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Hannover Messe - Power Plant Technology Forum 2011 5. –8. April 2011, Hannover, Deutschland

KA26 Combined Cycle Power Plant as Ideal Solution to Balance Load Fluctuations

Dr. Michael Ladwig, Mark Stevens Alstom Power, Baden, Switzerland

Abstract As commonly accepted, combined cycle operation regimes change over the lifetime of a plant. Especially in the last few years these changes have been noticeably evident as a consequence of the increasing proportion of electricity generation production from renewables, such as wind turbines. Therefore, today's modern combined cycle power plants need to be flexible in operation. This flexibility is not only requested regarding turndown capability, but also regarding operation regimes. Plants may operate one year in more or less base-load mode, and the next year in cyclic mode with daily starts and stops. Accurate modelling of thermal cycles is required to avoid premature fatigue failure of plant components for such requirements. Detailed temperature histories during transient events are required to determine maximum stress ranges for life assessment. These details are obtained from a dynamic simulation of the thermal process in the component of interest. The simulation of the HRSG, for example, is used to tune the operation concept and then to determine the temperature history for the four main types of cycles: hot start, warm start, cold start, and trip. Heat-up, steady state, and cool down phases of each cycle must be modelled. Thick walled components and those exposed to higher temperatures limit the life of the overall system. From the output of detailed analyses, a comparison to code-allowable stresses can be made to determine the fatigue usage factor. Due to the relative high flexibility in start-up and operation of combined cycle power plants, these power plants are expected to be called upon more and more to deliver greater operational flexibility. Components therefore within the CCPP that are designed for base-load applications are likely to be less tolerant to cycling than many of these plants will actually experience. This paper will show examples of the components of the KA26 combined cycle power plant, that demonstrate how Alstom achieved a fatigue-tolerant design to allow base-load and cycling plant operation.

© Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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Introduction Power markets around the world are facing new challenges as the need to build new power generation plant to meet growing demand is at the same time having to take in to consideration public pressure and measures to address the global environmental issues and of the impact the conventional power plants are having on our planet. This challenge is further complicated in many of the so-called ‘developed power markets’ by the trend-shift in structural set-up from one of regulated (closed) to de-regulated (open) power markets. The result of all these changes and developments is that many of the advanced gas-fired combined cycle power plants that where installed in the late 1990’s and 2000’s, which due to their relative high efficiencies were specified and designed based on base-load dispatch, are today being called on to operate under a wide ranging dispatch regime, including daily stop/starts and intermediate regimes, which was never foreseen at the outset. “Operational flexibility” is now becoming more and more a buzzword in the gas-fired power industry, and OEM’s and Operators alike have to re-define the way such power plants should be designed and the possible load regimes that could be expected today and in the future. The emergence and growth of renewable power, in particular wind-farms, also brings new challenges and issues for power companies and grid operators, who have to balance the power generation with the load demand. Although the emerging renewables and other low carbon technologies are expected to play an increasing role in the longer term, fossil energy supply from fuel gas, oil and coal is likely to remain for decades. The increasing installation of power generation systems using renewable energies and their dependency on ambient conditions (like wind power) calls for a balance with the reliable and rapidly available power resources covering periods of sudden supply shortage, peak demands or simply following the automated generation control over a wide range of relative load. Alstom’s modern combined cycle design concepts depict multi-purpose power plants, perfectly fitting plant operators’ needs for operation flexibility. It is understood that plants cannot be optimised for just one operation regime, as operators face different power market requirements and opportunities over the lifecycle of a power plant. Alstom has established a Plant Integrator™ approach in the way it designs and builds modern power plants. Alstom looks for the overall plant optimisation, rather than focusing at the component level, thus leading to complementary well-aligned development of Alstom’s in-house core components such as the gas turbine, steam turbine, HRSG, generators, and heat exchanger equipment. Plant Integrator™ thinking is reflected in the continuous improvement of the KA26 product family, based on the Alstom advanced-class GT26 gas turbine, the Alstom steam turbine portfolio for combined cycles and Alstom’s heat recovery steam generators (HRSG). In this paper we shall look specifically at the design considerations taken by Alstom with regards its gas turbine GT26, steam turbines and HRSG’s to ensure the KA26 products are able to deliver the very high operational flexibility needed in today’s power markets.

