Patent Publication Number: US-2017353037-A1

Title: Flexible Variable Speed Genset System

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to genset configurations, and more particularly, to systems and methods for controlling variable speed and constant speed gensets. 
     BACKGROUND 
     Genset systems, or networks of engine and generator combination sets, can be used to produce power in a variety of different applications. In marine vessels, for instance, multiple gensets can be harnessed together to drive primary loads, such as propellers or other drive mechanisms, as well as various auxiliary loads, such as heating, ventilation and air conditioning (HVAC) systems, lighting systems, pumps, and the like. In particular, the engine in each genset can be mechanically coupled to the corresponding generator and any mechanically-driven loads, while the generator can be electrically coupled to electrically-driven loads. Different genset configurations may be available for different applications, and selecting the appropriate configuration includes consideration of factors such as load optimization, load distribution, fuel economy, reliability, costs of implementation and maintenance, and the like. 
     In one configuration, each genset is operated at constant speeds to deliver constant voltage and frequency outputs. Constant speed genset configurations rely on control systems which activate or deactivate individual gensets in order to vary the otherwise constant output. One such configuration is disclosed in U.S. Pub. No. 2014/0309797 (“Frampton”), which discloses constant speed gensets that are controlled by a system that activates or deactivates individual gensets based on the load, fuel efficiency or noise level. Loads may be proportionally shared among active gensets. In other configurations, power demand is apportioned among a plurality of power sources based on performance goals and priorities, such as fuel consumption minimization. While constant speed genset configurations are relatively simple and easily installed, operating each genset at constant speed is not economical in terms of fuel consumption. Furthermore, apart from high noise levels, constant operation at high speeds leads to more wear and maintenance on the genset system. In addition, constant speed genset configurations are prone to uneven loading between gensets with the above described art, which can in turn cause uneven and premature wear within the genset system, if not properly accounted for. 
     In another configuration, each genset is operated at variable speeds, which in comparison to constant speed gensets, may provide better fuel economy. However, variable speed genset configurations require additional power electronics circuitry coupled to each genset in order to properly convert the variable voltage and frequency outputs to constant voltage with constant frequency for power generation applications. Moreover, it is not only costly to implement power electronics circuitry for each and every genset in the configuration, but it is also costly to maintain the various electronics involved. Also, if only a subset of the gensets are configured to be variable speed gensets, the fuel economy benefits of variable speed operation are reduced by uneven loading of gensets as well as downtime associated with failures and unplanned maintenance. Furthermore, the variable speed genset configurations, as with constant speed genset configurations with various load sharing schemes, are still prone to uneven load distribution, or uneven loading between gensets which can ultimately cause uneven and premature wear within the genset system and also lead to unplanned maintenance or failures. The problem is even more compounded when only one or a subset of gensets are variable speed gensets while others are constant speed gensets. For instance, to gain fuel economy benefits and allow long term even loading of gensets, manual electrical cabling modifications and follow up commissioning checks could be conducted to convert one or more constant speed gensets to variable speed gensets, and vice versa, for one or more variable speed gensets. This, however, negatively impacts operations and can be prone to error with significant failures and maintenance issues. 
     In view of the foregoing disadvantages associated with conventional genset systems and configurations, a need therefore exists for improved configurations and control schemes, which cost less to implement and maintain, reduce fuel consumption, and improve reliability by more evenly distributing the load over time on the individual gensets, while allowing flexible loading at a given instant. In addition, the configuration may allow for staggering maintenance schedules for gensets by appropriate loading over time. The present disclosure is directed at addressing one or more of the deficiencies and disadvantages set forth above. However, it should be appreciated that the solution of any particular problem is not a limitation on the scope of this disclosure or of the attached claims except to the extent expressly noted. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect of the present disclosure, a variable speed genset system is provided. The variable speed genset system may include a plurality of gensets, a switch assembly coupling one or more of the gensets to a common bus, a power electronics circuit selectively coupling the switch assembly to the common bus, and a controller in electrical communication with the gensets, the switch assembly and the power electronics circuit. The controller may be configured to designate any one or more of the gensets to operate as variable speed gensets and one or more of the remaining gensets to operate as constant speed gensets, engage the switch assembly to couple the variable speed genset to the power electronics circuit, and engage the switch assembly to couple the constant speed gensets to the common bus. An optional energy storage system may also be coupled to the power electronics circuit to couple to the common bus via the switch assembly. The energy storage may be engaged to provide better transient performance as well as provide a mechanism to maintain uninterrupted operation, such as when a constant or designated variable speed genset is shut down and until another genset can be brought on line to couple with the common bus. 
