Method for monitoring insulation state of stator winding of injection unit based on neutral point harmonic frequency sweep

Abstract

The invention discloses a method for monitoring the insulation state of a stator winding of an injection unit based on neutral point harmonic frequency sweeping. The invention provides an active online monitoring method based on harmonic injection for stator winding insulation of a full-power variable-speed pumped storage unit. A common-mode loop is constructed for a motor stator winding of a network side transformer neutral point ungrounded system through a motor neutral point grounded transformer, and therefore online monitoring of the insulation state of a motor stator is achieved. In order to solve the measurement problem of the full-power variable-speed pumped storage unit under the large-current working condition, a neutral point outgoing line current detection technology is adopted. A machine end large current interference area is avoided, monitoring is carried out on the primary side of the grounding transformer, transmission errors are avoided, and the measurement precision and reliability of insulation monitoring signals are remarkably improved. According to the invention, harmonic injection is carried out by avoiding an inherent common-mode component of a unit, a monitorable leakage current signal is enhanced, and the accuracy of insulation equivalent capacitance calculation is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of full-power variable-speed pumped storage unit, and particularly relates to a method for monitoring insulation state of stator winding of unit based on neutral point harmonic sweep injection. BACKGROUND

[0002] At present, the full-power variable-speed pumped storage technology system of hundreds of megawatts in China is not perfect, and its insulation state monitoring technology is directly related to the reliability and safety of unit operation. Compared with the alternating current excitation variable-speed technology, the full-power variable-speed technology has the advantages of simple structure and wide power regulation range, but due to the changeable operating condition and complex mechanical stress, the insulation aging problem is more prominent. The traditional insulation monitoring method mainly relies on redundant design or relay protection device, which can only respond passively after the insulation fault occurs, and cannot realize early warning of insulation aging. Although the existing offline detection method can evaluate the insulation state, it needs to stop operation, which is difficult to meet the demand of continuous operation of the unit.

[0003] In the neutral point ungrounded system of the network side transformer, real-time monitoring of the insulation state of the motor stator winding faces greater challenges. Due to the low harmonic potential content in the system, the common-mode loop of leakage current constructed by relying on the characteristics of the system has poor reliability, and the conventional monitoring method cannot effectively capture the early characteristics of insulation deterioration. The weak capacitive current change generated in the early stage of insulation aging is easily covered by system noise, resulting in insufficient monitoring sensitivity.

[0004] With large-scale grid connection of new energy, pumped storage units need to frequently adjust power to balance the fluctuation of power grid, which further accelerates the aging of insulation materials. The existing technology cannot realize online monitoring and accurate evaluation of insulation state, which brings potential risks to the long-term stable operation of the unit.

[0005] The existing technical solutions have the following technical defects: The data-driven monitoring method needs to collect a large amount of equipment field operation data, especially the operation data of the equipment before failure for algorithm training. This method has high cost of data collection and algorithm training.

[0006] The existing variable-frequency motor stator winding insulation monitoring method is only applicable to motors with common-mode loop, and cannot be directly applied to the neutral point ungrounded system of the network side transformer without common-mode loop; and for large-capacity units, the actual measurement of high-precision current sensors is difficult to realize due to the large rated current and the layout of phase lines.

[0007] The relay protection method of the generator stator winding single-phase grounding protection based on the third harmonic potential can realize rapid cutting after the insulation damage causes a fault, but it is difficult to perform online monitoring and fault early warning on the insulation state of the motor stator winding. The leakage current signal amplitude in the common mode loop constructed by using the third harmonic of the system itself is not high, and the current sensor accuracy is required to be high.

[0008] The harmonic injection method in the relay protection system is generally injected from the line entry end, and is mainly used for selecting and positioning the fault branch for the ground fault. The injection signal cannot be effectively coupled to the internal motor, and cannot perform online monitoring and early warning of the stator winding insulation state in the internal motor. SUMMARY

[0009] The present application aims to at least solve one of the technical problems in the related art to some extent.

