An off-line detection method and device for turn-to-turn short circuit of a hydro-generator rotor winding

Abstract

The application provides an off-line detection method and device for turn-to-turn short circuit of a hydro-generator rotor winding, and belongs to the field of generator fault detection. When the unit is stopped and the rotor winding is in an open circuit state, the wind shield is removed, the trapezoidal core winding the excitation coil is placed between two adjacent rotor magnetic poles, the double oblique edges thereof are tightly attached to the side surface of the pole shoe, a cross-pole magnetic circuit is formed, which spans the two rotor magnetic poles and is closed through the rotor yoke; AC current is injected into the excitation coil through an AC power supply, alternating magnetic flux is generated in the cross-pole magnetic circuit and the winding to be detected is excited; a weak magnetic detection sensor is tightly attached to the side surface of the winding of the two rotor magnetic poles, and the leakage magnetic field of each turn winding is scanned along the radial direction; when the local leakage magnetic field abnormally increases, it is determined that there is a turn-to-turn short circuit in the corresponding winding, fault positioning is realized according to the abnormal area, and otherwise, it is determined that there is no fault. The application has the advantages of rapid detection, high sensitivity, and accurate determination of the fault magnetic pole and the position of the short-circuit winding.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of generator fault detection, in particular to an off-line detection method and device for rotor winding inter-turn short circuit of a hydro-generator. BACKGROUND

[0002] Rotor winding inter-turn short circuit fault is one of the common faults of hydro-generators, and its causes mainly include manufacturing and operation. Local long-term overheating, rotor insulation damp, dirt and the like can all cause rotor winding inter-turn short circuit fault. Inter-turn short circuit can cause asymmetric distribution of the magnetic field of the hydro-generator, increase vibration of the stator and rotor and the like. If the inter-turn short circuit fault cannot be found and handled in time, serious problems such as damage of the stator core, damage of the stator insulation, accelerated damage of the bearing bush and the like can occur, which seriously threatens the safe operation of the unit. Therefore, it is of great significance to propose a hydro-generator rotor winding inter-turn short circuit fault detection method with high sensitivity and high reliability.

[0003] The salient pole rotor of the hydro-generator and the hidden pole rotor of the steam turbine generator have essential differences: 1) difference in magnetic pole structure. There is a large physical gap between adjacent salient poles, which is significantly different from the hidden pole steam turbine generator, and the magnetic circuit design of "cross-slot" excitation suitable for the steam turbine generator cannot be directly applied; 2) difference in magnetic circuit. The air gap size at each magnetic pole of the hydro-generator is different, and single-point detection (magnetic field measurement at the top of the magnetic pole) cannot locate the short circuit at which turn in the slot; 3) difference in winding arrangement. The magnetic pole winding of the hydro-generator presents a single-layer arrangement along the radial direction, with one side of the winding close to the magnetic pole and the other side exposed, which is different from the winding of the hidden pole steam turbine generator which is completely embedded in the slot.

[0004] For the diagnosis of rotor winding inter-turn short circuit fault of the hydro-generator, experts and scholars have made some practical research results. Generally, it is divided into two types of online monitoring and off-line detection. Online monitoring refers to determining whether there is an excitation winding inter-turn short circuit fault by analyzing the changes of fault characteristic quantities such as excitation current, stator harmonic current, active power, reactive power and the like of the generator.

[0005] Offline methods are performed when the generator is stationary, resulting in less interference. Traditional methods essentially infer faults indirectly by measuring macroscopic parameters (such as magnetic pole impedance and resistance), which are susceptible to external interference. The main technical approaches are as follows: 1) DC resistance method: Initial screening is performed during unit shutdown and maintenance. A micro-ohmmeter or DC double-arm bridge is used to measure the DC resistance of the rotor winding. The measured value is compared with the factory value or historical data. If the resistance deviation is greater than 1%, an early warning is issued. 2) AC impedance method: Combined with Class A maintenance, a static power frequency voltage is applied outside the chamber, and the test impedance value is compared with the factory data or historical data. A decrease in impedance value exceeding 10% should raise concern, indicating a high probability of an inter-turn short circuit fault. The improved impedance method compares the two-pole impedance Z1 / Z2, increasing sensitivity and setting the detection threshold to 2%. 3) When the AC impedance method fails to meet the standard, a diagnostic test is initiated, using a repetitive pulse method to determine whether an inter-turn short circuit fault exists in the rotor winding. 4) The AC / DC voltage divider method requires placing the rotor cavity horizontally outside, applying a DC current of 200-300A and an AC high voltage of 100V, comparing the measured value with the theoretical value, and judging that there is a short circuit if it is greater than the 5% threshold.

