Gas Turbine Technology- San Roque: a Spanish first

The San Roque power plant in Càƒ¡diz, Spain was the first KA26-1 combined cycle plant to enter commercial operation in Spain. A year later, the project set another ‘first’ when a gas turbine at the site became the first GT26 to receive the compressor upgrade for increased power output and efficiency.

Officially opened in July 2002, the San Roque power plant was the first KA26-1 combined cycle plant to enter commercial operation in Spain. The 800 MW plant, located in Càƒ¡diz province, southern Spain, is owned by Endesa and Gas Natural and is an important asset in the high load demand region of Andalucia. While significant as Spain’s first combined cycle plant of this type, the plant was also the first to see the installation of the compressor upgrade package on a GT26 gas turbine.

The construction contract for the plant followed an agreement between Endesa and Gas Natural signed on October 14, 1998. Under the agreement, the two power companies planned to build four 400 MW combined cycle units ” two at San Roque and two in Sant Adriàƒ  de Besàƒ³s (Barcelona). The construction contract for the projects at both sites was awarded as a turnkey contract to Alstom Power in July 1999.

San Roque comprises two 400 MW combined cycle blocks ” unit 1 owned by Gas Natural and unit 2, owned by Endesa. It represents a total investment by Gas Natural and Endesa of a340 million.

The plant was constructed during the March 2000 to February 2002 period. During this period, some 1.9 million man-hours were worked. Construction of the plant went according to schedule, with first synchronization of unit 1 to the grid in March 2002; and first synchronization of unit 2 to the grid in May 2002. Unit 1 began commercial operation in June 2002 and unit 2, a month later in July.

Plant design

The project sits on a plot size of 15 ha in Poligono Industrial Guadarranque, in Càƒ¡diz. The plant is based on Alstom’s standardised combined cycle reference plant concept. Under its contract, Alstom supplied two single shaft power trains each composed of one GT26 gas turbine, an associated heat recovery steam generator, a steam turbine and an electrical generator as well as the overall control system and balance of plant.

The single shaft concept means that the two units of the plant can be operated independently. Each block has its own control room and Endesa and Gas Natural can dispatch power to the grid as it suits them.

Figure 1. Site plan of the San Roque power station
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Power is dispatched from the plant via a 6.1 km, 220 kV line to Pinar del Rey substation.

As turnkey supplier, Alstom was responsible for the engineering, procurement and construction of the plant, additionally the two clients have an Alstom operation and maintenance contract.

The KA26-1 configuration chosen for San Roque has the following main characteristics:

  • Operational flexibility ” each unit is capable of running at full combined cycle load and part loads. Further, the plant can operate in baseload as well as in two-shift operation. Independent startup of the gas turbine reduces both startup times and startup fuel costs. No auxiliary boiler is required.
  • High efficiency at full load and part-load ” at 40 per cent load, the plant still achieves about 85 per cent of its nominal full load efficiency.
  • Low emission levels throughout a large load range
  • High reliability and availability ” all components are of proven, simple design. This minimizes operation and maintenance requirements and increases reliability and availability.
  • The plant can operate in frequency support mode
  • Low operating costs.

Unlike other power stations with more than one single shaft power train, San Roque was set up as a complete power station. With the exception of the make-up water system, nothing is shared between the two units. The plant is laid out with the two units adjacent to each other. The two sets of cooling towers are arranged parallel to the power trains. Each group of cooling towers is arranged on either side of the water treatment plant and demineralized water tank (see site layout). The cooling towers supplied by Hamon are 5-cell units of the wet mechanical draft cooling type. The back-up fuel oil tanks are at one end of the site.

Figure 2. Cross-section of the GT26 showing the sequential combustion system
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Gas turbine: The power trains use Alstom’s heavy duty GT26 gas turbines. They are designed to run on natural gas as the main fuel or on fuel oil No. 2 as a back-up. These turbines use a sequential lean pre-mix dry low NOx combustion system. Sequential combustion involves injecting fuel twice in the combustion process to increase output and cycle efficiency without increasing the gas turbine firing temperature.

The combustion system comprises two combustor-turbine pairs in series, where the exhaust gases from the first turbine feed the combustor of the second.

A 22-stage sub-sonic compressor feeds combustion air into the first combustor at twice the normal pressure. Here, fuel is mixed with the high pressure air and burns in the first combustor ” the annular EV (Environmental) combustor. The hot gases drive a first turbine, the single-stage high pressure turbine.

Unlike conventional turbines, fuel is injected in a second burner set and ignites spontaneously in the following annular combustion zone ” the SEV (sequential EV) combustor, thereby reheating the air before expanding it further through four low pressure turbine stages.

