Private power producer abandons status quo, reaps success

PowerGen, a CEGB dissolution offspring, adopted an innovative

strategy en route to building the award-winning Rye House Station

Edited from an article by PowerGen plc. and Siemens AG, Germany

The dissolving of Great Britain`s national electric utility, the Central Electricity Generating Board (CEGB), brought many changes to the power generation industry in that country. This new industry landscape was the setting for the emergence of a new private generating company, PowerGen. No longer bound to ways from the past, the utility was free to try new ideas in producing electricity.

Early in the new utility`s formation, PowerGen scrapped CEGB plans to build new 900-MW coal-fired generating units. Instead, the utility pursued its primary aim of becoming Great Britain`s lowest-cost electricity producer and decided to replace old units with modern, gas-fired, combined-cycle installations. The utility`s approach to building one of these installations, the Rye House Station, earned a 1994 Project of the Year Award from Power Engineering International.

New ways yield results

CEGB standard practice when building a new power station was to issue comprehensive specifications with stringent plant design criteria and standards. Then, CEGB would place a separate contract with a different British supplier for each plant area. PowerGen pursued a radically different contract strategy by issuing a single, concise, functional specification for a complete power station to a number of prequalified manufacturers from both within and outside Great Britain. The specification encouraged bidders to offer their best proven generating equipment in standard configuration to provide about 700 MW of station capacity with the least possible customization.

PowerGen used this strategy in pursuing a contractor for the Rye House Station. It carefully screened contract tenders based on through-life cost, including capital and operating costs, over a projected 25-year period.

On April 30, 1991, PowerGen awarded a turnkey contract for the entire 700-MW station to Siemens. A turnkey approach minimized PowerGen`s exposure to the financial and program risks associated with such a major construction project and greatly reduced its in-house project management staffing requirements. The contract also relieved PowerGen of detailed engineering design work, freeing management to monitor project progress closely as an informed client and to concentrate on applying for the numerous necessary consents and approvals.

Although PowerGen entrusted Siemens with the responsibility for completing the Rye House project in compliance with the contractual specifications and schedules, PowerGen played an active role in project management. PowerGen reserved the right to approve plant design principles, while waiving the right to approve detailed drawings, choosing to review them only. Monthly project review meetings, held alternately in England and Germany, promoted close cooperation between PowerGen and Siemens and kept the project firmly on track and ahead of the completion schedule.

Siemens completed the Rye House Station in 30 months (Figure 1). The company occupied the site in August 1991, and the first gas turbine was delivered on Jan. 6, 1993, six weeks ahead of schedule. Plant commissioning was strongly interlinked because, due to dispensing with bypass stacks, each boiler had to be capable of raising steam before the associated gas turbine could produce substantial power. The first gas turbine was synchronized on July 1, 1993, the day grid connection became available, and rapid commissioning of the other two gas turbines and boilers allowed the steam turbine to be synchronized on August 25.

Plant shake-down trials ensued with full-load operation beginning on September 22. After successful completion of the reliability-run period and verification of the guaranteed thermal performance, PowerGen officially took over the station on Nov. 9, 1993, more than two and one-half months in advance of the contractual commitment.

One-man show

The inherent simplicity of the unfired combined-cycle design together with fully automated functional-group control technology substantially reduced the necessary operating and maintenance personnel. PowerGen set the total permanent staffing levels for Rye House at 33, roughly one-tenth the staff of an ex-CEGB coal-fired plant of comparable size. The station manager has four senior staff members and an administration staff of five. Most of the remaining staff members have operational backgrounds. Divided into six teams of three, one of the operating teams works regular daytime hours, whereas the other five teams man three shifts every day. In addition, there is currently a staff of five dedicated to maintenance. PowerGen expects a high degree of flexibility in the workers, who are required to fill a variety of roles.

Rye House operators control the combined-cycle block in the station`s central control room from a single desk with eight color visual-display units (VDU) which allow touch-pen control. The station services and grid interface are controlled from an additional desk with VDU screens.

In the unlikely event of a total data-bus failure, operators can use analog instrumentation and switching devices to control the combined-cycle block. Fully automated startup and control permits one person to start and fully load the block in 68 minutes after an overnight shutdown (warm start), and in about 150 minutes after a lengthy standstill (cold start). Generally, only one operator is needed in the control room.