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The Alstom KA26 Combined Cycle Products The KA26 combined cycle power plants have been offered by Alstom since the mid 1990’s, so for around 15 years now. These combined cycle power plants have at their core the Alstom GT26 gas turbine, which is now well known for having the unique ‘sequential’ (2stage) combustion technology in the advanced-class GT/CC market. Alstom has focused on developing two “Reference Plant” products for the KA26:

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

- 3 1. KA26-1 (1-on-1) single-shaft combined cycle power plant (Figure 1) 2. KA26-2 (2-on-1) multi-shaft combined cycle power plant (Figure 2), also developed as Integrated Cycle Solution (ICS) utilizing once-through HRSG technology The primary 50 Hz advanced-class CCPP markets have tended to lean to date more towards the 1-on-1 (KA26-1) CCPP configurations, but at the same time, there are markets where a 2-on-1 (KA26-2) CCPP configuration is preferred. Todate, Alstom has received orders for a total of 36 CCPP projects utilising the KA26-1 (1-on-1) platform and 8 CCPP projects utilising the KA26-2 (2-on-1) platform.

Figure 1 – KA26-1 Single-Shaft CCPP

Figure 2 – KA26-2 Multi-Shaft CCPP

The current CCPP performance of these two KA26 “Reference Plant” offerings from Alstom at ISO ambient conditions are:

Net Output Net Efficiency

KA26-1

KA26-2

KA26-2 ICS

431

862

872

58.7%

58.7%

59.1%

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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Key Component Design

3.1 GT26 Gas Turbine The GT26 gas turbine (Figure 3), first introduced in the mid 1990’s, is Alstom’s advancedclass GT offering for the 50 Hz CCPP market. This GT26 is the only 50 Hz advanced-class gas turbine employing sequential (2-stage) combustion technology. This unique approach to the combustion process in the GT, applying the reheat principle, makes the GT26 particularly well suited for high CCPP base- and part-load performance. Combined with three (3) rows of variable inlet guide vanes at the inlet to the GT26 compressor, the GT26 is able to provide exceptionally high part-load efficiency in combined cycle operation, a factor that is becoming increasingly important as such large CCPP’s are being called upon to operate more and more at loads other than just base-load, including even for up to ca. 8 hours per day at minimum ‘parking’ loads during off-peak periods (such as during the night-time), which can see CCPP’s operating at loads down to 50% or below. It is easy to see therefore why partload performance is becoming a more and more important consideration for power companies planning to build new CCPP’s. The unique sequential combustion of the GT26 is also a key factor enabling the KA26 products from Alstom to be “parked” at very low CCPP load, typically around 20%. This Low Load Operation Capability (“LLOC”) will be looked at in more detail later in this paper. At the same time, in addition to being called on to operate at various loads, these advancedclass GT’s are also being expected to cycle (stop/start), potentially on a daily basis, and this in-turn poses a greater duty and burden on these engines. It is a well accepted and recognised fact that high cycling shortens the inspection interval and expected lifetime of the GT, and all OEM’s apply some form of “start” factor to their inspection interval formula to take this fact in to account. This is a factor that cannot be eliminated, but at the same time the OEM’s can and are looking at ways to mitigate this impact and to still enable reasonable lifetime expectations from the components, especially in the hot gas path zone. Advancements in the special exotic alloy materials and thermal barrier coatings are helping significantly in this respect.

