Abstract:
A method for regulating a short-term power increase of a steam turbine with an upstream waste-heat steam generator is provided. The steam turbine has a number of economizer, evaporator and super heater heating surfaces forming a flow path for a flow medium. The flow medium is tapped off from the flow path in a pressure stage and is injected into the flow path on the flow-medium side between two super heater heating surfaces of the respective pressure stage. Amount of flow medium injected is regulated with a characteristic value which is a discrepancy between the outlet temperature of the final super heater heating surface and a predetermined temperature nominal value. The temperature nominal value is reduced and the characteristic value is temporarily increased more than in proportion to the discrepancy for a time period of the reduction for achieving a short-term power increase of the steam turbine.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2011/065221 filed Sep. 02, 2011 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2010 040 623.6 filed Sep. 13, 2010, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a method for regulating a short-term power increase of a steam turbine with an upstream waste heat generator having a number of economizer, evaporator and superheater heating surfaces which form a flow path and through which a flow medium flows, in which flow medium is tapped off from the flow path in a pressure stage and is injected into the flow path on the flow-medium side between two superheater heating surfaces of the respective pressure stage, with a first characteristic value which is characteristic of the discrepancy between the outlet temperature of the final superheater heating surface on the flow medium side of the respective pressure stage and a predetermined temperature nominal value being used as a regulation variable for the amount of flow medium injected. 
       BACKGROUND OF THE INVENTION 
       [0003]    A waste heat steam generator is a heat exchanger which recovers heat from a hot gas flow. Waste heat steam generators are frequently used in gas and steam turbine plants (combined cycle power plants) which are used predominantly for electricity generation. In such cases a modern combined cycle power plant usually comprises one to four gas turbines and at least one steam turbine, wherein either each turbine drives a generator in each case (multi-shaft system) or a gas turbine drives the single generator with the steam turbine on a common shaft (single shaft system). The hot exhaust gases of the gas turbine are in such cases used in the waste heat steam generator for generation of water steam. The steam is subsequently fed to the steam turbine, usually around two thirds of the electrical power is allocated to the gas turbine and one third to the steam process. 
         [0004]    Similarly to the various pressure stages of a steam turbine, the waste heat steam generator also comprises a plurality of pressure stages with different thermal states of the water-steam mixture contained therein in each case. In the first (high) pressure stage the flow medium on its flow path initially passes through economizers, which use the residual heat to preheat the flow medium, and subsequently different stages of evaporator and superheater heating surfaces. In the evaporator the flow medium is evaporated, thereafter any possible residual moisture is separated in a separation device and the steam left behind is heated up further in the superheater. Thereafter the superheated steam flows into the high-pressure part of the steam turbine, is expanded there and supplied to the following pressure stages of the steam turbine. It is superheated again there and supplied to the next pressure section of the steam turbine. 
         [0005]    Because of load fluctuations the heat power transferred to the superheater can be heavily influenced. It is therefore frequently necessary to regulate the superheating temperature. In new plants this is mostly achieved by an injection of feed water between the superheater surfaces for cooling, i.e. an overflow line branches off from the main flow of the flow medium and leads to injection valves disposed accordingly there. The injection in such cases is usually regulated via a characteristic value predetermined for the temperature discrepancies from a predetermined temperature nominal value at the outlet of the superheater. 
         [0006]    The demands imposed on modern power plants include not only high levels of efficiency but also a method of operation that is as flexible as possible. In addition to short start-up times and high load change speeds, this also includes the option of compensating for frequency disruptions in the electricity grid. In order to fulfill these requirements the power plant must be in a position to be able to provide power increases of for example 5% within a few seconds. 
         [0007]    This is realized in previous usual combined cycle power plants by increasing the load on the gas turbines. Under certain circumstances however it can be possible, especially in the upper load range for the desired power increase not to be able to be provided exclusively by the gas turbines. Therefore in the interim solutions have also been pursued in which the steam turbine likewise can and is intended to make an additional contribution to frequency support in the first seconds. 
