Note: Descriptions are shown in the official language in which they were submitted.
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ThIs invention relates to a control system for a nuclear
power producing unit having a reactor in which a coolant, such
as water under high pressure, is heated and circulated in
parallel through a plurality of steam generators supplying steam
to a prime mover such as a turbine generator. As an order of
magnitude, the reactor in such a unit may have a heat output of
upwards of 3,400 Mw and a net electric output of 1,200 Mw.
In accordance with the invention a primary feed forward ~
control signal corresponding to the desired or demand power output ~;
10 adjusts, in paralle~, through separate discrete control loops, --
the reactor heat output required to satisfy the power demand,
and the total rate of feedwater flow to and steam flow from the
steam generators required to maintain critical system parameters
at set point.
Further in accordance with the invention the feed forward
control signal to each discrete control loop is modified by the
tLme integral of the difference between demand and actual power
outputs to thereby continuously calibrate, under steady state
conditions, changes in reactor heat output required to satisfy
the power demand because of changes in cycle efficiency and the
corresponding changes in total rate of feedwater flow to and
steam flow from the steam generators required to maintain
critical system p~rameters at set point. ~`
Further in accordance with the invention the feed forward
control signal to each discrete control loop is further modified
in proportion to transient changes in the difference between
demand and actual power outputs and critical system parameters.
Further in accordance with the invention the relative
rates of feedwater flow to the steam generators are additionally
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adjusted in proportion to changes in the relative rates of
coolant flows through the steam generators.
Further in accordance with the invention the relative rates
of feedwater flows to the steam generators are further adjusted
in accordance with the difference in te~peratures of the
feedwater entering the steam generators.
Further in accordance with the invention the relative
rates of feedwater flows to the steam generators are additionally
adjusted in accordance with the time integral of the difference
between the average coolant temperatures ~n the steam generators.
These and further objectives of the invention will be
apparent as the description proceeds in connection with the
drawings, in which:
IN T~E DRAWINGS
Fig. 1 is a schematic of a pressurized water nuclear
power producing unit in which are referenced the primary
controllers and final control elements utilized in the control
system shown in Figs. 2 and 3.
Fig. 2 is a logic diagram of a control system embodying
the invention
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as applied to the nuclear power producing unit shown in Fig. 1.
Fig. 3 is a logic diagram illustrating typical arrangements for
determining the deviation of critical system parameter~ from set
point.
DETAILED DESCRIPTION
. .
Referring to Fig. 1, there is shown a presYurized water reactor
1 which is maintained at a predetsrmined operating pressure by means
of a pressurizer 2. Reactor coolant, i.e. pressurized water, is
circulated through the reactor 1 and once-through steam generatorC 3
and 4 through parallel coolant flow loops A and B respectively.
Coolant flow through loop A i8 e~tablished and maintained by circul- ~-
ating pumps 7 and 8 arranged in parallel, whereas coolant flow
through loop B is established and maintained by similar circulating
pump~ 9 and 10.
Steam from the generators 3 and 4 is transported through a con-
duit 11 to a turbine unit, generally indicated at 12, having a high
pressure (HP) unit 13 and one or more intermediate and low pressure
(IP&LP) units as indicated at 14. The HP unit 13 and IP&LP units 14
drive a single generator 15 producing electric power transmitted
therefrom by conductors 16, 17 and 18. Alternately, each turbine
unit may be arranged to drive a separate generator all feeding into
a common buss,
Steam is admitted to the HP unit 13 through a conventional flow ~ -
control valve unit, shown diagramatically at 19, and di~charged
therefrom through a conduit 20 to a reheater 21 provided with heat-
ing steam from conduit 11 through branch conduit 22. The hot re-
heat ~team i8 then trahsported through a conduit 23 to the IP&LP
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8662unitsi14 and is discharged therefrom into a condenser 24. Conden-
sate from condenser 24 is pumped by condensate pump 25 through a
low pressure heater string 26, heated by extraction steam from
IP~LP units 14. Feedwater is drawn in parallel from the low pres-
sure heater string 26 by boiler feed pumps 27 and 28. Feedwater
discharged from boiler feed pump 27 passes through high pressure
heaters 29, heated by extraction steam from HP unit 13, into steam
generator 3. Feedwater discharged from boiler feed pump 28 passes
through high pressure heaters 30, heated by extraction steam from
HP unit 13, into steam gsnerator 4.
