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Measuring Efficiency in Modern Gas Turbine Power Stations Jeff Parmar Operational Performance Consultant Mechanical Systems National Power PLC Swindon England & Damian Gilmartin CCGT Support Group Killingholme Power Station National Power PLC North East Lincolnshire England3rd Annual American Industrial & Power Gas Turbine Operations & Maintenance Conference 25-26 February 1998, The Holiday Inn, San Diego, CA, USA.
ABSTRACTThe United Kingdom Government’s decision to put the publicly-owned Electricity Supply Industry intothe hands of private investors paved the way for the creation of a competitive electricity market, andencouraged the entry of Independent Power Producers (IPP). Competition in electricity generation hasincreased as new entrants have taken advantage of the latest Combined Cycle Gas Turbine (CCGT)technology, which offers rapid build times, low construction cost, and high cycle efficiency, as well aslower environmental emissions. National Power (NP) has invested in 4 large natural gas fired CCGTpower plant within the UK, totalling some 3200MW of capacity, and expanded internationally. NP’sinvestments overseas now total about £900 million, and the Company has equity in around7,500MW (oil/coal/gas) of plant in Australia, Pakistan, America, Portugal, Hungary, China, Spain andTurkey. As a major electricity generator NP brings not only financing to these projects, but also itstechnical expertise as an established operator and maintainer of power stations in the UK.It is of paramount importance that suitable Guarantees are in place before investing in either new build orexisting power plant projects. There will be Guarantees within all of the contracts that provide theframework for a viable power project, from the power purchase agreement (PPA), and fuel supplyagreement (FSA), to the turnkey contract (Engineer, Procure and Construct). The Plant PerformanceGuarantees, especially on module power output and heat rate, have a major impact on a projectseconomic viability, and although they are included within the turnkey construction contract, they may alsobe passed through to the PPA and FSA. These Performance Guarantees will be particularly important forinternational projects where the power and fuel purchase agreements can be in excess of twenty years.However each project is treated on its own merits and the criteria for testing and acceptance of the powerplant may change, dependent on the commercial and financial project drivers.This paper details the Performance Guarantee Tests that are carried out jointly by the Owner-Operator andthe Plant Constructor on a large CCGT plant in the UK, though the Tests are as valid for internationalsites. The methodology of the Performance Test is described, along with the applicable InternationalStandards. On-line efficiency monitoring at one of National Powers CCGT plant is also briefly outlined.A set of typical power and heat rate correction curves is provided in Appendix 1 for reference, whilstAppendix 2 contains an example calculation of turbine inlet temperature as per DIN 4341. Appendix 3 isa flow diagram for an online efficiency calculation cycle, using the EfficiencyMapTM PerformanceMonitoring system. Page 1
PERFORMANCE TEST METHODOLOGYINTRODUCTIONThe Performance Guarantee Tests are normally undertaken following the commissioning and reliabilitytests, as part of the plant Acceptance Tests. The aim of the Performance Guarantee Tests are to determinewhether the power plant is capable of achieving the power output and heatrate agreed within the turnkeycontract, by measuring actual power output and heatrate, and correcting to standard conditions. Theresults of these tests also provide a baseline against which future plant performance can be judged.Provided the plant can meet its contractual performance levels, and also passes all the other acceptancecriteria, then the plant can be handed over and commercial operation commences. The financialperformance of many of these projects depends on the thermal performance of the plant, and competitivemarkets increase the pressure on companies to operate at high levels of availability and thermal efficiency.These tests mainly focus on the power output, heat rate, emissions, noise and auxiliary power, thoughother tests may be carried out to satisfy the clients requirements.PERFORMANCE GUARANTEESThe terminology used for the Performance Guarantee Tests must be clearly defined, so that all partiesconcerned are fully aware and that no confusion is caused later on. The net power output is defined as theelectrical power delivered to the grid system at grid system voltage having taken into account all electricalpower consumed or dissipated within the station. The power consumed or dissipated includes, but is notlimited to, lighting, heating and ventilation, water treatment plant etc. The auxiliary power is powerconsumed within the station boundary and is required for the normal operation of the CCGT module. If asecond module is to be tested separately, then common auxiliaries are clearly identified.Net heat rate is stipulated as the fuel energy consumed to produce a unit of net output, expressed inkJ/kWh. This is based on the Lower Calorific Value (LCV) of the fuel. The LCV and the base density ofthe fuel are referenced to the