Coolant replenishing system for superconducting field windings

A coolant replenishing system for cooling the superconducting field winding of a machine, illustratively a turbo-generator. The machine is provided with a rotating antechamber in which is disposed a coolant in liquid and gaseous phases. Liquid coolant is delivered to the antechamber from a supply tank by a coolant supply line. The discharge opening of the coolant supply line in the antechamber is oriented away from the axis of rotation of the machine, and disposed at a predetermined radius with respect to the axis so that the input pressure of the coolant at the opening of the coolant feed line is in equilibrium with the pressure of the liquid coolant in the rotating antechamber.

BACKGROUND OF THE INVENTION 
This invention relates generally to machines which operate in cryogenic 
superconductive states, and more particularly, to a coolant feed system 
for a superconductive electric machine, illustratively a turbo-generator, 
in which a cryogenic coolant in liquid and gaseous phases is delivered to 
the machine by stationary coolant feed lines, connected to external 
coolant sources. 
One coolant replenishing system which allows flood or bath cooling of a 
superconducting winding in an electric machine, particularly a 
turbo-generator is described in German Offenlegungsschrift No. 29 23 496. 
The system described therein contains an antechamber which is disposed 
near the axis of rotation of the machine, and rotates therewith. A further 
cooling device, which contains a rotating mixing chamber in which coolant 
in liquid and gaseous phase states is contained, is disclosed in German 
Patentschrift No. 28 30 887 C3. In operation, the rotation of the mixing 
chamber causes the liquid and gaseous phases of the coolant to separate 
from each other by centrifugal forces. Thus, gaseous coolant settles in 
the portions of the mixing chamber which are near the axis of rotation, 
and the liquid coolant, which is used for cooling the field winding of the 
machine, remains in the portion of the mixing chamber away from the axis. 
The liquid coolant is distributed throughout the electric machine by a 
self-pumping effect utilizing thermo-siphon loops. Such thermo-siphon 
loops operate using the principle that the liquid and the gaseous phases 
of the coolant are characterized by different specific densities. A 
coolant distribution system is provided at the outer circumference of the 
field winding. The coolant distribution system is connected by a plurality 
of cooling canals which conduct the coolant through the field winding and 
to the outer portion of the mixing chamber. 
In an operating machine, all cooling canals in the field winding are 
completely flooded and filled with liquid helium from the coolant 
distribution system at the outer circumference of the winding. As the 
coolant absorbs heat from the field winding and the external environment, 
its density decreases. This decrease in density causes the coolant to flow 
in the cooling canals towards the mixing chamber. Simultaneously, colder 
and therefore denser coolant flows radially outward by means of coolant 
connecting lines into the coolant distribution system, and subsequently to 
the cooling canals. A pressure gradient develops along the cooling canals 
through the winding as a result of heat absorption. This produces 
convection flow in the system in the form of thermo-siphon loops (see: 
"Cryogenics", July, 1977, pages 429-433; and DE-OS No. 25 30 100). The 
rate of the circulating flow is increased as larger amounts of heat are 
absorbed by the coolant, thereby producing a fail-safe cooling operation. 
The gaseous coolant which is located near the axis of rotation in the 
mixing chamber is advantageously utilized to produce a counter flow for 
cooling connecting elements of the body of the rotor which carries the 
field winding. Such cooling substantially reduces the amount of heat which 
is introduced into the machine from the environment. In the course of such 
cooling, the gaseous coolant absorbs sufficient heat to be raised from a 
few degrees K to approximately room temperature, and becomes 
correspondingly less dense. Since the warming-up of the gaseous coolant 
takes place at a long radius from the axis of rotation, but the cold 
gaseous coolant enters the loop in the vicinity of the axis of rotation, a 
pumping effect is achieved. If the output pressure of the gaseous coolant 
from the mixing chamber is maintained at a constant pressure, 
illustratively 1.1 bar, the resulting pump would produce an under pressure 
in the mixing chamber of illustratively between 0.3 and 0.6 bar. As a 
result of the thermodynamic characteristics of helium, the reduction in 
pressure causes a drop of about 1 degree K, and, therefore a higher 
current-carrying capacity in the superconducting winding. 
