On-line monitoring method of variable frequency motor insulation to ground based on leakage current frequency domain characteristics

Abstract

The present invention discloses an online monitoring method for the insulation of a variable frequency motor to the ground based on the frequency domain characteristics of leakage current, the method comprising the following steps: step 1, establishing a frequency domain mathematical model of leakage current; step 2, implementing online monitoring of the insulation of the variable frequency motor to the ground based on the frequency domain mathematical model of leakage current. The beneficial effect of the present invention is that the decoupling evaluation of the insulation resistance and capacitance to the ground can be realized, and the evaluation result is more accurate.

Description

Technical Field

[0001] The invention relates to the technical field of on-line monitoring of the insulation of a variable frequency motor to the ground, in particular to an on-line monitoring method of the insulation of a variable frequency motor to the ground based on the frequency domain characteristics of leakage current. Background Art

[0002] Variable frequency motors are widely used in the industrial field, and their failures can cause huge economic losses and serious safety issues. The most common failures of variable frequency motors include stator insulation failure, bearing failure, and rotor failure. IEEE reports show that about 30%-40% of AC motor failures are related to stator insulation. The high-frequency, high-amplitude repetitive pulse voltage that variable frequency motors are subjected to accelerates the degradation process of their insulation to the ground. Therefore, real-time online monitoring of the insulation status of variable frequency motors to the ground is crucial to the safe and stable operation of variable frequency motor systems.

[0003] Insulation degradation is a long-term development process that may last for years or even decades. The motor stator insulation is continuously degraded due to factors such as electrical, thermal, and mechanical stress, resulting in a continuous and slow change in insulation impedance. In addition, moisture or surface contamination of the insulation causes the insulation resistance to decrease rapidly while the insulation capacitance to increase rapidly, while insulation delamination causes the insulation capacitance to decrease. Traditional testing methods evaluate the insulation state by measuring parameters such as insulation resistance, insulation capacitance, and polarization index offline. The reduction in insulation electrical strength and the transient overvoltage it withstands are both causes of insulation breakdown. Offline partial discharge (PD) testing or impulse voltage testing can also evaluate the stator insulation state and provide a basis for motor insulation maintenance. However, the offline testing method has the disadvantages of long downtime detection and test intervals. Therefore, researchers have proposed various online monitoring methods to evaluate the stator insulation state in real time, such as partial discharge online monitoring, magnetic signal online monitoring methods, and leakage current online monitoring methods.

[0004] The partial discharge online monitoring method is a widely used online monitoring method for insulation status. This method is often used for high-voltage motor insulation evaluation, but is rarely used for low-voltage motors. Wu Guangning et al. studied the aging mechanism of motor insulation under high-frequency pulse voltage. The results showed that metal chips and air gap defects in the motor insulator will cause electric field distortion, prompting the rapid development of partial discharge. The partial discharge online monitoring method tests the PD pulse signal and extracts the feature sequence to establish a PD feature fingerprint library, and then uses the BP neural network algorithm to evaluate the type and intensity of partial discharge of the motor stator insulation. In addition, the PD ultrasonic array positioning technology uses the ultrasonic signal generated by the partial discharge to locate the partial discharge source. The existing literature proposes a partial discharge positioning system based on a circular ultrasonic array sensor, which can locate the spatial position of the partial discharge source online. However, noise interference and credibility issues seriously limit the practical application of the partial discharge online monitoring method.

[0005] The online insulation monitoring method based on magnetic signals monitors the distortion of the main magnetic flux or leakage magnetic flux of the motor and evaluates the insulation state according to the even harmonic components of the magnetic flux. For example, the leakage magnetic flux of the motor is measured by Rogowski coils, and the insulation state is evaluated according to the changes in the harmonic content of the leakage magnetic flux; or a new magnetic field detection coil is placed in the air gap to distinguish the insulation faults of the stator winding and the rotor winding according to the distortion of the main magnetic flux of the motor; or a software and hardware system based on magnetic signal monitoring of the insulation state of the motor is studied, and a magnetic signal denoising method based on wavelet transform is studied. The motor magnetic signal contains rich insulation state information, but the leakage magnetic flux signal is weak and easily affected by electromagnetic interference. The main magnetic flux signal test requires changing the motor structure, so this method is greatly limited in application.

