A method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding

Abstract

The present invention relates to a method for extracting and decoupling the insulation state characteristics of the phase ends of a variable-frequency motor winding, comprising the following steps: Step S1, respectively collect the high-frequency switching oscillation currents on the motor side and the frequency converter side; Step S2, separate the high-frequency differential-mode current component from the high-frequency switching oscillation current, and extract the frequency characteristics of the differential-mode switching oscillation current; Step S3, calculate the first-level deterioration index of the phase-end insulation, and preliminarily judge whether the insulation state has deteriorated. If the index is within the specified range, the insulation state has not deteriorated; if the index exceeds the specified range, it indicates that the insulation state has deteriorated, and enter Step S4; Step S4, calculate the amplitude ratio of the differential-mode switching oscillation current; Step S5, calculate the decoupling index of the insulation state of the motor winding phase ends according to the frequency characteristics of the differential-mode switching oscillation current and the amplitude ratio of the differential-mode switching oscillation current, and judge the deteriorated part. Compared with the prior art, the present invention has the advantages of high sensitivity, solving the high-frequency coupling problem, etc.

Description

Technical Field

[0001] The invention relates to the field of variable frequency motor system state monitoring, and in particular to a method for extracting and decoupling insulation state characteristics of a phase end of a variable frequency motor winding. 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. Stator insulation failure is an important cause of motor failure, accounting for more than 30% of total motor failures. For variable frequency motors, the high dv / dt impact generated by the high-speed switching action of the inverter will accelerate the aging of the stator winding insulation. The first few coils connected to the incoming line end of each phase winding of the motor are subject to greater switching transient voltage overstress, which is more likely to cause phase end insulation failure. Therefore, it is necessary to obtain the status information of the phase end insulation of the variable frequency motor in a timely manner to improve the reliability of system operation.

[0003] Phase-end insulation refers to the insulation of the first few coils connected to the motor input terminal under the switching surge frequency band. The variable frequency motor is driven by a power electronic inverter. Due to the greater switching voltage overstress impact, the phase-end insulation of the motor winding is more likely to deteriorate, which is the key weak link of the variable frequency motor winding, especially when using silicon carbide devices with higher switching speed / frequency. However, most of the current motor insulation online monitoring methods are mainly aimed at the overall insulation status of the motor winding, and there are few studies on the phase-end insulation status monitoring of the variable frequency motor reliability weak point.

[0004] Patent CN 113391170 A proposes an online monitoring method for the insulation state of the end of an inverter-driven motor, and attempts to use the high-frequency common-mode switching oscillation current frequency characteristics to online monitor the changes in the insulation capacitance of the motor phase end. However, there is a complex high-frequency coupling effect between the common-mode switching oscillation mode and multiple factors of the variable-frequency motor system. The common-mode characteristics are not only sensitive to the insulation state of the motor phase end, but also related to factors such as the parasitic capacitance of the power device and the cable capacitance (especially the long cable system), which makes it difficult to distinguish the changes in the insulation state of the phase end.

[0005] Therefore, how to extract and decouple the characteristics of the insulation status of the phase ends of the variable frequency motor windings to achieve high-sensitivity online monitoring has become a technical problem that needs to be solved. Summary of the invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a method for extracting and decoupling the insulation state characteristics of the phase ends of the variable frequency motor winding.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] A method for extracting and decoupling insulation state characteristics of a variable frequency motor winding phase end, the method comprising the following steps:

[0009] Step S1: Collect the high-frequency switching oscillation currents on the motor side and the frequency converter side respectively;

[0010] Step S2: Use the modal decomposition method to separate the high-frequency differential-mode current component from the high-frequency switching oscillation current, and use the signal processing method to extract the frequency characteristics of the differential-mode switching oscillation current;

[0011] Step S3: Calculate the first-level deterioration index of the phase-end insulation according to the frequency characteristics of the differential-mode switching oscillation current, and preliminarily judge whether the insulation state has deteriorated according to the index. If the index is within the specified range, the insulation state has not deteriorated; if the index exceeds the specified range, it indicates that the insulation state has deteriorated, and go to Step S4;

[0012] Step S4: Calculate the amplitude ratio of the differential-mode switching oscillation current;

[0013] Step S5: Calculate the decoupling index of the motor winding phase-end insulation state according to the frequency characteristics of the differential-mode switching oscillation current and the amplitude ratio of the differential-mode switching oscillation current, and judge the deteriorated part.

