Electromagnetic Shielding of the Powerful Turbogenerator Stator End Zone Antonyuk O., Roytgarts M., Smirnov A. OJSC Power Machines St. Petersburg, Russian Federation Roytgarts_MB@els.power-m.ru Terms and definitions Electromagnetic shield is a construction made of metal with high electrical conductivity, displacing the magnetic field due to the eddy currents reaction (electromagnetic mirror) [1,2]. The shield is closed, if the magnetic field can get to the opposite side of the shield, just passing through the shield. If a part of the magnetic field can go around the shield, the shield is half-closed. Shielding effectiveness is estimated by the shielding factor and shielding attenuation. Shielding factor - is the ratio of the magnetic field behind the shield to the field at the same point without the shield. Shielding attenuation is a logarithm of the reciprocal value of the shielding factor module. 1. INTRODUCTION The end zone of the powerful turbogenerator stator is under serious electromagnetic, thermal and vibration loads. The turbogenerators operation experience shows that the accidents are the most frequent in the end zone. In this respect the design of the pressure plate and the shield, protecting the pressure plate and the stator core end packages from the penetration of electromagnetic fields of the stator and rotor windings, are of great importance and the greatest danger is the axial component of the magnetic field, which induces eddy currents and losses in the pressure plate and core end packages. The purpose of this work is a study the shielding effectiveness and losses in the shield, the pressure plate and the stator core end packages of the powerful turbogenerator under operational loads, considering the design features of the stator winding.
Results of numerical modeling and experimental data for shields and pressure plate in the end zone of the two-pole turbogenerators with rated power 800-1100 MVA are given in the this report. The standard numerical (ANSYS) and numerical-analytical methods, first of all, the so-called 2.5-dimensional model for calculating the rotating magnetic field, taking into account the complex geometry of the turbogenerator structure in the plane of the rotation axis and implying the symmetry and spatial periodicity of the structure in the direction of field rotation, are used as the methods of analysis of the electromagnetic processes in the end zone [3,4]. The spatial distribution of the stator and rotor windings, the configuration and electromagnetic properties of the core, the pressure plate and the stator electromagnetic shield, the rotor geometry, design of the hull and the end panels, spatiotemporal changes of the currents in the stator and rotor winding are taken into account. The harmonic representation of currents and electromagnetic fields in the direction of the machine rotation allowed combining an analytical approach to the problem in this area with the numerical analysis in the plane of the rotation axis. The results of calculation of three-dimensional electromagnetic field in such formulation of the problem are obtained as a superposition of the rotating waves. The adequacy of the calculation method is confirmed by comparing the calculation results with the experimental data obtained in the actual turbogenerators under load (Fig.1). a b c Fig.1 End zone model (а).the calculated and experimental values of the radial (b) and axial (с) induction components along the bottom surface of the horizontal shelf of the pressure plate at rated load.,i - experimental data The models and real shield structures are manufactured to verify the calculations, the design and technological solutions.
2. DESIGN OF WINDING AND LOSSES IN THE STATOR END ZONE 2.1 The layout of the winding layers Let us consider the impact of stator two-layer bar winding layout of laying on the losses in the end zone. Due to the reduction of the winding step, the spatial position of the upper and lower layers of winding does not coincide. In the slot winding due to high permeability of the core it does not matter, which side of the winding is leading and which is lagging behind. In the end zone the situation changes. If the top layer of the stator winding is advancing, the phase angle between the excitation field and the field nearest to the pressure plate the lower layer of the stator winding increases. At this the losses in the pressure plate and the shield decrease, however the losses in the stator ventilation panels increase. Changes of the magnetic fields, depending on the layout of the stator winding are clear from the vector diagram of Figure 2. Changes of the losses in pressure plate and the outer electromagnetic shield are shown in Figure 3. At the same time, regardless of the winding layout the losses in the mode of underexcitation increase, which is clear from the vector diagram for this mode. Fig.2. Stator winding layout and vector diagram of the stator and rotor currents 2.2. The bending angle of the end winding The impact of the angle at which the stator winding end are bent in the axial plane axis on the losses in the nonmagnetic pressure plate with an outer copper shield is shown in Table 1; the change of magnetic field distribution is shown in Figure 4. As the angle of bending increases, the losses in the shield are redistributed, reach the maximum value, and then slightly decrease due to the reduction of losses in the shield nozzle (cylindrical part of the shield). Thus the losses in pressure plate increase with the increase of the bending angle, but do not exceed 11% of the losses in the outer shield. The distribution of the magnetic field
