JPH0143201B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an integrated control scheme for large scale power generation plants such as those found in modern central stations. Such equipment, for example, may require more than 6,000,000 pounds per hour of main steam at 2,500 psig and 1,000 F at full load.
It can be estimated to be over 1000MW. Customarily, in such systems, the steam first passes through a high pressure turbine, then through a reheater passage in the steam generator that raises the steam to a hot reheat temperature of approximately 1000F, and then through a low pressure turbine. and flows to the condenser.

To improve the reliability of such power plants and to keep the steam generator dimensions reasonable, plants have recently been equipped with two steam generators operating in parallel to meet the total steam demand of the turbine. It has been proposed to do so. Such a combination of two steam generators and a single turbine generator presents unique control problems, as will be apparent from the detailed description below. The purpose of this invention is to solve these problems.

However, it will be appreciated that the novel control scheme of this invention has features that are equally applicable to many conventional single turbine generator/single steam generator configurations.

According to the invention, the total output of the two steam generators is maintained primarily in accordance with the unit MW or energy factor as established by an automatic load command or other device.

Furthermore, according to the invention, system energy requirements can be distributed between two steam generators as needed.

Further, in accordance with the present invention, if the output of one steam generator is limited, the output of the other steam generator is automatically increased to meet the system energy requirements.

Further, in accordance with the invention, if the total available power of the two steam generators is insufficient to meet the system energy demand, the demand is reduced to match the total available power.

Further, according to the invention, feed water, fuel and air are supplied to each steam generator primarily in accordance with the magnitude of the equipment energy requirement assigned to the steam generator.

Further, in accordance with the invention, the total reheat steam flow from the turbine is distributed between the two steam generators primarily in accordance with the allocation of the equipment energy requirements assigned to the steam generators.

These and other objects of the invention will become apparent from the following description of the accompanying drawings.

Referring now to FIG. 1, a nearly typical two-boiler, one-turbogenerator type system having steam generators A, B, commonly referred to as boilers, and a common turbine-generator system comprising a high-pressure turbine 9 and a low-pressure turbine 10 is shown. A basic steam-water cycle for power generation equipment is shown. As shown, both the high pressure turbine and the low pressure turbine may drive a single generator 11, or each turbine may drive a separate generator feeding a common bus. The specific construction of the turbine and generator that may be used does not form part of this invention. The steam released from the sub-superheating devices 12A and 12B of the steam generators A and B, respectively, passes through the high-pressure turbine 9 and is divided into streams, with a portion passing through the reheating device 13A and the remaining portion passing through the reheating device 1.
Pass 3B. And reheating devices 13A, 13B
The hot reheated steam from is passed through the low pressure turbine 10 and discharged to the condenser 14. What is shown as a low pressure turbine may actually be an intermediate pressure turbine and one or more low pressure turbines.

Condensate from the condenser 14 is fed by a condensate pump 15 to a low pressure heating device 16A, heated by extracted steam from the low pressure turbine 10 to an air separation heating device 17, and also from the low pressure turbine 10. It is also heated by extraction steam. The water supply to the boiler A is drawn from the air separation heating device 17 by the boiler feed water pump 18A, and then passes through the high pressure heating device 19A to the furnace 20A and the main superheating device 21A.
and a sub-heating device 12A. Air separation heating device 1 also supplies water to boiler B.
7 by the boiler feed pump 18B, and passes through the high pressure heating device 19B to the furnace 20B.
and is supplied to boiler B, which includes a main superheating device 21B and a sub-superheating device 12B.

Figures 2A and 2B show air/gas cycles for boilers A and B, respectively. These cycles are identical and for simplicity boiler A
Only the cycles for will be explained. Components of boiler A in FIG. 2A are designated with the letter A in the number series, and like components of boiler B in FIG. 2B are designated with the same number followed by the letter B.