[0006] Currently, offline detection methods suffer from bottlenecks such as insufficient sensitivity and poor applicability, as detailed in Table 1.

[0007] Table 1 Commonly used detection methods and their limitations

[0008]

[0009] Therefore, there is an urgent need to develop a new offline detection method and device for inter-turn short circuits in the rotor windings of hydro-generators that is highly reliable, sensitive, and safe. Summary of the Invention

[0010] In order to overcome the shortcomings of the prior art, the purpose of this invention is to provide an offline detection method and device for inter-turn short circuits in the rotor winding of a hydro-generator, which can solve the shortcomings of the prior art and improve the offline diagnosis level of inter-turn short circuit faults in the rotor winding of a hydro-generator.

[0011] To achieve the above objectives, the present invention provides the following solution:

[0012] An offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator includes:

[0013] When the unit is shut down and the rotor winding is in an open circuit state, open the top cover, remove the wind baffle, and place the trapezoidal iron core with the excitation coil wound between any two adjacent rotor magnetic poles, so that the double inclined sides of the trapezoidal iron core respectively fit the sides of the pole shoes of the two rotor magnetic poles, forming a transpole magnetic circuit that spans the two rotor magnetic poles and is closed by the rotor magnetic yoke.

[0014] An alternating current is injected into the excitation coil by an AC power supply connected in series with the excitation coil to generate an alternating magnetic flux in the transpole magnetic circuit, thereby exciting the rotor winding to be tested.

[0015] Using a weak magnetic field detection sensor, which is closely attached to the side surface of the windings of the two rotor magnetic poles respectively, the weak magnetic field detection sensor is moved along the radial direction of the generator to scan the leakage magnetic field of each winding.

[0016] When a local abnormal increase occurs in the leakage magnetic field, it is determined that there is an inter-turn short circuit in the winding corresponding to the rotor magnetic pole. The short-circuited winding is located in the area of ​​abnormal magnetic field increase, so as to realize fault location. Otherwise, it is determined that there is no fault.

[0017] Preferably, the alternating current injected into the excitation coil is provided by a 50Hz low-voltage power frequency current source.

[0018] Preferably, the excitation coil has 100 turns and is supplied with an alternating current of 2A to obtain the desired magnetic flux density.

[0019] Preferably, when the weak magnetic field detection sensor performs a leakage magnetic field scan of the rotor winding, it is in close contact with the rotor winding during measurement and moves at a preset distance from the rotor winding.

[0020] Preferably, when the relative deviation of the leakage magnetic field amplitude of a local rotor winding from the normal reference value under the same operating conditions exceeds 30%, it is determined that the local rotor winding has an inter-turn short circuit fault.

[0021] Preferably, the weak magnetic field detection sensor covers the two rotor magnetic poles that are attached to and connected to the trapezoidal iron core in one detection; wherein, the wind baffle is set corresponding to each rotor magnetic pole; during the detection, the wind baffle is removed along the circumference of the rotor at a rate of one wind baffle every other pole, so that the number of wind baffles to be removed is half the number of rotor magnetic poles.

[0022] Preferably, the weak magnetic field detection sensor scans the entire radial section of the rotor winding to eliminate measurement blind spots and obtain information on the range of the fault along the radial direction.

[0023] An offline detection device for inter-turn short circuits in the rotor winding of a hydro-generator includes:

[0024] A trapezoidal iron core wound with an excitation coil, wherein the double inclined sides of the trapezoidal iron core can fit against the side of the pole shoe of any two adjacent rotor magnetic poles, thereby forming a zero-air-gap transpolar magnetic circuit between the two magnetic poles.

[0025] An AC power supply is connected in series with the excitation coil to form an external circuit. The AC power supply is used to inject a 50Hz low-voltage AC current into the excitation coil.

[0026] A weak magnetic field detection device includes a weak magnetic field detection sensor and a weak magnetic field display device. The weak magnetic field detection sensor is attached to the rotor winding of the magnetic pole to be measured and moves in the radial direction to collect the leakage magnetic field induced voltage signal of the winding. The weak magnetic field display device is used to display the leakage magnetic field induced voltage signal and determine whether the rotor winding has an inter-turn short circuit based on the changes.

