Fast Cycling and Rapid Start-up US

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Answers for energy.

Fast cycling and rapid start-up:new generation of plantsachieves impressive results

Reprint from:

Modern Power Systems, January 2011

Author:

Lothar Balling, Siemens AG,

Erlangen, Germany


2/8www.modernpowersystems.com January 2010 Modern Power Systems

COMBINED CYCLE

Fast cycling and rapid start-up:new generation of plants

achieves impressive resultsIn last months issue an analysis of the operational characteristics of various power generation technologies

(pp 61-65) concluded that modern combined cycle power plants, with cycling and fast start capabilities, have

a number of advantages in a grid where a large percentage of renewables is envisaged, as in Germany, for

example. In this second article the theme of combined cycle plant operational flexibility is explored further.

Recent innovations in combined cycle technology are described, as well as newly commissioned power plants

that demonstrate what can be achieved.

The increased use of combined cycleplants for power generation over thepast decade can be attributed to theirhigh efficiencies, short execution times

and relatively low investment costs. But nowtheir potential for cycling and fast start-up isbecoming an increasingly important sellingpoint.

This need for increased flexibility firstemerged at the end of the 1990s in the UnitedStates and the United Kingdom. The price offuel continued to rise due to the large numberof plants being built during the boom. Plantsinitially planned to have a base-load role wereshifted to the load regime of an intermediate-load plant.

The challenge presented to projects by thischanged requirement gave birth to the idea oftrying to improve plant flexibility withoutcompromising plant service life or plantefficiency.

As the market continued to develop, ademand for quicker start-ups soon followedthe demand for more frequent start-ups. Thismarket demand finally resulted in the launchby Siemens of a development project called

FACY (FAst CYcling), which combined allthe initial engineering ideas into a singleintegrated plant concept. The aim of theresulting R&D programme was to design aplant for an increased number of starts and toreduce start-up times. If possible, no limitswere to be placed on the gas turbine by otherpower plant components, such as the heatrecovery steam generator or steam turbine,during hot and warm starts.

In the course of the project, potential areascame to light where further optimisation couldbe achieved, although these had to wait for asecond development generation to beimplemented.

The major improvement offered by thissecond generation involved the start-upprocedure. Hold points at which a plant waits

until certain steam parameters have beenreached were eliminated as part of theshortened Start on the Fly start-upprocedure. In this procedure, the steamturbine is started up in parallel to the gasturbine using the first steam which becomesavailable after a hot start.

While the first generation FACY reducedstart-up times for a hot start from 100 to 55minutes, the second generation succeeded inpushing start-up times down below the 40minute mark.

The first plants incorporating the features ofboth the first and second generations of the

Lothar Balling, Siemens,

Erlangen, Germany

Figure 1. Recent combined cycle projects with enhanced flexibility and fast start capabilities

Pont sur Sambre, France Sloe Centrale, Netherlands

Marchwood, England Pego, Portugal

Enecogen, Netherlands Hemweg and Diemen, Netherlands

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FACY concept have now entered commercialoperation (Figure 1).

A good example is Sloe Centrale, a 2 x 430MW F-class single shaft plant in theNetherlands, where 30-minute start-up

times were recorded during acceptance tests,while achieving over 59% net efficiency.Equally good results have been exhibited byother newly commissioned plants. Thismeans that the second generation of FACYhas far surpassed expectations in a numberof cases.

Shortening start-up times and improvingstarting reliability while increasing thenumber of starts was only one of many newrequirements with respect to plant flexibility.

An increasingly important driver (asdiscussed in last months article) is the growingpercentage of renewables envisaged on thegrid (see Figure 2). Wind and solar energy arenot continuously available and difficult topredict precisely. Reserve power generatingcapabilities must therefore be provided whichcan be activated quickly. Gas turbine basedplants are an obvious choice here as they canbe started up at relatively short notice. Theinherent inertia of other types of powergenerating facilities is usually much greater,making them largely unsuitable for use as arapidly available reserve source of power.There are, of course, other fast-respondingsources of power such as pumped storage. Butthey do not provide enough capacity to coverthe renewable generating capacity in theEuropean grid system, with the prospect of30% renewables by 2030.

