maximum temperature permitted in
and when this is done, a sacrifice in insulation life
must be expected and the risk of mechanical
damage to the machine must be considered.
If it is anticipated that the emergency overload
must be continued for 1 or more days, temporary
supplemental cooling, as discussed in
must be expected and the risk of mechanical
damage to the machine must be considered.
If it is anticipated that the emergency overload
must be continued for 1 or more days, temporary
supplemental cooling, as discussed in
10. LOAD LIMITATIONS OF ASSOCIATED
EQUIPMENT
EQUIPMENT
It is important that overloads should not be carried
on rotating machines without an investigation of
the limitations of associated equipment. Equipment
such as cables, buses, reactors, circuit breakers,
disconnecting switches, current transformers, and
power transformers should be checked. Any one of
these may constitute the practical limit in load
carrying ability of the unit. On the machine itself,
auxiliary equipment such as exciters or rheostats
should be checked. The exciter should have
sufficient margin while carrying the overload to
take care of small fluctuations in load and voltage
that may occur with minor system disturbances. In
some cases it may be possible to ease the burden
on the exciters of the machine being overloaded by
transferring reactive kVA to other units of the same
system.
on rotating machines without an investigation of
the limitations of associated equipment. Equipment
such as cables, buses, reactors, circuit breakers,
disconnecting switches, current transformers, and
power transformers should be checked. Any one of
these may constitute the practical limit in load
carrying ability of the unit. On the machine itself,
auxiliary equipment such as exciters or rheostats
should be checked. The exciter should have
sufficient margin while carrying the overload to
take care of small fluctuations in load and voltage
that may occur with minor system disturbances. In
some cases it may be possible to ease the burden
on the exciters of the machine being overloaded by
transferring reactive kVA to other units of the same
system.
11. COOLING WATER
If the mechanical limits of a machine are not
exceeded, overload capacity of air-cooled
machines with water cooled air coolers can be
increased in some cases by increasing flow of
cooling water so as to not exceed temperature
limits in
exceeded, overload capacity of air-cooled
machines with water cooled air coolers can be
increased in some cases by increasing flow of
cooling water so as to not exceed temperature
limits in
cooling water is available. It should be
emphasized that in addition to limiting the total
temperature, temperature rise should be held
within indicated limits. In a non-overload condition
for class B insulation, if the limiting stator
temperature rise is 60 °C, and the limiting total
temperature is 100 °C, the difference between the
temperature of the stator and the air leaving the
cooler should not be more than 60 °C even though
it may be possible to hold the total stator
temperature to less than 100 °C by increasing flow
or reducing the temperature of cooling water.
emphasized that in addition to limiting the total
temperature, temperature rise should be held
within indicated limits. In a non-overload condition
for class B insulation, if the limiting stator
temperature rise is 60 °C, and the limiting total
temperature is 100 °C, the difference between the
temperature of the stator and the air leaving the
cooler should not be more than 60 °C even though
it may be possible to hold the total stator
temperature to less than 100 °C by increasing flow
or reducing the temperature of cooling water.
(FIST 1-4 3/91) 8
12. COOLING WATER REGULATION
As stated earlier, winding insulation life (a) is
shortened by high temperature, and (b) is subject
to mechanical damage by temperature cycling.
Unfortunately, these facts place conflicting
demands on any cooling system which is
designed to lengthen the insulation service life,
since, if maximum cooling of insulation, the
possibility of mechanical damage due to
temperature cycling under varying load is
increased; likewise, if temperature cycling is to be
minimized, the insulation temperature must be
held constant at a relatively high value.
shortened by high temperature, and (b) is subject
to mechanical damage by temperature cycling.
Unfortunately, these facts place conflicting
demands on any cooling system which is
designed to lengthen the insulation service life,
since, if maximum cooling of insulation, the
possibility of mechanical damage due to
temperature cycling under varying load is
increased; likewise, if temperature cycling is to be
minimized, the insulation temperature must be
held constant at a relatively high value.
Therefore, there are presently in use two methods
for controlling cooling water, each of which
operates to lengthen insulation service life by
controlling one (but not both) of the above
conflicting requirements a. and b. as follows:
for controlling cooling water, each of which
operates to lengthen insulation service life by
controlling one (but not both) of the above
conflicting requirements a. and b. as follows:
a. In cooling systems not provided with
automatic regulation of water flow, the
cooling water should be adjusted to produce
minimum cooling air temperature without
exceeding temperature rise limits in
automatic regulation of water flow, the
cooling water should be adjusted to produce
minimum cooling air temperature without
exceeding temperature rise limits in
when the machine is carrying the maximum
expected load. This flow should be constant
for all other loads in order to maintain
minimum insulation temperature at all times.
However, winding temperatures should
always be held above
10 °C.
expected load. This flow should be constant
for all other loads in order to maintain
minimum insulation temperature at all times.
However, winding temperatures should
always be held above
10 °C.
Where cold cooling water is used, it is
important to watch for condensation on
piping and cooler surfaces within the
generator and see that water is not being
carried into the windings, causing rust or
corrosion of metal parts. The amount of
cooling water may have to be reduced to
prevent condensation, or mixed with the
warmer discharge water through a bypass
connection to raise its temperature.
important to watch for condensation on
piping and cooler surfaces within the
generator and see that water is not being
carried into the windings, causing rust or
corrosion of metal parts. The amount of
cooling water may have to be reduced to
prevent condensation, or mixed with the
warmer discharge water through a bypass
connection to raise its temperature.
b. On some units, an automatic cooling
water flow control system has been installed
to reduce the range of temperature
water flow control system has been installed
to reduce the range of temperature