Extra-high voltage converter transformer valve side bushing dielectric loss and capacitance detection method based on Hanning window function filtering and quasi-synchronous reconstruction method

Abstract

The invention provides an extra-high voltage converter transformer valve side bushing dielectric loss and capacitance detection method based on Hanning window function filtering and a quasi-synchronous reconstruction method. According to the method, quasi-synchronous sampling reconstruction is carried out on each non-synchronous sampling leakage current, and the reconstruction algorithm considers the accuracy of reconstructing the waveform of the leakage current and the calculated amount of reconstruction. Before quasi-synchronous sampling reconstruction, the fundamental wave period of the leakage current of the valve side sleeve needs to be accurately calculated. In order to improve the calculation accuracy of the fundamental wave period of the leakage current, a Hanning window function filter is designed, and other frequency components except the fundamental wave component are filtered out. According to the method, the dielectric loss and capacitance detection precision of the valve side bushing of the extra-high voltage converter transformer can be remarkably improved under the condition that hardware resource cost is not increased, and the technical problem of state abnormity diagnosis of the valve side bushing of the extra-high voltage converter transformer is solved.

Description

Technical Field

[0001] The invention belongs to the technical field of operation and maintenance of power transmission and transformation equipment, and particularly relates to a method for detecting the dielectric loss and capacitance of the valve side bushing of a UHV converter transformer based on the Hanning window function filtering and quasi-synchronous reconstruction method. Background Technique

[0002] The bushing is an important component of the UHV converter transformer. During normal operation, the bushing inside the converter transformer is subjected to the combined action of electricity, heat and stress, and the outside is affected by the extreme environment of high humidity and large temperature. Therefore, the bushing failure is one of the main failures of the converter transformer. Carrying out on-line monitoring of the insulation state of the converter transformer bushing is crucial for ensuring the stable operation of the converter transformer and even the entire converter station.

[0003] CN112946515A provides a set of methods and devices for on-line monitoring of the grid side bushing of the converter transformer. The device synchronously collects the leakage currents of multiple bushings under the same reference voltage, performs Fourier transform on the leakage currents to determine the fundamental wave phase, and calculates the relative dielectric loss values of each bushing.

[0004] CN115097745A discloses a transformer bushing fault diagnosis system and operation method based on digital twin. The multi-physical quantity state monitoring system of the bushing uses a low-frequency sensor installed on the end screen of the bushing to collect leakage current signals, and determines the relative capacitance and relative dielectric loss of the bushing through the ratio of the leakage current to the reference current amplitude and the phase difference.

[0005] CN112305319A discloses a method and system for on-line monitoring the parameters of the valve side bushing of the converter transformer. The system calculates the absolute dielectric loss factor according to the phase difference between the fundamental wave component and the harmonic component of the comparison signal and the reference signal, and the preset initial value of the dielectric loss, and determines the capacitance according to the amplitudes of the comparison signal and the reference signal, and the preset initial value of the capacitance. The above methods both use Fourier transform when analyzing the fundamental wave component and harmonic component of the leakage current. Fourier transform will produce spectral leakage and fence effect, and the reasons are: (1) The acquisition cards of the end screens of each converter transformer bushing cannot ensure completely synchronous sampling; (2) In addition to the fundamental wave, the voltage of the valve side bushing of the converter transformer also contains rich harmonics, and the sampling time cannot ensure an integer period. When there are spectral leakage and fence effect, there will be large errors in analyzing the amplitudes and phases of the fundamental wave and harmonics of the leakage current. Summary of the Invention

[0006] In view of this, the present invention aims to overcome the deficiencies of the above problems in the prior art, and proposes a method for detecting the dielectric loss and capacitance of the valve side bushing of a UHV converter transformer based on the Hanning window function filtering and quasi-synchronous reconstruction method. The leakage current collected by each acquisition card is filtered using the Hanning window function to improve the accuracy of fundamental wave period measurement. Then, the interpolation method is used to perform quasi-synchronous reconstruction on the asynchronous sampling data of each acquisition card, and Fourier analysis is performed on the quasi-synchronous reconstructed signal to calculate the dielectric loss and capacitance of the valve side bushing of the converter transformer.

