METHOD FOR OPERATING A COMBINED CYCLE POWER PLANT AND COMBINED CYCLE POWER PLANT FOR CONDUCTING SAID METHOD

TECHNICAL FIELD

The invention lies in the field of power generation. It relates to a method for operating a combined cycle power plant. It further relates to a combined cycle power plant for conducting such a method.

PRIOR ART

Fig. 1 depicts a typical steam turbine powertrain of a power plant PP 1 and the main associated steam lines. The steam turbine powertrain comprises high- pressure steam turbine 10, intermediate pressure steam turbine 1 1 and low- pressure steam turbine 12. High-pressure steam 13 (with or without

supplementary firing) enters high-pressure steam turbine 10. Intermediate pressure steam 16 exits high-pressure steam turbine 10, flows through cold reheat line 14 to reheat device 17 (which may be part of the heat recovery steam generator), is reheated and flows then through hot reheat line 15 to intermediate pressure steam turbine 1 1 . Finally, low-pressure steam 18 exits intermediate pressure steam turbine 1 1 and enters low-pressure steam turbine 12. If present, the degree of supplementary firing (SF) employed is limited by the inlet pressure of high-pressure steam turbine 10 and the associated water/steam cycle pressures.

Document US 6,442,924 teaches, within the context of supplementary firing, the art of injecting the steam generated by the duct burners, into locations

downstream of the inlet of high-pressure steam turbine 10 (see power plant PP 2 in Fig. 2). This measure permits a high-pressure steam turbine of a given design to always run at its design point (i.e. approximately constant pressure and mass flow rate), thereby performing optimally both with and without supplementary firing. Fig. 2 illustrates the various points A, B and C at which all of the additional steam raised by a supplementary firing can be injected (see arrows). However, this document does not describe increasing the mass flow rate through the high- pressure steam turbine 10.

Fig. 3 shows a typical arrangement of the so-called ISCC (Integrated Solar

Combined Cycle) power plant PP3, wherein the solar field of a solar thermal plant 20 (preferably via a central receiver technology) is used to generate additional solar steam 21 , which is then injected into (or slightly upstream of) the high- pressure steam turbine 10. An example of an ISCC is disclosed in document EP 2 372 1 16 A1 .

Configuring a combined cycle plant to operate in tandem with a solar steam generator and maximizing the overall plant flexibility leads to the identification of the issues 1 -3 listed below, whose resolution (a) increases the performance of the plant and (b) ensures high degrees of flexibility:

1 . The solar fields might produce more steam than can be accommodated by the high-pressure steam turbine 10. For instance, at 100% gas turbine load, a limited amount of solar steam 21 can be typically added so as to remain within the pressure limit at the inlet of the high-pressure steam turbine 10. Since oversizing the high-pressure steam turbine 10 leads to performance reductions in the absence of solar steam 21 , a significant quantity of solar steam 21 is rejected. This is a sub-optimal use of the expensive solar field.

2. It is potentially possible to run the ISCC in pure solar mode; i.e. the gas

turbines (not shown in the Figures) are switched off and the steam turbines 10, 1 1 and 12 are driven by steam that is generated exclusively by the solar field. Unfortunately, the cold reheat steam temperature falls during the long reheater section, resulting in steam that is saturated, or only a few degrees above saturation, at the inlet of the low-pressure steam turbine 12. The intolerance of such wet steam by the intermediate pressure steam turbine 1 1 (exit) and the low-pressure steam turbine 12 (inlet), therefore, precludes pure solar operation.

3. The solar steam quality and quantity cannot be constantly matched to the requirements of the steam turbine. This is problematic during planned transients (e.g. changing of plant load, changing of solar contribution) as well as during unplanned perturbations (e.g. rapid load changes, variable cloud cover), because large changes (> 50 °C) at the inlet of any of the steam turbine modules resulting in tripping of the steam turbine powertrain.

DISCLOSURE OF THE INVENTION

It is an object of the invention to propose a method for operating an ISCC power plant, whereby the output of the solar steam generator is used more effectively, and in which the overall plant performance, flexibility and operability are enhanced.

