INFLUENCE OF AMBIENT CONDITIONS AND OPERATION LOAD ON THE PERFORMANCE OF A COMBINED CYCLE THERMAL POWER PLANT

Transcription

1 INFLUENCE OF AMBIENT CONDITIONS AND OPERATION LOAD ON THE PERFORMANCE OF A COMBINED CYCLE THERMAL POWER PLANT Clecius Ferreira da Silva Thermal System Study Group, Federal Engineering School of Itajubá, Av. BPS 133 CP 5, Itajubá, MG, Brazil, CEP 37-93, Phone-Fax: Dalmo Massaru Wakabayashi Thermal System Study Group, Federal Engineering School of Itajubá, Av. BPS 133 CP 5, Itajubá, MG, Brazil, CEP 37-93, Phone-Fax: Felipe Raúl Ponce Arrieta Thermal System Study Group, Federal Engineering School of Itajubá, Av. BPS 133 CP 5, Itajubá, MG, Brazil, CEP 37-93, Phone-Fax: fponce@unifei.edu.br Electo Eduardo Silva Lora Thermal System Study Group, Federal Engineering School of Itajubá, Av. BPS 133 CP 5, Itajubá, MG, Brazil, CEP 37-93, Phone-Fax: electo@iem.efei.br Abstract. Combined cycle thermal power plants have shown themselves to be an excellent alternative for electric energy generation in a worldwide basis. The reasons for that are their high efficiency and their good performance environmentally speaking. These peculiarities become the preferred conversion technologies in the Priority Plan of Thermal Power Electricity, which was elaborated here in Brazil. This type of thermal power plant is characterized by its complex operation and the extreme sensitivity of its performance in relation to multiple parameters such as: ambient conditions, load variations, the use of supplementary firing, etc. Because of these reasons it is necessary to carry out studies that help us understand and mitigate the influence of these variables on the performance of this kind of generating unit. By using the software Thermoflex, produced by Thermoflow Inc., it was possible to simulate the influence of the ambient temperature, atmospheric pressure and the air relative humidity on the operation of a combined cycle thermal plant. The plant is composed by two Siemens-Westinghouse 51F gas turbines coupled to three pressure level HRSGs and re-heating with fuel supplementary firing, and a steam turbine, with a capacity of MW (ISO). A detailed analysis of the behavior of this type of plant was carried out, and several graphics were drawn indicating, in a quantitative way, how the efficiency and the generated power are severely affected by the variation of the above parameters. Also, a strategy for operating at partial loads aiming at maximizing the thermal efficiency of the installation was established. Curves presenting the variation of the plant s efficiency and machine power at partial loads are shown. Key Words: Combined Cycle, Energy Generation, Generated Power, Net Efficiency. 1. Introduction Some years ago, an accelerated process of decentralization of the transmission and electricity generating capacity started, and that introduced the competition to the electricity market. Decentralization and the energy profile have been requiring the introduction of new electricity generation technologies having high thermal efficiency and high reliability, so that they are able to operate at low generation costs. This way, market competitiveness will be maintained. The operation strategy defined in the project, which was carried out based on rigorous criteria and considering the configuration of the combined cycle plant, is determined by the conditions of the operation environment and it involves a high degree of complexity. For example: a plant set up with simple shaft units will have a different operation strategy from a plant with a multiple shaft arrangement. However, variables, such as the ambient parameters, plant location, operation load, type of load - peak and off-peak, type of fuel, etc., will determine the real operation strategy. The real time operation is always carried out in off-design, and will be determined mostly according to the behavior of the gas turbine facing the variation of these parameters. This turbine generates an average of 2/3 of the power of the plant. Nevertheless, one can adopt as a general rule that combined cycle thermal plants presenting similar configurations tend to have a similar off-design behavior (Kehlhofer, et al., 1999). Influence of the ambient temperature on the performance of a combined cycle plant: A rise in the ambient air temperature, in relation to its project value (ISO conditions), reduces the total power generated in the plant. This occurs because the gas turbine is designed so that the compressor operates with a constant air volume (Cohen, et all. 1987). When the air ambient temperature rises, its density decreases, so for the same volumetric air flow, the mass flow is reduced, as a consequence the power generated by the gas turbine falls. For combined cycle plants, a fall in the total generated power, at first, is less significant than the fall presented by gas turbines, which operate with simple cycles. Due to the rise in the ambient air temperature, the gas turbine exhaust gases present a higher temperature, favoring the

