Power requirements for unit auxiliaries
at startup (or shutdown) are dependent on the size and type of unit
and the degree of reliability. The design should include
considerations for continuous load, short time and transient
overload, and a margin for future growth.
Continuous Capability
The continuous capability should supply
maximum power requirements of all electrical auxiliaries in any
combination in which they may be used during plant operation from the
station service system.
Where more than one unit is supplied
from a common station service system, consideration should be given
to the possibility that a unit in normal operation may trip from full
load and transfer its shutdown auxiliaries to the common station
service system during the startup of another unit's auxiliaries.
Instead of designing a system to supply
auxiliaries requirements for two units, it may be acceptable to
impose engineering or administrative load limitations. The system
should also be able to react in the event that station load is
suddenly lost.
Short-Time Overload Capability
Certain auxiliaries served by station
service systems may be required, for limited periods, to carry loads
greater than those existing at plant full load.
Examples are circulating water pumps
filling the system before full syphon conditions are established,
induced-draft fans handling cold air, motor-driven startup boiler
feed pumps that will be replaced by turbine-driven pumps at a more
advanced stage of startup, and electric boilers not used during
full-load operation.
Affected equipment or components of the
service system should be capable of carrying these overloads for
appropriate periods without producing voltage profiles or temperature
rises outside prudent limits.
While reviewing equipment load
requirements that are greater during starting than at full-load
operating conditions, equipment whose load requirements are lower
during starting should also be noted.
A typical example would be the
forced-draft fans of a unit, which operate substantially below
nameplate rating during startup. Incorporating both types of load in
a common portion of the system may reduce the overall supply
requirements.
Transient Overload Capability
Station service systems should have the
capability of supplying electrical auxiliaries during the starting or
restarting of the largest load. When the largest load is a motor
driving high-inertia loads, such as an induced-draft fan, the
starting transient duration may be up to 60 s.
Voltage profiles during such transients
are not required to satisfy criteria established for continuous
operation or short-time overload, but they must be adequate to assure
that no running motor will stall and no motor-starter contactor will
drop out. Acceptable equipment operating voltage ranges should be
confirmed with the manufacturer.
If load tap-changing (LTC) transformers
are used in the power system, it should be assumed that these tap
changers do not operate fast enough to apply significant correction
during such transients. The system should be capable of supplying
unit auxiliaries required during an automatic transfer of the buses
from the normal to the startup/shutdown source.
Voltage profiles immediately following
such transfers should meet the same criteria as those established for
the motor-starting transient. Within approximately 60 s, the profiles
should meet criteria established for continuous operation. If
transformers with LTC are used, it should be assumed that significant
correction is not applied until after the 60 s period.
Margin for Load Growth
Station service system design should
allow for auxiliaries that may be added after the plant is placed in
service. An acceptable margin for future loads should be included in
transformer and feeder cable capacity and space should be provided
for switchgear and motor control center additions.
Short-circuit withstand capability of
switch-gear buses should be sufficient to avoid transformer voltage
regulation problems and allow for future growth. This last margin
generally will be created by increasing transformer impedance at the
expense of voltage regulation. For that reason, it is often
impractical to make large allowances for future load growth.
No comments:
Post a Comment