As shown in Fig. 2, which is a logic diagram of the control
system, unit load demand may be established by an automatic load
dispatch system, as shown at 32, or by other automatic or manual
means, inputing to a primary feed forward control signal generator
33, the purpose of which is to generate a feed forward control
signal corresponding to the desired or demand power output of the
power producing unit. The feed forward primary control signal,
with a maximum limit established in unit 33A correæponding to the
capability of the reactor under maximum load conditions wi~h all
auxiliary equipment in operation, transmitted over signal conductor
34, adjusts in parallel through individual discrete control loops,
steam flow to the HP turbine unit 13, total feedwater flow to the
steam generators 3 and 4 and neutron power or heat output (Nd) of
the reactor 1 to substantially maintain actual power output of
the power producing unit equal to the demand power output.
The feed forward prLmary control signal inputæ to the indi-
vidual discrete control loops through function generators 61A, 62A
and 63A, the purpose of which is to modify the primary feed forward
control signal so that under normal operating conditions and cycle
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efficiency there will be established the reactor heat release
required to satisfy the then existing demand for power and the
total rate of feedwater flow to and steam flow from the steam
generators required to maintain critical system parameters at
set point. Each control loop is further provided with indivi-
dual modifyi~g signals, as hereinafter described re in detail,
so that actual power output is maintained precisely equal to the
demand power output and the critical system parameters are main-
tained at set point, notwithstanding changes in cycle efficiency
and/or changes in operating conditions.
In reference to the drawings, it should be noted that con-
ventional control logic symbols have been used. The control
components, or hardware, as it is sometimes called, which such
symbols represent, are commercially available and their operation
well understood. Further, conventional logic symbols have been
used to avoid identification of the control system with a particu-
lar type of control, such a neumatic, hydraulic, electronic,
electric, digital or a combinationoof these, as the invention may
be incorporated in any one of these types. Further to be noted,
the primary controllers shown in the logic diagrams have been ref-
erenced into Fig. 1 as have the final control elements.
In Fig. 2 the modifying signals, one or more of which are
applied separately to each individual, discrete control loop are
identified as megawatt error (MWe), throttle pressure error lTPe),
feedwater temperature error (FWTe), and reactor coolant temperature
error (RCTe).
Fig. 3 is a logic diagram of the sub-loops for the generation
of ~ese ~odifying signals. The feed forward control signal is trans- -
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mitted over signal conductor 34 to function generators 35, 3637, and 39, the purpose of each being to condition the feed forward
control signal so that the output signal therefrom is representa- .
tive of the correct or set point value of the variable with which ~ -
it is associated for the then existing magnitude of the primary
feed forward control signal.
Function generator 35 generates a set point signal correspond-
ing to the correct throttle pressure for the existing primary feed
forward control signal which is compared in a difference unit 40
with a signal generated in throttle pressure transmitter 41 and
producing an output 8 ignal corresponding to throttle pressure
error (TPe)-
An error ~ignal corresponding to megawatt error (MWe) i9generated by comparing the output signal from function generator
36 to the output signal generated in megawatt transmitter 42 in
a difference unit 43.
A signal corresponding to average feedwater temperature error
(FWTe) is generated by averaging the feedwater temperature errors,
(FWTeA) and (FWTeB), in loops A and B respectively. Thus, as
~hown, the output signal from function generator 37, representing
the normal feedwater temperature in relation to load demand, with
all extraction feedwater heaters in service and operating normally,
is compared, in difference unit 44A, with a signal corresponding to
the actual feedwater temperature in loop A, generated in feed water
temperature tran~mitter 46. Similarly, the output ~ignal from
function generator 37 is compared in difference unit 44B with a
3ignal corre~ponding to the actual feedwater temperature in loop B,
generated in feedwater temperature transmitter 47. The output
signals from difference . . . . . . . . . . . . . . . . .