[0010] To this end, the present application proposes an online monitoring method for the insulation state of the stator winding of a large-capacity generator set which can be applied to a neutral point ungrounded system of a network side transformer. The method is based on the method of harmonic sweep injection of the motor neutral point, establishes a reliable common mode monitoring loop, and excludes the common mode voltage interference of the motor stator winding itself, and is not affected by the high-frequency switching harmonics of the frequency converter.

[0011] In view of the problem that the full-power variable-speed pumped storage unit generally has large capacity and is not easy to realize online measurement, a method of using a grounding transformer for harmonic injection and monitoring on the neutral point lead-out line is proposed, which can realize online measurement of the insulation state of the stator winding when the load current is large.

[0012] Another object of the present application is to propose an online monitoring device for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on neutral point harmonic sweep injection.

[0013] To achieve the above object, the present application proposes, in one aspect, an online monitoring method for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on neutral point harmonic sweep injection, comprising: Measuring the neutral point voltage on the neutral point lead-out line of the target motor, the neutral point voltage of the motor being the common mode voltage of the motor; Connecting the neutral line of the target motor to the grounding transformer, installing a through-type high-precision current transformer in the middle part, and making the neutral point lead-out line pass through the current transformer to measure the neutral point current; Performing FFT analysis on the common mode voltage, and dividing the frequency into 10 frequency intervals to obtain 10 frequency points; Using a signal generator to emit a sinusoidal signal according to the frequency points, inputting the power amplifier to obtain an amplified signal; According to the neutral point voltage and the neutral point current, an algorithm is executed to calculate insulation equivalent parameters of the full-power variable-speed pumped storage unit, for online monitoring of the insulation state of the stator winding.

[0014] In addition, the online monitoring method for the insulation state of the stator winding of the full-power variable-speed pumped storage unit based on the neutral point harmonic sweep injection according to the above-mentioned embodiment of the present application can further have the following additional technical features: Further, in an embodiment of the present application, the grounding transformer comprises: in the neutral point lead, the primary side is connected in series with the grounding transformer circuit to ground, and a common mode loop of the stator winding is constructed to provide a conduction loop for the leakage current; the secondary side is connected to the power amplifier to realize signal injection, and the primary side is connected to the neutral point lead of the motor to monitor the voltage and current signals.

[0015] Further, in an embodiment of the present application, the FFT analysis of the common mode voltage and the average division of the frequency into 10 frequency intervals to obtain 10 frequency points comprise: the measured common mode voltage is subjected to FFT analysis, and is averagely divided into 10 frequency intervals from the fundamental frequency to the upper limit of the frequency of the grounding transformer; one frequency point with the minimum voltage component in each interval is selected as the frequency of the output signal of the signal generator.

[0016] Further, in an embodiment of the present application, the use of the signal generator to emit sinusoidal signals according to the frequency points, input the power amplifier, and obtain amplified signals comprises: sinusoidal signals are generated according to the selected 10 frequency points and are injected into the power amplifier by sweep injection; the time window of signal injection at each frequency is ts, and a time window of ts is left empty without injection, the original three-phase voltage of the unit is measured, and the next sweep frequency is selected.

[0017] Further, in an embodiment of the present application, the method further comprises: the calculation formula of the total insulation equivalent capacitance of the unit is: the total insulation equivalent capacitance of the unit at different frequency points obtained by measurement and FFT transformation is averaged after removing the obvious difference points, and the average value is taken as the final result of the total insulation equivalent capacitance of the unit; when the final result of the total insulation equivalent capacitance of the unit deviates from the initial value by more than a preset threshold value during the operation of the target unit, an alarm signal is issued, and it is considered that the insulation of the stator winding or the insulation of the equipment at the unit end has reached a critical value.

[0018] To achieve the above-mentioned purpose, another aspect of the present application proposes an online monitoring device for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on neutral point harmonic sweep injection, comprising: a neutral point voltage measurement module, which measures the neutral point voltage at the neutral point lead of the target motor, and the neutral point voltage of the motor is the common mode voltage of the motor; A neutral point current measurement module connects the neutral line of the target motor to a grounding transformer, installs a high-precision through-type current transformer in the middle part, and makes the neutral point lead pass through the current transformer to measure the neutral point current. An FFT and frequency selection module performs FFT analysis on the common mode voltage, and divides the frequency into 10 frequency intervals to obtain 10 frequency points. A signal generation and amplification module uses a signal generator to emit a sinusoidal signal according to the frequency points, and inputs the power amplifier to obtain an amplified signal. A calculation and monitoring module calculates the insulation equivalent parameters of the full-power variable-speed pumped storage unit according to the neutral point voltage and the neutral point current, and executes an algorithm to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit for online monitoring of the insulation state of the stator winding.

