BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to twin-shaft gas turbines, and more particularly, to controlling a gas generator in a twin-shaft gas turbine. 
     2. Description of the Related Art 
     Generally, a twin-shaft gas turbine includes a gas generator constituted by a compressor, a combustor, and a high-pressure turbine. This gas turbine also includes a low-pressure turbine (power turbine) connected to a load, with a gas generator shaft (a rotary shaft of the gas generator) being separated from a rotary shaft of the low-pressure turbine. In the gas generator, the compressor generates compressed air and supplies the compressed air to the combustor in which a fuel mixedly with the compressed air is then burned, thus combustion gases are generated. The combustion gases that have thus been produced by the combustor rotationally drive the high-pressure turbine to generate a driving force of the compressor. After this, the combustion gases are further sent to the low-pressure turbine to drive it for rotation. 
     In such conventional twin-shaft gas generator, control that provides angle control of the compressor inlet guide vane (IGV), based on a corrected rotation speed of the gas generator shaft, that is, corrected rotation speed responsive IGV angle control has been totally applied as most common gas-generator control, irrespective of an operational state of the gas generator. 
     The disclosures given in JP-2007-40171-A, JP-08-82228-A, and JP-63-212725-A, for instance, are known as examples of a twin-shaft gas generator. 
     SUMMARY OF THE INVENTION 
     As discussed above, corrected rotation speed responsive IGV angle control is conducted in the gas generator of the conventional twin-shaft gas generator specifications. In this case, as shown in  FIG. 7A , the IGV angle changes according to the corrected rotation speed having a correlation with respect to an atmospheric temperature. As shown in  FIG. 7B , therefore, lines of operation also change, which in turn changes the gas generator shaft rotation speed according to the atmospheric temperature. In addition, since positions on the lines of operation vary, the load or deterioration of the turbine further changes the gas generator shaft rotation speed. 
     These changes in the rotation speed of the gas generator shaft cause resonance problems. In other words, the changes in the rotation speed of the gas generator shaft make this shaft rotation speed more likely to approach a resonance rotation speed. As the shaft rotation speed actually approaches the resonance rotation speed, resonance arises and shaft vibration increases. Such a resonance problem as this becomes particularly serious during high-speed rotation under high load operating conditions, and the resonance under the high-speed rotational state enhances a possibility of damage to rotor blade of turbine or rotor blade of compressor. For these reasons, the control scheme that totally applies corrected rotation speed responsive IGV angle control has required imparting a structure for avoiding the resonance at the speed assumed, or imparting to rotor blade a structure that allows the rotor blade to withstand the resonance, and consequently, costs have increased. 
     The present invention has been made with a backdrop of the above circumstances, and an object of the invention is to effectively resolve resonance problems in a twin-shaft gas turbine, associated with changes in a rotation speed of a gas generator shaft, and more particularly, a resonance problem under a high-speed rotational state of the gas generator shaft. 
     Corrected rotation speed responsive IGV angle control is effective for avoiding compressor surging. However, in the regions that the shaft rotation speed of the compressor, or the rotation speed of the gas generator shaft, reaches a constant value or more, the compressor increases in stability and surging does not pose too serious a problem. Therefore, while corrected rotation speed responsive IGV angle control is required under a low-speed rotational state of the gas generator shaft, such IGV angle control is not always required during the high-speed rotational state of the gas generator shaft in the regions that the stability of the compressor is ensured. The resonance problem, on one hand, becomes serious during such high-speed rotation of the gas generator shaft as in the regions that compressor stability can be obtained. 
     Accordingly, corrected-speed responsive IGV angle control is applied to operational states under which the gas generator shaft rotates at low speed (these operational states occur during operational starts, during operational stops, and during low load operation under fixed or lower load conditions), whereas control for maintaining a constant gas generator shaft rotation speed, that is, shaft rotation speed constant IGV angle control is applied to an operational state under which the gas generator shaft rotates at high speed (i.e., high load operation). 