shows that along with the increase in bending the greater part of the field impacts the disc part of the pressure plate and the shield. Fig.3. Ratio of the losses in the shield and the pressure plate depending on the winding layout. The rated load and underexcitation Table1. Losses, depending on the bending angle of the stator winding end, per unit. Zone 15 22,5 30 45 1 6,7 7,4 7,2 4,6 2 1,0 1,2 1,2 1,2 3 1,3 1,9 2,5 3,6 4 0,4 0,6 0,8 1,3 Shield 1 4 9,5 11,1 11,8 10,7 5 0,0 0,1 0,1 0,1 6 0,0 0,0 0,0 0,1 Pressure 7 0,2 0,3 0,5 1,1 Plate 5 7 0,3 0,4 0,6 1,2 Total 1 7 9,7 11,5 12,3 11,9
22,5 45 Fig.4. Magnetic induction when changing the bending angle of the winding end 3. THE PRESSURE PLATE OF NONMAGNETIC STEEL WITH INNER AND OUTER SHIELD If the pressure plate is made of nonmagnetic metal with low electrical conductivity and thermal conductivity (non-magnetic steel, titanium), to prevent the plate and the stator core end package from heating the additional shields, usually copper electromagnetic ones, are applied. The electromagnetic shield can be under the stator pressure plate on the core side or on the outer surface of the pressue plate. For the copper shield the thickness of 15-20 mm is usually chosen, exceeding the penetration depth by factor of 1.5-2. The non-magnetic pressure plate without the shield has a low efficiency of shielding, as shown in Figure 5, and significant inherent losses (Figure 6). 3.1. Varying the thickness of the shield Let us compare the construction of the pressure plate with the inner and outer shield by the effectiveness of shielding and stand out losses. The calculations show (Figure 5), that the pressure plates with inner and outer shield of the same thickness have similar shield attenuation values, but the spatial zone of effective shielding of the inner shield is wider. Along the edges, the magnetic field partially bypasses the shield and the plate, which means, the shield is a half-closed. Furthermore, near the inner diameter of the plate and the shield due to reversal of the eddy currents phase in the shield (Fig. 8), the field of the eddy currents increases the axial component of the external magnetic field. As a result, the losses in the area of base of the slots of the core end packages are reduced by the shield just twice in comparison with a plate without a shield, the overall effect of the plate and the shield is the reduction of losses by three times, using a shield with a nozzle results in the loss reduction of
5.5 times, which corresponds to a shielding attenuation 1.7. Due to the low efficiency of shielding of the last core stator packages it is made slits of teeth and bases of slots (fig. 9). a Fig.5. Shielding attenuation of nonmagnetic pressure plate with internal (a) and outer (b) shield of different thickness b Fig.6. Losses in nonmagnetic pressure plate and the shield depending on the thickness and placement of the shield
With internal location of the shield the greater part of losses is released in the pressure plate, with outer location of the shield the plate is protected, almost all the losses are released on the shield. At this the most loaded part of the shield is the nozzle and the shield minimal diameter zone, which is the nearest to it. Total losses in the pressure plate and the shield are always less with the outer location of the shield, than with the internal location. It can be seen from the histogram of figure 6. Figure 7 shows the distribution of the eddy currents and losses at the internal location of the shield. Figure 8 shows a distribution of the amplitude and phase of the eddy currents at the outer shield placement. Considering the fact that at the outer shield placement the currents in the pressure plate are small compared to the currents in the shield, in order to show those currents the color sensitivity is increased in the figure 8; that is why the amplitudes of the currents in the shield are in the top part of the color scale. Q, W/m3 Fig.7. Eddy currents and losses in the nonmagnetic plate and internal shield Fig.8. Amplitudes and phases of the eddy currents in the nonmagnetic plate with the outer shield