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units 44A and 44B input to s D ing unit 45 generating an output
signal (FWTe) corresponding to the average feedwater temperature
error.
A signal corresponding to reactor coolant tempsrature error
(RCTe) is generated by comparing, in difference unit 50, the out-
put signal from function generator 39 with a signal corresponding
to the average reactor coolant temperature generated in summing
unit 51 from signals generated in summing units 52, 53. S D ing
unit 52 averages the signals generated in temperature transmitters
54, 55 corresponding to the temperature of the coolant entering
and leaving the reactor 1 in coolant loop A. Similarly, summing -
unit 53 averages the signals generated in temperature transmitters
56, 57 corresponding to the temperature of the coolant entering
and leaving the reactor 1 in coolant loop B. - -
As evident from an inspection of Fig. 2, the itemized error
signals are applied to one or more computing units. To avoid un-
due complexity in the drawings, the error signal conductors from
difference units 40, 43, 45 and 50 have no~ been shown, it being
evident, for example, that signal (TPe) generated in difference
unit 40 is applied to those computing units showing a (TPe) input.
In regard to the discrete control loops shown in Fig. 2
for turbine steam flow, feedwater flow and reactor heat output ---
it will be no~ed that certain error signals are introduced through
integrating units 58, 5g and 60, the output signals therefrom
being transmitted to multiplyi~g units 61, 62 and 63 respectively
and serve to apply a steady state calibration correction to the
primary feed forward control signal. Other signals, a~ shown, are
introduced into s D ing units 64, 65 and 66 and serve to apply
bias corrections to the primary feed forward control signal pro-
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portional to transient changes in the error signals. The
particular error signals applied to make a steady state
calibration correction or applied to make a bias and transient
correction are dependent upon the discrete control loop in
question.
With respect of the reactor heat output control loop, a
modified feed forward control signal, as established by function
generator 63A, operates control rod drives 102 to maintain the
reactor heat output equal to that required to satisfy load
demand under steady state conditions. Megawatt error is intro-
duced, to provide a steady state calibration correction, through
integrating unit 60 and multiplying unit 63. Signals providing
a bias and transient correction corresponding to reactor coolant
temperature error, throttle pressure error and megawatt error
are introduced through summing unit 66. control rod drives 102
are positioned to maintain actual reactor heat output in
correspondence with the control signal from summing unit 66 by
means of a local feedback loop comprising a difference unit 103
in which the output signal from summing unit 66 is compared with
a signal corresponding to actual neutron power (~i) generated
in neutron power transmitter 48. The output signal from
difference unit 103 through proportional plus integral unit 104
controls the operation of control rod drives 102 to maintain
actual neutron power equal to that required to maintain ~he
power producing unit at that value established by summing unit 66.
With respect of the turbine steam flow control loop, the
dified feed forward control signal operates turbine control
valves 19. (The control signal transmitted to control valves
19 can, through
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analogue to digital circuitry and the like, be adapted to any particu-
lar type of turbine valve control mechanism). Throttle pressure error,
introduced through integrating unit 58 serves to decrease turbine valve
po~ition and thus the demand for steam flow upon a decrease in steam
pressure below set point and vice versaO In order to prevent improper
steady state corrections to steam flow, a signal proportional to mega-
watt error is subtracted from throttle pressure error in difference
unit 70. Signals corresponding to average coolant temperature error,
feedwater temperature error, throttle pres~ure error and megawatt
10 error are applied as properly gained bias corrections to the feed for-
~rd control signal in summing unit 64. In operation, a decrease inaverage coolant temperature effects a decrease in turbine steam flow;
a decrease in feedwater temperature, ~uch as caused by the outage of
a feedwater heater, effects a decrease in ~team flow to compensate for
the decrease in extraction flow and thus avoids the transient increase
in power output that would otherwise re~ult; a decrease in throttle
pressure effects a decrease in turbine steam flow; and a decrease in
power output effects a corresponding increase in turbine ~team flow.