2 steam cycle performance. At the same time, the rise in the ambient temperature leads to an increase in the condensing pressure, causing a reduction in the power generated by the steam turbine. The resulting effect on the total power generated in the combined cycle plant as a combined result of the exhaust gases highest temperature and the increase in pressure in the condenser, will depend on which factors will prevail. Influence of the atmospheric pressure on the performance of a combined cycle plant: The influence of the atmospheric pressure is related to the variation the air density. For low-pressure atmospheres, that is, high altitudes in relation to the sea level, the air density is reduced (Bathie, 1996) and the combined cycle plant presents a behavior that is similar to that was explained for ambient temperature. With the altitude increase, a reduction in the gas turbine exhaust gases mass flow also takes place, followed by a smaller steam generation in the HRSG and naturally by a reduction in the steam turbine and plant power. It is important to highlight that the effect of the ambient pressure on the performance must be considered, mainly during the project phase, because once the plant is installed, the variations of this variable is neglected. Influence of the relative humidity on the performance of a combined cycle plant: The rise in the air relative humidity leads to a fall in the power produced by the gas turbine because of the reduction in air density caused by an increase in its moisture. However, this results in a greater enthalpy in the gas turbine exhaust gases, and that may increase the power generated in the steam cycle. Plants presenting cooling towers deserve special attention. In these plants, the air relative humidity is directly related to the vacuum level in the condenser, and consequently to the steam enthalpy after the last stage. In these cases, a smaller air relative humidity results in a greater vacuum and, therefore in a greater generated power, once the steam enthalpy after the last stage is smaller. The influence of the air relative humidity in the combined cycle will depend on which of the factor is the most predominant, but the effect cause by this variable on the total generated power is considerably small if it compared with the effect caused by the variations of the two previous variables. Influence of the operation load on the performance of a combined cycle plant: At partial loads, up to 5 % of the nominal load, the cycle efficiency is reduced around 1 %. For partial loads, lower than 5 % of the nominal load, the reduction is more accelerated. This is a consequence, mainly, of the mechanisms implemented for load regulation and control in gas and steam turbines. The gas turbine can be designed with inlet guide vanes in the compressor, allowing the machine to present a good behavior up to, approximately, 6 % of the nominal load. The steam turbine allows a pressure variation when it operates with up to 5 % of the nominal load, which makes it possible to optimize the use of the gas turbine exhaust gas energy in the HRSG, generating steam at lower pressures. The steam pressure must be kept constant for smaller loads aiming at avoiding condensation in the steam turbine last stages. Under these conditions, the pressure is controlled by using governing stages valves, causing throttling losses that reduce the power generated in the steam turbine. Plants that use fuel supplementary firing present a more stable behavior facing load variations, once the supplementary firing allows the steam generation in the HRSG and the steam turbine operation to be kept in efficient levels. For any configuration (simple or multiple shaft), the operation sequence at partial loads is planned aiming at keeping the plant s maximum efficiency for any load variation range. The simulation of the off-design operation considered the variations of ambient air temperature at the compressors inlet, local atmospheric pressure, relative humidity, cooling water temperature, fuel supplementary firing in the HRSGs, as well as the load demand. 