Note: Descriptions are shown in the official language in which they were submitted.
~~ 17MT-2475
8~4
This invention relate~ to turbine overspeed control
systems for limiting turbine speed increases during the
occurrence of sudden decrea es in a load driven by the turbine,
where the turbine must be capable of stable operation at the
new load. More particularly, this invention relates to an
overspeed control system for a reheat turbine generator which
must be capa~le of stable operation upon separation from a
large electric system in which the generator normally operates
synchronously with other generators in the system.
In typical prior art reheat turbine overspeed control
systems, separate valves are controlled to regulate steam flow
to high pressure stages and reheat stages to maintain
turbine speed at a predetermined reference speed. A main
control valve regulates steam flow into high pressure ~tages
and an intercept control valve regulates steam flow into reheat
stages. On the occurrence of an overspeed condition while
the generator is separated from the system, by sudden loss
of at least part of the load, the main valve and intercept 'r
valve are partially closed to decrease steam flow into their
respective stages to effect a reduction of turbine speed to
the reference speed.
Control systems of the above-described type operate
effectively for small load losses or slowly decreasing loads
where both valves partially close and the reduced load is
proportionately distributed over all turbine stages. These
systems also operate effectively on the occurrence of sudden
severe load reductions where the load decrease~ to an amount
which the high pressure stages are capable of driving alone.
In this case a ram in the intercept valve is actuated causing
rapid complete valve closure and a consequent loss of the
major portion of driving power normally provided by the reheat
stages. Steam flow to the high pressure ctages is reduced
' 17MT-2475
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by the main control valve to that level necessary to support
driving of the reduced load.
On the occurrence of sudden severe load decreaqes
where the load decrea~e~ to an amount exceeding the driving
capability of the high-pressure stages alone, however, prior
art systems of the above-described type become unstable. Once
again the ram is actuated causing complete closure of the
intercept valve and loss of the load driving power normally
provided by the reheat stages. In this case, however, the
high pressure stages become overloaded and turbine speed drops
below the reference speed causing the intercept valve to
reopen. Steam again flows into the reheat stages and the
combined driving power of high pressure and reheat stages is
~ applied to the reduced load cau~ing turbine speed to again
- increase above the reference speed. The intercept valve then
alternates between closing and opening movement causing
alternate overloading of the high-pressure stages and over-
driving of the reduced load as the control system hunts for an
intercept valve position at which turbine speed will stabilize
at the reference speed for the reduced load.
The basic cause of control system instability during
severe load decreases is intercept valve nonlinearity. That
is, when valve movement is caused to occur at a constant rate
steam flow changes at a varying rate. In some cases steam -
flow decreaRes only 10% during the first 90h of valve closure.
~hus, when it is necesqary to rapidly decrease steam flow ~
during severe load decreases, prior art control systems which ~ -
cause cloqing movement of the intercept valve at a constant
rate cannot respond quickly enough to limit increasing
turbine speed and the intercept valve must be ~ubsequently
rammed closed. If the high pressure stages become overloaded
upon loss of the driving power of the reheat stages, prior ~
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` 17MT-2475
; 10384~4
art control Cystems begin to reopen the intercept valve. Valve
nonlinearity causes a cudden large increase in steam flow as
the valve begins to open resulting in the above-mentioned
overdriving of the load and consequent speed variation.
The present invention eliminates the instability
experienced with prior art systems during severe load changes
by improving intercept valve control and eliminating the need
for ~udden complete intercept valve closure. Thus the speed
instability caused by cycling of the intercept valve is
avoided. Intercept valve control is improved by development
of a nonlinear command signal and application o this ~ignal
to the valve causing rapid motion of the valve during that
portion of valve movement having little effect on steam flow
and causing less rapid motion of the valve during that portion
of valve movement having a significant effect on Qteam flow.