Figure 3. Cutaway of the GT26 gas turbine
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The EV combustor has an annular arrangement. The GT26 is fitted with 24 retractable EV burners. Each of these burners operate over the whole load range. Compared to other combustor arrangements, the annular combustor is designed to give a more even temperature distribution of hot gas in the circumferential direction.

Radial temperature uniformity is accomplished by pre-mixing virtually all incoming compressor air with the fuel in the EV burner, and by the absence of film cooling in the convection-cooled combustor walls. This produces a single, uniform flame ring in the free space of the EV combustion zone. The flame has no contact with the walls of the burner.

In the annular SEV combustor, the combustion process is repeated in a similar fashion as in the EV burner: vortex generation, fuel injection, pre-mixing and vortex breakdown. The SEV combustor consists of annularly distributed burners, followed by a single annular combustion zone surrounded by convection-cooled walls. Exhaust gas from the high pressure turbine enters the SEV combustor through the diffuser area. Combustion temperature uniformity in the SEV is determined by the spatial homogeneity of the fuel/air mixture which is again accomplished through the use of vortices. Each SEV burner contains wings, formed like ramps on all four of the burner interior walls, which swirl combustion air into the vortices.

Fuel is then injected through air-cooled fuel nozzles which distribute it to form a fuel/air mixture prior to combustion. The fuel jet is surrounded by cool carrier air which postpones spontaneous ignition until the combustion zone, beyond the burner area. Here the vortices breakdown, and like in the EV combustor, combustion takes place in a single, stable flame ring, operating across its entire load range.

The use of dry low NOx burners means there is no need for water or steam injection to control emission levels.

Water/steam cycle: The water/steam cycle is a state-of-the-art triple pressure reheat cycle with a drum type HSRG. The HP steam supplied by the HRSG is admitted to the HP turbine through one main stop valve and one main control valve. From the HP exhaust side, the cold reheat steam enters the reheater via a power-assisted check valve. The hot reheat steam is admitted to the IP-turbine by two intercept stop and two control valves. At the LP section, a third steam supply is admitted through butterfly-type stop and control valves. The expanded steam flows from the LP turbine into the condenser. The LP turbine extraction leads to the deaerator.

The HRSG is a natural circulation, triple pressure unit with reheat. It is a vertical type unit where exhaust gases from the gas turbine have an upward path while crossing the different heat exchangers. These are made of finned horizontal tubes, with return bends, between an inlet and an outlet header.

The reheater is used to reheat the medium pressure steam coming from the ST HP part exhaust and the medium pressure steam, up to the required superheater temperature (about 537à‚ºC and 116 bar). The steam cycle has a conventional deaerator mounted on the HRSG and a condensate preheater in the top of the unit. There is a recirculation loop to maintain the deaerator inlet temperature when burning oil and to prevent dew point corrosion at low temperatures.

Steam turbine: Each ST at San Roque is a floor-mounted, triple-pressure reheat steam turbine. Due to the high temperature and pressure, the HP turbine is of a double-shell design. The HP outer casing consists of an upper and lower half made of cast steel. The casing is split on the level of the shaft centre. The HP inner casing is also horizontally split and made of cast steel. The design is compact and has a high thermal flexibility. The live steam admission to the turbine is realised with a 360à‚º spiral into the inner turbine casing. This results in optimized steam flow to the blading and thus a low pressure drop.

The power range of the steam turbine and the exhaust pressure allows the use of a single flow LP turbine. An axial exhaust leads directly to the axially arranged condenser.

Due to the high temperature loading, the IP section also has a double-shell design. The IP outer casing consists of an upper and a lower half and is made from nodular cast iron. The inner casing is made of cast steel. The hot reheat steam admission to the turbine is realized with two 180à‚º inlet spirals into the turbine inner casing. Again, this promotes low pressure loss.

Table 1
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The LP outer casing part is of the welded design and split horizontally on the level of the turbine shaft centreline. To prevent overheating, a water injection point is located in the exhaust area of the casing.

Figure 4. Flow schematic
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Generator: Each power train uses a hydrogen-cooled generator IEC rated at 500 MVA and 21 kV. The generator is common to the gas turbine and steam turbine. One end is connected to the cold end of the gas turbine and the other end to the steam turbine. The ST to generator connection is through a synchronous self shifting clutch (SSS) clutch. It engages automatically at the nominal speed of the ST during startup and disengages automatically during shut down of the ST. This allows independent operation of the gas turbine during startup or steam turbine bypass operation. This eliminates the need for an auxiliary boiler, which would be required if the ST was directly connected to the GT.

