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 steam turbine
plant provided with turbine bypass systems.
The invention can be more fully understood from the following
detailed description taken in conjunction with the accompanying drawings in
which:
Figure 1 is a block diagram showing a prior art steam turbine
plant provided with turbine bypass systems;
Figures 2 and 3 are block diagrams showing prior art control
systems for a high pressure turbine bypass system and a low pressure turbine
bypass system respectively;
Figure 4 i6 a block diagram showing the control system embodying
the invention;
Figure 5 is a block diagram showing a combination of turbine bypass
control systems and a turbine control system embodying the invention;
Figure 6 is a block diagram showlng the detail of the control
system shown in Figure 4;
Figures 7 and 8 are graphs showing the manner of setting pressure
for the turbine bypass systems embodying the invention, in which the ordinate
shows the pressure in %, and the abscissa the turbine load in %; ant
Figure 9 is a graph showing the operating characteristics of the
main steam pressure, the steam flow quantity, the flow quantity through the
high pressure bypass line, the turbine speed, the turbine load, the reheated
steam pressure, and the flow quantity through the low pressure bypass line,
respectively.
It ia a recent trend to provide a turbine bypass system for an
electric power generating plant in the form of a combination of a sub-
critical pressure natural circulation type boiler and a reheat turbine or a
combination of a forced circulation type boiler and a reheat turbine. As
shown in Figure 1 the turbine bypass system utilized in such power plant
includes a hi8h pressure bypass line lO and a l~w pressure bypass line 11.
The high pressure bypass line 10 is connected to the inlet side of
a main steam stop valve 13 for a high pressure turbine 12 and includes a
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pressure reducing valve 14 and a desuperheater 15. The lower end of the
high pressure bypass line 10 is connected to a cold reheating line 16. The
purpose of the high pressure bypass line is to decrease the temperature and
pressure of the high temperature and high pressure steam generated by the
superheater 18 of a boiler 17. Cooling water from a feedwater pump 19 is
supplied to the desuperheater 15 through a temperature control valve 20.
This system permits the boiler to start and supply a certain load independ-
ently of the turbine so that the pressure and temperature of the boiler are
readily stabilized before starting the turbine. The steam passed through
the high pressure bypass system flows back to the reheater 26 of
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the boiler 17 is circulated through the boiler 17
The low pressure bypass line 11 is connected between
the inlet side of a reheat stop valve 22 and a condenser
25 via a pressure reducing valve 23 and a desuperheater
24 and operates to reduce the temperature and pressure
of the reheated steam. The cooling water from a conden-
sate pump 27 is supplied to the desuperheater 24 through
a temperature control valve 28 for the purpose of pre-
venting thermal deformation of the medium pressure tur-
bine 21 caused by the high pressure, high temperature
steam from the reheater 26 during starting. ~he result-
ing low temperature low pressure steam is circulated
through the boiler 17 together with the condensate from
condenser 25.
When the high pressure and low pressure bypass lines
are provided for an electric power plant the steam generat-
ed by boiler 17 flows, during starting, from superheater
18 through high pressure bypass line lO~ reheater 26,
low pressure bypass line ll, condenser 25, ~eedwater pump
19, back to the boiler. However, the quantity of steam
that flows through this circuit is about 3~/o of the rated
ste~m flow of the boiler. During the bypass operation,
the pressure of the main steam generated by the super-
heater 18 of the boiler 17 and reheat steam 26 are
controlled by pressure reducing valves 14 and 23 provided
for the high pressure bypass line 10 and the low pressure
byp~ss line 11, respectively. During the turbine startin~,
both the main and reheated steams flow into high pressure,
medium pressure and low pressure turbi~es through control
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valve 29, main stop valve 13 and intercept valve 30 and
reheat stop valve 22, respectively.
As the load of the turbines increases gradually the
quantity of the steam supplied to the turbines is in-
creased and the quantity of the steam circulating through
the high and low bypass lines 10 and 11 is decreased
correspondingly. ~hereafter, the total quantity of the
steam generated by the boiler flows through the turbines.
