Automatic steam generator feedwater control over full power range

A water level control for a steam generator of a pressurized water type of nuclear steam supply system varies the water level demand as a function of power. The control also varies the water feed rate after a reactor trip initially as a function of reactor coolant average temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to nuclear power and particularly to the control of 
recirculating steam generators in pressurized water nuclear steam supply 
systems (NSSS). More specifically, the present invention is directed to 
automatic water level controls for steam generators of nuclear power 
systems. Accordingly, the general objects of the present invention are to 
provide novel and improved methods and apparatus of such character. 
2. Description of the Prior Art 
The nuclear steam generator of a pressurized water nuclear power plant is 
typically controlled as a function of three primary operating parameters 
which are monitored, i.e., water level (L) steam flow (W.sub.S) and 
feedwater flow W.sub.fw). The signals corresponding to the monitored 
parameters are processed in proportional/integral and lead/lag circuits to 
generate a feedwater flow demand signal for controlling the amount of 
water introduced into the steam generator for the production of steam. The 
principal concern, and therefore the operating parameter on which the 
control action is primarily based, is the steam generator water level. 
In practice, the control of the steam generators of NSSS has proven to be 
an unusally difficult task. As a result, a significant proportion of major 
nuclear power plant outages have been caused by reactor trips due to steam 
generator operation outside the desired range. Many of these outages are 
due to reactor trips on low or high steam generator water levels. 
Typically, about 80 percent of steam generator low water level trips occur 
below 20 percent system rated power, and nearly 90 percent of the high 
water level trips occur below 20 percent power. The problem of maintaining 
steam generator water level within proper limits is particularly acute 
during plant startup, when the operators have had relatively little 
experience in steam generator water level control. 
A major complexity incident to steam generator control, particularly at low 
power levels, resides in the water recirculation characteristics of the 
system including the steam generator. Thus, during low power operation, 
the sensitivity of the steam generator water level to changes in feedwater 
flow increases. Also, at low power there is a seemingly anomolous 
behavioral characteristic which is manifested by an initial decrease in 
steam generator water level when there is an increase in the feedwater 
flow. This behaviour often confuses the operator, and usually causes the 
operator to further increase the feedwater flow, causing a further 
decrease in the water level and introducing "positive feedback" into the 
system which may lead to uncontrolled oscillation of the water level and 
to a reactor trip. 
Conventional controllers, even of the above-mentioned three parameter type, 
are unreliable at low power operation because the steam flow and feedwater 
flow signals are themselves not reliable under such operating conditions. 
In most instances, because of this known lack of reliability, the 
operators elect to manually control the water level. Attempts at manual 
control have met with only limited success to date. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to provide a control 
technique for a recirculating steam generator in a nuclear steam supply 
system and particularly a method which is capable of automatic water level 
control over the full power operating range of the steam supply system. 
In accordance with the invention, there is provided a control system and a 
method of control for a recirculating nuclear steam generator that takes 
into account the power related variations in the dynamic characteristics 
of the steam generator. 
Thus, the present invention automatically controls the feedwater flow rate 
to a steam generator to maintain satisfactory downcomer water level during 
steady-state operation and during the following load maneuvers: 
(a) 10 percent steps in NSSS load between 15 percent and 100 percent NSSS 
power. 
(b) 1 percent/minute ramps in NSSS load between 0 percent and 15 percent 
NSSS power and 5 percent/minute ramps in NSSS load between 15 percent and 
100 percent NSSS power. 
(c) Load rejections of any magnitude. 
The present invention also provides for automatic operation in the event of 
the following plant conditions: 
(a) Reactor trip 
(b) Loss of a Feedwater Pump during two feedwater pump operation. 
(c) High steam generator downcomer water level. 
In accomplishing the foregoing, the present invention automatically opens 
and closes, in a sequential manner, the downcomer and economizer feedwater 
control valves. Additionally, the invention coordinates the adjustment of 
the economizer feedwater control valve, the downcomer feedwater control 
valve and main feedwater pump speed setpoint to automatically regulate the 
feedwater flow between 0 percent and 100 percent NSSS power to control the 
steam generator water level. 
In accordance with a preferred embodiment, the invention regulates the 
feedwater flow rate to control the steam generator downcomer, i.e., steam 
generator, water level after a reactor trip by sensing the T.sub.AVG 
signal from the associated primary coolant loop. This action minimizes the 
possibility of overcooling the primary coolant loop after a reactor trip. 
The system automatically returns to the low power level control mode when 
steam generator water level returns to its setpoint. 