Referring to FIG. 2A, forced ventilation fan 22A
The combustion air supplied by the air heating device 2
3A and is supplied to the furnace 20A. oil, gas,
The fuel, which may be coal or a combination thereof, is supplied by any conventional supply system (not shown, but line 24).
(abbreviated as A) into the furnace 20A. Combustion gas, commonly called flue gas, coming out of the furnace is discharged into the atmosphere through a chimney (not shown).The sub-superheater section 12A, the reheater section 13A, the main superheater section 21A, and the air heating device. 23A and the suction fan 25A. The flue gas exiting the main superheater 21A may also be recycled into the furnace 20A by a gas recirculation fan 26A as a device to partially or totally control the reheat steam temperature.
Generally, the flow rate of recirculated gas is maintained inversely proportional to the heat or energy input to the boiler. The illustrated order in which the combustion products pass through the several heating sections is not an exclusive order. For example this order is the main superheating device,
The order may be the sub-superheating device and the reheating device, or the order of the sub-superheating device, the main superheating device and the reheating device. Similarly, economizing devices can also be provided at suitable locations.

FIG. 3 shows the configuration of the control system in a block diagram. Equipment load requests may be established by an automatic load command device 28 or other automatic or manual device. The purpose of this control is to generate a control signal commensurate with a desired energy output, which may be measured in MW, BTU or other energy units. The control signals thus generated are transmitted to a master controller 30, which can be any one of a number of types generally common in the art. One particularly suitable type is illustrated and described in "Steam", 38th edition, Chapter 35, published by Babcock & Wilcox. The purpose of the main control device is load command device 2
8 to generate a control signal for the steam generator, thereby maintaining the actual energy output of the generator equal to the desired energy output.

The control signal generated in the main controller 30 is transmitted to the ratio controller 32, and the function of this ratio controller 32 is to adjust the total load demand between the steam generators A and B within the performance range of the generators as necessary. It consists in distributing. That is, this ratio control device divides the control signal from the main control device 30 and sends it to the boiler A.
A first request signal for boiler B and a second request signal for boiler B are formed. The first request signal is utilized in boiler A's feed water control device 34A and ignition rate control device 36A as shown. The two request signals generated in the ratio controller 32 are also transmitted to the reheat flow controller 42, whose function is to distribute the reheat steam flow to boilers A and B roughly according to the boiler load. It's about doing.

Figures 4, 5, 6, and 7 show the ratio, water supply, ignition ratio, and reheat flow control device for boilers A and B. It will be seen that conventional control logic symbols are used in these figures. Control components, often referred to as hardware, with various symbols are commercially available and their operation is well understood by those skilled in the art. Additionally, the present invention may be combined with any special type of control device such as pneumatic control, fluid control, electronic control, electrical control, or a combination thereof, so that it is possible to incorporate such a special type of control device. Use conventional logic symbols to avoid identifying control schemes. The main controllers shown in Figures 4, 5, 6, and 7 are the 1st and 2nd
It has been referred to as the final control element in Figures A and 2B.

Referring to FIG. 4, the main controller 30 (third
A control signal corresponding to the total boiler demand generated by FIG. The portion of the total boiler demand to be scheduled for each boiler is formed in a ratio control 48 which generates a control signal which is transmitted to a multiplier 44 . The output signal from multiplier 44 is therefore the request signal for boiler A. In difference device 46, the boiler A demand signal is subtracted from the total boiler demand signal, and the output signal from difference device 46 is the boiler B demand signal.

If one or more factors in the operation of boiler A or boiler B are limited to or above a predetermined limit, the demand signal for the boiler must be limited accordingly.
The factors whose supply limits the boiler output represent fuel flow, air flow, feed water flow, reheat flow and superheat flow during bypass operation. The factor that limits the boiler output when a predetermined limit is exceeded represents the furnace draft, which can exceed the predetermined positive limit if the performance of the draft blower is limited.
The factors mentioned above will be considered to be merely representative of a number of factors that can effect limitations on boiler performance.

As shown in FIG. 4, such a factor may be considered a criticality generation signal that forms an input to a limit controller 49A of boiler A and a similar limit controller 49B of boiler B.

