Online monitoring method for variable frequency motor cable status based on switching oscillation damping characteristics

Abstract

The present invention relates to an online monitoring method for the cable status of a variable frequency motor based on the switching oscillation damping characteristic, comprising: collecting the high-frequency switching oscillation current of the variable frequency motor; extracting a differential-mode switching oscillation signal from the high-frequency switching oscillation current; performing frequency domain analysis on the differential-mode switching oscillation signal to obtain a spectrum, and further extracting the damping characteristic of the high-frequency differential-mode switching oscillation; and calculating a corresponding insulation degradation index based on the extracted damping characteristic, thereby determining the cable insulation degradation status monitoring result. Compared with the prior art, the present invention fully utilizes the high-frequency switching oscillation response generated by the rapid switching of the power devices of the variable frequency motor itself. Based on the characteristics that the participation factor of the high-frequency differential-mode oscillation is independent of stray capacitance and its amplitude response is more significant, the damping characteristic of the differential-mode switching oscillation is used as a characteristic parameter for cable insulation status monitoring, thereby online monitoring the cable insulation status, with the advantages of high sensitivity, accuracy, reliability, and good robustness.

Description

Technical Field

[0001] The present invention relates to the technical field of variable frequency motor cable status monitoring, and in particular to an online monitoring method for variable frequency motor cable status based on switch oscillation damping characteristics. Background Art

[0002] Variable-frequency motor systems are widely used in electric vehicles, high-speed train traction, wind power generation, and other fields, and their safe and reliable operation is crucial. In actual operation, electrical, thermal, and environmental factors, as well as the high dv / dt switching transient overvoltages of inverters, accelerate cable aging. Cable failures often develop from premature insulation degradation. Therefore, online monitoring of premature insulation degradation in variable-frequency motor systems is essential and an effective means of ensuring safe and reliable system operation. However, current research focuses primarily on online monitoring of power cables in distribution and transmission systems, lacking effective online monitoring methods for variable-frequency motor cables.

[0003] For example, Chinese patent CN202021163965.X proposes a transmitting cable intrusion detection device with microwave detection to prevent damage to the cable caused by external moisture corrosion. The moisture barrier can absorb moisture that enters the device from the outside, preventing damage to the device's internal parts caused by moisture corrosion, thereby facilitating normal operation of the device. However, this cable intrusion detection device can affect the normal operation of the system, making it difficult to distinguish the insulation status of the cable.

[0004] Chinese patent CN201510053432.3 discloses a device and method for detecting and evaluating partial discharge in DC XLPE cables. Based on the differences between DC partial discharge and AC partial discharge, this patent uses methods suitable for AC partial discharge detection and analysis to comprehensively evaluate the insulation condition of the cable. However, due to the complex field environment and the influence of various types of noise, obtaining a three-dimensional spectrum requires relatively professional manual experience, making it generally difficult to establish the required fingerprint library. Furthermore, due to the different types of noise, when only the raw data is directly analyzed in the frequency domain, the noise and discharge are likely to be in the same frequency band, and the noise interference is very serious.

[0005] In addition, in existing research, H. Pushpanathan and S. Grzybowski proposed an online diagnostic method based on the analysis of capacitor switching surge transient voltages on cables in the paper "Analysis of online capacitor switching transient voltages on experimental MV cables". Although they found that the switching transient voltages measured at both ends of the cable carry information about the overall health of the cable insulation, they did not conduct a quantitative analysis of the measurement results.

[0006] It can be said that the monitoring of the cable status of variable frequency motors currently still has problems such as low sensitivity and poor anti-interference ability, making it difficult to ensure the accuracy and reliability of the monitoring results. Summary of the Invention

[0007] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide an online monitoring method for the cable status of a variable frequency motor based on the switch oscillation damping characteristics.

