(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0010037 A1
`Vugdelija
`(43) Pub. Date:
`Jan. 16, 2003
`
`US 20030010037A1
`
`(54) PRIMARY FREQUENCY REGULATION
`METHOD IN COMBINED-CYCLE STEAM
`TURBINES
`
`(75) Inventor: Jorge Vugdelija, Buenos Aires (AR)
`
`Correspondence Address:
`Ira D. Finkelstein
`Howrey Simon Arnold & White, LLP
`750 Bering Drive
`Houston, TX 77057-2198 (US)
`
`(73) Assignee: PECOM Energia, SA.
`
`(21) Appl. No.:
`
`09/998,013
`
`(22) Filed:
`
`Nov. 30, 2001
`
`(30)
`
`Foreign Application Priority Data
`
`Jul. 13, 2001
`
`............................... .. P 01 01 03353
`
`Publication Classi?cation
`
`(51) Int. Cl.7 ...................................................... .. F02C 6/18
`(52) US. Cl. ......................................... .. 60/772; 60/39.182
`
`(57)
`
`ABSTRACT
`
`A method to provide Primary Frequency Regulation to a
`steam turbine in a combined cycle plant that comprises
`storing energy in the form of internal energy of the steam
`contained Within the piping and domes of the heat recovery
`boilers, and then using said energy When the poWer grid
`requires a sudden increase in output poWer. With the present
`method, the losses in poWer generation of the steam turbine,
`When said turbine is operating in the Primary Frequency
`Regulation mode, are reduced to a minimum. When the gas
`turbines in a combined cycle plant operate in PFR mode, the
`steam turbine reduces its output poWer because the gas
`turbines must operate beloW their rated poWer. The present
`method converts said decrease in output poWer of the steam
`turbine in a Spinning Reserve useful for PFR in said turbine.
`That is to say that the present method converts said Spinning
`Reserve in a rapid reserve, available after just a feW seconds;
`if necessary, response times of less than 10 seconds can be
`attained. The present method introduces several novel steps,
`Which constitute its essence, said steps permitting the con
`tinuous, long term operation of the steam turbine in a
`combined cycle plant in PFR mode, thereby ensuring at any
`time the effectiveness of mains frequency regulation and
`also guaranteeing the stability of the poWer generation
`process in said combined cycle plant.
`
`FREQUENCY
`SET-POINT
`
`MAINS
`FREGUENDV
`MEASUREMENT
`
`S1
`
`DEAD
`BAND
`
`i
`
`T
`
`STATISM
`
`i
`
`PFR
`ACTION
`LIMIT
`
`MAXIMUM
`CONTRIBUTION
`TO PFR
`
`STEAM CONTROL
`VALVE PRESSURE
`DIFFERENTIAL
`MEASUREMENT
`
`PFR REFERENCE
`POSITION
`
`STEAM TURBINE
`POWER SET POINT
`
`+
`'I-
`
`52
`
`+
`- S3
`
`PFR
`CONTROLLER
`
`VALVE
`CHAHACTEFW
`ZATION
`
`HIGH PRESSURE
`VAIgLErILZiIEON
`
`54
`
`I
`POWER
`AUTOMATIC
`UPDATING
`
`TO VALVES IN
`OTHER STAGES IF
`REQUIRED
`
`I
`
`STEAM TURBINE
`POWER
`MEASUREMENT
`
`TEMP 1011
`IPR of U.S. Pat. No. 8,008,804
`
`0001
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`

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`0
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`2
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`Z2
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`24
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`25
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`Time, in minutes
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`FIGURE 1
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`Patent Application Publication
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`US 2003/0010037 A1
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`IIFTFTjTTIYTIIWITIllilliilil;
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`Jan. 16, 2003 Sheet 1 0f 4
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`TV V‘ TG
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`0002
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`Patent Application Publication
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`Jan. 16, 2003 Sheet 2 0f 4
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`US 2003/0010037 A1
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`TV
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`VVA
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`llTIiFlliITIIFIIIIIIiFIIIjTI>
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`26
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`Time, in minutes
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`FIGURE 2
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`0003
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`Patent Application Publication
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`Jan. 16, 2003 Sheet 3 0f 4
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`US 2003/0010037 A1
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`...
