Abstract:
Power balancing techniques for a synchronous power generation system are provided. One exemplary method for balancing power in a synchronous generator system includes determining an output power characteristic of a synchronous generator driven by a prime mover and comparing the characteristic to a value derived from the output power of a plurality of synchronous generators. The method also includes providing a correction signal to the synchronous generator to modify the output power produced by that generator. A synchronous power generation system having a plurality of synchronous generators driven by a common prime mover is also provided.

Description:
BACKGROUND  
       [0001]     The present invention generally relates to electrical power generation systems. More particularly, the present invention relates to a technique for adjusting power output by each of a plurality of electrical generators driven by a single prime mover.  
         [0002]     Electrical power generation systems are employed in a number of diverse applications, including aviation, manufacturing, and commercial energy production, to name just a few. As will be appreciated, these systems are generally adapted to convert mechanical power to electrical power that may be used to operate one or more electrical devices or systems requiring such power. Generally, these power generation systems include one or more generators, each generator having a stator and a rotor that rotates with respect to the stator. Such power generation systems also typically include one or more prime movers that supply mechanical power to the rotors. In the case of aviation, for example, prime movers are often gas turbine engines of an aircraft. In some generators, a magnetic field is projected from rotor poles within the rotor and the rotation of the rotor and the magnetic field induces alternating electrical current in the windings of the stator that may be used to power electrical devices. While many generators are produced in which the rotor is configured to rotate within the stator, one skilled in the art will also appreciate that generators may be configured to allow the rotor to rotate about an interior stator.  
         [0003]     In the case of a power generation system having multiple generators, it may be desirable to control or balance the electrical power contribution of each generator, including its real and reactive components. Often, when such control is envisaged, a power generation system will include a separate prime mover that provides mechanical power to each generator. The use of independent prime movers for each generator allows the power output to be controlled via adjustment to the applied power of the prime mover and the relative phase angle of the alternating power between the generators. While the coupling of a single generator to a prime mover may be adequate for certain uses, such an arrangement has limitations. For instance, an aircraft includes a finite number of prime movers to drive electrical generators. If the aircraft included two gas turbine engines, only two electrical generators connected to the same grid could be driven by the turbines. If additional generators were needed, it would be possible to couple additional generators to a prime mover. However, as will be appreciated, the coupling of multiple generators to a single prime mover would preclude control of the power output by the generators via adjustments to the common prime mover or the mechanically coupled and non-adjustable phase angles of the generators. Further, the inability to make such adjustments aversely impacts the power rating of a given power generation system.  
         [0004]     There exists, therefore, a need for an improved power balancing and adjustment technique that would allow independent adjustment of output power, both real and reactive components, produced by each of a plurality of generators driven by a common prime mover.  
       BRIEF DESCRIPTION  
       [0005]     Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.  
         [0006]     Embodiments of the present technique generally relate to synchronous power generator systems and methods for balancing individual power outputs, including balancing the real, reactive, or apparent forms of these outputs, between generators within such systems. Certain embodiments of the presently disclosed technique facilitate balancing of power between multiple generators and independent adjustment of the output power produced by one of a plurality of generators driven by a single prime mover. In some embodiments, such as if the generators are of the same rating, balancing may be performed in order to equalize the output power of each generator of the plurality such that the output power produced by a single generator closely approximates the average output power of each generator of the plurality. However, in other embodiments, power balancing may be performed to establish a first output power from one generator driven by a prime mover and to establish a second, different, output power from another generator driven by the same prime mover. 
     
    
     DRAWINGS  
       [0007]     These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:  
         [0008]      FIG. 1  is a block diagram illustrating an exemplary power generation system having a plurality of synchronous generators in accordance with one embodiment of the present technique;  
         [0009]      FIG. 2  is a block diagram illustrating additional details and components of the generators and the power system provided in  FIG. 1  in accordance with one embodiment of the present technique;  
         [0010]      FIG. 3  is a block diagram illustrating further exemplary details with respect to control units depicted in  FIGS. 1 and 2 ;  
         [0011]      FIG. 4  is a graph illustrating exemplary apparent power output vectors of each of a plurality of generators;  
         [0012]      FIG. 5  is a graph depicting the balancing of real output power of the plurality of generators of  FIG. 4  in accordance with certain aspects of the present technique;  
         [0013]      FIG. 6  is a graph illustrating alternative exemplary apparent power output vectors of several generators; and  
         [0014]      FIG. 7  is a graph depicting the balancing of apparent output power of the generators of  FIG. 6  in accordance with certain aspects of the present technique. 