2. Scheme description and parameters The simulation was carried out on the base of a thermal cycle simulation to a typical combined cycle thermal plant operating in Brazil. The thermal cycle is similar, for example, with the Uruguaiana power plant. This item describes the assumptions adopted in order to evaluate the influence of the previously mentioned variables on the operation of a combined cycle thermal power plant. Figure (1) shows the simplified thermal scheme of the plant selected for the analysis. Its main features are: Two Siemens-Westinghouse 51F gas turbines; Two HRSGs presenting three pressure levels and fuel supplementary firing. A detailed scheme of the HRSG is displayed in Fig. (2). According to this figure fuel supplementary firing takes place after the two final highpressure superheating stages. The first re-heating stage and the first high-pressure superheating stage are placed after the supplementary firing. Just one pump is use for HP and IP pressure levels, in the real plant there is a pressure reduced valve at the IP tabulation but, it is not represented in the figure; A high, intermediate and low-pressure steam turbine, and the last one presents divided end flow; Dearator condenser with a wet tower cooling system and a water make up system; Cooling system pumps, low pressure pump at the condenser outlet and the high pressure pump, responsible for elevating the water pressure to high and intermediate levels; Natural gas supply. The fuel used by the gas turbines is heated, but the fraction used for the supplementary firing is not. Table 1 details the main data for the simulation of the combined cycle thermal plant at design point. It is possible to observe that the data shown in Tab. (1) are divided into four main groups. In the first group, besides the ISO parameters,

3 it is established that the net total electric power generated by the plant at design point is MW. The second data group defines the fuel used during the simulation, as well as its Lower Heating Value and the supplying conditions. The third group refers to the gas turbine. In this case, the data presented had already been implemented in the software Thermoflex when the turbine was selected. The last group is related to the steam cycle and the plant s auxiliary systems. The following values can be observed: steam pressure and temperature for different levels of pressure presented by the HRSG, condenser operating pressure, efficiency of the three-stages steam turbine and the imposition of a minimum steam quality at the low pressure turbine outlet, which is based on technical criteria. The other data that were shown are related to the condenser cooling system, the pumps and the BOP power consumption. It is important to highlight that the temperature of the gases after the fuel supplementary firing was limited at 675 C aiming at avoid the formation of steam in the final section of the economizer tubes (Ganapathy, 1991). Figure 1. Combined cycle plant simplified thermal scheme Figure 2. HRSG scheme

4 Table 1. Main data for the simulation of the combined cycle thermal plant at design point Parameter, unit Value Ambient Temperature, C 15 Atmospheric pressure, kpa Relative humidity.6 Net total electric power, MW Fuel: Natural gas, LHV, kj/kg 4656 Supply conditions, MPa/ C 2.68/25 Siemens-Westinghouse 51F gas turbines : Gross power, MW Compression isentropic maximum efficiency.9 Combustion efficiency.99 Turbine isentropic efficiency.9431 Electricity generator efficiency.985 Turbine inlet temperature, C Turbine outlet temperature, C 68 Cooling air fraction.178 Auxiliary power consumption, MW Steam cycle with three levels of pressure and re-heating High pressure steam, MPa/ C 15.6/526 Intermediate pressure steam and re-heating, MPa/ C 3.2/537 Low pressure steam, MPa / C (.976/.437)/(36/271) Condenser operating pressure, kpa 6.89 HRSG : Supplementary firing temperature, C 675 Supplementary firing efficiency.976 Minimal gas exiting temperature, C 7 Heat transfer coefficient, kj/s-m 2 -K Three-stage steam turbine: Net power with supplementary firing, MW High pressure turbine isentropic efficiency.898 Intermediate pressure turbine isentropic efficiency.9259 Low pressure turbine isentropic efficiency.8867 Minimum quality at the outlet of the low-pressure steam turbine (c).85 Total electromechanical efficiency Condenser: Heat transfer global coefficient, kw/ C 25/45 Cooling pump power consumption, kw 1172 Cooling tower: Number of fans 1 Cooling water inlet/outlet temperature 25/15 Pumps/Vanes power consumption, kw 146/1922 Balance of plant - BOP: Pumps isentropic efficiency,75 Total losses referring to the steam turbine power,198 Notes: a) Source: Gas Turbine World Handbook (1998); b) The temperature of the gas after the supplementary firing was limited at 675 C aiming at avoiding the formation of steam in the final section of the economizer tubes during the off-design operation (Ganapathy, 1991); c) According to Boyce (1999) this value avoids pallets erosion within the last stages of the turbine. 3. Methodology The study was carried out in accordance with the following steps: Step 1. Selection of a combined cycle thermal power plant. The thermal scheme was chosen by referring to a plant operating in Brazil; Step 2. Thermal cycle scheme was drawn. The analyzed thermal scheme was drawn in two parts. The Thermoflex graphic editor was used because it provides all the necessary components and connections; Step 3. Data input. It refers to the data input of all the components that form the thermal scheme: gas turbines, HRSGs surfaces, steam turbine, condenser, equipment and auxiliary sub-systems, etc.;