In such a cooling arrangement, coolant must be replenished at low pressure 
and low temperature. A refrigerator plant must therefore be provided which 
supplies undercooled helium at low pressure. In addition to costing more 
than a plant which operates at normal pressure, difficulties are 
encountered in the maintaining of undercooled helium. Moreover, the 
extremely cold machine parts draw warm gases from the environment, as a 
result of the underpressure, thereby creating difficulties in sealing the 
system. 
As a result of these problems, pressure reduction stages which rotate with 
the winding have been provided so as to permit the refrigeration plant and 
the coolant feed lines to be operated at an optimum pressure of 
approximately 1.2 bar. One such pressure reduction stage which is provided 
in a system for replenishing helium which is conducted from a helium 
supply tank to a helium bath in the rotor of a superconducting generator 
is described in the above-mentioned German patent application P No. 29 23 
496.6. In this system, a rotating antechamber which is located near the 
axis of rotation of the generator is provided in a coolant feed system. 
Helium is supplied to the antechamber from an external supply by means of 
a stationary coolant feed line. Since this helium is in liquid and gaseous 
phases, the gaseous coolant settles near the axis of rotation when the 
rotor rotates. The liquid coolant is separated from the gaseous coolant, 
and occupies a region in the antechamber which is radially away from the 
axis of rotation. Thus, the antechamber operates as a phase separator. 
In this arrangement, the pressure differential between the coolant pressure 
in the antechamber and the underpressure in the rotating helium bath for 
the field winding is achieved by a relatively warm coolant column in a 
feed line between the supply chamber and the helium bath. The coolant 
column is maintained at a pressure which equals the colder coolant column 
of the helium bath. The level of the liquid-gas phase boundary in the 
rotating helium bath for the field winding is determined by the radius of 
the liquid-gas phase boundary in the antechamber. Accordingly, if the 
proportion of liquid coolant in the helium bath decreases as a result of 
losses, then the pressure of the rotating coolant column in the bath must 
decrease correspondingly in order that coolant can flow from the 
antechamber into the bath. The liquid coolant flowing out of the 
antechamber must then be replaced with coolant which is fed in from the 
outside. The level of the liquid-gas phase boundary in the phase separator 
must therefore be controlled by level controllers in the coolant 
replenishing system. Such level controllers, which may be temperature 
monitoring sensors which operate in conjunction with control valves, are 
disadvantageously large, expensive, and unreliable. 
It is, therefore, an object of this invention to provide a coolant 
replenishing system for an antechamber in a superconducting electric 
machine, wherein the supply of coolant which is provided from an external 
source is controlled as a function of coolant loss. 
SUMMARY OF THE INVENTION 
The foregoing and other objects are achieved by this invention which 
provides a coolant replenishing system wherein the outlet of a stationary 
coolant feed line is located in an outer region of an antechamber, and 
oriented away from the axis of rotation. The outlet of the stationary 
coolant feed line is located at a predetermined radial distance from the 
axis of rotation so that the pressure of the coolant at the outlet is in 
equilibrium with the pressure of the liquid coolant in the antechamber 
during normal operation of the machine. 
As liquid coolant is drawn from the antechamber, the reduced supply of 
liquid helium remaining in the antechamber generates, as a result of 
antechamber rotation, a correspondingly lower counter pressure at the 
point where the outlet of the stationary coolant feed line is disposed. 
Thus, since the input pressure is higher than the counter pressure of the 
remaining liquid coolant supply, coolant will flow into the antechamber 
until the predetermined amount of liquid coolant is restored and the 
equilibrium pressure is reestablished. It can be seen, therefore, that the 
coolant replenishing system according to the present invention is 
self-regulating with respect to the demand for liquid coolant by the 
machine. Such self-regulation maintains a substantially constant level of 
liquid coolant in the antechamber without the need for additional 
controls. 
In a further embodiment of the invention, the coolant replenishing system 
may be incorporated as part of a cooling arrangement which contains 
coolant feed lines connecting the radially outer region of the antechamber 
to a coolant distribution system which is arranged at the circumference of 
the superconducting field winding. This embodiment further comprises a 
rotating mixing chamber near the axis of rotation of the machine. During 
normal operation of the machine, the mixing chamber contains liquid and 
gaseous coolant. Coolant canals which extend through the field winding, 
and coolant connecting lines outside the field windings, interconnect the 
coolant distribution system and the mixing chamber. At least one coolant 
discharge line is disposed near the axis of rotation of the mixing chamber 
for discharging gaseous coolant from the mixing chamber to the outside. 