It is a further object of the invention to provide the ISCC power plant for conducting such a method. These and other objects are accomplished by a method according to claim 1 and a combined cycle power plant according to claim 9. The inventive method relates to the operation of a combined cycle power plant, which comprises a steam turbine powertrain with a high-pressure steam turbine, an intermediate pressure steam turbine and a low-pressure steam turbine, whereby intermediate pressure steam flowing from the exit of the high-pressure steam turbine to the inlet of the intermediate pressure steam turbine is reheated by means of the reheat device, and which is connected to a solar thermal plant, that generates additional solar steam for being used by said steam turbine powertrain. The inventive method is characterized in that at least part of the additional solar steam reaches the intermediate pressure steam turbine without being reheated in said reheat device.

According to an embodiment of the inventive method at least part of the additional solar steam is directly injected into a hot reheat line connecting said reheat device with the inlet of said intermediate pressure steam turbine. According to another embodiment of the inventive method at least part of the additional solar steam is directly injected into the inlet of said intermediate pressure steam turbine.

Specifically, said part of the additional solar steam, which is directly injected into a hot reheat line or inlet of said intermediate pressure steam turbine, is controllably throttled, especially be means of a control valve.

According to an further embodiment of the inventive method, in order to avoid cooling of the cold reheat steam in pure solar mode, the additional solar steam is injected into the inlet of the high-pressure steam turbine, and at least part of the intermediate pressure steam bypasses the reheat device on its way to the intermediate pressure steam turbine. According to another embodiment of the inventive method, in order to cater for any circumstances potentially resulting in large temperature changes at the inlets of the steam turbines, part of the additional solar steam is directly injected into the cold reheat line connecting the exit of the high-pressure steam turbine and the reheat device and bypasses the reheat device through a reheat bypass line, or is directly injected into a hot reheat line connecting said reheat device with the inlet of said intermediate pressure steam turbine. Specifically, the high-pressure steam turbine bypass line is provided for bypassing the high-pressure steam turbine, and that part of the additional solar steam flows to the cold reheat line to said high-pressure steam bypass line.

More specifically, the extent of the intermediate pressure solar steam flow is adjusted to simultaneously ensure that the high-pressure steam turbine exit temperatures are within design limits.

The inventive combined cycle power plant for conducting a method according to the invention comprises a steam turbine powertrain with a high-pressure steam turbine, an intermediate pressure steam turbine and a low-pressure steam turbine, whereby a reheat device is provided between the exit of said high-pressure steam turbine and the inlet of said intermediate pressure steam turbine, and a solar thermal plant that generates additional solar steam for being used by said steam turbine powertrain, is connected to the inlet of said high-pressure steam turbine. Itis characterized in that a way for the additional solar steam is provided to reach the inlet of said intermediate pressure steam turbine without being reheated in said reheat device.

According to an embodiment of the invention, the solar thermal plant is directly connected to the inlet of said intermediate pressure steam turbine by means of a lower pressure solar steam line. Specifically, a control valve for throttling the solar steam is provided in said lower pressure solar steam line.

According to another embodiment of the invention a reheat bypass line is provided between the cold reheat side and the hot reheat side of the reheat device.

Specifically, a high-pressure steam turbine bypass line is provided at said high- pressure steam turbine. More specifically, said steam turbine powertrain has a common shaft , and that said high-pressure steam turbine is disconnectable from the rest of the shaft to permit more rapid operational transients.

Preferably, a self-synchronizing clutch is provided between the high-pressure steam turbine and the rest of the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS The invention shall now be explained by means of different embodiments in connection with the drawings.

Fig. 1 shows a simplified scheme of a typical turbine powertrain

according to the prior art;

Fig. 2 shows a simplified scheme of a steam turbine powertrain with injection of additional steam generated by supplementary firing, according to the prior art;

Fig. 3 shows a simplified scheme of a steam turbine powertrain, which incorporates solar steam into its water/steam cycle; Fig. 4 shows a simplified scheme of a steam turbine powertrain with independent introduction of lower pressure solar steam directly into the intermediate pressure steam turbine according to an embodiment of the invention;

Fig. 5 shows a simplified scheme of a steam turbine powertrain, similar to Fig. 3, but with an additional reheat bypass line according to another embodiment of the invention; and

Fig. 6 shows a simplified scheme of a steam turbine powertrain with means for preventing excessive temperature drops at the intermediate pressure steam turbine inlet according to a further embodiment of the invention.

