sequence obtained by time-frequency analysis of the turbine hydroacoustic signal, ; : Rational number sequence; : The distribution sequence of the spectrum power distribution of the turbine hydroacoustic signal with the corresponding frequency, .

[0036] The present invention discloses a method for evaluating the occurrence degree of cavitation phenomenon of a water turbine: first, the object of analysis is the water acoustic signal of the water turbine. Secondly, the mathematical method is a time-frequency analysis method. Finally, the variation trend of the spectral power distribution of the water acoustic signal with the frequency is analyzed. The variation trend of the spectral power distribution of the water acoustic signal of the water turbine with the frequency under the corresponding working conditions reflects the law of the energy of the water acoustic signal of the water turbine with the frequency, which is the mathematical basis for characterizing the cavitation degree of the water turbine.

[0037] Furthermore, in step 3, the frequency domain of the hydraulic turbine hydroacoustic signal is divided into two regions, a left region 1 and a right region 2, according to the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal using a piecewise linear function according to the following formula: A mathematical method is provided to determine how to divide the frequency domain of a hydraulic turbine hydroacoustic signal into two regions, a left region 1 and a right region 2, according to the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal: ① linear functions are used to represent the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal in the two regions, the left region 1 and the right region 2, respectively, and the intersection of the two linear functions is the dividing point between the left region 1 and the right region 2; ② the principle of minimum residual error is used, that is, the constructed piecewise linear function is closest to the actual energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal, and the piecewise linear function that best meets the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal is selected, and the intersection of the above piecewise linear functions is used as the dividing point to determine the left region 1 and the right region 2.

[0038] In the frequency range of the turbine hydroacoustic signal Inside, there is ,in, , Calculate the frequency resolution for the spectral power distribution of the turbine hydroacoustic signal. by , then the difference between the frequency domain data of the turbine hydroacoustic signal and the predicted value of the regression equation constructed by the piecewise linear function is defined as the residual error, Indicates that , respectively construct piecewise linear functions, then,

[0039] Where: : residual; : Coefficient term of piecewise linear function; : The constant term of the piecewise linear function; : The intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal; For each intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal ,when Sometimes, there are frequency value, and when Sometimes frequency value, then and Must satisfy . Residual The sum of squares It is expressed as follows: (3-5) make , , , to determine the corresponding , , and value, in: : right The partial derivative of : right The partial derivative of : right The partial derivative of : right The partial derivative of Determine all residuals The sum of squares Minimum value of , The corresponding intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal The frequency value represented This is the dividing point frequency of the left area (1) and the right area (2) of the turbine hydroacoustic signal frequency domain, which is the power distribution of the turbine hydroacoustic signal spectrum. exist The area enclosed by the interval and the horizontal axis of the coordinate axis is It is divided into two parts. The left side is the left zone (1), and the right side is the right zone (2). for The corresponding spectrum power distribution value of the turbine hydroacoustic signal.

[0040] Furthermore, in step 4, the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the spectrum power distribution of the water turbine hydroacoustic signal is calculated according to the following formula: Before and after turbine cavitation occurs, the energy distribution law of the hydroacoustic signal frequency domain is different: when the turbine does not cavitate, the energy occupied by relatively low-frequency hydroacoustic signals such as flow noise is relatively large, while the energy in the high-frequency band is relatively small; after cavitation occurs, the energy of relatively low-frequency hydroacoustic signals such as flow noise basically does not change, while the energy in the high-frequency band generally shows a significant upward trend.

[0041] The spectral power distribution data of the turbine hydroacoustic signal in the left area 1 of the frequency domain is The area enclosed by the interval and the horizontal axis represents the energy of low-frequency underwater acoustic signals such as flow noise, and the spectral power distribution data in the right area 2 is The area enclosed by the interval and the abscissa axis represents the energy of high-frequency hydroacoustic signals such as cavitation noise. By comparing the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine, it can be determined whether cavitation occurs in the turbine.

[0042] The dividing point frequency of the left area 1 and the right area 2 of the frequency domain of the hydraulic turbine hydroacoustic signal The frequency sequence obtained by time-frequency analysis of the turbine hydroacoustic signal Divided into two parts, the frequency sequence of left area 1 Contains frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are: , while the frequency series of right zone 2 Contains frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are: ,and ,but:

[0043] Where: : The ratio of the area of ​​right zone 2 to the area of ​​left zone 1 of the spectrum power distribution of the turbine hydroacoustic signal; : The area of ​​right zone 2 of the spectrum power distribution of the hydroacoustic signal of the turbine; : The area of ​​left zone 1 of the spectrum power distribution of the turbine hydroacoustic signal.