EV Combustor

Compressor

SEV Combustor

Solid Welded Rotor

3 rows Variable Guide Vanes (VIGV) High Pressure Turbine (HPT)

Exhaust Diffuser

Low Pressure Turbine (LPT)

Figure 3 – GT26 Gas Turbine

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

- 5 Although Alstom recognises that ‘cycling’ duty is a requirement and expectation of today’s advanced-class GT-engines, the company is also looking at ways to assist the power companies to increase the options / choices they have with respect to the way the CCPP is operated, so as to provide power companies with greater flexibility and higher part-load performance so as to possibly avoid or mitigate the need for cycling, and the associated increased maintenance cost impact associated with such stop/start regimes.

3.2 STF15C & STF30C Steam Turbine Modules The key features of the Alstom steam turbine portfolio, which make them ideally suited for high operational flexibility, are: • • • •

Inlet module portfolio for optimal fit to all steam turbine sizes and operation modes Shrink Ring design of HP Turbines Design of IP Turbines Welded Rotor design

Operators can take advantage of very significant additional payments for frequency response support via the operational flexibility advantages derived from these key features, in particular: • •

Low start-up times Rapid loading and de-loading ramp rates

Start-up Times While the ramp rate of a steam turbine depends substantially on boiler capacities and the overall margins available in the overload valves or the throttle reserve available in the main control valves, the start-up times of an Alstom 660 MW steam turbine generator set (cold start in 240 minutes) compares favourably even with existing 210 MW machines in India (going up to 360 minutes). Figure 4 shows typical start-up times, to full load, of Alstom’s steam turbines.

Figure 4 - Typical Alstom ST Start-up Times

Shrink-Ring design of the HP Turbine The unique shrink ring design of the Alstom HP turbine was introduced in the 1960s. It eliminates bolt flanges on the inner casing and results in a radially symmetrical structure with best thermo-elasticity. Therefore, the shrink ring design prevents casing distortions almost KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

- 6 completely, which is of increasing importance with the increasing steam temperatures of modern water-steam cycles. The benefits are long-term stable clearances and sustained efficiencies combined with long-term reliability and operational flexibility. Slots maintain the steam-extraction for the top heater in the inner casing and a simple and robust shrunk-on extraction chamber located between two shrink rings. The axial position of these slots is determined through the targeted final feed water temperature. Due to the double-shell design the outer-casing is exposed to the exhaust steam only, which allows relatively small flanges at the outer-casing. A pre-heating of the casings prior to a start-up is not required. Generally, Alstom’s reheat turbines are designed in such a way that the casings do not limit thermal transients. The assembly of the HP turbine with its shrink rings is shown in Figure 5.

Figure 5 - HP Turbine Assembly showing Shrink Rings

Design features of the IP Turbine The lower pressure level of the intermediate-pressure turbine allows a horizontal split of the inner and outer casing with conventional flanges. This standard double-shell design would however lead to large casing distortions with the increasing steam temperatures of modern water-steam cycles. Alstom has made intensive investigations to reduce the possible thermal casing distortion in particular in the inlet sections where the inlet scrolls cause additional asymmetries. The inner shell geometry of the IP modules in conventional double-shell design has been optimised to counterbalance the asymmetric mass distribution caused by the flange and the inlet scrolls. The inlet section with the inlet scrolls and some first stages build the inner-casing and supports the blade-carriers for the further steam path sections. The pressure drop over the blade-carriers is comparably small and therefore, their flanges can be kept small as well. The result is an overall more rotation-symmetrical design, which reduce thermal distortions almost completely. The benefits are stable clearances and sustained efficiencies. Thanks to the inlet scrolls with the integrated radial first stationary blade row and the welded rotor design a secondary steam cooling with its negative impact on the efficiency is not required, not even for the highest reheat temperatures of today.

Welded Rotor Design The welded rotor design is a key feature of Alstom steam turbines since the 1930s. It allows the material selection of each rotor section to match the respective temperature of the steam path. Therefore, welded rotors can be easily adapted to suit the increasing steam temperatures of modern water-steam cycles. Because of the cavity formed by the two KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

- 7 welded rotor sections, the stress levels due to temperature differences are lower. This design allows a faster start-up and/or lower life consumption rates compared to mono-block rotors and suits well the water-steam cycle parameters of today and for the future. Further, the small forgings (Figure 6) are easier to obtain on the market and to test than large forgings for mono-block rotors.