5 Step 4. Thermal scheme adjustment. Several adjustments were carried out in order to arrange the thermal scheme according to the ISO conditions and as similarly as possible to the operating conditions of the reference plant. This way, It was possible to obtain thermal scheme according to the design conditions and ready for the offdesign simulation; Step 5. Simulation of the off-design operation varying the ambient temperature between 5ºC and 4ºC. For this simulation, and others that were carried out, the macro tool available in the software was used. It allows the indication of initial, intermediary and final values of the variables that will be researched; Step 6. Simulation of the off-design operation varying the ambient pressure, from its value at sea level conditions (1.13 bar) to altitudes of about 1, meters; Step 7. Simulation of the operation off-design varying the air relative humidity with a range from 4 % to 9 % of humidity; Step 8. Simulation of the operation off-design varying the load. First, the gas turbine load from % down to 3 % of its project value was varied separately. In the second simulation, the rate of supplementary firing in the HRSGs was reduced, varying the temperature at the burners outlet/exit from 675 ºC to 585 ºC. In the third simulation, the plant s load started to be reduced with the decrease in the supplementary firing in one of the HRSGs, afterwards the same procedure was carried out for the other boiler. After reducing the burning in the two HRSGs the load of one of the turbines was varied down/up to 6 % of its nominal load, and after that the same thing was done to the other gas turbine; Step 9. Graphics elaboration showing the attained results using the Thermoflex graphic editor and the MS Excel; Step 1. Analysis of the results. It will be discussed below. It is important to remark that the calculations from step 5 to 8 are done varying one parameter each time; the others keep constant at the reference values shown in Tab. (1). 4. Analysis of the results In order to evaluate the performance of a combined cycle plant during off-design operation, the variations of generated power and efficiency were analyzed because of changes in the ambient temperature, atmospheric pressure, air relative humidity, operation load and supplementary firing. In Fig. (3a) it is possible to notice that the plant s net efficiency decreases from 53.6 % to % with the rise in the ambient temperature from 5 C to 4 C. The efficiency presents falls that keep getting higher (a fall of.41 percentage points for temperatures between 1 C and 2 C and.58 percentage points for temperatures between 3 C and 4 C). With a rise in the ambient air temperature, there is a fall in the compressor pressure ratio (the outlet pressure decreased from bar to bar) and the compressor work increases significantly causing a reduction in the gas turbine efficiency (from % to 35.4 %), surpassing the efficiency gain in the steam cycle. This result could be different if the gain in temperature of the gas turbine exhaust gases caused an increase in the efficiency of the steam cycle higher than the fall in efficiency of the gas turbine. In Fig. (3b), with the variation of the ambient air temperature from 5ºC to 4ºC, it is possible to notice that there is a fall in the plant s net power of % (119 MW). The fall in the power of the gas turbine was 2.32 % (75 MW) and of the steam turbine % (45 MW). Comparing the variation registered in the efficiency with the one registered in the power, one can notice that the ambient temperature has a greater impact on the electric power generated in the combined cycle plant. This is explained by the fact that the changes in gas turbine air mass and exhaust gas flows prevails over the changes in the temperature of the exhaust gases Net electric efficiency(lhv) [%] Ambient temperature [ C] Net power, MW Ambient temperature, C Gas cycle Steam cycle Figure 3. Variation of the net efficiency and net power of the plant in function of the ambient temperature

