On-line monitoring method for insulation state of stator winding of full-power variable-speed pumped storage unit

Abstract

The invention discloses a full-power variable-speed pumped storage unit stator winding insulation state online monitoring method, and provides a high-capacity generator unit stator winding insulation state online monitoring method which can be applied to a network side transformer neutral point ungrounded system. According to the method, an effective common-mode monitoring loop can be established, and monitoring is carried out by utilizing the third harmonic of the motor stator winding and is not interfered by the high-frequency switch harmonic of the frequency converter. In order to solve the problem that a full-power variable-speed pumped storage unit generally has large capacity and is not easy to realize online measurement, the invention provides a method for measuring on a neutral point outgoing line, which can realize online measurement of the insulation state of a stator winding when the load current is relatively large. The method is suitable for a full-power variable-speed pumped storage high-capacity unit, leakage current measurement can be carried out under the condition of large load current, and stator insulation information online monitoring is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of full-power variable-speed pumped storage unit, in particular to a kind of full-power variable-speed pumped storage generator stator winding insulation state monitoring technology that can be used in net side transformer neutral point ungrounded system. BACKGROUND

[0002] At present, a complete megawatt-level full-power variable-speed pumped storage technology system has not been established in China. As an important equipment for power system regulation, its insulation state monitoring technology is directly related to the reliability and safety of the unit operation. Compared with the AC excitation variable-speed technology, the full-power variable-speed technology has simple structure, low manufacturing difficulty, stronger water head amplitude adaptability and wider power regulation range. However, the operating conditions of the full-power variable-speed pumped storage motor are variable, the stress situation is more complex, the unit capacity is large, and the insulation wear is more serious, making the insulation state monitoring more challenging.

[0003] Stator winding insulation problem is one of the main challenges faced by full-power variable-speed pumped storage units. At present, redundancy design is mainly used to ensure the reliability of insulation, and there is a lack of online monitoring of winding insulation, which cannot timely track the insulation aging condition. The traditional offline detection method needs to be carried out during shutdown. The existing insulation problem processing method mainly uses relay protection device to immediately stop after short-circuit fault occurs. However, this method cannot realize early warning of insulation aging, which poses a potential threat to the stable operation of the unit.

[0004] With the grid connection of large-scale wind power, photovoltaic and other new energy sources, the power system needs to have greater range of reactive power compensation capability, so variable-speed pumped storage units are increasingly becoming an effective means of high proportion of new energy consumption. However, the related supporting design of full-power variable-speed pumped storage unit is not perfect, and the insulation problem limits the long-term stable operation of the unit. Therefore, the online monitoring system of the present application will provide a new insulation state monitoring method to meet the urgent needs of the market for efficient and stable power system.

[0005] At present, the method for monitoring the insulation of stator winding of variable-speed pumped storage unit mainly includes data-driven and insulation characteristic quantity measurement. The data-driven method uses the data provided by the motor monitoring system, while the insulation characteristic quantity measurement method directly measures the insulation health parameters through special sensors. On large synchronous generators, online partial discharge method has been widely used. However, in full-power variable-speed pumped storage units, the switching process of frequency converter causes interference to the partial discharge monitoring, and more effective monitoring method is needed.

[0006] The existing technical solutions have the following technical defects: (1) The data-driven monitoring method needs to collect a large amount of equipment field operation data, especially the operation data of the equipment in the stage before failure for algorithm training. The data collection and algorithm training cost of this method is high.

[0007] (2) The partial discharge online monitoring method is currently only applied to the equipment powered by the grid sinusoidal voltage. For the equipment powered by the modulated pulse width square wave controlled by the frequency converter, the partial discharge signal will be affected by the transient signal generated by the switching process of the frequency converter, and cannot work normally. Therefore, the partial discharge online monitoring method lacks the function of application in the equipment powered by the frequency converter.

[0008] (3) The existing frequency conversion motor stator winding insulation monitoring method is only applicable to the motor with a common mode loop itself, and cannot be applied to the neutral point ungrounded system of the grid side transformer without a 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 the phase line.