     In another aspect of the present disclosure, a controller for a plurality of gensets is provided. The controller may include a monitor module configured to monitor one or more operational parameters associated with the gensets, a designation module configured to designate one of the gensets as a variable speed genset and one or more of the remaining gensets as constant speed gensets based on the one or more operational parameters, a switch module configured to output a variable frequency output from the variable speed genset and one or more constant frequency outputs from the constant speed gensets, and a converter module configured to convert the variable frequency output into a converted constant frequency output. 
     In yet another aspect of the present disclosure, a method of controlling a plurality of gensets is provided. The method may include monitoring one or more operational parameters associated with the gensets, designating one of the gensets as a variable speed genset and one or more of the remaining gensets as constant speed gensets based on the one or more operational parameters, coupling the variable speed genset to a power electronics circuit, and coupling the constant speed gensets to a common bus. 
     These and other aspects and features will be more readily understood when reading the following detailed description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial perspective view of a marine vessel having a variable speed genset system constructed in accordance with the teachings of the present disclosure; 
         FIG. 2  is a diagrammatic view of one exemplary embodiment of a variable speed genset system; 
         FIG. 3  is a diagrammatic view of another exemplary embodiment of a variable speed genset system; 
         FIG. 4  is a diagrammatic view of one exemplary controller of a variable speed genset system; and 
         FIG. 5  is a flow diagram of one exemplary algorithm or method of controlling a variable speed genset system. 
     
    
    
     While the following detailed description is given with respect to certain illustrative embodiments, it is to be understood that such embodiments are not to be construed as limiting, but rather the present disclosure is entitled to a scope of protection consistent with all embodiments, modifications, alternative constructions, and equivalents thereto. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , one exemplary machine  100 , such as a marine vessel, having a variable speed genset system  102  is provided. In the particular machine  100  shown in  FIG. 1 , for example, the variable speed genset system  102  may be anchored to a platform  104  within a hull  106  of the machine  100 , and at least partially controlled from a bridge  108  or any other suitable location onboard and/or offboard the machine  100 . Moreover, the variable speed genset system  102  may be used to supply power to one or more loads  110  of the machine  100 . For example, the loads  110  may include any number of devices that consume mechanical and/or electrical power, such as motors for powering propellers or other drive mechanisms, lighting systems, heating, ventilation and air conditioning (HVAC) systems, water pumps, and any other primary or auxiliary load of the machine  100 . 
     Although the machine  100  shown in  FIG. 1  is depicted as a marine vessel, it will be understood that the machine  100  may include other types of mobile and/or stationary machines, such as machines typically used in mining, construction, farming, transportation, and other industries, or mobile or stationary power generation equipment used in many industries. Aside from marine vessels, for instance, the machine  100  may include an earth moving machine, an aircraft, a tractor, an off-road truck, an on-highway truck or passenger vehicle, a genset system for an oil drilling site or a power generation installation for a remote area or the like. Moreover, while the machine  100  in  FIG. 1  includes a variable speed genset system  102  that is configured to supply power to loads  110  which include propellers, it will be understood that the variable speed genset system  102  in other types of machines  100  may similarly be used to supply power to wheels, tracks, and/or any other drive and propulsion mechanisms. 