6 Figure (4a) shows that with the drop in the atmospheric pressure from 1.13 bar to.8455 bar it is possible to notice that there was a fall in the plant s net efficiency going from % to %. It is less remarkable when it is compared to the case of the temperature. This happens because the power consumption of the compressor is not affected, and only the air density decreases, reducing the air mass flow in the gas turbine compressor (from 433 kg/s to kg/s) for the same exhaust gas temperature (62.3 C). Figure (4b) shows a fall of % (17 MW) in the plant s net power, a fall in the gas turbine net power of % and a fall in the steam turbine power of %. The greatest drop in power of the steam cycle is explained by the reduction in the air mass flow at the inlet of the gas turbine compressor, resulting a smaller flow of exhaust gases (from 443 kg/s to 37 kg/s) Net electric efficiency(lhv) [%] Ambient pressure [bar] Net power, MW Atmospheric pressure, bar Gas cycle Steam cycle Figure 4. Variation of the efficiency and net power in function of the atmospheric pressure In Fig. (5a) it is possible to notice that the efficiency of plant presents a variation of only.4 percentage points for relatively wide limits of atmospheric air relative humidity (form 4 % to 9 %). The reason is that the steam cycle presents a fall in performance caused by a slight reduction in the vacuum in the condenser (from.629 bar it increased up to.785 bar). In Fig. (5b), for an air relative humidity variation between 4 % and 9 %, there was a fall in the plant s net power of.69 % (4.15 MW).The fall in the gas turbine net power was very small 7 kw, and can be disregarded. The steam turbine net power fell 1.51 % (46 kw). In the stage of high and intermediate pressure, there was an increase in the shaft power of 72 kw and 235 kw respectively, however the low pressure turbine presented a fall of 4149 kw, as a consequence of the rise in the condenser operating pressure, as it was mentioned above Net electric efficiency(lhv) [%] Ambient relative humidity [%] Net power, MW Ambient relative humidity, % Gas cycle Steam cycle Figure 5. Variation of the net efficiency and net power in function of the relative humidity

7 Figure (6a) shows the net efficiency variation in function of the load of the gas turbines. It is evident that the efficiency began to fall more sharply close to 55 % of the nominal load. For a load of 6 %, the fall in the plant s net efficiency was 1.95 percentage points of its initial value of %. For 5 % load in the gas turbine, the efficiency presented a sharper fall 9.44 percentage points; for a 4 % load the fall was percentage points; and a 3 % load presented a fall of percentage points of its initial value. This occurs because the gas turbine has a stage of inlet guide vanes that allow an excellent control of the turbine efficiency (from % to 31.3 %) keeping the temperature of the exhaust gases elevated (from 62.4 C to C) in spite of the reduction in the gas flow (from 443 kg/s to kg/s) to approximately 6 % of the load. In addition, the steam turbine operates with steam sliding pressure up/down to 5 %. In Figure (6b) it is possible to observe that for a load variation of the gas turbines from % to 3 % the net power of the cycle presented a fall of % (346 MW). The net power of the steam turbine fell by 4 %, which is mainly because of the reduction in the gas turbine exhaust gases flow caused by the load reduction. Net electric efficiency(lhv) [%] GT power as percentage of site rating [%] Net power, MW GT power as percentage of site rating, % Gas cycle Steam cycle Figure 6. Variation of the net efficiency and net power in function of the operating load in the gas turbine Figure (7a) shows that with the reduction in the supplementary firing, treated from the reduction in the temperature of the gases at the HRSGs burners outlet from ºC to 585 ºC, the efficiency of the plant increased from % to % because a smaller fuel consumption in the HRSG burners (4.39 kg/s and kg/s for ºC and 585 ºC, respectively). In Fig. (7b) there was a 9,26 % (55,6 MW) reduction in the plant s net power because of the fall in steam generation in the HRSG, resulting in a steam cycle smaller power. On the other hand, the gas turbines maintained the operation regime Net electric efficiency(lhv) [%] Duct Burners outlet temperature [ C] Net power, MW Duct burners oulet temperature, C Gas cycle Steam cycle Figure 7. Variation of the net efficiency and net power in function of the reduction in supplementary firing

