Process for drying the insulation of stator winding rods of electrical machines with direct liquid cooling

Winding rods, built into the stator of an electrical machine, are no longer safe to operate after leakages in a water chamber. After this waterchamber has been removed, as is necessary anyway, a flexible, gas-proof hose is pulled upon the rod end, and the machine-side end of the hose is attached to the rod in a gas-proof manner. The other hose end is connected to a vacuum pump and, by evacuating the hose, the water that has penetrated into the rod insulation is removed by the same route as it penetrated into the insulation. The quality of this drying process is monitored by dielectric measurements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for drying the insulation of stator 
winding rods of electrical machines with direct liquid cooling, which 
stator winding on both fronts of the machine comprises connection devices 
for power connection and for the supply or evacuation of cooling fluid or 
from the wave guides of the stator winding rods, at which connection 
device--and possibly in case of a stator winding that comprises not only 
wave guides but also massive subconductors--all subconductors are 
encompassed by a metallic structural component and are soldered together 
with the latter and among each other, which structural component protrudes 
over the subconductors, forming a water chamber, and which ends in a 
connection fitting for cooling liquid supply or cooling liquid evacuation. 
2. Discussion of the Background 
In the stator winding head of water-cooled electrical machines, the cooling 
fluid is supplied from ring-shaped collecting lines via insulation hoses 
to the so-called water chambers at the end of the conductor rod. The water 
chamber is formed by a connection device where the subconductors are 
encompassed by a metallic component and are soldered together with the 
latter and with each other. The metallic component protrudes over the 
subconductors, forming a water chamber. It ends in a connection fitting 
for cooling fluid supply or cooling fluid evacuation. The connection 
device at the same time serves as a power connection. 
The connection device is exposed to high mechanical stresses (vibrations 
oscillations!) during operation. Even minimal leakages cause damage to 
electrical machines. As a consequence, water penetrates into the winding 
insulation and, through hairline cracks between the conductor and the 
insulation, gets all the way into the iron part of the stator winding. 
When such damage occurs, then soldering connections must be renewed, if 
that is at all possible. 
Before these new soldering connections are made, it is compellingly 
necessary to remove the water that has penetrated through the leaks into 
the insulation. Drying the winding--such as this is done in new machines, 
for example, by downtime heating--is out of the question here because, 
during the drying process, the escaping water/water vapor cannot be 
completely removed from the winding via the same route by which it 
penetrated; instead, detailed investigations document the danger that the 
penetrated water will spread even more deeply in the winding. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a process for drying the stator 
winding coil! of the kind mentioned initially, that can be implemented in 
a simple manner without removal or even replacement of the stator winding 
but that facilitates extensive removal of the water that has penetrated. 
This problem is solved according to the invention in that, after the 
removal of one or both connection devices of a rod, the now exposed wave 
guides are temporarily closed, that a flexible, gas-proof hose is pushed 
over the rod end or ends, where the machine-side end of the hose is 
attached to the rod in a gas-proof manner, where the free hose end is 
connected to the suction connection of a vacuum pump, and where, by 
evacuating the hose, the water that has penetrated into the insulation of 
the conductor rod is removed; the criterion for the degree of drying 
achieved consists of dielectric measurements, especially recording of the 
insulation resistance between the conductor and the iron unit, the loss 
factor measurement, or capacity measurements. 
Using the process according to the invention, one can dry stator winding 
rods in the built-in state and thus make them safe again to operate. The 
invention will be described in greater detail below with the help of an 
exemplary embodiment illustrated in the drawing.

DISCUSSION OF THE PREFERRED EMBODIMENTS 
The initial object of the drying process described below is a 
turbogenerator with a directly water-cooled stator winding, as illustrated 
in a simplified manner in FIG. 1. 
The turbogenerator has a rotor 1, a stator with a sheet metal stator body 2 
with a stator winding 3. The winding ends of the stator winding are 
provided in the known manner with water chambers 4 for the electrical and 
hydraulic connection of the subconductors 5 whose structure is shown in 
FIGS. 2 and 3. 
Water chamber 4 is a single-part or multipart metallic component that 
surrounds the subconductors 5, which are combined into a cluster. 
Subconductors 5 are connected with each other and with the water chamber 4 
by means of hard soldering. The solder layers are marked with the 
reference number 6 in FIGS. 2 and 3. 
The free end of the water chamber is tapered and is transformed into a 
connecting fitting 7 (merely indicated). Connected to the fitting are 
insulation hoses 8 that connect the interior of all water chambers with 
ring lines 9 on both machine ends. Cooling water is supplied to or 
evacuated again from subconductors 5 through these lines. In FIGS. 2 and 
3, the stator winding is made up exclusively of subconductors 5. The basic 
structure remains the same in stator windings that have massive 
subconductors also in addition to wave guides. 
On the basis of the heavy stresses to which the winding heads of electrical 
machines are exposed, it happens again and again that the soldering 
connections on or in the water chamber are damaged locally. This causes 
leakages in the cooling system. Particularly dangerous are soldering 
points that have become defective when the cooling water, which is under 
pressure, penetrates from water chamber 5 in the stator winding in the 
direction toward the interior of the machine and when it spreads via 
hairline cracks between the surface of the subconductors and the main 
insulation 10 or between neighboring subconductors 5. Such winding rods 
are no longer safe to operate because the water has penetrated; this can 
be recorded, for example, during customary dielectric measurements, and 
the rods would really have to be exchanged. 
This is where the invention comes in by providing a simple process that 
smoothly fits into the repair process for the purpose of drying the stator 
winding without having to take it out. 
In case of leakages in or on water chamber 4, the latter must be removed 
anyway and must be replaced with a new one; this is why the invention 
provides closing the now exposed subconductors 5 temporarily on both rod 
ends. Then a flexible, gas-proof hose 11 is pushed over the rod end. In 
the process, the length of the overlap 1 is selected as large as possible, 
specifically at least 80 cm. The machine-side end of hose 11 is attached 
to the rod in a gas-proof manner, for example, by means of a hose clip 12. 
The free hose end is connected to the suction connection of a vacuum pump 
13. By evacuating hose 11 to values of around 0.1 mm Hg, the water that 
has penetrated into insulation 10 of conductor rod 3 is thus removed. The 
criterion for the degree of drying achieved is provided by dielectric 
measurements, especially recording of the insulation resistance between 
the conductor and the iron body, capacity measurements or loss factor 
measurement, with the usual diagnosis unit 14, such as it is also used for 
field measurements. The diagnosis unit 14 is therefore connected between 
subconductor 5 and the stator body that is on ground potential. 
During evacuation, one constantly records the moisture content F of the 
stator winding that is determined indirectly by one of the above-mentioned 
parameters. The drawing process can be suspended if one falls short of an 
empirical value F.sub.o that appears after a certain evacuation time 
t.sub.o. 
The evacuation is performed only on one machine side in the process 
described. To ensure that there will be a sufficiently high vacuum inside 
the insulation, it may be required to attach either hoses 11 on both rod 
ends and to evacuate on both sides, in other words, to dry simultaneously 
from both sides, or the other rod end is sealed by a flexible, gas-proof 
hose that is pushed over the rod and that protrudes beyond it. 
The procedure described is repeated for all stator winding rods where 
dielectric measurements or other observations revealed increased moisture 
content. Only then are new water chambers attached.