Gas turbine upgrade

Prior to acquisition by Alstom, ABB developed two similar combustion gas turbines: the GT24 for the 60 Hz market and the GT26 for the 50 Hz market. The “A” series was launched in 1995.

By the time the “B” version was introduced in late 1999, the main technology differentiator ” the sequential combustor ” had already proven itself in the “A” units. The “B” version includes higher output through modified burners and improved turbine dynamics and cooling. However, three technical issues became apparent at different stages during the beginning of the “B” series operation. While solutions to the problems were being developed, it was necessary to de-rate the units operating in the field. Alstom embarked on a programme which would address the technical issues, while improving performance and availability.

An agreement was put in place with Rolls-Royce (R-R) at the end of 2001, which allowed R-R engineers to make significant contributions in component evaluation methods, reviews and audits and thermal validation techniques. R-R methods have been used to cross-calibrate turbine component design, engine airflow modelling, and engine performance modelling.

In one of the programmes, the existing 22-stage compressor was upgraded for an increase in mass flow of about 5 per cent. This upgrade consists of an improved and optimised airfoil design and a re-staggering of the blades.

This means Alstom engineers have developed a design which requires no modifications to the rotor and only minor machining to the compressor vane carriers to optimize the blow off throat area. Blade and channel height and length, rotor vane fixation grooves and blade and vane materials all remained unchanged. The exhaust housing on the engine was also aerodynamically improved to accommodate the higher mass flow.

The improved compressor, re-designed in 2000/2001, was ready for validation in early 2002. It adheres strictly to a revised design procedure, which includes multiple external design reviews and a full validation phase.

Table 2
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To ensure comprehensive compressor testing, Alstom installed 400 additional probes along the full length of the compressor. The compressor was installed in the GT26 engine at Alstom’s Test Centre in Birr, Switzerland and testing began in June 2002 and ran until October 2002. The comprehensive test schedule covered startup and variable guide vane optimization, combustion mapping, part-load and base load measurements and transient and protection tests. Compressor mapping runs also included, as a final test, a surge test where the engine was run into surge conditions to confirm the safety margins. According to Alstom, the initial design settings were close enough that the engine achieved a successful startup to idle at the first attempt.

Results at Alstom’s Test Centre confirmed predictions for mass flow and therefore power output; gas turbine efficiency and start-up capability under various conditions. Mass flow increased by 5-6 per cent, resulting in a combined cycle power increase of about 5 per cent. Operation was also tested in all the required modes including dual fuel operation and fuel switch-over.

With the main programmes completed, implementation then began in the field.

Figure 5. GT24/GT26 upgrade programme
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In April 2003, the installation of a compressor upgrade package for the gas turbine was combined with a normal gas turbine overhaul outage on Endesa’s San Roque unit 2 to increase the power output of the unit.

At the time, the San Roque engine became the third field engine and the first GT26 to receive this power upgrade, following the GT24 engines at Monterrey (Mexico) and Lake Road (USA).

According to Alstom, installing the compressor itself was straightforward and consisted of exchanging the blading on the rotor and vane carriers. Only minor machining on the vane carriers was necessary and this was completed close to site.

The engine started hot commissioning on May 29, 2003 and by June 1, 2003 the compressor run-in process was completed. The run-in procedure advanced according to plan.

The remainder of the hot commissioning including combined cycle optimization, progressed according to schedule. Due to the high load demand in the Andalucia region, the plant commenced commercial operation on June 13, 2003, directly after completion of performance tests. The tests showed that the output was above the guaranteed values as defined in the original contract. Overall, the package led to a combined cycle output increase of about 25 MW (over 6 per cent). Before the upgrade, the net power output was about 375 MW and efficiency of just over 56 per cent. After the upgrade this increased to nearly 400 MW at an efficiency of more than 57.5 per cent.

In addition to the compressor, an inlet fogging system, developed by Alstom and AXEnergy, was installed and commissioned in October during the first B inspection i.e. a visual inspection of the hot gas path.

The inlet fogging system works by injecting water as a fine spay into the inlet air upstream of the GT inlet. Evaporation of this water causes cooling of the inlet air with a corresponding increase in mass flow and hence GT power. This system can deliver up to five per cent additional power depending on the ambient humidity conditions.

The engine was commissioned on fuel oil No. 2 in October 2003. Fuel switch-over from gas to oil and vice-versa is implemented and available.

With the implementation of the upgrade compressor, the San Roque plant is an important asset in the plant owner’s power generation portfolio. The additional benefit of the inlet cooling system allows for much greater flexibility and increased earning opportunities during periods of peak demand.

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