     
    
     DETAILED DESCRIPTION  
       [0015]     One or more specific embodiments of the present technique will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions will be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which can vary from one implementation to another. Moreover, it should be appreciated that such a development effort can be complex and time consuming, but would remain a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. It should be noted that illustrated embodiments of the present technique throughout this text represent a general case.  
         [0016]     Turning now to the figures, a block diagram illustrating an exemplary power generation system  10  in accordance with one embodiment of the present technique is provided in  FIG. 1 . The system  10  includes a plurality of synchronous generators  14  coupled to and driven by a single prime mover, such as turbine  12 . Particularly, turbine  12  provides mechanical power to generators  14  via a mechanical power divider, such as gear box  16 . As will be appreciated by one skilled in the art, generators  14  convert the mechanical input power received from turbine  12  via gear box  16  to electrical output power. As will be appreciated, the generators  14  may have equivalent or different power ratings in various embodiments.  
         [0017]     The system  10  also includes field control units  18 , each of which is associated with a generator  14 , and a master bus power control unit  20  coupled to the field control units  18 . The power output by generators  14  is transmitted to an output bus  22  that may provide power to a variety of other electrical devices or systems. A feedback bus  24  receives signals from sensors  25  coupled to the output bus  22 , and transmits these signals to master bus power control unit  20 . As explained in greater detail below, master bus power control unit  20  utilizes this information and applies correction or adjustment signals to the field control units  18  via a control bus  26  to regulate the active (i.e., real) and reactive power output by each generator in an independent fashion.  
         [0018]     It should be noted that the present system might find a wide range of uses and applications. For instance, the present system may be advantageously employed in connection with an aircraft electrical system. However, other implementations are also envisaged, including use in the oil and gas industry, ship power generation, directed energy defense systems, and radar systems, to name a few. Further, it will be appreciated that, although the present exemplary system  10  includes three synchronous generators of the same ratings, other systems may include a different number of generators, such as six, eight, or the like. Indeed, any systems including a plurality of generators may be employed in full accordance with the present technique. Still further, while the present embodiment includes three generators providing output power to a single output bus  22 , other embodiments may include multiple output busses that separately and independently receive output power from several sub groupings of generators. Additionally, one or more generators  14  may have loads attached to them independent of output bus  22 . In such an instance, the field control units  18  provide local control for independent operation of a generator  14 , while the master bus power control unit  20  facilitates parallel operation of the generators.  
         [0019]     Additional details of the exemplary system  10  may be more clearly described and understood with reference to  FIG. 2 . Notably, system  10  includes a plurality of generator groups  28 , each of which includes a generator system  14  and a field control unit  18 . The generator groups  28  produce electrical power that may be used to operate numerous devices or systems, such as load  30 . In the illustrated embodiment, generator system  14  includes a generator  32 , a rotating rectifier  34 , and an exciter  36 . The generator groups  28  may be one of a number of various generator types, including a brushless excited synchronous generator, a wound field synchronous generator, a synchronous generator with an excitation circuit, a homo-polar synchronous generator, to name but a few.  
         [0020]     Further, each field control unit  18  includes an inverter  38  and a field controller  40 . Each field controller  40  is configured to receive various input signals and to produce an output signal that controls a brushless exciter  36 . As will be appreciated, the control unit  18  can increase or decrease back EFM on the exciter coil to control internal field strength and output power produced by generator  32 . Characteristics of the output power produced by generator  32  are sensed via voltage sensors Va, Vb, and Vc, and current sensors Ia, Ib, Ic, and In. These sensed characteristics may then be input to field controller  40 . The field controller  40  is also configured to receive a control signal, which may include a status signal and a correction signal as discussed in greater detail below, from master bus power control unit  20  and a shunt voltage Vsh from a sensor  42 . Sensors  42  measure AC current produced by generator groups  28  and detect current imbalance between these groups. This information may be input to either or both of master bus power control unit  20  and the field controllers  40  to facilitate application of a correction or command signal to reduce such an imbalance. The field controller  40  processes these signals to generate a command signal to adjust the power output of generator  32 .  
