A method for on-line evaluation of the condition of the main insulation of a variable frequency motor

Abstract

The application discloses a kind of variable frequency motor main insulation state online evaluation method, first, by establishing variable frequency motor stator winding insulation model to determine the corresponding relationship between main insulation and leakage current.Second, based on the time-frequency domain characteristics of leakage current, the main insulation state of variable frequency motor is online evaluated, the degradation degree and position of main insulation are identified by combining leakage current common mode and differential mode harmonic, and the degraded phase is identified by using the initial oscillation amplitude of leakage current.Finally, the experiment verification is carried out on the permanent magnet synchronous motor system based on vector control.The beneficial effects of the application are that the initial degradation of motor main insulation can be effectively identified and the degradation position can be accurately located.

Description

Technical Field

[0001] The invention relates to the field of variable frequency motor technology, and in particular to an online assessment method for the main insulation status of a variable frequency motor. Background Technology

[0002] Variable frequency motors have been widely used in fields requiring high operational reliability, such as high-speed railways, wind power generation, and industrial machinery. Stator insulation failure, as one of the most serious motor failures, accounts for approximately 30%-40% of all failures. Therefore, to ensure the reliable operation of the motor system, it is necessary to conduct real-time assessment of the stator insulation condition of the variable frequency motor.

[0003] Traditional offline testing methods, such as insulation resistance and polarization index testing, AC / DC voltage testing, dissipation factor and offline partial discharge (PD) testing, have been widely used to detect the insulation condition of motors. However, offline methods have a long monitoring cycle, and the motor under test must be shut down for maintenance. To detect insulation degradation in a timely manner and avoid serious insulation faults, it is necessary to propose an online insulation condition assessment method.

[0004] Online partial discharge monitoring is a common method for real-time monitoring of motor insulation status. However, this method is susceptible to noise interference and requires the installation of specific PD current sensors. In recent years, online monitoring methods for motor insulation status based on phase current and leakage current have gradually gained attention. For example, the root mean square deviation of transient phase current is used as a monitoring index for stator winding insulation status; the influence of dv / dt on the root mean square deviation of phase current is further studied by adjusting the MOSFET switching speed; or a high-frequency model of electromagnetic wire is established to analyze the oscillation characteristics of transient phase current, and a method for monitoring the insulation status of stator winding input terminals based on the amplitude-frequency characteristics of transient phase current is proposed. However, the above phase current monitoring methods all require monitoring transient currents up to MHz and require the installation of high-precision and high-bandwidth current sensors. In addition, the above methods do not study the influence of degradation location on the degree of degradation.

[0005] Leakage current in motor systems is highly coupled with insulation impedance, and the leakage current can reflect the insulation state of the motor windings. For example, a differential-mode leakage current measurement method proposed in the prior art monitors the leakage current of each phase and obtains the equivalent resistance and capacitance of the main insulation to assess the degree of insulation degradation. This method requires three high-sensitivity current transformers, resulting in high cost, and also necessitates bringing out the motor neutral point. Those skilled in the art have also proposed a method for monitoring the equivalent capacitance of the motor's main insulation based on common-mode leakage current and common-mode voltage. This method can monitor the degree of degradation of the motor's main insulation, but it does not consider the influence of the location of insulation degradation. Those skilled in the art have also proposed using the time-domain characteristics of leakage current to identify the degradation state of the main insulation. However, this method cannot independently assess the degree and location of insulation degradation. Furthermore, a common-mode impedance monitoring method based on leakage current has been proposed in the prior art. This method can distinguish insulation degradation at different winding locations, but it requires monitoring common-mode leakage current above the resonant frequency, which places high demands on the monitoring bandwidth and sampling frequency of the leakage current.

[0006] To overcome the above-mentioned deficiencies, those skilled in the art need an assessment method that can improve the performance of online assessment of motor insulation condition, reduce monitoring signal bandwidth, and take into account the influence of degradation location on the degree of degradation. Summary of the Invention

[0007] The purpose of this invention is to solve the above-mentioned problems by designing an online evaluation method for the main insulation status of a variable frequency motor.