[0008] The purpose of the present invention can be achieved by the following technical solution: A method for online monitoring of the cable status of a variable frequency motor based on the switch oscillation damping characteristics, comprising the following steps:

[0009] S1, collects the high-frequency switching oscillation current of the variable frequency motor;

[0010] S2, extracting the differential mode switching oscillation signal from the high frequency switching oscillation current;

[0011] S3. Perform frequency domain analysis on the differential mode switch oscillation signal to obtain a spectrum, and further extract the damping characteristics of the high-frequency differential mode switch oscillation;

[0012] S4. According to the extracted damping characteristics, the corresponding insulation degradation index is calculated to determine the cable insulation degradation status monitoring result.

[0013] Furthermore, the step S1 specifically collects the high-frequency switching oscillation current of the variable frequency motor through a high-frequency current sensor.

[0014] Furthermore, the step S2 specifically uses a modal decomposition method to extract a differential-mode switching oscillation signal from the high-frequency switching oscillation current.

[0015] Furthermore, the method used for the frequency domain analysis in step S3 includes but is not limited to a fast Fourier decomposition method.

[0016] Furthermore, the step S3 specifically utilizes a half-power bandwidth algorithm to extract the damping characteristics of the high-frequency differential mode switching oscillation.

[0017] Furthermore, the damping characteristics in step S3 are specifically:

[0018]

[0019] Where ζ is the damping coefficient, f0 is the peak frequency of the spectrum, f1 and f2 are the left and right sides of the resonance peak of the spectrum respectively. The frequency corresponding to the peak.

[0020] Furthermore, the insulation degradation index calculation formula in step S4 is:

[0021]

[0022] Among them, K is the insulation degradation index of the variable frequency motor cable, ζ0 is the damping coefficient when the cable insulation is in a healthy state, and ζ i It is the damping coefficient of the cable insulation when it ages.

[0023] Furthermore, the step S4 specifically compares the calculated insulation degradation index with a preset threshold value, thereby determining the cable insulation degradation status monitoring result.

[0024] Furthermore, in step S4, if the insulation degradation index is less than a preset threshold, it indicates that the current cable is in a healthy state;

[0025] If the insulation degradation index is greater than or equal to the preset threshold, it indicates that the current cable is in a degraded state, and the larger the insulation degradation index value, the deeper the degree of cable insulation degradation.

[0026] Furthermore, the preset threshold is specifically 3%.

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

[0028] 1. High Sensitivity: The present invention collects the high-frequency switching oscillation current of the variable-frequency motor, extracts the differential-mode switching oscillation signal from the high-frequency switching oscillation current, performs frequency domain analysis on the differential-mode switching oscillation signal to obtain a spectrum, further extracts the damping characteristics of the high-frequency differential-mode switching oscillation, and calculates the insulation degradation index, thereby determining the cable insulation degradation status monitoring results. This fully utilizes the high-frequency switching oscillation response generated by the rapid switching of the variable-frequency motor's own power devices during operation. Based on the characteristics that the participation factor of high-frequency differential-mode oscillation is independent of stray capacitance and its amplitude response is more significant, the damping characteristics of the differential-mode switching oscillation are used as the characteristic parameter for cable insulation status monitoring. Under the same aging cycle, the oscillation damping has a larger variation range and higher sensitivity than the oscillation frequency, ensuring the sensitivity of the present invention.

[0029] 2. Non-invasive online monitoring: The present invention uses a high-frequency current probe to collect the switch oscillation signal. The system can non-invasively monitor the cable insulation status online without injecting additional excitation to ensure the normal operation of the system.