~hen pressure reducing valves 14 and 23 are fully closed
to terminate the operation of the bypass lines.
During the normal running of the power plant the
high and low bypass lines are not used, but when the
quantity of the steam flowing into the turbines decreases
rapidly due to the load reduction or turbine trip~ the
turbine bypass systems described above operate to prevent
pressure rise of steam and make it possible to continue
the operation of the boiler without tripping and starting
the turbine quickly.
~ he control system of the turbine bypass systems
arè shown in ~I~S. 2 and 3. FIG. 2 shows a control
system for the high pre6sure bypass system in which the
pressure of the main steam is detected by a pressure
detector 31. This pressure signal a is sent to a compara-
tor 32 to be compared with a pressure reference signal d ;~
which is produced by correcting a set signal b produced
by a manual se-tter 33 by an actually measured pressure
signal c detected at a turbine stage by a pressure set-
ter 34. Where there is a difference between signals a
and d, a deviation signal is created which is sent to
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pressure reducing valve 14 by a pressure controller 35
for opening or closing the pressure reducing valve 14.
The temperature of the steam leaving the desuper- `
heater 15 is sensed by a temperature detector 36 and a
detection signal e produced b~ it is sent to a comprarator
37 where signal e is compared with a set signal f pro-
duced by a temperature setter 38. When signals e and f
are not equal a deviation or error signal is sent to the
temperature control valve 20 through a temperature con-
troller 39.
The control system for the low pressure bypass sys-
tem is shown in FIG. 3. Since this control system is
similar to that shown in ~IG. 2 it would be unnecessary
to describe it in detail.
The control of the temperature and pressure has
been made by the control systems described above. It is
a recent tendency, however, to use electro-hydraulic
control system (EHC) for controlling a turbine control
valve because of its quick response characteristic.
Incorporation of EHC into the control system for the
turbine bypass system results in such advanatges as a
collective control of the turbine plant, cost saving due
to common use of the hydraulic system, easiness of the
maintainance and repair and improvement of the control
characteristics due to interlocked control of the turbine
system and the bypass system.
Accordingly,it is an object of this invention is to
provide a novel control system for a steam turbine plant
wherein the control system for the turbine bypass lines
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described above is combined with an electro-hydraulic
control system thereby enabling a collective control of
the entire turbine plant.
Another object of this invention is to provide a~
improved steam turbine control system capable of decreas- ;
ing the starting time of a steam turbine plant, simpli-
fying the control system, making easy the maintenance and
inspection of the control system and improving the con-
trol characteristic,
According to this invention there is provided a
control system for a steam turbine plant of the class
comprising a boiler provided with a superheater and a
reheater; a high pressure turbine co~nected to receive
steam generated by the boiler through first valve means
and to suppl~ exhaust stea~ to the reheater; a medium
pressure turbine connected to receive reheated steam ~rom
the reheater through second valve means; a high pressure
bypass line connected between the first ~alve means and
the reheater; and a low pressure bypass line connected
between the second valve means and a condenser;
wherein said control system comprises a first electric
control system for the high pressure bypass line including
mea~s responsive to ~he temperature and pr~ssure of the
steam supplied to the high pressure turbine for control-
ling the quantity of steam circulating through the high
pressure bypass line; a second electric control system
for the low pressure bypass line including means respon-
sive to the temperature and pressure of the ste~m supplied
to the medium pressure turbine for controlling the
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quantity of the steam circulating through the low pressure bypass line; a ~
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third electric control system responsive to the speed and load conditions of :~
the turbine plant for controlling the first and second valve means; an
electrical connection between the first, second and the third electric control
systems so that the operation of the first and second electric systems is
modified in accordance with the load of the turbine plant.
The invention will now be described in detail with reference to a
preferred embodiment illustrated in the accompanying drawings.