[0014] Furthermore, the high-frequency switching oscillation current in Step S1 is collected by a high-frequency current sensor.

[0015] Furthermore, the high-frequency current sensor has a high-bandwidth passband characteristic, with an upper bandwidth exceeding 1 MHz and a lower frequency greater than twice the frequency converter carrier frequency.

[0016] Furthermore, the modal decomposition method in Step S2 includes the empirical mode decomposition method and the variational mode decomposition method; the signal processing method in Step S2 includes the fast Fourier transform method.

[0017] Furthermore, the specific expression of the frequency characteristics of the differential-mode switching oscillation current is:

[0018]

[0019] where L cab is the high-frequency inductance of the cable, C cab2 is the distributed capacitance of the cable, and C e_DM is the high-frequency differential-mode equivalent capacitance.

[0020] Furthermore, the specific expression of the first-level deterioration index of the phase-end insulation in Step S3 is:

[0021]

[0022] where F HFDM0 is the frequency characteristic of the differential-mode switching oscillation when the phase-end insulation is healthy, and F' HFDMIt is the differential-mode switching oscillation frequency characteristic after the insulation state of the phase terminal changes.

[0023] Furthermore, the specified range of the first-level deterioration index of the phase-terminal insulation is 1% - 3%.

[0024] Furthermore, the specific expression of the differential-mode switching oscillation current amplitude ratio in step S4 is:

[0025]

[0026] where, i t is the high-frequency switching oscillation current on the motor side, and i sw is the high-frequency switching oscillation current on the frequency converter side; when the insulation state of the motor winding phase terminal changes, the high-frequency differential-mode equivalent capacitance C e_DM will change accordingly. At this time, the differential-mode switching oscillation current amplitude ratio and the differential-mode switching oscillation current frequency characteristic are K' and F' respectively HFDM .

[0027] Furthermore, the decoupling index of the insulation state of the motor winding phase terminal in step S5 is specifically expressed as:

[0028]

[0029] where, K is the differential-mode switching oscillation current amplitude ratio when the phase-terminal insulation is healthy, and K' is the differential-mode switching oscillation current amplitude ratio after the insulation state of the phase terminal changes.

[0030] Furthermore, the specific part for judging deterioration is: if the decoupling index of the insulation state of the motor winding phase terminal is close to 1, it is judged that the cable state deteriorates; if the decoupling index of the insulation state of the motor winding phase terminal deviates from 1, it is judged that the insulation state of the motor winding phase terminal deteriorates, and the greater the degree of deviation from 1, the more serious the phase-terminal insulation deterioration.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] First, the present invention extracts and decouples the insulation state characteristics of the variable-frequency motor winding phase terminal through the differential-mode switching oscillation current, solves the interference problem of the parasitic capacitance of the power device, the transient operating condition, and the cable state deterioration on the extraction of the insulation state characteristics of the variable-frequency motor phase terminal, realizes highly sensitive online monitoring of the weak state change of the phase-terminal insulation, and is non-invasive to the system.

[0033] Second, the present invention uses the amplitude ratio characteristics of the differential-mode switching oscillation current on the frequency converter side and the motor side to decouple the interference of cable parameters, such as different cable lengths and cable deterioration, etc., can eliminate the influence of the coupling effect, and accurately judge whether it is the insulation deterioration of the motor phase terminal or the cable state change.