Figure 6 - Alstom Rotor Forgings prior to Welding

Alstom has by far the most experience in welding of similar and dissimilar rotor materials. It is notable that Alstom’s welded rotors have never suffered rupture or similar failures since their introduction back in the 1930’s.

3.3 Heat Recovery Steam Generator (HRSG) Advanced large combined cycles continue to drive toward increased thermal efficiency. Correspondingly, the water/stream cycle, including the HRSG, has also had to evolve to capture more energy from the gas turbine exhaust. The HRSG has become larger while accommodating higher steam pressures and temperatures accepting increased gas turbine exhaust mass flow and temperature. Alstom HRSGs (ref. Figure 7) for such applications are typically triple pressure with reheat producing over 160 MW without supplementary firing. Figure 7 - Alstom 3PRH “Drum-Type” HRSG

Plant power output can be further increased with addition of (supplementary) firing in either the HRSG inlet-duct or inter-stage between HRSG tube banks. The range of operating modes for today’s plants add further demands for HRSG capability: − Grid frequency control participation, required in some countries (even when operating at nominal load), may mean transient grid frequency excursions leading to temporarily increased turbine outlet temperatures. Pressure parts and structural supports must be designed with consideration for creep at higher temperatures to endure these excursions. − Unlimited operation at a range of GT loads from 100% down to 10% requires a highly flexible means of high-pressure superheater and reheater temperature control. Alstom KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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HRSGs employ dual-stage desuperheating to maintain steam temperatures within metal design limits and avoid over-spraying to a saturated steam condition. Increased tube length, higher pressures and temperatures, frequent cycling, and different operation modes all work to fatigue HRSG pressure parts. Simple extension of existing technology from smaller HRSGs, operating under less demanding conditions, typically results in reduced availability. Alstom’s HRSG design with “Single Row Harps” and “Stepped Component Thickness” was specifically developed to be more flexible to meet the demands of advanced large combined cycles.

Design Methodology for Alstom Combined Cycle Products Figure 8 shows a typical annual load profile that is being witnessed on a large percentage of the Alstom KA26 (50 Hz) and KA24 (60 Hz) fleet. It shows that the load profile for the KA26 plants is varying greatly, including a large number of stop/starts as well as ‘parking’ at minimum load. Figure 9 is a 1 week snap-shot of a typical load regime being seen on the KA26 fleet in Europe, which also shows very clearly the different load points the plant is being called to operate at. Base-load is by no means the rule! The Combined Cycle Power Plants are being called upon very often by the grids to provide frequency support and being requested by the grid companies to run at reduced load so as to have ‘reserve capacity’ for fast response (load ramp-up) to support grid frequency disturbance restorations. Fast ramping from part-load and quick start-up from shutdown therefore is being seen more and more a factor for consideration in the design of the combined cycle power plants.

1 Year

Figure 8 – Typical KA26 Annual Load Profile (Europe)

1 Week

Figure 9 - Typical KA26 Weekly Load Profile (Europe)

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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4.1 Plant Base- and Part-Load Efficiency Continuously increasing plant efficiency levels, always striving for a better use of natural resources, is a clear trend in the power industry and Alstom is certainly one of the leading global players in this field. Fluctuating market power demand, offering an operation reserve, a frequency response reserve or low load parking during periods of low demand, all means that CCPP’s will spend more and more time at part loads. Part-load efficiency nowadays is therefore becoming a significant criterion in new CCPP projects. In contrast to other OEMs / GT suppliers, Alstom has focused its GT/CCPP development to ensure high overall plant efficiency from base-load down to very low part-loads. With its sequential combustion concept combined and multiple variable inlet guide vanes the GT26 offers a very high turn-down ratio at low NOx emissions and high exhaust temperatures, thereby permitting todate higher part-load efficiency levels than can be achieved with single combustor technology.

