Further advanced versions of the Siemens V94.3A gas turbine are being developed and tested at full output under commercial power generation conditions at the Cottam Development Centre, which is in the neighbourhood of PowerGen’s existing Cottam coal fired power station in Nottinghamshire, United Kingdom. The prolonged trials constitute a preparatory phase for the introduction of new models.

The new plant was synchronized on 1 March 1999 and has been feeding power to the National Grid Co’s system since then. It is operated by Cottam Development Centre Ltd, a joint venture between PowerGen and Siemens Project Ventures, in which both companies own equal shares.

Through evolutionary advances in the design of the major components and further development of the combined cycle process, the immediate objective is to use the experience gained at the Cottam facility to raise plant efficiency to around 60 per cent with an increase in power output from 390 MWe to about 500 MWe, while at the same time reducing the plant’s power generating costs.

The joint venture

The contract to build the Cottam Development Centre was signed by the joint venture between Siemens Project Ventures and PowerGen on 6 May 1997 following receipt of full planning clearance including section 36 planning consent and DTI consent to build the new gas pipeline required.

PowerGen has a long track record of working with Siemens since its inception following the privatization of the UK power supply industry, starting with the first major combined cycle power plants at Killingholme and Ryehouse and continuing with key overseas power projects such as Tapada do Outeiro in Portugal, Paiton in Indonesia and Paguthan in India.

CDC – the Cottam Development Centre – is a profit centre which competes in the competitive power market as the generating station operator. PowerGen and Siemens have financed the plant from internally generated funds with funding nominally split 80 per cent debt and 20 per cent equity.

Operation is divided into two distinct phases under the agreements: a test phase during which the current state of the art V94.3A is operated in conjunction with the innovative once-through Benson type HRSG; and a second phase in which the more advanced new gas turbine is installed for development and testing.

The first phase is intended to raise revenue which will go a long way towards covering the investment costs out of profits while at the same time verifying the performance and reliability of the plant. An extraordinary level of additional instrumentation and monitoring equipment will be installed and operated at an early stage in this phase.

In the second phase, the new advanced large scale gas turbine combined cycle system will be tested and operated under commercial conditions to verify its design and performance before introducing it to the market. The plant will be supplied with natural gas from the North sea via a new 24 km underground pipeline from the national gas transmission system at Blyborough in Lincolnshire. This pipeline is owned and operated by PowerGen.

When the pipeline was conceived, PowerGen planned to convert two of the coal fired units at Cottam, and also two units at their Fiddlers Ferry coal fired station, to natural gas firing, but these applications have been rejected under the UK government’s de facto moratorium on gas fired power plants, reducing the prospects of fuel arbitrage.

PowerGen supplies natural gas to the joint venture under the terms of a toll process agreement (TPA) under which PowerGen pays CDC a capacity fee based on the availability of the CDC plant. In return, PowerGen gains the income from the electricity generated. The TPA also places certain obligations on PowerGen to operate the plant in a way which will facilitate Siemens’ development activities.

PowerGen supplies the gas to CDC from its general gas portfolio, part of which derives from long-term supplies already contracted by PowerGen. Although PowerGen has no long term gas contracts associated directly with the supply of gas to CDC, it does manage the total gas requirements of CDC, including day-to-day management of the gas markets. In effect, this is a toll process agreement in which PowerGen take the electricity and gas market risks in return for the market rewards.

Project philosophy

With a view to proving commercial operational performance and competitive market penetration, technological advances for Siemens’ new large utility gas turbines as well as in-service O&M strategies and economic optimization will be tested in fully commercial, competitive field service conditions over significant service life duration in the United Kingdom electricity supply industry.

Equally as important as the gas turbine development, a new, first-of-a-kind, concept of once through heat recovery steam generator will be tested in actual service. Optimization of the entire combined cycle system will also be a major function of the development programme.

The facility is located near to the site of PowerGen’s 2000 MWe Cottam coal fired power station, some 17 km north-west of the city of Lincoln in the heart of England’s most intensely power station populated region between the South Yorkshire and Nottinghamshire coal fields. It will have to compete with coal fired power plants which are now fully amortized and therefore supply electricity at little more than the cost of the fuel and very high availability.

Although Siemens operates a gas turbine test facility at its development and manufacturing centre in Berlin, on which the V84.3 prototypes were originally tested, this test bed can no longer accommodate the increased full output power of the advanced V94.3A machines. Furthermore gas turbines can only be tested on this facility for a limited number of hours due to temporarily limited gas supply, a disadvantage in case of creep effects, which only become evident under continuous operation.

Valuable earlier experience with the V94.3 and first generation V94.3A systems was gained on the commercial installations at, Didcot and Seabank in the UK. The first V94.3A for gas turbines for commercial service were supplied to National Power’s Didcot B site in Autumn 1996. The first and second generation of the V94.3A gas turbines.

It is the second generation of V94.3A gas turbines, which is now running in Cottam during Phase 1. Three similar gas turbines of the same output are being installed in Scottish HydroElectric’s Peterhead repowering project described in Modern Power Systems, August 1998.

In Phase 2, a further advanced gas turbine will be used in a combined cycle in which the power output will be raised from the present 390 MWe to nearly 500 MWe with a thermal efficiency increase from 58 per cent to 60 per cent.

In order to exceed the 60 per cent efficiency mark, it is necessary to get the maximum benefit out of the higher exhaust-gas temperatures and exhaust-gas mass flow rates of advanced gas turbines. To achieve this, the cycle as well as the key components, including new designs of once through heat-recovery steam generators and steam turbines with more advanced aerodynamics, have to be further improved.