Deadband departures of less than 5% are ignored to avoid momentary small departures beyond the scheduled limits that unnecessarily stress boiler demand. However, if the sustained deviation is greater than 5%, the deadband error signal is reduced to 1%, so that only errors less than 1% are ignored. The demand signal for the boiler is therefore returned to match the forced boiler capacity. The output signals from limit controllers 49A, 49B are integrated with respect to time in integrators 52A, 52B, respectively, and low selectors 56A, 5, respectively.
Sent to 6B. Therefore, the signal from multiplier 44 or the signal from integrator 52A, whichever is smaller, is the actual request signal for boiler A.
Similarly, the signal from the difference device 46 or the integrator 5
The smaller of the signals from 2B is the actual request signal for boiler B.

Even if the output of one boiler is stressed, it is desirable that the total output of the two boilers should meet the unit energy requirement. Therefore, this invention:
This includes cases in which the output of one boiler is limited and the output of the other boiler is automatically compensated for by increasing the output. For this reason, the difference device 60
A is the output signal of the multiplier 44 and the low signal selector 5
Generates a signal proportional to the difference with the 6A output signal,
This signal passes through a direct proportional device 64 to a summation device 6.
Sent to 8. Similarly, the difference device 60B is the difference device 4
6 and the output signal of low signal selector 56B, which signal is sent through inverse proportional device 66 to summing device 68.

If both boilers A, B carry their assigned loads, i.e. neither output is limited, the output signal generated by the summing device 68 is zero. If the output of boiler A is limited, the output signal from summing device 68 activates transmission device 70 and then multiplier 4
Acts as an input signal for 4. Since the output signal from multiplier 44 changes to be equal to the output signal from integrator 52A, the demand signal for boiler B is correspondingly increased by the operation of difference device 46.

Squeezing at the output of boiler B operates similarly,
The output signal from the inverse proportional device 66 is sent to the summation device 68.
and acting via switching device 70 to increase the demand signal for boiler A and decrease the demand signal for boiler B until the output signal from difference device 60B becomes zero.

If the load changeover from one boiler to the other boiler cannot be made to satisfy the total boiler demand after a scheduled time lapse of 1 to 5 minutes, the unit load demand will be equal to the actual performance of the two boilers. will be reduced to The unsatisfied demand at boiler A, represented by a sustained output signal from difference device 60A, is transmitted to high signal selector 72 and, after a predetermined time delay set in time delay device 73, by the load command device. The unit load demand generated at the time is returned to and matched with the overall performance of the two boilers. Also sent to device 72 is an output signal from differential device 60B representing a sustained unsatisfactory demand on boiler B. Obviously, the high signal selector 72 connects the device load command device 28 to the difference device 60A,
It acts to pass the larger of the two error signals generated at 60B. Therefore, the difference device 6
If the greater of the two signals representing an unsatisfied demand in either boiler from either 0A or 60B is reduced to zero by a return to equipment load demand, the unsatisfied demand in both boilers will be reduced to zero by a return to equipment load demand. decreases to zero.

In the normal structure of a single boiler and single turbine,
Feed water temperature varies in a directly predictable function of equipment load. In a two-boiler, one-turbine configuration, as in the case of the present invention, the two boilers operate at the same load for as long as possible. However, this relationship is broken whenever the two boilers operate at different loads since the feedwater temperature is a function of the turbine load. The relationship between the feedwater temperature and the actual steam flow entering the turbine, which is equal to the total feedwater flow to both boilers, is directly related to the total turbine extraction flow to the high pressure, low pressure air separation feedwater heating system. Therefore, a feedwater temperature lower than that expected for the existing boiler demand is directly related to the existing low steam flow to the turbine which is equal to the low required boiler feedwater flow. Therefore, if the feedwater temperature is higher than expected for the existing boiler demand, the feedwater demand is increased to result in an actual increase in steam flow to the turbine resulting from the increased extraction flow.

In the control system shown in Figure 5, the flow rate of feed water to each boiler is maintained in direct proportion to boiler demand. The control signal generated in ratio controller 32 is therefore sent to feedwater control valve 74A. a flow transmission device 76A, a difference device 78A, and an integration device 80A;
A feedback loop with multiplier 82A maintains the actual feed water flow equal to the requested feed water flow.