A control system in accordance with the invention is configured to minimize 
the necessity for separate adjustments by the operator during manual 
operation of the feedwater pump speed setpoint and the feedwater control 
valves. This minimizes operator actions and thus minimizes possible 
operator error. 
A particularly unique feature of the invention is its ability to position 
the feedwater control valves as a function of power speed such that at low 
flow rates feedwater flow is predominantly regulated by valves while pump 
speed control is the primary mechanism for feedwater flow adjustment at 
high flow rates. 
In the practice of the invention, a signal commensurate with the measured 
steam generator water level is passed through a lead-lag circuit. The lead 
improves the control responsiveness and compensates for the delays in the 
steam generator process, while the lag improves the steady state response 
and the stability margin. The lead and lag settings are automatically 
varied with power level to compensate for the dynamic characteristics of 
the steam generator. The thus processed water level signal is then passed 
through a proportional-integral controller, where the gain and reset rate 
are also adjusted as a function of power to further compensate for the 
steam generator dynamic characteristics.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a schematic illustration of a typical recirculating steam 
generator installation in a NSSS while FIG. 2 schematically shows a single 
steam generator 10. Feedwater is, under certain operating conditions, 
delivered to the generator via the downcomer nozzle 12 and flows into the 
downcomer 14 where it mixes with the recirculating saturated liquid. A 
downcomer valve 15 is located upstream of nozzle 12. The combined flows 
move down the downcomer and enter the tube bundle region 16 at the bottom 
of the generator. As the fluid rises through the tube bundle 18, it 
absorbs heat from the primary loop, exiting the bundle region as a two 
phase fluid. It then flows upward through the riser region 20 to the 
separators 22. The separators remove the saturated liquid from the steam, 
returning the liquid to the downcomer 14 and allowing the steam to rise to 
the dryers 24, before exiting the steam exits the steam generator 10 and 
enters the main steam line 26. 
As is well known in the field of nuclear power, the nuclear reactor (not 
shown) and the associated piping to and from the steam generator are 
usually referred to as the primary system, and the reactor vessel and 
associated piping contain the primary coolant volume. The hot leg of the 
reactor contains water which has been heated by the reactor and which 
enters the steam generator through inlet nozzle 30. The steam generator 
output nozzle 32 returns water from the steam generator through the cold 
leg piping to the reactor vessel. 
The recirculation process which includes the steam generator 10 is 
sustained by the imbalance in the hydraulic heads of fluid between the 
downcomer 14 and the tube bundle 18 and riser region 20. During high power 
operation, the difference in these driving heads is significant and leads 
to relatively stable operation. As the power is dropped, however, the 
amount of boiling is reduced in the tube bundle 18, causing a reduction in 
the quality, i.e., the amount of steam in the mixture flowing through the 
tube bundle, of the fluid and thus an increase in its density. This in 
turn reduces the amount of driving head which, in turn, reduces the amount 
of recirculation. As this occurs, the generator 10 approaches a manometer 
type condition, where the hydraulic head of the downcomer 14 and the 
hydraulic head of the riser 20 and tube bundle 18 approach each other. 
Under these conditions, the downcomer water level becomes very difficult 
to control. 
The normal water level in the steam generator is indicated at 36. The 
instrumentation for water level measurement is conventional and is 
indicated in FIG. 1 at 40. Steam flow is measured at, for example, 42. 
Feedwater is also and primarily supplied to steam generator 10 through a 
feedwater nozzle 44 which is connected to an economizer 50. An economizer 
valve 52 is located upstream of nozzle 44. 
A principal objective of the steam generator water level control system is 
to prevent the water level from rising too high, and causing a high level 
plant trip, or dropping too low, causing a low level plant trip. The water 
level is controlled primarily by regulating the feedwater flow through 
nozzles 12 and 44 through modulation of the settings of valves 15 and 52, 
and by varying the speed of pumps 54. 
With reference to FIG. 3, a feedwater flow and steam generator water level 
control is generally illustrated. A set point 54, representing the desired 
water level, is a first input to a comparator 56. The difference between 
the set point and the actual steam generator water level, as provided by 
level instrumentation 40, is outputted from comparator 56 and fed to a 
controller 60. Controller 60 generates an electrical control signal which, 
after suitable processing, to convert the electrical signal into a fluidic 
signal is delivered to one or more valve actuators 64. Each actuator 
operates a valve or the like, having characteristics as represented at 66. 
System operation is also influenced by flow relationships, as represented 
at 68, and by the feedtrain characteristics, as represented at 70. Both 
the individual flow relationship 68, as influenced by the valve 
characteristics, and the feed train characteristics 70 determine the steam 
generator performance represented at 72. Steam generator performance is 
also influenced by the operating parameters and design of the primary 
system 74. The changes in the steam generator performance 72 are measured 
by the level instrumentation 40 and the signal is returned to the 
comparator 56 to complete the closed control loop. 