[0007] To achieve the above object, the technical solution of the present invention is realized as follows:

[0008] A method for detecting the dielectric loss and capacitance of the valve side bushing of a UHV converter transformer based on the Hanning window function filtering and quasi-synchronous reconstruction method includes the following steps:

[0009] S1: Input the comparison and reference leakage currents;

[0010] S2: Design a Hanning window function filter;

[0011] S3: Calculate the fundamental wave period of the filtered leakage current;

[0012] S4: Perform quasi-synchronous reconstruction on the filtered leakage current using the second-order Newton forward interpolation;

[0013] S5: Perform discrete Fourier transform on the quasi-synchronous sampling reconstructed signal;

[0014] S6: Calculate the relative capacitance and relative dielectric loss of the valve side bushing of the converter transformer.

[0015] Further, in step S1, the leakage currents collected by the acquisition cards of the end screen sensors of the valve side bushing of the UHV converter transformer are input, including the leakage currents of the comparison bushing and the reference bushing.

[0016] Further, in step S2, the Hanning window function is selected as the FIR filter to filter out other frequency components except the fundamental wave component

[0017] Further, in step S2, the expression of the Hanning window function is

[0018]

[0019] where N is the sampling length, n (0 ≤ n ≤ N - 1) is the sampling point, I N (n) is the nth sampling data of the leakage current, and I Filt (n) represents the leakage current data after Hanning window filtering.

[0020] Further, in step S2, the frequency characteristics of the designed Hanning window function filter are as follows: the center frequency is 50 Hz, the stopband edge frequencies are 43.6 Hz and 56.4 Hz, and the passband edge frequencies are 49.2 Hz and 50.8 Hz. The stopband attenuation is 30 dB, and the passband ripple is 0.4 dB.

[0021] Further, in step S3, let the filtered leakage current be I Filt near the threshold I th be the points {I Filt (k n -1), k n -1} and {I Filt (k n ), k n}, then let I Filt (k n -1) = I 0 , f(I 0 ) = k n -1, I Filt (k n ) = I 1 , f(I 1 ) = k n , I Filt (k n +1) = I 2 , f(I 2 ) = k n +1. Use the second-order divided-difference Newton interpolation polynomial P(I) to approximate the function f(I):

[0022] P(I) = f(I 0 ) + f[I 0 , I 1 (I - I 0 ) + f[I 0 , I 1 , I 2 (I - I 0 )(I - I 1 )

[0023]

[0024] Further, in step S3, use the time when the filtered leakage current crosses the threshold I th for the second time and the time when it crosses the threshold I th for the tenth time to calculate the fundamental wave period T * and the frequency f * ;

[0025] Calculate the time t th when it crosses the threshold I 2 for the second time as

[0026] t 2 =P(I th ) (6)

[0027] Similarly, the time t th when crossing the threshold I for the 10th time can be calculated 10 , and then the fundamental period T * of the leakage current after filtering is

[0028]

[0029] The fundamental frequency f * of the leakage current after filtering is

[0030]

[0031] Furthermore, the step S4 specifically includes:

[0032] (a) Calculate the number of sampling points L and the sampling period T s in one signal period of the leakage current after filtering;

[0033] (b) Calculate the quasi-sampling period λ s

[0034]

[0035] where L * is the integer power of 2 closest to L;

[0036] (c) Let any sampling point {I Filt (g 1 ), g 1} of the leakage current after filtering be the first sampling point {I Rec (q 1 ), q 1} of the quasi-synchronous reconstruction signal;

[0037] (d) The m-th sampling point {I Rec (q m ), q m} of the quasi-synchronous reconstruction signal is after the leakage current after filtering {I Filt (g u ), g u}, and calculate the interval

[0038] τ m =(m - 1)λ s -(u - 1)T s (10)

[0039] (e) Let I Rec (q k ) = h(qk (1 ≤ k ≤ L * ),calculate I using a second-order Newton forward interpolation polynomial Rec (q m )

[0040]

[0041]

[0042] (f) Repeat step (e) to calculate L * - 1 quasi-synchronous reconstruction sampling points. These L * - 1 quasi-synchronous reconstruction sampling points and the initial point {I Rec (q 1 ),q 1} together constitute a quasi-synchronous reconstruction leakage current signal with a length of L * .

[0043] Furthermore, in step S5, perform a Fourier transform on the quasi-synchronous reconstruction leakage current signal with a length of L * to calculate the frequencies f m , amplitudes A m and phases φ m of each harmonic. The calculation formulas are

[0044]

[0045] where X(f m L * λ s ) is the discrete Fourier transform result of the m-th harmonic of the quasi-synchronous reconstruction leakage current signal.