[0006] The insulation condition of the motor can also be evaluated by measuring the leakage current and calculating the insulation capacitance and dielectric loss factor (capacitance and dissipation factor, C / DF). During the degradation of the motor insulation, the insulation resistance gradually decreases, the insulation capacitance gradually increases, and the time-frequency characteristics of the leakage current flowing through the motor insulation also change continuously. Existing papers Analysis of leakage current in PWM motor system [J]. Transactions of China Electrotechnical Society, 2019, 34(08): 1599-1606. The published literature analyzes the flow path and time-frequency characteristics of the leakage current in the variable frequency motor system, and concludes that the leakage current contains a DC component, a switching frequency component and its multiple frequency component. Lee SB, Younsi K, Kliman G B. An online technique for monitoring the insulation condition of AC machine stator windings [J]. IEEE Transactions on Energy Conversion, 2005, 20(4): 737-745. The published literature proposes an online monitoring method for evaluating the insulation condition of the motor using differential mode (DM) leakage current, which is implemented by calculating C / DF. In view of the limitation that the DM leakage current measurement method can only be applied to six-tap motors, Zhang Pinjia et al. proposed an online monitoring method for motor insulation to ground based on common mode (CM) leakage current, which can be applied to three-tap variable frequency motors. Further research shows that by monitoring CM voltage and DM current, the insulation state of the motor to ground and the insulation state between phases can be evaluated respectively. The researchers analyzed the evolution of the time domain characteristics of the variable frequency motor leakage current as the insulation to ground degrades, and proposed a method for identifying the degraded phase, degradation position and degradation degree of the insulation to ground, but this method cannot achieve the decoupled evaluation of insulation resistance and capacitance.

[0007] Domestic and foreign scholars have conducted a lot of research on online monitoring methods for motor insulation. For example, there is a motor insulation monitoring method based on partial discharge and magnetic signals. However, this method can only evaluate the overall insulation status and cannot be used for decoupling evaluation of insulation resistance and capacitance. Summary of the invention

[0008] The purpose of the present invention is to solve the above problems and to design an online monitoring method for the insulation of a variable frequency motor to the ground based on the frequency domain characteristics of leakage current.

[0009] A method for online monitoring of the insulation of a variable frequency motor to ground based on the frequency domain characteristics of leakage current, the method comprising the following steps:

[0010] Step 1: Establish a frequency domain mathematical model of leakage current:

[0011] The equivalent three-phase square wave voltage is decomposed by Fourier, and the results are as follows:

[0012]

[0013] Where U a ,U b ,U c Respectively represent the three-phase PWM voltage, U dc is the DC bus voltage, a k and b k is the coefficient of the Fourier series, ω 0 =2πf 0 is the fundamental frequency f 0 The corresponding angular frequency, the fundamental frequency is the frequency of the PWM voltage, i.e. the inverter switching frequency, and t is the test time of the electrical signal;

[0014] The leakage current of the variable frequency motor can be expressed as the superposition of the response of the stator winding equivalent circuit under the action of the harmonic components of the three-phase PWM voltage:

[0015]

[0016] In the formula, I leak Indicates the leakage current of the variable frequency motor, i k Represents the harmonic components of leakage current,

[0017] When the insulation between the C-phase stator winding and the ground is degraded, the characteristic equations of the harmonic components of the variable frequency motor leakage current are as follows:

[0018]

[0019] Where U ak ,U bk ,U ck Respectively represent the sinusoidal components of the three-phase PWM voltage, ω=kω 0is the angular frequency corresponding to each sine wave component, R s and L s Respectively represent the stator resistance and inductance of the variable frequency motor, R g and C g Respectively represent the equivalent resistance and capacitance of insulation to ground, U N Represents the sinusoidal component of the neutral point voltage of the variable frequency motor, U D It represents the sinusoidal component of the stator winding voltage at point D where the insulation to ground is degraded, I a , I b and I c Respectively represent the sinusoidal components of the three-phase stator current, I c1 and I c2 Respectively represent the sinusoidal components of the stator winding current on both sides of point D, I k Frequency domain expressions for harmonic components of leakage current;

[0020] According to formula (3), the time domain expression of each harmonic component of the leakage current can be obtained. The time domain expression of the harmonic component of the leakage current is converted into the frequency domain expression as follows:

[0021]

[0022] Step 2: Online monitoring of the insulation of the variable frequency motor to the ground based on the leakage current frequency domain mathematical model:

[0023] According to formula (4) and the simulation results of the amplitude of each harmonic component of the variable frequency motor leakage current, it can be seen that the amplitude of the leakage current harmonic decreases with the increase of the harmonic order, and the amplitude of the harmonic component after the fifth order is very small. Therefore, the DC component, fundamental frequency component and triple frequency component of the leakage current can be selected to characterize the insulation state of the variable frequency motor to the ground; by analyzing the changes in the DC component, fundamental frequency component and triple frequency component in the variable frequency motor leakage current, the change of the variable frequency motor to the ground insulation resistance is evaluated, and then the DC component expression of the leakage current is obtained as follows:

[0024]

[0025] Where U a0 ,U b0 ,U c0 They represent the DC component of the three-phase PWM square wave voltage respectively. It can be seen from formula (5) that the DC component of the leakage current is only related to the insulation resistance and the insulation degradation position, and has nothing to do with the insulation capacitance;

[0026] According to equations (4), (5) and the simulation results of the harmonic components of the leakage current of the variable frequency motor changing with the insulation resistance, it can be seen that the DC component, fundamental frequency component and triple frequency component of the leakage current all increase continuously with the decrease of the insulation resistance, and as the insulation resistance decreases, its influence on the harmonic components of the leakage current becomes more and more significant; when the insulation resistance is very small, the harmonic components of the leakage current increase rapidly with the decrease of the insulation resistance; therefore, by measuring the DC component, fundamental frequency component and triple frequency component of the leakage current, the real-time online monitoring of the insulation status of the variable frequency motor to the ground can be realized.

[0027] Each harmonic component of the variable frequency motor leakage current contains information on insulation resistance to ground, insulation capacitance and insulation degradation position. By measuring and calculating the leakage current harmonic components in real time, the insulation status of the variable frequency motor to ground can be monitored online.

[0028] The insulation capacitance monitoring can be achieved by measuring the fundamental frequency component and the triple frequency component of the variable frequency motor leakage current, and the DC component is independent of the insulation capacitance.

[0029] As the insulation capacitance increases, the harmonic components of the leakage current of the variable frequency motor increase, and the fundamental frequency component and triple frequency component of the leakage current increase approximately linearly, while the DC component remains a constant value.

[0030] The change of the insulation degradation position has no effect on the harmonic components of the leakage current.

[0031] Beneficial Effects

[0032] The on-line monitoring method for ground insulation of a variable frequency motor based on the leakage current frequency domain characteristics produced by the technical solution of the present invention has the following advantages:

[0033] 1. This method can realize the decoupling evaluation of ground insulation resistance and capacitance by monitoring the frequency domain characteristics of leakage current, through the characteristics that the fundamental frequency component and triple frequency component of the variable frequency motor leakage current increase with the decrease of insulation resistance and the increase of insulation capacitance, and the DC component increases with the decrease of insulation resistance and has nothing to do with insulation capacitance;

[0034] 2. Since the DC component, fundamental frequency component and triple frequency component of the variable frequency motor leakage current are independent of the insulation degradation location, the ground insulation monitoring method proposed in this application is not affected by the insulation degradation location, and the evaluation result is more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a flow chart of the method for online monitoring of the insulation of a variable frequency motor to ground based on the frequency domain characteristics of leakage current according to the present invention;

[0036] Figure 2 is a graph of the amplitudes of the harmonic components of the leakage current of the present invention;

[0037] Figure 3is a simulation result diagram of the leakage current component according to the present invention changing with the insulation resistance;

[0038] Figure 4 is a simulation result diagram of the leakage current component according to the present invention changing with the insulation capacitance;

[0039] Figure 5 is a simulation result diagram of the leakage current component according to the present invention changing with the insulation degradation position;

[0040] Figure 6 It is a wiring diagram of the experimental platform of the present invention;

[0041] Figure 7 is a spectrum diagram of leakage current under different ground insulation states of the present invention;

[0042] Figure 8 is a graph showing the experimental results of leakage current components under different insulation resistances according to the present invention;

[0043] Fig. 9 is a graph showing the experimental results of leakage current components under different insulation capacitances according to the present invention;

[0044] Fig.10 is a graph of experimental results of the leakage current component of the present invention at different degradation positions; DETAILED DESCRIPTION

[0045] The present invention will be described in detail below in conjunction with the accompanying drawings. Figure 1-10 As shown;

[0046] The equivalent three-phase square wave voltage is decomposed by Fourier, and the results are as follows:

[0047]

[0048] Where U a ,U b ,U c Respectively represent the three-phase PWM voltage, U dc is the DC bus voltage, a k and b k is the coefficient of the Fourier series, ω 0 =2πf 0 is the fundamental frequency f 0 The corresponding angular frequency, the fundamental frequency is the frequency of the PWM voltage, that is, the inverter switching frequency.