8 In Fig. (8a) it is possible to observe the plant s net efficiency behavior in function of the load reduction (see load reduction stages I to IV at the figure foot note). With the variation on the supplementary firing of the two HRSGs there was an increase in the plant s efficiency from % to %. By reducing the load in one of the gas turbines from % to 6 %, the plant s net efficiency decreased to %. By reducing the load of the other gas turbine, the plant s net efficiency fell to %. This operation sequence resulted in a total efficiency variation of 6.74 percentage points of its initial value, which was % and became %. In Fig. (8a) the straight lines indicate the sequence in which the load reduction operations were carried out. Figure (8b) shows the variation of the total net power during the operation at partial loads according to the same strategy shown in Fig. (8a). A fall of 4.7 % (28.2 MW) was observed when the supplementary firing was reduced in the first HRSG. When the firing was reduced in the second HRSG, the fall in the plant s net power was 4.56 % (27.4 MW). By reducing the load from % to 6 % in the first gas turbine, the plant s net power fell by % (96. MW). By reducing the load from % to 6 % in the gas turbine, the plant s power presented a load reduction of more than % (95.66 MW), resulting in a total variation of % (247.2 MW) in the plant s net power. So, the regulation range goes from up to % Net efficiency, % I II III IV Power plant operation load, % Net plant power, MW %GT1 %GT2 %ST %GT1 %TG1 6%GT1 I %GT2 II %GT2 III %GT2 IV 89.3%ST 78.95%ST 68.43%ST Tubines operation coditions First GT Second GT ST 6%TG1 6%GT2 58.4%ST Figure 8. Variation of the net efficiency and net power during the load reduction sequence of the plant I Reduction in the rate of supplementary firing in the first HRSG, II Reduction in the rate of supplementary firing in the second HRSG, III Reduction in the load of the first gas turbine, from % to 6 % of the nominal load, IV Reduction in the load of the second gas turbine, from % to 6 % of the nominal load.

9 It is important to highlight that the load reduction sequence established here, in spite of being logical, it is not, necessarily, the only one that can be implemented for this type of generating unit. Contrariwise to the steam cycle thermal plants, combined cycle plants offer greater operation flexibility in terms of load reduction strategies making it possible to operate the gas turbines separately. Any of the turbines can be used at a certain moment during the real operation since the equipment specifications and the guidelines established by the manufacturer are respected. Other criteria, for example, the real performance condition of the plant s equipment, the possibility of damages, etc., must be considered during the decision-making process that will define the reduction strategy. 5. Conclusions On account of the great importance that combined cycle thermal plants have within the scenario of energy generation in Brazil and in the world, the parametric study of the influence of ambient parameters on the plant s operation, which is considerably important and complex, must be observed attentively. Based on this study it is possible to foresee occurrences that may harm the plant from a economic and financial profitability point of view and define a strategy to mitigate them. In order to maximize the profit, it is necessary to maintain the generation of electricity at low costs during the whole operating time. This way, an operation methodology must be established. It must consider an activity sequence in order to reduce the influence of ambient parameters on the plant s load and efficiency. This methodology will determine of the operating strategy that allows the achievement of the best efficiency values in real load and ambient parameters situations. After carrying out this study, the following conclusions can be expressed: The plant s net efficiency decreases with the rise in the ambient temperature; The rise in the ambient temperature reduces the plant s net power. The fall registered in the gas turbine within the 5 C to 4 C interval was 2.32 %. On the other hand, the steam turbine registered %; The installation of combined cycle plants in relatively highland areas leads to smaller operation efficiency values because of the drop in the atmospheric pressure. In this sense, it was observed that the fall in the plant s net efficiency ranges from % to %, and the fall in the net generated power was %; The combined cycle plant s net efficiency and power present small variation in relation to the atmospheric air relative humidity. It is possible to say that this parameter has meaningless influence on the performance of this type of generating unit; The load reduction in the gas turbines causes a slight fall in the plant s efficiency, as long as the minimum values of 6 % are not surpassed. Lower