[0008] The technical solution of the present invention to achieve the above objectives is an online assessment method for the main insulation status of a variable frequency motor, the method comprising the following steps:

[0009] Step 1: An equivalent circuit model of stator winding main insulation degradation was established in the low-frequency band. The common-mode frequency and differential-mode frequency were obtained by harmonic analysis of the three-phase voltage in the equivalent circuit model of stator winding main insulation degradation using double Fourier integral.

[0010] Step 2: Based on the equivalent circuit model of the main insulation degradation of the variable frequency motor, obtain the frequency domain mathematical expression and the time domain mathematical expression of the leakage current.

[0011] Step 3: Based on the time-domain and frequency-domain mathematical expressions of the leakage current, the degree of degradation, location of degradation, and phase of degradation of the main insulation of the variable frequency motor are evaluated online.

[0012] The equivalent circuit model for the degradation of the stator winding main insulation in step one is an equivalent parallel circuit of capacitor and resistor, which represent capacitive coupling and dielectric loss, respectively. a,b,c Represents three-phase voltage, R s and L s R represents the stator resistance and inductance, respectively. g and Cg The equivalent resistance and capacitance of the main insulation are represented by N, the neutral point, and D, the location of main insulation degradation. x represents the ratio of the number of turns from the input terminal to point D to the number of turns in the phase winding. The three-phase voltages are replaced by equivalent voltage sources, and the three-phase branches are combined to obtain an RLC series circuit, where k, Z, and U... g These represent the stator impedance coefficient, equivalent circuit impedance, and equivalent voltage source to ground, respectively. The specific circuit parameters are as follows:

[0013] (1)

[0014] (2)

[0015] (3).

[0016] In step one, a double Fourier integral is used to perform harmonic analysis on the three-phase voltage. The Fourier expression for phase A voltage is as follows:

[0017] (4)

[0018] In the formula E d ω is the DC bus voltage, ω0 is the fundamental angular frequency, ω c J is the carrier angular frequency (the switching angular frequency of the inverter), α is the modulation depth, and J is the carrier angular frequency (the switching angular frequency of the inverter). k(x) Denotes the k-th order Bessel function;

[0019] According to equation (3), the expression for the equivalent voltage source to ground at the common-mode frequency is as follows:

[0020] (5)

[0021] Furthermore, combining equations (3) and (5), the equivalent voltage source to ground at the differential mode frequency is obtained as follows:

[0022] (6)

[0023] From equations (5) and (6), it can be seen that the common-mode equivalent voltage to ground U g,CM The common-mode voltage of the phase C winding is equal to that of the phase C winding and is unaffected by the degraded position x, but the differential-mode equivalent voltage to ground U g,DM It decreases linearly with increasing degenerate position x;

[0024] Based on the inverter's base frequency f0 and switching frequency f c The obtained common-mode frequency and differential-mode frequency are shown in equations (7) and (8), respectively:

[0025] (7)

[0026] (8)

[0027] In the formula, m = 1, 2, 3, ..., n = 0, 1, 2, .... When n is a multiple of 3, f1 represents the common-mode frequency; otherwise, f1 represents the differential-mode frequency. When 2n+1 is a multiple of 3, f2 represents the common-mode frequency; otherwise, f2 represents the differential-mode frequency.

[0028] The frequency domain mathematical expression for the leakage current in step two is:

[0029] (9)

[0030] In the formula, ω = 2πf0. When f is the common-mode frequency, I... g Indicates common-mode leakage current, otherwise I g Indicates differential mode leakage current;

[0031] The time-domain mathematical expression for the leakage current is:

[0032] (10)

[0033] in style Indicates that it is on the rising edge, when This indicates that it is at the falling edge.

