Method for non-destructive testing of stator winding insulation

Description

[0001] The present invention relates to a method for the non-destructive testing of at least partial areas of an insulation of a stator winding of an electrical machine, in particular a generator or a high-voltage motor.

[0002] The winding bars of the stator winding of generators, such as those of turbo and hydropower generators, are provided with insulation. This insulation usually comprises several layers of insulating tape wound one above the other, which are wound in an overlapping manner around the winding bars. The layers of insulating tape are impregnated with a resin that is cured after winding. In terms of their shape, the winding bars each comprise a straight section, most of which is inserted into the slots of the stator or stator core, and two involute-shaped sections adjoining the two free ends of the straight section, which protrude outwards from the stator slots. Since the stator core is grounded, the insulation in the slot area contains a weakly electrically conductive outer layer, the so-called external corona shield, to control the potential distribution in the insulation.In the edge area of ​​the straight section, the external corona shielding outside the grooves is extended by a weakly conductive end corona shielding so that the electrical potentials on the rod surface are gradually controlled and the electric field inside the insulation is increasingly reduced.

[0003] During generator operation, the stator winding insulation undergoes structural changes caused by electrical, thermal, mechanical, and chemical stresses. This results in accelerated aging of the insulation, leading to long-term damage, for example, in the form of microcracks or delamination. To prevent generator damage, it is necessary to detect such aging phenomena and their extent early and, if necessary, to remedy them promptly. For this reason, stator winding insulation is inspected at regular intervals and repaired as needed.

[0004] The detection of aging phenomena in stator winding insulation is currently carried out using, for example, partial discharge measurements. However, this proven method, which is generally considered an integral measurement, can only cover those sections of the stator winding insulation where the electric field is sufficiently strong—that is, those sections where the winding bars are arranged within the stator slots, as well as the sections comprising approximately 30% of the front insulation length under the respective corona shield. However, aging phenomena also occur in the remaining areas of the winding bars, so complete detection of aging phenomena in stator winding insulation is not possible using partial discharge measurements alone.In addition, localization of detected aging phenomena is not possible to the required extent during partial discharge measurement, since the accuracy of the transit time measurement of the partial discharge pulses is approximately 1.2 m.

[0005] Another known method for detecting signs of aging in stator winding insulation is high-voltage testing, but if significant signs of aging are present, this can lead to irreparable breakdown, which is why it is rarely used.

[0006] Furthermore, it is possible to detect superficial defects or damage to the insulation using an endoscope or mirror. However, deeper defects cannot be detected with these methods and therefore remain undetected.

[0007] CIMINO ALESSANDRO ET AL: "Analysis of fault detection in the electrical insulation system of rotating machines" discloses a method for the non-destructive testing of at least partial areas of an insulation of a stator winding of an electrical machine, in particular a generator or a high-voltage motor, using terahertz measurement technology.

[0008] Terahertz measurement technology is based on electromagnetic waves in the frequency range of 0.1 to 10 terahertz at wavelengths from 3 mm to 30 µm. Due to its low photon energy, terahertz radiation can penetrate non-conductive materials. Its short wavelengths simultaneously offer spatial precision, allowing defects to be resolved and precisely localized in terahertz images. Furthermore, terahertz technology enables depth-based analysis and can detect defects located deep within the insulation. Since the radiation is non-ionizing and therefore harmless to biological matter, no special protective measures are required, making the technology well-suited for use in industrial environments. Thanks to the inventive on-site inspection and diagnosis of the insulation condition, repair measures can be initiated immediately if existing defects are detected.In this way, the risk of an unexpected failure of the corresponding electrical machine during operation can be significantly reduced, thus avoiding the costs associated with such a failure. Since terahertz technology enables precise statements to be made about the type, extent, and location of existing insulation defects, the development of detected defects that were not immediately remedied due to their minor extent can be monitored and recorded. Accordingly, it is possible to predict relatively reliable statements about the influence of various defects on the service life of the insulation. This is becoming increasingly important, especially given that the operating mode of generators has changed significantly due to the steadily increasing share of renewable energies, which is why previous empirical values ​​can only be relied upon to a limited extent.The test data and findings obtained with terahertz technology can also be used for future insulation designs.

