Method for starting up and loading a combined power plant

When loading a power station in a grid without any energy, this power station must be able to cover all the short-term power requirements, and at the same time to keep the grid frequency within a permissible tolerance band. The invention specifies a method of providing this capability by means of an efficient combination system. According to the invention, the gas turbine (1) is loaded in a lower power range such that it is controlled in accordance with a predetermined load program, without having to react to load transients in the grid (50). Before the power station is connected to the grid, the heat recovery steam generator (5) is heated up as an energy store, and a steam turbine (2) is raised at least to its rated rotation speed, with the steam control valves (9, 15) being highly restricted. The steam turbine reacts in the event of short-term load requirements from the grid, which cannot be covered by the gas turbine.

TECHNICAL FIELD

The invention relates to a method for operating a combined cycle power station. It relates in particular to a method for starting and loading a combination system. This method relates especially to power control and to power sharing within the combination system, when loading the combination system within an black electrical grid.

PRIOR ART

When an electrical grid without any energy (black grid) is brought into use once again, particular requirements are placed on the capabilities of a power station to react quickly to load transients and to support the frequency. When loading a grid, and when the loads are connected successively, the grid load does not rise continuously but, for example when an industrial concern with a large number of electric-motor loads is connected, sudden changes occur in the real power requirement and in the wattless component requirement of from several hundred kilowatts up to the order of magnitude of megawatts. Even if these sudden load changes are considered to be small in absolute terms, they must nevertheless be covered by a single power station. If the total power consumption of the grid is only a few megawatts, for example 5 MW, then the requirements which are placed on the control system quality and the capability to support the frequency in the lower partial load range of a power station in these conditions become clear.

In principle, these requirements can be covered very well by a gas turbine system, which intrinsically has short response times. Nevertheless, the development toward the use of modern premixed burner technology with low hazardous emission levels in gas turbines has led, at least in the lower load range, to a limitation to the possible load gradients, for reasons of flame stability. In addition, some of these modern types of gas turbines cannot operate in certain power bands, which normally have a width of several megawatts.

Thus, according to the prior art, special units must be provided in the electricity budget which are able to cover the requirements for restarting an electrical grid without any energy. It is thus possible in this case to provide appropriate capacities in the form of peak-load gas turbines and diesel units. However, these are frequently comparatively uneconomic systems which produce high emission levels and which otherwise are operated for only short times. A high maintenance penalty is involved in maintaining their starting availability, for very rare operation down to them being effectively permanently stationary. Furthermore, the investments are not amortized via the electrical power that is sold.

Thus, according to the prior art, the provision of the appropriate reserves is highly costly overall. On the other hand, in the liberalized electricity markets in particular, power station operators can receive large amounts of money from the grid operators, to provide these black starting capacities.

DESCRIPTION OF THE INVENTION

For the power station operators, the above statements result in the aim of providing the appropriate black starting capacities by means of modern, economic systems which are operated continuously.

The object of the present invention is thus to specify a method which allows a frequency supporting capability to be provided for any given system load by means of a combination system which is based on a modern gas turbine equipped with low-emission premixing burner technology.

According to the invention, this object is achieved in that, for a method for operating a combined cycle power station when starting up an electrical grid without any energy, a water-steam circuit is charged with energy and a steam turbine is started before the grid is loaded, and the gas turbine is loaded at least in a part of its load range in accordance with a predetermined load program, with transient load requirements in the grid being covered by changes in the steam turbine power.

In this context, the expression charging the water/steam circuit with energy means, for example, heating up the water/steam circuit. On the other hand, this may also comprise a successive increase in the fresh steam pressure in the boiler, or successive charging of a container with a steam volume. A pressurized volume of water close to its boiling point likewise represents a very efficient energy reservoir, which has the capability to provide large amounts of steam spontaneously by reducing the pressure. Normally, a combination of these options is used for the implementation of the idea of the invention, although those skilled in the art will also be familiar with other possible ways of operating a water/steam circuit as an energy store.

The essence of the invention is thus not, as is normal in the prior art, to control the supply of fresh steam to the steam turbine such that the thermal stresses in the steam turbine are limited, but actually also to use the fresh steam supply for power control in the starting phase. In this case, mechanical overloading of the steam turbine on a temporary basis is deliberately accepted, although this reduces the life. Nevertheless, an event such as this occurs only rarely in practice, and the probability of a combination system actually being operated in accordance with the method according to the invention is intrinsically very low. Nevertheless, it is important for the system to be prepared for the operating method according to the invention, and for the appropriate programs to be implemented in the control technology which, as mentioned above, can in its own right generate considerable cash flows.