International System Organisation (ISO) conditions of 15C and 1.01325bara.The Guarantees, provided by the contractor, on net output and net heat rate are given for specific siteambient conditions, which are defined within National Powers Contract Specification. In fact thehistorical weather data for at least five years is provided to the contractor, on which they base thePerformance Guarantees. Since it is unlikely that all operating conditions at the plant boundary during thePerformance Guarantee Tests will correspond exactly with the Guaranteed Specified values, it isnecessary to correct the test performance to the Guarantee values. The contractor provides correctioncurves to correct from test net output and net heat rate conditions to the site-specific conditions. A typicalset of such Vendor correction curves is contained in Appendix 1.The scope of the Commissioning of the plant includes Performance Guarantee Tests and tests todemonstrate compliance with noise, emissions and the 30 day reliability run.In addition to the full load net output test, tests are also normally undertaken at 75% load, 50% load andminimum stable generation (MSG). The MSG is defined as operation of the combined cycle plant in safemode with continuous operation without any steam bypass in service, meeting all noise and emissionlimits set by the Regulatory Bodies.The absolute overall tolerances on net output and net heat rate are set at ±0.5% and ±1.0% respectively. Page 2
These tolerances include all measurement and instrumentation tolerances and no other tolerances areallowed in the derivation of the corrected net output and heat rate. Table 1 shows a typical PerformanceGuarantee Plant Schedule which is filled in by the contractor during the plant tender stage.In conjunction with the Performance Guarantee tests, there are other ancillary tests carried out todemonstrate the plant is fully compliant with the contract conditions, such as the derivation of the gasturbine inlet temperature (which has an impact on the life of the gas turbine components), frequencyresponse tests, load rejection, vibration level tests etc. An example of the derivation of the gas turbine inlettemperature based on the Test Standard DIN 4341 is given in Appendix II.Table 1. THE PERFORMANCE GUARANTEE SCHEDULE PARAMETER SITE CONDITION ISO CONDITIONSITE AMBIENT TEMPERATURE CSITE AMBIENT PRESSURE baraRELATIVE HUMIDITY %COOLING WATER TEMPERATURE CGROSS OUTPUT MWAUXILIARY POWER MWNET OUTPUT MWNET HEAT RATE kJ/kWhNox EMISSION LEVEL ppmFUEL CONSUMPTION MWthGAS SUPPLY TEMPERATURE CGT INLET FILTER PRESS DROP mbGT EXHAUST PRESS LOSS mbGT INLET TEMPERATURE CLCV OF FUEL AT 15C/1.013bar kJ/kgAVAILABILITY (OPTIONAL) %30-Day Reliability Run TestsThe reliability of the power plant, including all ancillaries that are essential for its safe and efficientoperation, is proved by running the plant for three 10 day periods. For each 10 day reliability run the plantshall respond to a demanding continuous running regime which is specified by National Power. Therunning regime includes start-ups, load changes, on-line and off-line GT compressor washes andshutdown of the plant. The final 10-day period consists of ten consecutive days of two-shift operation inwhich the plant is shut down for cold, warm and hot start-ups. During these tests the contractor is alsorequired to demonstrate; the plant dynamic characteristics, which include start-up and shut-down times,ramp-rates, that component integrity levels are those specified by the contractor, that plant conditionmonitoring equipment operates satisfactorily, and a six hour full load alternator heat soak test at specifiedpower factor. The details of the downtime and the allowed plant trips during the reliability run aredepicted in Table 2. If the allowance is exceeded, the 10-day reliability run is repeated.Table 2. 30-DAY RELIABILITY RUN SCHEDULE PERIOD (days) DOWN-TIME (hours) ALLOWED TRIPS 0-10 8 2 11-20 8 1 21-30 0 0 Page 3
PERFORMANCE GUARANTEE TEST METHODOLOGYThe contractor normally submits a detailed Performance Test Procedure for review and acceptance 90days prior to conducting the Tests. National Power then provides a written response to the procedurewithin 20 working days of receipt of the document, after which the Test Procedure is agreed and approvedby both parties. The Performance Guarantee Tests are carried out in accordance with the approved TestProcedure which is based upon the International Test Codes and Standards listed below:Gas Turbine Acceptance Tests: ISO2314/DIN 4341Heat Recovery Steam Generator: ASME PTC 4.4Acceptance Test Code for Steam Turbine: IEC 953-2 (The Thermodynamic values of steam and water are taken from the “Properties of steam and water in SI Units”, edition 1982, IFC-Formulation 1967)Flow measurement: ISO 5167Measurement of fluid flow by means of pressure differential devices: ISO 5167 (or ISO 9951 Turbine Type Meters)Natural Gas flow measurement: ISO 6976/AGA Report No.8 Gas compressibility Measurement of Electrical Output: IEC 953-2Gas Turbine exhaust gas emission, measurement and evaluations: ISO/TC 192/WG2Agreed Contractual Noise ProcedureSome of the measurement criteria to be met during the preparation for the Performance Guarantee Testsare summarised below:-All salient instruments are precision calibrated and have accredited calibration certificates.