     Turning to  FIG. 2 , one exemplary embodiment of the variable speed genset system  102  that may be used with the machine  100  of  FIG. 1  is provided. In general, the variable speed genset system  102  may include a plurality of gensets  112  and interface circuitry  114  for electrically coupling the gensets  112  to the loads  110  of the machine  100 . As shown, each genset  112  may include an engine  116  that is mechanically coupled to a generator  118 . The engine  116  may include a diesel engine, a gasoline engine, a natural gas engine, or any other type of combustion engine capable of mechanically driving or rotating the generator  118 . Moreover, as an added benefit to the variable speed genset system  102  shown, the individual engines  116  need not be similarly sized and can have varying load capacities. The generator  118  may include any suitable electric generator that can generate electrical energy, such as multi-phase alternating current (AC) voltage, using the mechanical energy supplied by the engine  116 . For instance, the generator  118  may include an electrically-excited brushless synchronous generator, an electrically-excited brush type synchronous generator, a permanent magnet generator, an induction generator, or the like. 
     Furthermore, the interface circuitry  114  of  FIG. 2  may receive the electrical energy supplied by each of the gensets  112 , condition and/or convert the electrical energy into a form more suitable for the loads  110 , such as constant frequency and constant alternating current (AC) voltage, and communicate the electrical energy to a common bus  120  associated with the machine  100 . More specifically, the common bus  120  may be used to communicate the electrical energy to the various loads  110  of the machine  100 . Additionally or optionally, an energy storage device  122 , such as batteries, ultracapacitors, and the like, may be coupled to the interface circuitry  114 . In other embodiments, the common bus  120  may additionally incorporate one or more energy storage devices  112  within which the common bus  120  can store excess energy for later use. Depending on the type of loads  110  present, the common bus  120  may also incorporate additional circuitry, such as inverters, converters, rectifiers, filters, and the like, that may be used to convert the electrical energy in the common bus  120  into forms more suited for the individual loads  110 . 
     Referring now to  FIG. 3 , another exemplary embodiment of the variable speed genset system  102  is provided in more detail. As shown, the variable speed genset system  102  may generally include a plurality of gensets  112 , a switch assembly  124 , a power electronics circuit  126 , and a controller  128 . In particular, each genset  112  may include an engine  116  and a generator  118  mechanically coupled thereto. The switch assembly  124  may electrically couple one or more of the outputs of the gensets  112  to one or more loads  110  of the machine  100  via the common bus  120 , or the like. Furthermore, the power electronics circuit  126  may be electrically and selectively coupled between the switch assembly  124  and the common bus  120 . Still further, the controller  128  may be in electrical communication with and operatively coupled to one or more of the gensets  112 , the switch assembly  124  and the power electronics circuit  126 . Furthermore, the power electronics circuit  126  may be coupled to the energy storage device  122 . 
     Still referring to  FIG. 3 , each genset  112  may include an engine  116  and a generator  118  mechanically coupled thereto. Furthermore, each engine  116  and thus each genset  112  may be operated in either a variable speed mode or a constant speed mode of operation. Moreover, one of the gensets  112  may be designated to operate as a variable speed genset and electrical output having variable frequency and possibly variable voltage, while the remaining gensets  112  are designated to operate as constant speed gensets and output electrical signals having constant frequency and constant voltage. As will be discussed in more detail further below, the mode of operation of each genset  112  may be designated via instructions electrically communicated from the controller  128 , and determined based on a variety of factors, such as operational parameters of the machine  100  and/or preprogrammed or real-time optimized profiles configured to optimize fuel consumption, load distribution, and the like. Furthermore, the designation of the ] gensets  112  may be performed in real-time and automatically during operation. 