Fig.9. Losses in the last core stator packages without shield (a), plate 80 mm with outer shield (b), plate 160 mm with outer shield (c) 3.2. Varying the thickness of the pressure plate The thickness of the pressure plate is chosen on the basis of the construction and mechanical properties to provide the required press forming of the core and minimize the crushing of the teeth of the stator core end packages. Let us review the impact of changing the thickness of the pressure plate on the shielding efficiency of the axial component of the magnetic field and the losses. As one can see in fig.10 with the unchanged shield thickness the shielding attenuation is weakly dependent on the thickness of plate with the internal placement of the shield. When an external shield with increasing thickness of the plate most of the field walks the plate and shield in the zone of minimum diameters. In this area is decreasing of the shielding attenuation, losses in the last package are increasing. The total losses in the shield and the plate increase with increasing the thickness of the plate but they are always less with outer placement of the shield, than with internal one (fig.11). The shield nozzle increases the losses in the shield and decreases the losses in the plate. Fig.10. Shielding attenuation when varying the thickness of the nonmagnetic pressure plate
with internal and outer shield Fig.11. Losses in the nonmagnetic pressure plate and the shield depending on the plate thickness 4. PRESSURE PLATE OF MAGNETIC STEEL The special case is when the pressure plate is made of magnetic steel. The losses in the magnetic pressure plate are 1.7 times much as the losses in the nonmagnetic plate (fig. 11, 13) wherein the shielding attenuations of the plates due to the edge effect differ insignificantly (fig. 10, 12). Due to the low depth of penetration into the magnetic steel the losses are released in the surface layer, the density of losses is high and the electromagnetic shields are required to protect the plates from heating. In the presence of the outer magnetic shield the losses in the magnetic pressure plate are reduced by several times, however, they exceed the losses in a similar construction with the nonmagnetic pressure plate. With increasing thickness of the magnetic pressure plate the losses in the plate and the shield increase. Due to the alignment of the edge effect, connected with a half-closed nature of the shield and the pressure plate, with the surface effect in the magnetic plate the thickness of the plate does not influence the shielding attenuation. The presence of "nozzle" in the shield increases shielding attenuation and decreases the losses in the plate and the stator core end package at least by twice.
Fig.12. Shielding attenuation of the magnetic pressure plate without shield and with shield Fig.13. Losses in the magnetic pressure plate and the shield at the rated load and at underexcitation
Losses Fig.14. Distribution of the eddy currents and losses in the magnetic pressure plate with shield 5. THE PRESSURE PLATE MADE OF ALUMINUM ALLOY The pressure plate of aluminum alloy (Silumin) possesses electrical and thermal conductivity which is less than the copper's one, but is quite high, and is an order of magnitude greater than that of non-magnetic steel. The thickness of the plate, selected for mechanical reasons, provides high shielding efficiency, and the additional shield is not required. The peak value of shielding attenuation of the plate made of Silumin (fig. 16) is higher, than the one of the nonmagnetic or magnetic plates with the shields, in the middle zone the axial magnetic field is reduced by hundreds of times. mm mm mm mm Thickness of the pressure plate Fig.15 Distribution of losses by zones of the pressure plate made of aluminum alloy
At the same time, the edge effect is also expressed here, in the plate minimal diameter zone the field is not shielded, the protection of the end package from the axial magnetic field is required. Losses in the pressure plate of Silumin (fig. 15) is 1.5 times less than in nonmagnetic steel plate and 2.5 times less than in the magnetic steel plate. The losses in the end package are of the same order of magnitude as those of non-magnetic steel plate with the shield. Fig.16. Shielding attenuation of the pressure plate made of aluminum alloy 6. THE INFLUENCE OF THE WELDING SEAMS OF THE SHIELD ON ITS EFFECTIVENESS In the manufacture of the actual structures of complex shape copper shields the deviations from the ideal solid shell are inevitable. It is necessary to manufacture a disk turning into the cylindrical "nozzle" for the shields under the pressure plate. For the outer shields outside the pressure plate the disk turns into the flared section and then if necessary into the cylindrical "nozzle". The simplest way to manufacture such structures at the minimum consumption of materials is to weld it from the segments. The eddy currents in the shield are the concentric tangential-angular contours eddy currents in the shield are the concentric tangential-radial contours rotating together with the exciting magnetic field. The number of the contours equals to the number of the poles of the machine. For effective shielding the resistance to the flow of these currents should be minimized. The welding seams