- All measuring equipment necessary for the Net output and heat rate tests, including the determination of the performance of the individual plant components, is supplied by the contractor.-The net electrical power output is measured on the high voltage busbar side of the generator transformers. The contractor normally provides calibrated current and voltage transformers and the Watt-hour meters, which give an overall accuracy of better than ±0.2%. These instruments become a permanent part of the installation.-The contractor supplies turbine flow meters to monitor the gas flows in accordance with ISO 9951 with on-lining monitoring of fuel characteristics which has its calorific value referenced to 15C and 1013.25mbar, although orifice plates have been installed on some of NP’s combined cycle power plants. As this is one of the most important items of plant for monitoring performance, it is vital that the layout and the design of the gas metering stations are fully compliant with the appropriate design standards prior to manufacture and installation. Three fuel gas samples are taken for evaluating the composition and the heating value of the fuel consumed during the performance tests.-During the tests, the operating conditions are close as possible to the rated operating conditions.-The power plant is operated in steady state condition and the test measurements are performed after power output stabilisation period of at least two hours. The boiler and the steam turbine cycle is fully isolated from the makeup water system, and the blowdown and anti-icing systems are also isolated.-The test duration for the Guaranteed load points following the stabilisation period must be in accordance with ISO 2314, and for the combined cycle the duration is 60 minutes. Page 4
-Measurement of ambient pressure, ambient temperature and ambient humidity is undertaken in accordance with the appropriate standards. The measurement position for these ambient conditions is upwind of the plant, at a position representative of the average site conditions. and which has been agreed between National Power and the contractor prior to the tests.-The emission sampling probe is normally temporarily installed in the duct downstream of the gas turbines. The multihole stainless steel probe with its measuring equipment is capable of extracting and analysing the following components within the turbine exhaust gas: NOX (NO, NO2), UHC (unburnt hydrocarbons), CO, CO2, and O2. The probe allows integral sampling through the holes such that equal duct areas are covered hence ensuring a representative sampling of the gas turbine exhaust duct. The lines to the analyzers are heated to prevent any condensation of moisture and unburned hydrocarbons. There are filters and moisture traps in the probe lines to remove dust particles and water before the samples are analysed. The NOX figures are corrected to 15% oxygen on a dry basis.-Defined by national and international Standards, the average surface sound pressure level is derived from the measured values after applying corrections for the background noise and the influence of reflecting sound. A sound level meter that meets the requirements of IEC Publication 804 or better is used for these tests.-Generally, the plant is tested in the ‘new and clean’ condition; however, if the tests are prolonged for whatever reasons then degradation curves are utilised for correcting to the new and clean conditions. The use of degradation curves is dependent on the contract procedure and in some cases they may not be used. NP’s recent view is to discard the degradation curves if the delay in testing the plant is caused by the plant supplier.-Preliminary test results are presented within one week after completion of the tests. National Power responds with comments within ten days of receiving the draft and the contractor issues the final report within 30 days of completion of the tests.EXAMPLE OF PERFORMANCE EVALUATIONSkWm=actual measured kW of power on the high voltage side of the generator transformers by the 2 Wattmeter method, which represents the net output of the power module.HICm= actual heat input, this is the product of the fuel gas mass flow and the lower heating value of the fuel.kWc=corrected kW of power on the high voltage side of the generator transformers by the 2 Wattmeter method, which represents the net output of the power module.HICc= Corrected Heat input, this is the product of the fuel gas mass flow and the lower heating value of the fuel. Correction to specified conditions:If the conditions during the Performance Tests are different to the Guaranteed values, the test results arecorrected using the appropriate correction factors (see Appendix I).Therefore, the corrected combined cycle net power kWc = kWm x p1 x p2 x p3 x p4 x p5, wherep1 = output correction for ambient temperature Page 5
