Abstract:
A power generation and distribution system utilizes two or more AC generators each of which may be driven by a separate prime mover such as a turbine. The generators may be driven at different rotational speeds. AC power from the generators may be rectified and applied to a common DC bus. Electrical loads may be applied to the common bus and may establish an electrical power requirement. Allocation of electrical power requirement may be made among the generators based on power available from the turbines.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention is in the field of electrical power generation and distribution systems and, more particularly, systems which may be employed in aerospace vehicles. 
         [0002]    In a typical prior-art aerospace vehicle such as an aircraft, there may be many different requirements for electrical power. Many functions may be performed with electrical motors and controls. 
         [0003]    On-board generators may be driven with various prime movers such as turbine engines. In many cases, a prime mover may drive a generator only as an ancillary function. A typical primary function for a prime mover, such as a turbine engine, may be to provide propulsion thrust for the aircraft. In the context of its primary function, the prime mover may operate at varying rotational speeds. A generator coupled to a shaft of such a variable-speed prime mover may rotate at varying speeds. 
         [0004]    As aircraft designs evolve, more of the ancillary power requirements are being met with electrical systems instead of previously used bleed air and hydraulic systems. An evolving design concept has become known as “more electric aircraft” (MEA). In the context of MEA designs, electrical loads on generators may be become quite large. Indeed, a generator load may become large enough to negatively affect engine thrust output. Because of these increased electrical power demands in MEA design, a single generator driven by a single prime mover may not be capable of producing all of the electrical power for an aircraft. Consequently, an aircraft may be provided with multiple generators, each driven by different prime movers. 
         [0005]    Because prime movers have varying rotational speed during operation of the aircraft, rotational speed of any particular generator may differ from rotational speed of other generators on the aircraft. In the case of alternating current (AC) generators, each AC generator may produce AC power at a frequency and phase angle different from the other AC generators. It may be said that, each generator may produce “variable frequency” electrical power. 
         [0006]    Certain aircraft operating conditions may arise in which a particular generator may be subjected to a particularly high load demand during a time when its associated prime mover may be performing its primary function (e.g. producing thrust) at a relatively low speed. In order to meet the high electrical power requirement of an attached generator, it may be necessary to increase the speed of the prime mover, even though such an increase in speed may not otherwise be required for the primary function of the prime mover. 
         [0007]    Excessive fuel may be consumed if and when a prime mover is operated at a speed greater than required for its primary role. Certain design efforts have been directed to this issue. For example U.S. Pat. No. 7,285,871 (Jean Luc Derouineau) issued Oct. 23, 2007, discloses multiple generators that may be driven on different shafts of a turbine machine. The turbine machine may have a low-pressure turbine output shaft and a high-pressure turbine output shaft. A separate generator may be driven by each of the shafts. Electrical outputs of the generators may be shared and controlled so that electrical loads may be allocated to either the low-pressure turbine or the high-pressure turbine as a function of turbine operating speed. This allocation may facilitate efficient operation of the turbine machine. 
         [0008]    This prior-art power allocation method may require paralleling of two or more AC generators onto a common power bus. Successful paralleling of AC generator outputs may require matching of frequency of the generators. Thus this prior-art method, when employed with AC generators, may be practical only when the AC generators operate at the same rotational speed. Alternatively, as in well understood prior art, the generators may be driven via a constant speed transmission to match their frequency and phase, or may use power electronics to synthesize a matched AC output. Both of these techniques require large, complex and expensive devices to facilitate paralleling. 
         [0009]    Many MEA aircraft employ multiple turbines that may operate at different speeds. Each of the turbines may drive AC generators. It has heretofore not been practical to allocate electrical power requirements of multiple-engine aircraft to all of the generators of the aircraft as required by the operational conditions. 
         [0010]    As can be seen, there is a need to provide power generation and distribution systems in which AC power produced by multiple generators operating at different speeds may be paralleled to a common bus. Additionally, there is a need to provide such a system in which electrical loads may be allocated to any prime mover of a multiple-engine aircraft, or any turbine of a multiple-turbine prime mover. 