The proportional corrections applied through summing unit 64 act to
20 stabilize operation of the power producing unit during transient con- ~
dition~. The control signal from summing unit 64 i8 transmitted to
the final control element, turbine control valves 19,
In order that a consistent relationship will exi~t between the
control signal from unit 64 and rate of turbine steam flow~ a local
feedback loop is provided. ~ signal corresponding to actual turbine
steam flow is generated in first stage pressure transmitter 71 and
compared with the output signal from unit 64 in difference unit 72.
The signal generated in proportional plus integral unit 73 adjusts the
turbine control valves as required to maintain the signal generated
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mitter 71 equal to the output signal from summing unit 64.
As apparent from the foregoing description turbine steam
flow control valves 19 are positioned as required to maintain the
turbine steam flow demanded by the output signal from summing
unit 64. If desired, and as described in United States patent
3,894,396 which issued on July 15, 1975, to the applicant herein,
limit controls may be introduced into the steam flow control loop
whereby the rate of steam flow to the turbine is adjusted as
required to prevent throttle pressure excursions from set point
exceeding pred~termined limits.
With respect of the feedwater flow control, total feedwater
flow is maintained in proportion to a discrete modified feed
forward control signal and the feedwater flow to one steam gener-
ator relative to the feedwater flow to~-Sthe other steam generator
adjusted as required to maintain the average coolant temperatures
in loops A and B equal.
The feed forward control signal as modified in function
generator 62A operates, in parallel, a valve 75 regulating the
feedwater flow to steam generator 3 and a valve 76 regulating the
feedwater flow to steam generator 4. Total feedwater flow to steam
generators 3 and 4 is maintained equal to the demand by a feed-
back loop comp~ sing flow transmitter 77, difference unit 78 and
proportional plus integral unit 79. The output signal generated
in proportional plus integral unit 79 is transmitted through
conductors 80 and 81 to summing units 82 and 83 respectively and
hence adjusts feedwater flow to steam generators 3 and 4 equally.
Thus, under normal conditions, the steam outputs from the steam
generatores are maintained equal.
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A local feedback loop is provided for steam generator 3 comprising
flow transmitter 84, difference unit 85 and proportional plu8 inte-
gral unit 86. A 9 imilar feedback loop iq provided for steam genera-
tor 4 comprising flow transmitter 87, difference unit 88 and proport- -
ional plus integral unit 89. Thus the feedwater flow to steam genera-
tor 3 is maintained proportional to the output signal from summing
unit 82 and the feedwater flow to ~eam generator 4 i8 maintained pro-
portional to the output signal from summing unit 83. In summation,
the feedwater control 80 far described operates to maintain the total
fe~dwater flow to -~team generators 3 and 4 in proportion to the sig-
nal generated in ~umming unit 65 while maintaining the feedwater flow
to steam generator 3 proportional to the output signal from summing
unit 82 and the feedwater flow to steam generator 4 pr0portional to
the output signal from summing unit 8~.
Such equality of feedwater flow to cteam generators 3 and 4 is, ~ -
however, continuously modified, as required, to maintain the average
coolant temperatures in loop~ ~ and B equal. As shown in Fig. 3,
the output 8 ignal from summing unit 52 i~ proportional to the
average of the coolant temperature~ in loop A entering and leaving
the reactor 1 and the output signal from 3umming unit 53 is propor-
tional to the average of the coolant temperatures in loop B enter-
ing and leaving the reactor 1. As ~hown in Fig. 2 an output 9 ignal
proportional to the difference in loop A and loop B coolant tempera-
tures i8 generated in difference unit 90 and through proportional
plu9 integral unit 91 and summing unit 92 inputs to a multiplying
unit 93 receiving the output signal from summing unit 65. Thus the
feed forward control signal establishing the rate of feedwater flow
to stream generator 3 is modified in accordance with the difference
between the average coolant temperatures in . . . . . . . . . .
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loops A and B. If the average coolant temperature in loop A is
less than the average coolant temperature in loop B, the feedwater
flow to steam generator 3 will be dec~eased and vice versa.
