Electromagnetic Shielding of the Powerful
Turbogenerator Stator End Zone
Antonyuk O., Roytgarts M., Smirnov A.
OJSC “Power Machines”
St. Petersburg, Russian Federation
[email protected]
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.
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
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
Zone
Shield
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
b
Fig.5. Shielding attenuation of nonmagnetic pressure plate with internal (a) and outer (b)
shield of different thickness
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
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
Losses
zone the axial magnetic field is reduced by hundreds of times.
mm
mm
mm
Thickness of the pressure plate
mm
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
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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.
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