Turning now to FIG. 4, the improved extended range automatic feedwater 
control system of the present invention will be described in detail. In 
the following description, it should be appreciated that the invention 
comprises apparatus and a method for accomplishing novel control 
techniques. The particular components necessary to implement these 
techniques are known in the general art of control engineering, but the 
particular variables used in the control algorithms, and the manner in 
these variables are employed are novel. Accordingly, where only method 
steps are described, the associated hardware is either conventionally 
found in a nuclear power plant, or the selection thereof is self-evident 
from the function it is to perform. 
Three parameter steam generator control is known in the art. The present 
invention uses the known three parameter controller as the base upon which 
the present invention is constructed. A comparator 76 receives a steam 
flow signal WS from the instrument 42 that measures steam flow and a 
feedwater flow signal. The feedwater flow W.sub.fw /signal is provided by 
a flow sensor 77 (FIG. 1) that measures the feedwater flow. The difference 
between steam and feedwater flow is a mass flow difference signal which is 
dynamically compensated in a filtered derivative network 82 and the 
resulting flow error signal is delivered as a bias signal to a comparator 
86. Network 82 has a steady state gain equal to zero so that, when the 
flow difference is not changing, the compensated output signal will be 
zero and thus will not contribute to the flow demand signal. A signal 
commensurate with the measured water level L is delivered to an adaptive 
lead/lag network 90, and the dynamically compensated level signal from 
network 90 is delivered as the second input to comparator 86. The network 
90 gives higher amplification to rapidly changing inputs than to slowly 
changing inputs and has a steady state gain of unity. The output of 
comparator 86 is a water level signal biased by the input to output mass 
flow difference. This biased level signal is delivered as the first input 
to another comparator 96 where it is compared with a water level set point 
(LSP). The result of the comparison performed in comparator 96 is a 
compensated level error signal. This signal has previously been employed 
as the primary control signal to maintain steam generator water level. 
While the prior control scheme works reasonably well for relatively high 
power operation, i.e., above 20% of rated power, the control becomes 
unreliable as the power level of the NSSS decreases. 
In accordance with one aspect of the invention, the lead/lag circuit 90 
varies the water level signal L as a function of reactor power level to 
compensate for dynamic characteristics of the steam generator. The level 
error signal from comparator 96, i.e., the sum of the compensated mass 
flow difference and level error signals, is passed through a proportional 
integral controller 102, where the gain and reset time constant are also 
adjusted as a function of power to further compensate for the 
characteristics of the steam generator. The output of controller 102 is a 
flow demand signal and is the sum of the level error signal plus the 
integral of the level error signal. Circuit 102 has steady state 
characteristic such that when the summed error input signal is zero, the 
controller output signal is constant. A non-zero summed error input signal 
is integrated causing the output signal to move toward its maximum (for a 
positive summed error signal) or minimum (for a negative summed error 
signal) valve. Thus, any level error is forced in steady-state to equal 
zero. 
A signal proportional to nuclear reactor power is filtered in circuit 106. 
The thus processed power signal is delivered to function generators 110 
and 112, where the lag coefficient T4 and the ratio of the lead and lag 
coefficients T3/T4 is determined as a function of power. The thus 
determined ratio and lag coefficient are inputted to circuit 90. The 
filtered power signal is also delivered to function generators 114 and 116 
which in turn generate the coefficients T8 and K1 for delivery to the 
proportional-integral controller 102, the gain control signal K being 
filtered in circuit 117 before inputting to controller 102. The power 
adjusted set point for cirucit 90 and controller 102 make it possible to 
control steam generator level over the full power range by accounting for 
the non-linear characteristics of the steam generator. The feedflow demand 
signal from controller 102 is the main signal for establishing the set 
points for the feedwater pump speed, downcomer valve position, and 
economizer valve position. 
The steam generator water level is thus controlled, as a function of the 
measured level and reactor power level. The control scheme is further 
refined by utilizing a logic scheme that depends on where the reactor 
power lies relative to three power operating regimes, i.e., a low power 
regime, typically covering the range of about 0-15% of the reactor power, 
an intermediate power range, preferably in the range of about 15-50% 
power, and a high power range, preferably in the range of about 50-100% 
power. The power range determines the combination of feedwater pump speed, 
downcomer valve position, and economizer valve position utilized to 
control water level. The reactor power signal from circuit 106 is also 
delivered to switchover control circuits 126 and 128. In the manner to be 
described below, the state of switchover control circuits 126 and 128 in 
part determines the analog input signals to the pump speed and valve 
position function generators 130, 132, and 134 and also exercises on-off 
control over the economizer valve position control signal outputted from 
function generator 134. 