[0034] In step three, the assessment of the degradation degree of the main insulation of the variable frequency motor is identified by the common-mode leakage current. According to equations (5) and (9), the mathematical expression of the common-mode leakage current is as follows:

[0035] (11)

[0036] In the formula, ω = 2mπf c m=1,3,5,…;

[0037] According to equation (11), the common-mode leakage current I under different insulation capacitances and degradation positions is obtained. g,CM I g,CM For C g Highly sensitive, but almost unaffected by the location of degradation, the degree of primary insulation degradation can be identified using common-mode leakage current.

[0038] The assessment of the degradation location of the main insulation of the variable frequency motor in step three is performed by combining the common-mode leakage current and the differential-mode leakage current. The mathematical expression of the differential-mode leakage current is obtained through equations (6) and (9) as follows:

[0039] (12)

[0040] In the formula, ω = 2mπf c +2πf0, m=2,4,6,…;

[0041] According to equation (12), the differential mode leakage current I under different insulation capacitances and degradation positions in the frequency band from 8050Hz to 48050Hz is obtained. g,DM I g,DM I decreases linearly with increasing x. g,DM Not only with C g It is related to, and also affected by, x; therefore, the location of primary insulation degradation x can be determined by I. g,DM To identify.

[0042] In step three, the assessment of the degraded phase of the main insulation of the variable frequency motor is identified by the initial oscillation amplitude of the leakage current, and its mathematical expression is as follows:

[0043] (13)

[0044] in style This indicates that the PWM voltage is at the rising edge, when This indicates that it is at the falling edge. From equation (13), we know that when E d R s and L s When fixed, the initial oscillation amplitude A of the leakage current mp Only affected by the insulating capacitance C g And the equivalent circuit impedance coefficient k has an effect; therefore, A mp It can assess the degradation phase of the main insulation.

[0045] Beneficial effects

[0046] A method for online assessment of the main insulation status of a variable frequency motor, based on the technical solution of the present invention, has the following advantages:

[0047] 1. This method establishes a stator winding insulation model in the low-frequency range, clarifies the relationship between the time-frequency domain characteristics of leakage current and the degradation of main insulation, and proposes an online assessment method for the insulation status of variable frequency motors based on the time-frequency domain characteristics of leakage current. This method can assess the degree of degradation, location of degradation, and phase of degradation of the main insulation.

[0048] 2. In the low-frequency range, both the common-mode and differential-mode harmonics of leakage current are affected by the degree of insulation degradation, while the differential-mode harmonics are also affected by the location of degradation. Compared with previous methods, this method can identify the degree of degradation, location of degradation, and phase of degradation of the main insulation at a lower monitoring bandwidth, providing a reference for the accurate maintenance of motor insulation systems. Attached Figure Description

[0049] Figure 1 This is a flowchart illustrating the online assessment method for the main insulation status of a variable frequency motor according to the present invention.

[0050] Figure 2This is an electrical model diagram of the degradation of the main insulation of the C-phase winding described in this invention;

[0051] Figure 3 This is a simplified electrical model diagram of the degradation of the C-phase main insulation described in this invention;

[0052] Figure 4 These are simulation results of common-mode leakage current at different frequencies as described in this invention, where (a) is a graph showing the change with insulation capacitance, and (b) is a graph showing the change with degradation location;

[0053] Figure 5 These are simulation results of differential mode leakage current at different frequencies as described in this invention, where (a) is a graph showing the change with insulation capacitance; and (b) is a graph showing the change with degradation location.

[0054] Figure 6 This is the transient leakage current diagram of the three-phase step voltage excitation described in this invention, wherein the degradation position is the C-phase winding x = 0.5;

[0055] Figure 7 This is a framework diagram of the insulation condition monitoring described in this invention;

[0056] Figure 8 The different insulation capacitances ΔC described in this invention g And the experimental results of common-mode leakage current at degradation location x;

[0057] Figure 9 The different insulation capacitances ΔC described in this invention g And the experimental results of differential mode leakage current at degradation position x;

[0058] Figure 10 The different insulation capacitances ΔC described in this invention g And the experimental results of K value at degradation position x;