High availability and reliability powerplants, such as combined cycle units, arerequired in order to compensate forfluctuating renewables. The requirementswith respect to grid support, which are usually

defined in a country-specific grid code, haverecently become more rigorous for this reason.Some of the most stringent requirements are

to be found in the UK grid code. Requirementsin the areas of load stabilisation at lowfrequencies, primary and secondary frequencyresponse, and island operation capability havepresented a particular challenge to UKoperators for quite some time. However, therecently handed over 840 MW multi-shaft F-class Marchwood plant has finallydemonstrated that the problem can be solvedwithout compromising efficiency (over 58%)by introducing additional technical featuresand optimising the plant concept.

A decisive factor in the success of Marchwoodwas the integrated approach, which combinedthe potentials of several systems andcomponents in a single solution, including useof gas turbine compressor optimisation, firingreserves, fast wet compression and othermeasures, combined with an optimisedI&C/closed-loop control concept.

The new demand for extremely fast powergenerating availability is also becomingapparent in CCGT developers economicassessments. Only a few years ago there wereprojects in which start-up times did notfigure at all in the assessment, whereas nowwe are seeing over 100 000 /min for someprojects.

'"TU$:DMJOHDPODFQU'"$:The idea of focusing plant design on anincreased number of fast starts originatedfrom market conditions and from specificprojects. A multidisciplinary team of

component and plant experts (for the steamturbine, gas turbine, balance of plant andauxiliary systems, I&C and steam generator)was formed by Siemens around 2002 toidentify improvement potential in the existingplant concepts.

The team identified the following priorities: Maintaining pressure and temperature in

the main components during shutdowns, byusing stack dampers, auxiliary steam, etc.

Ready-for-operation water/steam cycleusing a fully automated start-up conceptwithout manual operation or interventionduring hot start.

Optimised component design (eg, highcapacity and fast acting de-superheaters)and plant operation to reduce materialfatigue caused by load cycling.

Flexible operation concept to allow theoperator to predetermine componentfatigue and to choose start-up time andramp rate.

Optimisation of the automation and controlconcept.

New start-up sequence, Start on the Fly,to allow a nearly unrestrained ramp-up.

Figure 3 summarises the main features of theFACY concept. These measures help reducestart-up time significantly. They are modularand are offered, configured and implementedon a project-specific basis.

Preserving warm start conditions

Major heat loss from the HRSG occursthrough the stack and therefore a stackdamper is deployed to limit heat loss duringshut-down. Cooling down of the HRSG isconsiderably reduced and delayed.

Furthermore, auxiliary steam can be used toheat the HRSG. These measures increase theshut-down periods for which the criteria forhot and warm starts remain applicable.

Ready-for-operation mode of water/steam cycleAuxiliary steam is also used to maintain thewater/steam cycle in a ready-for-operationmode. This means auxiliary steam is fed intothe gland steam system of the steam turbine.Keeping the gland steam system in operationprevents air from being drawn into the steamturbine and the condenser. Since the steam

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turbine and the condenser are sealed off fromthe ambient air, the condenser vacuum pumpscan maintain the vacuum.

To enhance the start-up procedure, thecondensate polishing plant can be used tobring the water/steam cycle within specifiedchemistry limits faster.

Optimised component design and plantoperation to reduce material fatigueIn a conventional HRSG the high pressuredrum is one of the most critical componentsin the start-up and ramping procedure. As athick-walled component it is exposed to largetemperature gradients and high operatingpressures. Thermal stress in the high-pressuredrum walls limits the load-, start up- and shutdown- gradients of the HRSG.

However there is no high pressure drum ina Benson-type boiler, so these limits do notapply. The Benson boiler technologyemploys once-through steam generation,which means that conventional separation ofsteam and boiling water inside a drum is notnecessary. Instead, steam is generateddirectly within the evaporator tubes, asshown in Figure 4.