[0009] The invention relates to a method according to claim 1 for the non-destructive testing of at least partial areas of an insulation of a stator winding of an electrical machine and a corresponding testing device according to claim 8.

[0010] According to one embodiment of the method according to the invention, the non-destructive testing is carried out on those sections of the insulation that are accessible from the outside, in particular exclusively on those sections of the insulation that are accessible from the outside. This applies primarily to those sections of the insulation that surround involute-like bent sections of the stator winding or winding bars that protrude outward from the stator slots. For such sections, as already explained above, testing using partial discharge measurement is unsuitable because the electric field in these sections is not sufficiently strong.

[0011] Advantageously, the non-destructive testing is carried out using a terahertz sensor that is moved along the surfaces of the insulation sections to be tested and performs measurements at different measuring positions.

[0012] The sensor signals emitted by the terahertz sensor are preferably introduced into the surfaces of the insulation sections to be tested at predetermined angles, with the predetermined angles preferably being 90°. Due to the limited space available for positioning the terahertz sensor, it may well be the case that the preferred angle of 90° cannot be easily maintained at certain measurement positions and must be varied accordingly.

[0013] According to one embodiment of the present invention, the signals received by the terahertz sensor in response to the transmitted signals are stored together with the respective position and orientation of the terahertz sensor at the time of transmission of the signals relative to a predetermined coordinate system. In this way, detected insulation defects can be precisely localized based on the coordinate system.

[0014] Preferably, the surfaces of the insulation sections to be tested are scanned using the terahertz sensor, and individual two-dimensional sectional planes or sectional images through the insulation are generated by recording complete depth information of the insulation, so-called depth sweeps, at individual measurement points. The depth information in the terahertz measurement is obtained, in particular, using the frequency-modulated continuous wave (FMCW) radar method. In this method, the difference frequency between a transmitted frequency ramp and a frequency ramp reflected by the insulation is detected in the receiver path of the measuring system by frequency mixing. From the frequency data thus acquired, travel time or depth information of the individual reflective layers of the insulation is generated.

[0015] Advantageously, 3D images of the insulation sections to be tested are generated based on the generated two-dimensional sectional planes or sectional images.

[0016] The movement of the terahertz sensor is automated using an electro-mechanical displacement unit, in particular in the form of an articulated-arm robot holding the terahertz sensor, which enables very precise, flexible and reproducible guidance of the terahertz sensor during the non-destructive testing.

[0017] The electromechanical displacement unit is mounted on the rotor of the electric machine and is moved along it. To move the electromechanical displacement unit along the rotor, the displacement unit can have a driven drive unit, for example, equipped with wheels. The drive unit can be equipped with magnets on its underside to allow it to be moved overhead on the rotor.

[0018] Alternatively, it is possible to attach the electromechanical displacement unit to the stator of the electrical machine at a first position. After performing a number of measurements, detach it and attach it to the stator of the electrical machine at a second position, different from the first position, to perform further measurements. Compared to the first variant, this variant has the disadvantage that the displacement unit must be manually repositioned several times. The advantage, however, is that the structure of the electromechanical displacement unit is less complex and therefore cheaper to manufacture due to the fact that no drive is required.

[0019] Furthermore, the present invention provides a testing device which is designed to carry out the method according to the invention, comprising an electro-mechanical displacement unit and a terahertz sensor arranged on the latter.

[0020] The electro-mechanical displacement unit is preferably an articulated arm robot.

[0021] According to a first variant, the electro-mechanical displacement unit is designed to be arranged on a rotor of an electrical machine and to be moved thereon by a motor.