To account for the deviation of the actual feedwater temperature from the temperature expected for the existing boiler demand, a signal corresponding to the expected feedwater temperature generated from the function generator 83A is transmitted to the temperature transfer device 84A in the difference device 86A. is compared with a signal corresponding to the actual feed water temperature generated by. The output signal from difference device 86A is sent through summing device 88A to multiplier device 87A to modify the demand control signal according to the deviation of the actual feedwater temperature from the expected temperature for the existing boiler demand. .

Also, the desired feedwater flow rate for a given boiler demand is affected by changes that may occur in the steam temperature set point from the normal or design set point. Therefore, an increase in the desired steam temperature from the normal temperature will reduce the feedwater flow rate and vice versa. A control signal responsive to the change in steam temperature set point generated in device 90A changes the output of summing device 88A to effect the desired compensation.

Although the water supply control device for boiler A has been illustrated and described above, it should be understood that the water supply control device for boiler B is also the same as that for boiler A in all respects.

The demand signals for boilers A and B adjust the required boiler heat release to meet the required energy output of the power plant. Heat release is directly related to the fuel BTU input to the boiler's furnace. The boiler demand signal thus regulates fuel flow at the same time as air flow. It is observed that the required fuel air flow for a given fuel and fuel BTU input remains essentially constant. The present invention directly matches air flow to boiler demand because the boiler demand signal is directly equated with the required energy output and the overall cycle efficiency at each load is relatively constant and predictable. Fuel flow is readjusted as necessary to maintain the two boiler superheater outlet temperatures equal and at the turbine throttle temperature set point.

As shown in FIG. 6 and in accordance with the above description, the boiler A demand signal is transmitted to a final control element 91A, such as a damper drive, which controls the flow rate of air supplied for combustion. A feedback signal from flow transfer device 92A proportional to the measured airflow maintains the actual airflow equal to the desired airflow. The signal generated in transmission device 92A is sent to differential device 93A, and the output signal is passed through proportional-integral device 132A to adjust airflow damper 91A as necessary to adjust the actual airflow to the desired airflow. It acts to maintain equality.

A signal proportional to the temperature error from the set point and the rate of change of the superheater outlet temperature of boiler A is superimposed on the demand signal in a direction opposite to the direction of temperature change, thereby providing an instantaneous change in the superheater outlet temperature. Compensate for fluctuations. As shown, a signal corresponding to the superheater outlet temperature is generated in temperature transfer device 94A and transferred to proportional-differentiator device 95A and thus to summing device 96A.

The demand signal for boiler A, which is modified according to the rate of change of the superheater outlet temperature, also controls the fuel flow rate to maintain the fuel flow rate in the required relationship to the air flow rate. As shown, the output signal from the summing device 96A is connected to the second A as a fuel control valve.
The particular type of fuel flow control device used and sent to the final control element 97A shown in the figure is related to the type of fuel. Fuel flow transmission device 98A and differential device 99A
A feedback loop having a proportional and integral device 100A maintains the actual flow rate of fuel equal to the requested flow rate of fuel.

A relatively long-term change in the turbine throttle temperature, which represents a change in the BTU content of the fuel, for example, will cause the ratio of air flow to fuel flow to change, thus effectively changing the fuel flow to air flow automatically and Adjust continuously. This is achieved by introducing the signal generated in the turbine throttle temperature transfer device 103 into an integrator 101A, from which the output signal is transferred to the multiplier 102.
Sent to A. Thus, the output signal from multiplier 102A inferentially changes to fuel flow transfer device 98 according to long-term changes in turbine throttle temperature, which is a measure of the change in the relationship between heat input to the boiler and fuel flow rate.
It acts to modify the feedback signal from A.

The ignition ratio control device explained above is for boiler A.
However, as can be seen from FIG. 6, the control device for boiler B is also the same, and the same components are indicated by the same number with the letter B added.