[0019] The full-power variable-speed pumped storage unit stator winding insulation state online monitoring method and device based on neutral point harmonic sweep injection can enhance the signal-to-noise ratio of the leakage current under the condition of large load current, avoid the transmission error, and realize more reliable online monitoring of the stator insulation information.

[0020] The beneficial effects of the present application are: 1) According to the characteristics of the stator winding of the full-power variable-speed pumped storage unit in the neutral point ungrounded system of the network side transformer, a neutral point grounding transformer and a harmonic injection system that can provide a stator insulation leakage current conduction loop are designed, the selection of the injection frequency excludes the influence of the inherent common mode component of the unit, and high-precision online measurement of the stator winding leakage current is realized.

[0021] 2) It is suitable for large-capacity units of full-power variable-speed pumped storage, can enhance the signal-to-noise ratio of the leakage current under the condition of large load current, avoid the transmission error, and realize more reliable online monitoring of the stator insulation information.

[0022] Additional aspects and advantages of the present application will be partially given in the following description, partially will become obvious from the following description, or will be understood by the practice of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0023] The above and / or additional aspects and advantages of the present application will become apparent and easily understood from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which: Figure 1 is the equivalent circuit diagram of the existing generator third harmonic electromotive force and ground capacitance; Figure 2 is the equivalent circuit diagram of the existing generator single-phase grounding third harmonic electromotive force distribution; Figure 3 is the curve diagram of the existing neutral point voltage and terminal voltage with the change of the fault point; Figure 4 is a schematic diagram of the principle of the existing S injection method using voltage transformer injection signals; Figure 5 is a flow chart of a neutral point harmonic sweep injection-based online monitoring method for the insulation state of a stator winding of a full-power variable-speed pumped storage unit according to an embodiment of the present application; Figure 6 is a system structure diagram of a neutral point harmonic sweep injection-based online monitoring method for the insulation state of a stator winding of a full-power variable-speed pumped storage unit according to an embodiment of the present application; Figure 7 is a common-mode equivalent circuit of a unit according to an embodiment of the present application; Figure 8 is a flow chart of a neutral point harmonic sweep injection-based online monitoring method for the insulation state of a unit according to an embodiment of the present application; Figure 9 is a structure diagram of a neutral point harmonic sweep injection-based online monitoring device for the insulation state of a stator winding of a full-power variable-speed pumped storage unit according to an embodiment of the present application. DETAILED DESCRIPTION

[0024] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0025] In order for those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor should be within the scope of protection of the present application.

[0026] A neutral point harmonic sweep injection-based online monitoring method and device for the insulation state of a stator winding of a full-power variable-speed pumped storage unit according to an embodiment of the present application will be described below with reference to the accompanying drawings.

[0027] A method for monitoring leakage current and calculating insulation equivalent capacitance through common-mode voltage generated by a frequency converter has been successfully applied to the insulation monitoring of a motor stator winding in a neutral point grounding system of a network-side transformer. However, in a non-grounding system, there is a lack of necessary common-mode leakage current loop. At the same time, the large load current, thick wire diameter and wide spacing of the phase lines of the unit end make the installation and detection of high-precision current sensors face great challenges. Therefore, this method cannot be directly applied to the online monitoring of the insulation state of the stator winding of a variable-speed pumped storage unit in a neutral point non-grounding system of a network-side transformer.

[0028] For the insulation fault of generator stator winding, the current engineering practice mainly relies on the relay protection device to deal with the short circuit accident. The third harmonic voltage has been used to construct the single-phase grounding protection system of generator stator winding. Due to the combined effect of the non-sinusoidal distribution of generator air gap flux density and the core magnetic saturation effect, the induced electromotive force of stator winding contains high harmonic components. Among them, the third harmonic component has common mode characteristics, which is canceled in the online electromotive force, but still exists in the phase electromotive force.