This discharge line may extend radially from the axis of rotation so as to 
produce a reduction in the pressure between the antechamber and the mixing 
chamber, and thereby improve coolant flow through the machine.

DETAILED DESCRIPTION 
The FIGURE shows a cooling arrangement which is intended for use in a 
superconducting field winding in the rotor of an electric machine, 
illustratively a turbo-generator. Portions of the rotor and machine which 
are not show in the FIGURE are described in DE-OS No. 24 39 719, or DE-OS 
No. 25 03 428. The FIGURE shows an upper half of a rotor with a coolant 
arrangement which comprises a coolant replenishing system. Rotating parts 
which are to be cooled are disposed in a vacuum, so as to limit the amount 
of heat which is conducted to these parts from the environment. 
A superconducting field winding 4 is arranged on a rotor body 3 which is 
supported concentrically about a shaft (not shown) having an axis 2. The 
rotating parts are surrounded by evacuated spaces 5 which are contained 
within a rotating cylindrical vacuum housing 6 which is at room 
temperature or warmer. An end portion 7 of vacuum housing 6 is designed 
integrally as part of a connecting head 9 of the rotor. Connecting head 9, 
as will be discussed hereinbelow, is provided with means for receiving and 
discharging coolant which is required for cooling field winding 4. 
The cooling arrangement contains a rotating mixing chamber 11, which is 
near axis 2 and which contains a bath of coolant which is boiling at 
underpressure. In this embodiment, helium is provided as the coolant 
because winding 4 is comprised of superconductive material. As the rotor 
rotates, a phase separation is produced in mixing chamber 11 as a result 
of centrifugal forces. Thus, liquid helium A.sub.1 settles concentrically 
about gaseous coolant B.sub.1 which is held near the axis rotation. A 
similar arrangement is described in German patent application P No. 28 30 
887.4. 
A boundary 12 in mixing chamber 11 is a surface which separates the gaseous 
and liquid coolant phases. A coolant distribution system 14 is connected 
to mixing chamber 11 by cooling canals 15 and coolant connecting lines 16. 
Cooling canals 15 conduct coolant from distribution system 14, through 
winding 4, and to mixing chamber 11. Coolant connecting lines 16 are 
outside of winding 4 and supply liquid coolant from mixing chamber 11 to 
distribution system 14. 
The cooling of superconducting field winding 4 is achieved by a 
self-pumping effect in thermo-siphon loops. In essence, cold liquid helium 
A.sub.1 is pumped from mixing chamber 11 to the coolant distribution 
system 14 by means of the radially arranged cooling connecting lines 16. 
As indicated, the cold liquid helium is then utilized to cool 
superconducting field winding 4 by cooling canals 15. As a result of heat 
which is accumulated in the coolant from the environment and heat 
dissipation from field winding 4, the density of the coolant in the 
coolant connecting lines and in the cooling canals is reduced, thereby 
producing a corresponding reduction in the hydrostatic pressure. Such 
hydrostatic pressure differences cause the coolant to return to mixing 
chamber 11 radially inward by means of cooling canals 15. 
Evaporated coolant B.sub.1, which is collected in mixing chamber 11 during 
normal operation of the machine is suctioned off to the outside by exhaust 
gas lines 18 and 19. Such gas is exhausted by also using the self-pumping 
effect described hereinbelow with respect to the liquid coolant. Gaseous 
coolant B'.sub.1 which is in exhaust lines 18 and 19, and which has been 
drawn from mixing chamber 11 at a point near axis of rotation 2, is warmed 
by being utilized for cooling rotor elements such as connecting elements 
20 and 21, which are relatively warm, and other parts of the machine 
rotor. Such other rotor parts may, for example, be a cylinder 22 which 
operates as an electromagnetic damper and is only partially shown in the 
FIGURE. Connecting elements 20 and 21 which are disposed at a substantial 
radius from axis of rotation 2, are cooled by coolant gas B'.sub.1 to the 
temperature of vacuum housing 6. Such cooling is achieved in a direction 
countered to the heat flow, thereby permitting these elements to be 
considered as counter flow coolers. Coolant gas B'.sub.1 which absorbs the 
heat from connecting elements 20 and 21 is discharged centrally from the 
rotor at connecting head 9 and conducted to refrigeration plant 24, as 
indicated in the FIGURE. 
