Arrangement for cooling a super conducting field winding and a damper shield of the rotor of an electric machine

A cooling arrangement for a superconducting field winding of an electric machine, especially a turbo-generator, has a coolant space which contains a cryogenic coolant fed into it via a feed line, for cooling the field winding, and a coolant supply chamber which contains liquid coolant for cooling a damper shield. So that no separate feed and discharge lines are required for cooling the damper shield independently of the cooling of the field winding, the coolant supply chamber is connected directly to the coolant feed line via a radial connecting line, and the flow cross sections of the connecting line and of the section of the coolant feed line between the connecting head and the connecting point of the connecting line are made large.

BACKGROUND OF THE INVENTION 
This invention relates to electric machines with superconducting windings 
in general and more particularly to an arrangement for cooling a 
superconducting field winding in the rotor of an electric machine. 
An arrangement for cooling the superconducting field winding of a rotor of 
an electric machine, especially a turbo-generator, with at least one 
coolant chamber which contains, in the operating condition, a vaporous and 
a liquid phase of a cryogenic coolant is known. The coolant is fed at a 
connecting head under subcritical pressure into a corotating feed line 
which is connected to the coolant chamber. Coolant paths extend through 
the field winding and are connected to the liquid space of the coolant 
chamber occupied by the liquid phase. At least one coolant discharge line 
is connected to the vapor space of the coolant chamber occupied by the 
vaporous phase. There is at least one coolant supply chamber containing 
liquid coolant. At least one further coolant path, which is arranged in 
loop-fashion, is thermally connected to a damper shield arranged around 
the field winding and is connected at its start and end to the coolant 
supply chamber. Such a cooling arrangement is described in the report by 
M. T. Brown et al. entitled "Rotor Cooling System for a 10-MVA 
Superconducting Generator" from Proceedings of the 1979 Cryogenic 
Engineering Conference, Madison, Wis, U.S.A., Aug. 21 to 24, 1979, Paper 
IC 9. 
The superconducting field winding of a generator must be kept at a 
sufficiently low temperature during the operation of the machine that its 
superconductors do not undergo a transition into the normally conducting 
state. In general, cooling with liquid helium as the cryogenic coolant is 
therefore provided, and the dissipation heat produced in the 
superconductors and the heat introduced from the outside into the 
torque-transmitting parts of the rotor body lead to partial evaporation of 
the coolant. The heat flux introduced into the low temperature region of 
the rotor, however, can be greatly reduced, according to the principle of 
counterflow cooling, if the evaporated but still cold coolant exhaust gas 
is passed along the torque-transmitting rotor parts in good thermal 
contact, absorbs from these parts a large part of the inflowing heat, is 
warmed up in the process and leaves the rotor as warm gas at a connecting 
head. 
In the superconducting field winding itself, dissipation heat is produced, 
among other things, if the superconductors are subjected to an alternating 
magnetic field. This takes place in the rotor, for instance, if, in the 
case of unbalanced load, not all current phases of the stator winding are 
loaded uniformly, and also after a short circuit switching sequence or in 
the case of oscillations when the rotor speed temporarily does not agree 
with the predetermined network frequency. In order to keep the alternating 
field amplitudes in the superconductors small in such transient operating 
states, the field winding may be surrounded by one or more damper shields 
which shield the magnetic field and at the same time damp oscillations. 
For shielding against low frequency fields, damper shields of thermally 
and electrically highly conductive material, kept at low temperature, are 
advantageous. For, during steady state generator operation, practicaly no 
dissipation heat is produced in a damper shield. A large amount of 
dissipation heat is temporarily released only during the relatively brief 
transient operating states, especially in the case of oscillations. 
Contrary to the superconductors of the field winding, the temperature of 
which must remain below the transistion temperature of its superconductive 
material even during the transient operating states, the temperature of 
the cold damper shield is allowed to rise higher, for instance, to 20 to 
30K. 