     The switch assembly  124  of  FIG. 3  may be electrically coupled to each of the outputs of the gensets  112 , and configured to connect or disconnect each of the gensets  112  to the common bus  120 . In addition, the switch assembly  124  can be configured to connect one of the gensets of  112  operating in variable speed mode to the power electronics circuit  126  while passing through the remaining gensets  112  operating in constant speed mode directly to the common bus  120 . In addition, the switch assembly  124  may include switches that engage or disengage each genset from the common bus  120 . For instance, the switch assembly  124  may use interlocking switches, or any other switch arrangement, that is engageable by the controller  128  and capable of selectively switching any one of the gensets  112  to the power electronics circuit  126 . The power electronics circuit  126  may include an AC to DC converter  130 , for example, a DC link with a capacitor, a DC to AC converter  132 , and/or any other circuitry such as filters, inductors, configured to convert the variable frequency and/or variable voltage output by one of the gensets  112  into a constant frequency and/or constant voltage signal appropriate for the connected loads  110 . For instance, the AC to DC converter  130  may include an active front end, a diode rectifier, such as 6-pulse, 12-pulse, 18-pulse, 24-pulse rectifier, or the like, a phase controller rectifier, a diode rectifier followed by a chopper, and any other circuitry suited to the generator type and its characteristics as well as the choice of power electronics technology. Ultimately then, the common bus  120  may receive a plurality of constant frequency and/or constant voltage inputs from the switch assembly  124  and the power electronics circuit  126 . 
     Optionally or additionally, the switch assembly  124  of  FIG. 3  may include a bypass switch  134  electrically coupling the variable frequency output of the switch assembly  124  to either the power electronics circuit  126  or the common bus  120 . More particularly, the bypass switch  134  may be used to enable the output of the designated variable speed genset  112  to pass straight through to the common bus  120  during instances when the power electronics circuit  126  is not needed, but otherwise couple the designated variable speed genset  112  to the power electronics circuit  126 . The power electronics circuit  126  may not be needed, for instance, when the genset  112 , although operating in a variable speed mode, is operating at a substantially constant operating speed, such as a maximum operating speed, or where the output signal is effectively a constant frequency and/or constant voltage signal without being passed through the power electronics circuit  126 . The bypass switch  134  may include a contactor, a motor-actuated selector switch, or any other suitable means for enabling a controllable switch. 
     In such cases, the bypass switch  134  may directly couple the designated variable speed genset  112  to the common bus  120 , and thereby reduce unnecessary losses from the power electronics circuit  126 . Furthermore, because the bypass switch  134  can relieve the power electronics circuit  126  from the higher loads associated with maximum operating speeds, the maximum load capacity of the power electronics circuit  126  as well as the costs associated therewith may be reduced. Although the variable speed genset system  102  in  FIG. 3  is depicted with one possible switch arrangement, it will be understood that other switch arrangements may be used to provide comparable results. For instance, any one or more of the switch assembly  124  and the power electronics circuit  126  may be automatically implemented, manually implemented, or any combination thereof. Moreover, any one or more of the connections between the gensets  112  and common bus  120  may be automatically and/or manually switched or engaged. Still further, the power electronics circuit  126  may include optional or additional circuitry, such as a DC to DC converter  127  and any supporting circuitries including inductors, fuses, contactors, and the like, configured to convert DC voltage from the optional energy storage device  122 , for example, batteries and ultracapacitors, into a DC link voltage that is appropriate for interfacing with the DC link between the power electronics circuit  126  and the common bus  120 , and ultimately converted via the DC to AC converter  132  into AC output for coupling with the common bus  120 . 