         [0021]     The power output by each generator group  28  is supplied to the output bus  22 , as discussed above. The characteristics of the aggregate power transmitted over the bus, including I R , I S , I T , V R , V S , and V T , are sensed and input to master bus power control unit  20 . As discussed below, the power control unit  20  generates a status or control signal that is applied to each field controller  40 . Further, the power control unit  20  may also output a control signal to one or more protective circuits, such as switches  44  and  46 .  
         [0022]     Further details of master bus power control unit  20  and field controllers  40  are illustrated in  FIG. 3  in accordance with one embodiment of the present technique. Various signals described above are communicated between master bus power control unit  20  and field controllers  40  via a communication link  52 . The voltage from output bus  22  is input to master bus power control  20  and transformed, as indicated in block  54 . This transformed voltage is combined with a reference voltage  56  and a reference voltage correction factor  58 , which are input into an integrator circuit  60 . In one embodiment, reference voltage  56  is 115 Vrms, however, other voltages may be utilized in accordance with the present techniques. Integrator  60  outputs a bus voltage correction factor  62  which is combined with a power balance correction factor  68 , as described below.  
         [0023]     A system configuration module  64  may receive or contain configuration data regarding the generators and the buses. For instance, the system configuration module  64  may include the status of the various generators, such as which generators are active or inactive, and which are running in parallel or in an independent fashion, for example. As discussed below, a generator may be removed from parallel operation due to loading issues or operating error, to operate independently from other generators, or for other reasons. A target value control module  66  receives data from system configuration module  64 , as well as the apparent power from each generator  32 , and provides a power balance correction factor  68  for each generator. System configuration module  64  may also output a signal to various protection circuitries  74 , which may include switches  44  and  46 , to activate or deactivate the power generation system  10  or individual generator groups  28  or the system.  
         [0024]     In some embodiments, it may be desirable to equalize the power generated by each generator  32 . In such embodiments, a power balance correction factor  68  is provided to each generator  32 , based on the difference between or deviation of the actual output power of each generator  32  from the desired output power of that generator, to reduce or increase the power output by that generator in order to more closely conform with the average power output of all generators  32  in parallel operation. As will be appreciated, each power balance correction factor  68  is based on the particular output characteristics of that generator. A generator  32  that is outputting power of a greater magnitude than the average of a group of generators may result in application of a power balance correction factor  68  that effects a reduction in the output power produced by that particular generator. Likewise, a generator  32  that is producing less power than the group average could receive a power balance correction factor  68  that results in an increase in the power output of that generator. In other embodiments, it may be desirable to operate one generator  32  to produce output power that is higher or lower than that produced by the other generators  32 . In these embodiments, a power balance correction factor  68  is applied to each generator  32  to reduce deviation of the output power of each generator  32  from the desired output power level for that generator.  
         [0025]     Each power balance correction factor  68  and the bus voltage correction factor  62  are combined, as indicated in summation block  70 , to produce a correction command signal or factor  72  for the respective generator  32 . The correction or adjustment signal  72  is input from the master bus power control unit  20  to a field controller  40 , and combined with a transformed output voltage, as indicated in block  76 , and a local reference voltage  78 . This combined signal may then be fed to a voltage regulator  80 .  
         [0026]     Additionally, output current of a particular generator  32  is input to its respective field controller  40  and transformed as indicated in block  82 . A processing block  84  receives both the transformed voltage and transformed current to calculate the apparent power  86  of the generator  32 , which is input to the target value control module  66 , as described above. The input of the updated apparent power of each generator  32  to target value control module  66  facilitates the comparison of the output power of each generator  32  to a desired level, such as an average level. Further, it allows for simultaneous or near-simultaneous correction or adjustment of each individual output power via a correction or adjustment signal  72 .  
         [0027]     The transformed current may also be used to provide a load feed forward signal  88  that is combined with an output signal from voltage regulator  80  to improve its dynamic performance at summation block  90 . The combined signal may be output from block  90  to a field regulator  92 . As will be appreciated, voltage regulator  80  and field regulator  92  may also include various error compensation or integration techniques that may be desirable for a given application. It should be noted that, in certain embodiments, field controllers  40  may also include protection and ground fault interrupter modules  94  to control various protection circuitry  96  of the system  10 .  