[0030] 3. Strong robustness: The present invention adopts the damping ratio as the intrinsic parameter of the cable insulation status monitoring system, which is not affected by the working conditions, has high robustness, and can achieve the purpose of online continuous monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the method flow of the present invention;

[0032] Figure 2Schematic diagram of the application process of the embodiment;

[0033] Figure 3 This is the principle diagram of the half-power bandwidth algorithm;

[0034] Figure 4 Schematic diagram of the experimental platform built in the embodiment;

[0035] Figure 5 Graph showing the experimental results of the switch oscillation current in the embodiment;

[0036] Figures 6a-6b Graphs of cable oscillation current at different aging levels in the embodiment;

[0037] Figures 7a-7b 2. Schematic diagram of the switching oscillation frequency characteristics at different aging levels in the embodiment;

[0038] Figures 8a-8b Graph showing the switching oscillation damping characteristics at different aging levels in the embodiment;

[0039] Figure 9 The variation of the damping ratio with the aging degree at different carrier frequencies in the embodiment;

[0040] Figure 10 The variation of the damping ratio with the aging degree under different load currents in the embodiment;

[0041] Figure 11 2 is the variation law of the damping ratio with aging degree at different fundamental frequencies in the embodiment. DETAILED DESCRIPTION

[0042] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] Example

[0044] like Figure 1 As shown, a method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics includes the following steps:

[0045] S1, collects the high-frequency switching oscillation current of the variable frequency motor;

[0046] S2, extracting the differential mode switching oscillation signal from the high frequency switching oscillation current;

[0047] S3. Perform frequency domain analysis on the differential mode switch oscillation signal to obtain a spectrum, and further extract the damping characteristics of the high-frequency differential mode switch oscillation;

[0048] S4. According to the extracted damping characteristics, the corresponding insulation degradation index is calculated to determine the cable insulation degradation status monitoring result.

[0049] This embodiment applies the above technical solution, such as Figure 2 As shown:

[0050] 1. Use high-frequency current sensor to collect high-frequency switching oscillation current i of variable frequency motor system sw .

[0051] 2. Use VMD mode decomposition method to extract the high-frequency switching oscillation current i sw Separate the high-frequency differential mode switching oscillation current i sw-DM , using frequency domain analysis methods (including but not limited to fast Fourier decomposition) to analyze i sw-DM Perform spectrum analysis to obtain the high-frequency differential mode switch oscillation current i sw-DM Spectrum diagram of

[0052] 3. Using half-power bandwidth algorithm (such as Figure 3 As shown) calculate the high frequency differential mode switch oscillation current i sw-DM The damping coefficient ζ is as shown in formula (1), Figure 3 As shown, f0 is i sw-DM The spectrum peak frequencies, f1 and f2, are on the left and right sides of the resonance peak respectively. The frequency corresponding to the peak;

[0053]

[0054] 4. Define the variable frequency motor cable insulation degradation index K as formula (2), where ζ0 is the damping coefficient when the cable insulation is in a healthy state, ζ i is the damping coefficient of the cable insulation when it ages;

[0055]

[0056] 5. Determine whether the cable insulation condition has deteriorated based on the indicator K: In this embodiment, if the indicator K is less than 3%, the cable insulation is in a healthy state; if the indicator K exceeds 3%, it means that the cable insulation condition has deteriorated. The larger the K value, the more serious the cable insulation deterioration.

[0057] To verify the effectiveness of this technical solution, this embodiment has built Figure 4The experimental bench is shown. The experimental motor is a three-phase 380V / 3kW permanent magnet synchronous motor with a rated current of 7.5A. The load motor is a three-phase 380V / 3kW induction motor. Two commercial inverters (Delta CH2000 and C2000) are used to control the permanent magnet synchronous motor and the induction motor in speed mode and torque mode respectively. The motors are star-connected, and the active power circulates between the two motors by connecting the inverter DC bus in parallel. A high-speed digital oscilloscope (Pico5444D, sampling frequency 125MHz) and a host computer are used for signal acquisition and processing. The current probe in the experiment is a CYBERTEK CP8030B high-frequency current probe with a bandwidth of 50MHz. The experimental cable is a 20m three-phase XLPE four-core shielded cable (YJVP 0.6 / 1kV, 4×2.5mm 2 ).