A turbo-generator with a field winding surrounded by such a damper shield 
is known from the report of the Electric Power Research Institute U.S.A.: 
"ERPI EL-577, Project 429-1, Final Report", November 1977, pages 3-258 to 
3-270. In cooling the field winding, a so-called self-pumping effect is 
utilized, as is described, for instance, in the dissertation of A. Bejan: 
"Improved Thermal Design of Cryogenic Cooling Systems for a 
Superconducting Synchronous Generator", Ph.D Thesis, Massachusetts 
Institute of Technology (U.S.A.), December 1974, pages 148 to 159. The 
damper shield of the machine known from the ERPI report is fastened to a 
support cylinder of the rotor body, in which individual canals are 
provided which serve as damper cooling canals. For this indirect cooling 
of the damper shield, liquid helium is provided which flows in these 
canals due to a co-called thermo-siphon effect. Heat produced in the cold 
damper shield penetrates the support cylinder, gets into the helium 
contained in its cooling canals and starts convection there. Due to this 
convection, the warmed helium flows back into a helium bath present in the 
center of the rotor, where a corresponding amount of liquid helium 
evaporates. 
In this known cooling arrangement it is assured that the heat from the cold 
damper shield is not transported directly into the area of the 
superconducting field winding since the damper cooling canals are 
thermally insulated from the winding cooling canals. In addition, good 
heat transfer from the support cylinder to the helium contained in its 
cooling canals makes rapid cooling of the damper possible. In this cooling 
arrangement, however, removal of generated gaseous helium is done using 
exhaust gas lines with a relatively small flow cross section. These 
exhaust gas lines run along the torque-transmitting parts of the rotor 
body. A large flow of helium vapor which occurs suddenly in the case of a 
disturbance then cannot leave the rotor fast enough, so that the pressure 
in the central helium bath rises accordingly. A consequence thereof is 
also an increase of the saturation temperature of the helium of this bath, 
used for cooling the superconducting field winding, and therefore, of the 
temperature of the winding itself. 
These difficulties are largely avoided in the above-mentioned cooling 
arrangement for a corresponding turbo-generator known from the report from 
the "Proceedings of the 1979 Cryogenic Engineering Conference". For, in 
this cooling arrangement, the cooling of two damper shields and the 
winding cooling are largely separated from each other. Accordingly, there 
is provided, in addition to a central coolant chamber for receiving the 
coolant required for cooling the superconducting field winding, a separate 
cooling supply chamber in which the liquid helium required for cooling the 
damper shields is stored. In addition, a phase separator for separating 
gaseous helium components is provided in the coolant loop for cooling the 
damper shield. Now, if heat is suddenly released in the damper shields, a 
coolant circulation is developed, due to a thermo-siphon effect, from the 
helium bath contained in the coolant supply chamber through the first 
damper shield to a phase separator and through the second damper shield 
back to the helium bath. Due to the supply of heat, helium vapor is then 
generated in the helium bath of the coolant chamber, and in the phase 
separator; this helium vapor can escape through special gas outlets and 
further, through an emergency outlet provided in the separator. A return 
valve in the connection between the coolant supply chamber and the coolant 
space containing the coolant bath for the winding prevents any reaction of 
the pressure rise in the coolant supply chamber on the coolant bath of the 
winding. This valve also makes it possible to fill the coolant supply 
chamber with liquid helium. However, a difficulty still exists with this 
cooling arrangement, in that the helium vapor, which is formed in the 
coolant bath of the winding, cannot escape as long as the return valve of 
the coolant supply chamber is closed. Then, the pressure and therefore, 
the temperature of the coolant provided for cooling the field winding, can 
rise however. In addition, proper operation of the return valve with its 
moving parts can be assured at low temperatures only at relatively high 
cost. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to simplify a cooling 
arrangement of the kind mentioned at the outset, and to improve it in such 
a manner that the discharge of coolant vapor caused by losses in the 
damper shield from the rotor is facilitated without appreciable 
concomitant warming of the liquid coolant which must be provided for 
cooling the field winding. 
According to the present invention, this problem is solved by connecting 
the coolant supply chamber via a radial connecting line directly to the 
coolant feed line and for removing gaseous coolant components from the 
coolant supply chamber toward the connecting head, by making the flow 
cross section of the radial connecting line as well as the flow cross 
section at least of the section of the feed line between the connecting 
head and the point of connection of the feed line accordingly large. 
The advantages of this design of the cooling arrangement are, for one, that 
already existing coolant lines are utilized for feeding the coolant 
required for the loop to cool the damper shield into the rotor as well as 
also for removing this coolant. Therefore, separate coupler parts of 
elaborate design between rotating and stationary machine parts are not 
required for the damper coolant at the connecting head of the rotor. 