[0009] (4) The generator stator winding single-phase grounding protection relay protection method based on the third harmonic potential and the fundamental zero sequence voltage can realize rapid removal after the fault caused by insulation damage, but cannot monitor and give early warning of the fault of the motor stator winding insulation state, so as to ensure the operation stability of the unit. SUMMARY

[0010] The present application aims to at least solve one of the technical problems in the related art to some extent.

[0011] To this end, the present application provides an online monitoring method for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on the third anti-potential harmonic. This method can establish an effective common mode monitoring loop, and use the third harmonic of the motor stator winding itself for monitoring, which is not affected by the high-frequency switching harmonic of the frequency converter.

[0012] 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 measuring on the neutral point lead line is proposed, which can realize online measurement of the insulation state of the stator winding when the load current is large.

[0013] Another object of the present application is to provide an online monitoring device for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on the third anti-potential harmonic.

[0014] To achieve the above object, the present application provides an online monitoring method for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on the third anti-potential harmonic, which comprises: measuring the three-phase winding voltage of the motor end of the target motor to obtain a motor end voltage signal; A neutral point lead-out wire of the target motor is drawn out, a voltage of the neutral point is measured, and a neutral point voltage signal is obtained; A grounding transformer is connected in series on the neutral point lead-out wire, wherein the neutral point lead-out wire passes through a through-type high-precision current transformer to obtain a neutral point current signal; The collected motor terminal voltage signal, neutral point voltage signal and neutral point current signal are subjected to an algorithm to calculate insulation equivalent parameters of the full-power variable-speed pumped storage unit, and the insulation state of the stator winding is monitored online.

[0015] 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 third-order counter electromotive force harmonic 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: selecting a suitable type of grounding transformer so that the current on the neutral point lead-out wire is within the range of the high-precision current transformer; and connecting the grounding transformer circuit in series on the neutral point lead-out wire to ground, thereby constructing a common-mode loop of the stator winding; and the common-mode loop provides a conduction loop for the common-mode frequency of the third-order harmonic.

[0016] Further, in an embodiment of the present application, the algorithm executed on the collected motor terminal voltage signal, neutral point voltage signal and neutral point current signal to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit comprises: determining the number of coils of each phase winding of the target motor, performing Fourier transform on the motor terminal voltage signal, neutral point voltage signal and neutral point current signal to obtain the amplitude and phase of the third-order harmonic, thereby constructing a vector, the third-order harmonic of the motor terminal voltage, the third-order harmonic of the neutral point voltage, the single-phase third-order harmonic potential and the primary equivalent current of the neutral point grounding transformer.

[0017] Further, in an embodiment of the present application, the algorithm executed on the collected motor terminal voltage signal, neutral point voltage signal and neutral point current signal to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit comprises: obtaining sample data of a multi-modal feature map, performing data labeling of a target detection frame using the multi-sample data, training the multi-modal fusion target detection model using the labeled data; inputting a two-dimensional image and a three-dimensional point cloud to be tracked into the trained multi-modal fusion target detection model to detect the target trajectory, and obtaining a three-dimensional target detection result at each prediction time.

[0018] Further, in an embodiment of the present application, the method further comprises: the third-order harmonic potential of each coil is:

[0019] ; The third harmonic voltage of each coil to ground is: ; The third harmonic current of each coil coupled to ground capacitance is: ; The third harmonic current coupled to ground capacitance is: ; The measured third harmonic current of the neutral point to ground is , two complex equations are solved simultaneously ; Solving the real part and the imaginary part respectively, two unknowns can be solved.

[0020] Further, in an embodiment of the present application, the online monitoring of the stator winding insulation state comprises: measuring the final calculated value of the two unknowns during the operation of the full-power variable-speed pumped storage unit; and issuing an alarm signal when the final calculated value deviates from the initial value by more than a preset threshold, considering that the insulation of the stator winding or the insulation of the machine terminal device of the current unit has reached a critical value.