[0058] First, use an impedance analyzer (HIOKI IM7581) to measure the differential mode impedance of a 20m long cable and the common mode impedance of the motor. The differential mode impedance of the cable at low frequency is 1021Ω, and the cable capacitance is 4 / 3C. cab It can be calculated as 0.78nF. In addition, the cable inductance is estimated to be 3 / 2L based on the cable differential mode resonant frequency of 2.43MHz. cab The star-connected terminal capacitance C can be calculated based on the common-mode impedance of the high-frequency motor. t The system high-frequency differential mode resonant frequency can be calculated to be 1.96MHz.

[0059] The cables were then suspended in an incubator for accelerated thermal aging. The maximum operating temperature of XLPE cables is 90°C, so the cable operating conditions at 90°C were used as a reference. A step-by-step degradation experiment was designed with different aging temperatures between 90°C and 140°C. Each cycle of aging lasted three hours, with the aging temperature increasing by 10°C. The cable insulation after each cycle of aging was defined as healthy, initial, mild, moderate, severe, and critical states, respectively, within the early degradation spectrum.

[0060] Figure 5 The waveform of the differential mode switch oscillation current was experimentally measured. Fast Fourier transform (FFT) was used to analyze the differential mode switch oscillation in the frequency domain. The experimentally measured differential mode switch oscillation frequency was 1.829MHz, which is close to the theoretical calculated value of 1.96MHz (the error is less than 6.6%), verifying the correctness of the high-frequency differential mode circuit model. Feature extraction was performed on the differential mode switch oscillation current under different cable insulation aging conditions. Figure 6a 、 6b Figure 2 is the time domain and frequency domain waveform of the differential mode switch oscillation current. It can be seen that with the increase of aging, the frequency of the differential mode switch oscillation current decreases, which is consistent with the formula in the theoretical analysis.

[0061] Further analysis of the cable health and dangerous states, the differential mode switching oscillation frequency characteristics f in 200 switching events during motor operation are performed. HFDM The scatter plot and average value of the damping ratio characteristic ζ. The switching oscillation frequency characteristics under different aging degrees are as follows Figure 7a 、 7b As shown, f HFDM As the cable insulation ages, it decays linearly, which can characterize the slight changes in the cable insulation state. Figure 8a 、 8b The medium damping ratio characteristic ζ also decreases with the deepening of cable insulation aging, which is consistent with the analysis content. Both can be used as sensitive characteristics to evaluate the insulation status of cables, verifying the correctness of the theory.

[0062] Finally, through comparison, it is not difficult to find that as the aging degree deepens, within an aging cycle, the resonant frequency characteristic f HFDM The damping ratio characteristic ζ decreases by 2.09%, while the damping ratio characteristic ζ decreases by 13.08%. It can be seen that the damping ratio ζ is relatively close to the resonant frequency f HFDM It is more sensitive to cable insulation aging and can effectively evaluate the cable insulation status.

[0063] In addition, this embodiment also proves the stability of the above method in the robustness study. The inverter carrier frequency mainly affects the switching speed and the amount of oscillation generated in a time period, such as Figure 9 It shows the changing rules of the average values ​​of the damping scatter points at different aging degrees when the load current is 11.36A, the fundamental frequency is 25Hz, and the carrier frequencies are 4kHz, 5kHz, and 6kHz respectively. The method of using damping for early online monitoring of cable status is robust when the switching frequency changes.

[0064] The change of load is mainly achieved by adjusting the torque of the asynchronous motor. The greater the torque, the greater the load and the greater the load current. Figure 10 The results show that the change pattern of the average value of the damping scatter points at different aging degrees when the carrier frequency is 4kHz, the fundamental frequency is 25Hz, and the load current is 8.95A, 11.36A, and 14.26A respectively. The method of using damping to monitor the cable status online in the early stage when the load changes is robust.