[0059] Figure 11 The three-phase leakage current ΔA described in this invention mp With insulation capacitance ΔC g Change diagram. Detailed Implementation

[0060] The present invention will now be described in detail with reference to the accompanying drawings, such as... Figure 1 As shown;

[0061] To analyze the coupling relationship between leakage current and main insulation, an equivalent circuit model of stator winding main insulation degradation was established in the low-frequency range (approximately 0Hz to 100kHz). The electrical model of the stator winding with degraded C-phase main insulation is shown below. Figure 2 As shown, the main insulation model is an equivalent parallel circuit of capacitor and resistor, which represent capacitive coupling and dielectric loss, respectively. U a,b,c Represents three-phase voltage, Rs and L s R represents the stator resistance and inductance, respectively. g and C g This represents the equivalent resistance and capacitance of the main insulation. Point N is the neutral point, point D is the location of main insulation degradation, and x represents the ratio of the number of turns from the input terminal to point D to the number of turns in the phase winding.

[0062] The three-phase voltages are replaced with equivalent voltage sources, and the three-phase branches are combined to obtain... Figure 3 The RLC series circuit shown has k, Z, and U. g These represent the stator impedance coefficient, equivalent circuit impedance, and equivalent voltage source to ground, respectively. The specific circuit parameters are as follows:

[0063] (1)

[0064] (2)

[0065] (3)

[0066] Harmonic analysis of the three-phase voltage of the variable frequency motor is performed using double Fourier integrator. The Fourier expression for the A-phase voltage is as follows:

[0067] (4)

[0068] In the formula E d ω is the DC bus voltage, ω0 is the fundamental angular frequency, ω c J is the carrier angular frequency (the switching angular frequency of the inverter), α is the modulation depth, and J is the carrier angular frequency (the switching angular frequency of the inverter). k(x) Let represent the k-th order Bessel function.

[0069] Pulse width modulation (PWM) voltage can be divided into common-mode voltage and differential-mode voltage. Common-mode voltage is uniformly distributed across the three-phase windings and has the same amplitude and phase angle at the same frequency. It only contributes to leakage current and is not used for power transfer. In contrast, the differential-mode voltages of each phase have the same amplitude at the same differential-mode frequency, but their phase angles differ by 120 degrees. They contribute to both leakage current and are the sole excitation for the load current.

[0070] The common-mode voltage can be calculated from the average value of the three-phase voltages. Based on equation (3), the expression for the equivalent voltage source to ground at the common-mode frequency is as follows:

[0071] (5)

[0072] Furthermore, combining equations (3) and (5), the equivalent voltage source to ground at the differential mode frequency is obtained as follows:

[0073] (6)

[0074] From equations (5) and (6), it can be seen that the common-mode equivalent voltage to ground U g,CM The common-mode voltage of the phase C winding is equal to that of the phase C winding and is unaffected by the degraded position x, but the differential-mode equivalent voltage to ground U g,DM The value decreases linearly with increasing degenerate position x.

[0075] Based on the inverter's base frequency f0 and switching frequency f c The obtained common-mode frequency and differential-mode frequency are shown in Equation (7) and Equation (8), respectively.

[0076] (7)

[0077] (8)

[0078] In the formula, m = 1, 2, 3, ..., n = 0, 1, 2, ... When n is a multiple of 3, f1 represents the common-mode frequency; otherwise, f1 represents the differential-mode frequency. When 2n+1 is a multiple of 3, f2 represents the common-mode frequency; otherwise, f2 represents the differential-mode frequency.

[0079] according to Figure 4 The simulation results of the common-mode leakage current yield the following frequency domain mathematical expression for the leakage current:

[0080] (9)

[0081] In the formula, ω = 2πf0. When f is the common-mode frequency, I... g Indicates common-mode leakage current, otherwise I g This indicates the differential mode leakage current.