In order to avoid the interaction, commonly referred to as hunting, between the superheater outlet temperatures of the two boilers while maintaining the desired turbine throttling temperature, a push-pull control device is additionally shown in FIG. The fuel flow to the boiler with the superheater outlet temperature is reduced and at the same time the fuel flow to the other boiler is increased, thus reducing the fuel flow to both boilers as the throttle steam temperature varies from the set point. Maintain the same outlet temperature of the superheater while adjusting at the same time. As shown, a signal proportional to the difference between the superheater outlet temperatures of boiler A and boiler B is generated in difference device 122 and via direct acting proportional device 124 and inverse acting proportional device 125 to summing device 105A, 10
5B, which biases the input signals to the integrators 101A and 101B in opposite directions.

The present invention balances the reheat steam from the high pressure turbine 9 between boilers A and B according to the forward feed signal derived from the demand signal for each boiler, and at the same time makes the reheat flow to each boiler proportional to the main steam generation. This includes maintaining the In FIG. 7, the demand signal for boiler A positions the final control element, flow control valve 107A. Similarly, a demand signal for boiler B positions flow control valve 107B. Any desired linear or non-linear functional relationship between reheat flow and demand signal for boilers A and B, respectively, can be programmed by function generators 108A and 108B. Proper distribution of the total available reheat flow is obtained by a push-pull feedback control loop, thereby maintaining equal differences between the actual and programmed reheat flows through each boiler. As shown, reheat flow through boiler A and boiler B is measured by flow transfer devices 109A and 109B. Although the main elements of the reheat flow have been schematically shown as orifices in FIG. 1 for simplicity, the present invention uses arbitrary pressure drops that vary in a functional relationship to the flow to minimize unrecovered pressure losses. A pressure drop through a reheat device may be utilized or a main element such as a ventilator or flow nozzle may be utilized. Known flow measurement devices other than those that generate differential pressures that vary with flow may also be used. Furthermore, as is well known, the pressure and temperature of the reheat stream, which varies with the reheat stream and the flow transfer device, can be compensated for both of these conditions by any suitable device.

The difference between the actual reheat flow and the programmed reheat flow through each boiler is determined by differential devices 110A, 110B. As an example, an actual reheat flow greater than the programmed reheat flow may be considered to generate a positive output signal from differential device 110A or 110B. The generated signal is sent to difference device 111. The output signals generated here are connected to the proportional-integrator 112 and the inverse and direct proportional device 113.
A and 113B respectively to the summation device 114.
A, 114B. Total summation device 114
The output signals from A, 114B are therefore used to reverse positioning valves 107A, 107B to maintain the difference between the actual and programmed reheat flows equal.
is changed in the opposite direction. As is clear,
If necessary, the amount of adjustment allowed to maintain equal differences may be limited.

As mentioned above, by appropriate distribution of the total reheat flow between the two boilers, the desired reheat temperature can be obtained by gas recirculation as shown in FIG. Circulation is supplemented if necessary by atomizing means or other known devices. However, if the hot reheat temperature of a boiler exceeds the set point, the invention increases the reheat flow to that boiler while decreasing the reheat flow to the other boiler, and the hot reheat temperature of both boilers exceeds the set point. Exceeding the point involves increasing the reheat flow to the larger boiler while simultaneously decreasing the reheat flow to the other boiler.

This action therefore maintains the excess hot reheat temperatures from both boilers equal.

Temperature transfer devices 115A and 115B are boiler A,
A signal corresponding to the hot reheat temperature of B is generated. These signals are sent to high signal selection devices 116A, 116B.
is compared against the setpoint signal at . As long as the signal generated at transmission device 115A is below the set point, for example, device 116A sends the set point signal to difference device 117. However, when the corresponding signal of the hot reheat temperature exceeds the set point signal,
It is sent to difference device 117. At the same time, as long as the signal generated by transmission device 115B is below the set point, the signal sent to difference device 117 is the set point signal. However, transmission device 115
If the signal generated at B is greater than the setpoint signal, that signal is sent to difference device 117. As is clear from the above discussion, the output signal from the difference device 116 has a neutral value, such as zero, as long as both hot reheat temperatures are at or below the set point.
However, when the hot reheat temperature of one boiler exceeds the set point, device 117 generates a signal in one direction, and when the hot reheat temperature of the other boiler exceeds the set point, device 117 generates a signal in the opposite direction. occurs. If the hot reheat temperatures of both boilers exceed the set point, the device 117 outputs an output signal in the direction associated with the boiler with the higher hot reheat temperature and a value commensurate with the difference between the two hot reheat temperatures. occurs.