[0029] When the generator adopts the neutral point ungrounded operation mode, its equivalent circuit is shown in Figure 1 . The self-ground capacitance C f of each phase of the generator is equivalent to be concentrated between the neutral point N and the machine end S, while the self-ground capacitance C w of each phase of the generator machine end outlet electrical equipment (including lead-out wire, step-up transformer, plant transformer and voltage transformer, etc.) is equivalent to be concentrated at the machine end.

[0030] The third harmonic voltage of the neutral point and the machine end is (1) (2) The ratio of the third harmonic voltage U S3 of the machine end to the third harmonic voltage U N3 of the neutral point is (3) When the metallic single-phase grounding of the generator stator winding occurs, it is assumed that the grounding occurs at a distance of from the neutral point, as shown in Figure 2 .

[0031] At this time, there are and , and by comparison, we get (4) Figure 3 The curves of the third harmonic voltage of the neutral point and the machine end with the fault position are shown. Based on this characteristic, the is used as the action quantity, and the is used as the braking quantity to construct the grounding protection criterion . According to (3), the protection does not act under normal operation, but has very high sensitivity to grounding faults near the neutral point. Specifically, the third harmonic protection can effectively cover the single-phase grounding fault in the neutral point side area, and the closer to the neutral point, the higher the sensitivity; the grounding protection composed of the fundamental zero sequence voltage can reflect the The single-phase ground fault in the range is detected, and the sensitivity is improved as the fault point is close to the machine end.

[0032] The signal injection method is also commonly used in the relay protection system for ground line selection and positioning protection. Figure 4

[0033] In addition to the TV signal injection, the signal can also be injected through the auxiliary coil of the arc suppression coil, the neutral point of the star-connected three-phase capacitor, and the like, and the basic principle is similar. Figure 4 After the single-phase ground fault occurs in the system, the injection signal circuit automatically injects a diagnostic signal into the TV secondary winding of the fault phase according to the change of the TV secondary voltage, as shown by the dashed line 1 in FIG.

[0034] Figure 5 FIG. 1 is a flowchart of a neutral point harmonic sweep injection-based online monitoring method for the insulation state of a full-power variable-speed pumped storage unit stator winding according to an embodiment of the present application.

[0035] As shown in FIG. Figure 5 , the method includes but is not limited to the following steps: S1, a neutral point voltage is measured by leading out a line from the neutral point of a target motor, and the neutral point voltage of the motor is a common-mode voltage of the motor; S2, the neutral line of the target motor is connected to a grounding transformer, a high-precision current transformer is installed in the middle part, the neutral point lead-out line passes through the current transformer, and a neutral point current is measured; S3, the common-mode voltage is subjected to FFT analysis, and the frequency is evenly divided into 10 frequency intervals to obtain 10 frequency points; S4, a signal generator is used to emit a sinusoidal signal according to the frequency points, and the signal is input into a power amplifier to obtain an amplified signal; S5, according to the neutral point voltage and the neutral point current, an algorithm is executed to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit, which are used for online monitoring of the insulation state of the stator winding.

[0036] ​The online monitoring method for the insulation state of the stator winding of the full-power variable-speed pumped storage generator set based on neutral point harmonic sweep injection can enhance the signal-to-noise ratio of the leakage current under the condition that the load current is large, avoids the transmission error, and realizes more reliable online monitoring of the stator insulation information. The harmonic injection is avoided to the inherent common-mode component of the unit, the monitorable leakage current signal is enhanced, and the accuracy of the insulation equivalent capacitance calculation is ensured.

[0037] The online monitoring method for the insulation state of the stator winding of the full-power variable-speed pumped storage generator set based on neutral point harmonic sweep injection will be described in detail below with reference to the accompanying drawings.