Use of Benson technology allows thenumber of permissible starts and cyclingevents during the plant lifetime to

be significantlyincreased, byreducing stressinduced fatigue in thehigh pressure sectionof the HRSG.

Also, a

t e m p e r a t u r e -controlled start-upprocess, using anoptimised high-capacity de-superheater to limit steam temperaturesduring the start-up, has been developed forwarm and cold starts. This reduces thermalstress in critical components of the steamturbine.

Optimising the automationThere are essentially two ways in whichautomation system is optimised to supportimproved start-up: Design limits are fully exploited through the

use of closed-loop control instead of earlierempirically based approaches. A turbinestress controller is used to determinethermal stress based on temperaturedifferences measured within the steamturbine and ensures that stress limits are notexceeded. The turbine stress controller

makes it possible toshorten the start-uptime without reducingthe lifetime of heat-critical turbinecomponents.

Two additional start-up modes FASTand COST-EFFECTIVE in addition tothe NORMAL mode have beenintroduced. The operator has the option ofchoosing the appropriate start-up modedepending on such factors as currentelectricity market prices. Maintenanceintervals can be extended using the COSTEFFECTIVE setting, while the FASTmode permits controlled fast start-up, butentails increased maintenancerequirements.

The start-up procedure is automated to a levelthat enables hot starts with only a few operatoractions, the aim being to minimise inefficientand unproductive periods during start-uppreparations. Draining and venting are largelyautomated, for example.

Second-generation FACY Start on the FlyIn addition to the features included in theoriginal FACY concept, a procedure forparallel start-up of gas and steam turbines hasbeen developed. It is based on monitoring and

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controlling the temperature gradients withinlimits acceptable for all critical plantcomponents and on long term turbineoperating experience over a range of steamconditions. The new concept enables the plantto be started-up without any gas turbine loadhold points, enabling a new start-up sequence

to be implemented see Figure 5. The maininnovation is the early steam turbine startingpoint with earlier acceleration and loading ofthe turbine.

3FDFOUPQFSBUJOHSFTVMUTThe features described above have beenimplemented in plants across Europe andexcellent results have been achieved in singleshaft as well as in multi shaft configurations.Two notable examples are the Pont sur SambreF-class single shaft plant in France and the F-class multi shaft configuration at Irsching 5 inGermany, Figures 6 and 7, respectively.

Both units have demonstrated the capabilityto start-up and reach full load in about 30minutes folowing an overnight shut down,without compromising efficiency, achievinglevels of over 58% and over 59%, respectively.

It is noteworthy, that the single shaftoperating concept allows parallel start up ofseveral units at a site, resulting in multiples of,eg, 430 MW, being available in around 30minutes, as has been demonstrated at SloeCentrale, with its two units.

(SJETVQQPSUIn liberalised electricity markets, theminimum requirements for power plantdynamics are set out in grid codes. Some of the

most stringent requirements imposed on plantdynamics are to be found in the grid code ofthe UK, reflecting its island geography. Threeof the most critical considerations are: loadstabilisation at low frequencies; primary andsecondary frequency response; and islandoperation capability.

Load stabilisation at low frequenciesNormal fluctuations in the balance betweengeneration and consumption are reflected influctuations in grid frequency which can becompensated for by means of routinefrequency control measures. The frequencycan, however, also decrease or even increasesignificantly in the event of unusually largedisturbances.

Unfortunately a decrease in grid frequencyalso means a reduction in turbine speed andsubsequently a decrease of power output. Thisdecrease in speed causes the compressor in agas turbine to produce a reduced volumetricflow, thus decreasing gas turbine output ifappropriate compensatory measures are notimplemented.