[0027] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0028] 1) No historical data is required. It can directly measure leakage magnetic distortion caused by short circuit between rotor winding turns, which overcomes the systematic error problem of traditional AC impedance method relying on historical data and has a very low dependence on the operator's personal experience level.

[0029] 2) Traditional AC / DC voltage divider methods require the application of high voltage (100V AC) or high current (200-300A DC), which is complex to operate, cumbersome to wire, and poses risks of high-voltage electric shock and arcing, resulting in poor safety. When pursuing the detection signal strength, those skilled in the art are prone to thinking of using higher voltage or current. However, this solution achieves high-sensitivity detection under low-voltage and low-current conditions, which is something that existing technologies have not yet achieved.

[0030] 3) The rotor magnets of the hydro-generator have a salient pole structure, and the air gaps of each pole are different. Traditional steam turbine generator testing methods typically perform single-point magnetic field measurements at the slot openings of the straight sections of the windings, which can lead to misjudgments in hydro-generators. This invention can achieve full-range testing on the side of the rotor winding ends, eliminating blind spots in the testing.

[0031] 4) The steam turbine generator detects the slot leakage magnetic field. This invention directly detects the side leakage magnetic field of the rotor magnetic end winding of the hydro turbine generator. The detection range is larger and more direct. It can locate the specific part of the short-circuited winding and detect the fault range based on the leakage magnetic field at different locations.

[0032] 5) Unlike the cross-slot excitation path of steam turbine generators, this method is more suitable for hydro turbine generators. Existing hydro turbine generator methods have failed to effectively solve the problem of gap magnetic circuit closure; for example, the AC impedance method is easily affected by severe electromagnetic interference coupling from adjacent magnetic poles. The cross-pole magnetic circuit design solves the problem of salient pole gap differences. The trapezoidal core with double inclined surfaces efficiently couples the excitation magnetic field to the target adjacent magnetic poles and the rotor winding region between them, significantly improving excitation efficiency, preventing the excitation magnetic field from crossing the damping winding, eliminating the influence of eddy currents in the damping winding, and significantly increasing the intensity of the excitation magnetic field.

[0033] 6) The sensitivity is greatly improved. The characteristic magnetic field strength of the fault is significantly greater than that of the normal condition, which can identify minor faults such as a one-turn short circuit.

[0034] 7) The existing offline methods for hydro-generators described above do not directly detect the magnetic field, but rely on indirect changes in system-level electrical parameters (overall resistance, impedance, voltage distribution) to infer faults. These methods have low sensitivity, cannot accurately locate faults, are susceptible to interference, and are insensitive to minor early-stage faults. This invention can directly capture changes in the leakage magnetic field on the rotor winding side caused by inter-turn short-circuit faults, which is simpler, more direct, and allows for fault location. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 A flowchart of the method provided in an embodiment of the present invention;

[0037] Figure 2 A schematic diagram of the detection device and rotor provided in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the technical route provided in the embodiments of the present invention;

[0039] Figure 4 The Maxwell 2D model of the turbine generator rotor provided in this embodiment of the invention;

[0040] Figure 5 This is a partial enlarged view of the 2D model of the hydro-generator rotor provided in an embodiment of the present invention;

[0041] Figure 6 This is a schematic diagram of the radial leakage magnetic flux density at measuring point 1 of the rotor winding of a hydro-generator provided in an embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram of the tangential leakage flux density at measuring point 1 of the rotor winding of a hydro-generator provided in an embodiment of the present invention.

[0043] Figure 8 This is a schematic diagram of the radial leakage magnetic flux density at measuring point 2 of the rotor winding of the hydro-generator provided in an embodiment of the present invention;

[0044] Figure 9 This is a schematic diagram of the tangential leakage flux density at measuring point 2 of the rotor winding of the hydro-generator provided in an embodiment of the present invention;

[0045] Figure 10 This is a schematic diagram of the radial leakage magnetic flux density at measuring point 3 of the rotor winding of the hydro-generator provided in an embodiment of the present invention;

[0046] Figure 11This is a schematic diagram of the tangential leakage magnetic flux density at measuring point 3 of the rotor winding of the hydro-generator provided in an embodiment of the present invention. Detailed Implementation

