CA1038474A - Turbine overspeed control system - Google Patents
Turbine overspeed control systemInfo
- Publication number
- CA1038474A CA1038474A CA236,430A CA236430A CA1038474A CA 1038474 A CA1038474 A CA 1038474A CA 236430 A CA236430 A CA 236430A CA 1038474 A CA1038474 A CA 1038474A
- Authority
- CA
- Canada
- Prior art keywords
- amplifier
- speed
- valve
- turbine
- signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
- F01K7/00—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating
- F01K7/16—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating the engines being only of turbine type
- F01K7/22—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating the engines being only of turbine type the turbines having inter-stage steam heating
- F01K7/24—Control or safety means specially adapted therefor
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01D—NON-POSITIVE DISPLACEMENT MACHINES OR ENGINES, e.g. STEAM TURBINES
- F01D21/00—Shutting-down of machines or engines, e.g. in emergency; Regulating, controlling, or safety means not otherwise provided for
- F01D21/02—Shutting-down responsive to overspeed
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- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Combustion & Propulsion (AREA)
- Control Of Turbines (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Steam flow to a turbine is reduced by partially closing intercept control valves to reduce turbine speed to a reference speed after sudden increases above this speed caused by sudden decreases in turbine load. Circuit-ry for developing a command signal for closing the valves in response to a difference between the reference speed and the actual speed of the turbine is provided. The command signal magnitude is varied to cause rapid closing motion of the valves during that portion of valve movement having little effect on steam flow and less rapid closing motion during that portion of valve movement having substantial effect on steam flow, thereby compensating for a nonlinear relationship between valve movement and change in steam flow.
Steam flow to a turbine is reduced by partially closing intercept control valves to reduce turbine speed to a reference speed after sudden increases above this speed caused by sudden decreases in turbine load. Circuit-ry for developing a command signal for closing the valves in response to a difference between the reference speed and the actual speed of the turbine is provided. The command signal magnitude is varied to cause rapid closing motion of the valves during that portion of valve movement having little effect on steam flow and less rapid closing motion during that portion of valve movement having substantial effect on steam flow, thereby compensating for a nonlinear relationship between valve movement and change in steam flow.
Description
~~ 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 - :IQ38~7~
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 ~
:
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 - :IQ38~7~
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 ~
:
- 2 -.
` 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 :
~038~q4 ~
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. ~-~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 . `. 17MT-2475 ~`
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, .. ,,. ~ ., .
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.
. .
:, ~ .
.. . . .
- . , . .. , .. ~. . ~
. - . .. . . .
, . . . : . . -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 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, . .
- 8 _ - ~
... . ~ , ... ~ . ;
~~~ 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 ;
- 10 -- ~
.
.' ;. 17MT-2475 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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i. :
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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 :
~038~q4 ~
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. ~-~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 . `. 17MT-2475 ~`
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, .. ,,. ~ ., .
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.
. .
:, ~ .
.. . . .
- . , . .. , .. ~. . ~
. - . .. . . .
, . . . : . . -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 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, . .
- 8 _ - ~
... . ~ , ... ~ . ;
~~~ 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 ;
- 10 -- ~
.
.' ;. 17MT-2475 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
,y - .
. . ,:
~;
, T ~`
!i : ' J
i. :
'~
' '
Claims (6)
1. In an overspeed control system for steam turbines of the type wherein steam flow to the turbine is reduced by a valve to decrease speed to a reference speed after sudden undesired increases above this reference speed, the improvement comprising:
(a) a circuit for developing a command signal for controlling the position of the valve in response to a difference between the reference speed and the actual speed of the turbine;
(b) said circuit including signal generating means for developing a command signal which initially increases at a rapid rate causing rapid closing motion of the valve during that portion of valve movement having little effect on steam flow and later increases at a less rapid rate causing rapid closing motion of the valve during that portion of valve move-ment having substantial effect on steam flow, whereby said circuit compensates for a nonlinear relationship between valve movement and change in steam flow.
(a) a circuit for developing a command signal for controlling the position of the valve in response to a difference between the reference speed and the actual speed of the turbine;
(b) said circuit including signal generating means for developing a command signal which initially increases at a rapid rate causing rapid closing motion of the valve during that portion of valve movement having little effect on steam flow and later increases at a less rapid rate causing rapid closing motion of the valve during that portion of valve move-ment having substantial effect on steam flow, whereby said circuit compensates for a nonlinear relationship between valve movement and change in steam flow.