[0082] The leakage current is mainly generated by the high voltage change rate (dv / dt) at the rising and falling edges of the PWM voltage. Therefore, the equivalent voltage source U to ground... g It can be equivalent to a series of square wave pulses, so the time-domain mathematical expression of the leakage current can be regarded as the step response of the RLC circuit, as follows:

[0083] (10)

[0084] in style Indicates that it is on the rising edge, when This indicates that it is at the falling edge.

[0085] Based on the time-domain and frequency-domain characteristics of the leakage current, the degree of degradation, location of degradation, and phase of degradation of the main insulation can be evaluated respectively.

[0086] The degree of degradation of the main insulation can be identified by the common-mode leakage current. The mathematical expression for the common-mode leakage current is obtained from equations (5) and (9) as follows:

[0087] (11)

[0088] In the formula, ω = 2mπf c m=1,3,5,…

[0089] During the operation of the variable frequency motor, the DC bus voltage E d The modulation depth α is usually fixed, and the stator resistance R s and stator inductance L s It can be obtained through online parameter identification methods. According to equation (11), the common-mode leakage current I under different insulation capacitances and degradation locations is obtained. g,CM like Figure 4 As shown. From Figure 4 (a) It can be seen that I g.CM With the insulation capacitance C g The common-mode frequency increases linearly from 0 to 1 nF, reaches a maximum at 4000 Hz (switching frequency), and then tends to decrease with increasing common-mode frequency. Furthermore, as... Figure 4 As shown in (b), when the degenerate position x changes, I g,CM It remains almost unchanged. Therefore, I g,CM For C g Highly sensitive, but almost unaffected by the location of degradation, the degree of primary insulation degradation can be identified using common-mode leakage current.

[0090] The location of degradation in the main insulation is assessed by combining the common-mode leakage current and the differential-mode leakage current. The mathematical expression for the differential-mode leakage current is obtained from equations (6) and (9) as follows:

[0091] (12)

[0092] In the formula, ω = 2mπf c +2πf0, m=2,4,6,…

[0093] According to equation (12), the differential mode leakage current I under different insulation capacitances and degradation positions in the frequency band from 8050Hz to 48050Hz is obtained. g,DM like Figure 5 As shown. From Figure 5 (a) It can be seen that, with C g Increasing from 0 to 1 nF, I g,DM Gradually increase, and achieve I at 8050Hz g,DM Maximum value. Figure 5 (b) indicates that I g,DMIt decreases linearly with increasing x, and is almost zero near the neutral point. From the above, we can see that I... g,DM Not only with C g It is related to, and also affected by, x. Therefore, the location of primary insulation degradation x can be determined by I. g,DM To identify.

[0094] The degraded phase of the main insulation can be identified by the initial oscillation amplitude of the leakage current, and its mathematical expression is as follows:

[0095] (13)

[0096] in style This indicates that the PWM voltage is at the rising edge, when This indicates that it is at the falling edge. From equation (13), we know that when E... d R s

[0097] and L s When fixed, the initial oscillation amplitude A of the leakage current mp Only affected by the insulating capacitance C g And the equivalent circuit impedance coefficient k. Therefore, using A mp It can assess the insulation status to ground.

[0098] Using MATLAB / Simulink Figure 3 The RLC circuit shown is simulated, where C g Set to 220 pF, the degradation location is phase C (x=0.5). From Figure 6 It can be seen that the leakage current begins to oscillate at the rising edge of the PWM voltage and decays to zero after several oscillation cycles. Since the winding insulation conditions of phase A and phase B are the same, their leakage current at the moment of the PWM voltage step is... mp Both are 58mA. In addition, the C-phase winding A... mp The amplitude is 150mA, significantly larger than the initial oscillation amplitude of the corresponding leakage current in the other two phases. Therefore,

[0099] By comparing the leakage current A corresponding to the voltage step moment of the three-phase PWM, mp To assess the degradation phase of the main insulation.

[0100] Example 1

[0101] The monitoring framework of the insulation condition assessment method proposed in this invention is as follows: Figure 7 As shown, the motor under test is a 3kW vector control permanent magnet synchronous motor. The If value of the variable frequency motor in its healthy state is... g,CM I g,DM and A mp As a reference value, the incremental value of leakage current signal characteristics is monitored in real time to assess the degradation status of the main insulation.

