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Describtion Parallel
Operation Switching
Devices PLC
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 Removing
alternator from the bus
To
remove an alternator from the bus transfer all the loads to the remaining
machine
By
adjusting the governer controls of both then trip the
CB of the outgoing
machine
And
adjust the voltage of the remaining alternator to the desired value
The
transfer of the load by tripping the breaker of the outgoing machine ,instead of
through governer adjustment .should be avoided ,because
it exerts as ever strain
On
the mechanical parts of the machine that takes the load from safety stand points
If
the generator is out of service for an extended period of time ,the disconnect
switch or links should be opened (after
generator shut down) and kept open
until the generator is to go back on the line this prevents damage to the
machine in the events that the breaker is inadvertently closed
Motorization
Of Alternator
If
the energy input to the prime mover of an alternator is decreased
to a point where the energy admitted is insufficient to keep the machine
running in synchronism with the bus the other machine on the bus will feed into
the machine and run it as motor
at
the same speed and in the same direction as before ,this is known as
”motorization” although . motorisation is not harmful
to additational load on the bus
A
reverse power relay also called A power direction relay is provided to trip the
machine
Off
the bus if this occur
Instrumentation
And Control Of AC Generator
the
functional diagram for
a two generator system shown in the figure
the
minimum instrument ,switches and adjustable control that an operating engineer
must
be acquainted with when operating AC generator single or in parallel
the
connecting lines indicate what each control device operates and where each meter
gets its signal
operating
controls and connection are drawn with heavy line meter and instrumentation
connection are drawn with light lines panel 1 and
3 are identical
generator
panels and contain the instruments and controls for the particular machine
panel
2 contain the instruments
and control necessary for paralleling
the
watt meter and Pf meter each
require both current and voltage
signals .and the
synchronoscope
require voltage signal from each generator to determine the error angle
power
factor problems Of Alternator In Parallel
individual
alternator supplying power to the distribution system have the same power factor
as the connecting load however when operating in parallel ,the alternator may
have entirely different power factor .when machine are in parallel their
terminal voltage are the same yet the acceptance of load by a machine causes its
internal voltage to decrease below the bus voltage .hence the machine is said to
be under excited for its particular load the machine that gave up the has
its internal voltage increased above the bus voltage and its said to be over
excited for its particular load
If
the bus load has lagging power factor, which is the general case the internal
voltage difference brought about by transfer of load will cause the machine that
absorb
some
load to take less than its proportional share of the Kvar
thus the machine that accept some load has a lagging power factor than
the bus and may be even leading where as the machine that lost some load has
amore lagging Pf
than the bus
although
the active power may be equally
divided between machines ,the equal division of reactive power required
adjustment of the internal voltage
of each machine ,to do this the DC
field excitation of the machine that took some load
should
be increased and that of other machine
decreased until the power factor indication of each alternator reads the
same and lagging .
some
installation may use varmeter for the same purpose .the excitation of each
machine is adjusted until both machines have the same amount of Kvar
the field excitation of each machine is adjust with its respective field
rheostat
machine
that are equipped with (avr) don’t require manual adjustment of their field
rheostat
to balance the Kvar a compensating device
in the regulator automatically
adjust
the Kvar distribution and hence the power factor of each machine .
If
they are not equal the voltage
adjusting control of the voltage regulator may be used to provide equalization.
By
other ward when the alternator work in over excited mode that making circulating
current bass through the to machine that will the super posed of the original
current distribution this current
should be added to the load current of machine (1) and subtract from the load
current of machine (2)
Effect
Of Change In Steam Supply For Two Alternator In Parallel
for
two identical machine sharing the load equally ,and the steam supply of machine
(1) to be increased the increased power in take has to be utilized
so the alternator (1)
has
rotor will decrease in the position with respect to rotor of alternator (2)
the
difference between E1&E2 cause
the current is to followes is
almost =90
is
almost in phase with E1 the power output of machine 1 increase
by E1 is watt per phase since the external load is constant the power
output of machine 2 is reduced by the same amount its has a small
reactive component so that the reactive loading of two machine remain unchanged
it
will be necessary to make small adjustment
in excitation and reduction in steam supply of alternator
2 to keep bus voltage and
frequency constant
synchronised
method and problem
…
there
is no advantage in adjusting the incoming
machine to have the exact frequency of the bus for if paralleled in such manner
the machine would merely float on the line and slight decrease in its input
energy would immediately result in motorization
hence
for added protection the frequency of the incoming machine prior to
paralleling should be adjusted to above 0.1
cycle higher than the bus frequency .
two
generator can not operate at different frequency
when they are in parallel .
hence
if the frequency of the incoming machine before paralleling is lower than that
of the bus the on the bus will drive the incoming machine
at the high electrical speed
as soon as it is paralleled the energy to motorize the incoming machine comes
from the other generator on the bus
if these machine is heavly loaded
the additional kilowatt
required to motorize the incoming machine may be sufficient to trip them off the
bus and black out the plant .
if
the frequency of the incoming machine before paralleling is higher than of the
bus it will take some of the load from the other machine at the instant its
paralleled the absorption of
load by the incoming machine causes its frequency to drop.