[0022] Alternatively, the electro-mechanical displacement unit is designed to be attached to a stator of an electrical machine at a first position and, after performing a number of measurements, to be released and attached to the stator of the electrical machine at a second position different from the first position for performing further measurements.

[0023] The terahertz sensor preferably has at least one dielectric measuring tip, which is angled, particularly in the region of its free end, to accommodate the narrow space available for positioning the terahertz sensor during the implementation of the method according to the invention. The at least one measuring tip can be surrounded by a rigid sheath to prevent damage and / or bending of the measuring tip.

[0024] Alternatively, the terahertz sensor can have a quasi-optical free-beam system designed to direct the terahertz radiation toward the measurement position. The quasi-optical free-beam system can be implemented, for example, in the form of miniaturized mirror optics.

[0025] Further features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings. Fig. 1a perspective schematic partial view of a stator winding of an electrical machine, in the form of a generator; Fig. 2 a schematic perspective view of a single column bar of the Figure 1 shown stator winding; Fig. 3 an enlarged view of section III in Figure 2, showing an involute-like bent section of the stator rod; Fig. 4 a schematic perspective view of four winding bars of the Figure 1 shown stator winding; Fig. 5 a sectional view showing an upper winding bar and a lower winding bar arranged in a common slot of the stator of the Figure 1 shown stator winding; Fig. 6 a perspective view showing a testing device according to a first embodiment of the present invention during the performance of a testing method according to the invention; Fig. 7a perspective view showing a testing device according to a second embodiment of the present invention during the performance of a testing method according to the invention, and Fig. 8 a schematic view showing a terahertz sensor 16 whose measuring tip is inserted between two winding bars.

[0026] The Figures 1 to 5show parts of a stator winding 1 of an electrical machine 2, which in this case is a generator, for example a turbo or hydropower generator. The stator winding 1 comprises a plurality of winding bars 3. With reference to Figures 2 and 3, each winding bar 3 is made from several firmly consolidated conductor strands 4 and has a central straight section and two involute-like bent sections 6 that adjoin the straight section 5 on both sides. Each winding bar 3 is covered with insulation 7. The insulation 7 comprises a base insulation 8, which completely covers the straight section 5 and the bent sections 6. The base insulation 8 in this case consists of insulating tape that is wound in several overlapping layers around the winding bar 3. The insulating tape is impregnated with resin that was cured after winding.Furthermore, the insulation 7 comprises an outer corona shield 9, which covers the straight section 5, but not the bent sections 6, and an end corona shield 10 in the transition areas between the straight section 5 and the bent sections 6. Both the outer corona shield 9 and the end corona shield 10 are made of a conductive strip.

[0027] The straight sections 5 of the winding bars 3 are, as shown in Figure 1is shown, inserted into slots 11 formed in an annular stator 12 of the electrical machine 2. In the present case, two winding bars 3 are arranged radially one above the other in a single slot 11, namely first a lower winding bar 3a and then an upper winding bar 3b, which are fixed in the associated slot 11 using a wedge piece 13. The bent sections 6 of the lower winding bars 3a are aligned crossing the bent sections 6 of the upper winding bars 3b, as shown in Figure 1 indicated by arrows 14 and 15, wherein the bent sections 6 of both free ends of each lower winding bar 3a are connected to the free ends of the bent sections 6 of the upper winding bars 3b, and vice versa. The connection is realized via connecting elements not shown in detail.

[0028] During generator operation, a structural change in insulation 7 occurs, caused by electrical, thermal, mechanical, and / or chemical stresses. This results in accelerated aging phenomena of insulation 7, leading to long-term damage, for example, in the form of microcracks, delaminations, defects, or the like. To prevent generator damage, it is therefore necessary to detect such aging phenomena and their extent early on and, if necessary, to remedy them in a timely manner.