[0046] Furthermore, in step S6, calculate the relative capacitance C relat and relative dielectric loss tanδ relat

[0047]

[0048] where A 1 and A 1,Ref are the fundamental wave amplitudes of the comparison and reference UHV converter transformer valve side bushings respectively, and φ 1 and φ 1,Ref are the fundamental wave phases of the comparison and reference UHV converter transformer valve side bushings respectively.

[0049] Compared with the prior art, the UHV converter transformer valve side bushing dielectric loss and capacitance detection method based on the Hanning window function filtering and quasi-synchronous reconstruction method of the present invention has the following advantages:

[0050] 1. The quasi-synchronous sampling reconstruction method based on second-order Newton interpolation proposed by the present invention performs quasi-synchronous sampling reconstruction on the asynchronous sampling leakage current signal, ensuring that the length of the sampled data is an integer multiple of the fundamental wave period and avoiding the spectrum leakage phenomenon in spectrum analysis.

[0051] 2. The quasi-synchronous sampling reconstruction of the leakage current signal of the present invention only needs to calculate 10 fundamental wave periods, greatly reducing the calculation amount. While taking into account the calculation accuracy, compared with the fourth-order Newton interpolation, the addition and multiplication calculation amounts of the quasi-synchronous sampling reconstruction using the second-order Newton interpolation algorithm are significantly reduced.

[0052] 3. The accurate calculation of the fundamental wave period is required for the quasi-synchronous sampling reconstruction of the leakage current signal. On the one hand, before processing the collected leakage current signal, the present invention designs a Hanning window function filter to filter out other frequency components except the fundamental wave component and optimizes the parameters of the Hanning window function filter; on the other hand, a second-order divided difference Newton interpolation polynomial is used to approximate the actual leakage current signal. Compared with the linear interpolation method, the second-order divided difference Newton interpolation polynomial method has higher accuracy in dealing with larger harmonic components. Compared with the Lagrange interpolation and the fourth-order divided difference Newton interpolation polynomial method, the second-order divided difference Newton interpolation polynomial method has less calculation amount and is easier to implement in programs.

[0053] 4. When calculating the fundamental wave period, the average value of 8 signal fundamental wave periods (as shown in Equation (7)) is used as the final calculation result of the fundamental wave period. This method is equivalent to performing average value filtering on the leakage current signals of 8 fundamental wave periods. Due to the strong electromagnetic interference in the UHV converter station, this method can significantly suppress Gaussian white noise. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0055] Figure 1 It is a schematic flow chart of the method for detecting the dielectric loss and capacitance of the valve side bushing of the UHV converter transformer based on the Hanning window function and the quasi-synchronous reconstruction method of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other.

[0057] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first", "second", etc. may explicitly or implicitly include one or more of such features. In the description of the present invention, unless otherwise specified, the meaning of "a plurality" is two or more.

[0058] In the description of the present invention, it should be noted that, unless otherwise clearly specified and defined, the terms "mounted", "connected", "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood through specific situations.

[0059] The present invention will be described in detail below with reference to the drawings and in conjunction with embodiments.

[0060] Different from the AC power transformer bushings and the line side bushings of converter transformers, the voltage and current waveforms of the valve side bushings of converter transformers contain rich harmonics, and the traditional absolute measurement method cannot be used to measure the actual values of the dielectric loss and capacitance of the valve side bushings. In a converter substation, the voltage waveforms borne by the valve side bushings of different converter transformers are not the same, and there is no device such as a voltage transformer for measuring the voltage reference signal of the valve side bushings, resulting in the inability to use the relative measurement method to measure the relative values of the dielectric loss and capacitance of the valve side bushings either.

[0061] The present invention proposes to calculate the relative dielectric loss and relative capacitance of the valve side bushing by using the leakage current waveform of the valve side bushing of the converter transformer. The Fourier transform of the leakage current is required in this calculation process. During the data acquisition process, the acquisition cards configured at the end shields of each valve side bushing cannot ensure completely synchronous sampling, and the length of the sampled data cannot be guaranteed to be an integer multiple of the period. Therefore, spectral leakage and the fence effect will occur, affecting the fundamental frequency amplitude and phase calculated by the spectral analysis. The inaccurate fundamental frequency amplitude affects the accuracy of the relative capacitance detection, and the inaccurate fundamental frequency phase affects the inaccuracy of the relative dielectric loss. Since the capacitance and dielectric loss change little during the early faults of the valve side bushing, spectral leakage and the fence effect will lead to insufficient sensitivity to the early fault identification.