     Using appropriate one of different control modes for a particular operational state in this way makes it possible to resolve the resonance problem effectively and to respond to compressor surging effectively. This means that during the gas generator shaft high-speed rotation that renders the resonance problem particularly serious, since shaft rotation speed constant IGV angle control keeps the gas generator shaft rotation speed constant, the situation where the gas generator shaft rotation speed approaches the resonance rotation speed can be avoided effectively and the resonance problem can therefore be resolved effectively. In the meantime, compressor surging that becomes a problem during the low-speed rotation of the gas generator shaft can be avoided by using corrected rotation speed responsive IGV angle control. 
     The present invention solves the foregoing problem in line with the concepts described above. More specifically, a twin-shaft gas turbine with a gas generator including a compressor to generate compressed air, a combustor to generate combustion gases by burning a fuel mixedly with the compressed air supplied from the compressor, and a high-pressure turbine rotationally driven by the combustion gases supplied from the combustor, the high-pressure turbine being used to generate a driving force of the compressor, is configured described below. A first control mode and a second control mode are selectively usable for control of the gas generator. In the first control mode, an IGV angle in the compressor is controlled in accordance with a corrected shaft rotation speed of the gas generator, and in the second control mode, the IGV angle is controlled to maintain a constant gas generator shaft rotation speed. Furthermore, the first control mode is used to start, to stop, and to operate the turbine under fixed or lower load conditions, and the second control mode is used under operational states other than those to which the first control mode is applied. 
     Under a state of high gas-generator shaft rotation speeds and small IGV angles, deterioration of performance due to a separated flow of air at a blade of the compressor is prone to occur, which, in turn, easily causes icing as well. It is preferable, therefore, that such situations be made avoidable. For these reasons, in a preferred embodiment of such twin-shaft gas turbine of the present invention as outlined above, the gas turbine allows a third control mode to intervene during a mode change between the first control mode and the second control mode, and in the third control mode, a constant IGV angle is maintained without relying upon the rotation speed of the gas generator shaft. 
     According to the present invention outlined above, the resonance problem arising under a high-speed rotational state of a gas generator shaft in a twin-shaft gas turbine can be resolved effectively. Effective response to compressor surging can also be implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a configuration of a twin-shaft gas turbine according to a first embodiment; 
         FIG. 2  is a diagram showing an IGV angle controller configuration in the first embodiment; 
         FIGS. 3A and 3B  are diagrams that represent relationships of an IGV angle with respect to a corrected rotation speed and actual rotation speed of a gas generator shaft in the first embodiment; 
         FIG. 4  is a diagram showing a configuration of a twin-shaft gas turbine according to a second embodiment; 
         FIG. 5  is a diagram showing an IGV angle controller configuration in the second embodiment; 
         FIGS. 6A and 6B  are diagrams that represent relationships of an IGV angle with respect to a corrected rotation speed and actual rotation speed of a gas generator shaft in the second embodiment; and 
         FIGS. 7A and 7B  are diagrams that represent relationships of an IGV angle with respect to a corrected rotation speed and actual rotation speed of a gas generator shaft in a conventional control technique. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereunder, embodiments of the present invention will be described. A twin-shaft gas turbine  1  according to a first embodiment is shown in schematic form in  FIG. 1 . The twin-shaft gas turbine  1  includes a gas generator  2  and an output turbine  3 . 
     The output turbine  3  includes a low-pressure turbine  4  and a load  5  as its major constituent elements, the load  5  being connected to the low-pressure turbine  4  via an output turbine shaft  6  which also operates as a rotor of the turbine  4 . 
     The gas generator  2  includes a compressor  7 , a combustor  8 , a high-pressure turbine  9 , and a gas generator control unit  10 , as its major constituent elements. 