p2 = output correction for ambient pressurep3 = output correction for ambient relative humidityp4 = output correction for generator speedp5 = additional increase in filter differentialSimilarly, the corrected heat input consumption, HICc = HICm x h1 x h2 x h3 x h4 x h5, whereh1 = heat rate correction for ambient temperatureh2 = heat rate correction for ambient pressureh3 = heat rate correction for ambient relative humidityh4 = heat rate correction for generator speedh5 = heat rate increase in filter differentialThus, the corrected heat rate, HRc = HICc /kWcThe corrected net output, kWc and the corrected net heat rate, HRc are compared with the Guaranteedvalues and should agree within the specified tolerances. If the corrected values of power and heat rate ,andall the other performance criteria, are within the specified Guarantees, then the plant is taken over fromthe contractor and is prepared for commercial operation. If the plant fails to achieve the Guaranteedvalues, the contractor is asked to repeat these tests with plant modifications if necessary until all theGuaranteed levels are met. If the Guarantee points are not satisfied, then the contractor may incur financialpenalties, in the form of Liquidated Damages.ONLINE EFFICIENCY MONITORING SYSTEMINTRODUCTIONAs an operator of gas turbine power stations in the UK market National Power aims to maximise bothplant availability and thermal efficiency. However the measurement of efficiency on a CCGT is mademore difficult as ambient conditions have a significant impact on both module output and heatrate, and thefact that a fall in efficiency in the GT cycle will be offset to a lesser extent by a gain in the steam cycle.Efficiency reports are prepared at the end of each month, and the offline Thermal Efficiency MonitoringProgramme (TEMP) is used to identify the thermal losses during the period. Typical losses on a CCGTare those due to compressor fouling, and an increase in inlet filter differential pressure, as well as the morefamiliar steam/water side losses eg. HRSG stack loss, and deviation from target exhaust pressure in thecondenser.This information is required by management and should bring to light any inaccuracies in the meteringsystems, but the offline, once a month nature of this reporting makes it much less useful from anoperational standpoint. The answer is to develop efficiency trends that are readily available to the plantoperators via the station’s Operational Information System, which acts as the plant data storage, retrievaland display system. By monitoring important performance parameters such as gas turbine correctedpower, HRSG efficiency and target (or expected) condenser pressure, then plant performance can beoptimised wherever possible, and problems identified at an early stage.The decision was taken by the management at NP’s Killingholme A power station towards the end of1995 to look for an Online Efficiency Monitoring System, which would calculate performance parametersmore accurately than they could be done using raw measured data, and would correct these parameters toISO standard conditions. The end result of this work was that a contract was placed with ENTER Page 6
Software Inc (USA) and their agent, SMCI Services Ltd (UK) in early 1996 for the supply and installationof the EfficiencyMapTM Performance Monitoring System.DEVELOPMENT AND INSTALLATIONKillingholme A has a rated net output of 652MW at site conditions (12oC and 1013.25 mbar) andcomprises 3 ABB Type 13E1 gas turbines (gross output of 145.4 MW ea), 3 two pressure HRSGs (HeatRecovery Steam Generators) and the steam turbine, rated at 227MW. The station was commissioned in1993, and is situated on the south bank of the Humber River, in the north-east of England. One of thesystems supplied by ABB was intended to monitor plant performance and it is called PCO (PerformanceCharacterisation and Optimisation). The system is entirely reliant on measured data for its calculations,and there is no data validation in the system. So any transducer errors will be reflected in the calculatedvalues. The other problem has been that PCO data cannot be sent to the plant historical database (OIS)and so is lost after 3 months, making trending impossible.However EfficiencyMap uses the plant measured data set as a starting point for its calculations, whichinvolves satisfying a complete set of mass and energy balance equations, as well as simple constraints,using a non-linear equation solver, LINGOTM . EfficiencyMap also uses an Optimisation Equation, basedon the Least Squares Error (LSE) method, to validate some of the plant data, notably measurements ofpressure and flow, which reduces its sensitivity to transducer inaccuracy. This iterative fitting process ofsatisfying the laws of conservation of mass and energy, and minimising the Optimisation Equation, resultsin an Optimal validated dataset, upon which all of the performance calculations are based. This model-based approach, using data validation, should allow performance parameters to be calculated moreaccurately. The final advantage the system offered was that it could interface with OIS and not only takemeasured data for its calculations, but also send EfficiencyMap calculated data back to OIS, for trendingover time. A fuller explanation of an EfficiencyMap online cycle is given below, and Appendix IIIcontains a flow diagram of the execution cycle.The first stage in the development of the system was for ENTER to build a software model of theKillingholme thermodynamic cycle, using the GateCycleTM heat balance software. Using vendor suppliedperformance curves, plant drawings, heat balance diagrams, and plant historical data, including thePerformance Guarantee Test data, ENTER built and tested a GateCycle model. From this model theydeveloped a set of mass and energy