[0034] III. The present invention utilizes high-frequency differential-mode switching oscillation as the sensitive mode for the insulation of the motor phase terminals, which can eliminate the influence of the distributed capacitance of the inverter power devices. Moreover, the frequency characteristics of the differential-mode switching oscillation current do not change with the switching time, and the influence of the transient operating point can be decoupled. Description of the Drawings

[0035] Figure 1 Schematic diagram of the motor system test bench built for the present invention;

[0036] Figure 2 Schematic diagram of the method flow of the present invention;

[0037] Figure 3 Schematic diagram of the differential-mode switching oscillation current on the inverter side and the motor side of the present invention;

[0038] Figure 4 Schematic diagram of the PWM voltage and the switching oscillation current waveforms during the operation of the motor of the present invention;

[0039] Figure 5 Schematic diagram of the variational mode decomposition result of the switching oscillation current of the present invention;

[0040] Figure 6 Schematic diagram of the differential-mode switching oscillation current waveforms under different motor phase terminal insulation states of the present invention;

[0041] Figure 7 Frequency characteristic diagram of the differential-mode switching oscillation current under different motor phase terminal insulation states of the present invention;

[0042] Figure 8 Schematic diagram of the influence of the transient operating point on the amplitude-frequency characteristics of the differential-mode switching oscillation current of the present invention;

[0043] Figure 9 Schematic diagram of the feature extraction results under different cable capacitances of the present invention;

[0044] Figure 10 Schematic diagram of the feature extraction results under different cable lengths of the present invention;

[0045] Figure 11 Schematic diagram of the feature extraction results under different end-winding phase terminal insulation capacitances of the present invention. Detailed Embodiment

[0046] The present invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0047] Embodiment

[0048] The present invention provides a method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding. In this embodiment, an experimental bench as shown in Figure 1 is built. The motor under test in the experiment is a three-phase 380V / 3kW permanent magnet synchronous motor with a rated current of 7.5A. The load motor for the back-to-back test is a three-phase 380V / 3kW induction motor. Two commercial frequency converters (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 motor under test is connected in a star connection, and the length of the cable between the motor and the frequency converter is 1.5m. By connecting the DC buses of the frequency converters in parallel, the active power circulates between the two motors. A high-speed digital oscilloscope (Pico 5444D, sampling frequency 125MHz) and a host computer are used for signal acquisition and processing. In the experiment, a differential voltage probe CYBERTEK DP6150A with a bandwidth of 100MHz and a high-frequency current probe CYBERTEK CP8030B with a bandwidth of 50MHz are used. The high-frequency current sensor is a self-made self-integrating Rogowski coil sensor.

[0049] As shown in Figure 2 , a method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding includes the following steps:

[0050] Step S1: Collect the high-frequency switching oscillation currents on the motor side and the frequency converter side respectively.

[0051] The high-frequency switching oscillation current i t on the motor side and the high-frequency switching oscillation current i sw on the frequency converter side are collected respectively through high-frequency current sensors. The high-frequency current sensor should have a high-bandwidth passband characteristic, and the upper limit bandwidth should exceed 1MHz to achieve the measurement of high-frequency switching oscillation current. The lower limit frequency should be more than twice the carrier frequency of the frequency converter, so as to filter out the interference of the system fundamental wave and switching harmonics. As shown in Figure 3 , a variable capacitor ΔC cab2 (100pF, 220pF, 330pF) is connected in parallel between the motor-side cable and the ground to simulate the deterioration of cable insulation. Two high-frequency current sensors are installed on the frequency converter side and the motor side respectively to collect the high-frequency current i sw on the frequency converter side and the high-frequency current i t on the motor side. As shown in Figure 3 , i sw and i t have the same frequency, and the difference in amplitude is mainly caused by the leakage current i Ccab2 generated by the cable distribution parameters.

[0052] Step S2: Use the modal decomposition method to separate the high-frequency differential-mode current component from the high-frequency switching oscillation current, and use the signal processing method to extract the frequency characteristics of the differential-mode switching oscillation current.