2. A system as in claim 1 wherein said circuit comprises:
(a) a first amplifier functioning at speeds above the reference speed to develop a first electrical output signal varying at a rapid rate with respect to turbine speed;
(b) a second amplifier functioning at speeds above a predetermined overpseed to develop a second electrical output signal varying at a less rapid rate than the first output signal with respect to turbine speed;
(c) means for applying the second output signal to said first amplifier causing a reduction in the rate of increase of the signal at the output of said first amplifier at speeds above the predetermined overspeed; and (d) the output signal from said first amplifier constituting said command signal.
(a) a first amplifier functioning at speeds above the reference speed to develop a first electrical output signal varying at a rapid rate with respect to turbine speed;
(b) a second amplifier functioning at speeds above a predetermined overpseed to develop a second electrical output signal varying at a less rapid rate than the first output signal with respect to turbine speed;
(c) means for applying the second output signal to said first amplifier causing a reduction in the rate of increase of the signal at the output of said first amplifier at speeds above the predetermined overspeed; and (d) the output signal from said first amplifier constituting said command signal.
3. A system as in claim 2 further comprising:
(a) a third amplifier electrically connected to the output of said first amplifier acting to prevent the command signal from rising above a predetermined upper voltage limit;
and (b) a fourth amplifier also electrically connected to the output of said first amplifier acting to prevent the command signal from falling below a predetermined lower voltage limit.
(a) a third amplifier electrically connected to the output of said first amplifier acting to prevent the command signal from rising above a predetermined upper voltage limit;
and (b) a fourth amplifier also electrically connected to the output of said first amplifier acting to prevent the command signal from falling below a predetermined lower voltage limit.
4. A system as in claim 3 further comprising a fifth amplifier acting to prevent the second amplifier output signal from falling below a predetermined lower voltage limit.
5. A system as in claim 2 including means for providing an input to said first amplifier utilized for periodic application of a test signal causing development of an output signal from said first amplifier resulting in partial closure of the valve.
6. The invention as defined in any of the preceding claims including any further features of novelty disclosed.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US512808A US3913329A (en) | 1974-10-04 | 1974-10-04 | Turbine overspeed control system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1038474A true CA1038474A (en) | 1978-09-12 |
Family
ID=24040665
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA236,430A Expired CA1038474A (en) | 1974-10-04 | 1975-09-25 | Turbine overspeed control system |
Country Status (5)
| Country | Link |
|---|---|
| US (1) | US3913329A (en) |
| JP (1) | JPS5162203A (en) |
| CA (1) | CA1038474A (en) |
| CH (1) | CH599454A5 (en) |
| GB (1) | GB1519833A (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CH621179A5 (en) * | 1977-06-29 | 1981-01-15 | Bbc Brown Boveri & Cie | |
| DE2939534B2 (en) * | 1979-09-28 | 1981-06-25 | Kraftwerk Union AG, 4330 Mülheim | Control device for steam turbines with reheating |
| AR029828A1 (en) * | 2001-07-13 | 2003-07-16 | Petrobras En S A | METHOD FOR PRIMARY FREQUENCY REGULATION IN COMBINED CYCLE STEAM TURBINES |
| US7343744B2 (en) * | 2005-07-27 | 2008-03-18 | General Electric Company | Method and system for controlling a reheat turbine-generator |
| EP2466077A1 (en) * | 2010-12-17 | 2012-06-20 | Alstom Technology Ltd | Steam turbine overspeed protection |
| US9765636B2 (en) | 2014-03-05 | 2017-09-19 | Baker Hughes Incorporated | Flow rate responsive turbine blades and related methods |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CH524091A (en) * | 1970-05-26 | 1972-06-15 | Sulzer Ag | Device for shutting off a work medium |
-
1974
- 1974-10-04 US US512808A patent/US3913329A/en not_active Expired - Lifetime
-
1975
- 1975-09-25 CA CA236,430A patent/CA1038474A/en not_active Expired
- 1975-09-25 GB GB39331/75A patent/GB1519833A/en not_active Expired
- 1975-09-26 CH CH1254175A patent/CH599454A5/xx not_active IP Right Cessation
- 1975-10-03 JP JP50119031A patent/JPS5162203A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| DE2542936B2 (en) | 1977-04-21 |
| DE2542936A1 (en) | 1976-04-22 |
| CH599454A5 (en) | 1978-05-31 |
| JPS5162203A (en) | 1976-05-29 |
| AU8276875A (en) | 1977-01-06 |
| US3913329A (en) | 1975-10-21 |
| GB1519833A (en) | 1978-08-02 |
| JPS5624763B2 (en) | 1981-06-08 |
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