balance equations for the plant, as well as tables of correction factors,and the performance calculations, that make up the EfficiencyMap model. Further tuning of the modelwas carried out once EfficiencyMap went online, whilst trends to display the data in OIS were built.EFFICIENCYMAP ONLINE CALCULATION CYCLEThe flow diagram in Appendix III and the accompanying text explains the sequence of events in theEfficiencyMap execution cycle. This sequence is repeated every 6 minutes in the Killingholmeinstallation.1. Read current measured dataMeasured data is read from the OIS database and the cross-reference file specifies tags to read and theircorresponding EfficiencyMap variable names.2. Calculate thermal propertiesUsing measured pressures and temperatures, and gas compositions calculated after combustors then theenthalpies for exhaust gas, steam and water in the cycle are calculated.3. Run data validationThe equation file specifies the mass and energy balance equations, and the cross-reference file specifiesthose measurements which are to be included in the Optimisation Function. The LINGO equation solveris the used to solve the optimisation problem and produce a complete heat balance. This process is Page 7
iterative and only stops when the value of the optimisation function has been minimised. The result ofthese calculations is the Optimal Validated dataset.4. Performance CalculationsAll of the logic and calculations are stored in the performance script file, as are the equations specified forheat rates and component efficiencies. Table look-ups are used to access vendor supplied performancecurves, such as variation of GT output with ambient temperature. Formulas are also specified forparameters such as gas turbine expected power and heat rate at part load. In the latest version ofEfficiencyMap GateCycle models are also called to allow the calculation of parameters such as HRSGexpected steam flows and stack temperature, and steam turbine expected power. The end results of thesecalculations is that three performance datasets are produced.Calculated / Validated Dataset: Represents the current performance of the plant, and may be directly measured or validated (such as gas turbine gross power), or calculated directly from measurements or validated data (such as heat rate).Expected Dataset: Represents the plant performance that would be produced if the plant were in a “new and clean state” at the current ambient and operating conditions. The expected is obtained by taking the reference value (typically the value at the Performance Guarantee Test) and correcting it to current operating conditions.“Corrected to Standard Day” (CSTD) Dataset: Represents the current performance of the plant, corrected to ISO standard conditions.5. Write calculated data from Heat Balance file to OIS DatabaseAll of the results from steps 2, 3 and 4 are then stored in the heat balance file, and the cross-referencefile is used to specify those variable that are written back to the plant database.EFFICIENCYMAP: CONCLUSIONThe existing system has been in operation for just over a year, and provides station staff with onlineefficiency data via OIS. For performance monitoring it is normally any deviation from the norm that theoperator will be interested in, and so the power and heatrate deviations are trended, as well as GTcorrected power and heatrate, expected condenser pressure etc. Costs are also attributed to the power andheatrate losses. Work is continuing to improve the calculation of expected performance for the HRSG andsteam turbine, as well as for part load performance, though this can be hampered by lack of plant data atlower loads.As a baseload CCGT power station, with modern controls, there is normally little that the operators can doon-load to optimise plant thermal efficiency. Offline compressor washes and air inlet filter changes are thebest ways to recover lost gas turbine output, but these require outages. However with the greaterknowledge of current and expected plant performance that an Online Performance Monitoring Systemprovides there should be scope for efficiency optimisation, as the operating regime at Killingholmechanges.CONCLUSIONPerformance testing on CCGT plant has become increasingly important as the amount of gas turbinegenerated electricity has dramatically increased all over the world, and new markets have been opened upto independent power producers. This paper has outlined the methodology for testing CCGTs and therelevant standards, and following this approach any operator can ensure that they are getting exactly whatthey contracted to pay for, in terms of output and heatrate. However performance testing and monitoringshould not stop once commercial operation begins, and to optimise the substantial investment they havemade the operator would be expected to look at online performance monitoring, as National Power has Page 8
with the EfficiencyMap system at Killingholme A.ACKNOWLEDGEMENTThe authors wish to thank the management at National Power for their kind permission to publish thispaper. Page 9
APPENDIX IOUTPUT & HEAT RATE CORRECTION CURVES USED FOR CORRECTING TEST CONDITIONS TO PERFORMANCE GUARANTEED VALUES
APPENDIX IIEVALUATION OF GAS TURBINE INLET TEMPERATURE BASED ON METHODOLOGY AS PER DIN 4341
APPENDIX IIIFLOW DIAGRAM FOR EFFICIENCYMAP ONLINE CALCULATION CYCLE