`T
`TlfllllilillliTlillllfllllTl;
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`Time, in minutes
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`FIGURE 3
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`0004
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`Patent Application Publication
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`Jan. 16, 2003 Sheet 4 0f 4
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`US 2003/0010037 A1
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`0005
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`

`

`US 2003/0010037 A1
`
`Jan. 16, 2003
`
`PRIMARY FREQUENCY REGULATION METHOD
`IN COMBINED-CYCLE STEAM TURBINES
`[0001] This application claims the bene?t of Argentine
`Patent Application No. P 01 01 03353, ?led on Jul. 13, 2001.
`
`FIELD OF THE INVENTION
`
`[0002] The present invention relates to a method for
`providing Primary Frequency Regulation in the steam tur
`bines of combined-cycle poWer generation plants, such
`plants comprising at least one gas turbine and at least one
`steam turbine.
`
`PRIOR ART
`
`[0003] Before the development of the present invention,
`most poWer generators associated With steam turbines form
`ing part of combined cycle plants based on gas turbines Were
`not capable of being provided With the type of control
`knoWn in the art as Primary Frequency Regulation (PFR).
`PFR is a function required by the agencies in charge of
`dispatch in poWer netWorks of many countries and is con
`sidered to be one of the parameters characteriZing the quality
`of poWer delivered to customers.
`
`[0004] The problem to be solved in steam turbines of
`combined cycle poWer plants is basically that the steam
`turbine is unable to supply signi?cant amounts of additional
`poWer (betWeen 3 and 10% of its rated capacity) Within a
`short period of time (response time on the order of a feW
`seconds, e.g., from 10 to 30 seconds), to effectively com
`pensate for any variation in the mains frequency caused by
`sudden changes in demand. “Response time” means the time
`it takes the turbine to reach the required ?nal output level
`upon the occurrence of a step-type frequency deviation of a
`given magnitude.
`[0005] Primary frequency regulation is not a major prob
`lem in steam turbines associated With conventional thermal
`cycles, that is, those Where the steam used to drive the
`turbine is generated in a ?red boiler. In these cases, the
`fuel-feeding loops into the boiler’s burners are adjusted to
`meet the required transient response, and in most cases a
`satisfactory performance is obtained.
`[0006] On the contrary, steam turbines in combined cycles
`With gas turbines pose a quite different problem. In these
`cycles, the steam needed to drive the steam turbines comes
`from heat recovery boilers, Which generate steam out of the
`heat content in gas turbine eXhaust gases. In these cases, the
`time constants associated With the energy transfer from the
`eXhaust gases to the Water-steam circuit in the heat recovery
`boiler are extremely high. Therefore, upon a variation in the
`energy content of the eXhaust gases from the gas turbines,
`the corresponding variations in the Water-steam circuit Will
`make them reach their ?nal state only after several minutes.
`Response times of this order of magnitude are not useful to
`attain PFR because, as stated above, response times of
`several seconds are necessary.
`
`[0007] Unlike conventional cycle steam turbines, in com
`bined cycle steam turbines it is not possible to adjust the
`amount of energy fed into the boilers at Will. This is because
`the energy fed into the heat recovery boiler comes from the
`eXhaust gases of the gas turbine, the properties of Which
`change according to the poWer output of the gas turbine.
`Since the gas turbine must supply Primary Frequency Regu
`
`lation service, its output cannot be taken to a condition
`different than that required to provide said PFR service.
`[0008] Consequently, steam turbines in combined cycle
`facilities are usually eXcluded from the netWork’s PFR, and
`this degrades the quality of poWer currently delivered by
`combined cycles.
`[0009] Some combined cycle plants are equipped to per
`form an additional combustion step at some point in the gas
`piping connecting the gas turbine With the heat recovery
`boiler. This particular type of combined cycle can potentially
`perform a PFR function in the steam turbine by modulating
`fuel injection into the burners. This is a Well knoWn and
`valid Way of providing PFR in steam turbines, Which is
`derived from the operating procedure of steam turbines in
`conventional cycles, and has been Widely disclosed in the
`related technical literature. HoWever, the use of an additional
`combustion system in combined cycle plants originates in
`economic criteria unrelated to PFR, and only a small number
`of such plants make use of this system. On the other hand,
`due to unstable combustion as Well as other technical
`considerations, it is not common to ?nd combined cycle
`plants With additional combustion systems that provide PFR
`service in their steam turbines.