Through ample design of the flow cross sections of the line parts carrying 
coolant exhaust gas of the damper cooling loop, it is ensured at the same 
time that large amounts of exhaust gas which are produced in the event of 
operating transients can be removed from the rotor without difficulty. 
Secondly, cooling the damper shield is made possible with this cooling 
arrangement in such a manner that it is substantially independent of the 
cooling system for cooling the superconducting field winding. In the event 
of temperature increases in the cooling loop of the damper shield, a heat 
transport via the coolant to the conductors of the field winding is 
therefore largely precluded. 
According to a further embodiment of the cooling arrangement of the present 
invention, the coolant path of the damper shield includes at least one 
pair of substantially axial coolant lines which are tied together at the 
point which is farthest removed from the associated coolant supply 
chamber. Advantageously, one coolant line of each pair is arranged 
radially further outward while the other coolant line is located radially 
further inward. In this manner, a very effective coolant flow through the 
coolant lines, due to a thermo-siphon effect, is made possible. 
Other advantageous embodiments of the cooling arrangement according to the 
present invention are also described.

DETAILED DESCRIPTION 
In the FIGURE, the path of a coolant through only part of the rotor 
adjoining a connecting head, of an electric machine, especially of a 
turbo-generator, is indicated. The rotor parts not specifically detailed 
in the FIGURE may correspond, for instance, to parts of a machine which is 
described in U.S. Pat. No. 3,942,053. From the FIGURE, only a part of the 
upper half of the rotor with a cooling arrangement according to the 
present invention can be seen. 
A rotor body 3, supported rotatably about an axis 2, contains a winding 
carrier 4 which is provided with slots 5 into which conductors of a 
superconducting field winding 6 are inserted. Only some of the slots with 
the winding parts arranged therein are detailed in the FIGURE. The 
conductors of the winding 6 contain superconductive material; liquid 
helium is provided as the cryogenic coolant. For thermal insulation of the 
winding, which must be cooled to a low temperature, toward the outside, it 
is surrounded by the vacuum spaces 7, 8, and 9 which are provided in the 
rotor body 3 and which are located within a cylindrical outer part 10 and 
substantially radial parts 11 on the end face of the rotor body 3. The 
rotor parts 10 and 11 are generally approximately at room temperature. 
The helium required for cooling is taken from a coolant supply device, not 
shown in the FIGURE, at subcritical pressure, i.e., for instance, in the 
normal boiling state at normal pressure or at a slight overpressure of, 
say, 1.1.times.10.sup.5 Pa and a temperature of about 4.4 K, and is fed 
into the rotor via a helium coupler at a connecting head, not shown in 
detail. By means of such a helium coupler which is known, for instance, 
from the publication "Siemens Forschungs-und Entwicklungsberichte", vol. 5 
(1976), no. 1, page 13, the coolant is transferred from stationary to 
rotating machine parts. The coolant thus is transported into a corotating 
feed line 13 which extends in the axial direction in the interior of the 
rotor body. This feed line is a double tube which is concentric with the 
axis 2 and has a coolant guiding space of annular cross section. However, 
it can advantageously also consist of several individual tubes which are 
jointly located on the surface of a cylinder, or a single, centered tube 
if no other lines are required in the center. 
Since part of the coolant evaporates when fed into the rotor body, a liquid 
component of the coolant, designated as A, accumulates within the feed 
line 13 in a radially outer region 14 under the influence of the 
centrifugual force, and a gaseous component B accumulates in the region 15 
of the feed line 13 facing the rotor axis 2. 
The liquid helium required for cooling the field winding 6 is taken from 
the region 14 of the feed line 13, containing the liquid component A, for 
instance, in the rotor center indicated by a dashed line 16, and is first 
transfered via a coolant feed line 17 to a coolant space 18 which is 
arranged concentrically about the feed line 13. 
For cooling the superconducting field winding, a loop is advantageously 
provided in which the well-known self-pumping effect is utilized. Such a 
coolant loop is provided, for instance, in the cooling arrangement known 
from DE-OS No. 28 30 887. 