As indicated above with respect to the flow of liquid coolant, the 
self-pumping effect of gaseous coolant B'.sub.1 is caused by differences 
in density between cold and warm coolant gas. If the output pressure gas 
is held constant, a coolant underpressure in the mixing chamber of 
illustratively 0.4 bar is produced. This results in a lowering of the 
temperature of the coolant by approximately 1 degree K. 
The liquid helium which is required for cooling the superconducting field 
winding is supplied to distribution system 14 from a rotating antechamber 
26 which is arranged near the axis of rotation and connecting head 9. 
During normal operation of the machine, antechamber 26 contains liquid 
coolant A.sub.2 which is settled in regions away from axis of rotation 2 
by centrifugal forces caused by rotation of the antechamber. The liquid 
coolant surrounds gaseous coolant B.sub.2 which is located near the axis 
of rotation. A boundary surface 27 separates the liquid and gas coolant 
phases. A coolant feed line 28 is connected to rotating antechamber 26 at 
a region away from axis of rotation 2 and extends radially so as to 
conduct liquid coolant A'.sub.2 from the antechamber to coolant 
distribution system 14 at the outer circumference of field winding 4. 
A pressure reduction stage (not shown) is provided between antechamber 26 
and field winding 4 so as to create a state of equilibrium between 
pressure P.sub.1 of the coolant at the outer circumference of field 
winding 4, and pressure P.sub.2 at the radially outer end of feed line 28. 
Such a pressure equalizing system is required because the respective 
helium columns from mixing chamber 12 and antechamber 26 are at different 
pressures densities and temperatures. One pressure reduction stage which 
may be utilized to achieve the desired pressure equalization is described 
in Offenlegungsschrift No. 29 23 496. The levels of liquid-gas boundaries 
27 and 12 are in a feedback relationship with respect to one another. 
Thus, a lowering of the level of boundary 12 in mixing chamber 11 toward a 
larger radius causes pressure P.sub.1 to drop. Consequently, helium can 
flow through feed line 28 of the pressure reduction stage until 
equilibrium between pressures P.sub.1 and P.sub.2 is restored. 
Liquid coolant A'.sub.3 is supplied to rotating antechamber 26 from an 
external supply tank 29. Supply tank 29 is arranged geodetically higher 
than antechamber 26, and contains liquid coolant A.sub.3 and gaseous 
coolant B.sub.3, at a pressure of about 1.2 bar. The gaseous and liquid 
phases are separated by a boundary 30. A line 33 is provided between 
antechamber 26 and supply tank 29 which is controlled by a valve 32 for 
conducting liquid coolant. Line 33 begins at the bottom region of supply 
tank 29, extends into antechamber 26, and is provided with a radially 
outward extending stationary section 34 which is provided with an outlet 
50. A further line 36 having a valve 35 is provided for exchanging cold 
coolant gas between the gas space of supply tank 29, which is formed above 
coolant A.sub.3, and the region near axis of rotation 2 in antechamber 26. 
Such a gaseous exchange may be achieved even though gaseous coolant 
B.sub.3 in supply tank 29 and gaseous coolant B.sub.2 in antechamber 26 
are at approximately the same pressure of 1.2 bar. In some embodiments, 
line 26 can be arranged concentrically around line 33 which conducts 
liquid helium so as to improve the thermal insulation of the liquid 
helium. 
Liquid coolant A.sub.4 from an external refrigeration plant 24 can be fed 
into supply tank 29 by means of a connecting line 38 having a shut off 
valve 37. Gaseous coolant B'.sub.3 can be returned from the supply tank to 
the refrigeration plant by means of a line 40 having a shut off valve 39. 