[0065] The change of speed is mainly achieved by adjusting the fundamental frequency of the permanent magnet synchronous motor. The higher the fundamental frequency, the higher the speed of the permanent magnet motor. Under the same load, the speed of the asynchronous motor on the other side of the coupling is also higher (the oscillating current only flows through the asynchronous motor as the load and not the synchronous motor). Figure 11It shows the change pattern of the average value of the damping scatter points at different aging degrees when the load current is 11.36A, the carrier frequency is 4kHz, and the fundamental frequencies are 10Hz, 25Hz, and 40Hz respectively. The method of early online monitoring of cable status using damping has good robustness when the speed changes.

[0066] In summary, this technical solution considers the high-frequency switching oscillation currents generated in variable-frequency motor systems. These are resonant responses caused by the high dv / dt and di / dt generated by power electronic devices during the switching process acting on the system's distributed parameters. Therefore, to address the weak insulation state characteristics of cables in variable-frequency motor systems, an online cable insulation condition monitoring method based on the damping characteristics of differential-mode switching oscillations is proposed. This method utilizes the inherent excitation of the variable-frequency motor to generate characteristic oscillation parameter damping for condition monitoring. This method offers the advantages of high sensitivity, accuracy, reliability, and robustness, enabling online and continuous monitoring of early-stage cable aging in variable-frequency motor systems.

Claims

1. A method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics, characterized in that: The following steps are involved: S1, collects the high-frequency switching oscillation current of the variable frequency motor; S2, extracting the differential mode switching oscillation signal from the high frequency switching oscillation current; S3. Perform frequency domain analysis on the differential mode switching oscillation signal to obtain a spectrum, and further extract the damping characteristics of the high-frequency differential mode switching oscillation; S4. According to the extracted damping characteristics, the corresponding insulation degradation index is calculated to determine the cable insulation degradation status monitoring result.

2. The method for online monitoring of the cable status of a variable frequency motor based on the switch oscillation damping characteristics according to claim 1 is characterized in that: The step S1 specifically involves collecting the high-frequency switching oscillation current of the variable frequency motor through a high-frequency current sensor.

3. The method for online monitoring of the cable status of a variable frequency motor based on the switch oscillation damping characteristics according to claim 1 is characterized in that: The step S2 specifically uses a modal decomposition method to extract a differential-mode switching oscillation signal from the high-frequency switching oscillation current.

4. The method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics according to claim 1 is characterized in that: The method used for frequency domain analysis in step S3 includes but is not limited to the fast Fourier decomposition method.

5. The method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics according to claim 1, characterized in that: The step S3 specifically uses a half-power bandwidth algorithm to extract the damping characteristics of the high-frequency differential mode switching oscillation.

6. The method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics according to claim 5, characterized in that: The damping characteristics in step S3 are specifically: Where ζ is the damping coefficient, f0 is the peak frequency of the spectrum, f1 and f2 are the left and right sides of the resonance peak of the spectrum respectively. The frequency corresponding to the peak.

7. The method for online monitoring of the cable status of a variable frequency motor based on the switch oscillation damping characteristics according to claim 6 is characterized in that: The calculation formula for the insulation degradation index in step S4 is: Among them, K is the insulation degradation index of the variable frequency motor cable, ζ0 is the damping coefficient when the cable insulation is in a healthy state, and ζ i It is the damping coefficient of the cable insulation when it ages.

8. The method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics according to claim 1, characterized in that: The step S4 specifically compares the calculated insulation degradation index with a preset threshold value, thereby determining the cable insulation degradation status monitoring result.

9. The method for online monitoring of the cable status of a variable frequency motor based on the switch oscillation damping characteristics according to claim 8, characterized in that: In step S4, if the insulation degradation index is less than the preset threshold, it indicates that the current cable is in a healthy state; If the insulation degradation index is greater than or equal to the preset threshold, it indicates that the current cable is in a degraded state, and the larger the insulation degradation index value, the deeper the degree of cable insulation degradation.

10. The method for online monitoring of the cable status of a variable frequency motor based on switch oscillation damping characteristics according to any one of claims 8 to 9, characterized in that: The preset threshold is specifically 3%.

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