[0049] The leakage current of the variable frequency motor can be expressed as the superposition of the response of the stator winding equivalent circuit under the action of the harmonic components of the three-phase PWM voltage:

[0050]

[0051] In the formula, I leakIndicates the leakage current of the variable frequency motor, i k Indicates the harmonic components of leakage current.

[0052] When the insulation between the C-phase stator winding and the ground is degraded, the characteristic equations of the harmonic components of the variable frequency motor leakage current are as follows:

[0053]

[0054] Where U ak ,U bk ,U ck Respectively represent the sinusoidal components of the three-phase PWM voltage, ω=kω 0 is the angular frequency corresponding to each harmonic component.

[0055] According to formula (3), the time domain expression of each harmonic component of the leakage current can be obtained. The time domain expression of the harmonic component of the leakage current is converted into the frequency domain expression as follows:

[0056]

[0057] It can be seen from formula (4) that the harmonic components of the variable frequency motor leakage current contain the ground insulation resistance, insulation capacitance and insulation degradation location information. Therefore, by measuring and calculating the harmonic components of the leakage current in real time, the ground insulation status of the variable frequency motor can be monitored online.

[0058] According to formula (4), the simulation results of the amplitude of each harmonic component of the variable frequency motor leakage current are as follows: Figure 2 As shown in the figure, the amplitude of the leakage current harmonic decreases with the increase of the harmonic order, and the amplitude of the fifth and above harmonic components is very small. Therefore, the DC component, fundamental frequency component and triple frequency component of the leakage current are selected to characterize the insulation state of the variable frequency motor to the ground.

[0059] The insulation state to ground is mainly characterized by insulation resistance and insulation capacitance. By analyzing the changes in the DC component, fundamental frequency component and triple frequency component in the variable frequency motor leakage current, the changes in the variable frequency motor insulation resistance to ground can be evaluated. The expression of the DC component of the leakage current is as follows:

[0060]

[0061] Where U a0 ,U b0 ,U c0 They represent the DC component of the three-phase PWM square wave voltage. It can be seen from formula (5) that the DC component of the leakage current is only related to the insulation resistance and the insulation degradation position, and has nothing to do with the insulation capacitance.

[0062] According to equations (4) and (5), the simulation results of the harmonic components of the variable frequency motor leakage current changing with the insulation resistance are as follows: Figure 3As shown in the figure. The DC component, fundamental frequency component and triple frequency component of the leakage current all increase as the insulation resistance decreases, and as the insulation resistance decreases, their influence on the harmonic component of the leakage current becomes more and more significant. When the insulation resistance is very small, the harmonic components of the leakage current increase rapidly as the insulation resistance decreases. Therefore, by analyzing the changing trend of the harmonic components of the leakage current, the insulation status of the variable frequency motor to the ground can be evaluated, insulation breakdown can be warned, and the insulation failure of the motor to the ground can be avoided.

[0063] Insulation capacitance monitoring can be achieved by measuring the fundamental frequency component and triple frequency component of the variable frequency motor leakage current. The DC component has nothing to do with the insulation capacitance. The simulation results of the harmonic components of the variable frequency motor leakage current changing with the insulation capacitance are shown in Figure 2. Figure 4 As shown in Figure 2, as the insulation capacitance increases, the fundamental frequency component and triple frequency component of the leakage current increase approximately linearly, while the DC component remains a constant value.

[0064] According to the stator winding and ground insulation equivalent circuit, the variable frequency motor leakage current is the sum of the leakage current flowing through the ground insulation resistance and the insulation capacitance. The leakage current components flowing through the ground insulation resistance mainly include DC component, fundamental frequency component and triple frequency component. As the ground insulation degrades, the insulation resistance decreases, its equivalent impedance relative to the leakage current components decreases, and the amplitude of the leakage current components increases accordingly. For the DC component of the leakage current, the equivalent capacitive reactance X of the ground insulation capacitance is c =1 / jwC g is infinite, the DC component of the leakage current does not flow through the insulation capacitance to ground, so the DC component of the leakage current is not affected by the insulation capacitance to ground. For the fundamental frequency component and triple frequency component of the leakage current, the equivalent capacitive reactance of the insulation capacitance to ground gradually decreases as the insulation capacitance increases, and the amplitude of the fundamental frequency component and triple frequency component of the leakage current increases accordingly.