[0021] To achieve the above purpose, another aspect of the present application provides a kind of full-power variable-speed pumped storage unit stator winding insulation state online monitoring device based on third anti-electromotive force harmonic, comprising: Machine terminal voltage measurement module, the three-phase winding voltage of the machine terminal of target motor is measured, and machine terminal voltage signal is obtained; Neutral point voltage measurement module, the neutral point voltage signal is obtained by measuring the voltage of the neutral point lead-out line of target motor; Neutral point current measurement module, in neutral point lead-out line, series connection ground transformer, wherein, neutral point lead-out line is threaded through through-type high-precision current transformer, and neutral point current signal is obtained; Acquisition and calculation module, the machine terminal voltage signal, neutral point voltage signal and neutral point current signal collected are executed algorithm to calculate the insulation equivalent parameter of full-power variable-speed pumped storage unit, and the stator winding insulation state is monitored online.

[0022] The full-power variable-speed pumped storage unit stator winding insulation state online monitoring method and device based on third anti-electromotive force harmonic of the embodiment of the present application can establish an effective common-mode monitoring loop, monitor using the third harmonic of the motor stator winding itself, is not interfered by the high-frequency switching harmonic of frequency converter, and can realize online measurement of the stator winding insulation state when load current is large.

[0023] The present application has the following advantages: 1) According to the stator winding characteristics of the full-power variable-speed pumped storage unit in the non-grounded system of the neutral point of the network-side transformer, a neutral point grounding transformer capable of providing a conduction loop of the stator insulation leakage current is designed, and the stator winding leakage current is measured online based on the third harmonic component.

[0024] 2) It is suitable for the full-power variable-speed pumped storage of a large-capacity unit, and can measure the leakage current under the condition of large load current, and realize online monitoring of the stator insulation information.

[0025] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter in the description. BRIEF DESCRIPTION OF DRAWINGS

[0026] The above and / or additional aspects and advantages of the present application will become apparent and be readily understood from the following description, taken in conjunction with the accompanying drawings, in which: Figure 1 It is an equivalent circuit diagram of the third harmonic electromotive force and the ground capacitance of the existing generator; Figure 2 It is an equivalent circuit diagram of the third harmonic electromotive force distribution when the existing generator is single-phase grounded; Figure 3 It is a curve graph of the change of the existing neutral point voltage and the terminal voltage with the fault point; Figure 4 It is a flow chart of a third counter electromotive force harmonic-based full-power variable-speed pumped storage unit stator winding insulation state online monitoring method according to an embodiment of the present application; Figure 5 It is a system structure diagram of a third counter electromotive force harmonic-based full-power variable-speed pumped storage unit stator winding insulation state online monitoring method according to an embodiment of the present application; Figure 6 It is a third harmonic equivalent circuit diagram of a motor according to an embodiment of the present application; Figure 7 It is a phasor diagram of the third harmonic electromotive force of the motor stator winding according to an embodiment of the present application; Figure 8 It is a structure schematic diagram of a third counter electromotive force harmonic-based full-power variable-speed pumped storage unit stator winding insulation state online monitoring device according to an embodiment of the present application. DETAILED DESCRIPTION

[0027] 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.

[0028] In order to make the personnel in the technical field better understand the present application, the technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work should belong to the protection scope of the present application.

[0029] A method and device for on-line monitoring of insulation condition of stator winding of full-power variable-speed pumped storage unit based on third-order back EMF harmonic are described below according to an embodiment of the present application.

[0030] A method for on-line monitoring of stator insulation of a motor applied to an inverter-driven motor is proposed in the prior art. The method utilizes the common-mode component voltage harmonic generated by the inverter to monitor the corresponding leakage current harmonic component.

[0031] (1) wherein, ILCM is the common-mode leakage current, Cg is the a-phase-to-ground insulation equivalent capacitance, Va is the a-phase-to-ground voltage, and ω is the angular frequency corresponding to the selected frequency component.

