Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-12-06
2004-09-21
Freay, Charles G. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S787000, C060S790000, C060S039170, C060S039183, C060S039270, C060S778000
Reexamination Certificate
active
06792760
ABSTRACT:
TECHNICAL FIELD
The invention pertains to a method of operating a turbine arranged in a compressed air energy storage (CAES) power generation plant and in particular to a method for a controlled start-up and shut-down of such a turbine.
BACKGROUND ART
A typical compressed air energy storage (CAES) power generation plant as shown in
FIG. 1
comprises a cavern (C) for compressed air, which is used to drive a single-shaft expansion turbine consisting of an air expansion turbine (AT) followed by a combustion chamber (CC). It furthermore comprises a gas turbine (GT), which during normal operation drives a fixedly coupled generator (G) in synchronisation with the mains.
Speed and power control of the turbine is usually realised by a coordinated control of valves, in particular of the air inlet valves (AV) at the air turbine inlet as well as the fuel inlet valves (FV) at the combustion chamber. The following disclosure pertains to a CAES power generation plant as shown in the schematic of
FIG. 1
as well as variants thereof. These are for an example CAES power plants with an additional combustion chamber arranged preceding the air turbine (AT) or a recuperator prior to the air turbine for preheating the pressurized air from the cavern.
The single-shaft turbine is started up from a stand-still or from a turning operation to synchronization, for which an open or closed-loop speed control is implemented. For part of this start-up procedure, the CAES plant comprises in addition a static frequency converter (SFC), that is arranged in parallel to the generator-to-mains breaker switch (S). The electrical brake torque of the SFC counteracts the drive torque of the turbine while the generator is not yet synchronized. It can also be used in order to shorten the coast-down procedure following a separation from the mains (M). The power output after the sychronization of the turbine occurs in conventional manner via the generator to the electrical consumer mains.
The start-up of a turbine comprises several steps:
Purging of the turbine and combustion chamber using preheated air from the cavern and via the air inlet valve.
Reduction of the purge air flow rate through the air inlet valve followed by ignition of the combustor.
Acceleration of the turbine by control of the air flow rate coordinated with the control of the combustor fuel flow rate while maintaining given process values such as gas turbine inlet temperature and amounts of toxic components in the exhaust gases from the turbine. At this step, critical operation parameters that are specific to this turbine are observed. These critical operation parameters are, for example, the mechanical stresses of the gas turbine final stage by windage effects due to insufficient flow rate at high speeds, and any possible critical speed ranges.
Synchronization of the turbine with the electrical consumer mains at rated speed as well as operation at minimal load.
The power operation at constant speed occurs by means of control of the electrical power output while given limits for the gas turbine inlet temperature and the amounts of toxic components in the exhaust gases of the gas turbine are observed. For this and a suitable operation procedure with regard to the combustion, the air mass flow and the gas fuel mass flow are given reference values, which are both power and temperature dependent and realized by means of the position control values of the air inlet and combustion fuel inlet valves. The shut-down of the plant following the separation from the mains occurs by means of the SFC as a brake in order for the turbine to pass rapidly through any critical speed ranges as well as for the coast-down time to be shortened.
A specific problem encountered during start-up of the turbine of this type is that the turbine, that is accelerated by means of a relatively small air mass flow, can quickly reach speeds at which windage effects due to insufficient flow volume can occur in the final stages of the gas turbine. Windage is caused by a faulty air flow about the blades, which causes unusual mechanical stress in the blades. Furthermore, this type of turbine tends to develop an “astatic” behavior with respect to speed, which is essentially due to the lack of a counter torque. For this reason, a controlled operation with regard to speed is only possible by means of a particular braking torque, which in this case is produced electrically at the generator by the SFC. (In other turbine plants such as steam turbines or gas turbines, there are braking mechanisms provided by means of the long blades in the steam turbine and the ventilation of the generator cooling air or the compressor for the gas turbine). According to the state of the art, the air mass flow during start-up may be given and the speed is strictly controlled by means of equilibrium between drive and brake. This kind of speed control is ambiguous from the control point of view as either the drive (air inlet) or the brake (SFC) or both can be used as control means.
In order to avoid final stage windage due to particular air mass flows in the air expansion turbine, it is usually required that the air mass flow and the turbine speed are functionally connected. The setting of any controlled speed requires, depending on the current brake torque, a particular air mass flow. In the case of a small brake torque, the airflow rate can be so low such that windage can set in. In other words, for every air flow rate, there is a particular maximum speed, at which windage sets in. If during a start-up, the speed is chosen as a control variable and the brake torque is very small, then uncontrolled windage can easily occur. In order to set the speed and avoid windage, either the air mass flow or the SFC brake torque or both can be varied.
SUMMARY OF INVENTION
In view of the described state of the art, it is an object of the invention to provide a method of operation of turbines arranged in a CAES power generation plant, in particular for start-up and shut-down of the turbines. The method shall be particularly suitable for the operation of turbines that have no braking mechanism other than a SFC.
It is a particular object of the invention to provide a method that is unambiguous in the choice of variables for speed control. It is a further particular object of the invention to provide a turbine speed control by which windage effects and astatic behaviour with respect to speed are prevented in an air expansion turbine.
According to the invention, a method for operating a turbine arranged in a CAES power generation plant is based on two different concepts applied to different turbine speed ranges. In a lower turbine speed range, it comprises an open-loop control of the air mass flow alone according to which the turbine speed is left free to develop. Specifically, the method comprises for the lower speed range, the control of the air mass flow to the air expansion turbine by means of setting the air inlet valve as a function of time and/or any possible operational state requirements of the turbine. These requirements are in accordance with the pertaining operation concept of the CAES plant, such as turbine and recuperator warm-up, purging, acceleration etc. In a higher turbine speed range it comprises a closed-loop control of the turbine speed where the turbine speed is controlled by a speed controller acting on a static frequency converter (SFC). The turbine speed is limited, if necessary, by a SFC such as in the case when the speed reaches a given determined speed range that is critical in regard to windage effects or rotor dynamics. In the closed-loop control, the turbine speed is limited according to the momentary airflow rate that results from the air inlet valve position. As the airflow generates a high driving torque that tends to accelerate the turbine rotor, the turbine speed is limited to an airflow dependent set point value by means of the speed controller, which activates the variable braking torque of the SFC.
The method according to the invention is particularly applicable to the start-up of an expansion tur
Mathias Gernot
Seketa Bozidar
Alstom Technology LTD
Burns Doane Swecker & Mathis L.L.P.
Freay Charles G.
LandOfFree
Method for operating a turbine does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for operating a turbine, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for operating a turbine will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3264047