[0053] As shown Figure 4 in the figure are the PWM voltage and the switching oscillation current waveforms during the normal operation of a permanent magnet synchronous motor. The modal decomposition method is used to separate the high-frequency differential mode current component from the high-frequency switching oscillation current. The modal decomposition method includes the empirical mode decomposition method and the variational mode decomposition method. In this embodiment, the variational mode decomposition method (VMD) is used to adaptively decompose the high-frequency switching oscillation current, and the decomposition layer number k = 3 is set. The modal decomposition result is as Figure 5 shown. IMF1 to IMF3 respectively correspond to the three dominant modes in the high-frequency switching oscillation current. The central frequencies of IMF2 and IMF3 are 4.3 MHz and 9.1 MHz respectively. Since the oscillation component of 4.3 MHz can be observed in both the motor phase current and the ground wire current, while the oscillation component of 9.1 MHz only exists in the phase current, it can be judged that IMF3 of 9.1 MHz is the high-frequency differential mode switching oscillation current component. The extraction of the differential mode switching oscillation current frequency F HFDM includes but is not limited to using signal processing methods such as fast Fourier transform to obtain the frequency at the peak of the spectrum. F HFDM is defined as follows:

[0054]

[0055] where L cab is the high-frequency inductance of the cable, C cab2 is the distributed capacitance of the cable, and C e_DM is the high-frequency differential mode equivalent capacitance.

[0056] The high-frequency equivalent common mode capacitance (3C g1 ) of the permanent magnet synchronous motor is measured to be approximately 1.15 nF at 1 MHz using an impedance analyzer (HIOKI IM7581). The phase angle of the common mode impedance is closer to -90 degrees (capacitive) near 1 MHz. It can be approximately considered that the measured value in this frequency range is the end capacitance of the motor. Thus, the high-frequency equivalent differential mode capacitance C e_DM of the motor can be estimated to be approximately 255 pF. The high-frequency inductance L cab and the distributed capacitance C cab2 of the cable are 40 μH / m and 30 pF / m respectively. The theoretical value of the differential mode switching oscillation frequency F HFDM is calculated to be 9.44 MHz. A variable capacitance ΔC g1 is connected in parallel between the end of phase C of the motor and the ground, and the capacitance value is selected from 25 pF to 680 pF to simulate different degrees of insulation deterioration at the motor phase end. The experimentally measured differential mode switching oscillation frequency of 9.1 MHz is close to the theoretically calculated value of 9.44 MHz (the error is less than 4%), thus verifying the correctness of the high-frequency differential mode circuit model.

[0057] Step S3: Calculate the first-level deterioration index of the phase-terminal insulation based on the differential-mode switching oscillation current frequency characteristics, and preliminarily determine whether the insulation state has deteriorated according to the index. The specified range of the first-level deterioration index of the phase-terminal insulation is 1% - 3%. If the index is within the specified range, the insulation state has not deteriorated; if the index exceeds the specified range, it indicates that the insulation state has deteriorated, and proceed to Step S4.

[0058] The specific expression of the first-level deterioration index of the phase-terminal insulation is:

[0059]

[0060] where F HFDM0 is the differential-mode switching oscillation frequency characteristic when the phase-terminal insulation is healthy, and F' HFDM is the differential-mode switching oscillation frequency characteristic after the change of the phase-terminal insulation state.

[0061] Extract the characteristics of the differential-mode switching oscillation current under different phase-terminal insulation states of the motor. Figure 6 are the time-domain and frequency-domain waveforms of the differential-mode switching oscillation current. It can be seen that as the phase-terminal insulation capacitance increases, the amplitude of the differential-mode switching oscillation current increases while the frequency decreases. Further, extract the scatter plot and average value of the differential-mode switching oscillation frequency characteristic F HFDM from 300 switching events during motor operation, as shown in Figure 7 . F HFDM decays approximately exponentially with the increase of the phase-terminal insulation capacitance, and can sensitively characterize the weak changes of the motor end capacitance.