         [0008]    This can occur for example by opening partly throttled turbine valves of the steam turbine or of a so-called step valve, through which the steam pressure upstream of the steam turbine is reduced. Steam from the steam reservoir of the upstream waste heat steam generator is stored by this process and supplied to the steam turbine. With this measure a power increase in the steam part of the combined cycle power plant is achieved within a few seconds. 
         [0009]    This additional power can be released within a relatively short time so that the delayed heat power increase by the gas turbines (limited by their maximum speed of load change resulting from design and operational constraints) can be at least partly compensated for. The entire block makes a direct jump in performance through this measure and through a subsequent increase in power of the gas turbine can also maintain or exceed this power level permanently, provided the plant is in the part load range at the point at which the additional power reserves are requested. 
         [0010]    A permanent throttling of the turbine valves to maintain a reserve however always leads to a loss of efficiency so that, for cost effective operation the degree of throttling should be kept as low as is absolutely necessary. In addition a few designs of waste heat steam generator, thus for example once-through circulation steam generators, under some circumstances have a significantly smaller storage volume than for example natural circulation steam generators. The difference in the size of the boiler has an influence in the method described above on the behavior during changes in power of the steam part of the combined cycle power plant. 
       SUMMARY OF THE INVENTION 
       [0011]    The object of the invention is thus to specify a method for regulation of a short-term power increase of a steam turbine with an upstream waste-heat steam generator of the type given above, in which the efficiency of the steam process is not disproportionately adversely affected. At the same time the short-term power increase should be made possible independently of the design of the waste-heat steam generator without invasive structural modifications to the overall system. 
         [0012]    This object is achieved in accordance with the invention, for a short-term power increase of the steam turbine, by reducing the temperature nominal value and increasing the characteristic value for the period of the reduction of the temperature nominal value temporarily disproportionately to the discrepancy. 
         [0013]    The invention is based on the idea that additional injection of feed water can make a further contribution to a rapid variation in power. Through this additional injection in the area of the superheater the steam mass flow can namely be temporarily increased. The injection is triggered in such cases by the temperature nominal value being reduced. A jump in the temperature nominal value is linked via a corresponding characteristic value to a jump in the regulator discrepancy, which causes the regulator to vary the degree of opening of the injection regulation valve. Thus a power increase of the steam turbine can be realized precisely by such a measure, i.e. a sudden reduction in the temperature nominal value. 
         [0014]    This power increase and thus also the injection mass flow should further be provided as quickly as possible. In such cases however attenuating properties of the regulation system can be a hindrance, which prevents disproportionately fast variations of the injection mass flow which, although desirable in normal load operation, are not desirable for an increase in power to be provided quickly. Therefore the regulation should be adapted accordingly for the case of a short-term power increase. This is possible in an especially simple manner by the regulation signal for the injection mass flow being amplified accordingly, and by this being done for the period of the desired short-term power increase. For this purpose the characteristic value for the discrepancy between the outlet temperature of the last superheater heating surface on the flow medium side and a predetermined temperature nominal value is temporarily increased disproportionately to the discrepancy for the period of the reduction of the temperature nominal value. 
         [0015]    In the method described above in a corresponding regulation system an actual-nominal comparison between desired and measured steam temperature is made via a subtractor element. Depending on the regulation concept used, this signal can still be modified further by additional information from the process, before it is subsequently switched as an input signal (regulation discrepancy) to a PI controller for example. Advantageously the temperature can additionally be used as a regulation variable directly after the injection point of the flow medium, i.e. at the inlet of the last superheater heating surface. In this type of so-called dual circuit regulation sudden variations in the injection mass flow which have occurred as a result of the regulator intervention are damped out. Under these circumstances the regulation optimized for rapid interventions can be stabilized by preventing an overshoot. 