4.2 KA26 Turn-Down Ratio and Low Load Operation Capability Utilising the competitive turndown ratio of the GT26, plant operators have a wide field of possible operation points. In principle a turndown to around 30% GT load is possible, subject to emission limit constraints, whilst still being able to provide primary and secondary frequency response. Where the outlook on future NOx and CO emission legislation makes catalysts installation/use (or their retrofit-ability at a later date) advisable, Alstom can provide proven and reliable solutions. Period of low power demand or reduced power tariff 110 100

General Possibilities of Reducing Load Typical minimum GT load given by emission regulations & GT emission characteristic *

GT relative load (%)

90 80 70 60

KA26 LLOC: Reduced GT load at low emission levels

50 40 30

GT Shutdown and Restart: Thermal stress cycles, starting reliability, no online power reserve.

20 10 0

Time * Typical for single combustion engines

Figure 10 – The KA26 “LLOC Concept”

Alstom can even offer an option reducing the steady-state power output to around 20% relative plant load or lower for the sole purpose of “parking” the CCPP fully on-line instead of shutting down. The KA26 Low Load Operation Capability (“LLOC”) concept permits the fuel consumption during operation at minimum parking load to be reduced to a minimum. Todate, power companies have had only two options during off-peak periods, to park at a minimumload point of approximately 50% CCPP load or shut-down. Figure 10 shows the three options for the GT26/KA26 plant, namely normal operation at the high part-load point (around 50% CCPP load), complete shut-down plus the ability to park at a much lower load point (around 20% CCPP load). The KA26 “LLOC” works by having only the first-stage environmental (EV) burners of the GT26 in operation, the second-stage SEV-burners are switched off, thereby offering a feature that is “special” to the KA26. In the LLOC operation

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

- 10 mode, the KA26 plant is “parked” and the GT26 still produces sufficient exhaust energy to permit with the water/steam cycle to remain in full operation at the very low load point, and importantly the plant is still able to meet close to base-load NOx and CO emission limits. Figure 11 shows the significant difference in CCPP part-load heat consumption of the LLOC concept compared with the normal operating range.

Heat Input [MJ/s]

It is re-emphasised that the LLOC is a specific operating point for the pure purpose of providing full on-line reserve power, and when in this mode the CCPP is “parked”.

Combined Cycle Power Plant Load [% ]

Figure 11 – Typical KA26-1 Heat Consumption vs. CCPP Load Graph

Some of the salient customer benefits that can be brought by this LLOC feature are: 1. 2. 3. 4. 5. 6.

provides a true “on-line” reserve capacity due to fully functional water/steam cycle; affords shorter re-loading time to full / high load compared to a full plant shut-down; avoids risk of potential (uncertain) start-up failure at critical stage; reduces the cumulative NOx / CO emissions compared to parking at the higher more traditional part-load points; project specific solutions combining the LLOC with district heating / cogeneration can be worked-out, hence improving the fuel utilization at times of low power demand. reduces fuel consumption during off-peak ‘parking’ periods, thereby saving fuel costs during the low or negative spark spread periods.

4.3 Cycling Capability In addition to the availability of the LLOC concept, Alstom KA26 plants offer competitive solutions for daily start and stop (DSS) operation. An overall assessment of process parameters of the topping (GT) and bottoming (water/steam) cycles has led to advanced start-up concepts. Alstom’s GT26 with its sequential combustion in combination with the multiple and variable GT inlet guide vanes offers two unique characteristics.