Target performance on the mark

PowerGen tracks the operating performance of the Rye House Station, which has a nominal capability of 704 MW, on a monthly basis. In two periods covering the first 14 months of commercial operation, the station`s generation factor (total MWh sold divided by the product of 704 MW and hours in the month) averaged 89.0 percent and 90.4 percent. Planned and unplanned outages, less than full-load demand, high ambient temperature, low atmospheric pressure and gas supply interruptions prohibit achieving a 100-percent generation factor. However, PowerGen is confident that by resolving the problems that arise as the equipment matures in service and by carrying out promptly and thoroughly all the maintenance required to ensure optimum performance, it can surpass its long-term generation-factor targets (89 percent annual, 92 percent during winter peak months). Not a single forced outage of the whole block occurred during the last nine months of 1994.

The maintenance recommendations for all Siemens gas turbines are based on equivalent operating hours (EOH). EOH reflect the actual design-life expenditure incurred by all steady-state and non-steady-state thermal stresses during operation. All fired service hours up to base load are considered to be simple EOH, whereas all fired hours at higher load output (peak-load operation) are multiplied by four to give an EOH figure which takes into account the greater stresses due to higher turbine-inlet temperature.

Appropriately larger weighting factors are applied to rapid temperature transients. For example, a factor of 10 is used for each startup and for each subsequent emergency loading at a steep gradient (30 MW/min). In addition, a computer evaluates the impact of all sudden operational temperature changes, typically caused by large load steps and trips. The most severe events, rejecting full load down to unit auxiliaries load or to zero, amount to a maximum of 90 or 140 EOH respectively.

Table 1 lists the EOH accumulated by all three gas turbines from first synchronization in July 1993. The operating data indicate the periods before and after Siemens` officially handing over the station to PowerGen. Optimizing the overall combined-cycle block performance necessitated frequent starting up of the gas turbines and resulted in numerous trips. Hence, during plant commissioning and the trial-run periods, the EOH are typically more than double the number of service hours. This ratio dropped below 1.2 after the combined-cycle block commenced commercial operation. It should sink in time toward 1.1, which is typical of base-loaded gas turbines with a reliable fuel supply and a stable non-erratic system load demand.

PowerGen carried out the first routine minor inspections of all three gas turbines at Rye House during a nine-day station outage in May 1994. Each turbine had accumulated more than 8,000 EOH. Inspectors found the machines to be in satisfactory condition. A few flame-cylinder tiles with surface cracks needed replacing, and there was some fretting of combustion-chamber internals. Although there was evidence of minor foreign-particle damage to one of the compressors, the VPS-coated turbine blades were in good condition. PowerGen had all three boilers inspected simultaneously. Inspectors granted certificates for 38 months of further operation before the next statutory boiler inspections. The gas turbines are scheduled for major inspection in 1996, when they will have accumulated about 25,000 EOH.

Despite problems, expectations fulfilled

Comprehensive factory testing of the entire control system was conducted before shipment to the site. This resulted in its working well from the beginning. Although minor problems occurred with spurious closures of the gas supply-pressure slam-shut valve, the first hot start took only two hours, and spurious trips have since become a rare occurrence. The closures were due to leakage past the control valves when the valves were in the closed position. Currently, about two-thirds of block starts are trouble free, with the other starts resulting in an almost 0.5-percent generation-factor deterioration. Since the station is continuously base-loaded and thus is rarely off line, there is little opportunity to make and test the software modifications required to optimize the start-up performance under all operating conditions.

The most significant problem at Rye House was the repeated failure of the gas-turbine generator flexible connections. These connections link the stator to the isolated-phase busbars. An improved connector design rectified the problem. Another problem was leaky boiler tube-to-header welds. This required the reworking of about 20 welds on site, and there have been no subsequent boiler tube leaks. The utility has also encountered difficulties with water chemistry. However, this has not adversely affected boiler operation. In addition, operators experienced resin fouling in the water-treatment plant.

Deaeration at Rye House is done by extracting air from the air-cooled condenser. The condenser is remarkably quiet in operation; low-speed (68 rpm) fan noise is barely audible at ground level directly below the fans. Heat-exchange surface fouling is minimal even when the semiautomatic cleaning system is not employed. Although the condenser has failed to establish the predicted backpressure levels under summer conditions and target dissolved-oxygen levels are not being achieved, its performance is satisfactory.

To date, Rye House Station has fulfilled PowerGen`s expectations in all aspects. Since the combined-cycle block operated satisfactorily with high reliability from startup, the utility is confident that the station will significantly reduce the cost of generating electricity to the mutual benefit of PowerGen and its customers.