       SUMMARY OF THE INVENTION 
       [0011]    In one aspect of the present invention, an apparatus for generating and distributing electrical power comprises a first alternating current (AC) generator driven at a first rotational speed, a second AC generator driven at a second rotational speed different from the first speed, a common direct current (DC) bus, fed from either of the generators ( e.g., via a rectifier), supplying electrical power to electrical loads connected to the common bus, and a controller for allocating portions of the electrical load among the first and the second generators. 
         [0012]    In another aspect of the present invention, an apparatus for generating and distributing electrical power in an aircraft with multiple turbines comprises a first alternating current (AC) generator driven by a first turbine at a first rotational speed, a second AC generator driven by a second turbine at a second rotational speed different from the first speed, and a common direct current (DC) bus interconnected with the first and second generators ( e.g., via rectifiers). The common bus is selectively connected to electrical loads that produce the electrical power demand. A controller is provided to selectively allocate portions of the electrical power demand among the first and the second generators. 
         [0013]    In still another aspect of the present invention, a method for producing and distributing electrical power in an aircraft comprises the steps of driving a first AC generator at a first rotational speed, driving a second AC generator at a second rotational speed different from the first speed, supplying electric power from the first and second generators to a common DC bus (e.g. via rectifiers), supplying electric power from the DC bus to electrical loads that produce the electrical power demand, and allocating the electrical power demand among the first and second generators. 
         [0014]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is block diagram of a power system in accordance with the invention; 
           [0016]      FIG. 2  is a block diagram of the power system of  FIG. 1  in an engine starting mode of operation in accordance with the invention; 
           [0017]      FIG. 3  is a block diagram of the power system of  FIG. 1  in a failed-generator mode of operation in accordance with the invention; 
           [0018]      FIG. 4  is a block diagram of the power system of  FIG. 1  in a failed-bus mode of operation in accordance with the invention; and 
           [0019]      FIG. 5  is a flow chart of a method of generating and controlling electrical power in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0021]    Broadly, the present invention may be useful for distributing electrical power demands among various prime movers. More particularly, the present invention may provide a power allocation system that may distribute electrical power requirements among multiple prime movers of a vehicle. The present invention may be particularly useful in vehicles such as aircraft with multiple turbines. 
         [0022]    In contrast to prior-art aircraft electrical power systems, among other things, the present invention may provide for combining, on a common bus, electrical power produced by multiple AC generators which may be driven at rotational speeds which may differ for each generator. The present invention, instead of paralleling AC power from different generators as in the prior art, may convert AC power from individual generators into DC power and then parallel the resultant DC power of multiple generators onto a common bus. Additionally, the present invention may provide generator output controls to allocate electrical power demands to various prime movers having differing rotational speeds. 
         [0023]    Referring now to  FIG. 1 , an exemplary electrical power system is designated generally by the numeral  100 . The power system  100  may be utilized in an aircraft (not shown) which may have multiple turbines and/or multiple engines. The power system  100  may comprise multiple electrical generators, such a left-hand HP starter/generator  12  which may be driven by a high-pressure turbine  112  of a left engine  140 , a left-hand LP generator  14  which may be driven by a low-pressure turbine  114  of the left engine  140 , an APU starter/generator  16  which may be driven by an auxiliary power unit  116 , a right-hand HP starter/generator  18 , which may be driven by a high-pressure turbine  118  of a right engine  180 , and a right-hand LP generator  20  which may be driven by a low-pressure turbine  120  of the right engine  180 . 
         [0024]    The power system  100  may also comprise electrical buses. A left bus  22  may be interconnected with the generators  12  and  14 . A main power bus  24  may be interconnected with the generator  16  and may also be selectively interconnected with a ground power unit (GPU) at an external power entry point  25 . A right power bus  26  may be interconnected with the generators  18  and  20 . The buses  22 ,  24  and  26  may also be interconnected with one another with bus-tie contactors  28  and  30 . In that regard, the buses  22 ,  24  and  26  may be considered to be sub-buses of a common bus  27 . The buses  22 ,  24  and  26  may be direct current (DC) buses and may operate with an exemplary voltage of about ±270 volts DC 
         [0025]    During normal flight operation of the aircraft the contactors  28  and  30  may be closed so that the buses  22 ,  24  and  26  may be electrically interconnected. Contactors  12 - 1 ,  14 - 1 ,  16 - 1 ,  18 - 1  and  20 - 1  may also be closed during normal flight conditions. It may be seen that all of the generators  12 ,  14 ,  16 ,  18  and  20  may be interconnected with all of the buses  22 ,  24  and  26  in normal flight conditions. 