Simultaneously, with the change in feedwater flOW to steam genera-
tor 3 the control operates to produce an esual but opposite change
in feedwater flow to steam generator 4. The output signal from
multiplying unit 93, representative of the demand for feedwater
flow to steam generator 3 is applied throuyh signal conductor 94
to difference unit 95 and thus subtracts from the output signal
from summing unit 65 an amount corresponding to the feedwater flow
demand to steam generator 3. The output signal from difference
unit 95, proportional to the difference between total feedwater
demand and the steam generator 3 feedwater demand, is thus the
correct feedwater demand for steam generator ~.
The override control from average coolant loop temperature dif-
ference, while being of high accuracy, is relatively slow in re-
sponse for the reason that it is dependent upon temperature mea-
surements which have a relatively long time constant. Such
changes in average coolant temperatures as may be caused by a
gradual fouling of one steam generator as compared to the other
one are satisfactorily handled. In order to handle rapid, violent,
and possibly catastrophic changes in average coolant temperatures
as might be caused, for example, by the outage of a coolant pump,
my invention further comprehends making immediate changes in the
relative rates of feedwater flow to the steam gene.ators in a push-
pull fashion to approximate the resulting change in difference in
averaga coolant loop temperatures. Yollowing such approximation,
the control operating from the difference in average coolant loop
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temperatures, introduces a relatively slow, continuing change until
the average coolant loop temperatures are restored to equality.
In Fig. 2 this principle is illustrated as applied to an abrupt
change in coolant flow as might be caused by the loss o~ a coolant
pump in loop A or B. Any change in coolant flow in one loop as com-
pared to the coolant flow in the other loop produces a proportional
change in the relative rates of feedwater flow to the ~team genera-
tors, anticipating the change in average coolant temperatures which
would result from such a change in the relative coolant loop flows.
Function generator 96 generates an output 5 ignal corresponding
to coolant flow through loop A as determined by a flow transmitter 97.
Function generator 98 generates an output 5 ignal corresponding to
coolant flow through loop B as determined by a flow transmitter 99
~hese two output signals are compared in a difference unit 100 and
the output signal therefrom inputs to summing unit 92. Upon a de-
crease in coolant flow through loop A, as caused, for example, by
the outage of coolant pump 7, the control operates to proportionate-
ly decrease the flow of feedwater to ~team generator 3 and effect a
proportionate increase in feedwater flow to steam generator 4 and
vice ver~a. Thereafter the control from the difference in average
loop temperatures modifie~ the change in the relative rates of feed-
water flow on a continuing basis until the average loop temperatures
are equal.
This principle i8 further illu3trated in Fig. 2 as applied to
anticipate differences in average coolant loop temperatures caused by
changes in the relative feedwater temperatures to steam generators
3 and 4, re3ulting, for example, from the outage of a feedwater
heater. A~sume, for example, the outage of feedwater heater 29. The
expected result would be a lowering of the temperature of the coolant
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entering reactor 1 from steam generator 3 and thus lowering the
average loop A coolant temperature. My invention anticipates this
change by making an immediate change to the feedwater flow rates to
the steam generators. Biac control action from difference unit 101
acting on summing unit 92 to the multiplying unit 93 increases the
feedwater flow to steam generator 4 and decreases the feedwater flow
to steam generator 3.
In the interest of brevity, there has been omitted from the
drawings and description, details where such details are not
germane to the invention and subject to alternate well known typesO
Thus, for example, in Fig. 1 steam generators 3 and 4 would be pro-
vided with multiple parallel tubes as customarily employed in once-
through steam generators. Similarly, the control rod drives 102
and nuclear power transmitter 48 are shown in block diagram to indi-
cate that the control system may be applied to any one of the
~everal types available for adjusting nuclear power level and the
mea~urement thereof. Further, the usual and well known protective
systems and limiting controls employed in a nuclear power plant
~ould be included. As such systems and controls form no part of the
present invention they have been omitted from the drawings and
description.
It will be apparent that the control system illu~trated and
de~cribed i9 by way of example only and that various modification-
~can be made within the scope of the invention as defined in the
appended claims.
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