     Although the variable speed genset system  102  of  FIG. 3  is shown with four gensets  112 , it will be understood that the variable speed genset system  102  may include fewer or more gensets  112 . Furthermore, the switch assembly  124  may be correspondingly configured to selectively couple more than one genset  112  to the power electronics circuit  126  while passing the outputs of the remaining gensets  112  directly through to the common bus  120 . This may be accomplished by operating all of the variable speed gensets  112  to run at substantially the same speed, producing similar output voltages, and employing the controller  128  for paralleling and synchronization of outputs. Additionally, while the embodiment of  FIG. 3  is shown with one power electronics circuit  126  coupled to the common bus  120 , other embodiments may include additional power electronics circuits  126  coupled to the common bus  120 , so long as the total number of power electronics circuits  126  does not exceed the total number of gensets  112  available. Furthermore, in configurations having more than one power electronics circuit  126 , the switch assembly  124  may be correspondingly configured to selectively couple more than one genset  112  to the power electronics circuits  126  while passing the outputs of the remaining gensets  112  directly through to the common bus  120 . 
     Turning to  FIG. 4 , one exemplary embodiment of a controller  128  that may be used in association with the variable speed genset system  102  is diagrammatically provided. The controller  128  may be implemented using one or more of a processor, a microprocessor, a microcontroller, an electronic control module (ECM), an electronic control unit (ECU), and any other suitable device for operatively communicating with one or more of the gensets  112 , interface circuitry  114 , engines  116 , generators  118 , common bus  120 , switch assembly  124 , power electronics circuit  126 , and the like. The controller  128  may be configured to operate according to predetermined algorithms or sets of instructions designed to control the variable speed genset system  102  and the gensets  112  thereof in an efficient and reliable manner. For example, the controller  128  may determine an optimum mode of operation based on a set of performance goals, priorities and constraints using the operational parameters of the gensets  112  and/or the machine  100 , and according to fuel and/or load optimization maps or profiles preprogrammed therein. 
     Still referring to  FIG. 4 , the controller  128  may be configured to function according to one or more preprogrammed algorithms, which may be generally categorized into, for example, a monitor module  136 , a designation module  138 , a switch module  140 , and a converter module  142 . The controller  128  may also include the functions of paralleling and synchronizing the outputs of the constant speed gensets  112  in as well as the outputs of the variable speed genset  112  via the power electronics circuit  126  coupled to the common bus  120 . The controller  128  may also be configured to include paralleling and synchronization of a plurality of variable speed gensets  112  from running at substantially the same speed producing similar voltages to the power electronics circuit  126 , if such a configuration is desired. In addition, the controller  128  may also decide the distribution of loading among gensets  112  based on any operational constraints and performance goals, such as fuel economy and long term load distribution among gensets  112 . 
     The monitor module  136  may be configured to monitor one or more operational parameters of the gensets  112  and/or the machine  100 . The monitor module  136  may receive or detect feedback from the gensets  112  and the machine  100  using sensor systems commonly used in the art, and monitor operational parameters, such as engine operating speeds, runtime, load capacity, load duty cycles, fuel consumption rates, fuel economy, and any other relevant information. Additionally, the operational parameters may be individualized and specific to each genset  112  or engine  116  within the variable speed genset system  102 . Furthermore, the monitor module  136  may have access to, or have the ability to make updates or additions to, historical data previously logged for each individual genset  112  or engine  116 . 
     In turn, the designation module  138  of  FIG. 4  may be configured to designate one or more of the gensets  112  as variable speed gensets, and designate the remaining gensets  112  as constant speed gensets based on one or more of the operational parameters provided by the monitor module  136 . For instance, the designation module  138  may cause one of the gensets  112  in  FIG. 3  to operate in a variable speed mode, and operate the remaining three gensets  112  to operate in a constant speed mode based on a comparison between the operational parameters and preprogrammed optimization profiles. Among other things, a fuel optimization profile may indicate optimum engine operating speeds for a particular load and a target fuel consumption rate. The load optimization profile may indicate the relative loading or runtime histories for each of the individual gensets  112 , which allows the designation module  138  to avoid overloading one genset  112  over another. The designation module  138  can thereby promote more even distribution of load between the gensets  112 , and help extend the life of the variable speed genset system  102 . Furthermore, the designation module  138  may perform such designations in real-time and automatically during operation. 