[0029] According to the invention, a non-destructive test of the insulation 7 is carried out in-situ at the installation site of the electrical machine 2 on the externally accessible sections of the stator winding 1 using terahertz measurement technology. This primarily concerns those sections of the insulation 7 that surround the bent sections 6 of the winding bars 3, but possibly also other sections of the insulation 3 that are accessible, for example, through cooling openings or the like.

[0030] Terahertz measurement technology operates with electromagnetic radiation in the terahertz range, i.e., in a frequency range between 0.1 and 10 terahertz. Electromagnetic radiation in this frequency range can penetrate materials opaque to visible and infrared light, especially non-metallic materials, making damage to insulation 7 detectable. The wavelengths of the radiation are in the range of 30 µm to 3 mm, thus providing good spatial resolution. Accordingly, detected defects can also be easily localized. The radiation is very low-energy, which is why it is not harmful to humans.

[0031] To carry out the procedure, as described in the Figures 6 to 8As shown, a testing device 24 comprising a terahertz sensor 16 is used here, wherein the terahertz sensor 16 is moved along the surfaces of the partial areas of the insulation 7 to be tested and carries out measurements at different measuring positions. The sensor signals emitted by the terahertz sensor 16 in the form of electromagnetic radiation in the terahertz range are introduced into the surfaces of the partial areas of the insulation 7 to be tested at predetermined angles, which are preferably 90°. In order to take into account the narrow installation space between the adjacent bent sections 6 of the winding bars 3 in combination with the preferred predetermined angles, the terahertz sensor 16, as shown in Figure 8shown by way of example, advantageously has one or more dielectric measuring tips 25 which are angled in the region of their free end. Accordingly, the terahertz sensor 16 can be inserted between adjacently arranged curved sections 6 of the winding bars 3 such that the free end of the measuring tip 25 is aligned perpendicular or at least almost perpendicular to the surface 26 of the insulation 7 over which or along which the measuring tip is to be guided. In the present case, the measuring tip 25 is surrounded by a rigid sheath 27 which protects the measuring tip 25 and prevents bending. Alternatively, instead of curved dielectric measuring tips, the terahertz sensor 16 can also have a miniaturized quasi-optical free-beam system which is designed to direct the emitted terahertz radiation in the range of the preferred predetermined angles onto the respective measuring positions.

[0032] The movement of the terahertz sensor 16 is automated using an electro-mechanical displacement unit 17, which is designed in the form of an articulated-arm robot holding the terahertz sensor 16, the arm members 18 of which are connected to one another via a plurality of joints 19. The electro-mechanical displacement unit 17 can, as shown in Figure 6 shown, be arranged on the rotor 20 of the electrical machine 2 and be movable thereon. For this purpose, a motorized drive unit 21 is provided, which is equipped with wheels or the like. On the underside, the drive unit 21 is provided with magnets (not shown in detail) in order to be able to move it overhead on the rotor 20. Alternatively, it is possible to use the electro-mechanical displacement unit 17 as in Figure 7shown, to be attached to the stator 12 of the electrical machine 2 at a first position and, after performing a number of measurements, to be released and, for performing further measurements, to be attached to the stator 12 at a second position, which is different from the first position. For this purpose, the electromechanical displacement unit 17 has a base unit 22, which is provided with suitably designed fastening means 23, which enable the base unit 22 to be releasably attached to the stator 12.

[0033] The signals received by the terahertz sensor 16 in response to the transmitted signals are then stored together with the respective position and orientation of the terahertz sensor 16 or its measuring tip(s) 25 at the time the signals were transmitted, relative to a predetermined coordinate system. For each measurement, a depth sweep of the insulation 7 is performed in the direction of the transmitted signals, and a two-dimensional cross-sectional plane of the insulation 7 is generated accordingly. The depth information is preferably obtained using the frequency-modulated continuous wave radar method. Quasi-3D images of the tested sections of the insulation 7 can then be compiled from a multitude of such cross-sectional planes in the direction of movement of the terahertz sensor 17. The positions of any defects contained therein can then be easily located based on the coordinate system.

