Power plants – Motive fluid energized by externally applied heat – Process of power production or system operation
Reexamination Certificate
2002-07-30
2003-11-18
Nguyen, Hoang (Department: 3748)
Power plants
Motive fluid energized by externally applied heat
Process of power production or system operation
C060S662000, C060S663000, C060S679000
Reexamination Certificate
active
06647727
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for controlling a low-pressure bypass system for a steam turbine installation having a reheater, in particular for run-up from the cold condition, for idling and low-load operation and after load rejection procedures.
PRIOR ART
Steam turbine installations with reheaters are steam turbines in which the steam emerging from turbines of certain pressure stages is heated again in a reheater before its introduction into a turbine with lower inlet pressure and, after passing the lower pressure units, is supplied to a condenser.
In order to start the block, it has been found advantageous, for various reasons and as a function of the mode of operation, to initially load the boiler to a minimum output by means of a steam bypass system, and then to begin the turbine starting procedure. Thus, in the case of a cascading bypass or wet reheater bypass system, the live steam is guided by means of a high-pressure bypass valve with downstream water injection to the “cold” reheater rail and, after superheating in the reheater, is led from the “hot” reheater rail via a low-pressure bypass valve into the condenser. This bypass system is also employed if, after a load rejection (for example when the mains circuit-breaker is opened) or in the case of a rapid shut-down of the turbine, the steam generated cannot be completely accepted by the turbine or cannot be accepted at all. Whereas the high-pressure bypass system depends essentially on the requirements of the boiler, the design and control of the low-pressure bypass system is determined by the steam turbine. The core feature in controlling a low-pressure bypass control system is the pressure control of the hot reheater main. As a rule, the required value control for the low-pressure bypass system consists of a minimum pressure and a floating pressure characteristic curve, which are triggered by a choice of maximum.
In the past, there were various concepts for forming the floating pressure characteristic curve for the required value control for the reheater pressure:
Derivation from the output.
Derivation from the pressure before the blading of the high-pressure turbine.
Derivation from the reheater steam flow, which can be calculated or measured.
A common feature of all these concepts is the fact that a certain required value characteristic curve is formed by means of suitable measures, which required value characteristic curve is located, during turbine operation, above the “natural” floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine, so that the low-pressure bypass station remains closed.
PRESENTATION OF THE INVENTION
The invention is therefore based on the object of making available a method for controlling a steam turbine installation having a reheater arranged between high-pressure turbine and medium-pressure turbine or low-pressure turbine, the steam turbine installation also comprising a low-pressure bypass with a low-pressure bypass valve, which low-pressure bypass leads from the reheater outlet into a condenser. The method is intended to permit the determination of the control of the reheater pressure value required for the low-pressure bypass station in such a way that, on the one hand, the low-pressure bypass station remains closed in turbine operation but, on the other hand, account is taken of the requirements arising from the starting and low-load operation of the high-pressure steam turbine so that, for example, windmilling of the high-pressure turbine is prevented or limited.
The present invention achieves this object in that characteristic curves for the required value of the reheater pressure are used for controlling the low-pressure bypass valve during run-up, during (partial) load rejection procedures or during idling, which characteristic curves depend on the load applied to the installation, and/or on the pressure before the high-pressure turbine blading and/or on the reheater steam flow, and also on the high-pressure turbine exhaust steam temperature, and/or on the temperature and/or on the pressure of the live steam introduced into the high-pressure turbine, and/or on the reheater pressure.
The core of the invention therefore consists in the provision not only of one single characteristic curve for the required reheater pressure value of the low-pressure bypass device as a function of one of the parameters known for this from the prior art, namely the load applied to the installation and/or the pressure before the high-pressure turbine blading and/or the reheater steam flow but, rather, of a characteristic curve field which, on the basis of one or a plurality of the previously mentioned parameters according to the prior art, additionally determines the required reheater pressure value as a function of the high-pressure exhaust steam temperature from the high-pressure turbine and/or of the temperature and/or of the pressure of the live steam introduced into the high-pressure turbine, and/or of the reheater pressure in the cold reheater rail.
In this arrangement, the swallowing capacity of the low-pressure bypass station is preferably designed in such a way that the associated floating pressure characteristic curve is located below the natural floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine, when the low-pressure bypass valve is fully open.
According to a first preferred embodiment of the method, the characteristic curves are located, as a function of the load applied to the installation and/or of the pressure before the high-pressure turbine blading and/or of the reheater steam flow, between a maximum characteristic curve, determined by the required pressure value during turbine operation, and a minimum characteristic curve, determined by the minimum permissible pressure at full reheater flow, for different values of high-pressure turbine exhaust steam temperature and/or of the temperature and/or of the pressure of the live steam introduced into the high-pressure turbine and/or of the reheater pressure. The maximum characteristic curve determined by the required pressure value during turbine operation is typically located in this arrangement approximately 5 percent above the natural floating pressure characteristic curve, which is determined by the swallowing capacity of the medium-pressure turbine. In addition, the characteristic curves have, in the usual manner, a maximum value which is provided by the opening pressure of the safety valves.
According to a further preferred embodiment of the method, the characteristic curves are essentially linear with respect to the load applied to the installation, and/or with respect to the pressure before the high-pressure turbine blading, and/or with respect to the reheater steam flow. In a further preferred embodiment, the characteristic curves are also essentially linear with respect to the high-pressure turbine exhaust steam temperature, and/or with respect to the temperature and/or with respect to the pressure of the live steam introduced into the high-pressure turbine, and/or with respect to the reheater pressure. In addition, the characteristic curves preferably extend through a common origin, i.e. rotations about this common origin result for different values of high-pressure turbine exhaust steam temperature, and/or temperature and/or pressure of the live steam introduced into the high-pressure turbine, and/or reheater pressure.
According to another preferred embodiment of the method, the pressure in the cold reheater main before reheater inlet is used as a further control parameter for determining the characteristic curves.
In a particularly preferred method, the required value for the high-pressure exhaust steam temperature is determined by the live steam temperature, the high-pressure exhaust steam temperature being controlled to a value which is located around a constant value of preferably, in particular, in the region of 100 degrees Celsius below the live steam temperature. In t
Klatt Stefan
Schnaithmann Kurt
Alstom (Switzerland) Ltd.
Cermak Adam J.
Nguyen Hoang
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