Background of the invention

[0001] The invention relates to a method for operating a steam turbine power plant.

[0002] In an electrical grid, consumption and production must be in balance. Owing to fluctuations in consumption, the electrical grid needs to be provided with regulating power to enable the production in the electrical grid to be adjusted according to the consumption. It is known to use for instance hy- dropower, gas turbine power plants and import of electricity as such regulating power.

Brief description of the invention

[0003] An object of the invention is to provide a novel way of producing regulating power. This object is achieved by a method for operating a steam turbine power plant, which is characterized by what is disclosed in the independent claim 1 . Preferred embodiments of the invention are disclosed in the dependent claims.

[0004] The invention is based on using a steam turbine power plant as regulating power in an electrical grid by adjusting at least one process parameter. The method according to the invention enables the electric power of the steam turbine power plant to be reduced while at the same time the thermal power remains constant. In prior art, it is the thermal load of the steam turbine power plant that in practice dictates the volume of electricity production, i.e. no electric power reduction potential suitable for use as regulating power exists.

[0005] An advantage of the method according to the invention for operating a steam turbine power plant is that the method enables steam turbine power plants to be used as regulating power. Utilization of the method increases the amount of potential regulating power in electrical grids. Main grid companies pay the producers of regulating power, so the invention provides the owners of the steam turbine power plants with an opportunity to sell regulating power. Brief description of the figures

[0006] The invention is now described in closer detail in connection with preferred embodiments and with reference to the accompanying drawings, in which:

Figure 1 shows a load duration curve of a steam turbine power plant;

Figure 2 shows how dropping the temperature of live steam reduces the electric power of a steam turbine power plant;

Figure 3 illustrates the time within a year during which the drops in the temperature of live steam according to Figure 2 enable the electricity production to be reduced; and

Figure 4 shows the electric power of a steam turbine power plant and a reduction in the electric power obtainable through adjustment of the temperature of live steam as a function of time during one year.

Detailed description of the invention

[0007] A steam turbine power plant comprises a steam turbine and a generator connected to the steam turbine. The generator is connected to an electrical grid in order to feed electricity produced by the generator to the electrical grid. In the method according to the invention, the electric power of the steam turbine power plant is reduced by changing at least one process parameter in order to keep the electricity production and electricity consumption of the electrical grid in balance. In other words, the steam turbine power plant is used as regulating power.

[0008] A process parameter herein refers to a temperature at a given point in a power plant process. It is possible to adjust the process parameters by controlling valves in the steam turbine power plant. The adjustment to be thus implemented is accurate and fast, and it is simple to carry out.

[0009] In the invention, the steam turbine power plant is a condensing power plant, and the electric power of the steam turbine power plant is reduced by raising the temperature of cooling water entering a condenser. In an embodiment, the cooling water entering the condenser is heated by means of bled steam of a steam turbine.

[0010] By utilizing the method according to the invention, it is possible to achieve an operating state wherein the power of the steam turbine power plant can be adjusted quickly both higher and lower. In an embodiment, the rated electric power of the steam turbine power plant is 100 MW, from which the electric power may be reduced by 10 MW by means of the method according to the invention. By adjusting the electric power by means of the method according to the invention to a value of 95 MW, an adjustment range is achieved which extends 5 MW both downwards and upwards from an operating point.

[0011] In some embodiments, the reduction of electric power taking place through a change in at least one process parameter may be supported by one or more adjustment procedures. An adjustment procedure herein refers to a procedure used for influencing a power plant process otherwise than by changing one or more process parameters. The adjustment procedure may comprise for instance bypassing a feed water preheater, utilizing bleeding or bypassing the steam turbine.

[0012] During bypassing of a steam turbine, at least some of the live steam is led past the steam turbine. It is advisable to employ the bypassing of a steam turbine with caution since a bypass valve intended for implementing the bypassing operation is seldom designed for carrying out adjustment procedures, a breakdown thereof very likely resulting in the entire steam turbine power plant having to be shut down.

[0013] Often a steam turbine power plant comprises at least one bleed via which steam can be discharged from the turbine, from between an inlet of a high-pressure part and an outlet of a low-pressure part. The pressure of the steam taken via a bleed is lower than the pressure of the steam taken via the bypass valve but higher than the pressure of the steam discharged through the outlet of the turbine.