[0047] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0048] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0049] Figure 1 The method flowchart provided in the embodiments of the present invention is as follows: Figure 1 As shown, the present invention provides an offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator, comprising:

[0050] Step 100: With the unit shut down and the rotor winding in an open circuit state, open the top cover, remove the wind baffle, and place the trapezoidal iron core with the excitation coil wound between any two adjacent rotor magnetic poles, so that the double inclined sides of the trapezoidal iron core respectively fit against the sides of the pole shoes of the two rotor magnetic poles, forming a transpole magnetic circuit that spans the two rotor magnetic poles and is closed by the rotor magnetic yoke.

[0051] Step 200: Inject AC current into the excitation coil through an AC power supply connected in series with the excitation coil to generate alternating magnetic flux in the transpole magnetic circuit, thereby exciting the rotor winding to be tested.

[0052] Step 300: Using a weak magnetic field detection sensor, place it close to the side surface of the windings of the two rotor magnetic poles respectively, move the weak magnetic field detection sensor along the radial direction of the generator, and scan the leakage magnetic field of each turn winding.

[0053] Step 400: When the leakage magnetic field shows a local abnormal increase, it is determined that there is an inter-turn short circuit in the winding of the corresponding rotor pole. The short-circuited winding is located in the area of ​​abnormal magnetic field increase, so as to realize fault location. Otherwise, it is determined that there is no fault.

[0054] The offline detection device in this embodiment consists of a trapezoidal iron core (with an excitation coil wound around it), an AC power supply, and a weak magnetic field detection device. The external circuit consists of the trapezoidal iron core (with an excitation coil wound around it) and the AC power supply. The weak magnetic field detection device includes a weak magnetic field sensor and a weak magnetic field display device. During operation, the two inclined sides of the trapezoidal iron core with the excitation coil wound around it can be precisely engaged and fixed with the two tooth edges of any adjacent rotor magnetic pole, ensuring a zero-air-gap magnetic circuit closure with the tooth edges of any adjacent magnetic pole. The weak magnetic field detection device detects the bypass magnetic field of the winding in the slot on the other side of the same winding, the principle of which is as follows... Figure 2 As shown. Changes in the output of the weak magnetic field display device are used to determine whether an inter-turn short circuit has occurred in the rotor winding.

[0055] The trapezoidal core adopts a double-beveled chamfer design, and its inclination angle matches the geometric parameters of the rotor magnetic pole edge, ensuring that a zero-air-gap magnetic circuit is formed with the tooth edge of any adjacent magnetic pole. The magnetic flux path is optimized as follows: trapezoidal core → magnetic pole A tooth edge → rotor yoke → magnetic pole B tooth edge → trapezoidal core, forming a cross-pole magnetic circuit (compared to the same-pole cross-slot path of a steam turbine generator).

[0056] The AC power supply used for the wound coil is a 50Hz industrial frequency power supply. Low-voltage industrial frequency current is used to avoid the risks of overcurrent and overheating.

[0057] Figure 3 This is a schematic diagram of the technical route provided in the embodiments of the present invention, such as... Figure 3 As shown, the technical approach of this embodiment includes the following steps:

[0058] A. During testing, a trapezoidal iron core with a double-beveled chamfer design is used. The inclination angle matches the geometric parameters of the rotor magnetic pole edge, ensuring a zero-gap magnetic circuit closure with the tooth edge of any adjacent magnetic pole. An AC power supply is connected in series with an external circuit to inject AC current into the wound coil windings, generating an alternating magnetic field linking a certain adjacent magnetic pole.

[0059] B. By placing a weak magnetic field sensor close to the rotor winding under test and moving it in a direction perpendicular to the winding direction, the radial and tangential leakage magnetic fields near the winding are simultaneously collected. The radial and tangential weak magnetic field data U are then read through a display device. r U t .

[0060] C. The radial and tangential weak magnetic field data U r U t Normal voltage value U under the same conditions r0 U t0 Compare.

[0061] D. Calculate the short-circuit deviation rates a and b respectively:

[0062] (1)

[0063] (2)

[0064] The fault detection threshold is set to 30%, and compared with the short-circuit deviation rates a and b. If a > 30% or b > 30%, it is determined that there is an inter-turn short-circuit fault in the rotor winding of the hydro generator; if a ≤ 30% or b ≤ 30%, it is determined that there is no inter-turn short-circuit fault.