A command signal controlling intercept control
valves in such a manner a~ to compensate for intercept valve
nonlinearities is generated by two amplifiers. The command
signal is developed at the output of a first amplifier and
increases in magnitude at a rapid rate in response to an
increa~e in turbine ~peed above the reference speed. ~his
rapid signal increase causes rapid initial intercept valve
closing motion during that portion of valve movement having
little effect on steam flow. At a predetermined speed above
the reference speed a second amplifier interacts with the
first to cause a reduction in the rate of increase of command
signal. This reduction in command signal rate of change causes
less rapid valve closing motion during that portion of valve
movement having ~ignificant effect on steam flow. The net
effect of varying valve movement in this manner is a rapid,
steady decrease in steam flow resulting in effective control
of sudden speed increases by intercept valves. The speed
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instability caused by intercept vaLve cycling is thus avoided.
FIGURE 1 iS a block diagram of a steam turbine
generator in which the overspeed control sy~tem of this .: .
invention is incorporated.
FIGURE 2 is a curve repre~enting the command signal
utilized to compensate for control valve nonlinearities.
FIGURE 3 is a schematic diagram of a circuit for
generating the command signal.
Referring to FIGURE 1, steam produced in ~team
generator 11 flows through main stop valve 13 and main control ~-
valve 14 to a high pressure turbine stage 15. The direction
of steam flow i8 indicated by the direction of the arrowq ;-
along steam linec. Steam flows through the high pre3sure
stage 15 to reheat section 17 of steam generator 11 where
it i9 reheated and then to intercept control valvec 19 and
21. From valves 19 and 21 steam is applied to a first reheat
stage 23, through the first reheat stage and then through
second and third reheat stages 25 and 27. Steam from the
second and third stages 25 and 27 is collected in condenser
29 where it is condensed and then returned to steam generator
11. The turbine drives generator 31 to generate electric ~;
power.
With the turbine rotating at a constant speed, main
stop valve 13 and intercept control valves 19 and 21 are full
open. ~he amount of steam required to rotate the turbine at ~-
a particular speed is dependent upon generator loading and
is normally controlled by main control valve 14 which is
governed by control 37 in response to speed changes detected
by qpeed sensor 39. If the speed increases above a predetermined
reference speed control 37 applies a command signal to intercept
control valves 19 and 21 causing closing movement of the valves ~`~
until turbine speed decrease~ to the reference speed. ~-
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~Q 17MT-2475
. 1038474
In accordance with the invention, a command ~ignal
is generated which has the characteristic of increa~ing
rapidly during a first period to effect rapid closing movement
of the intercept control valves, and then less rapidly during
a second period to effect a less rapid closing movement of
the valves. ~hese characteristics are illustrated in FIGURE 2.
VOUt represents the command signal voltage and v repreQents
the speed sensor output voltage. Each particular V t also
corresponds to a particular position of the intercept valves.
Below reference speed A a zero volt command signal i9 generated
and intercept control valves 19 and 21 remain full open. If
turbine speed increases above the reference speed the command
signal voltage rises at a rapid rate B proportional to the
rate of increase of turbine speed. Closing movement of the
intercept control valves at this same rapid rate compensates
for the ff~mall effect valve movement has on ffffteam flow during
initial closure. At overspeed C the valves have moved to that
partially closed position where they begin to have significant
effect on steam flow and the rate of change of command signal
fi 20 voltage i9 decreased to a less rapid rate D. Although the
valves are also moving toward their closed positions at this
slower rate D, they have significantly more control over steam
flow and essentially the same rate of change of steam flow as
that occurring during rate of movement B is maintained. The
net effect is a rapid steady decrease of steam flow until the
turbine speed is under control.
FIGURE 3 shows a circuit for generating the command
signal function of FIGVRE 2. frhe command signal is yenerated
at circuit output 41 by amplifier 42 in response to a turbine
speed signal applied to circuit input 43. The speed signal is
applied through a resistor 44 to a summing node 45 at a first
input of amplifier 42. A reference speed signal is also
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1038474
applied to summing node 45 through resistor 46~ This reference -
speed signal is in the form of an adjustable voltage level
provided at potentiometer 47 which has one end connected to a
positive voltage supply and the oppo~ite end connected to a
zero volt reference or ground. A second input of amplifier
42 is referenced to ground. Resistor 48 and potentiometer
49 provide a feedback path from circuit output 41 to summing
node 45. Potentiometer 49 i~ included in the feedback path
for the purpose of adjusting amplifier gain. A resistor 50
is connected in series with the amplifier output to effectively
increase amplifier output impedance so that limiting amplifier
circuitry, to be described, can override the output of this
amplifier if the amplifier output exceeds certain predetermined
voltage limits.