load values in the gas turbines may lead to sharp falls in combined cycle plants. In this sense, it was observed that for a 6 % load in the gas turbine, the plant s net efficiency fell 1.95 percentage points, for 5 % the fall in efficiency was 9.44 percentage points, for 4 % the fall was percentage points and for a 3 % load the efficiency presented a fall of percentage points. The good performance of the combined cycle plant within the range of load reduction in the turbine down to 6 % takes place because, within this operation range, this machine has inlet guide vanes that allow the turbine efficiency control keeping the temperature of the gases elevated in spite of the reduction in their flow; The load reduction in the gas turbines will cause a higher fall in power in the combined cycle, as a set, than in the steam cycle, if they are analyzed separately. The plant s net power decreased by % with the load reduction in the gas turbines, whereas the net power of the steam turbine presented a fall of approximately 4 %. This is explained by the reduction in the gas flow in the gas turbine as well as by the increase in their temperature up to 6 % of the load, and the latest is a positive factor for the steam cycle; By reducing the supplementary firing, establishing the temperature at the outlet of the HRSG burners as the control variable, the efficiency of the combined cycle plant increased from % to %. This can be explained by a lower fuel consumption in the bottoming cycle; With the reduction in the supplementary firing, there was a 9.26 % reduction in the plant s net power because of the fall in the steam generation in the HRSG resulting in a smaller power of the steam cycle. On the other hand, the gas turbines maintained their operation regime without suffering any changes; By reducing the load in the first gas turbine from % to 6 %, the plant s net efficiency was reduced by %. By reducing the load in the second gas turbine, the plant s net efficiency was reduced by %. This operation sequence resulted in an efficiency total variation of 6.74 percentage points of its initial value that was %, going to %; The plant s net power fell 4.7 % (28.2 MW) when the supplementary firing in the first HRSG was reduced. The same thing occurred when the supplementary firing in the second HRSG was reduced, presenting a fall in the plant s net power of 4.56 % (27.4 MW); By reducing the load in the first gas turbine from % to 6 %, the plant s net power was reduced by % (96. MW). The reduction in the second gas turbine showed a fall in the plant s net power of % (95.66 MW). The sum of the procedure of reducing load in both gas turbines resulted in a total variation of the combined cycle plant s net power of % (247.2 MW). To conclude, it is important to highlight that the theme concerning the operation of combined cycle thermal plants is a complex one, and also that this study, the second one of a sequence (see Arrieta and Lora, 2), is far from establishing definitive criteria about it. In this sense, NEST s researchers will continue to carry out studies to

10 characterize, in a more rigorous way, the variables that influence the operation and the performance of this type of generating unit. In the future, other types of project configurations will be studied in the light of thermodynamic and economic points of view. And these projects may represent the thermal plants installed in Brazil, as well as real cases referred from existing installations. 6. Acknowlodgements The authors thank the Thermal Systems Study group (NEST), FINEP and the CNPq (National Council for Scientific and Technological Development) for their technical and financial support for the accomplishment of this study. 7. References Arrieta, F. R. P, Lora, E. E. S., 2, Operação das CTEs de ciclo combinado: influência das condições ambientais, Proceedings of IX CONGRESS OF BRAZIAN ENGINEERING AND THERMAL SCIENCES, Caxambu MG, Brazil, Outubro, Bathie, W., Fundamentals of Gas Turbine, 2 nd Edition, John Wiley & Sons, p. Boyce, M., 1999, Performance monitoring of large combined cycle power plants, PWR- Vol. 34, Joint Power Generation Conference, Vol. 2, ASME, pp Cohen, H., Rogers, G. F. C., Saravanamutto, H. I. H, Gas Turbine Theory, 3rd Edition, Eighth Impression, Longman Scientific & Technical, p. Ganapathy, V. Waste Heat Boiler Deskbook; USA, Ed. The Fairmont Press, Inc., USA, p. Gas Turbine World Handbook, 1998, Pequot Publishing, Fairfield, CT, USA. Kehlhofer, R. H.; Warner, J.; Nielsen, H.; Bachmann, R., 1999, Combined Cycle Gas-Steam Turbine Power Plants, USA, Ed. Pennwell, USA, 288 p. 8. Copyright Notice The authors are the only responsible for the printed material included in his paper.