[0038] First, the following symbols are explained: : a selected common-mode frequency component in the voltage on the primary side of the grounding transformer; : a selected common-mode frequency component in the current on the primary side of the grounding transformer; : the total capacitance of the machine end equipment to ground (mainly the coupling capacitance of the frequency converter and the secondary side of the network-side transformer to ground); : the total capacitance of the stator winding to ground; : the leakage current to ground generated by the insulation of the stator winding; : the leakage current to ground generated by the insulation of the machine end equipment; : the parameters of the neutral point grounding transformer; : 10 selected frequency points; : the total insulation equivalent capacitance of the unit.

[0039] Specifically, the specific implementation steps of the present application, as shown in Figure 6 , include: 1. Neutral point voltage measurement module: measure the neutral point voltage on the neutral point lead-out line of the motor. The neutral point voltage of the motor is the common-mode voltage of the motor.

[0040] 2. Neutral point current measurement module: install a through-type high-precision current transformer in the middle part of the neutral point to the grounding transformer, and make the neutral point lead-out line pass through the current transformer.

[0041] 3. FFT and frequency selection: perform FFT analysis on the measured common-mode voltage of the neutral point, divide the frequency range from the fundamental frequency f0 to the upper limit fmax (about 100 Hz) of the frequency of the grounding transformer into 10 frequency intervals, select a frequency with the smallest voltage component in each interval as the frequency of the output signal of the signal generator.

[0042] 4. Grounding transformer: On the neutral point lead, the primary side is connected in series with the grounding transformer circuit for grounding, and the common mode loop of the stator winding is constructed to provide a conduction loop for the leakage current. The primary and secondary sides of the transformer ensure electrical isolation, and the secondary side is connected to the power amplifier to realize signal injection. The primary side is connected to the neutral point lead of the motor to monitor the voltage and current signals, which can avoid the transformation error of the grounding transformer.

[0043] 5. Signal generator: A sinusoidal signal is generated according to the selected 10 frequency points and is swept to inject. The time window for signal injection at each frequency is ts, and another ts time window is left empty for measurement of the original three-phase voltage of the unit to select the next sweep frequency. Therefore, the length of a monitoring period is Ts = 11ts.

[0044] 6. Power amplifier: The power of the signal generator output signal is amplified to make the current on the primary side of the grounding transformer, i.e. the neutral point lead, within the range of the high-precision current transformer.

[0045] 7. Other calculation modules: According to the signals measured by the voltage and current measurement modules, algorithms are executed to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit for online monitoring of the insulation state of the stator winding The related principles of the present application are as follows: The injected harmonic voltage is used for online insulation monitoring. Since the network side transformer of the power grid is not grounded, the primary side capacitor of the power transformer cannot be coupled to the secondary side after the neutral point of the motor is grounded through the grounding transformer. Since the injected is a common mode voltage, the voltage drop along the winding is very small and can be ignored. The insulation resistance is usually very large and can also be ignored. After harmonic injection, the common mode equivalent circuit of the unit is as shown in Figure 7 .

[0046] Therefore, the calculation formula of the total insulation equivalent capacitance of the unit is:

[0047] The Call at different frequencies fn obtained by measurement and FFT is averaged after removing the obvious difference points, which is the final result of the total insulation equivalent capacitance of the unit. When the final calculation value of the total insulation equivalent capacitance of the unit deviates from the initial value by more than a certain threshold during the operation of the measured full-power variable-speed pumped storage unit, an alarm signal will be sent, indicating that the insulation of the stator winding or the insulation of the machine terminal equipment of the current unit has reached the critical value.

[0048] The flowchart of each monitoring period is shown in 8.

[0049] In order to realize the above-mentioned embodiments, as Figure 9As shown, the embodiment also provides an online monitoring device 10 for insulation state of stator winding of full-power variable-speed pumped storage unit based on neutral point harmonic sweep injection, which comprises a neutral point voltage measurement module 100, a neutral point current measurement module 200, an FFT and frequency selection module 300, a signal generation and amplification module 400, and a calculation and monitoring module 500.