These voltage limits are provided by two circuits,
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one including amplifier Sl and the other including amplifier
56. The circuit of amplifier 51 functions to place a pre-
determined upper limit on circuit output voltage in accordance
with a predetermined reference voltage. The circuit output
voltage and the reference voltage are applied to a fir3t input
of amplifier 51 through resistors 52 and 53, respectively.
~he reference voltage is provided at potentiometer 54 which
has one end connected to a negative voltage supply and the
opposite end connected to ground. A second input of amplifier
51 is referenced to ground~ Diode 55 is connected in series
with the output of amplifier 51 to effectively disconnect the
amplifier output from circuit output 41 at all circuit output
voltages below the upper limit. If the circuit output voltage
at 41 rise~ above the predetermined limit, an output voltage
developed by amplifier 51 decreases ~ufficiently to forward
bias diode 55 and reduce circuit output voltage to the
predetermined limit.
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17MT-2475
1~38474
The circuit of amplifier 56 functions to place a
predetermined lower limit on circuit output voltage in accordance
with a predetermined reference voltage. Normally the lower
limit is the zexo volt level. The circuit output voltage and
the reference voltage are applied to a first input of amplifier
56 through resistors 57 and 58, respectively. The reference
voltage is provided at potentiometer 59 which has one end
connected to a positive voltage supply and the opposite end
connected to a negative voltage supply. A second input of
amplifier 56 is referenced to ground. Diode 60 is connected
in serie~ with the output of amplifier 56 to effectively
disconnect the amplifier output from circuit output 41 at
all positive circuit output voltages. If the circuit output
voltage at 41 goes negative, an output voltage developed by
amplifier 56 increases sufficiently to forward bias diode 60
and increase circuit output voltage to the zero volt level.
An additional input signal to amplifier 42 is provided
by amplifier 61 at speeds above C to cause a reduction in the
rate of change of circuit output voltage. This signal, which
increases at a constant rate with respect to turbine speed,
is applied to summing node 45 through resi tors 62 and 63.
Amplifier 61 is connected in a summing configuration similar
to that of amplifier 42. The turbine speed signal from circuit
input 43 is applied through resistor 64 to a summing node at
a first input of amplifier 61. A reference speed signal
representing turbine speed C is also applied to the summing
node through resistor 65. This reference speed signal is in
the form of an adjustable voltage level provided at potentio-
meter 66 which has one end connected to a positive voltage
supply and the opposite end connected to ground. A second
input of amplifier 61 is refer to ground. ~esistor 67 and
potentiometer 68 provide a feedback path from output
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17MT-2475
1038g~r~4 .
resistor 62 to the summing node. Potentiometer 68 is included
in the feedback path for the purpose of adjusting amplifier
gain. Output resistor 62 is connected in series with the
amplifier output to effectively increase amplifier output ;
impedance.
In order to prevent the signal applied to resistor
63 from dropping below zero volts when the output from amplifier
61 goes negative limiting amplifier 69 i9 provided. The cignal
; applied to resistor 63 and a reference voltage level are applied
to a fir~t input of amplifier 69 through resistors 70 and 71,
respectively. me reference voltage is provided at potentio- ;~
meter 72 which has one end connected to a positive voltage
supply and the opposite end connected to a negative voltage
upply. A second input of amplifier 69 is referenced to
ground. Diode 73 is connected in series with the output of ~ -
amplifier 69 to effectively di~connect the amplifier output
,.. . .
from the amplifier 42 input at resistor 63 when positive
voltages are applied to resi~tor 63. When the voltage at the
output of amplifier 61 goes negative an output voltage developed
by amplifier 69 increases sufficiently to forward bias diode
73 and increase the voltage applied to resistor 63 to the zero
volt level.