Figure 4 shows a block diagram of a control system wherein a novel
control system for the turbine bypass system is incorporated into a prior ~`art electro-hydraulic control system (FHC) for controlling a turbine control
valve. In Figure 4 the main control system 40 comprises a speed control
circuit 41 and a load control circuit 42.
The speed control circuit 41 compares a speed setting signal g sent
from a speed setter 43 with an acceleration signal h sent from an acceleration
setter 44a and an actual turbine speed signal i and sends a deviation or error
signal ~ to the load control circuit 42.
The load control circuit 42 is constructed to process and amplify -
the deviation signal ~ from the speed control circuit 41, a load reference
signal k from a load setter 44, a loading rate setting signal Q sent from a
loading rate setter 45, a pressure signal m actually measured at the first
stage of the turbine, a full arc-partial arc transfer signal n sent from a
full arc-partial arc transfer circuit and a load
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limiting signal p sent from a load limiter 47 for sending
processed and amplified signals ~1~ q2~ q3 and q4 to the
control circuit 48 of the main steam stop valvel the
control circuit 49 for the steam control valve, the con-
trol circuit 50 for the intercept valve, and the control
cirucit 51 for the reheat stop valve, respectively, for
controlling the degree of opening of these valves.
Above description relates to a well known main con- :
trol system 40 for controlling the turbine control valve.
According to this invention a control system designated
by reference numerals 52 through 62 for controlling the
turbine bypass system is added to the control system 40.
More particularly, in FIG. 4, the turbine bypass
control system 52 for controlling the turbine bypass
system 52 comprises a high pressure bypass control cir-
cuit 5~ and a low pressure bypass control circuit 54.
~ he high pressure bypass control circuit 5~ is
constructed to process and amplify a pressure settin~
signal rl sent from a pressure set-ter 55, a temperature
setting signal sl sent from a temperature setter 56, an
actual steam pressure signal tl and an actual steam
temperature signal ul for producing output signals vl
which are sent to a reducing valve control circuit 57 and
a temperature control valve control circuit 58 which
control the pressure reducing valve 14 and temperature
control valve 20 respectively contained in the high pres-
sure bypass line 10.
Similarly, the low pressure bypass control circuit
54 is constructed to process and amplify a pressure
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setting signal r2 sent from a pressure setter 59, a
temperature setting signal s2 sent from a temperature
setter 60, an actual reheated steam pressure signal t2
and an actual reheated steam temperature signal u2 for
producing output signals v2 which are sent to a reducing
valve control circuit 61 and a temperature control valve
control circuit 62 which control the pressure reducing ~:
valve 23 and the temperature control valve 28 respective-
ly contained in the low pressure bypass line 11 shown in
--j FIG. 1. The high pre6sure bypass control circuit 53 and
the low pressure bypass control circuit 54 are electrical-
ly connected to the load control circuit 42 so as to
operate in accordance with the turbine load.
By combininK the control systems of turbine bypass
lines with the control system for controlling the turbine
control valve as above described, it is possible to
commonly use an EHC circuit and a hydraulic power unit as
shown in ~IG. 5.
~ IG. 6 is a block diagram ~howing the detail of a
turbine bypass control circuit and a load control circuit
when the control system for the turbine bypass lines is
combined with an electro-hydraulic control system for
controlling the turbine control valves shown in FIG. 4.
B More particularly in ~IG. 6, a deviation signal/~ent
from speed control circuit 41 is passed through gain
setters 70a and 70b which set a reciprocal of the turbine
speed regulation and then added to a load setting signal
k sent from load setter 44 by adders 71a and 71b to form
valve opening setting signal~. ~uring the normal running
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of the turbine the output signal from adder 71a operates
as a control signal for a control valve 73, whereas the
output from the adder 71b operates as a control signal
for an intercept valve 74.