`[0010] Sometimes combined cycle plants that are required
`to provide PFR service do so only With their gas turbines due
`to the dif?culties encountered to comply With this require
`ment for steam turbines. Some of these plants provide
`“over-regulation” by means of their gas turbines combined,
`so as to make up for the de?ciency of the steam turbine.
`HoWever, none of these operational modes is ef?cient. To
`provide PFR With a gas turbine, it must be dispatched under
`its rated capacity. The reason Why a turbine providing PFR
`must operate beloW its rated capacity, in normal frequency
`conditions, is that the turbine needs to have some eXtra
`capacity to deliver additional poWer When the frequency
`decreases. This eXtra capacity is called Spinning Reserve.
`The decrease in output in the gas turbines causes a reduction
`in the output of the associated steam turbine. HoWever, such
`loss of poWer generated by the steam turbine is completely
`useless, because the combined cycle plant Will generate less
`poWer and the managers of the system Will not consider it as
`Spinning Reserve for PFR purposes.
`
`[0011] Another aspect of the current state of the art is that
`in all combined cycle plants, irrespective of their origin,
`steam turbines operate normally in the “sliding pressure”
`mode. In this operational mode, the steam turbine control
`valves remain totally open once the starting procedure of the
`turbine is over. Thus, all of the steam that the heat recovery
`boilers are capable of producing enters into the steam
`turbine Without eXerting any modulation action on the steam
`turbine. It is generally accepted idea any throttling of the
`steam turbine control valves Would cause a dramatic loss in
`output poWer, as Well as introduce instability in the process,
`because the steam turbine Would not be using all of the
`steam generated in the heat recovery boiler. This idea has
`frequently led combined cycle plant manufacturers and
`operators to think that the provision of PFR by means of
`steam turbines in combined cycle plants is not practical.
`
`SUMMARY OF THE INVENTION
`
`[0012] To solve the above-described problems, the method
`of the present invention makes it possible to supply PFR
`
`0006
`
`

`

`US 2003/0010037 A1
`
`Jan. 16, 2003
`
`service to the power network by means of steam turbines
`operating in combined cycle plants, regardless of Whether
`these plants are equipped With additional combustion instal
`lations in their exhaust piping. Such purpose can be achieved
`ef?ciently both from the standpoint of netWork frequency
`control, and from the standpoint of plant operation. Thus, the
`method of the present invention increases the quality of
`poWer delivered by combined cycle poWer plants.
`[0013] The basic operating principle of the invention
`consists in storing energy in the form of internal energy of
`the steam contained in the dome and in the piping of the heat
`recovery boiler, then using the stored energy When the poWer
`demand in the netWork increases sharply. According to the
`principles of Primary Frequency Regulation, the occurrence
`of such an increase is marked by a decrease in frequency
`beloW its rated value. To put this basic principle to Work in
`an efficient Way, several complex controls must be per
`formed, all of Which are covered by the present method.
`[0014] A further object of the present invention is to
`reduce to a minimum any losses in the poWer generated by
`the steam turbine, When the turbine operates in the Primary
`Frequency Regulation mode. As explained above, When gas
`turbines in a combined cycle plant operate in PFR mode, the
`steam turbine must be operated beloW its rated capacity,
`since gas turbines must also be operated beloW their rated
`capacity. The method of the present invention converts the
`decrease in the poWer generated by the steam turbine into a
`Spinning Reserve useful to implement PFR in that turbine.
`Thus, the Spinning Reserve is converted into readily avail
`able back-up poWer, available in just a feW seconds.
`Response times as loW as 10 seconds or less can be achieved
`if necessary. By means of the present invention, it is no
`longer necessary to decrease the output poWer of the steam
`turbine to a greater extent to achieve a Spinning Reserve
`useful for PFR.
`[0015] Operating Principle
`[0016] In order to supply PFR in a steam turbine operating
`in a combined cycle plant by the method of the present
`invention, the related gas turbines must also be operated in
`PFR mode.