     The compressor  7  generates compressed air by letting air in from the atmosphere and compressing this air. Also, the compressor  7  has an inlet guide vane (IGV)  11  at its air inlet side. The IGV  11  is constructed to make its opening angle changeable via an IGV driver  12 , thus changing an air inlet rate of the compressor  7 . 
     The combustor  8  generates combustion gases  17  by receiving a fuel  15  from a fuel supply  13  via a fuel control valve  14  and burning the fuel  15  mixedly with the compressed air  16  from the compressor  7 . 
     The high-pressure turbine  9  adapted to transmit a driving force to the compressor  7  via a gas generator shaft  18  which is also a rotor of the turbine  9  is rotationally driven by the combustion gases  17  from the combustor  8  to generate the driving force. The combustion gases  17  that have acted upon the rotational driving of the high-pressure turbine  9  to decrease in pressure are further sent therefrom to the low-pressure turbine  4  to drive it for rotation. 
     The gas generator control unit  10  includes a fuel controller  19  and an IGV angle controller  20 . 
     The fuel controller  19  provides control of the fuel control valve  14 , based upon data from a rotation speed detector  27  which detects a rotation speed of the output turbine shaft  6 , and upon load state data obtained about the load  5 . Thus, the fuel controller  19  controls the supply of the fuel  15  from the fuel supply  13  to the combustor  8 . 
     The IGV angle controller  20  controls the angle of the IGV  11  through the control of the IGV driver  12 . An example of an IGV angle controller configuration is shown in  FIG. 2 . The IGV angle controller  20  in this example includes a first controller  21 , a second controller  22 , an operational state discriminator  23 , and a mode selector  24 . 
     The first controller  21  executes control in a first control mode. In the first control mode, the first controller  21  conducts corrected rotation speed responsive IGV angle control to adjust the IGV angle on the basis of the corrected rotation speed of the gas generator shaft  18 . This corrected rotation speed of the gas generator shaft  18  is obtained by normalizing an actual rotation speed value thereof (this value is given by a speed detector  25  that detects actual rotation speeds of the gas generator shaft  18 ) with an atmospheric temperature value (this value is given by a thermometer  26  that measures atmospheric temperatures). More specifically, the corrected rotation speed Nt is obtained using the following expression, with the actual rotation speed being represented as N and the atmospheric temperature as T:
 
 Nt=N·[ 288.15/(273.15+ T )] 1/2  
 
     The second controller  22  executes control in a second control mode. In the second control mode, the second controller  22  conducts IGV angle adjustments by shaft rotation speed constant IGV angle control to obtain a constant gas generator shaft rotation speed. This constant rotation speed by shaft rotation speed constant IGV angle control is a rated rotation speed, for example. 
     The operational state discriminator  23  discriminates a particular operational state on the basis of data such as the load data. More specifically, the operational state discriminator  23  discriminates whether the operational state of the turbine is a first operational state (either a starting operational state, a stopping operational state, or a low load operational state) or a second operational state (an operational state other than the first operational state, i.e., a high load operational state). This discrimination process assumes that IGV angle data on a stable operational region of the compressed air  16  is used as a measure for the discrimination between the low load operational state and the high load operational state. That is to say, an appropriate target IGV angle for a stable operational region according to particular characteristics of the compressed air  16  is set and whether the operational state is the low load operational state or the high load operational state is discriminated on the basis of the target IGV angle. 
     The mode selector  24  selects a control mode appropriate for discrimination results in the operational state discriminator  23 . More specifically, when the discriminated operational state is the first operational state, the first controller  21  is started, and when the discriminated operational state is the second operational state, the second controller  22  is started. Briefly, the appropriate mode is selected so that corrected rotation speed responsive IGV angle control, that is, the first control mode, will be used for the first operational state, and so that shaft rotation speed constant IGV angle control, that is, the second control mode, will be used for the second control mode. 