During the turbine bypass operation, the output
signal from adder 71a is sent to a low value priority
circuit 76a which compares the output signal, a load ~-
limiting signal p sent from load limiter 47, and a high
pressure bypass pressure signal sent from a high pressure
bypass control circuit 53 to be described later Por
selecting a signal having the lowest value. A portion
of the output signal from the low value priority circuit
76a is added to a bias signal sent from full arc - partial
arc transfer circuit 46 by adders 77a and 77b, and the
outputs of the adders are sent to main steam stop valve
72 and control valve 7~, respectively. The other portion
oP the output of the low value priority circuit 76a is
sent to another low value priority circuit 76b via a gain
setter 78 and an adder 79 to be compared with a low pres-
sure bypass pressure signal sent from a low pressure by-
pass control circuit 54 so that a signal having lower
value is selected by the low value priority circuit 76b.
The output oP this circuit is added to a bias signal sent
from Pull arc - partial arc trans~er circuit 46 by
adders77c and 77d to control intercept valve 74 and
reheat stop valve 75. The gain setter 78 connected bet-
ween the low value priority circuit 76a which interlocks
the main stop valve 72 and the reheat stop valve 75, and
the low value priority circuit 76b which produces a
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cortrol signal for interlocking the control valve 73 and
the intercept valve 74 determines the ratio of openings
of the control valve 73 and the intercept valve 74 during
the turbine bypass operation. A bias signal adder 80 is
provided for the purpose of releasing the interlock bet-
ween the main stop valve 72 and the reheat stop valve
and the interlock between the control valve 73 and the
intercept valve 74 during running.
The control system for the high pressure bypass line
,~, operates to decrease the opening of the pressure reducing
valve 81a of the high pressure bypass line when the quan-
tity of the steam flowing into the turbine increases during
the bypass operation so that it operates as a pilot element
for pressure control. Accordingly, the output signal
from the high pressure bypass control circuit 58 is com-
pared with the output signal from the low value priority
circuit 76a by a comparator 82a and its deviation or error
output signal is used for operating the high pressure
bypass pressure reducing valve 83a. This deviation
~O signal and the temperature signal sent from a temperature
control circuit 84a are compared by a high value priority
circuit 85a~ and a signal having a higher value is sent
to a high pressure bypass temperature control ~alve 81a
from the high value priority circuit 85a. The pressure
setting of the high pressure bypass line is effected in
the following manner. Thus, the signal from a mc~nual
setter 86a and the output signal sent from a pressure
setter 87a and corresponding to the turbine l~ad are
compared by a high value priority circuit 88a, and one
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of the signals having higher value is sent to the high
pressure b~pass co~trol circuit 53 to be compared with
actual main steam pressure seignal xl. This relation is
shown by the graph shown in ~IG.7. Accordingly, this high
pressure bypass control system acts as a safety valve
at the time of load interruption and to prevent decrease
in the pressure of the main steam.
~ he control system for the low pressure line is
similar to the control system for the high pressure by-
pass line described aboveO Thus, during the bypass
operation of the turbine, when the quantity of the steam
flowing into the turbine increases the opening of the low
pressure bypass pressure reducing valve 81b is decreased
80 that the control system acts as a pilot element.
Accordingly, the output signal from the low pressure by-
pass control circuit 54 is compared with the output from
the low value priority circuit 76b by a comparator 82b
and its output deviation signal is used to control low
pres~ure bypass pressure reducing valve 81b. ~urther,the
deviation signal from comparator 82b is compared with a
temperature signal from temperature control circuit by
a high value priori-ty circuit 85b. The signal having a
higher value is used to control a low pressure bypass
temperative control valve 83b. The pressure setting of
the low pressure bypass line is made in the following
manner. More particularly, the output signal from a
manual setter 86b is compared with the output sginal sent
from a pressure setter 87b and corresponding to the tur-
bine load b~ a high value priorit~ ciucuit 88b so that a
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signal having a higher value is se~t to the low pressure
bypass control circuit 54 to be compared with actual
reheat steam pressure signal x2. ~his relationship is
shown by a graph shown in ~IG. 8.
~ he operation of the control system of this inven-
tion under normal running will now be described with
reference to ~IGS. 6 and 9.