A
synchroscops ,shown in fig (1) always
indicate the condition of the incoming machine with respect to the bus if
the frequency of the incoming machine is higher than the bus frequency the
syncronoscope pointer will revolve in the direction marked “fast ” ,and if
the frequency of the incoming machine is lower than the bus frequency the
pointer will revolve in the direction marked
“slow” if the pointer stop at
the position after than 0 degree it
is indication that the incoming machine is at the same frequency as the bus .
but
out of phase by an error angle indicated by the position of the pointer,
to
correct this adjust the governer
control of the incoming machine to admit more energy to its prime mover this
will result in a slightly higher frequency and will cause the syncronoscope
pointer to revolve slowly in the fast direction the machine should be paralleled
when the pointer reaches the 0
degree position whill travelling in the fast direction when paralleled the
syncroscope pointer will not revolve but will stay at the
0 degree position because there is slight lag in the operation of the
breaker or switching mechanism it
is good practice to start the breaker
closing operation 1 or 2
before the 0
degree position thus assuming that the actual closing occurs at about
0 degree a breaker with a 15 cycle closing time takes
15/60 sec for the closing
mechanism to complete its operation a 30 cycle closing device takes
½ sec
for closing to occur ( assume 60
hertz system) .
if
the breaker is closed at an angle other than
0 degree (12 o clock ) a cross current will occur between the generator
the severity of this cross current depends on how far out of the synchronism
the machine are when the breaker is closed the grater the
error angle the higher the
cross current the worst condition corresponds to an error angle of 180 degree
and has the same effect as a short circuit
on both machines the resulting tremendous force on the generator winding and
other conductors could be disastrous .
fig
(2 ) show how the error angle
changes with time when the voltage of the incoming machine has a higher Frequency
than that of the bus. under no circumstances, emergency or otherwise, should the
breaker be closed when the error angle exceeds 15 '.
Synchronizing
lamps provid ameans for checking the synch. Roscope.and may be used as asccondry
method for determining the correct instant for paralleling.if the synchroscope.
is defective synchronizing-lamp. connections are shown in fig(3).
If
the frequncyof the incoming machine up to speed, both lamps will go alternately
brigh and dark in unison.the significant correspondence between the
synchroscope. and the synchronizing lamps is the 0' –lamp dark relation
ship.when the lamps are dark, the synchroscope pointer must be at 0'. or the
synchroscope is defective. the lamps are always correct.
assuming
that they are not burn ed out. when the using lamps for synchronizing the
duration of the dark period should
be timed, and the breaker closed midway through the dark period. timing is
important because the dark period may extend over 8' or more of error angle.
lamps used for this purpose are generally of the nonfrosted type. Both lamps
must have same voltage and wattage relatings. The voltage
Rating
must be as specified by the manufacture.
In
the most application the generator voltage is greater than 250 volt rms.
Potential transformer are connected
between the generator and the synchroscope [or lamps] to reduce the voltage to
the rated voltage of the synchroscope or the lamps. Accidental
Reversing
of the connection in the primary of secondary of the transformer will cause the
synchroscope and the synchronizing lamps to give false indications. The
synchroscope will indicate to 0' and lamp will be dark when the generator is
180' out of phase with the bus.
voltage
failure of AC generator.
voltage
failure of an AC generator may be caused by an open in the field circuit, an
open in the field rheostat, or failure of the exciter generator. A voltage check
of the exciter generator will determine if it is at fault. To test the rheostat,
short circuit its terminals and observe the alternator voltage. A Buildup of
voltage indicates an open rheostat. If the exciter is operating at rated voltage
and the alternator fails to buildup when the rheostat
is short-circuited, the trouble is in the field circuit
or the wires connecting the exciter to the field. The loss of field
excitation to generator operating in parallel with others causes it to lose load
and over speed.
Unless
tripped off the bus is a short time, the machine may be damaged by excessively
high temperature; high armature current caused by the high voltage differential
between the armature and the bus and the high currents induced in the field iron
and field windings by the armature current. Will cause rapid heating of the
apparatus. However, if the loss in excitation is identified as caused by
accidental tripping of the voltage-regulator switch or field breaker the
immediate reclosing of the respective
switch
or breaker should restore service.
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