In the method according to the invention, it is advantageous for as much energy as possible to be stored in the water/steam circuit so that it can be made available at short notice in the form of steam for the steam turbine. It is thus advantageous for the method for the gas turbine first of all to be operated with the grid switch open once the gas turbine has been accelerated to its rated rotation speed, but with the generator switch closed in island operation in order to provide the in-house supply for the combined cycle power station. During this period, the exhaust gas heat from the gas turbine is used to charge the water/steam circuit with energy and to make it ready for operation. It is advantageous for the steam turbine to have already been started at this time. The grid switch is closed only when the water/steam circuit has reached a specific state. Only then need the combined cycle power station cover the power requirements of the grid; on the basis of the method according to the invention, the water/steam circuit and the steam turbine are now ready to assist the gas turbine in covering transient load requirements, by supplying steam to the steam turbine. It is also advantageous for further holding points to be provided in the load program, at which the water/steam circuit is successively charged with further energy. This further increases the steam turbine power that is available at short notice.

The water/steam circuit in the method according to the invention is operated as an accumulator for thermal energy which can be made available quickly in the form of hot, compressed steam.

If the combination system is provided with a drum boiler, a high-pressure drum and/or a low-pressure drum may be used as energy stores. In a system for carrying out the method, the drums may advantageously be derated as energy stores. Saturated water is then stored in the drums, as well as steam. In one preferred method variant, in which steam must be supplied when a power demand is made on the steam turbine, and a main control valve that is arranged between the boiler and the steam turbine is opened, the backpressure in the boiler falls spontaneously by a small amount. The saturated water boils, and is thus able to provide large amounts of steam quickly.

In a further advantageous refinement of the method, steam is passed at least at times from the high-pressure section of the steam generator to the low-pressure section or medium-pressure section of the steam turbine. This is done either by temporarily reducing the pressure in the high-pressure section of the steam generator or by restricting the pressure between the steam generator and the steam turbine. One advantage of this method variant is that the moisture in the steam does not rise excessively during the expansion in the steam turbine and when the fresh steam temperature is low with a low gas turbine load.

The steam turbine may, of course, also be supplied with additional steam from an auxiliary boiler.

In addition, the power of a black starting diesel may also be used to compensate for transient load requirements.

APPROACH TO IMPLEMENTATION OF THE INVENTION

The FIGURE shows, by way of example, a combination system with black starting capability, with reference to which the method according to the invention will be explained in the following text. A gas turbine1and a steam turbine2drive a common generator3. In this case, the steam turbine2, which comprises a high-pressure housing201and a medium-pressure/low-pressure housing202, is coupled in a manner known per se via an automatically acting clutch4to the generator3. The exhaust gas from the gas turbine flows through a heat recovery steam generator5, in which steam is generated from the steam turbine. In this case, condensate from a condenser6is raised to a first pressure level by a low-pressure/medium-pressure feed pump7. This feed water flows through a feed water preheater501, and flows into a low-pressure/medium-pressure drum505. Water from the low-pressure/medium-pressure drum505circulates through a low-pressure/medium-pressure evaporator502, and is separated into boiling water and saturated steam in the drum505. Some of the boiling water circulates further through the low-pressure/medium-pressure evaporator502, where some of it is evaporated in each case. Another flow element of the boiling water in the drum is raised to a second pressure by a high-pressure feed pump8. The high-pressure feed water is evaporated in a positive flow evaporator504, and the resultant steam is superheated. The high-pressure steam finally flows via a steam control valve9to the inlet connection of the high-pressure housing201of the steam turbine2, where it is expanded, with useful power being produced, to a pressure which corresponds approximately to the first pressure level in the water/steam circuit. The saturated steam which is deposited in the drum flows out of the drum and is mixed with the partially expanded first flow element before it is passed to a low-pressure/medium-pressure superheater. From there, the superheated steam is passed to the inlet connection of the low-pressure/medium-pressure housing202of the steam turbine, and is expanded to the condenser pressure, with useful power being emitted. Finally, the expanded steam is liquefied again in the condenser6. The water/steam circuit described above is illustrated in highly simplified form. It could likewise be constructed differently in terms of the number of pressure stages, preheating stages and evaporator types, without affecting the idea of the invention. In particular, the low-pressure/medium-pressure evaporator could be in the form of a positive flow evaporator and superheater. The boiler could also be designed with a high-pressure drum. A large number of further variations will be familiar to those skilled in the art. During normal operation of the system, the generator switch S1and the grid switch S2are closed, while the switches S3to S6are open. The power station is connected to the electrical grid50via the grid switch S2. This is generally a large grid, to which a large number of further power stations and a large number of loads V1, V2, V3, . . . are connected, which are switched on and off via switches SV1, SV2, SV3, . . . The exemplary embodiment indicates that the power station is connected via the grid switch S2to a high-voltage rail51, and that a number of loads can be connected to this rail via switches. Apart from this, the electrical circuitry is illustrated in a highly schematic form; transformers, circuit breakers and further elements which are not essential for immediate understanding of the invention are omitted. A high-power electrical grid is generally very insensitive to fluctuating power demands, and power stations which are running under load have appropriate reserves for frequency support in their own right.

