FIELD OF THE INVENTION
The present invention generally relates to method of operation of air cooled
turbo generator with closed circuit air cooling system. More particularly the
invention relates to an improved method of operation of air-cooled turbo
generators with closed circuit air cooling system.
BACKGROUND OF THE INVENTION
The existing technology for up gradation of turbo generators mainly focus on
cooling design and insulation technology. When the intent is either to improve
the reliability of operation or upgrade the machine to match with its prime mover
with upgraded technology or providing a retrofit solution, the easier option is to
attempt improvement in the existing ventilation system of the turbo generator.
Enhancing capability of air cooled generators through increase of air pressure
inside the generator, improving the ventilation system through optimizing the air
flow paths with the aid of Computational Fluid Dynamics software are some of
the approaches taken up worldwide. These approaches do improve the machine
capability, but they relatively a long drawn process.
In a generator operation with closed circuit air cooling system, the losses
induced during the operation are converted into heat and results in temperature
rise of various components of the generator including machine winding. The heat
generated is dissipated to a cooling medium, usually air in case of small to
medium sized generators. In a turbo-generator operating with closed circuit air

cooling system, cold air is circulated into the machine which absorbs the above
heat and becomes hot. This hot air is allowed to flow past a plurality of heat
exchangers, where the cold water flowing through the tube absorbs the heat and
turns hot air into cold air. This cold air is circulated back into the machine. The
water flowing through the heat exchanger tubes is drawn from the cooling
towers where water gives away heat to the atmosphere. Hence the inlet water
temperature is very much dependent on ambient air temperature; higher the
ambient air temperature, higher is the inlet water temperature, the temperature
of water during summer being always high. This results in increased temperature
of cold air and resulting higher winding temperatures. As the specific heat of
water is more than four times that of air, the rise in inlet air temperature is as
many times more during the other seasons. Generator winding temperature is
one of the parameters that limit the output of a generator as the electrical
insulation cannot sustain a temperature beyond a certain limit, above which the
insulation losses its properties. Hence the capability of a turbo generator,
installed at a place, is less during the summer.
Rating of any Turbo Generator is defined at a particular Standard Temperature
(ST) and varies in inverse relation to the ambient temperature. Hence, during
summer, the maximum mega volt amperes (BVA) that can be drawn from a
generator is reduced.
During summer, when the machine operates at or nearer to rated loads, the
generator inlet air temperature is more than that during winter, in certain
circumstances which reduces the generator performance.

Data collected from some of the air cooled turbo generators show that the cold
air temperature is of the order of 50 deg C when ambient air temperatures
recorded is 37 deg C and the winding temperature reached its alarming limit of
115 deg C even when the machine is operating at a load of 96 MW. Rating of it
prime mover is a gas turbine with rating of 105 MW. Higher inlet air temperature
than ambient temperatures is also possible, when there is hot air bypassing
coolers and mixing with cold air, or when the coolers are inadequately designed.
In such an event when there exists a difference between cold air inlet and
ambient air temperatures, there is no way to utilize the additional margin
available with the prime mover as per prior art.
As per prior art (patent No. US 6,657,332 B2 dated December 2, 2003) the turbo
generator cooling system comprises a cylindrical heat sink with axially extending
fins on both inside and outside of its surface with radial holes and the generators
stator is press fit inside it. The stator external surface is in contact with the
above internal fins and when the turbo-generator rotates air enters and passes
along the fins. The amount of air can be adjusted by modifying the space
between the fins. Necessary head for the above is given by the fans mounted on
the rotor. Hence, the system requires an additional heat sink which is
cumbersome, occupies space and deprives the maintenance personnel of access
to some of zones of stator to provide man holes for accessing the machine
internals. There is hence a need to have a simpler system which improves the
machine ventilation and cheaper to install at the same time. The system shall
improve the generator performance during summer or when the ambient
temperature is lea than the cold air inlet temperature.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of
operation of air-cooled turbo generators with closed circuit air cooling system.
Another object of the invention is to propose an improved method of operation
of air-cooled turbo generators with closed circuit air cooling system, which
reduces the air inlet temperature from ambient temperature to enable the
generator to produce more power.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved method of operation of air-cooled
turbo generators with closed circuit air cooling system, the air-cooling system of
the turbo generator essentially comprising a plurality of air to water heat
exchangers, a plurality of axial flow fans disposed on the generator rotor, and a
cooling medium is circulated through the generator to exhaust heat generated to
a heat exchanger, the method comprising providing a device for introducing a
second steam of air through the openings configured on the stator frame,
wherein the inlet device comprising a flanged cover plate with air filter
cartridges, and an actuation means to independently operate the inlet device,
wherein the device is disposed on the suction side chamber of the stator frame,
and wherein the air-filter cartridges are removable.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Exemplary embodiments are described hereinbelow with reference to the
accompanying drawings which is for the purpose of illustrating the embodiments
and not for the purpose of limiting the scope of the invention, in which
Figure 1 shows a schematic representation of a turbo generator ventilation
system, shown in its longitudinal view.
Figure 2 shows the side view of a generator ventilation system of the prior art
and that of the invention.
Figures 3 and 4 respectively show the generator stator of prior art and that as
per invention.
DETAIL DESCRIPTION OF THE INVENTION
The present invention provides a method through which another stream of air is
introduced through the openings provided on stator frame of the generator from
the other side, as shown in Figure 1, 2, 4. The existing fans mounted on the
rotor give the necessary head and atmospheric air is drawn into the machine. To
ensure cleaner air, air is drawn through air filters assembled to a plate fastened
to the air entry opening. As the temperature of the air drawn is almost same as
ambient air, the capacity of the new ventilation system gets enhanced.