[0062] The present invention proposes a method for detecting the dielectric loss and capacitance of the valve side bushing of a UHV converter transformer based on the Hanning window function filtering and quasi-synchronous reconstruction method. This method performs quasi-synchronous sampling reconstruction on the leakage currents of each asynchronous sampling, and the reconstruction algorithm takes into account both the accuracy of reconstructing the leakage current waveform and the computational complexity of the reconstruction. Before performing the quasi-synchronous sampling reconstruction, it is necessary to accurately calculate the fundamental period of the leakage current of the valve side bushing. In order to improve the accuracy of calculating the fundamental period of the leakage current, a Hanning window function filter is designed to filter out other frequency components except the fundamental wave component. This method can significantly improve the accuracy of detecting the dielectric loss and capacitance of the valve side bushing of the UHV converter transformer without increasing the hardware resource cost, and solves the technical problem of abnormal state diagnosis of the valve side bushing in the UHV converter station.

[0063] A method for detecting the dielectric loss and capacitance of the valve side bushing of a UHV converter transformer based on the Hanning window function filtering and quasi-synchronous reconstruction method, the process of which is as Figure 1 shown, specifically including:

[0064] (1) In step S1, input the leakage currents collected by the acquisition cards of the end shields of the valve side bushings of the UHV converter transformer, including the leakage currents of the comparison bushing and the reference bushing.

[0065] (2) In step S2, design a Hanning window function filter. The expression of the Hanning window function is

[0066]

[0067] where N is the sampling length, n (0≤n≤N - 1) is the sampling point, and I N (n) is the nth sampling data of the leakage current, and I Filt(n) represents the leakage current data after Hanning window filtering. Preferably, the frequency characteristics of the Hanning window function filter are designed as follows: the center frequency is 50 Hz, the stopband edge frequencies are 43.6 Hz and 56.4 Hz, and the passband edge frequencies are 49.2 Hz and 50.8 Hz. The stopband attenuation is 30 dB, and the passband ripple is 0.4 dB.

[0068] (3) In step S3, calculate the fundamental period of the filtered leakage current. Assume the filtered leakage current I Filt is near the threshold I th . The adjacent points are {I Filt (k n - 1), k n - 1} and {I Filt (k n ), k n}. Then let I Filt (k n - 1) = I 0 , f(I 0 ) = k n - 1, I Filt (k n ) = I 1 , f(I 1 ) = k n , I Filt (k n + 1) = I 2 , f(I 2 ) = k n + 1. Use the second-order divided-difference Newton interpolation polynomial P(I) to approximate the function f(I)

[0069] P(I) = f(I 0 ) + f[I 0 , I 1 (I - I 0 ) + f[I 0 , I 1 , I 2 (I - I 0 )(I - I 1 ) (2)

[0070]

[0071] Preferably, use the time when the filtered leakage current crosses the threshold I th for the second time and the time when it crosses the threshold I th for the tenth time to calculate the fundamental period T * and the frequency f * .

[0072] Calculate the time t when it crosses the threshold I th for the second time2 For

[0073] t 2 = P(I th )(6)

[0074] Similarly, the time t th when the threshold I 10 is crossed for the 10th time can be calculated. Then, the fundamental period T * of the leakage current after filtering is

[0075]

[0076] The fundamental frequency f * of the leakage current after filtering is

[0077]

[0078] (4) In step S4, a second-order Newton forward interpolation is used to perform quasi-synchronous reconstruction on the leakage current after filtering. The specific steps include:

[0079] (a) Calculate the number of sampling points L and the sampling period T s within one signal period of the leakage current after filtering;

[0080] (b) Calculate the quasi-sampling period λ s

[0081]

[0082] where L * is the integer power of 2 closest to L.

[0083] (c) Let any sampling point {I Filt (g 1 ), g 1} of the leakage current after filtering be the first sampling point {I Rec (q 1 ), q 1} of the quasi-synchronous reconstruction signal.