         [0016]    For the provision of an immediate reserve by the injection system this damping effect of the dual-circuit regulation is however rather a hindrance. Therefore, with dual-circuit regulation in particular, it is especially advantageous to perform the described amplifying adaptation of the characteristic value. What the artificial regulation-side increase of the discrepancy created thereby of the actual temperature from the predetermined nominal value namely achieves is that the subsequent correction by the temperature at the inlet of the last superheater heating surfaces, i.e. directly after the injection point, ends up comparatively lower in dual-circuit regulation. This means that a greater regulation discrepancy remains, which directly results in a greater regulator response, i.e. a greater increase in the injection mass flow, which is desired in this case. The fact that the characteristic value however is only increased temporarily disproportionately for the period of the reduction of the temperature nominal value means that the influence of this overincrease disappears again, so that the steam temperature set via the nominal value can also actually be achieved. Thus the advantage of dual-circuit regulation, of avoiding impermissible drops in steam temperature, is still retained. 
         [0017]    In an especially simple manner the temporary increase of the characteristic value can be advantageously created by the characteristic value characteristic for the discrepancy between the temperature and the nominal value being formed from the sum of this discrepancy and a second characteristic value characteristic for the change over time of the temperature nominal value. In this case, in an especially advantageous embodiment, the second characteristic value is essentially the change over time of the temperature nominal value multiplied by an amplification factor. In regulation technology terms, this is achieved by the predetermined steam temperature nominal value being used as an input signal of a first order differentiation element and the output of this element being subtracted, after suitable amplification from the difference between measured and predetermined temperature at the heater surface outlet. The desired artificial increase of the discrepancy is realized especially easily in this way and via the additional first-order differentiation element the injection mass flow and thus the power additionally released via the steam turbine is increased significantly faster. 
         [0018]    Because of the differential character, i.e. taking account of the change in the nominal value over time, the influence of such regulation on the system as a whole decreases over the course of time (disappearing pulse). This means that the differentiation element has no further influence on the regulation discrepancy and the current temperature set via the nominal value is also reached. Even in the event of the nominal value of the steam temperature not changing (the normal situation in conventional load operation) such an embodiment has no influence on the remaining regulation structure. Thus in conventional load operation no differences occur in the regulation behavior of the steam temperature regulation between the regulation structure with or without this additional differentiation element. 
         [0019]    In an advantageous embodiment a parameter of one of the characteristic values is determined specifically for the plant. This means that the level of the amplification, the parameters of the differentiation element etc. are to be determined specifically on the basis of the plant concerned in the individual case. This can be done for example in advance with the aid of simulation calculations or can happen during commissioning of the regulation. 
         [0020]    In the waste heat steam generators normally used today there is also injection on the flow medium side downstream from the superheater heating surfaces into the flow path (end injection). However the use of the injection (intermediate injection) described above disposed between the superheater heating surfaces during use for providing a power reserve produces a higher energy yield, since only here can there be an explicit utilization of the thermal energy which is stored in the heating surfaces located upstream. However system conditions mean that it takes some time until the additional intermediate injection makes itself felt in the form of additional power at the steam turbine, since first of all the entire superheater path downstream from the intermediate injection must be loaded up before an increased steam mass flow becomes evident as a result of an additional injection at the turbine inlet. 
         [0021]    For this reason it is of advantage to also use the final injection and thus also the thermal energy stored in the steam line pipe wall of the fresh steam line to the steam turbine. Because of the fact that the final injection is disposed directly at the inlet into this fresh steam line, the reaction namely takes place directly, i.e., when an injection regulating valve of the final injection is opened, a higher steam mass flow is present relatively rapidly at the turbine inlet and thus ensures a rapid power increase. This however functions only as long as the thermal reservoir of the fresh steam line is not yet fully utilized for the present application, however this store is expected to be sufficient until the additionally obtained power via the intermediate injection comes into play. In concrete terms this means that the dead time or reaction time of the intermediate injection in respect of the provision of the immediate reserve can be effectively compensated for by inclusion of the final injection. 