[0044] Furthermore, in step six, the turbine cavitation determination index value is determined according to the following formula: like Figure 2As shown in Figure 3, the energy distribution law of the frequency domain of the hydroacoustic signal of the turbine before and after cavitation occurs is different: when cavitation does not occur, the energy distribution of the frequency domain of the hydroacoustic signal of the turbine presents the law shown in line 3, that is, ① the overall energy is low; ② the value of the area of ​​the right zone 2 of the hydroacoustic signal spectrum power distribution of the turbine is the smallest; ③ the value of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine is relatively low. At the beginning of cavitation, the energy distribution of the frequency domain of the hydroacoustic signal of the turbine presents the law shown in line 4, that is, ① the overall energy increases; ② the value of the area of ​​the right zone 2 of the hydroacoustic signal spectrum power distribution of the turbine increases; ③ the value of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine increases relatively. As the degree of cavitation further deepens, the energy distribution of the turbine hydroacoustic signal frequency domain presents a pattern as shown in line 5, that is, ① the overall energy further increases; ② the value of the area of ​​the right zone 2 of the turbine hydroacoustic signal spectrum power distribution also further increases; ③ the value of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the turbine hydroacoustic signal spectrum power distribution also further increases.

[0045] Under normal circumstances, cavitation will not occur during the operation of the turbine. Cavitation may only occur in cases such as when the unit output is extremely large. In view of this, by comparing the numerical values ​​of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the turbine hydroacoustic signal spectrum power distribution under different operating conditions of the turbine, it is possible to search for an operating condition that satisfies the minimum ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the turbine hydroacoustic signal spectrum power distribution, that is, a condition where no cavitation occurs. As a benchmark condition, the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the turbine hydroacoustic signal spectrum power distribution is used as a benchmark value and compared with the numerical values ​​of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the turbine hydroacoustic signal spectrum power distribution under other conditions. The dimensionless value obtained is used as the judgment index value for determining whether cavitation occurs in the turbine. The turbine cavitation judgment index value is determined according to the following formula:

[0046] Where: : A natural number sequence representing the number of stable operating states of all non-transition conditions experienced by the turbine; : The first time the turbine experienced The cavitation determination index value under the non-transitional stable operation state; : The first time the turbine experienced The ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution under a non-transitional stable operation state; : The minimum value of the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution under all non-transition conditions experienced by the turbine in stable operation.

[0047] Furthermore, whether cavitation occurs in the turbine in step 7 is determined by the following formula:

[0048] Where: : coefficient, generally taken as .

[0049] After the turbine cavitation occurs, the area of ​​the right zone 2 of the hydroacoustic signal spectrum power distribution increases significantly while the area of ​​the left zone 1 remains basically unchanged, resulting in a significant increase in the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 compared to when cavitation does not occur. According to the results of multiple tests on different units, once the ratio of the area of ​​the right zone 2 to the area of ​​the left zone 1 of the hydroacoustic signal spectrum power distribution of the turbine increases by 1.3 times or more compared to the corresponding value of the working condition without cavitation, it can be determined that the turbine has cavitation.

[0050] Furthermore, the cavitation degree of the turbine in step eight is calculated according to the following formula:

[0051] Where: : Characterization coefficient of turbine cavitation degree.

[0052] The cavitation degree of the turbine is characterized by constructing a dimensionless value. The more severe the cavitation, the greater the value of the turbine cavitation degree characterization coefficient; conversely, the smaller the value of the turbine cavitation degree characterization coefficient.

Claims

1. A method for characterizing the degree of cavitation in a hydraulic turbine, characterized in that: The method is implemented by the following steps: Step 1: Collect the hydroacoustic signal when the turbine is in a non-transitional stable operating state; Step 2: Analyze the time-frequency characteristics of the above turbine hydroacoustic signal to obtain the trend of the spectral power distribution of the turbine hydroacoustic signal with frequency under this working condition: Step 3: using a piecewise linear function, the frequency domain of the hydraulic turbine hydroacoustic signal is divided into two regions, a left region (1) and a right region (2), according to the distribution trend of the spectral power distribution of the hydraulic turbine hydroacoustic signal frequency domain; Step 4: Calculate the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) in the frequency domain of the hydroacoustic signal of the turbine under the working condition; Step 5: Change the operating conditions of the turbine and repeat steps 1 to 4; Step 6: Determine the turbine cavitation determination index value under each stable operating condition of the turbine according to the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) in the frequency domain of the hydroacoustic signal; Step 7: Determine whether cavitation occurs in the turbine according to the turbine cavitation determination criteria; Step 8: Determine the degree of cavitation of the turbine.

2. A method for characterizing the degree of cavitation of a hydraulic turbine according to claim 1, characterized in that: The step 2 performs time-frequency characteristic analysis on the water turbine hydroacoustic signal, and obtains the following trend of the spectrum power distribution of the water turbine hydroacoustic signal with frequency under this working condition: Where: F i :The changing trend of the spectrum power distribution of the turbine hydroacoustic signal with frequency; f1,f2,…,f j ,…,f n : frequency sequence obtained by time-frequency analysis of turbine hydroacoustic signal, Hz; 1,2,…,j,…,N: rational number sequence; E1,E2,…,E j ,…,E N : The distribution sequence of the spectral power distribution of the turbine hydroacoustic signal with the corresponding frequency, W / Hz.