[0084] (d) The m-th sampling point {I Rec (q m ), q m} of the quasi-synchronous reconstruction signal is located after the leakage current after filtering {I Filt (g u ), g u}. Calculate the interval

[0085] τ m = (m - 1)λ s - (u - 1)T s (10)

[0086] (e) Let \(I\) Rec (q k ) = \(h(q\) k )(1 ≤ \(k\) ≤ \(L\) * ). Calculate \(I\) using a second-order Newton forward interpolation polynomial for \(I\) Rec (q m )

[0087]

[0088]

[0089] (f) Repeat step (e) to calculate \(L\) * - 1 quasi-synchronous reconstruction sampling points. These \(L\) * - 1 quasi-synchronous reconstruction sampling points and the initial point \(\{I\) Rec (q 1 ), \(q\) 1}\) together constitute a quasi-synchronous reconstruction leakage current signal with a length of \(L\) * .

[0090] (5) In step S5, perform a Fourier transform on the quasi-synchronous reconstruction leakage current signal with a length of \(L\) * to calculate the frequency \(f\) m , amplitude \(A\) m and phase \(\varphi\) m of each harmonic. The calculation formula is

[0091]

[0092]

[0093] where \(X(f\) m \(L\) * \(\lambda\) s ) is the discrete Fourier transform result of the \(m\)th harmonic of the quasi-synchronous reconstruction leakage current signal.

[0094] (7) In step S6, calculate the relative capacitance \(C\) relat and relative dielectric loss \(\tan\delta\) relat

[0095]

[0096] where \(A\) 1 and \(A\) 1,Ref are the fundamental wave amplitudes of the comparison and reference UHV converter transformer valve side bushings respectively, and \(\varphi\) 1 and \(\varphi\) 1,Ref are the fundamental wave phases of the comparison and reference UHV converter transformer valve side bushings respectively.

[0097] The leakage current I measured by acquisition card 1 in the present invention 1 and the leakage current I measured by acquisition card 2 2 are subjected to discrete Fourier transform (when the number of sampling points is 32,000), and only the frequency components from 0 to 500 Hz are retained, which can be expressed by the following formula

[0098]

[0099] where k is the acquisition card serial number, n is the sampling data point, i is the harmonic order, A i is the amplitude of the i-th harmonic, f i is the frequency of the i-th harmonic, φ i is the phase of the i-th harmonic, f s is the sampling frequency. The parameters of the leakage current I 1 are shown in Table 1, and the parameters of the leakage current I 2 are shown in Table 2.

[0100] Table 1

[0101]

[0102]

[0103] Table 2

[0104]

[0105] When the sampling length N of the leakage current is 256, calculated by the direct Fourier transform method: the parameters of the leakage current I 1 are A 1 = 61.7, φ i = -17.8°; the parameters of the leakage current I 2 are A 1 = 57.8, φ i = -21.8°. The relative error of the fundamental wave amplitude by the direct Fourier transform method is 0.52%, and the relative error of the fundamental wave phase is 15.18%.

[0106] When the sampling length N of the leakage current is 256, calculated by the method of the present invention: the parameters of the leakage current I 1 are A 1 = 61.9, φ i = -20.9°; the parameters of the leakage current I 2 are A 1 = 58.0, φ i = -25.6°. The relative error of the fundamental wave amplitude by the method of the present invention is 0.17%, and the relative error of the fundamental wave phase is 0.39%.

[0107] After calculation, the relative capacitance of the two valve-side bushings is 1.067, and the relative dielectric loss is 0.069.

[0108] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for detecting dielectric loss and capacitance of bushing on valve side of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method, characterized in that: The steps include: S1: input comparison and reference leakage current; S2: Design Hanning window function filter; S3: Calculate the fundamental period of the filtered leakage current; S4: Quasi-synchronous reconstruction of the filtered leakage current using second-order Newton forward interpolation; S5: performing discrete Fourier transform on the quasi-synchronous sampling reconstructed signal; S6: Calculate the relative capacitance and relative dielectric loss of the bushing on the converter transformer valve side.

2. According to claim 1, a method for detecting dielectric loss and capacitance of a UHV converter transformer valve-side bushing based on Hanning window function filtering and quasi-synchronous reconstruction method, characterized in that: In the step S1, the leakage current collected by the sensor acquisition card of the bushing end screen on the valve side of the UHV converter transformer is input, including the leakage current of the comparison bushing and the reference bushing.

3. According to claim 1, a method for detecting dielectric loss and capacitance of a UHV converter transformer valve-side bushing based on Hanning window function filtering and quasi-synchronous reconstruction method, characterized in that: In step S2, the Hanning window function is selected as the FIR filter to filter out other frequency components except the fundamental wave component.