[0062] Then, to analyze the influence of the transient operating point, extract the IGBT switching time, differential-mode switching oscillation amplitude and frequency characteristics under different transient currents within a fundamental wave period (fundamental wave frequency 30 Hz, peak current 7.5 A). From Figure 8 (a), it can be seen that within a fundamental wave period, the IGBT turn-on / off time changes with the transient current. The turn-off time is generally longer than the turn-on time, and it turns off more slowly at small currents. From Figure 8 (b), it can be seen that the amplitude characteristic changes negatively with the IGBT switching time, that is, the smaller the IGBT switching time, the larger the oscillation amplitude. However, regardless of how the switching time t sw varies dynamically, the frequency characteristic F HFDM of the differential-mode switching oscillation current always remains stable, as shown in Figure 8 (c). This is because the modal frequency is the intrinsic characteristic of the system and is independent of the excitation. Therefore, the frequency characteristic can decouple the influence of the transient operating current on the IGBT dynamic switching time.

[0063] Step S4: Calculate the amplitude ratio of the differential-mode switching oscillation current.

[0064] Considering the shunt effect of the cable distributed capacitance, calculate the differential-mode switching oscillation current amplitude ratio K:

[0065]

[0066] where i t is the high-frequency switching oscillation current on the motor side, and i sw is the high-frequency switching oscillation current on the frequency converter side.

[0067] When the insulation state of the motor winding phase terminal changes, the high-frequency differential-mode equivalent capacitance C e_DM will change accordingly. At this time, the differential-mode switching oscillation current amplitude ratio and the differential-mode switching oscillation current frequency characteristic are K' and F' HFDM .

[0068] Step S5: According to the differential-mode switching oscillation current frequency characteristic and the differential-mode switching oscillation current amplitude ratio, calculate the decoupling index of the motor winding phase terminal insulation state, and judge the deteriorated part.

[0069] The decoupling index of the motor winding phase terminal insulation state is specifically expressed as:

[0070]

[0071] where K is the differential-mode switching oscillation current amplitude ratio when the phase terminal insulation is healthy, and K' is the differential-mode switching oscillation current amplitude ratio after the phase terminal insulation state changes. If the decoupling index of the motor winding phase terminal insulation state is close to 1, it is judged that the cable state is deteriorated; if the decoupling index of the motor winding phase terminal insulation state deviates from 1, it is judged that the insulation state of the motor winding phase terminal is deteriorated, and the greater the degree of deviation from 1, the more serious the phase terminal insulation deterioration.

[0072] As Figure 9 shown, the change of the cable capacitance will change the differential-mode switching oscillation frequency F HFDM , so it is difficult to accurately characterize the insulation state only by using F HFDM . The phase terminal insulation state index Flag_C g1 designed in the present invention remains near the reference value of 1, indicating that the insulation state of the motor phase terminal is not deteriorated. Considering that the cable length in the actual system may change, the characteristic extraction results of 1.5 m cables and 10 m cables are compared in the experiment. As Figure 10 shown, the differential-mode switching oscillation frequency F HFDM of the 10 m cable decreases significantly. However, the average values of Flag_C g1 under the 1.5 m cable and the 10 m cable are 0.964 and 0.962 respectively, indicating that the proposed index Flag_C g1 is not affected by the cable length of the variable-frequency motor system.

[0073] AsFigure 11 As shown, the differential-mode switching oscillation frequency F HFDM and the phase-terminal insulation state index Flag_C g1 both shift with the deterioration degree of the motor phase-terminal insulation. Connecting a parallel capacitor ΔC at the motor phase end g1 will change ΔC e_DM , and further calculate the theoretical value of Flag_C g1 and compare it with the measured value. As shown in Table 1, the error between the theoretical value and the measured value of Flag_C g1 is less than 4%. Experiments show that combining the amplitude ratio characteristics of the differential-mode oscillation mode can effectively decouple the influence of cable distribution parameters and accurately reflect the change of the motor phase-terminal insulation state.