The United Kingdom grid code stipulatesthat power output must be maintained for aminimum of 5 minutes in the event of afrequency drop, down to 49.5 Hz so as toavoid further taxing of the grid due to under-frequency. If a greater decrease in frequencyoccurs, the grid code permits a maximumdecrease in output of 5%, down to 47 Hz, asillustrated in Figure 8.

To counteract this decrease in power output,several measures for increased output can beimplemented at short notice. The decrease in

output can be compensated for by rapidlyopening the guide vanes on the compressor.The fuel flow is increased at the same time.This can compensate for a drop in power ofaround 6 MW.

In unfavourable operating conditions,however, this increase in output will not be

sufficient on its own. In this case Fast WetCompression (a patented Siemens concept)can be used to mobilise a further powerreserve of around 12 MW.

Fast Wet Compression consists of sprayingdemineralised water into the compressorinlet. The mass of the injected water increasesthe mass flow through the compressor. Theevaporating water also cools the air flow atthe compressor inlet. The air density andconsequently the mass flow through thecompressor increase due to this coolingprocess. Rapid activation of the systemconstitutes a challenge to control systems, asthe fast increase in power output requiresperfect co-ordination between the gasturbine control system and the water

injection.These grid support features have been

validated and demonstrated in theMarchwood F-class multi shaft plant in theUK, at a power output of about 840 MW andover 58% efficiency (Figure 9).

Plots from the Marchwood tests are shownin Figure 10. It can be seen that an 18 MWincrease was achieved (for each gas turbine)by opening the compressor IGVs and theninitiating fast wet compression, thus meetingthe requirement of the United Kingdom gridcode.

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1SJNBSZBOETFDPOEBSZGSFRVFODZSFTQPOTFThe purpose of load stabilisation at lowfrequencies is to prevent furtherdestabilisation of the grid when the frequencydecreases due to major disturbances. Primaryand secondary frequency responses are now

required for grid support during normaloperation. For this purpose the UK grid codestipulates that a power plant operating at partload must be capable of making additionalpower available. Figure 11 illustrates therelevant part of the UK grid code. We can seethat a power plant operating at under 80%load must be able to make available at least10% of its rated power within 10 seconds inthe event of a decrease in frequency. Forsecondary frequency response 10% of its ratedpower must be made available within 30seconds. Figure 11 shows that therequirements are reduced when the plant isoperating at loads over 80%.

Unlike load stabilisation at low frequency,there is no need to look for a further powerreserve in this case. The challenge lies more inthe speed at which the power must be madeavailable.

To meet the requirements of the grid code,we rely on fast repositioning of the compressorIGVs coupled with fuel control optimised tosuch an extent that load ramps are possiblewithout destabilising combustion.

Figure 12 illustrates the results of tests doneon the Marchwood plant and clearly showsthat the required additional power is achievedboth after 10 seconds and after 30 seconds. Infact, performance is significantly better thanthat required by the grid code in bothinstances.

Another aspect of the grid code, also shownin Figure 11, is high frequency response,namely that load must be reduced by 10% ofrated power within 10 seconds in the event ofover-frequencies of up to 500 mHz. However,when it comes to load reduction the islandoperation requirement (see next section) iseven more stringent.

Island operation capabilityThe primary objective of island operationcapability is to stabilise the grid, in the eventof excess power and an abrupt drop inconsumption within an islanded portion of thegrid, resulting in a very rapid frequencyincrease. The power plant must react to thisfrequency increase by throttling back tostabilise the frequency, thus avoiding a forcedshut-down due to over-frequency.

Uncontrolled shut-down of power plantscan result in a grid collapse, which is why theUK grid code stipulates that power plantsmust be capable of rapidly decreasing fromrated power to the design minimum operatinglevel (DMOL). The DMOL must not be

smaller than 55% of rated power in this case.This load reduction must be effectedsufficiently quickly that the island frequencyremains below 52 Hz. Grid studies based onthe UK National Grid requirements show thatthe load reduction must take place withinaround 8 seconds.