         [0022]    For this purpose the temperature nominal value is also reduced during the end injection for a short-term increase in the power of the steam turbine, which is also used here as a regulation variable for the amount of flow medium M injected. This change however is applied in usual systems with a slight time delay (e.g. in control technology terms by a PTn element). This time delay models the temporal behavior of the superheater path between intermediate and final injection, i.e. it is advantageously characteristic of the throughflow time of the flow medium M through the superheater heating surfaces and of its thermal behavior between the two injection points. Under these circumstances the intermediate injection regulation valve opens first, since this first experiences the change in the temperature nominal value. Because of the injection amount introduced the temperature before the final injection reduces with the temporal behavior of the superheater path. Thus in the usually favorable case the final injection is not active, which under certain circumstances is desirable in conventional operation. If however the final injection is to be used as a result of the said advantages, this must become active directly after the application of the nominal value change to the assigned characteristic value. For this purpose the time delay of the temperature nominal value is advantageously deactivated in the definition of characteristic value. 
         [0023]    In any event it should be guaranteed for this measure that the final injection is optimized in respect of the injection quality, so that a finer spray mist is created. This avoids large water droplets entering into the steam turbine and being able to damage the turbine. With an appropriately fine spray mist all water droplets are already evaporated as soon as they reach the steam turbine. 
         [0024]    In an advantageous embodiment a regulation system for a waste heat steam generator with a number of economizer, evaporator and superheater heating surfaces forming a flow path through which a flow medium flows comprises means for executing the method. In a further advantageous embodiment a waste heat steam generator for a combined cycle power plant has such a regulation system and a combined cycle power plant has such a waste heat steam generator. 
         [0025]    The advantages obtained by the invention especially consist of enabling, through the explicit reduction of the steam temperature nominal value using the injection regulation method, the thermal energy stored in the metal masses downstream from the injection to be used for a temporary power increase of the steam turbine. If in such cases the adapted regulation method described is used, in the event of a sudden reduction of the steam temperature nominal value, significantly faster power increases are able to be realized with the aid of the injection system. 
         [0026]    In addition the method for providing a temporary power increase of the steam turbine is independent of other measures, so that also for example throttled turbine valves can additionally be opened in order to further amplify the power increase of the steam turbine. 
         [0027]    The effectiveness of the method remains largely unaffected by these parallel measures. 
         [0028]    In such cases it should be stressed that, for a fixed predetermined requirement for additional power, the degree of throttling of the turbine valves can be reduced, should the use of the injection system be applied for increasing the power. The desired power release can under these circumstances then be achieved with a lower, in the most favorable case entirely without, additional throttling. Thus in normal load operation in which it must be available for an immediate reserve, the plant can be operated with a comparatively high level of efficiency, which also lowers operational costs. 
         [0029]    A further advantage of the method consists of being able to lower the actual steam temperature by the temperature regulation concept. Maximum allowable temperature transients of the steam turbine are not exceeded under these circumstances with a maximum possible power increase. Precisely in respect of the additional use of the final injection, the fresh steam temperature can be adjusted very precisely. 
         [0030]    Finally the method is also to be realized without any invasive constructional measures, but can merely be undertaken by an implementation of additional components in the regulation system. Greater plant flexibility and benefit without additional costs is achieved by this method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    An exemplary embodiment of the invention will be explained in greater detail with reference to a drawing, in which: 
           [0032]      FIG. 1  shows a flow-medium-side schematic of the high-pressure part of a waste heat steam generator with data-side connection of the intermediate injection regulation system for use for an immediate power release, and 
           [0033]      FIG. 2  shows a flow-medium-side schematic of the high-pressure part of a waste heat steam generator with data-side connection of the final injection regulation system for use for an immediate power release. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The same parts are provided in both figures with the same reference characters. 
         [0035]      FIG. 1  for example shows the high-pressure part of the waste heat steam generator  1 . The invention can naturally also be used in other pressures stages for regulation of the intermediate superheating.  FIG. 1  schematically represents a part of the flow path  2  of the flow medium M. Of the heating surfaces of the economizer, evaporator and superheater usually disposed in the high-pressure part of the waste heat steam generator  1  only the last superheater heating surfaces  4  are shown. The spatial arrangement of the individual superheater heating surfaces  4  in the hot gas duct is not shown and can vary. The superheater heating surfaces  4  shown can each be representative of a plurality of serially connected heating surfaces which, for reasons of clarity, are not shown differentiated. 