[0074] Table 1 Comparison between the theoretical value and the measured value of Flag_C g1

[0075] <![CDATA[ΔC g1 > <![CDATA[ΔC e_DM > <![CDATA[Flag_C g1 Theoretical value]]> <![CDATA[Flag_C g1 Measured value]]> 100 pF 39 pF 1.15 1.18 220 pF 80 pF 1.32 1.36 330 pF 112 pF 1.44 1.43

[0076] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations according to the concept of the present invention without creative work. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field based on the concept of the present invention through logical analysis, reasoning or limited experiments on the basis of the prior art should be within the protection scope determined by the claims.​

Claims

1. A method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding, characterized in that, The method described above includes the following steps: Step S1: Collect the high-frequency switching oscillation currents on the motor side and the frequency converter side respectively; Step S2: Use the modal decomposition method to separate the high-frequency differential mode current component from the high-frequency switching oscillation current, and use the signal processing method to extract the frequency characteristics of the differential mode switching oscillation current; Step S3: Calculate the first-level insulation deterioration index of the phase terminal according to the frequency characteristics of the differential mode switching oscillation current, and preliminarily judge whether the insulation state has deteriorated according to the index. If the index is within the specified range, the insulation state has not deteriorated; if the index exceeds the specified range, it indicates that the insulation state has deteriorated, and go to Step S4; Step S4: Calculate the amplitude ratio of the differential mode switching oscillation current; Step S5: Calculate the decoupling index of the insulation state of the motor winding phase terminal according to the frequency characteristics of the differential mode switching oscillation current and the amplitude ratio of the differential mode switching oscillation current, and judge the deteriorated part; The specific expression of the frequency characteristics of the differential mode switching oscillation current described above is: Among them, L cab is the high-frequency inductance of the cable, C cab2 is the distributed capacitance of the cable, C e_DM is the high-frequency differential-mode equivalent capacitance; The specific expression of the first-level insulation deterioration index of the phase terminal in Step S3 described above is: Among them, F HFDM0 is the differential-mode switching oscillation frequency characteristic when the phase-terminal insulation is healthy, F ＇ HFDM is the differential-mode switching oscillation frequency characteristic after the change of the phase-terminal insulation state; The specific expression of the decoupling index of the insulation state of the motor winding phase terminal in Step S5 described above is: Among them, K is the differential-mode switching oscillation current amplitude ratio when the phase terminal insulation is healthy, K ＇ the differential-mode switching oscillation current amplitude ratio after the phase terminal insulation state changes; The specific expression of the amplitude ratio of the differential mode switching oscillation current in Step S4 described above is: Wherein, i t is the high-frequency switching oscillation current on the motor side, i sw is the high-frequency switching oscillation current on the frequency converter side; when the insulation state of the motor winding phase terminal changes, the high-frequency differential-mode equivalent capacitance C e_DM will change accordingly. At this time, the amplitude ratio of the differential-mode switching oscillation current and the frequency characteristics of the differential-mode switching oscillation current are respectively K ＇ and F ＇ HFDM .

2. A method for extracting and decoupling the insulation state characteristics of the phase ends of a variable-frequency motor winding according to claim 1, characterized in that, The high-frequency switching oscillation current in Step S1 is collected by a high-frequency current sensor.

3. A method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding according to claim 2, characterized in that, The high-frequency current sensor has a high-bandwidth passband characteristic, with an upper bandwidth exceeding 1 MHz and a lower frequency greater than twice the carrier frequency of the frequency converter.

4. A method for extracting and decoupling the insulation state characteristics of the phase ends of a variable-frequency motor winding according to claim 1, characterized in that, The modal decomposition method in Step S2 includes the empirical mode decomposition method and the variational mode decomposition method; the signal processing method in Step S2 includes the fast Fourier transform method.

5. A method for extracting and decoupling the insulation state characteristics of the phase terminals of a variable-frequency motor winding according to claim 1, characterized in that, The specified range of the first-level insulation deterioration index of the phase terminal is 1% - 3%.

6. The method for extracting and decoupling the insulation state characteristics of the phase ends of a variable-frequency motor winding according to claim 1, wherein The specific method for judging the deteriorated part is: if the decoupling index of the insulation state of the motor winding phase terminal is close to 1, it is judged that the cable state has deteriorated; if the decoupling index of the insulation state of the motor winding phase terminal deviates from 1, it is judged that the insulation state of the motor winding phase terminal has deteriorated, and the greater the degree of deviation from 1, the more serious the insulation deterioration of the phase terminal.

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