Due to heat inflow from the outside and also due to the dissipation heat 
generated in the winding parts of the field winding 6, part of the coolant 
provided for cooling the field winding evaporates. These components 
accumulate in a region 19 near the axis, designated as the vapor space, of 
the coolant space 18. In the operating state, there is thus a two-phase 
mixture of liquid coolant A.sub.1 and gaseous coolant B.sub.1 in the space 
18. During rotation, the phases are separated under the influence of 
centrifugal forces, so that the heavier liquid coolant A.sub.1 accumulates 
in a liquid space 20 concentrically about the gaseous coolant B.sub.1 
which is held in the region 19 facing the rotor axis 2. 
The coolant vapor B.sub.1 contained in the coolant space 18 is discharged 
to the outside via a specially designed exhaust gas line 22. Here, too, a 
self-pumping effect is utilized for drawing the gaseous coolant off. For, 
the coolant gas taken via a radial tube 23 at a point of the vapor space 
19 near the axis is warmed up by being used for counter-flow cooling of a 
torque-transmitting connecting piece 24 of the rotor body 3. This 
connecting piece 24 is the end piece at the end face of a hollow 
cylindrical support body part 25 of the rotor body 3 and extends between a 
cold intermediate piece 26 of this support body part and the warm side 
portion 11 of the rotor body. The exhaust gas line 22 further contains an 
axial line section 28 which is thermally connected to the piece 24 away 
from the axis and acts as a heat exchanger. The connecting coolant exhaust 
gas warmed up in this manner and designated as B.sub.2, is then returned 
to the vicinity of the axis and discharged from the rotor at a connecting 
head, not detailed in the FIGURE, and is fed to a refrigerating machine. 
Due to the self-pumping effect for pumping off the coolant exhaust gas, an 
underpressure is then advantageously established in the exhaust gas tube 
23 and therefore in the coolant space 18, which is, for instance, less 
than 10.sup.5 Pa. 
Since the coolant in the feed line 13 is at normal pressure or at a slight 
overpressure, but underpressure prevails in the coolant space 18, pressure 
equalization must take place between these two spaces. The pressure 
equalization is achieved in a manner known per se by the particular design 
of the coolant feed line 17 as a pressure equalization line (see, for 
instance, "Cryogenics", 1977, pages 429 to 433). Accordingly, the coolant 
feed line 17 contains a tubular line section 30, which is connected to the 
feed line 13, leads radially outward, and to the outer end of which a 
tubular line section 31 leading radially inward is connected which opens 
into the coolant space 18. It can thereby be achieved that pressure 
equilibrium prevails at the connecting point 32 away from the axis between 
the tubular line sections 30 and 31, because the two helium columns in 
these line sections have different temperatures and therefore, densities, 
due to the different pressures. Lowering the level in the coolant space 18 
toward a larger radius causes, furthermore, a pressure unbalance at the 
connecting point 32. Then, helium can flow in via the feed line 17 until 
the equilibrium condition between the pressures is reestablished. 
To limit the alternating field amplitudes at the superconducting conductors 
of the field winding 6, for instance, in the event of a pulse short 
circuit or oscillations, the field winding is concentrically surrounded by 
two corotating damper shields 34 and 35. The damper shield 34 is mounted 
on the outside of the hollow cylindrical outer part 10 of the rotor body 3 
and is at room temperature, while the damper shield 35 is arranged on the 
outside of the hollow cylindrical support body part 25 and is cooled to a 
low temperature. In this shield 35, which advantageously consists of a 
material with very high thermal and electric conductivity such as copper, 
practically no dissipation heat is produced during steady state generator 
operation. A very large amount of dissipation heat is temporarily released 
there only during the relatively brief cases of transient operation. 
The cooling system required for cooling the inner damper shield 35 is 
advantageously largely separated from the system for cooling the field 
winding 6. This damper cooling system contains one or more pairs of axial 
damper cooling canals 36 and 37, at least one coolant supply chamber 38 
and at least one radial, tubular connecting line 39. The coolant supply 
chamber 38 is connected to the helium feed line 13 and thereby, to the 
cooling system of the field winding 6, only via this connecting line 39. 