[0065] Unlike the field weakening sensors in steam turbine generators, which can only be placed at the slot openings, the field weakening sensors in this method can scan each turn of the rotor magnetic pole windings. The scanning position can be on a straight section or at the end. The field weakening sensor should be in close contact with the rotor windings during measurement.

[0066] The number of turns and the current supplied to the trapezoidal iron core coil winding are set according to the required magnetic flux density and winding current carrying capacity. As the best choice, the winding coil is set to 100 turns and a 2A AC current is supplied.

[0067] During offline testing, the generator rotor needs to be removed, the wind deflector on the unit removed, and the rotor windings in an open-circuit state. An alternating current is then applied to the coil windings wound around the trapezoidal iron core. Under normal circumstances, the alternating current passing through the coil windings would generate an alternating magnetic field, but because the rotor windings are in an open-circuit state, no induced current is generated.

[0068] When an inter-turn short circuit occurs in the rotor winding, a closed loop is formed inside the rotor winding. At this time, the alternating magnetic field will induce a current and an induced magnetic field in the rotor winding, which can be detected by a weak magnetic field detection sensor.

[0069] To verify the method and apparatus, this invention uses a hydro-generator as an example; the basic parameters are shown in Table 2. A two-dimensional simulation model of the generator rotor was established using Maxwell software, and finite element simulation analysis was performed. The Maxwell two-dimensional model of the rotor is shown in [Table 2]. Figure 4 See enlarged view for details. Figure 5 .

[0070] Table 2 Basic Parameters of Hydro-generator

[0071]

[0072] When a short circuit occurs between turns in a hydro-generator, the rotor winding is divided into two parts: the short-circuited excitation winding and the remaining excitation winding. In the Simplier external circuit, switching elements are used to control the number of short-circuited turns in the rotor winding. The external circuit and control section of the hydro-generator are constructed, and a field-circuit coupling joint simulation is performed, with a simulation time of 0.36 seconds. In offline testing, with the rotor stationary, a trapezoidal iron core with 100 turns of coil is placed between magnetic poles 1 and 2, and a 2A AC current source is applied to the coil. Simulations are performed for normal operation, 2-turn short circuit, 4-turn short circuit, and 6-turn short circuit fault conditions, respectively, to detect the side leakage flux of the short-circuited and remaining excitation windings.

[0073] Steam turbine generators measure the leakage flux at a single static point at the slot opening, while hydro turbine generators have ample space between the magnetic poles and on the end faces of the magnetic poles, allowing for two-dimensional leakage flux measurement on the side of the rotor windings, covering the entire area of ​​the magnetic pole windings and solving the problem that single-point measurement cannot locate the faulty turn.

[0074] The radial and tangential magnetic flux densities of the turbine generator rotor windings were measured at a location near the short-circuited winding (measuring point 1), such as... Figure 6 , Figure 7 As shown in Table 3, the deviation rates a and b of the weak magnetic field under different short-circuit conditions relative to the normal condition are as follows. Comparing the fault deviation rate with the threshold, it was found that the deviation rates a and b under the fault conditions were both greater than 30%, successfully identifying the presence of an inter-turn short-circuit fault in the rotor winding.

[0075] Table 3. Comparison of Weak Magnetic Field Deviation Rates under Different Inter-turn Short Circuit Degrees

[0076]

[0077] To verify the positioning performance of this method, the radial and tangential magnetic flux density of the turbine generator rotor winding was measured at a measuring point (measuring point 2) located relatively far from the short-circuited winding position. Figure 8 , Figure 9 As shown, the increase in the weak magnetic field under different short-circuit conditions is significantly smaller than that under other conditions. Figure 6 , Figure 7 The situation indicates that the short-circuit turn is not near measuring point 2.

[0078] The radial and tangential magnetic flux densities of the turbine generator rotor windings were measured at the measuring point (measuring point 3) furthest from the short-circuit winding location, as follows: Figure 10 , Figure 11 As shown, the increase in the weak magnetic field under different short-circuit conditions is also significantly smaller than that under different short-circuit conditions. Figure 6 , Figure 7 The situation indicates that the short-circuit turn is also not near measuring point 3.

[0079] The location of an inter-turn short circuit can be accurately determined by moving a weak magnetic sensor and observing changes in the weak magnetic field.