[0065] Based on the above analysis, by measuring the DC component, fundamental frequency component and triple frequency component of the leakage current, real-time online monitoring of the insulation status of the variable frequency motor to the ground can be achieved. In addition, the DC component of the leakage current increases as the insulation resistance decreases, but has nothing to do with the insulation capacitance. Therefore, the decoupled evaluation of the insulation resistance and insulation capacitance can be achieved.

[0066] According to formula (4), the change of insulation degradation position has almost no effect on the harmonic components of leakage current. The simulation results of the DC component, fundamental frequency component and triple frequency component of the leakage current of the variable frequency motor with the change of insulation degradation position are shown in Figure 5 As shown in the figure, as the insulation degradation position changes, the DC component and triple frequency component of the leakage current do not change, while the change degree of the fundamental frequency component is very small and can be almost ignored. Therefore, the ground insulation status monitoring method proposed in this paper is not affected by the ground insulation degradation position.

[0067] Example 1

[0068] In order to verify the proposed variable frequency motor insulation status monitoring method, an experimental platform was built. The wiring taps were drawn out from different positions of the stator winding of the faulty prototype and fixed in the terminal box. An RC parallel circuit with adjustable impedance was connected between the wiring taps and the ground to simulate the insulation degradation to the ground. Figure 6 The wiring diagram of the experimental platform is shown in Figure 1. The current sensor is installed at the motor input terminal to measure the leakage current. g and insulation capacitance C g Represents the parameters of the RC regulation circuit.

[0069] Figure 7 The figure is the spectrum of the leakage current of the variable frequency motor under different insulation conditions to the ground. The leakage current spectrum mainly contains DC components and high-frequency components, among which the high-frequency components are mainly the 4kHz fundamental frequency component and its multiple frequency components. When the insulation capacitance increases from 0 to 1.5nF, the amplitudes of the DC component, fundamental frequency component and triple frequency component of the leakage current of the variable frequency motor increase significantly. When the insulation resistance decreases from 100kΩ to 4kΩ, the amplitudes of the DC component, fundamental frequency component and triple frequency component of the leakage current also increase significantly, among which the fundamental frequency component changes most significantly. The experimental results show that when the insulation of the variable frequency motor to the ground degrades, the harmonic components of the leakage current will change accordingly, and the change law is consistent with the theoretical analysis results.

[0070] In order to verify the proposed variable frequency motor insulation status monitoring method, the leakage current under different insulation resistance and capacitance is tested and the spectrum analysis is performed. The experimental results of the harmonic components of the leakage current under different insulation resistance and insulation capacitance are as follows: Figure 8 and Fig. 9 shown. Figure 8 The DC component, fundamental frequency component and triple frequency component of the leakage current increase as the insulation resistance decreases, and the smaller the insulation resistance, the more significant its impact on the leakage current component. Fig. 9 The fundamental frequency component and triple frequency component of the leakage current increase with the increase of insulation capacitance, while the DC component is approximately constant. The variation law of each harmonic component of the leakage current in the experimental results is consistent with the theoretical analysis results.

[0071] Fig.10 The experimental results of the harmonic components of the leakage current at different insulation degradation positions show that the DC component, fundamental frequency component and triple frequency component of the leakage current do not change significantly with the change of insulation degradation position. Therefore, the insulation degradation position to the ground will not affect the insulation status monitoring method proposed in this paper, which is consistent with the theoretical analysis results.

[0072] When implementing the proposed insulation monitoring method, current transformers and sampling circuits are used to test and sample leakage current, and Fourier transform is performed in DSP to obtain the amplitudes of each component of the leakage current, thereby achieving decoupling evaluation of insulation resistance and capacitance to ground, providing guidance for variable frequency motor insulation maintenance.

[0073] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions. The sentence "includes an element defined by ... does not exclude the existence of other identical elements in the process, method, article or device including the element".