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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The more or less constant exhaust gas temperature over a wide operation range (30100% GT load) reduces thermal transients during load changes as far as possible. − Due to the second sequential combustor system, the GT26 exhaust volume flow and exhaust temperature can be adapted to the actual start-up conditions of the underlying water/steam cycle. Amply dimensioned steam de-superheaters and steam-bypass systems (a tradition in Alstom plants) also plays an important role in the KA26 design for cycling capability. The Alstom HRSG pressure parts design is well prepared for any transient operation, which is addressed in section 5 of this Paper. Alstom developed ways to fulfil the EN12952 restrictions regarding the protection of drum magnetite layers, which allows Alstom to offer EN12952 compliant solutions with HP-drums rather than once-through evaporation systems. Further improvement of the Alstom combined cycle steam turbine portfolio will allow Alstom to offer fast start-up options from hot and warm turbine conditions (e.g. after overnight shutdown or weekend shutdown). The same design principles contribute to a reduced expenditure of steam turbine fatigue lifetime. It is worth noticing that Alstom plants can realise all of these plant characteristics without additional auxiliary systems such as: − auxiliary steam boilers − condensate polishers (for neither cycles employing drum-type nor for once-through HP evaporation concepts) Reduced capital expenditure (first, operational and maintenance) is the positive outcome for plant operators. Alstom is able to offer HRSG with either fully drum-type or partial (i.e. only high pressure) once-through evaporation technology. Once-through evaporators are currently set in operation in two plants in Germany and UK, where high efficiency 850 MW blocks are installed and commissioned. Here the selected high operation pressures had driven the decision for once-through technology. For the base fleet with single-shaft 400 MW blocks, however, Alstom does not currently see a necessity for once-through evaporation systems: − For 1-on-1 CCPP applications the impact of temperature on efficiency exceeds the influence of HP system pressure. Actually the pressure / efficiency curve is relatively flat with a cost / efficiency optimum around 140 bar (depending on alloy material cost). − Drum-type applications provide a more robust chemical/steam purity behaviour characteristic, since drum blow-down is always possible. Condensate polishing is neither required for commissioning nor for commercial operation. − Alstom drum design and operation concepts allow for fast-start-up procedures and daily start-stop operation regimes. − The start-up and part load behaviour of a drum-assisted evaporator is easier to control than the one of a once-through evaporator with separation bottle. − Drum-type HRSG’s achieve at least the same part-load efficiency as those equipped with once-through technology. − Alstom HRSG design employing HP-drums even allows for additional supplementary firing sufficient for most markets’ needs, as and when required. − The HP once-through evaporation system, at the selected moderate pressure levels, has no considerable first or secondary cost advantage over the drum-type design. − Once-through evaporators can be subject to dry-out at elevated steam qualities, which leads to reduced inner heat transfer coefficient and requires additional heat exchange surface.

KA26 Combined Cycle Power Plant Hannover Messe - Power Plant Technology Forum 2011 as Ideal Solution to Balance Load Fluctuations 5. –8. April 2011, Hannover, Deutschland © Alstom 2011. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

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4.4 Grid Frequency Support With its competitive turndown ratio outlined above, Alstom KA26 plants offer a unique frequency response load range. Alstom HRSG’s in KA26 plants are ready to accept high GT exhaust temperatures, which allows maintaining power output characteristics according UK National Grid Code requirements, one of the most demanding grid codes worldwide. Steam turbine assistance in frequency primary response is another optional feature developed for KA26 plants. It allows the water/steam cycle to contribute to the active grid frequency support of the plant. As an additional feature, the steam turbine may also be used to limit frequency excursions during partial load rejection in small island grids.

4.5 Power Augmentation Air inlet cooling (AIC) by water injection is a well-established measure for gas turbine power augmentation. By cooling down the inlet air, the GT air mass flow is increased and the compressor work reduced. While fogging saturates the intake air, high fogging injects additional water, which evaporates primarily in the compressor. Alstom started back in 2000 its in-house GT inlet cooling development program for evaporative coolers, fogging systems and high fogging systems, with the goal to achieve the optimum performance and availability with a safe operation of the respective systems together with the gas turbine. Theses systems have been validated and today are offered by Alstom as standard options for the Alstom KA26 product offerings. These GT inlet cooling options permit more than 20% power boost. Figure 12 shows the locations in the air intake for the GT inlet cooling options.

Figure 12 – GT Inlet cooling option locations

The fogging and high fogging systems inject small water droplets (Dv90