[0032] According to formula (1), the following can be obtained (2) The method realizes on-line monitoring of main insulation. It can be applied to insulation monitoring of motor stator winding in a neutral-point grounded system of a network-side transformer. However, in a non-grounded system, there is no common-mode loop itself. Meanwhile, the load current of the three-phase motor terminal phase lines is large, the wire diameter is thick, and the spacing is far, so it is difficult to directly use a high-precision current sensor for on-line monitoring of leakage current. Therefore, the method cannot be directly applied to on-line monitoring of insulation condition of stator winding of a variable-speed pumped storage unit in a neutral-point non-grounded system of a network-side transformer.

[0033] At present, in the problem of insulation damage of motor stator winding, a relay protection device is directly used to deal with short-circuit fault. A single-phase ground protection of generator stator winding can be realized by using third-order harmonic voltage. Due to the non-sinusoidal distribution of air-gap flux density of the generator and the influence of ferromagnetic saturation, the electromotive force induced in the stator winding contains high-order harmonic components. The third-order harmonic component is of common-mode nature, which is eliminated in the on-line electromotive force, but still exists in the phase electromotive force.

[0034] When the neutral point of the generator is not grounded, an equivalent circuit as shown in Figure 1 is obtained. Wherein Cf represents the ground capacitance of each phase, which is equivalent to being concentrated in the neutral point N and the motor terminal S of the generator. Cw represents the ground capacitance of each phase of the generator terminal lead-out line, step-up transformer, auxiliary transformer, voltage transformer and other devices, which is equivalent to being concentrated at the motor terminal.

[0035] The third harmonic voltage at the neutral point and the machine terminal are respectively (3) (4) The ratio of the third harmonic voltage at the machine terminal US3 and the third harmonic voltage at the neutral point UN3 is (5) When the metallic single-phase grounding occurs in the stator winding of the motor, it is assumed that the grounding occurs at a distance of from the neutral point, as shown in Figure 2 .

[0036] At this time, there are and , and by comparison, we get (6)The third harmonic voltage at the neutral point and the third harmonic voltage at the machine terminal change with the fault point , as shown in . Using Figure 3 as the action quantity, using as the braking quantity to form the grounding protection, and using as the protection action condition. As can be seen from equation (5), the protection cannot act in normal operation. When grounding occurs near the neutral point, it has high sensitivity. The grounding protection formed by the third harmonic can reflect single-phase grounding faults in the range of , and the closer the fault point is to the neutral point, the higher the sensitivity of the protection; the grounding protection formed by the fundamental zero sequence voltage can reflect single-phase grounding faults in the range of , and the closer the fault point is to the machine terminal, the higher the sensitivity of the protection. Therefore, the combination of the third harmonic voltage ratio and the fundamental zero sequence voltage can form 100% stator winding single-phase grounding protection.

[0037] Figure 4 is a flow chart of an online monitoring method for the insulation state of the stator winding of a full-power variable-speed pumped storage generator set based on the third harmonic of the counter electromotive force according to an embodiment of the present application.

[0038] As shown in Figure 4 , the method includes but is not limited to the following steps: S1, measuring the three-phase winding voltage at the machine terminal of the target motor to obtain a machine terminal voltage signal; S2, measuring the voltage at the neutral point of the target motor through a neutral point lead to obtain a neutral point voltage signal; S3, on the neutral point lead-out line, a series ground transformer is connected, wherein the neutral point lead-out line is passed through a through-type high-precision current transformer to obtain a neutral point current signal; S4, the collected machine terminal voltage signal, neutral point voltage signal and neutral point current signal are subjected to an algorithm to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit, and the insulation state of the stator winding is monitored online.

[0039] The embodiment of the present application is a kind of based on three times back EMF harmonic full-power variable-speed pumped storage unit stator winding insulation state online monitoring method, designed can provide the neutral point grounding transformer of stator insulation leakage current conduction loop, based on three harmonic components realize online measurement stator winding leakage current.It is suitable for full-power variable-speed pumped storage large-capacity unit, can carry out leakage current measurement under the condition of large load current, realize the online monitoring of stator insulation information.