4. According to claim 1, a method for detecting dielectric loss and capacitance of a UHV converter transformer valve-side bushing based on Hanning window function filtering and quasi-synchronous reconstruction method, characterized in that: In step S2, the Hanning window function expression is: Where N is the sampling length, n (0≤n≤N-1) is the sampling point, I N (n) is the nth sampling data of leakage current, I Filt (n) represents the leakage current data after Hanning window filtering.

5. According to claim 4, a method for detecting dielectric loss and capacitance of a UHV converter transformer valve-side bushing based on Hanning window function filtering and quasi-synchronous reconstruction method, characterized in that: In step S2, the frequency characteristics of the Hanning window function filter are designed as follows: center frequency 50 Hz, stopband edge frequencies 43.6 Hz and 56.4 Hz, passband edge frequencies 49.2 Hz and 50.8 Hz, stopband attenuation 30 dB, and passband ripple 0.4 dB.

6. The method for detecting dielectric loss and capacitance of valve-side bushing of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method according to claim 1 is characterized in that: In step S3, the filtered leakage current I Filt At threshold I th The nearby points are {I Filt (k n -1), k n -1} and {I Filt (k n ), k n }, then let I Filt (k n -1)=I0,f(I0)=k n -1,I Filt (k n )=I1,f(I1)=k n , I Filt (k n +1)=I2,f(I2)=k n +1Use the second-order mean error Newton interpolation polynomial P(I) to approximate the function f(I): P(I)=f(I0)+f[I0,I1](I-I0)+f[I0,I1,I2](I-I0)(I-I1) 7. The method for detecting dielectric loss and capacitance of valve-side bushing of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method according to claim 6 is characterized in that: In step S3, the filtered leakage current is used to cross the threshold I for the second time. th The time and the 10th crossing of threshold I th The time to calculate the fundamental period T * and frequency f * ; Calculate the second crossing threshold I th The time t2 is t2=P(I th ) (6) Similarly, the 10th crossing threshold I can be calculated th Time t 10 , then the fundamental period of the filtered leakage current is T * for The fundamental frequency of the leakage current after filtering is f * for 8. The method for detecting dielectric loss and capacitance of bushing on valve side of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method according to claim 7 is characterized in that: The step S4 specifically includes: (a) Calculate the number of sampling points L and sampling period T in one signal cycle of the filtered leakage current s ; (b) Calculate the quasi-sampling period λ s Where L * is the integer power of 2 closest to L; (c) Let any sampling point of the filtered leakage current {I Filt (g1), g1} is the first sampling point of the quasi-synchronous reconstructed signal {I Rec (q1), q1}; (d) Quasi-synchronous reconstruction signal mth sampling point {I Rec (q m ), q m } is located at the filtered leakage current {I Filt (g u ), g u }After that, calculate the interval between the two t m =(m-1)λ s -(u-1)T s (10) (e) Order I Rec (q k )=h(q k )(1≤k≤L * ), using the second-order Newton pre-interpolation polynomial to calculate I Rec (q m ) (f) Repeat step (e) to calculate L * -1 quasi-synchronous reconstruction sampling point, this L * -1 quasi-synchronous reconstruction sampling point and the initial point {I Rec (q1, q1} together form a length of L * Quasi-synchronous reconstruction of the leakage current signal.

9. The method for detecting dielectric loss and capacitance of bushing on valve side of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method according to claim 8 is characterized in that: In step S5, the length L * Quasi-synchronously reconstruct the leakage current signal and perform Fourier transform to calculate the frequency f of each harmonic m , Amplitude A m and phase φ m , the calculation formula is Where X(f m L * λ s ) is the mth harmonic discrete Fourier transform result of the quasi-synchronous reconstruction of the leakage current signal.

10. The method for detecting dielectric loss and capacitance of bushing on valve side of UHV converter transformer based on Hanning window function filtering and quasi-synchronous reconstruction method according to claim 1, characterized in that: In step S6, the relative capacitance C of the bushing on the converter transformer valve side is calculated. relat and relative dielectric loss tanδ relat Where A1 and A 1,Ref are the fundamental amplitudes of the bushings on the valve side of the UHV converter transformer for comparison and reference, φ1 and φ 1,Ref They are the fundamental wave phase of the bushing on the valve side of the UHV converter transformer for comparison and reference respectively.

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