The signal generated in difference device 117 is sent to direct acting proportional device 118A and inverse acting proportional device 118B. These devices are total summation devices 120A, 12
0B to reversely change the forward feed signal to boilers A, B, thus reversing the positioning of valves 107A, 107B and setting the hot reheat temperature from both boilers. and further maintain the hot reheat temperatures from both boilers equal when both such temperatures are above the set point.

When reheat attenuators are rarely used and the reheater absorption and loading characteristics are such that the two boilers are often operated with unbalanced steam flows and therefore unbalanced reheater outlet temperatures. Another construction that is particularly applicable is to improve reheater temperature control performance by biasing the steam flow through the reheater whenever the two reheater outlets have a temperature difference below or above a set point. It is about maintaining. To accomplish this, difference devices 130A, 130 form the reheater exit temperature error from the set point for comparison in difference device 117 in high signal selection devices 116A, 116B as shown in FIG. 7A.
Replaced by B. Whenever either reheater exit temperature error differs from a comparable error from the other boiler, reheat steam flow is increased through the reheater with a higher exit temperature.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the basic steam and water flow cycle of a typical power generation plant with two single reheat steam generators supplying steam to the turbine;
Figure A shows the basic air flow of steam generator A shown in Figure 1.
2B is a block diagram showing the basic air/gas flow cycle of the steam generator B shown in FIG. 1. FIG. 3 is a block diagram showing the configuration of the control system. 4 is a logic diagram showing in detail the ratio control device indicated by block 32 in FIG. 3, and FIG.
FIG. 6 is a logical diagram showing in detail the feed water flow control device for each of the steam generators A and B shown in FIG. FIG. 7 is a logic diagram detailing the control system (fuel and air control system); FIG. 7 is a logic diagram detailing the reheat steam control system shown as block 42 in FIG.
The figure is a basic logic diagram showing a modification of the reheat steam control device shown in FIG. 7. In the drawing, 28 is a load command device, 30 is a main control device, 32 is a ratio control device, 34A, 34B are feed water flow control devices, 36A, 36B are ignition ratio control devices, and 42 is a reheat flow control device.

Claims (1)

[Claims] 1. A device that has boilers A and B that supply steam to a single turbine in parallel, and that generates a request signal for boiler A and a device that generates a request signal for boiler B;
A device that maintains the amount of fuel supplied to boiler A in a functional relationship with the request signal of boiler A in response to the request signal of boiler A, and a device that maintains the amount of fuel supplied to boiler B in response to the request signal of boiler B, a device for maintaining a functional relationship between the signals and a device for changing the functional relationship between the two demand signals and the amount of fuel supplied to the boilers A and B in accordance with the integral over time of the fluctuation of the turbine throttle steam temperature from the set point; A control system for power generation equipment that combines the following. 2 A boiler A that supplies steam to the high pressure section of the turbine and reheated steam to the low pressure section of the turbine, and a boiler A that supplies steam to the high pressure section of the turbine and reheated steam to the low pressure section of the turbine in parallel with the boiler A. It has a boiler B that supplies steam, a device that generates a request signal for the boiler A, a device that generates a request signal for the boiler B, and a flow rate of reheated steam that passes through the boiler A in response to the request signal for the boiler A. and a device that changes the function into a functional relationship with respect to changes in the request signal of boiler A, and boiler B.
and a device that changes the flow rate of reheated steam passing through boiler B in a functional relationship with respect to changes in the demand signal of boiler B in response to a demand signal of boiler B.