[0014] Figure 1 shows a load duration curve of the electric power of a steam turbine power plant. For almost half a year the plant operates at an over 80% production capacity, and for about 3 000 hours at a 60 to 80% production capacity. A significant electric power reduction potential thus exists for the most part of the year.

[0015] Figure 2 shows a reduction of electric power as a function of live steam quantity, calculated for a steam turbine power plant. The unit for live steam quantity q _m is kg/s, and the unit for electric power reduction P _reg is MW. Figure 2 shows two graphs, graph 21 depicting a situation wherein the temperature of live steam has been dropped by 10°C and graph 22 depicting a situation wherein the temperature of live steam has been dropped by 20°C. It can be seen in Figure 2 that by dropping the temperature of the live steam by 20°C, approximately a double reduction in electric power in its maximum is achieved as compared with a situation wherein the temperature of the live steam has been dropped by 10°C. It can also be seen in Figure 2 that when the quantity of live steam exceeds 54 kg/s, the temperature of the live steam can no longer be dropped by 20°C since as the efficiency of the turbine improves, the temperature limits for outlet steam will be faced. When the temperature drop is 10°C, the electric power can still be reduced when the live steam quantity is 56.5 kg/s.

[0016] In some embodiments, it is possible to drop the temperature of the live steam by more than 20°C. Correspondingly, in some embodiments it is possible to drop the temperature of the live steam by less than 10°C. In an embodiment, the dropping of the temperature of the live steam is adjusted steplessly, according to the operating situation. It is for instance possible to achieve an operation program wherein the electric power is reduced by changing at least one process parameter continuously as much as it is technically possible. This may mean for instance that the temperature of the live steam is dropped in each operating situation as low as possible. According to Figure 2, in most embodiments, a technically feasible drop in the temperature of the live steam decreases as the live steam quantity increases.

[0017] Figure 3 illustrates the time within a year during which the drops in the temperature of live steam according to Figure 2 enable the electricity production to be reduced. Figure 3 shows two graphs, graph 31 depicting a situation wherein the temperature of live steam has been dropped by 10°C and graph 32 depicting a situation wherein the temperature of live steam has been dropped by 20°C. Figure 3 is drawn by using the load duration curve of Figure 1 . Hours on the left-hand side in Figure 3 during which no reduction whatsoever of electric power is possible designate the shutdown hours of the steam turbine power plant. The right-hand side in Figure 3 shows the same as Figure 2, i.e. that by decreasing a drop in the temperature of the live steam the amount of hours during which the method may be employed for reducing the electric power can be increased.

[0018] Figure 4 shows the actual electric power P _e i of a steam turbine power plant and a reduction potential P _reg of the electric power obtainable through adjustment of the temperature of live steam as a function of time dur- ing one year. The horizontal axis in Figure 4 shows the hours of a year in chronological order.

[0019] The electric power reduction graph P _reg in Figure 4 depicts the greatest possible reduction in electric power. At a given moment in time, the method enables the electric power to be reduced between zero and the disclosed maximum value. The data according to Figure 4 make it possible to determine an annual total energy capacity obtainable by the method. The total energy capacity herein refers to an amount by which the amount of electric energy fed by a steam turbine power plant to an electrical grid in one year is reduced if the full potential of the method according to the invention is utilized.

[0020] Naturally, the load duration curve is not identical year after year but the load duration curve actualized for many steam turbine power plants predicts relatively accurately the shape of a future load duration curve. Consequently, it is possible to use the load duration curve for estimating the amount of regulating power a given steam turbine power plant might be able to sell in a year.

[0021] Usually, a main grid company only buys regulating power which is available at any time. In addition, the regulating power purchased by the main grid company in practice always has a lower limit. By applying the method according to the invention to a group of power plants comprising a plurality of steam turbine power plants, it is possible to improve the likelihood that at a given moment in time there will be regulating power available and to increase the reduction potential of electric power at each moment in time. If the power plant group comprises power plants which feed loads of different types and whose shutdowns are scheduled to take place at different times, it is very likely that regulating power according to the invention will be available at any hour of the year. The power plant group may comprise a plurality of condensing power plants which may be located geographically far away from one another.

[0022] It is apparent to one skilled in the art that the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are thus not restricted to the examples described above but may vary within the scope of the claims.