When the power station is started up in the normal way, the switches S1, S2, S5and S6are opened first of all. The switch S4is closed, and a starting apparatus10, for example a static frequency converter, is connected to the high-voltage rail51. The starting apparatus10is connected via the switch S3to the generator3, which is now operated as a motor, and the gas turbine is first of all accelerated to its self-sustaining rotation speed and remains in operation up to a certain rotation speed in order to assist the shaft acceleration. The switches S3and S4are opened at the latest when the gas turbine reaches its rated rotation speed, and the gas turbine runs on no load. The switch S1is closed, and the power station can be supplied by the generator via the in-house supply rail52, with the system now having the capability to run in an island mode, without any external power supply. By way of example, the illustration shows here that the feed pumps7and8are connected to the in-house supply rail52. In a next step, the power station is synchronized to the grid, and the grid switch S2is closed. The power from the gas turbine is increased in a controlled manner, at least in a lower load range, in accordance with a predetermined load program. The steam turbine2is started up; as soon as it reaches its rated rotation speed, it is connected to the generator via the automatically acting clutch4. The steam control valve9increases the amount of steam slowly in accordance with a predetermined load program for the steam turbine and/or as a function of measured material temperatures, in order to limit the mechanical load on the steam turbine caused by thermal stresses. In this way, the combination system is driven to produce a predetermined power requirement irrespective of the grid load; the frequency support for the grid50is provided by other systems which are connected to the grid.