As per prior art shown in Figure III, the generator openings on the side away
from the cooler duct is closed with a blind cover plate. In the arrangement as per
invention as shown in Figure IV, envisages a flange fitted with air filter cartridges
and in normal course of operation, the flange assembly is blinded with a dummy
cover plate. When there is requirement to draw more power from generator and
in the event the ambient is lower than the inlet air temperature, then the dummy
cover plate is removed.
The present invention provides means and method to improve rating of turbo
generators operating substantially at base loads and inlet air temperature is
higher than the ambient temperature.
According to the invention a cooling medium is circulated through the generator
to absorb the heat generated during its operation and is let out to a separate
heat exchanger.
Axial flow fans are fitted to the generator rotor on either side of it to force cold
air into the generator.
The air, flows past the internals of the machine and during its course, absorbs
heat and becomes hot. The hot air then flows past a plurality of air to water heat
exchangers and circulated back into the machine with the aid of the above fans.

The rating of a Turbo generator is dependent on the inlet air temperature. In a
generator operated with closed circuit air cooling system, the inlet air
temperature is dependent on the cooler feed water temperature. The feed water
temperature in turn is dependent on ambient air temperature.
The present invention proposes introducing another stream of air to bring down
the inlet air temperature to enable the generator to produce more load,
particularly under the conditions where difference exists between temperatures
of inlet cold air and the atmospheric air.

WE CLAIM :
1. An improved method of operation of air-cooled turbo generators with
closed circuit air cooling system, the air-cooling system of the turbo
generator essentially comprising a plurality of air to water heat
exchangers, a plurality of axial flow fans disposed on the generator rotor,
and a cooling medium is circulated through the generator to exhaust heat
generated to a heat exchanger, the method comprising :
providing a device for introducing a second steam of air through the
openings configured on the stator frame, wherein the inlet device
comprising a flanged cover plate with air filter cartridges, and an actuation
means to independently operate the inlet device, wherein the device is
disposed on the suction side chamber of the stator frame, and wherein
the air-filter cartridges are removable.

ABSTRACT

The invention relates to method of operation of air cooled turbo generators with
closed circuit air cooling system, the air-cooling system of the turbo generator
essentially comprising a plurality of air to water heat exchangers, a plurality of
axial flow fans disposed on the generator rotor, and a cooling medium is
circulated through the generator to exhaust heat generated to a heat exchanger,
the method comprising providing a device for introducing a second steam of air
through the openings configured on the stator frame, wherein the inlet device
comprising a flanged cover plate with air filter cartridges, and an actuation
means to independently operate the inlet device, wherein the device is disposed
on the suction side chamber of the stator frame, and wherein the air-filter
cartridges are removable.