Second generation V94.3A

In the annular combustion chamber with its 24 hybrid burners for diffusion and premix combustion of natural gas and distillate, all-ceramic combustion-chamber tiles are used instead of the ceramic-coated metallic heat shields. These tiles need less air for cooling, which in turn makes more combustion air available for the hybrid burners, which produce less thermal NOx as a result. Potential thus exists for increasing the turbine inlet temperature.

The first and second stage turbine blades are protected by improved ceramic thermal barrier coating which lowers the temperature of the underlying metal blade surface, thus also providing potential for increasing the turbine inlet temperature.

The compressor of the introductory V84.3A gas turbine featured the advanced flow path design with 15 compressor stages but it still had a welded outer casing design with four adjustable guide vane rows.

The current design of units for both 60 and 50 Hz applications features the most advanced 15 stage compressor design with the cast casing with only the first compressor guide vane row being adjustable. The compressor casing has been designed as a three dimensional system which provides not only stiffness to carry static and dynamic loadings, but also expands in a fully concentric manner without adversely affecting the operating clearance in axial and radial directions. Blade carriers with external clearance adjustment are utilized for the latter section of compressor blading and for the turbine section to handle high temperatures and still keep minimum clearances.

Single-shaft combined-cycle

The Cottam combined cycle plant is based on the single-shaft, combined-cycle concept as shown in the block schematic.

The gas turbine, generator and steam turbine are coupled together on a single shaft and form a modular, standardized system. The single-shaft concept is also characterized by simplification and reduction in the number of components of auxiliary plant systems, low plant complexity and the resulting simplification of open and closed loop control functions.

Plant complexity was further reduced by eliminating redundancies of pumps and auxiliary and ancillary systems, and this is not expected to adversely affect plant availability, but installation space, auxiliary power requirements and investment costs are substantially reduced.

Combustion air enters the gas turbines through huge side entry air filter units, and the exhaust steam from the steam turbines exits axially through to the water-cooled condenser.

The high-pressure section of the water/steam cycle was designed for an operating pressure of up to 160 bar so that better use is made of the high temperature heat due to the lower evaporation enthalpy resulting from the higher boiling temperature. If volumetric flow rates remain sufficiently high, no decrease in expansion efficiency is expected.

An SSS clutch is used to allow operation of the gas turbine independently of the steam turbine.

A monumentally large exhaust gas diverter valve, installed to enable gas turbine testing without the steam system running, was originally intended to be removed for the second phase of operation. It has now been decided to retain the damper and diversion stack for possible future use. Built by Rako in Düren, the damper has a single flap, for open/close operation only, driven by two gigantic hydraulic cylinders an each side of the casing.

New HRSG concept

A triple-pressure reheat steam cycle with gas preheating was selected as the optimum solution.

The increase in efficiency gained by further increasing the number of evaporator stages in the heat-recovery steam generator drops off as the gas-turbine exhaust-gas temperature increases, making it no longer economic. The horizontal triple-pressure heat recovery steam generator supplied by Babcock Borsig makes use of a new concept combining a natural-circulation system for the low-pressure stage with a Benson type once-through, forced-circulation system for the intermediate and high pressure stages. The Deutsche Babcock group held a Benson technology licence from Siemens AG from the earliest days.

All major utility gas turbine manufacturers are currently having to resort to some form of once through HRSG design to withstand the higher exhaust gas temperatures now being experienced. It is the use of the high exhaust gas temperatures which places more emphasis on the complete gas cycle system for improvements in thermal efficiency rather than the gas turbine.

This trend calls for the use of high grade stainless steels in the construction of the HP boiler tubes, but the new systems benefit from greater simplicity and reliability. Some manufacturers claim that, in an emergency, the boilers can be allowed to run dry without damage.

The design of the once-through system is similar to that of the natural-circulation system, but does not require a drum. In terms of flow dynamics, the evaporators are designed for low pressure losses so that the mass flow rate in the series-connected tube bundles adapts itself to the amount of heat provided by the exhaust-gas mass flow. A cross section of the boiler showing how accommodation for the SCR, and ammonia feed if required, is located between the IP evaporator and HP evaporator tube banks. The advantage of this arrangement is that despite the decreasing amount of heat in the latter rows of tubes, the differences in steam content and temperature at the outlets of the tube bundles remain very small. The new boiler exhibits improved dynamic behaviour, short start-up times and great potential for future technical development.

Steam turbine improvements

The currently installed steam turbine is designed for main steam conditions of up to 160 bar/600°C. The turbine is able to withstand the stresses produced by the higher pressures and temperatures primarily because high grade steel is used, but also through the incorporation of targeted design features in the inlet areas of the turbine cylinders.

The single-flow high-pressure turbine cylinder has a service-proven barrel-type casing. The rotationally symmetrical construction without interfering mass concentrations caused by axial flanges results in a highly compact design. The advantage of this concept over jointed casings becomes more significant as steam conditions increase.

The combined intermediate-/low-pressure turbine cylinder is currently designed as a reverse-flow turbine with a split casing. The use of a new IP/LP turbine cylinder with welded rotor, straight-through flow path and longer titanium blades for the last stage is planned.

Operating regime

In the first phase, beginning in September 1999, the power plant began producing electrical power commercially under grid guidance.

When the author visited the plant, the gas turbine had already been run up to full load, and the commissioning staff declared that they were quite satisfied with the performance so far. Steam blow-out had been completed on the heat recovery steam generator. Efforts to increase the gas turbine inlet temperature and to optimize the combustion process will continue in accordance with a well-defined schedule. New maintenance concepts will be tested, and the operating performance of the new components will be observed and further improved.

A breakdown of the plant areas in which the performance improvements are expected to be accrued.

By the end of the year 2000, enough meaningful data should have been gathered to allow further increases in efficiency and power output to be achieved.