         [0026]    The left engine HP generator  12  may be an AC generator operating at a first speed and the right engine HP generator  18  may be an AC generator operating at a second speed different from the first speed. But the generators  12  and  20  may be interconnected with their respective DC buses  22  and  26  though rectifiers  12 - 2  and  18 - 2  respectively. Similarly the APU generator  16  may be an AC generator operating at still another speed and its output power may be applied to the DC bus  24  through a rectifier  16 - 2 . 
         [0027]    The generators  14  and  20  may be DC generators and may supply power to their respective buses  22  and  26  in parallel with the AC generators  12 , and  18 . It may be seen then, that electrical power from the generators  12 ,  14 ,  16 ,  18  and  20  may be pooled together on the buses  22 ,  24  and  26  during normal flight operations, irrespective of whether the generators produce AC or DC power. 
         [0028]    The bus  24  may be interconnected to provide power to various motor controllers or other loads, symbolically designated herein as motor controllers  31  and  32 . Any number of motor controllers may be interconnected with the bus  24  in accordance with the present invention. An exemplary number of two motor controllers,  31  and  32  are illustrated in  FIG. 1 . The exemplary motor controllers  31  and  32  may control exemplary motors  31 - 1  and  32 - 1  which may perform general aircraft operating functions that may be associated with normal flight conditions. 
         [0029]    The buses  22  and  26  may be interconnected with engine starting controllers  34  and  36  respectively. The controllers  34  and  36  may be employed during APU and engine starting operations for the aircraft, which operations are hereinafter described. Additionally, the motor controller  34  and  36  may control other exemplary motors designated by the numerals  34 - 1  and  36 - 1  respectively. 
         [0030]    During normal flight operations, the exemplary motor controllers  31 ,  32 ,  34  and  36  may provide control for their respective exemplary motors  31 - 1 ,  32 - 1 ,  34 - 1  and  36 - 1 . The motors may be either AC or DC motors and they may be configured to operate at high or low voltages. The motor controllers may extract ±270 volt DC power from the bus  27  and convert the power into a form that may be properly used by the motors. 
         [0031]    A supervisory controller  38  may be interconnected with sensors (not shown) to monitor generator power and engine control units (not shown) so that proper portions of loads may be allocated to any one or more of the generators  12 , 14 ,  16 , 18  or  20  during normal flight operations. Allocation may be performed by appropriate signaling from the supervisory controller  38  to generator control units (GCU)  12 - 3 ,  14 - 3 ,  16 - 3 ,  18 - 3  and  20 - 3 . Each of the GCU&#39;s  12 - 3 ,  14 - 3 ,  16 - 3 ,  18 - 3  and  20 - 3  may provide control of electrical output of their respective generators  12 ,  14 ,  16 ,  18  and  20 . 
         [0032]    By way of example, the GCU&#39;s may define a power share their respective generators. Each of the generators may produce power in according to its commanded share. Thus a generator that is assigned an exemplary share of 25% may produce 25% of the total power demand. Power share of the generators  12 ,  14 ,  16 ,  18  and  20  may be changed continuously by the GCU&#39;s in response to changes of electrical power requirements and availability of turbine power. 
         [0033]    Allocation of electrical power requirement may be performed to optimize turbine efficiency. For example, if flight conditions demand particularly low power extraction from the high-pressure turbines  112  and  118 , electrical load may be reduced on generators  12  and  18  by reducing their duty cycle. The motors  31 - 1 ,  32 - 1 ,  34 - 1  and  36 - 1  may still consume an undiminished amount of electrical energy during this period, but the balance of the total electrical energy may be provided by the low pressure turbine generators  14  and/or  20 . In other words, a larger portion of the overall electrical power requirements of the aircraft could be extracted from the low-pressure turbines during this period. 
         [0034]    Conversely, electrical power requirements may be allocated to the high-pressure turbines  112  and/or  118  at times when this may provide the best engine operating performance. Additionally electrical power requirements may be re-allocated or shifted from the low pressure generators  14  and/or  20  or the high pressure generators  12  and/or  18  to the APU generator  16 . 