It is to be noted that at high power levels the steam flow rate is a 
reliable measure of power. The steam flow rate signal W.sub.S, in addition 
to delivery to comparator 76, is filtered in circuit 170 and applied to a 
switch control circuit 172. Control circuit 172 provides a digital control 
signal for a switching circuit 174 when the system power exceeds a 
predetermined level. In one reduction to practice, this control level, on 
rising power, was 55% of rated power. The circuit 172 simulates a 
hysteresis effect such that the switching control signal is discontinued, 
on decreasing power, when the power falls below 50% of rated. The presence 
of the control signal at the output of circuit 172 causes application of a 
steady state demand signal from downcomer bias signal generator 176 to the 
input of downcomer valve position demand function generator 132 via 
switching circuit 178. 
From the discussion above, and as will be described in greater detail 
below, under high power level conditions the downcomer valve will be open 
but will not be modulated. Also, the economizer valve will be open and 
modulated. The feedwater pump(s) speed will similarly be modulated. In the 
intermediate power range, the downcomer valve will be closed and the 
economizer valve position and pump speed will be modulated. In the low 
power range, the economizer valve will be closed, the pump speed will be 
constant and the downcomer valve will be modulated. Closing of the 
economizer valve during low power operation is achieved by exercising 
control over a switching circuit 180 to select a "zero" level economizer 
valve position demand signal when either of the binary inputs to an OR 
gate 182 are positive. It may thus be seen that the present invention is 
effectively a single element system at low power levels and a 
three-element system at high power levels. 
It should be noted that the steady state steam generator liquid inventory 
is greater when feeding the downcomer than when feeding the economizer. 
For this reason, the switchover control circuit 128 is programmed to 
provide a hysteresis effect so that there is a delay in the valve action 
to compensate for the two different inventories. The output of circuit 128 
is a digital command signal. 
Below a predetermined power level, fifteen (15) percent in one reduction to 
practice, the steam flow and feed flow signals are unreliable. For this 
reason, the maximum demand signal provided at the output of controller 102 
is limited in circuit 140 when the power level is below the predetermined 
level. This limiting action is obtained by employing the digital control 
signal from switchover control circuit 126, which functions as a level 
detector to control a switching circuit 142. Thus, when the NSSS is 
operating at less than 15% rated power in the example being discussed, the 
output of controller 102 will be limited. When the NSSS power is above 15% 
of rated power, the limiting circuit 140 is bypassed. 
Similarly, the output of level detector 126 is used to control a switching 
circuit 144 so that the steam flow/feed flow bias signal is removed from 
comparator 86 in the low power operating regime. 
Another feature of the invention is the automatic control of refilling the 
steam generator. After a reactor trip, a tripped override signal will 
appear, after a delay, at a first input of AND gate 152 and a first input 
of a trip set/reset circuit 190. If the water level is below a threshold 
level as established by a level detector 156, the level detector output 
being delivered as the second input to gate 152, refill of the steam 
generator is accomplished by a control scheme based on the difference 
between the average temperature T.sub.AVG of the primary loop and a 
constant commensurate with the average temperature under no load 
conditions T.sub.AVGNL. The difference in these temperature signals is 
generated by a comparator 160 and processed in a proportional/integral 
controller circuit 162. The output of controller 162 will be a flow demand 
signal. If this demand signal is between upper and lower limits, as set in 
a limiting circuit 168, it will be passed to switch 166. The state of 
switch 166 is controlled by the output of circuit 190 to pass only the 
limited output of controller 162 until circuit 190 is reset by the change 
of state of one of the inputs to AND gate 152. The input to gate 152 
provided by level detector 156 will, of course, change when the sensed 
water level exceeds the threshold level. Thus, if the reactor has been 
tripped and the water level is below normal, the primary loop average 
water temperature controls the refill rate with the water being supplied 
via the downcomer valve. Once the water level reaches normal, the control 
is returned to the automated system described above. 
The output of set/reset circuit 190 is also applied as an input to OR gates 
182 and 192. Thus, until circuit 190 is reset after having been set in 
response to a reactor trip, gate 182 will apply a signal to switching 
circuit 180 which will close the economizer valve. Similarly, gate 192 
will apply a signal to switching circuit 178 which will cause the output 
of controller 162 to be applied to function generator 132. 