     Based on the designations made by the designation module  138 , the switch module  140  of  FIG. 4  may be configured to output a variable frequency signal or output from the designated variable speed genset  112  and one or more constant frequency signals or outputs from the designated constant speed gensets  112 . For instance, consider the goal to provide more even loading among the gensets  112  over time, while optimizing fuel economy throughout and if the designation module  138  determines that the second genset  112 - 2  in  FIG. 3  historically has the least amount of running hours than the other gensets  112 - 1 ,  112 - 3  and  112 - 4 , the designation module  138  may assign that genset  112 - 2  to operate in a variable speed mode for at least the given iteration and allow it to be among the first active and loaded gensets  112 , and assign each of the other gensets  112 - 1 ,  112 - 3  and  112 - 4  to operate in a constant speed mode with a lower loading priority until changes in the conditions or operational parameters suggest otherwise. Based on those designations, the switch module  140  may control or engage the switch assembly  124  to connect the output of the designated variable speed genset  112 - 2  to the power electronics circuit  126 , and to pass through the outputs of the designated constant speed gensets  112 - 1 ,  112 - 3  and  112 - 4  directly to the common bus  120 . 
     Correspondingly, the converter module  142  of  FIG. 4  may be configured to convert the variable frequency signal or output, originally supplied by the designated variable speed genset  112 - 2  and output by the switch assembly  124 , into a converted constant frequency signal or output. For example, the converter module  142  may be adapted to control the power electronics circuit  126  of  FIG. 3  to convert the variable frequency and/or variable voltage signal into a constant frequency and/or constant voltage signal that is appropriate for the common bus  120  or the loads  110  connected thereto. Additionally, the converter module  142  may disable the power electronic circuit  126 , or the switch module  140  may simply engage the bypass switch  134  of  FIG. 3 , when conversions are not necessary, such as when the designated variable speed genset  112 - 2 , although operating in a variable speed mode, outputs a signal that is effectively a constant frequency and/or constant voltage signal without being passed through the power electronics circuit  126 . In addition, the converter module  142  may charge and/or discharge the optional energy storage device  122  via the DC to DC converter  127  so as to enable the energy storage device  122  to maintain uninterrupted power through the DC link of the power electronics circuit  126  during transient events, or planned or unplanned shutdown events. 
     INDUSTRIAL APPLICABILITY 
     In general, the present disclosure finds utility in marine applications, but can also find utility in various other applications, such as in mining, construction, farming, transportation, and other industries. More particularly, the present disclosure provides a simple and cost-effective solution for operating multiple gensets in an efficient and reliable manner. For instance, by enabling one or more gensets to partially operate in variable speed modes as necessary, the present disclosure improves overall fuel economy. Also, by providing the ability to designate different variable speed gensets based on loading or runtime, the present disclosure is able to more appropriately distribute the load among the gensets over time to allow for proper maintenance without downtime as well as prolong the life of the genset system. Still further, by requiring as few as one power electronics circuit, the present disclosure also reduces implementation and maintenance costs typical of conventional constant speed genset configurations. 
     Turning now to  FIG. 5 , one exemplary algorithm or method  146  for controlling the variable speed genset system  102  is provided. In particular, the method  146  may be implemented in the form of one or more algorithms, instructions, logic operations, or the like, and the individual processes thereof may be performed or initiated via the controller  128 . Although the method  146  may be substantially automated, it will be understood that certain portions of the method  146  may also be amenable to be performed manually. As shown in block  146 - 1 , the method  146  may initially monitor operational parameters of the individual gensets  112  and/or the machine  100 . For example, the method  146  may receive, detect and monitor operational parameters, such as engine operating speeds, runtime, load capacity, load duty cycles, fuel consumption rates, fuel economy, and any other relevant information. While receiving new operational parameters, the method  146  may additionally access, update and/or make additions to historical data previously logged for each genset  112 . 