I. From boiler starting to pressure rise.
Since the turbine is at standstill, both output
signals from the speed control circuit 41 and the load
setter 44 are zero with the result that the main stop
valve 72, the control valve 73, the intercept valve 74
and the reheat stop valve 75 are fully closed.
The high pressure bypass control system operates
such t~at although the high pressure bypass pressure
reducing valve 8~a is fully closed while the steam pres-
sure is low as the pressure of the main steam generated
by the boiler increases the output from the high pressure
bypass control circuit is increased to gradually open the
high pressure ~ypass pressure reducing valve 83a. ~he
low pressure bypass control system operates in the same
manner to gradually open the low pressure bypass pressure
reducing valve 81b.
II. ~urbine starts
As the turbine starts, the speed control circuit 41
functions and the output or valve opening signal produced
by the low value priority circuit 76a increases gradually.
When the turbine is sarted with the all ~ull arc
admission mode, the bias signal from the full arc -partial
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arc transfer circuit 46 is applied to the steam adjust-
ing valve 73 and the intercept valve 74 thus maintaining
these valves in fully opened state.
~ he main steam stop valve 7~ and the reheat stop
valve 7~ control the quantities of the main steam and the
reheated steam respectively by a small internal bypass
valve (not shown).
At this time, the control signal sent to the reheat
stop valve 75 is sent from the low value priority circuit
76a through gain setter 78, adder 79 and low value priority
circuit 76b thus interlocking each other the main stop
valve 72 and the reheat stop valve 75, the ratio of the
flow quantities of valves 72 and 75 being determined by
the gain qetter 78.
III. Turbine loading
As the turbine load increases the quantity of stec~m
is increased by interlocked main stop valve 72 and the
reheat stop valve 75 (the output from the low value
priority circuit 76a increases) so that the deviation
signals produced by adders 82a and 82b decrease whereby
the degree of opening of the high pressure bypass pres-
sure reducing valve 83a and the low pre~sure bypass pres-
sure reducing valve 8~b is decreased thus limiting pres-
sure variation.
IV. Full arc mode to partial arc mode transfer
In this case control is transferred from the inter-
locked control o~ the main stop valve 72 and the reheat r'
stop valve 75 to the interlocked control of the control
valve 73 and the intercept valve 74. The degree of
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openings of the control valve 73 and the intercept 74which have been maintained in their fully opened state
is decreased by the bias signal from the full arc-partial
arc control circuit 46. ~hereafter the main s-top valve
and the reheat stop valve 75 are fully opened thus trans-
ferri~g the control to the turbine control wherein the
turbine is controlled by the steam control valve 73 and
the intercept valve 74. However this control is not
related directly to the invention.
V. ~urbine bypass running complete
When the load increases under these conditions, as
the openings of the control valve 73 and the intercept
valve 74 increase, openings of the high pressure bypass
pressure reducing valve 83a and the low pressure bypass
pressure reducing valve 83b decrease. Finally,they are
completely closed.
VI. Normal running
The control of the turbine bypass systems becomes
inoperative and the normal running of the turbine using
reheated steam commences.
VII. Interruption of the turbine load
Due to the rapid closure of the steam control valve
73 and the intercept valve 74 the pressures of the main
steam and t~e reheat steam generated by the boiler in-
crease so that the high pressure bypass pressure reducing
valve 81 and the low pressure bypass pressure reducing
valve 81b are opened quickly. At the same time,the high
pressure bypass temperature control valve 83a and the
low pressure bypass temperatur control valve 83b are
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opened quicklythus controlling the pressure and temper_
ature of the turbine bypass systems.
Thus,according to this invention a turbine bypass -
control system for controlling the turbine bgpass lines
are incorporated to the conventional main control system
which controls the main steam stop valve, steam control
valve, interception valve and the reheat stop valve so
that it is possible to provide an overall control of the
turbine plant. Accordingly, it is possible to improve
the control characteristics, to reduce the starting time
and cost of maintenance and to make easy to mantain and
inspect.
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