WAYS OF CARRYING OUT THE INVENTION

According to the invention it is proposed to implement a direct line, complete with its own control valve 22, from the main solar steam line directly to the inlet of the intermediate pressure steam turbine 1 1 , as illustrated in Fig. 4. Such a

configuration permits any excess solar steam that cannot be accommodated by the high-pressure steam turbine (due to pressure limits at the high-pressure steam turbine inlet) to be throttled as needed and injected into the hot reheat line 15 and/or into the intermediate pressure steam turbine; alternatively, part of the solar steam can be injected along the high-pressure steam turbine (between inlet and exit), as shown with the dashed line in Fig. 4). The performance benefit is highlighted in table 1 below. Table 1

At 100% GT load, this technique increases the solar electric power by up to 15MW (from 71 MW to 86MW), thereby halving solar wastage. Extending the inlet pressure limits of the intermediate pressure steam turbine 1 1 (a relatively minor matter, since the nominal pressure is only 45 bar - compared to 160 bar at the inlet of the high-pressure steam turbine 10) would permit more, or all, the excess solar steam to be accommodated. Such a minor upgrading of the respective

intermediate pressure drum (i.e. without step change in technology) permits standard drum wall thicknesses to be used and therefore do not hinder the high cycling capacity of a modern combined cycle power plant CCPP.

This idea can be used for single-shaft as well as multi-shaft power plant

configurations.

In order to avoid cooling of the cold reheat steam and in pure solar mode, it is further proposed to bypass the reheat line (reheat device 17) by means of a reheat bypass line 24 such that the cold reheat steam of cold reheat line 14 is fed directly to the inlet of the intermediate pressure steam turbine 1 1 as indicated for power plant PP5 in Fig. 5. Implementing this technique ensures a satisfactory margin above saturation at the inlet of the low-pressure steam turbine 12. Effectively, this method is indispensable for enabling operation in pure solar mode. This is because, if the cold reheat steam temperature falls by only 30 °C, then the intermediate pressure steam turbine exhaust steam and the low-pressure steam turbine inlet steam are saturated. Closer inspection of the CCPP process flow diagram reveals that the reheat bypass line 24 is also necessary for permitting solar operation during transients involving the heat recovery steam generator (e.g. gas turbine startup or shut down), because routing of the recovery steam generator bypass steam is completely isolated from that of the solar steam 21 .

The methodology described in Fig. 4 can also be applied to cater for any circumstances potentially resulting in large temperature changes at the steam turbine inlets. For instance, during the transition from ISCC to pure solar mode, in which the gas turbines are turned off and the reheat function thus ceases, the temperature at the intermediate pressure steam turbine inlet falls by as much as 150°C. In order to prevent this, part of the hot solar steam 21 is added (with the help of the existing high-pressure steam turbine bypass, 25 in Fig. 6, including attemperation to cold reheat conditions, see Fig. 6) directly to the cold reheat flow (line 14), which bypasses the reheat line (17) or to the hot reheat flow (line 15). The extent of intermediate pressure solar flow must be adjusted so as to ensure that the high-pressure steam turbine exit temperatures are within design limits. Disconnecting of the high-pressure steam turbine 10 from the rest of the shaft is an additional measure that could be taken to permit more rapid transients to be followed. Such a technique could be realized by adding a self-synchronizing clutch (SSC) 26 (Fig. 6). List of reference numerals

10 high-pressure steam turbine

1 1 intermediate pressure steam turbine

12 low-pressure steam turbine

13 high-pressure steam

14 cold reheat line

15 hot reheat line

16 intermediate pressure steam reheat device (e.g. heat recovery steam generator) low-pressure steam

gas turbine plant

solar thermal plant

solar steam

valve

lower pressure solar steam line

reheat bypass line

high-pressure steam turbine bypass line

self-synchronizing clutch (SSC)

power plant