     According to block  146 - 2  of  FIG. 5 , the method  146  may further compare the operational parameters to one or more preprogrammed optimization profiles. For example, the method  146  may compare the operational parameters to a fuel optimization profile, a load optimization profile, and/or any other profile provided as a reference by which the gensets  112  can be controlled. Moreover, a fuel optimization profile may indicate optimum engine operating speeds for a particular load and a target fuel consumption rate, while a load optimization profile may indicate the relative loading or runtime histories for each of the individual gensets  112 . Based on the comparisons, the method  146  in block  146 - 3  may be configured to designate which gensets to be in operation, and have one of the gensets  112  to operate in a variable speed mode, and further, designate each of the remaining gensets  112  that are designated to operate, to operate in a constant speed mode, and as well as apportionment of the loads among these gensets, based on performance goals, priorities such as fuel economy, load distribution as well as operating constraints for the gensets. In  FIG. 3 , for instance, if the operational parameters indicate that the runtime of the second genset  112 - 2  is significantly below the runtimes of the other three gensets  112 - 1 ,  112 - 3  and  112 - 4 , the method  146  may designate the second genset  112 - 2  to operate in a variable speed mode as well as designate it to be among the first gensets to be operational and loaded. 
     Based on the designations, the method  146  in block  146 - 4  of  FIG. 5  may be configured to engage a switch assembly  124  to couple the output of the genset  112  operating in a variable speed mode through a power electronics circuit  126  as shown in  FIG. 3 . Using the power electronics circuit  126 , the method  146  in block  146 - 5  may in turn convert the variable frequency output by the genset  112  into a converted constant frequency output. For example, the power electronics circuit  126  may be controlled in a manner which outputs a converted constant frequency output that is appropriate for the common bus  120  and any connected loads  110 . The method  146  in block  146 - 6  may further communicate the converted constant frequency output to a common bus  120 . Additionally or optionally, the method  146  may selectively bypass the power electronics circuit  126 , such as via control of the bypass switch  134  of  FIG. 3 , when conversions are not necessary to reduce the losses from the power electronics circuit  126 . 
     Still referring to  FIG. 5 , the method  146  in block  146 - 7  may be configured to couple the gensets  112  operating in a constant speed mode directly to the common bus  120 . As shown in  FIG. 3 , for example, the method  146  may be configured to engage the switch assembly  124  such that the outputs of the gensets  112  are connected directly to the common bus  120 , while the output of the genset  112  operating in a variable speed mode is connected to the power electronics circuit  126 . Furthermore, the method  146  in block  146 - 8  may be configured to communicate the constant frequency outputs directly to the common bus  120  without substantial power conversions. Although the method  146  in  FIG. 5  designates one genset  112  to operate in a variable speed mode, in other embodiments or modifications, the method  146  may designate more than one of the gensets  112  to operate in a variable speed mode, so long as either the variable speed gensets  112  are paralleled and synchronized by the controller  128  while operating at substantially the same speed to an appropriate power electronics circuit  126  or a separate power electronics circuit  126  is available for each such genset  112  operating in a variable speed mode. 
     In addition, the method  146  of  FIG. 5  may maintain any designations for one or more iterations, or at least until the operational parameters indicate changes in the gensets  112  and/or the machine  100  urge otherwise. For instance, if the operational parameters indicate that the runtime of a genset  112  previously operating in a variable speed mode significantly exceeds all of the runtimes of gensets  112  previously operating in a constant speed mode, the method  146  may reassign the gensets  112  in a subsequent iteration. In the next iteration, for example, the genset  112  which previously operated in a variable speed mode may be reassigned to operate in a constant speed mode, while one of the gensets  112  which previously operated in a constant speed mode is reassigned to operate in a variable speed mode. Such reassignments may be made based on changes in fuel economy, load distribution, load optimization guidelines and other changes. 
     From the foregoing, it will be appreciated that while only certain embodiments have been set forth for the purposes of illustration, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.