Coolant line 42 branches off of line 38 at a point between valve 37 and 
the refrigeration plant, and is itself branched off into two sections 45 
and 46 having respective valves 43 and 44. These lines lead into main 
lines 33 and 36 which extend between antechamber 26 and supply tank 29, 
the discharge points 47 and 48 being located between valves 32 and 35, and 
the antechamber. 
The coolant replenishing system disclosed above insures that a sufficient 
quantity of liquid helium A.sub.2 is always available in antechamber 26 
even if large quantities of liquid helium A'.sub.2 flow into coolant 
distribution system 14 through coolant feed line 28. The maintaining of a 
suitable level of liquid A.sub.2 in antechamber 26 is achieved by the 
advantageous orientation of stationary coolant feed line 33 in the 
antechamber. Coolant feed line 33 is provided with a radial section 34 
having a discharge opening 50 so as to discharge coolant into the region 
of antechamber 26 which is away from rotating axis 2. The discharge 
opening is disposed a predetermined radius R from axis of rotation 2. 
Predetermined radius R is selected so that the pressure of coolant 
A'.sub.3 at discharge opening 50 is in equilibrium with the pressure of 
liquid coolant A.sub.2 which rotates in the antechamber during the normal 
operation of the machine. As previously noted, such rotation produces a 
gradient of hydrostatic pressure in the antechamber which varies radially 
from the axis of rotation. The magnitude of the hydrostatic pressure is a 
function of the level of liquid-gas phase boundary 27, and the rate of 
rotation of the antechamber. Thus, if the level of boundary 27 is 
displaced outward, then the pressure at discharge point 50 drops 
accordingly, and liquid helium can flow into antechamber 26. If, on the 
other hand, the radius of the phase boundary level recedes toward the axis 
of rotation, the pressure at the discharge point rises, and the supply of 
liquid helium is discontinued. In this manner, the supply of liquid helium 
in antechamber 26 is automatically regulated. In one embodiment, 
antechamber 26 rotates at 50 revolutions per second and liquid coolant 
discharge outlet 50 is disposed at a radius of 5 centimeters from the axis 
of rotation. In such an embodiment, the acceleration of liquid coolant 
along portion 34 of the coolant supply line is 500 times the acceleration 
which would be due only to gravity. In addition, the radial position of 
phase boundary 27 changes only very little with a change in the level of 
boundary 30 in supply tank 29. A drop of 1 meter in the level of boundary 
30 results in a drop of approximately only 2 millimeters in the level of 
boundary 27. Such minor variations in the level of phase boundary 27 in 
view of large excursions in the level of phase boundary 30 in supply tank 
29 permit extended operation of the machine during temporary failure of 
refrigeration plant 24. 
In the embodiment shown in the FIGURE, a radially disposed partition 52 is 
provided at one end of the rotor shaft in connecting head 9, and near 
discharge opening 50. Partition 52 prevents liquid coolant A.sub.2 in 
antechamber 26 from communicating with seals 53. Seals 53 are disposed at 
a radius with respect to axis of rotation 2 which is greater than that of 
phase boundary 27. Seals 53 communicate between rotating and stationary 
parts of connecting head 9. In this manner, it is assured that seals 53, 
as is the case with seals 54, serve only to prevent an exchange of warm 
and cold gas. If the pressure of the cold gas is somewhat higher than that 
of the warm gas, for example, 1.2 bar as against 1.1 bar, then the cold 
gas escaping through the gaps at these seals cools the walls according to 
the counterflow principle. This further reduces the losses due to the 
introduction of heat from the environment. 
One exhaust gas, designated with the symbol "C", which escapes from exhaust 
gas lines 18 and 19 at the seals, is combined in a plenum 55 of connecting 
head 9, and conducted from there to refrigeration plant 24 by means of 
collecting line 56. A valve 57 may be located in collecting line 56. 
In operation without disturbance, valves 32 and 35 are open, while valves 
33 and 44 are closed. Refrigeration plant 24 supplies a sufficient 
quantity of liquid helium A.sub.3 to supply tank 29. Two process 
variations are available for cooling-down the machine. In the first 
variation, the warm rotor is flooded with liquid coolant A.sub.3 by 
opening valves 32 and 35. Although very short cooling times can be 
accomplished by such flooding, the rapidity of temperature change causes 
large temperature gradients in the rotor. Moreover, cooling efficiency is 
relatively poor. In the second variation, valves 32, 35, 37 and 39 are 
kept closed, and valves 43 and 44 are opened. This permits refrigeration 
plant 24 to communicate directly with main feed lines 33 and 36. 