         [0036]    Before it enters the part shown in  FIG. 1 , the flow medium M is conveyed from a feed pump at the corresponding pressure into the high-pressure flow path  2  of the waste heat steam generator  1 . During this process the flow medium M initially passes through an economizer, which can comprise a plurality of heating surfaces. The economizer is typically disposed in the coldest part of the hot gas duct, in order to obtain a use of residual heat here to increase the efficiency. Subsequently the flow medium M passes through the heating surfaces of the evaporator and a first superheater. In such cases a separation device can be disposed between the evaporator and superheater, which removes the residual moisture from the flow medium M so that only pure steam gets through to the superheater. 
         [0037]    Disposed on the flow medium side downstream from a first superheating heating surface not shown in the figure is an intermediate injection valve  6 , a further final injection valve  8  is disposed after the last superheater surface  4 . Here cooler and unevaporated flow medium M for regulating the outlet temperature at the outlet  10  of the high-pressure part of the waste heat generator  1  can be injected. The amount of flow medium M introduced into the intermediate injection valve  6  is regulated via the injection valve  12 . The flow medium M in this case is supplied via an overflow line  14  branching off beforehand in the flow path  2 . Disposed in the flow path  2  for regulating the injection are a number of measurement devices, namely a temperature measurement device  16  upstream from the intermediate injection valve  6 , a temperature measurement device  18  and a pressure measurement device  20  downstream from the intermediate injection valve  6  and upstream from the superheater heating surfaces  4 , as well as a temperature measurement device  22  downstream from the superheater heating surfaces  4 . 
         [0038]    The other parts of  FIG. 1  show a regulation system  24  for the intermediate injection. First of all a temperature nominal value is set at a nominal value generator  26 . This temperature nominal value is switched together with the temperature measured at the temperature measurement device  22  downstream from the superheater heating surface  4  to a subtractor element  28 , where the discrepancy between the temperature at the outlet of the superheater heating surfaces  4  and the nominal value is thus formed. This discrepancy is corrected in an adder element  30 , wherein the correction models the time delay of a temperature change during passage through the superheater heating surfaces  4 . To this end the temperature at the inlet of the superheater heating surfaces  4  is switched from the temperature measurement device  18  to a time-delayed PTn element  32 . The signal produced is switched together with the value from the temperature measurement device  18  to a subtractor element  34 , the output of which is supplied to the adder element  13 . The subtractor element  34  consequently only supplies a value other than zero for a certain time after a change of the temperature at the temperature measurement device  18 , which corrects the discrepancy present at the adder element. 
         [0039]    The signal present at the adder element  30  is switched together with further signals to a minimum element  36 , which takes account of further parameters: On the one hand the temperature downstream from the intermediate injection must have a certain distance from the pressure-dependent boiling temperature. For this purpose the pressure measured at the pressure measurement device  20  is switched into a functional element  38 , which outputs the boiling temperature of the flow medium M corresponding to this pressure. In an adder element  40  a preset constant from a generator  42  is added, which can amount for example to 30° C. and guarantees a safety gap to the boiling line. The minimum temperature thus determined is switched together with the actual temperature determined at the temperature measurement device  18  to a subtractor element  44  and the discrepancy thus determined is given to the minimum element  36 . For reasons of clarity a few switching connections are not shown in  FIG. 1  but are indicated by appropriate connection symbols &lt;A&gt;, &lt;B&gt;, &lt;C&gt;. 
         [0040]    Furthermore following injection, a certain enthalpy of the flow medium M must be guaranteed which must not fall below a certain level for operational reasons. To this end the signals of the pressure measurement device  20  as well as those of the temperature measurement devices  16 ,  18  upstream and downstream from the intermediate injection are switched to the enthalpy module connected upstream from the minimum element  36 . The enthalpy module  46  for its part calculates an associated temperature difference on the basis of these parameters which will be connected as an input signal to the following minimum element  36 . The signal determined in the minimum element  36  is connected to a regulation element  48  for controlling the injection regulation valve  12 . 