The situation is fundamentally different when there is no energy in the grid50. In this case, the black starting capability of a system which can be connected to the grid50is required. For this purpose, the combined cycle power station illustrated by way of example is equipped with a black starting diesel11. The switches S1to S6are opened. The black starting diesel11is started using a starter motor12, which is supplied from a battery13. The black starting diesel11drives an auxiliary generator14. The switch S5is closed, and the starting apparatus10is supplied from the auxiliary generator14. As described above, the switch S3is closed, and the gas turbine1is accelerated to its rated rotation speed. The switches S3and S5are then opened. Before the gas turbine is started, the auxiliary generator14is connected to the in-house supply rail52by means of the switch S6. This makes it possible to ensure that the boiler is ready to start. The feed pumps7and8may be started up for this purpose. The condenser6is also evacuated as soon as possible. Once the gas turbine is running at its rated rotation speed without any load, the black starting diesel11initially, in one preferred variant, remains idle, or at a small load, which supplies electrical power to the most necessary power station components. The generator switch S1is closed, and the generator3which is driven by the gas turbine1supplies the power for the in-house supply for the power station. The power station is now running independently of any external or auxiliary power supply, autonomously in an island mode. If the feed pumps7and8have not yet been started, they are now started, and the condenser6is evacuated. In the island mode, the heat recovery steam generator5is heated up as much as possible, and the steam production process is started. The steam control valves9and15are closed, or are greatly restricted. When an adequate steam state is reached, the steam turbine is accelerated to its rated rotation speed. The switch S2is then closed, and the power station is connected with a minimum power load to the high-voltage rail51of the grid50, and feeds electrical power into the grid50which, until this point, has had no energy in it. Most of the load switches SV1, SV2, SV3, . . . are open; in this case, the loads may also be entire grid segments. The power from the power station is increased slowly. Loads must, of course, be connected in a comparable way such that the power demand on the grid corresponds at least approximately to the generator power; the system responds to small discrepancies by fluctuations in the grid frequency, to which the power station must react with power changes in order to keep the grid frequency within a tolerable range. The loads V1, V2, V3, . . . , which are connected successively, are not infinitesimal and, particularly in the starting phase of the grid load, are invariably significant relative to the total power. At a first moment, the rotating inert masses of the power station damp the load fluctuations before they can be regulated out by varying the useful power emitted from the turbines. According to the previous prior art, the gas turbine line control acts in order to support the frequency in response to certain load changes. As mentioned initially, this is not always possible with modern gas turbines, at least in a lower load range up to, for example, 40% or 50% of the relative gas turbine load; the load requirement could also fall into an unacceptable gas turbine power range. To a person skilled in the art, the expression relative gas turbine load or gas turbine power in fact means the ratio of the currently emitted useful power from the gas turbine to its maximum power at that time, and this is highly dependent on environmental conditions and the operating parameters of the gas turbine. According to the invention, the steam turbine is being operated at its rated rotation speed, at a low power or on no load. The valves9and15are highly restricted. Energy is stored in the heat recovery steam generator5in the form of hot, pressurized water and steam. In one preferred embodiment, the low-pressure/medium-pressure drum505is derated, and acts as an additional store for boiling water. This could in principle also be achieved by means of a high-pressure drum; in practice, however, a high-pressure reservoir of appropriate size results in relatively major problems, particularly with regard to the strength. The energy which is stored in the drum in the form of boiling water can be used by opening the control valve15: the drop in the backpressure causes this water to evaporate, and the saturated steam that is produced is superheated in the superheater503, and is supplied to the medium-pressure/low-pressure turbine. In the same way, the steam control valve9is opened, and the power emitted from the high-pressure steam turbine201likewise rises spontaneously. Although a procedure such as this produces thermal shocks and increases the thermal stresses within the steam turbine components, thus considerably shortening the life in some circumstances, this can invariably be tolerated, however, if the described method allows problem-free starting of an electrical grid with no energy in it, without the risk of emergency disconnections of the power station resulting from excessively low or high frequencies. To this extent, it is also preferable for the steam turbine to be operated under load as early as possible during the process of starting the electrical grid, in order that the steam turbine power control can react in both directions, that is to say not only when there is a threat of an excessively low frequency but also when there is a threat of an excessively high frequency. The gas turbine power is increased, controlled in accordance with a predetermined load program. During the starting process, the gas turbine does not react to load fluctuations, at least in a lower load range. The frequency support, that is to say the compensation for transient sudden load changes, is carried out by the steam turbine in the described manner. It is advantageous in this case for the load program for the steam turbine to include a number of holding points, at which the heat recovery steam generator is charged with further energy as an energy store, which improves the short-notice power potential of the steam turbine. In a further preferred method variant, the switch S6is closed. In this case, the power of the black starting diesel can then also be used for frequency support. Once the gas turbine line has reached a specific limit value—values above around 40% of the relative power, and preferably more than 50% of the relative power, are typical—the gas turbine is able to react to short-term changes in the power demand. In consequence, the gas turbine is now no longer loaded in a controlled manner but is operated in a regulated manner for frequency support, and, if required, is loaded further up to its maximum power. The steam turbine is removed from the control process, and is loaded conventionally, slowly, and avoiding excessive thermal stresses. The load is distributed between the gas turbine and the steam turbine from then on in accordance with a specific system operating concept. Furthermore, any diesel which is still being operated can also be stopped.

In a further method variant, it is possible to provide, if the boiler temperatures are low, for steam to be supplied from the high-pressure section504of the heat recovery steam generator to the low-pressure/medium-pressure section202of the steam turbine, for which purpose the pressure which the high-pressure feed pump8supplies is temporarily reduced, in a particularly advantageous manner. One major advantage is that the moisture in the steam in the steam turbine does not rise excessively when the exhaust gas temperature, and hence the fresh steam temperature, are low. The steam lines and restriction and shut-off devices required for this purpose are not shown in the FIGURE, for reasons of clarity. However, based on knowledge of the present description, a person skilled in the art will be able without any problems to implement this variant of the invention in an appropriately constructed combination system.

In addition to the specific requirement in the exemplary embodiment, a wide range of further system forms will be evident without any effort to the average person skilled in the art, by means of which the invention can be implemented; in particular, the described water/steam circuit does not represent any restriction: those skilled in the art will be directly familiar with a large number of different embodiments; advantageous method variants which, although not described explicitly, are still within the scope of the invention as it is characterized in the claims, result herefrom within the scope of the idea of the invention.

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