Refrigeration plant 24 then supplies initially warm and then increasingly 
colder gas so as to cool the rotor gradually. This requires large amounts 
of gas, for which reason two lines, 33 and 36, are advantageously 
connected in parallel so as to provide a sufficiently large flow cross 
section. After the cooling process has advanced to the point that liquid 
coolant A.sub.2 accumulates in antechamber 26, valves 43 and 44 are closed 
and valves 32, 35, 37 and 39 are opened. The supply of liquid coolant 
A.sub.3 in supply tank 29 accelerates the cooling-down process which would 
otherwise be limited by the output rating of refrigeration plant 24. After 
thermal equilibrium and normal phase boundary levels 12 and 27 are 
achieved, the cooling arrangement makes an automatic transition to normal 
continuous operation. 
During times when it is desired to warm-up the rotor, the speed of the 
rotor is reduced to, for example, 2.25 revolutions per second, so that the 
centrifugal force on liquid coolant A.sub.2 in antechamber 26 is just 
sufficient to balance the pressure at discharge point 50 which results 
from the force of gravity. Valves 32, 35, and 44 are closed and liquid 
helium A.sub.2 is either conducted into a geodetically lower tank (not 
shown) or, with valves 43 and 47 open, liquid coolant A.sub.2 is pumped by 
a pump, which may be part of refrigeration plant 24, into supply tank 29 
by means of lines 33, 45, 42 and 38. Since the pumping effect of 
connecting elements 20 and 21 as counterflow coolers is virtually 
eliminated as a result of the low speed of rotor rotation, the gas glow in 
exhaust gas lines 18 and 19 is reversed, and the pressure in the rotor 
rises to approximately 1.1 bar, which is the pressure of the exhaust gas 
system. If the pump in refrigeration plant 24 lowers the pressure on its 
intake side to about 1 bar, the differential pressure of 0.1 bar is 
sufficient to empty the liquid helium out of the entire rotor. Thereupon, 
the warming-up can be accomplished by blowing gas through lines 33 and 36 
in a manner analogous to the cooling-down process. 
In the event of a so-called "quench", wherein a portion of field winding 4 
goes from super conducting to normal conducting states, the local 
temperature rise associated therewith would lead to increased helium 
evaporation. Such evaporation would produce an increase in the internal 
pressure, thereby causing liquid helium to be transported back to supply 
tank 29. Since the cross section of lines 33 and 36 can be advantageously 
selected at will, contrary to exhaust gas lines 18 and 19, the cross 
section of which in counterflow coolers 20 and 21 must be kept small 
because of heat transfer, a reduction in the pressure of the overall 
coolant feed system can be assured to a large degree. If the internal 
pressure of the rotor is high, antechamber 26 is flooded with liquid, and 
both lines 33 and 36 transport this liquid equally to supply tank 29. If 
the quench causes the rotor to be emptied completely, line 36 would 
conduct the gas which continues to flow and which is increasing in 
temperature, because line 33 would remain filled with liquid from the 
supply tank. The gas therefore does not need to flow through the liquid 
coolant in the supply tank, which would lead to additional evaporation. 
The pressure increase resulting from a quench would also produce a rise in 
the temperature of the helium. This would increase the velocity of 
propagation of liquid helium through the winding. Since liquid helium 
would be pushed out of the rotor, the level of liquid-gas boundary 12 
would drop toward a larger radius. As soon as inner-most conductors of 
winding 4 are no longer covered by liquid helium, such conductors would be 
cooled to a much lesser degree, thereby permitting a more uniform 
distribution of thermal energy throughout the winding. 
Although the inventive concept disclosed herein has been described in terms 
of a specific embodiment and applications, other applications and 
embodiments will be obvious to persons skilled in the pertinent art 
without departing from the scope of the invention. The drawing and the 
description of the specific illustrative embodiment of the invention in 
this disclosure are illustrative of applications of the invention and 
should not be construed to limit the scope thereof.