Each pair of damper cooling canals 36 and 37 is connected to the coolant 
supply chamber 38 via two connecting lines 40 and 41, the connecting line 
40 opening into the chamber 38 radially further outward and the connecting 
line 41 radially further inward. The two cooling canals of a pair of 
cooling canals are also advantageously arranged at unequal distances 
relative to the rotor axis 2. The damper cooling canal 36 is at a larger 
radius with respect to the axis 2 than the cooling canal 37. 
The two cooling canals 36 and 37 of a pair of cooling canals are also 
connected to each other at a point 42 which is farthest away from the 
coolant supply chamber 38 assigned to them. According to the illustrated 
embodiment this junction point is at the center of the rotor as indicated 
in the FIGURE by the dashed line 16, since it is assumed that a 
corresponding cooling loop for the cold damper shield is also provided in 
the part of the rotor not detailed. Thus, a flow of the coolant provided 
for cooling the damper shield 35 in a closed circuit is made possible. 
Each pair of cooling canals together with the connecting lines 40 and 41 
forms a thermo-siphon loop. For, the heat released in the damper shield 35 
imparts buoyancy to the helium in the cooling canals 36 and 37 and the 
connecting lines 40 and 41 toward the rotor axis 2. Thereby, a flow is 
developed: colder, heavier helium flows from the coolant supply chamber 38 
to the damper shield 35, while warmer, lighter helium flows back into the 
supply chamber. There, the warmed up helium tends to flow toward the rotor 
axis 2 and gets into the helium feed line 13 via the radial connecting 
line 39 at a connecting point 43. Part of the helium is evaporated there 
and leaves the rotor in the gas space 15 near the axis. 
In the course of a transient in the operation, especially during a case of 
oscillation, a large part of the coolant contained in the helium supply 
chamber 38 evaporates. Removal of the gas quantities produced can 
advantageously be ensured by making the flow cross section of the tubular 
connecting line 39, and at least of the section 44, extending between the 
connecting head and the mouth 43 of the connecting line 39, of the helium 
feed line 13, large enough. After the disturbance, the coolant supply 
chamber 38 is then refilled with liquid helium which is taken from an 
external helium supply unit. 
With this cooling arrangement, degradation of the winding cooling by the 
damper cooling is largely made impossible. Since the axial helium feed 
line 13 is made large enough, a suddenly appearing quantity of helium 
vapor can escape from the rotor without a major increase in the pressure. 
A slight pressure rise in the axial feed line 13, at most, has the 
consequence that the relatively small amount of liquid helium A present in 
the feed line is pumped via the feed line 17 into the coolant space 18. 
Helium vapor cannot get from the feed line 13 into this coolant space 18 
since the liquid in the radial canals 30 and 31 of the feed line 17 acts 
as a barrier. The gas pressure in the axial feed line 13 and in the radial 
feed line 30 counteracts the pressure of the liquid column, caused by the 
centrifugal force, in the other radial feed line 31. As long as the gas 
pressure does not become larger than the pressure of the liquid at the 
deflection point 32 away from the axis, no gas can therefore get from the 
axial feed line 13 into the coolant space 18. The pressure of the 
deflection point 32 depends mainly on the radial distance of the helium 
level in the coolant space 18 from the axis of rotation 2 and on the 
distance of the deflection point 32 from the axis of rotation 2. If the 
latter is, for instance, 0.4 m and is large as compared to the radial 
distance of the helium level in the coolant space 18, this pressure is 
about 11.times.10.sup.5 Pa at a rotor speed of 50 revolutions per second. 
In this cooling arrangement, separate feed and discharge channels for 
feeding and discharging coolant which is provided for cooling one or more 
damper shields, are thus advantageously avoided without causing an 
impairment of the winding cooling system. 
The rotor body 3 indicated in the FIGURE also contains a hollow cylindrical 
part 46 which acts as a radiation shield and is located between the outer 
part 10 of the rotor body 3 and its hollow cylindrical support body part 
25 which surrounds the cold damper shield 35. This radiation shield 46 has 
its end faces mechanically and thermally connected to the torque 
transmitting connecting parts 24 of the support body part 25. 
In the FIGURE a coolant exhaust gas line 48 near the axis is also shown. 
The exhaust gas is removed from the rotor through line 48. The exhaust gas 
is used for counterflow cooling the end piece, not detailed in the FIGURE, 
on the turbine side of the torque-transmitting hollow cylindrical rotor 
body part 25.