`[0017] As explained above, When gas turbines provide
`PFR, a certain amount of poWer from the steam turbine is
`lost because the gas turbines must operate beloW their rated
`capacity. Under conventional prior art methods, the Spin
`ning Reserve created in the steam turbine is not readily
`available by the netWork as required by the PFR. The
`method of the invention converts the Spinning Reserve into
`readily available back-up poWer Within the short response
`times necessary for PFR. Therefore, it is no longer necessary
`to decrease the output poWer of the steam turbine to a greater
`extent to operate the steam turbine in PFR mode.
`[0018] When gas turbines are operated in PFR mode,
`variations in the output poWer of gas turbines because of
`PFR Will cause variations in the output poWer of steam
`turbines as Well. This is because, upon a change in output
`poWer in gas turbines, the available energy in exhaust gases
`Will also change, Which Will in turn provoke changes in the
`internal energy of steam. The latter Will in turn modify the
`output poWer of the steam turbine in the same direction as
`those in the gas turbines.
`[0019] FIG. 1 is a representation of a rapid change in the
`output poWer of a gas turbine in a typical combined-cycle
`
`plant, and the associated change in output poWer caused in
`the steam turbine, versus time. In a non-limiting example, a
`rapid increase of approximately 5 percent in output poWer of
`the gas turbine Was assumed, based on the output poWer of
`the gas turbine before the change. Avalue of 5 percent Was
`chosen because it is a typical Spinning Reserve value
`required by the authorities in charge of poWer dispatch.
`HoWever, should other values be used in the example, the
`results obtained should be very similar. Curve TG in FIG. 1
`shoWs the evolution of gas turbine output poWer versus time,
`While curve TV corresponds to the steam turbine. It can be
`seen that the time constant of the change in output poWer of
`the steam turbine for a given change in the output poWer in
`the gas turbine is very high. This example suggests a
`response time by the steam turbine of approximately 20
`minutes. In other Words, once an increase in output poWer of
`the gas turbine occurs, the output poWer in the steam turbine
`Will reach its ?nal value only after 20 minutes. This response
`time is not acceptable to achieve PFR in the system, since,
`as stated above, response times of just a feW seconds are
`required.
`[0020] HoWever, if long term frequency deviations are
`considered, the gas turbines Will readily reach their neW load
`condition, and Will remain in that condition as long as the
`frequency deviation persists. Once the response time char
`acteriZing the transfer of energy from gas turbine exhaust
`gases to steam has elapsed, the steam turbine Will reach its
`neW load condition, Which Will then have changed in the
`same direction as the gas turbine (as seen in the example of
`FIG. 1), thus complying With PFR requirements.
`
`[0021] Therefore, long-term response of the steam turbine,
`or steady-state response, should be considered completely
`satisfactory. Short-time response, or transient response,
`Which is most important for PFR, must be noW enhanced.
`
`[0022] It must also be considered that, When operating gas
`turbines associated With a steam turbine in PFR mode, the
`steam pressure in piping and boiler components Will be
`loWer than normal. This is because the gas turbines Will be
`operating beloW their base output poWer. The decrease in
`steam pressure is extremely useful in the present method, as
`Will be explained beloW.
`
`[0023] To provide a better transient response, the method
`of the present invention is based on the innovative use of the
`stored energy principle. Energy is stored Without decreasing
`the output poWer of the steam turbine to a greater degree
`than is necessary When gas turbines are operating in PFR
`mode.
`
`[0024] Stored energy is used to readily supply additional
`poWer required by the PFR function, Which used to be a
`shortcoming of combined cycle steam turbines in the prior
`art. In case a decrease in output poWer is required, said
`decrease can be readily achieved, according to a Well-known
`practice, by throttling the turbine control valves as neces
`sary.
`
`[0025] To achieve the storage of energy, the steam turbine
`control valves should be throttled. HoWever, valve throttling
`should not be indiscriminate. Valve throttling is necessary to
`implement the method; and Within an acceptable control
`valve throttling range, removing the steam turbine from the
`“sliding pressure” mode Will not cause signi?cant energy
`losses, as Was believed in the prior art. On the contrary, any
`
`0007
`
`

`

`US 2003/0010037 A1
`
`Jan. 16, 2003
`
`such losses are negligible and thus very dif?cult to measure.
`It can be concluded that no losses in output poWer or in
`combined cycle ef?ciency occur.
`
`[0026] By throttling the steam turbine control valves,
`steam pressure in piping and boiler components increases.