Inside Rye House Station

The Rye House Station lies in the River Lea valley near Hoddesdon in Hertfordshire to the north of London. It covers an area of six hectares and features a 700-MW GUD 3.94.2 block comprising three Model V94.2 gas turbosets with associated heat-recovery boilers and a single non-reheat steam turboset with an air-cooled condenser. All the gas turbines are in one building under a single traveling crane with a lifting capacity of 60 tons. A separate building houses the heat-recovery boilers. Boiler stacks are 65-meters (m) tall. The steam turbine building with its control-building annex is located beside the gas turbine building. This configuration allowed all four main step-up transformers to be lined up in a row opposite the 400-kV banking compound.

Rye House Station?s three Model V94.2 gas turbines have a nominal International Standards Organization (ISO) rating of 154 MW at their generator terminals. Despite the heat-recovery boilers elevating their exhaust pressure, the turbines? site output amounts to about 155 MW each in combined-cycle operation. This higher output is due to the design ambient temperature being only 8 C, i.e. 7 C lower than the 15 C ISO level. Each turbine is equipped with a pair of off-board silo-type combustion chambers with a total of 16 hybrid burners for dry low-NOx emissions control. The design temperature of the 513 kilograms per second (kg/s) exhaust-gas flow is 548 C at base load.

Babcock Energy Ltd. manufactured the three heat-recovery boilers. The boilers are unfired, dual-pressure, assisted-circulation, drum boilers with finned-tube heat-transfer surfaces. The heat-transfer surfaces are arranged in horizontal banks in the vertical gas-flow path. Separate high-pressure (HP) and low-pressure (LP) boiler feedwater pumps draw from a storage tank common to all three boilers, and condensate from the steam turbine is deaerated in the air-cooled condenser. The design boiler stack exit temperature is 102 C.

The 254.5-MW, two-casing, condensing steam turbine is designed for full-arc admission of 195.5 kg/s main steam to the single-flow HP section and 236.5 kg/s steam to the two-flow LP section, with a total exhaust annulus area of 20 m2. Steam condition at the HP and LP control valves is 76 bar/528 C and 6.1 bar/194 C respectively. The steam bypass station is designed to handle 100 percent of the full-load steam output of all associated boilers. A 7-m duct exhausts the steam at 0.092 bar to the air-cooled condenser, supplied by Balcke-D?rr. The condenser consists of 100 modules, each with finned tubing arranged in a Oroof-trussO configuration, through which the steam and condensate flow. A 6.1-m-diameter fan blows air through the finned-tube banks of each module to promote condensation. Six tapered sections reduce the steam duct from 7 m to 2.3 m so that steam is distributed evenly over the 10 rows of 10 modules. The nominal auxiliary power consumption associated with the 100 fans is 2.7 MW. This condenser is the largest of its kind in Europe.

Each of the three air-cooled generators coupled to the gas turbines is rated 178 MVA/ 11 kV at 8 C, while the hydrogen-cooled generator belonging to the steam turbine is rated 295 MVA/ 15.75 kV. The four step-up transformers rated 180 MVA and 305 MVA respectively are brought together on the high-voltage side in a banking compound. From there, the total station output is transmitted by a single 400-kV cable to the new National Grid Co. substation, 500 m away.

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PowerGen chose gas turbines (above) in a combined cycle to pursue its goal of being Great Britain`s lowest-cost electricity producer.

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One operator can run the entire station from this state-of-the-art central control room.

Not a single forced outage occurred during the last nine months of 1994.

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Authors

Terence E. Chappell is director of projects for PowerGen with responsibility for all major construction projects, including three new gas-fired, combined-cycle stations and a large flue-gas desulfurization retrofitting program. Chappell, a chartered mechanical and electrical engineer, was deputy director of engineering at the Atomic Energy Authority in England prior to his position with PowerGen. Chappell studied at Manchester and Salford Universities, obtaining his master`s degree in advanced control theory.

John S. Joyce is deputy director of marketing for the Siemens Power Generation Group (KWU) with responsibility for overseas thermal power plant projects, mainly turnkey gas-turbine and combined-cycle installations. Joyce began his career with Siemens in Germany in 1954. Between 1970 and 1981, he was manager of engineering for Allis-Chalmers Power Stations Inc. and later for Utility Power Corp., Milwaukee, Wis., USA. Joyce graduated from the National University of Ireland in Dublin, where he studied mechanical and electrical engineering.