[0074] The above technical solutions only reflect the preferred technical solutions of the technical solutions of the present invention. Some changes that may be made to certain parts thereof by technicians in this technical field all reflect the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A method for online monitoring of the insulation of a variable frequency motor to the ground based on the frequency domain characteristics of leakage current, characterized in that: The method comprises the following steps: Step 1: Establish a frequency domain mathematical model of leakage current: The equivalent three-phase square wave voltage is decomposed by Fourier, and the results are as follows: Where U a ,U b ,U c Respectively represent the three-phase PWM voltage, U dc is the DC bus voltage, a k and b k is the coefficient of the Fourier series, ω0=2πf0 is the angular frequency corresponding to the fundamental frequency f0, the fundamental frequency is the frequency of the PWM voltage, that is, the inverter switching frequency, and t is the test time of the electrical signal; The leakage current of the variable frequency motor can be expressed as the superposition of the response of the stator winding equivalent circuit under the action of the harmonic components of the three-phase PWM voltage: In the formula, I leak Indicates the leakage current of the variable frequency motor, i k Indicates the harmonic components of leakage current; When the insulation between the C-phase stator winding and the ground is degraded, the characteristic equations of the harmonic components of the variable frequency motor leakage current are as follows: Where U ak ,U bk ,U ck They represent the sine wave components of the three-phase PWM voltage respectively, ω=kω0 is the angular frequency corresponding to each sine wave component, R s and L s Respectively represent the stator resistance and inductance of the variable frequency motor, R g and C g Respectively represent the equivalent resistance and capacitance of insulation to ground, U N Represents the sinusoidal component of the neutral point voltage of the variable frequency motor, U D It represents the sinusoidal component of the stator winding voltage at point D where the insulation to ground is degraded, I a , I b and I c Represent the sinusoidal components of the three-phase stator current, I c1 and I c2 Respectively represent the sinusoidal components of the stator winding current on both sides of point D, I k Frequency domain expressions for harmonic components of leakage current; According to formula (3), the time domain expression of each harmonic component of the leakage current can be obtained. The time domain expression of the harmonic component of the leakage current is converted into the frequency domain expression as follows: Step 2: Online monitoring of the insulation of the variable frequency motor to the ground based on the leakage current frequency domain mathematical model: According to formula (4) and the simulation results of the amplitude of each harmonic component of the variable frequency motor leakage current, it can be seen that the amplitude of the leakage current harmonic decreases with the increase of the harmonic order, and the amplitude of the harmonic component after the fifth order is very small. Therefore, the DC component, fundamental frequency component and triple frequency component of the leakage current can be selected to characterize the insulation state of the variable frequency motor to the ground; by analyzing the changes in the DC component, fundamental frequency component and triple frequency component in the variable frequency motor leakage current, the change of the variable frequency motor to the ground insulation resistance is evaluated, and then the DC component expression of the leakage current is obtained as follows: Where U a0 ,U b0 ,U c0 They represent the DC component of the three-phase PWM square wave voltage, and I0 is the DC component of the variable frequency motor leakage current. It can be seen from formula (5) that the DC component of the leakage current is only related to the insulation resistance and the insulation degradation position, and has nothing to do with the insulation capacitance; According to equations (4), (5) and the simulation results of the harmonic components of the leakage current of the variable frequency motor changing with the insulation resistance, it can be seen that the DC component, fundamental frequency component and triple frequency component of the leakage current all increase continuously with the decrease of the insulation resistance, and as the insulation resistance decreases, its influence on the harmonic components of the leakage current becomes more and more significant; when the insulation resistance is very small, the harmonic components of the leakage current increase rapidly with the decrease of the insulation resistance; therefore, by measuring the DC component, fundamental frequency component and triple frequency component of the leakage current, the real-time online monitoring of the insulation status of the variable frequency motor to the ground can be realized.

2. The method for online monitoring of the insulation of a variable frequency motor to the ground based on the frequency domain characteristics of leakage current according to claim 1 is characterized in that: Each harmonic component of the variable frequency motor leakage current contains information on insulation resistance to ground, insulation capacitance and insulation degradation position. By measuring and calculating the leakage current harmonic components in real time, the insulation status of the variable frequency motor to ground can be monitored online.

3. The method for online monitoring of the insulation of a variable frequency motor to ground based on the frequency domain characteristics of leakage current according to claim 1 is characterized in that: The insulation capacitance monitoring can be achieved by measuring the fundamental frequency component and the triple frequency component of the variable frequency motor leakage current, and the DC component is independent of the insulation capacitance.

4. The method for online monitoring of the insulation of a variable frequency motor to ground based on the frequency domain characteristics of leakage current according to claim 3 is characterized in that: As the insulation capacitance increases, the harmonic components of the leakage current of the variable frequency motor increase, and the fundamental frequency component and triple frequency component of the leakage current increase approximately linearly, while the DC component remains a constant value.

5. The method for online monitoring of the insulation of a variable frequency motor to ground based on the frequency domain characteristics of leakage current according to claim 1, characterized in that: The change of the insulation degradation position has no effect on the harmonic components of the leakage current.

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