`Consequently, care should be taken not to exceed the design
`pressure of installations such as piping and boiler compo
`nents. HoWever, this is not a major obstacle against the
`application of the present method, since the operation of the
`associated gas turbines in PFR mode, together With the
`decrease in output poWer in the steam turbine, both cause a
`decrease in steam pressure.
`
`[0027] Therefore, upon throttling the steam turbine control
`valves, the equipment Will operate at a steam pressure above
`the pressure corresponding to the same load condition but
`With the steam turbine in the “sliding pressure” mode.
`Nevertheless, the steam pressure Will not exceed the design
`pressure corresponding to operation at rated output poWer.
`
`[0028] The storage of energy derives from the fact that
`steam pressure in the facility is greater than the pressure
`corresponding to the same load condition in normal turbine
`operation.
`[0029] The amount of stored energy is in general terms
`given by the total volume of steam contained in the facility
`at an increased pressure.
`
`[0030] FIG. 2 illustrates the process undergone by the
`steam turbine output poWer When the turbine control valves
`are throttled.
`
`[0031] FIG. 2 depicts the behavior of the steam turbine
`output poWer upon the throttling of the turbine control
`valves. Both variables are shoWn against time. The TV curve
`represents the variations of the steam turbine output poWer
`With time. The VVA curve represents the aperture of the
`steam turbine control valves With time. Initially, a sudden
`throttling is introduced, reducing the aperture of the control
`valves. The output poWer in the steam turbine initially
`decreases. With time, output poWer steadily recovers its
`original value. The reason for this is that the increased steam
`pressure upstream the control valves causes the inlet pres
`sure to the turbine to be restored, even With the control
`valves throttled. The loss of head in the control valves causes
`no signi?cant losses in ef?ciency or in output poWer, since
`differential steam pressure values required in control valves
`are comparatively loW.
`
`[0032] HoWever, if a certain degree of throttling in the
`control valves is eXceeded, the behavior of the system Will
`no longer be the one depicted in FIG. 2. On the contrary,
`upon throttling the valves, the steam turbine output poWer
`Will decrease, With no further recovery.
`
`[0033] To determine the optimal steam turbine control
`valve throttling degree for every situation, tWo basic con
`siderations should be made. The ?rst consideration concerns
`the amount of Spinning Reserve useful for PFR that needs
`to be obtained from the steam turbine—that is, hoW much
`additional output poWer the machine should readily supply
`Within a response time of a feW seconds. This Spinning
`Reserve value is directly linked to the requirements of the
`local poWer grid to Which the combined cycle plant is
`connected. The second basic consideration concerns the
`length of time during Which such increment in the steam
`
`turbine output poWer should be sustained by the eXtra energy
`stored in the steam, until the additional energy coming from
`the gas turbines operating at a higher output poWer is
`available to the steam turbine. Once a permanent increase in
`the output poWer of the steam turbine is attained, in response
`to a decrease in frequency, such permanent increase in the
`output poWer can be sustained for as long as the frequency
`deviation persists.
`[0034] In general terms, the ?rst of the tWo above-men
`tioned basic considerations Will require a greater degree of
`throttling of the steam turbine control valves, as compared
`to the throttling required by the second above-mentioned
`basic consideration. Therefore, the present method intends
`?rst to determine the degree of control valve throttling
`necessary to comply With the Spinning Reserve require
`ments for PFR, and later to check that such control valve
`throttling degree is enough to sustain over time the output
`poWer requirements derived from the PFR function. Should
`this not be the case, a greater throttling degree Will be
`necessary.
`[0035] Typically, the greater the requirement for instanta
`neous Spinning Reserve, the greater the control valve throt
`tling degree. When the aperture of the control valves is
`reduced, the pressure differential across the valve increases.
`On the other hand, there is a direct relationship betWeen the
`pressure differential across the turbine control valves and the
`additional output poWer the turbine is capable of supplying
`readily. HoWever, this relationship varies according to the
`construction characteristics of each plant.