with the resistance increased in the weld zone decrease eddy currents and reduce the shielding attenuation. For checking purposes the model of the welded copper shield in full size was manufactured and the shielding efficiency of the axial component of the pulsating magnetic field was determined (Fig.17a). Welding was performed in argon with the wire of CuSi3 grade having electric conductivity less than that of the copper. As a source of the magnetic field two concentric coils with opposite direction of current were used. The diameters of coils correspond to the big and small diameters of the shield. The coils are powered by alternating current of industrial frequency (Fig.17b). The shielding attenuations (fig. 18) were determined by measuring the axial field along the welding seam and along a continuous surface of the shield, juxtaposed with the measurements of the field without the shield else. The measurement results show that in the zone of the welding seam the shielding effectiveness is reduced by 5-7 times compared with the solid copper, that is proportional to the reduction in the electric conductivity in that zone. This conclusion is valid for a single shield. In the presence of an electrical contact of the shield with pressure plate the eddy currents in the welding seam high-resistance area will pass into the pressure plate, thus the resistance for them will be reduced, a shielding effect will be restored (Fig. 19). This means that if during the operation of the turbogenerator the local microcrack appears on the shield, the emergency situation does not happen, if the additional local heating of the pressure plate in the zone of currents flow does not exceed the generator cooling system capacity and does not worsen the vibration condition of the end zone. a b Fig.17. Shield model on a test facility (a). Winding sources of the magnetic field (b)
Fig.18. Shielding attenuation of the shield model Fig.19. Overflow of the eddy currents along the welding seam and on the solid copper into the pressure plate in the welding zone 7. CONCLUSION 1. The layout of the stator winding influences the losses in the end zone. Depending on the placement of the winding layers the losses can be redistributed between the pressure plate and the end shields. The advancing top layer of winding decreases the losses in the pressure plate in increases the losses in the ventilation shield. 2. The increase in bending of the stator end winding results in change of losses distribution in the pressure plate; the greater part of losses are in the cylindrical part of the pressure plate. 3. The load mode significantly influences the losses in the end zone. At the underexcitation with power factor of 0.95 with regard to the rated load, the losses in the structure of the stator end zone increase up to 40% and more. 4. The pressure plate of nonmagnetic steel possesses low shielding effectiveness of the magnetic field in the stator end zone. To protect the pressure plates of nonmagnetic steel and the stator core end packages from heating it is advisable to use the electromagnetic shield made of materials with high electric and thermal conductivity. 5. Regardless of the material of the pressure plate and the shield the shielding effect is absent in the plate minimal diameter zone; the increase of the axial component of the magnetic field happens. To protect the last packages from heating slits in the teeth and the slot bases are used. 6. At internal arrangement of the shield a zone of effective shielding of the last package the widest, however, such arrangement of the shield does not protect a pressure plate from losses and heating. At outer arrangement of the shield with increase in a thickness of a
pressure plate the edge effect is amplified, losses in the last package of the core are increased. At the same time the pressure plate is protected from losses. 7. The pressure plate of magnetic steel for powerful turbogenerators cannot be used without outer shield, maximally covering the plate, including from the side of the air gap. 8. The pressure plate of aluminum alloy does not require additional shields, and possesses the highest shielding efficiency of the considered structures. 9. The welding seams as well as the local micro cracks in the shields reduce the shielding effectiveness, increase heating of the pressure plate; their admissibility is determined by the efficiency of ventilation and vibration level in the area. References 1. H.Kaden. Wirbelstrome und Schirmwirkung in der Nachrichtentechnik. Berlin. Springer, 1959. 354s. 2. Janusz Turowski. Elektrodynamika techniczna. Warszawa, WNT, 1968, 487s. 3. V.Chechurin, I.Kadi-Ogly, M.Roytgarts, Yu.Varlanov. Computation of Electromagnetic Field in the End Zone of Loaded Turbogenerator. IEMDC 2003. Proceedings of the International on Electrical Machines and Drives Conference, Madison, Wisconsin, USA, June 1-4, 2003. 4. M.Roytgarts, Yu.Varlamov, A.Smirnov. Electromagnetic computation in the end zone of loaded turbogenerator. ADVANCED COMPUTER TECHNIQUES IN APPLIED ELECTROMAGNETICS/ Springer, 2008, IOS PRESS. Chapter C. Applications/ C1.Electrical Mashines and Transformers.