[0040] The embodiment of the present application is a kind of based on three times back EMF harmonic full-power variable-speed pumped storage unit stator winding insulation state online monitoring method, designed can provide the neutral point grounding transformer of stator insulation leakage current conduction loop, based on three harmonic components realize online measurement stator winding leakage current.It is suitable for full-power variable-speed pumped storage large-capacity unit, can carry out leakage current measurement under the condition of large load current, realize the online monitoring of stator insulation information.

[0041] First, the following symbols are explained: : machine terminal voltage three harmonic; : neutral point voltage three harmonic; : single-phase three harmonic potential; : the number of coils of each phase winding; : three harmonic potential of each coil; : three harmonic current of each ground capacitance coupling branch; : primary equivalent current of neutral point grounding transformer; : total capacitance of machine terminal equipment to ground (mainly the coupling capacitance of frequency converter and network side transformer secondary to ground); : total capacitance of stator winding three-phase to ground; : parameters of neutral point grounding transformer.

[0042] Specifically, the specific implementation steps of the present application, as shown in Figure 5 , include: 1. Machine terminal voltage measurement module: measure three-phase winding voltage at the machine terminal of the motor.

[0043] 2. Neutral point voltage measurement module: measure the neutral point voltage on the neutral point lead-out line of the motor.

[0044] 3. Neutral point current measurement module: install a through-type high-precision current transformer in the middle part of the motor neutral point to the grounding transformer, and pass the neutral point lead-out line through the current transformer.

[0045] 4. Grounding transformer: a grounding transformer circuit is connected in series on the neutral point outgoing line to ground, thereby constructing a common mode loop of the stator winding. By selecting a suitable model of grounding transformer, the current on the neutral point outgoing line is within the range of the high-precision current transformer. The grounding loop can provide a conducting loop for common mode frequencies such as the third harmonic, thereby realizing the calculation and monitoring of the insulation parameters.

[0046] 5. Acquisition and calculation module: acquiring the signals measured by the voltage and current measurement module, and performing 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.

[0047] The relevant principles of the present application are as follows: Insulation online monitoring is performed using the third harmonic potential. Since the network side transformer of the power frequency power grid is not grounded, the primary side capacitor of the power frequency transformer cannot be coupled to the secondary side after the neutral point of the motor is grounded through the grounding transformer. As shown in the equivalent circuit diagram of the third harmonic of the motor. Figure 6 The equivalent circuit diagram of the third harmonic of the motor is shown.

[0048] For a common 60° phase belt motor, the third harmonic potential is 180° phase belt, so the third harmonic of each phase forms a semicircle. The potential amplitudes of each turn are equal, but the phases are different. As shown in the phasor diagram of the third harmonic potential of the motor stator winding. Figure 7 The phasor diagram of the third harmonic potential of the motor stator winding is shown.

[0049] The calculation process of the present application is as follows: The number of turns of each phase winding of the motor is known. Fourier transform is performed on the signals acquired by the current and voltage sensors to obtain the amplitude and phase of the third harmonic. Thereby, the phasor and .

[0050] The third harmonic potential of each turn is:

[0051]

[0052] The third harmonic voltage of each turn to ground is:

[0053] The third harmonic current of the coupling capacitor of each turn to ground is:

[0054] The third harmonic current of the coupling capacitor to ground is:

[0055] ​​The third harmonic current of the neutral point to ground can be measured as Two complex equations are solved simultaneously. By solving the real and imaginary parts separately, two unknowns and can be solved.

[0056]

[0057] When the final calculated value of or deviates from the initial value by more than a certain threshold during the operation of the full-power variable-speed pumped storage unit being measured, an alarm signal will be issued, indicating that the insulation of the stator winding or the insulation of the terminal equipment of the current unit has reached a critical value.

[0058] In order to realize the above-mentioned embodiments, as shown in Figure 8 , the embodiment also provides an online monitoring device 10 for the insulation state of the stator winding of a full-power variable-speed pumped storage unit based on the third anti-potential harmonic, which comprises a terminal voltage measurement module 100, a neutral point voltage measurement module 200, a neutral point voltage measurement module 300 and a collection and calculation module 400.