`[0036] In order to determine the relationship betWeen the
`pressure differential across the turbine control valves and the
`amount of the Spinning Reserve useful for PFR, practical
`tests must be ran. To run these tests it is necessary in the ?rst
`place to operate the gas turbines With the same amount of
`Spinning Reserve they Would have in case the turbines Were
`operated in the PFR mode, eXcept that they Will not be
`actually operated in such a mode, but to a constant output
`poWer. Starting With the turbine in the “Sliding pressure”
`mode (i.e., With the control valves fully open), a certain
`throttling degree Will then be introduced in the control
`valves. After throttling the control valves, the process must
`be alloWed to stabiliZe. A stable condition for the process
`Will be reached When both the output poWer in the steam
`turbine and the steam pressure upstream the control valves
`reach values that remain constant over time. For this pur
`pose, small oscillations near the constant values, considered
`normal in the operation of these plants, Will not be taken into
`account. The neW steam turbine output poWer value Will
`match very closely the value of the output poWer of the
`turbine prior to the throttling of the control valves. Once the
`process is stable again, the amount of the pressure differ
`ential across the turbine control valves must be recorded.
`AfterWards, a sudden aperture of the throttled control valves
`Will be effected. The test shall be repeated for several other
`throttling positions of the control valves, trying to attain an
`amount of Spinning Reserve useful for PFR, equal to the
`maXimum value With Which it is desired to operate the PFR
`of the steam turbine. The pairs of values for Steam Differ
`ential Pressure/obtained Spinning Reserve useful for PFR,
`Will be used in the application of the present method, as
`disclosed in the section Description of the Method beloW.
`[0037] In case the Spinning Reserve requirement is not
`very severe, typically only the control valves in the high
`
`0008
`
`

`

`US 2003/0010037 A1
`
`Jan. 16, 2003
`
`pressure stage of the steam turbine shall be throttled. HoW
`ever, When the requirements of Spinning Reserve for PFR
`exceed a certain value, Which has to be determined for each
`facility, it Will become necessary to throttle the control
`valves at different pressure stages of the steam turbine. This
`requirement Will become self-evident either When the maXi
`mum Working steam pressure is reached in any of the
`components in the facility, or When, notWithstanding a
`signi?cant loss in steam turbine output poWer induced by
`throttling the control valves in the high pressure stage of the
`turbine, the desired Spinning Reserve useful for PFR is not
`attained. In these cases the above mentioned test must be
`repeated by simultaneously throttling the control valves in
`different steam pressure stages in the turbine, and reopening
`them also at the same time. If, even under these conditions,
`the maximum Working steam pressures are reached, but the
`desired amount of Spinning Reserve useful for PFR is not
`achieved, then the amount of Spinning Reserve useful for
`PFR of the steam turbine shall be limited to the maXimum
`value attainable.
`
`[0038] Regarding the loss of output poWer in the steam
`turbine, and the loss of ef?ciency of the combined cycle
`plant, if it Was necessary to throttle the steam valves to a
`point Where output poWer is being lost to comply With a
`certain amount of Spinning Reserve, a ?nancial evaluation
`shall be performed for every particular case, by comparing
`the cost of the loss in output poWer With the eXtra revenue
`resulting from participating in the PFR function. The con
`venience of using the present method Will result from such
`evaluation.
`
`[0039] The present method is based on the above
`described tests used to sustain at any moment a differential
`pressure across the steam turbine control valves, in order to
`supply the amount of increased output poWer required by the
`PFR function.
`[0040] TWo principal operational modes are currently
`available for varying output poWer of a turbine-driven
`generator operating in the PFR mode, upon the occurrence
`of variations in the mains frequency. In both cases, the
`variations to be forced on the turbines as a function of
`variations in the mains frequency, depend upon a factor
`called Statism. The main difference betWeen both modes
`relates to the Way this factor is derived, and the Way
`variations in the turbine are subsequently controlled.
`[0041] The most traditional mode, deriving from the old
`turbine mechanical governor technology, de?nes Statism as
`a percent ratio betWeen the mains frequency variation and
`the corresponding variation that must be imposed on the
`turbine control valves. In this mode, every change in the
`mains frequency causes corresponding changes in the posi
`tion of actuators in the turbine control valves. The changes
`in the position of the actuators in the turbine control valves
`Will undoubtedly cause corresponding changes in the output
`poWer. HoWever, these latter changes are not precisely
`predictable, since there is not a univocal relationship
`betWeen the aperture of the steam turbine control valves and
`the corresponding output poWer. The relationship betWeen
`control valve position and steam turbine output poWer is
`affected by a large number of factors such as pressure and
`temperature of the steam, condition of the different turbine
`components, current mains frequency, and so on. Therefore,
`When operating With PFR under this mode, the actual
`contribution of these turbines to the control of the mains
`frequency cannot be predicted accurately. This is indeed not
`Welcome by the authorities in charge of poWer dispatch.