     As set forth above, the IGV angle controller  20  selectively uses the corrected rotation speed responsive IGV angle control mode or the shaft rotation speed constant IGV angle control mode according to the particular operational state. A relationship between the corrected rotation speed of the gas generator shaft  18  and IGV angle under such control by the IGV angle controller  20  is represented in  FIG. 3A , and a relationship between the actual rotation speed of the gas generator shaft  18  and the IGV angle, in  FIG. 3B . As can be seen from these graphs, under low load conditions, lines of operation are the same, regardless of the atmospheric temperature, but under high load conditions, the corrected rotation speed changes with the atmospheric temperature. Meanwhile, however, the lines of operation under the low load conditions change with the atmospheric temperature, the rotation speed of the gas generator shaft  18  becomes constant under the high load conditions. 
     Use of such control allows effective resolution of the resonance problem, that is, effective reduction of an increased likelihood of damage to the turbine and/or the compressor due to the resonance arising during high-speed rotation of the gas generator shaft  18  when the rotation speed approaches the resonance rotation speed. Such control also allows effective response to compressor surging during low-speed rotation. These advantages allow resonance-associated design loads to be relieved and costs to be reduced. 
     A second embodiment is described below. A configuration of a twin-shaft gas turbine  31  according to the second embodiment is shown in schematic form in  FIG. 4 . The twin-shaft gas turbine  31  of the present embodiment is substantially the same as the twin-shaft gas turbine  1  of  FIG. 1 , except that a gas generator control unit  10  of the turbine  31  includes an IGV angle controller  32  instead of the IGV angle controller  20  in  FIG. 1 . Configurational features and characteristics of the twin-shaft gas turbine  31 , therefore, are mainly described below, with the description of the foregoing embodiment being invoked for configurational features and characteristics common to those of the twin-shaft gas turbine  1 . 
     The IGV angle controller  32 , as its configuration is shown in  FIG. 5 , includes a third controller  33  in addition to substantially the same first controller  21 , second controller  22 , operational state discriminator  23 , and mode selector  24 , as those of  FIG. 2 . 
     The third controller  33  executes control in a third control mode. In the third control mode, the third controller  33  conducts IGV angle constant hold control to maintain a constant IGV angle, independently of the rotation speed of the gas generator shaft  18 . This third control mode of the third controller  33 , that is, the IGV angle constant hold control mode is used during a mode change between the first control mode and the second control mode. This means that when the operational state discriminator  23  discriminates a shift in operational state between the first operational state and the second operational state, the third controller  33  will be started to execute the control in the IGV angle constant hold control mode. 
     The relationship between the corrected rotation speed of the gas generator shaft  18  and IGV angle under the control of the IGV angle controller  32  is represented in FIG.  6 A, and the relationship between the actual rotation speed of the gas generator shaft  18  and the IGV angle, in  FIG. 6B . As can be seen from these graphs, since the control in the IGV angle constant hold control mode can also be conducted, decreases in IGV angle at high rotation speeds of the gas generator shaft can be avoided. That is to say, under the state of high gas-generator shaft rotation speeds and small IGV angles, the deterioration of performance due to the separated flow of air at the blade of the compressor  7  is prone to occur, which, in turn, easily causes icing as well. Such situations can be effectively avoided by making the third control mode intervene during a mode change between the first control mode and the second control mode. Reliability can also be improved. 
     While embodiments of the present invention have been described above, these embodiments are only typical examples and the invention can be embodied in various forms without departing from the scope of the invention.
A twin-shaft gas turbine 1, which has a gas generator 2 including a compressor 7, a combustor 8, and a high-pressure turbine 9, is configured to make a first control mode and a second control mode selectively usable for control of the gas generator. In addition, in the first control mode, an IGV angle in the compressor is controlled in accordance with a corrected shaft rotation speed of the gas generator, and in the second control mode, the IGV angle is controlled to maintain a constant gas generator shaft rotation speed. Furthermore, the first control mode is used to start, to stop, and to operate the turbine under fixed or lower load conditions, and that the second control mode is used under operational states other than those to which the first control mode is applied.