[0050] The neutral point voltage measurement module 100 measures the neutral point voltage of the target motor through the neutral point lead-out wire, and the neutral point voltage of the motor is the common-mode voltage of the motor. The neutral point current measurement module 200 connects the neutral line of the target motor to the grounding transformer, installs a through-type high-precision current transformer in the middle part, and makes the neutral point lead-out wire pass through the current transformer to measure the neutral point current. The FFT and frequency selection module 300 performs FFT analysis on the common-mode voltage and divides the frequency into 10 frequency intervals to obtain 10 frequency points. The signal generation and amplification module 400 uses a signal generator to send a sinusoidal signal according to the frequency points, inputs a power amplifier, and obtains an amplified signal. The calculation and monitoring module 500 calculates the insulation equivalent parameters of the full-power variable-speed pumped storage unit according to the neutral point voltage and the neutral point current, and executes an algorithm to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit for online monitoring of the insulation state of the stator winding.

[0051] Further, the neutral point voltage measurement module 100 is also used for: The primary side of the neutral point lead-out wire is connected to the grounding transformer circuit for grounding to construct a common-mode loop of the stator winding and provide a conduction loop for the leakage current. The secondary side is connected to the power amplifier to realize signal injection, and the primary side is connected to the motor neutral point lead-out wire to monitor the voltage and current signals.

[0052] Further, the FFT and frequency selection module 300 is also used for: The measured common-mode voltage is subjected to FFT analysis, and is divided into 10 frequency intervals from the base frequency to the upper limit frequency of the grounding transformer, and each interval selects a frequency point with the smallest voltage component as the frequency of the output signal of the signal generator.

[0053] Further, the signal generation and amplification module 400 is used for: Generating sinusoidal signals according to the selected 10 frequency points and sweeping injection into the power amplifier; The time window of signal injection at each frequency is ts, and another ts time window is left empty without injection, the original three-phase voltage of the unit is measured, and the next sweep frequency is selected.

[0054] Further, the above-mentioned computing and monitoring module 500 is configured to: The formula for calculating the total insulation equivalent capacitance of the unit is: ; The total insulation equivalent capacitance of the unit at different frequency points obtained by measurement and FFT transformation is averaged after removing the obvious difference points, and the average value is taken as the final result of the total insulation equivalent capacitance of the unit. When the final result of the total insulation equivalent capacitance of the unit deviates from the initial value by more than a preset threshold during the operation of the target unit, an alarm signal will be sent, and it is considered that the stator winding insulation or the insulation of the machine terminal device of the current unit has reached a critical value.

[0055] The online monitoring device for the stator winding insulation state of the full-power variable-speed pumped storage unit based on the neutral point harmonic sweep injection can enhance the signal-to-noise ratio of the leakage current under the condition of large load current, avoid the transformation error, and realize more reliable online monitoring of the stator insulation information. The harmonic injection is performed by avoiding the inherent common-mode component of the unit, the monitorable leakage current signal is enhanced, and the accuracy of the insulation equivalent capacitance calculation is ensured.

[0056] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

[0057] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise specifically limited.

Claims

1. A method for on-line monitoring of insulation condition of stator winding of full-power variable-speed pumped storage generator set based on neutral-point harmonic sweep injection, characterized in that, The method comprises the following steps: A neutral point voltage of a target motor is measured, the neutral point voltage being a common-mode voltage of the motor; A neutral line of the target motor is connected to a grounding transformer, a high-precision current transformer is installed in the middle part, and the neutral point lead-out wire passes through the current transformer to measure a neutral point current; FFT analysis is performed on the common-mode voltage, and the frequency is evenly divided into 10 frequency intervals to obtain 10 frequency points; A signal generator is used to emit a sinusoidal signal according to the frequency points, and the signal is input into a power amplifier to obtain an amplified signal; Based on the neutral point voltage and the neutral point current, an algorithm is executed to calculate insulation equivalent parameters of a full-power variable-speed pumped storage unit, which are used for online monitoring of the insulation state of a stator winding.

2. The method of claim 1, wherein, The grounding transformer comprises: The neutral point lead-out wire is grounded by connecting the primary side of the grounding transformer circuit in series, thereby constructing a common-mode loop of the stator winding to provide a conduction loop for the leakage current; The secondary side is connected to the power amplifier to realize signal injection, and the primary side is connected to the motor neutral point lead-out wire to monitor the voltage and current signals.