         [0035]    Referring now to  FIG. 2 , it may be seen how the power system  100  may be employed during a start-up on an exemplary engine. In this case, startup of the left engine  140  may be illustrated. In  FIG. 2 , it may be seen that a starter bus  40  may be provided power for starting from the motor controller  34  through a contactor  40 - 1 . Power to the motor controller  34  may be provided from the generators  16 ,  18  and/or  20  because the contactors  16 - 1 ,  18 - 1  and  20 - 1  may be closed. For main engine starting, power from the starter bus  40  may be provided to the starter/generator  12  through a contactor  40 - 2  which may be closed. The GCU  12 - 3  may produce commands which actuate the starter/generator  12  as a starter motor. 
         [0036]    For APU starting, power from the starter bus  40  may be provided to the APU starter/generator  18  through a contactor  40 - 3  which may be closed. 
         [0037]    If starting with ground power is required, a contactor  25 - 1  may also be closed so that power from the ground power unit may be supplied through the entry point  25 . The external power may be converted to DC by rectifier  25 - 2 , to the common bus  27  and the motor controller  34  and then on to the starter/generator  12 . 
         [0038]    It may be noted that the bus-tie contactors  28  and  30  may remain closed during starting operations, just as they may remain closed during normal flight operation of the aircraft. Thus the buses  22 ,  24  and  26  may continue to provide collective pooling of electrical power for the aircraft. 
         [0039]    Referring now to  FIG. 3 , it may be seen how the power system  100  may operate in the event of a failure of a generator. By way of example a failure of the generator  14  may be illustrated. In this case, the contactor  14 - 1  may be opened and the generator  14  may be disconnected from the bus  22 . The contactors  28  and  30  may remain closed so that the buses  22 ,  24  and  26  may remain interconnected. Thus in spite of a partial loss of power to the bus  22 , the motor  34 - 1  may be provided with power from the other generators  12 ,  16 ,  18  and  20 . 
         [0040]    Referring now to  FIG. 4 , it may be seen how the power system  100  may operate in the event of a short-circuit failure of a bus. By way of example, a short-circuit failure of the bus  22  may be illustrated. In this case, the contactor  12 - 1 ,  14 - 1  and  28  may be opened. The bus  22  may be thus electrically isolated from the electrical power of the aircraft. The contactor  30  may remain closed so that the buses  24  and  26  may remain interconnected. Thus in spite of failure of the bus  22 , the motors  31 - 1 ,  32 - 1  and  36 - 1  may still be provided with power from the generators  16 ,  18  and  20 . This is because the buses  22 ,  24  and  26  may selectively be isolated from one another electrically by the contactors  28  and  30 . 
         [0041]    Referring now to  FIG. 5 , in one embodiment of the present invention, a method may be provided for generating and distributing electrical power on an aircraft. In step  502  of a method  500 , electrical power may be generated in a first generator (e.g. the AC generator  12  driven by the left hand high-pressure turbine  112 ). In a step  504 , electrical power may be generated in a second generator (e.g. the AC generator  18  driven by the right hand high-pressure turbine  118 ). 
         [0042]    In a step  506 , power generated in steps  502  and  504  may be supplied to a common bus (e.g. by rectifying AC outputs of generators  12  and  18  and supplying power to interconnected buses  22 ,  24  and  26 ). In a step  508 , power may be delivered to electrical loads (e.g. through interconnections between motor controller  31 ,  32 ,  34  and  36  and the buses  22 ,  24  and  26 ). 
         [0043]    In a step  510 , a load allocation calculation may be performed (e.g. with the supervisory controller  38 ). In a step  512 , a calculated power requirement allotment may be provided to a GCU for the first generator (e.g. the GCU  12 - 3  for the generator  12 ). In a step  514 , a calculated power requirement allotment may be provided to a GCU for the second generator (e.g. the GCU  18 - 3  for the generator  18 ). 
         [0044]    In a step  516 , a power share for the first generator may be produced on the basis of the power requirement allotment provided in step  512 . Step  502  may then be performed in accordance with the power share produced in step  516 . Similarly, in a step  518 , a power share for the second generator may be produced on the basis of the power requirement allotment provided in step  514 . Step  504  may then be performed in accordance with the power share produced in step  518 . 
         [0045]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.