Claims

1. Method for the non-destructive testing of at least partial regions of an insulation (7) of a stator winding (1) of an electric machine (2), in particular a generator or a high-voltage motor, wherein the non-destructive testing is carried out in situ at the installation site of the electric machine (2) using terahertz measurement technology, wherein the non-destructive testing is carried out using a terahertz sensor (16), which is moved along the surfaces of partial regions of the insulation (7) that are to be tested and carries out measurements at different measurement positions, wherein the terahertz sensor (16) is moved in an automated manner using an electromechanical displacement unit (17), in particular in the form of a buckling arm robot holding the terahertz sensor (16) and wherein the electromechanical displacement unit (17) is arranged on the rotor (20) of the electric machine (2) and is moved on said rotor.

2. Method according to Claim 1, characterized in that the non-destructive testing is carried out on those partial regions of the insulation (7) which are accessible from the outside, especially on partial regions of the insulation (7) which surround involutely curved sections (6) of the stator winding (1) that protrude outwardly from grooves (11) in the stator (12).

3. Method according to Claim 1, characterized in that the sensor signals emitted by the terahertz sensor (16) are introduced at predetermined angles into the surfaces of partial regions of the insulation (7) that are to be tested, wherein the predetermined angles are preferably 90°.

4. Method according to Claims 1 and 3, characterized in that the signals received by the terahertz sensor (16) in response to the emitted signals are stored together with the respective position and orientation of the terahertz sensor (16) at the time at which the signals are emitted, based on a predetermined coordinate system.

5. Method according to one of Claims 1, 3 and 4, characterized in that the terahertz sensor (16) is used to scan the surfaces of partial regions of the insulation (7) that are to be tested and individual two-dimensional sectional planes or sectional images through the insulation (7) are generated by recording complete depth information relating to the insulation (7) at individual measurement points in each case, wherein the depth information in the terahertz measurement is obtained in particular by way of the frequency-modulated continuous wave radar method.

6. Method according to Claim 5, characterized in that quasi-3D images of the partial regions of the insulation (7) that are to be tested are generated based on the generated two-dimensional sectional planes or sectional images.

7. Method according to Claim 1, characterized in that the electromechanical displacement unit (17) is attached to the stator (12) of the electric machine (2) at a first position and detached after a number of measurements have been carried out, and is attached to the stator (12) of the electric machine (2) at a second position, different from the first position, in order to carry out further measurements.

8. Testing device (24) designed for carrying out non-destructive testing of at least partial regions of an insulation (7) of a stator winding (1) of an electric machine according to one of the preceding claims, comprising an electromechanical displacement unit (17) arranged on the rotor (20) of the electric machine (2), and a terahertz sensor (16) arranged on said unit.

9. Testing device (24) according to Claim 8, characterized in that the electromechanical displacement unit (17) is a buckling arm robot.

10. Testing device according to Claim 8 or 9, characterized in that the electromechanical displacement unit (17) is designed to be arranged on a stator (12) of an electric machine (2), in particular a generator or a high-voltage motor, and to be moved on said stator in a motor-driven manner.

11. Testing device (24) according to Claim 8 or 9, characterized in that the electromechanical displacement unit (17) is designed to be attached to a stator (12) of an electric machine (2), in particular a generator or a high-voltage motor, at a first position and detached after a number of measurements have been carried out, and to be attached to the stator (12) of the electric machine (2) at a second position, different from the first position, in order to carry out further measurements.

12. Testing device (24) according to one of Claims 8 to 11, characterized in that the terahertz sensor (16) has at least one dielectric measurement tip (25), which is angled in particular in the region of its free end.

13. Testing device (24) according to one of Claims 8 to 11, characterized in that the terahertz sensor (16) has a quasi-optical free-beam system designed to direct the terahertz radiation onto the measurement position.