[0102] An adjustable capacitor ranging from 0.1 to 1 nF was connected between the C-phase winding tap and ground to simulate the degradation of the motor's main insulation. Different insulation capacitances ΔC were obtained. g The common-mode leakage current increment at degradation location x is as follows Figure 8 As shown. It can be seen that ΔI g,CM With ΔC g It increases with the increase of, and ΔI g,CM The slope of change is almost the same at different degradation locations x. Therefore, ΔI g,CM Unaffected by x, it can be used to assess the degree of degradation of the main insulation.

[0103] Different insulation capacitances ΔC g The differential-mode leakage current increment ΔI at degradation position x g,DM like Figure 9 As shown, when x is fixed, ΔIg,DM increases with the increase of ΔCg, but its growth slope gradually decreases from its maximum value and approaches zero as x increases. Therefore, ΔI g,DM It contains degradation location information. In this application, the degradation location is determined by the ratio K of the common-mode leakage current increment to the differential-mode leakage current increment, as shown in Equation (14), where the superscript * indicates that the variable is the leakage current measurement result under degradation state.

[0104] (14)

[0105] Different ΔC g The slope K value under x is as follows Figure 9 As shown, the slope K is unaffected by the insulation capacitance at any degradation location x, but it increases linearly with increasing x. Therefore, the value of K is only related to x and can be used to assess the degradation location of the main insulation.

[0106] Different capacitance values ​​of ΔCg were inserted at x=0.5 on phase C winding to simulate the degree of insulation degradation of phase winding, and the initial oscillation amplitude increment ΔA of leakage current was obtained. mp like Figure 11 As shown, where ΔA mp-A,B,C These represent the initial oscillation amplitude increments of the leakage current excited by the three-phase voltage. It can be seen that...

[0107] Degenerate phase ΔA mp-C With ΔC g The value gradually increases with the increase of ΔA, while the healthy phase ΔA mp-A and ΔA mp-B The value compared to ΔA mp-C Smaller, and almost unaffected by ΔC g The impact of this. Therefore, the degradation phase of the main insulation can be compared by ΔA. mp-A,B,C The difference in amplitude between them is used for identification.

[0108] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the phrase "comprising an element defined as..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0109] The above technical solutions only embody the preferred technical solutions of the present invention. Any modifications that may be made by those skilled in the art to certain parts thereof embody the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A method for online assessment of the main insulation condition of a variable frequency motor, characterized in that, The method includes the following steps: Step 1: An equivalent circuit model of stator winding main insulation degradation was established in the low-frequency band. The common-mode frequency and differential-mode frequency were obtained by harmonic analysis of the three-phase voltage in the equivalent circuit model of stator winding main insulation degradation using double Fourier integral. Step 2: Based on the equivalent circuit model of the main insulation degradation of the variable frequency motor, obtain the frequency domain mathematical expression and the time domain mathematical expression of the leakage current. Step 3: Based on the time-domain and frequency-domain mathematical expressions of the leakage current, the degree of degradation, location of degradation, and phase of degradation of the main insulation of the variable frequency motor are evaluated online. The equivalent circuit model for the degradation of the stator winding main insulation in step one is an equivalent parallel circuit of capacitor and resistor, which represent capacitive coupling and dielectric loss, respectively. a,b,c Represents three-phase voltage, R s and L s R represents the stator resistance and inductance, respectively. g and C g The equivalent resistance and capacitance of the main insulation are represented by N, the neutral point, and D, the location of main insulation degradation. x represents the ratio of the number of turns from the input terminal to point D to the number of turns in the phase winding. The three-phase voltages are replaced by equivalent voltage sources, and the three-phase branches are combined to obtain an RLC series circuit, where k, Z, and U... g These represent the stator impedance coefficient, equivalent circuit impedance, and equivalent voltage source to ground, respectively. The specific circuit parameters are as follows: (1) (2) (3)。 2. The method for online assessment of the main insulation status of a variable frequency motor according to claim 1, characterized in that, In step one, a double Fourier integral is used to perform harmonic analysis on the three-phase voltage. The Fourier expression for phase A voltage is as follows: (4) In the formula E d ω is the DC bus voltage, ω0 is the fundamental angular frequency, ω c Where α is the carrier angular frequency, and J is the modulation depth. k(x) Denotes the k-th order Bessel function; According to equation (3), the expression for the equivalent voltage source to ground at the common-mode frequency is as follows: (5) Furthermore, combining equations (3) and (5), the equivalent voltage source to ground at the differential mode frequency is obtained as follows: (6) From equations (5) and (6), it can be seen that the common-mode equivalent voltage to ground U g,CM The common-mode voltage of the phase C winding is equal to that of the phase C winding and is unaffected by the degraded position x, but the differential-mode equivalent voltage to ground U g,DM It decreases linearly with increasing degenerate position x; Based on the inverter's base frequency f0 and switching frequency f c The obtained common-mode frequency and differential-mode frequency are shown in equations (7) and (8), respectively: (7) (8) In the formula, m = 1, 2, 3, ..., n = 0, 1, 2, ..., when n is a multiple of 3, f1 represents the common-mode frequency; otherwise, f1 represents the differential-mode frequency. When 2n+1 is a multiple of 3, f2 represents the common-mode frequency; otherwise, f2 represents the differential-mode frequency.