At power levels in excess of that predetermined as being in the low range, 
the output of switchover control circuit 128 will operate a switching 
circuit 194 to cause closing of the downcomer valve until the bias signal 
from source 176 is applied in the manner described above. The output of 
circuit 128, after inversion and delay, is applied to OR gate 182 and 
closes the economizer valve at low power. The delay in opening and closing 
the economizer valve provides compensation for mass changes. 
A high level (HLO) will result in a zero flow demand signal and both the 
economizer and downcomer valve will be closed. The pump(s) will, however, 
continue to operate. 
The disclosed control provides for operator intervention, M/A, at various 
points in the system whereby full or partial manual control may be 
exercised. 
It can be appreciated that, in order to implement the control system 
described above, the dynamic characteristics of the steam generator and 
other components have to be considered in order to provide appropriate 
compensation in the various circuits described above. This information can 
be obtained by analytical studies or field testing, where the steam 
generator is subjected to primary side perturbations, including steps, 
ramp and sinusoidal perturbations. The perturbation techniques are 
intended to characterize the non-linear behavior of the steam generators. 
The steps and sinusoids provide an indication of increasing responsiveness 
of the steam generators at low power and also provide an indication of the 
delays in the system. 
FIG. 5 demonstrates some of the steam generator characteristics which 
adversely affect the controlability of the steam generator water level at 
low power. The initial influx of feedwater tends to cause a small level 
rise due to the mass increase in the downcomer. As fluid is accelerated to 
the tube bundle, because of the increase in the downcomer head, the amount 
of fluid entering the tube bundle is increased. The net effect is that the 
steam generator reacts opposite to intuition, at least initially, and has 
an inherent delay which provides a negative phase shift in an 
uncompensated control scheme. The present invention takes this phenomena 
into account. 
A slow sinusoidal perturbation of feedwater flow rate results in a large 
phase lag in the steam generator water level response. Thus, as shown in 
FIG. 6, a relatively slow sinusoidal flowrate perturbation at low powers 
is manifested by a change in water level delayed by a phase lag 
approaching 180 degrees, indicating a need for compensation in the 
controller design. This phonomona is also represented by FIG. 8. 
FIG. 7 shows the rapid increase in the delay time (time for level to 
recover back to the initial level) as feedwater temperature is reduced. 
Since most pressurized water reactors feed relatively cold water into the 
steam generator at low powers (before there is adequate steam to bring the 
main turbine on line) the delay time is large and can lead to an 
instability if the control system is not designed to compensate for the 
delay. 
FIG. 8 shows the process gain increasing at low power. For an optimum 
control system, variable gain is required to maintain the controller 
performance without affecting stability. Particularly, gain should be 
varied to compensate for the steam generator response characteristics, 
including the response characteristics of components such as valves which 
affect the feedwater flow in the system. Other considerations which should 
be taken into account include the downcomer level versus water level 
relationship and the state of the fluid where the feedwater is being 
injected into the steam generator. 
Of course, the variation of steam generator response as a function of power 
level is of major importance and the function generators which provide 
control signals for power level compensation purposes may need to be 
adjusted during life of the NSSS. 
FIG. 9 graphically shows actual startup of a typical NSSS where the thermal 
power of the system is increased from hot standby to 12%. Initially, steam 
generator level is maintained manually using an auxiliary feedwater 
system. At approximately 2% power, feedwater control is manually 
transferred to the feedwater control system of the present invention. 
Throughout this period, the steam generator level oscillates as the 
operator attempts to maintain it. After the system is placed in automatic, 
at about 3% power, the steam generator level stabilizes at its setpoint. 
Furthermore, it continues to operate in this stable manner, as power is 
increased and disturbances are imposed on it due to the placing steam 
reheater in service, driving control rods and changing blowdown rates. 
FIG. 10 demonstrates how maintaining the feedwater system under the 
automatic control of the present invention can help to avoid a steam 
generator level trip. In this case, the operator took manual control of 
the system and closed the downcomer valve on the steam generator without 
simultaneously opening the economizer valve. As noted above, the downcomer 
valve is used during low power operation and the economizer valve is used 
for operation between 15 and 50% power. Since both valves were closed the 
steam generator water level dropped rapidly. The transient was mitigated 
by opening both control valves and placing the system in automatic. In 
this mode the system was again able to maintain steam generator level. 
FIG. 11 demonstrates how the present invention is able to restore steam 
generator water level following a large perturbation. During the 
transient, feedwater flow is transferred from the economizer valve to the 
downcomer valve causing the steam generator operating characteristic to 
change. This causes an initial swell in steam generator level due to the 
non-equilibrium condition. The feedwater control system limits the 
overshoot in level and then successfully restores it to its normal 
setpoint during this low power operation. 
While a preferred embodiment has been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustration and 
not limitation.