Provision i8 made for periodically checking the
interce~t control valves for proper operation by application `~
of a negative going test signal to circuit input 74. The
test signal i5 applied through resistor 75 to cause a negative
unbalance at summing node 45 resulting in development of a
command ~ignal at circuit output 41 and consequent partial
closing of the intercept control valves,
In operation, the intercept control valve command
signal is generated by amplifier 42 in response to the turbine
speed signal applied to circuit input 43. The speed signal,
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~~~ 17MT-2475
- 103~4~4
which becomes increasingly more negative as turbine speed
increases, is algebraically summed with a positive reference
speed signal applied to summing node 45 through re~istor 46.
If the turbine ic operating at the reference speed, the turbine
speed signal and reference signal balance and a zero volt
output signal iq developed by amplifier 42. At speeds below
the reference speed A the speed signal becomes less negative
and the output of amplifier 42 goes negative. Circuit output
41 also attempts to go negative but limiting amplifier 56
prevents this by forward biasing diode 60.
If turbine speed increases above the reference speed
the speed signal becomes more negative than the level at which
it balances the reference signal causing the circuit output
voltage to go positive to prevent an unbalanced condition at
the summing node. The circuit output voltage rises rapidly
at a rate proportional to the rate of increase of turbine speed
as illustrated at B of FIGURE 2. The ratio of these two rates
can be set at a particular value by adjustin~ the gain of
amplifier 42 at potentiometer 49.
If turbine speed increases above a predetermined
overspeed value C a positive voltage signal is applied to
resistor 63 by amplifier 61. Prior to the occurrence of this
predetermined overspeed a zero volt signal was maintained at
the amplifier 42 input at resistor 63 by limiting amplifier
69. At all speed below C the amplifier 61 output is negative
becau~e of an unbalance between the reference signal applied
to resistor 65 and the speed signal applied to resistor 64, but
limiting amplifier 69 prevents the input signal to resistor 63
from going negative. At speeds above C amplifier 61 applies
a positive signal to resistor 63 which increases at a rate
directly proportional to the rate of increase of turbine speed.
This applied signal assists amplifier 42 in balancing the
`17MT-2475
lQ38~74
negative going speed signal at the resistor 44 input causing
a less rapid rate of rise of voltage at circuit output 41.
This decreased rate of rise of output voltage is indicated
at D in FIGURE 2. ~he ratio between the rates of ri~e of
turbine ~peed and circuit output voltage during transition D
can be set at a particular value by adjusting the gain of ;
... .. . . .amplifier 61 at potentiometer 68. The output voltage continue~ -
to increase until level E of FIGURE 2 is reached at which time
limiting amplifier Sl stops the voltage rise.
10Aside from limiting circuit voltage levels, the
limiting amplifiers effect sharp transition points at each
change of the command signal rate. Of particular importance
iB a sharp transition at point C where the rate of change of
valve motion must be altered to compensate for valve non-
linearities. The sharpness of these transition points is `
dependent on the stability of the references utilized to
determine these points. Since the references utilized with
the limiting amplifiers can be selected to be as stable as
desired, b~ proper choice of positive and negative voltage
supplies, very sharp tran~ition points are obtainable.
Circuit input 74 is utilized for the periodic appli-
cation of a test signal to input re~istor 75. Usually this
signal i~ applied during operation of the turbine at normal
operating speed A. The signal takes the form of a negative
going voltage and iB algebraically added to the actual speed
signal applied to resistor 44 to cau~e a negative unbalance
at summing node 45. This negative unbalance simulates an
overspeed condition and should cau~e the intercept valves to
close to the degree required to offset the simulated over~peed.
30It will be appreciated from the above description
that the present invention overcomes deficiencies of prior art
turbine speed control ~ystems related to ineffective ;
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1038~74 ;~-
.' control of speed during load changes. The effectiveness of ::-
typically used intercept control valves is greatly increased
allowing rapid ~peed control during severe load changes without :~
,' the in~tability problems experienced with the prior art
,, systems. ~I
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