The power plant must detect islandformation automatically and take immediateaction. As soon as island operating mode isactivated, permitted load change ramps areset to the maximum value. The inlet guidevanes in the gas turbine compressor areclosed without delay. At the same time thevarious closed-loop controls ensure that thepower is decreased at the maximum rate ofchange for load. Maintaining flame stabilityand avoiding potential flash backs in thecombustion system are the main objectivesof closed-loop control optimisation, soas to avoid emergency shutdown of thegas turbine.

Figure 13 illustrates an island operation testat Marchwood. The gas turbine output wasdecreased by 52% within 4 seconds as the resultof a simulated fast frequency increase of 0.9Hz, without initiating a plant trip. A furtherdecrease of 4% was achieved in the following4 seconds. Thus performance again exceededgrid code requirements.

Transfer to the HMeanwhile, these basic plant featuresdemonstrated in F-class plants are beingtransferred also to H-class technology andhave already been validated in open cycleoperation at Irsching 4 (Figure 14),demonstrating that even this latest andhighest efficiency technology is capable ofsupporting the same stringent grid code

requirements.

5IFCPUUPNMJOFCFOFGJUTUPUIFPQFSBUPSThe previous sections clearly demonstrate thatFACY and Start on the Fly permit a reductionin start-up times as well as an increasednumber of start-ups, enabling nightly powerplant shut-downs. The latter offers twoadditional benefits: Carbon dioxide emissions are minimised by

shortening inefficient plant start-ups.Maximum electrical efficiency is reachedfaster and total emissions are reduced.

Since nightly shutdowns and reliable start-ups become economically feasible, overallcarbon dioxide emissions are furtherreduced as inefficient overnight parking at

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load is avoided. Other power plants withinthe grid can then be operated at full load andmaximum efficiency.

Operators benefit from this, primarilythrough fuel savings and a reduction in carbondioxide emissions during the start-up phase.Shortening the start-up time by using Start onthe Fly for a hot start offers an estimatedadded value of more than 3 million eurosalone, assuming that the savings described

above are realised over the service life of a 430MW power plant.

The option of disconnecting the plant fromthe grid overnight offers enormous potentialin the form of savings in operating costs.

Night-time electricity prices have been atsuch a low level in the European system thata combined cycle power plant could no longerbe operated at a profit during the night due tohigh gas and carbon dioxide costs (see Figure15). To minimise these losses, power plants areoperated at part load or are shut downaltogether at night.

Reducing the load already brings about asignificant reduction in losses. Howeverwhen the load decreases, so does overall

efficiency, meaning that gas and carbondioxide costs can only be reduceddisproportionately.

In addition to the positive effect of loadreduction, shutting a power plant down atnight can achieve other significant benefits.Only shut-down and start-up costs areincurred, for example. Restrictions relating tothe permitted number of start-ups for the planthave been significantly reduced, thanks to theFACY programme. FACY and Start on theFly have also significantly reduced start-uptimes. The result is lower gas consumption andlower carbon dioxide emissions, providing the

power plant operator with an additional

economic benefit for every start.Figure 16 shows the carbon dioxide and fuelsavings which can be achieved by night-timeshutdown using FACY compared with night-time part-load operation at about 25%. Wecan see that the power plant in this examplecan avoid up to 130 tons of gas consumptionand 362 tons of carbon dioxide emissions pernight through night-time shutdown. Thisincreases the annual power plant profit by 4.8million euros as compared with night-timepart-load operation.

Today grid support features arise primarilyfrom the grid access requirements of the

individual countries. No monetary valuation

of the additional plant flexibility is included intender specifications as yet. For this reasontodays plants are designed purely based ongrid code specifications. Depending on thelevel of electricity market liberalisation,however, the various flexibility features enableadditional earnings to be generated, inparticular by participating in the frequencyreserve market.

Another potential benefit is that plants withhigh reliability and operational flexibility,able to cope well under disturbed gridconditions, can expect to be prioritised fordispatch. MPS

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Modern Power Systems

January 2011, Page 35 41

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Modern Power Systems

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