[0059] The terminal voltage measurement module 100 measures the three-phase winding voltage of the terminal of the target motor to obtain a terminal voltage signal; The neutral point voltage measurement module 200 measures the voltage of the neutral point through the neutral point lead-out line of the target motor to obtain a neutral point voltage signal; The neutral point voltage measurement module 300 is used on the neutral point lead-out line and is connected in series with a grounding transformer, wherein the neutral point lead-out line passes through a through-type high-precision current transformer to obtain a neutral point current signal; The collection and calculation module 400 executes an algorithm on the collected terminal voltage signal, neutral point voltage signal and neutral point current signal to calculate the insulation equivalent parameters of the full-power variable-speed pumped storage unit and monitor the insulation state of the stator winding online.

[0060] Further, the neutral point voltage measurement module 300 is further used for: Selecting a suitable type of grounding transformer so that the current on the neutral point lead-out line is within the range of the high-precision current transformer; Connecting the grounding transformer circuit in series on the neutral point lead-out line to ground and construct a common-mode loop of the stator winding; the common-mode loop provides a conduction loop for the common-mode frequency of the third harmonic.

[0061] Further, the collection and calculation module 400 is further used for: The number of coils of each phase winding of the target motor is determined, Fourier transform is performed on the machine terminal voltage signal, the neutral point voltage signal and the neutral point current signal to obtain the amplitude and phase of the third harmonic, thereby constructing a vector, the third harmonic of the machine terminal voltage, the third harmonic of the neutral point voltage, the third harmonic potential of the single phase and the equivalent current of the primary side of the neutral point grounding transformer.

[0062] Further, the above-mentioned acquisition and calculation module 400 is further used for: The third harmonic potential of each coil is:

[0063] The third harmonic voltage of each coil to ground is: The third harmonic current of the coupling capacitor of each coil to ground is: The third harmonic current of the coupling capacitor to ground is: The measured third harmonic current of the neutral point to ground is Two complex equations are established The two unknowns are solved by solving the real part and the imaginary part respectively.

[0064] Further, the device 10 further comprises a junction monitoring result output module, configured to: During the operation of the full-power variable-speed pumped storage unit, the final calculation values of the two unknowns are measured; When the final calculation values deviate from the initial values by more than a preset threshold value, an alarm signal is sent, and it is considered that the insulation of the stator winding or the insulation of the machine terminal device of the current unit has reached a critical value.

[0065] According to the online monitoring device for the insulation state of the stator winding of the full-power variable-speed pumped storage unit based on the third anti-electromotive force harmonic, the device is suitable for the full-power variable-speed pumped storage unit with large capacity, can measure the leakage current under the condition of large load current, and realizes online monitoring of the stator insulation information.

[0066] ​​​​​In the description of the 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 specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Also, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. Furthermore, the person skilled in the art can combine and combine the different embodiments or examples described in the specification and the features of the different embodiments or examples, without contradiction.

[0067] In addition, the terms "first", "second", are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Thus, the features defined with "first", "second" can include at least one of the features, explicitly or implicitly. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise explicitly specified.

Claims

1. A method for on-line monitoring of insulation condition of stator winding of a full power variable speed pumped storage generator set based on third anti-potential harmonic, characterized in that, The method comprises the following steps: measuring three-phase winding voltage of a machine end of a target motor to obtain a machine end voltage signal; measuring voltage of a neutral point to obtain a neutral point voltage signal; connecting a grounding transformer in series on the neutral point lead, wherein the neutral point lead passes through a high-precision current transformer to obtain a neutral point current signal; performing an algorithm on the collected machine end voltage signal, neutral point voltage signal and neutral point current signal to calculate insulation equivalent parameters of the full-power variable-speed pumped storage unit, and monitoring insulation state of the stator winding online.