[0080] In summary, when a short circuit fault exists between rotor winding turns in a hydro-generator, the magnetic field detected by the weak magnetic field detection sensor is significantly different from the normal state and exceeds the set threshold. Therefore, this method can effectively detect short circuit faults between rotor winding turns. By comparing the weak magnetic fields near each winding turn laterally, the specific location of the faulty winding turn can be determined.

[0081] This invention, based on the principle of electromagnetic induction, proposes an innovative method and device for offline detection of inter-turn short circuits in the rotor windings of hydro-generators. This approach effectively identifies the presence of inter-turn short circuit faults, solving the problem of traditional methods relying on indirect impedance calculations for verification. It directly measures the changes in the weak magnetic field near the magnetic pole windings without requiring historical data support. It offers advantages such as convenient operation, significant comparison of detection results, high reliability and sensitivity, and location capability, making it suitable for offline detection of inter-turn short circuits in the rotor windings of hydro-generators.

[0082] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0083] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. An offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator, characterized in that, include: When the unit is shut down and the rotor winding is in an open circuit state, open the top cover, remove the wind baffle, and place the trapezoidal iron core with the excitation coil wound between any two adjacent rotor magnetic poles, so that the double inclined sides of the trapezoidal iron core respectively fit the sides of the pole shoes of the two rotor magnetic poles, forming a transpole magnetic circuit that spans the two rotor magnetic poles and is closed by the rotor magnetic yoke. An alternating current is injected into the excitation coil by an AC power supply connected in series with the excitation coil to generate an alternating magnetic flux in the transpole magnetic circuit, thereby exciting the rotor winding to be tested. Using a weak magnetic field detection sensor, which is closely attached to the side surface of the windings of the two rotor magnetic poles respectively, the weak magnetic field detection sensor is moved along the radial direction of the generator to scan the leakage magnetic field of each winding. When a local abnormal increase occurs in the leakage magnetic field, it is determined that there is an inter-turn short circuit in the winding corresponding to the rotor magnetic pole. The short-circuited winding is located in the area of ​​abnormal magnetic field increase, so as to realize fault location. Otherwise, it is determined that there is no fault.

2. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, The alternating current injected into the excitation coil is provided by a 50Hz low-voltage power frequency current source.

3. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, The excitation coil has 100 turns and is supplied with an alternating current of 2A to obtain the desired magnetic flux density.

4. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, When the weak magnetic field detection sensor performs a leakage magnetic field scan of the rotor winding, it is in close contact with the rotor winding during measurement and moves at a preset distance from the rotor winding.

5. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, When the relative deviation of the leakage magnetic field amplitude of a local rotor winding from the normal reference value under the same operating conditions exceeds 30%, it is determined that the local rotor winding has an inter-turn short circuit fault.

6. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, The weak magnetic field detection sensor covers the two rotor magnetic poles that are attached to and connected to the trapezoidal iron core in a single detection; wherein, the wind baffle is set corresponding to each rotor magnetic pole; during the detection, the wind baffle is removed along the circumference of the rotor at a rate of one wind baffle every other pole, so that the number of wind baffles to be removed is half the number of rotor magnetic poles.

7. The offline detection method for inter-turn short circuits in the rotor winding of a hydro-generator according to claim 1, characterized in that, The weak magnetic field detection sensor scans the entire radial section of the rotor winding to eliminate measurement blind spots and obtain information on the range of the fault along the radial direction.

8. An offline detection device for inter-turn short circuits in the rotor winding of a hydro-generator, characterized in that, include: A trapezoidal iron core wound with an excitation coil, wherein the double inclined sides of the trapezoidal iron core can fit against the side of the pole shoe of any two adjacent rotor magnetic poles, thereby forming a zero-air-gap transpolar magnetic circuit between the two magnetic poles. An AC power supply is connected in series with the excitation coil to form an external circuit. The AC power supply is used to inject a 50Hz low-voltage AC current into the excitation coil. A weak magnetic field detection device includes a weak magnetic field detection sensor and a weak magnetic field display device. The weak magnetic field detection sensor is attached to the rotor winding of the magnetic pole to be measured and moves in the radial direction to collect the leakage magnetic field induced voltage signal of the winding. The weak magnetic field display device is used to display the leakage magnetic field induced voltage signal and determine whether the rotor winding has an inter-turn short circuit based on the changes.

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