         [0041]    To enable the injection system to be used not only for regulation of the outlet temperature, but also to provide an immediate power reserve, said system comprises corresponding means for executing the method for regulating a short-term power increase of the steam turbine. Initially for this purpose the temperature nominal value is reduced at the nominal value generator  26 , which results in an increase in the intermediate injection amount. However so that this leads directly to a power increase the, a rapid regulator response of the PI regulator element  48  should be guaranteed. The discrepancy caused between the actual temperature and the temperature nominal value will however be ameliorated by the PTn element  32  shortly after the change. 
         [0042]    In order to prevent this in the event of a desired rapid power increase, the signal of the nominal value generator  26  for the temperature nominal value is to be switched to a first-order differentiation element (DT1). For this a PT1 element  50  has the signal of the nominal value generator  26  applied to it on the input side and is switched together with the original signal of the nominal value generator  26  to a subtractor element  52  on the output side, the output of which is connected to a multiplier element  54  which amplifies the signal by a factor, e.g. 5,from a generator  56 . This signal is in its turn added by a subtractor element  58  into the signal to the adder element  30 . In the event of a change in the nominal value, the circuit outputs a signal differing from one via the PT1 element  50 , which is amplified by the multiplier element  54  and artificially disproportionately amplifies the value characteristic for the discrepancy. The signal via the loop with the PTn element  32  is then relatively smaller and a faster regulator response of the PI regulator element  48  is forced. Thus a steam amount increase is rapidly achieved and the power of the downstream steam turbine is increased. 
         [0043]      FIG. 2  now shows the parts of the regulation system  24  relevant to the final injection. There is a further temperature measurement device  60  here in the flow path  2  downstream from the final injection valve  8  Likewise the temperature nominal value of the nominal value generator  26  is used here as a regulation variable. Its signal is sent to a PTn element  62  which, like the PTn element  32 , models the time delay through the superheater heating surfaces  4 . Its output signal is sent together with the signal of the nominal value generator  26  to a maximum element  64 , of which the output signal together with the signal from the temperature measurement device  60  is sent into a subtractor element  66 . The discrepancy determined there is sent to a PI regulation element  68  which regulates the injection regulation valve  70  of the final injection. 
         [0044]    In the event of a change in the temperature nominal value via the nominal value generator  26 , the PTn element, in combination with the maximum element  64 , here delays the regulator response of the PI regulator element  68 . In order to prevent this for the case of a final injection desired quickly, the time delay, i.e. the PTn element  62 , is deactivated for a time in such a case. This speeds up the regulator response accordingly and a fast power release is possible. 
         [0045]    A waste heat steam generator  1  regulated in this way is now used in a combined cycle power plant. Here the hot waste gases of one or more gas turbines are routed on the flue gas side through the waste heat steam generator  1 , which thus provides steam for a steam turbine. The steam turbine in this case comprises a number of pressure stages, i.e. the steam heated up by the high-pressure part of the waste heat steam generator  1  and expanded in the first stage (high-pressure stage) of the steam turbine is routed into a medium-pressure stage of the waste heat steam generator  1  and is superheated there once again, but to a lower pressure level however. As already mentioned, the exemplary embodiment shows the high-pressure part of the waste heat steam generator  1  to illustrate the invention by way of example, but this can also be used in other pressure stages. 
         [0046]    A combined cycle power plant equipped with such a waste heat steam generator is able not only to provide a short-term power increase of the gas turbine which is restricted by the allowed maximum load change speed, but also to rapidly provide a power increase via an immediate power release of the steam turbine, which serves to support the frequency of the electricity grid. 
         [0047]    The fact that this power reserve is achieved by a double use of the injection valves as well as the usual temperature regulation also enables a permanent throttling of the steam turbine valves in order to provide a reserve to be reduced or dispensed with entirely, whereby an especially high level of efficiency during normal operation is achieved.