2. The method of claim 1, wherein, The grounding transformer in series comprises: selecting a grounding transformer of a proper type so that current on the neutral point lead is within a range of the high-precision current transformer; connecting a grounding transformer circuit in series on the neutral point lead to ground, thereby constructing a common-mode loop of the stator winding; the common-mode loop provides a conducting loop for a common-mode frequency of the third harmonic.

3. The method of claim 1, wherein, The algorithm performed on the collected machine end voltage signal, neutral point voltage signal and neutral point current signal to calculate insulation equivalent parameters of the full-power variable-speed pumped storage unit comprises: determining the number of coils of each phase winding of the target motor, performing Fourier transform on the machine end voltage signal, neutral point voltage signal and neutral point current signal to obtain amplitudes and phases of the third harmonic, thereby constructing vectors of the machine end voltage third harmonic, neutral point voltage third harmonic, single-phase third harmonic potential and primary equivalent current of the neutral point grounding transformer.

4. The method of claim 3, wherein, The method comprises: third harmonic potential of each coil is: ； third harmonic voltage of each coil to ground is: ； third harmonic current of each coil to ground coupling capacitance is: ； third harmonic current of the capacitance to ground coupling is: ； The third harmonic current to ground for the available neutral is measured as , the two complex equations are solved simultaneously ； two unknowns can be solved by solving real and imaginary parts respectively.

5. The method of claim 4, wherein, The online monitoring of the insulation state of the stator winding comprises: measuring final calculation values of the two unknowns during operation of the full-power variable-speed pumped storage unit; when the final calculation values deviate from initial values by more than a preset threshold, an alarm signal is sent, and it is considered that insulation of the stator winding or insulation of a machine end device of the current unit 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 set based on triple-reverse-potential harmonic, characterized in that, The method comprises: a machine end voltage measurement module for measuring three-phase winding voltage of a machine end of a target motor to obtain a machine end voltage signal; a neutral point voltage measurement module for measuring voltage of a neutral point to obtain a neutral point voltage signal; a neutral point current measurement module for connecting a grounding transformer in series on the neutral point lead, wherein the neutral point lead passes through a high-precision current transformer to obtain a neutral point current signal; a collection and calculation module for performing an algorithm on the collected machine end voltage signal, neutral point voltage signal and neutral point current signal to calculate insulation equivalent parameters of the full-power variable-speed pumped storage unit, and monitoring insulation state of the stator winding online.

7. The apparatus of claim 6, wherein, The neutral point current measurement module comprises: selecting a grounding transformer of a proper type so that current on the neutral point lead is within a range of the high-precision current transformer; On the neutral point lead, a series ground transformer circuit is connected to ground, and a common mode loop of the stator winding is constructed; the common mode loop provides a conducting loop for the common mode frequency of the third harmonic.

8. The apparatus of claim 6, wherein, The acquisition and calculation module comprises: Determine the number of coils of each phase winding of the target motor, perform Fourier transform on the motor terminal voltage signal, neutral point voltage signal and neutral point current signal to obtain the amplitude and phase of the third harmonic, thereby constructing a vector, the third harmonic of the motor terminal voltage, the third harmonic of the neutral point voltage, the third harmonic potential of the single-phase and the equivalent current of the primary side of the neutral point grounding transformer.

9. The apparatus of claim 8, wherein, The method comprises: The third harmonic potential of each coil is: ； The third harmonic voltage of each coil to ground is: ； The third harmonic current of the coupling capacitor of each coil to ground is: ； The third harmonic current of the coupling capacitor to ground is: ； The third harmonic current to ground for the available neutral is measured as , the two complex equations are solved simultaneously ； Solving the real part and the imaginary part respectively can solve two unknowns.

10. The apparatus of claim 9, wherein, The online monitoring of the insulation state of the stator winding comprises: During the operation of the full-power variable-speed pumped storage unit, the final calculation values of the two unknowns are measured; When the final calculation value deviates from the initial value by more than a preset threshold, an alarm signal is sent, and it is considered that the insulation of the stator winding or the insulation of the motor terminal equipment of the current unit has reached a critical value.

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