3. The method for online assessment of the main insulation status of a variable frequency motor according to claim 1, characterized in that, The frequency domain mathematical expression for the leakage current in step two is: (9) In the formula, ω = 2πf0. When f is the common-mode frequency, I... g Indicates common-mode leakage current, otherwise I g Indicates differential mode leakage current; The time-domain mathematical expression for the leakage current is: (10) in style Indicates that it is on the rising edge, when This indicates that it is at the falling edge.

4. The method for online assessment of the main insulation status of a variable frequency motor according to claim 1, characterized in that, In step three, the assessment of the degradation degree of the main insulation of the variable frequency motor is identified by the common-mode leakage current. According to equations (5) and (9), the mathematical expression of the common-mode leakage current is as follows: (11) In the formula, ω = 2mπf c m=1,3,5,…; According to equation (11), the common-mode leakage current I under different insulation capacitances and degradation positions is obtained. g,CM I g,CM For C g Highly sensitive, but almost unaffected by the location of degradation, the degree of primary insulation degradation can be identified using common-mode leakage current.

5. The method for online assessment of the main insulation status of a variable frequency motor according to claim 1, characterized in that, The assessment of the degradation location of the main insulation of the variable frequency motor in step three is performed by combining the common-mode leakage current and the differential-mode leakage current. The mathematical expression of the differential-mode leakage current is obtained through equations (6) and (9) as follows: (12) where ω = 2mπf c + 2πf0, m = 2, 4, 6, …; According to equation (12), the differential mode leakage current I under different insulation capacitances and degradation positions in the frequency band from 8050Hz to 48050Hz is obtained. g,DM I g,DM I decreases linearly with increasing x. g,DM Not only with C g It is related to, and also affected by, x; therefore, the location of primary insulation degradation x can be determined by I. g,DM To identify.

6. The method for online assessment of the main insulation status of a variable frequency motor according to claim 1, characterized in that, In step three, the assessment of the degraded phase of the main insulation of the variable frequency motor is identified by the initial oscillation amplitude of the leakage current, and its mathematical expression is as follows: (13) in style This indicates that the PWM voltage is at the rising edge, when This indicates that it is at the falling edge. From equation (13), we know that when E d R s and L s When fixed, the initial oscillation amplitude A of the leakage current mp Only affected by the insulating capacitance C g And the equivalent circuit impedance coefficient k has an effect; therefore, A mp It can assess the degradation phase of the main insulation.

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