3. The method of claim 1, wherein, The FFT analysis of the common-mode voltage and the even division of the frequency into 10 frequency intervals to obtain 10 frequency points comprise: The measured common-mode voltage is subjected to FFT analysis, and is evenly divided into 10 frequency intervals from the base frequency to the upper limit of the grounding transformer frequency, and one frequency point with the minimum voltage component in each interval is selected as the frequency of the output signal of the signal generator.

4. The method of claim 1, wherein, The use of the signal generator to emit a sinusoidal signal according to the frequency points and the input of the signal into the power amplifier to obtain an amplified signal comprise: Sinusoidal signals are generated according to the 10 selected frequency points and are swept to inject the power amplifier; The time window for signal injection at each frequency is ts, and a time window of ts is left empty for the next frequency selection.

5. The method of claim 1, wherein, The algorithm comprises: The calculation formula of the total insulation equivalent capacitance of the unit is: ； The total insulation equivalent capacitance of the unit at different frequency points obtained by measurement and FFT transformation is averaged after removing the obvious difference points, and the final result of the total insulation equivalent capacitance of the unit is obtained; When the final result of the total insulation equivalent capacitance of the unit deviates from the initial value by more than a preset threshold during the operation of the target unit, an alarm signal is sent, and it is considered that the insulation of the stator winding or the insulation of the equipment at the unit end has reached a critical value.

6. A device for on-line monitoring of insulation condition of stator winding of a full-power variable-speed pumped storage generator unit based on neutral-point harmonic sweep injection, characterized in that, It comprises: A neutral point voltage measurement module is arranged at the neutral point lead-out wire of a target motor to measure the neutral point voltage, which is a common-mode voltage of the motor; A neutral point current measurement module is arranged to connect the neutral line of the target motor to a grounding transformer, install a high-precision current transformer in the middle part, and pass the neutral point lead-out wire through the current transformer to measure the neutral point current; An FFT and frequency selection module is arranged to perform FFT analysis on the common-mode voltage and evenly divide the frequency into 10 frequency intervals to obtain 10 frequency points; A signal generation and amplification module is arranged to use a signal generator to emit a sinusoidal signal according to the frequency points, input the signal into a power amplifier, and obtain an amplified signal; A calculation and monitoring module calculates insulation equivalent parameters of the full-power variable-speed pumped storage unit according to the neutral point voltage and the neutral point current, so as to monitor the insulation state of the stator winding online.

7. The apparatus of claim 6, wherein, The grounding transformer is used for: The primary side of the grounding transformer circuit is connected in series and grounded on the neutral point lead-out line, a common-mode loop of the stator winding is constructed, and a conduction loop is provided for the leakage current. The secondary side is connected to the power amplifier to realize signal injection, and the primary side is connected to the neutral point lead-out line of the motor to monitor the voltage and current signals.

8. The apparatus of claim 6, wherein, The common-mode voltage is subjected to FFT analysis, and the frequency is evenly divided into 10 frequency intervals to obtain 10 frequency points, including: The measured common-mode voltage is subjected to FFT analysis, and is evenly divided into 10 frequency intervals from the fundamental frequency to the upper limit of the grounding transformer frequency, and each interval selects a frequency point with the minimum voltage component as the output signal frequency of the signal generator.

9. The apparatus of claim 8, wherein, The signal generator generates sinusoidal signals according to the frequency points, inputs the power amplifier, and obtains amplified signals, including: The signal generator generates sinusoidal signals according to the frequency points, inputs the power amplifier, and obtains amplified signals, including: The time window of each frequency signal injection is ts, and another ts time window is left empty without injection, the original three-phase voltage of the unit is measured, and the next frequency scanning frequency is selected.

10. The apparatus of claim 9, wherein, The algorithm includes: The calculation formula of the total insulation equivalent capacitance of the unit is: ； The total insulation equivalent capacitance of the unit at different frequency points obtained by measurement and FFT transformation is averaged after removing the obvious difference points, and the final result of the total insulation equivalent capacitance of the unit is obtained. When the final result of the total insulation equivalent capacitance of the unit deviates from the initial value by more than a preset threshold during the operation of the target unit, an alarm signal will be sent, and it is considered that the insulation of the stator winding or the insulation of the equipment at the unit end has reached a critical value.

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