`
`[0042] In the second mode, introduced mainly With elec
`tronic turbine governors, Statism is used to derive the
`variation in the output poWer the turbine must supply for a
`given frequency variation. AfterWards, the PFR controller
`eXerts a control action that ?nally alters the control valve
`position, so that the turbine Will actually undergo the desired
`output poWer variations. This second mode of operation is
`much more ef?cient and predictable. Care must be taken
`here that the PFR controller have a reduced proportional
`action, While its action should be mainly integral, in order to
`avoid malfunctioning during sharp variations in the mains
`frequency.
`
`[0043] In the method of the present invention, the inven
`tors have decided to determine Statism and control accord
`ing to the second mode, because of its better efficiency to
`control the mains frequency.
`
`[0044] Another problem must be solved to alloW com
`bined cycle steam turbines to leave the “sliding pressure”
`control mode and control the output poWer, and thus meet
`PFR requirements. It is necessary to knoW exactly What
`output poWer the turbine must deliver in idling conditions,
`to avoid causing generation limitations. These limitations
`should be avoided, and the steam turbine shall not be forced
`to deliver an immoderate output poWer. Both situations
`Would cause poor performance of the PFR function, and
`Would also cause instability in the poWer generation process,
`like over-pressures or sharp changes in system parameters.
`To avoid the above-mentioned problems, the method of the
`present invention incorporates a special control structure.
`
`[0045] In order to introduce the output poWer variations
`needed for the PFR function, the method comprises modu
`lating the turbine control valves. The response times that can
`be achieved When varying the steam turbine output poWer by
`reopening the control valves and liberating the stored energy
`are extremely loW (less than 10 seconds). In general terms,
`these response times comply With the most stringent PFR
`requirements currently in practice. For eXample, the United
`Kingdom’s Grid Code calls for response times of less than
`10 seconds, While the Argentine code requires less than 30
`seconds. The goal is to adopt a response time slightly beloW
`the time required by the code. This is made by adjusting the
`variables of a proportional plus integral controller, described
`in the present method, thus avoiding unnecessary sharp
`movements.
`
`[0046] FIG. 3 depicts the response of a combined cycle
`composed of tWo gas turbines and one steam turbine,
`operating under PFR according to the method of the present
`invention, upon a simulated long time reduction in the mains
`frequency. Curve F represents frequency versus time. An
`initial value of frequency equal to the rated frequency is
`assumed, Which diminishes later on. Curves TGl and TG2
`represent respectively the time behavior of the output poWer
`of both gas turbines. Curve TV represents the time behavior
`of the steam turbine. The steam turbine output poWer
`reaches its ?nal condition after response times similar to
`those of the gas turbines. On the other hand, the increase in
`output poWer is sustained during all of the recording time.
`
`[0047] According to the present method, the gas turbines
`can operate in PFR mode, under the design parameters
`established by the manufacturers. It is not necessary to
`introduce changes in the design parameters. The only
`requirement is to select the appropriate Statisms both for the
`gas turbines and for the steam turbine.
`
`0009
`
`

`

`US 2003/0010037 A1
`
`Jan. 16, 2003
`
`DETAILED DESCRIPTION OF THE METHOD
`
`FIG. 4 illustrates a general block diagram of the
`
`[0048]
`method.
`[0049] The plant operator must in the ?rst place ?x the
`value of tWo parameters according to the requirements. One
`of these parameters is “Frequency Set Point.” Frequency Set
`Point is the desired value for frequency, Which normally is
`set as the rated frequency of the system. The second param
`eter that the operator must determine is “Maximum Contri
`bution to PFR”. Maximum Contribution to PFR is the
`absolute value of the maximum output poWer that PFR Will
`contribute to control the mains frequency. This second value
`is determined by the operator on the basis of the local
`regulatory requirements for PFR in the system.
`[0050] Once the “Maximum Contribution to PFR” value
`has been set, the block “Reference Position for PFR” cal
`culates the degree of throttling of the control valves that is
`necessary to obtain such an increase in output poWer,
`Whenever needed. In order to determine the throttling de