         [0028]     A signal from each field regulator  92  is transmitted from field controller  40  to an inverter  38  associated with a respective generator  32 . As will be appreciated, the output of inverter  38  affects the back EMF on exciter coil  36  that, in turn, affects the output power of generator  32 . Thus, the signal output from field controller  40  to the inverter  38  directly impacts the output power of each generator  32 . Further, feedback is provided from exciter  36  to inverter  38  for closed loop control of the magnetic field.  
         [0029]     The present techniques allow for balancing of various power characteristics, including real power, reactive power, and apparent power. The balancing of power output by each of several generators may be made clear with reference to  FIGS. 4-7 . Notably, these figures illustrate graphs depicting the output power of several exemplary generators. As will be appreciated, the apparent power of a generator is the vector sum of a real or active power component and a reactive power component. In the graphs of  FIGS. 4-7 , the real power produced by the exemplary generators is measured along the horizontal axis, while the reactive power is measured along the vertical axis. The resulting apparent power is represented by the length of the vectors  108 ,  110 , and  112  in  FIG. 4 .  
         [0030]     As illustrated in graph  102  of  FIG. 4 , having axes  104  and  106 , three exemplary generators may produce equal individual apparent output powers  108 ,  110 , and  112 . It will be noted that the present apparent output powers  108 ,  110 , and  112  are based on a load lagging power factor of 0.95. One skilled in the art will appreciate that, holding the apparent power constant, decreasing the power factor increases the reactive power component and decreases the real power component of the generated power. In present case, with a relatively high power factor of 0.95, it may be desirable to balance the real power component of the power output by the generators, such as by equalizing the real power components of output powers  108 ,  110 , and  112 , at a level indicated by line  114 . Such balancing may be achieved through the technique disclosed above. Particularly, an adjustment signal may be applied to Generator  1 , such as via a field controller  40 , to increase the apparent power  108  produced by the generator. Similarly, an adjustment signal may be applied to Generator  3  to reduce the apparent power  112  produced by this generator. Through use of such correction signals, the real power components of output powers  108 ,  110 ,  112  may be balanced, as provided in  FIG. 5 .  
         [0031]     Notably, in other embodiments, balancing power may include operating one or more generators at one power output level, one or more generators at a second output level, or operating each generator at a different output level. Further, in these or other embodiments, one or more of the generators may be disconnected from the system and operated independently of the parallel generators, or deactivated, and correction signals may be applied to the remaining parallel generators to compensate for the power not being produced by the disconnected generators. As will be appreciated, this would allow one or more of the generators to power an independent load or be serviced without impacting the system as a whole, and also provides redundancy in case a generator unexpectedly fails.  
         [0032]     If the power factor is reduced below a certain threshold, such as 0.80 or 0.85, it may be desirable to balance apparent output powers of several generators instead of balancing the real power components. For instance,  FIG. 6  illustrates the apparent powers produced by several generators at a power factor 0.70. In graph  118 , the real power is measured along horizontal axis  120  and the reactive power is measured along vertical axis  122 . Generators  1 ,  2 , and  3  produce apparent output powers  124 ,  126 , and  128 , respectively. In some applications, as noted above, it may be desirable to balance the magnitudes of output powers  124 ,  126 , and  128 , such as at a level represented by curve  130 . Individual correction signals maybe applied to each of Generators  1 ,  2 , and  3  to increase or decrease the apparent output power produced by these generators, as needed, to balance the magnitudes of these output powers along the curve  130 , as provided in graph  132  of  FIG. 7 .  
         [0033]     In still further applications it may be desirable to balance the reactive power components of output power generated by a plurality of generators instead of balancing either the apparent power or real power components. Such reactive power balancing may find particular application in those situations in which the power factor falls below a certain threshold, such as 0.25 or 0.50, in which the reactive power components are larger than in the instance of higher power factors. As in the examples above, individual control signals can be applied to each of a plurality of generators to adjust the output power produced by the generators and to balance the reactive power components thereof.  
         [0034]     While the present technique may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.