3. The method for characterizing the degree of cavitation of a hydraulic turbine according to claim 1, characterized in that: The step 3 uses a piecewise linear function according to the following formula to divide the frequency domain of the hydraulic turbine hydroacoustic signal into two regions, a left region (1) and a right region (2), according to the energy distribution trend of the frequency domain of the hydraulic turbine hydroacoustic signal: In the frequency range of the turbine hydroacoustic signal (f1, f N ) has f j =f1+j·Δf, where j=0,1,2,…,N-1, and Δf is the frequency resolution of the turbine hydroacoustic signal spectrum power distribution calculation. j ,E j ) with (x j ,y j ), then the difference between the frequency domain data of the turbine hydroacoustic signal and the predicted value of the regression equation constructed by the piecewise linear function is defined as the residual error, and δ j Indicates that, let x m =x j+1 , respectively construct piecewise linear functions, then, Where: δ j : residual; a1, a2: coefficients of piecewise linear function; b1, b2: constant terms of piecewise linear function; x m : The intersection point of the frequency domain piecewise linear function of the turbine hydroacoustic signal; For each intersection point x of the frequency domain piecewise linear function of the turbine hydroacoustic signal m , when x i ≤x m When x i >x m When there are n2 frequency values, then n1 and n2 must satisfy n1+n2=N. Residual δ j The sum of squares R j It is expressed as follows: make To determine the corresponding a1, b1, a2 and b2 values, in: R j Partial derivative with respect to a1; R j Partial derivative with respect to b1; R j Partial derivative with respect to a2; R j Partial derivative with respect to b2; Determine all residuals δ j The sum of squares R j The minimum value R jmin , R jmin The corresponding intersection point x of the frequency domain piecewise linear function of the turbine hydroacoustic signal mmin The frequency value f BP It is the dividing point frequency of the left area (1) and the right area (2) of the turbine hydroacoustic signal frequency domain, that is, the spectral power distribution of the turbine hydroacoustic signal E1, E2, ..., E j ,…,E N In [f1,f N ] The area enclosed by the interval and the horizontal axis of the coordinate axis is (f BP ,E BP ) is divided into two parts, (f BP ,E BP ) The left side is the left area (1), the right side is the right area (2), E BP f BP The corresponding spectrum power distribution value of the turbine hydroacoustic signal.

4. A method for characterizing the degree of cavitation of a hydraulic turbine according to claim 1, characterized in that: The step 4 calculates the ratio of the area of ​​the right region (2) to the area of ​​the left region (1) of the spectrum power distribution of the water turbine hydroacoustic signal according to the following formula: The dividing point frequency f between the left area (1) and the right area (2) of the frequency domain of the turbine hydroacoustic signal BP The frequency sequence f1, f2, ..., f obtained by time-frequency analysis of the turbine hydroacoustic signal j ,…,f n Divided into two parts, the frequency sequence f1,f2,…,f in the left area (1) BP Containing n1 frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are {(f1,E1),(f2,E2),…,(f BP ,E BP )}, and the frequency series f BP+1 ,f BP+2 …,f n Containing n2 frequency values, the frequency of the turbine hydroacoustic signal and its corresponding spectral power distribution sequence are {(f BP+1 ,E BP+1 ),(f BP+2 ,E BP+2 ),…,(f n ,E n )}, and n1+n2=N, then: Where: ζ: The ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the spectrum power distribution of the turbine hydroacoustic signal; S R : The area of ​​the right zone (2) of the spectrum power distribution of the turbine hydroacoustic signal; S L : The area of ​​the left zone (1) of the spectrum power distribution of the turbine hydroacoustic signal.

5. The method for characterizing the degree of cavitation of a hydraulic turbine according to claim 1, characterized in that: The step 6 determines the turbine cavitation determination index value according to the following formula: Where: 1,2,…,i,…,M: a natural number sequence representing the number of all non-transitional stable operating states experienced by the turbine; α i : The cavitation determination index value of the turbine under the stable operation state of the i-th non-transition condition; ζ i : The ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the hydroacoustic signal spectrum power distribution under the i-th non-transitional condition and stable operation of the turbine; min(ζ1,ζ2,…,ζ i ,…,ζ M ): The minimum value of the ratio of the area of ​​the right area (2) to the area of ​​the left area (1) of the hydroacoustic signal spectrum power distribution under all non-transitional operating conditions experienced by the turbine in stable operation.

6. A method for characterizing the degree of cavitation of a hydraulic turbine according to claim 1, characterized in that: Whether cavitation occurs in the turbine in step 7 is determined by the following formula: Where: K: coefficient, generally K=1.

3.

7. A method for characterizing the degree of cavitation in a hydraulic turbine according to claim 1, characterized in that: The cavitation degree of the turbine in step eight is calculated as follows: Where: β i : Characterization coefficient of turbine cavitation degree.

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