Patent Publication Number: US-2020295574-A1

Title: Distribution systems using incongruent load imbalance response

Description:
TECHNICAL FIELD 
     This document pertains generally, but not by way of limitation, to electrical power distribution systems that can receive power from multiple power plants, including those operating gas turbine engines. More specifically, but not by way of limitation, the present application relates to control systems for electrical power distribution systems and power plants having a load imbalance response to changing grid conditions. 
     BACKGROUND 
     Power plants typically supply power to a grid system within a distributed network where voltage is provided at a constant amplitude or magnitude. The grid system is managed to maintain frequency regulation, such as at a control frequency of, for example, 60 Hertz (Hz), so that the frequency and voltage magnitude maintain stability across a broad range of power input and load conditions. Each power plant can separately provide power to the grid system using a controlled frequency that can coincide with the control frequency. Put another way, each power plant is expected to contribute power to meet the demand such that the grid system operates with the desired degree of frequency regulation, such as at the control frequency. Typical loading on the grid system will not vary enough to cause the system frequency to change from the control frequency. However, when the load on the grid system changes sufficiently, such as during a load imbalance event, the system frequency will change from the control frequency. For example, when the grid system suddenly becomes heavily loaded, the system frequency will drop as each power plant correspondingly becomes more heavily loaded. That is, the additional load on each electrical generator will cause the generator to slow down. The frequency of a synchronous generator is governed by Equation [1]. 
     
       
         
           
             
               
                 
                   F 
                   = 
                   
                     
                       P 
                        
                       N 
                     
                     120 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                   [ 
                   1 
                   ] 
                 
               
             
           
         
       
     
     In Equation [1], F is frequency in Hertz (Hz), P is the number of poles in the generator, and N is the speed of the generator in revolutions per minute (RPM). Some power plants operate gas turbine engines as prime movers to operate electrical generators. In order to produce the additional power required by the grid system, a control system for each power plant can provide additional fuel to gas turbine engine combustors according to a predetermined schedule corresponding to a prescribed “droop response.” As additional power is provided to the grid system to accommodate the increased power demand, the speed (N) of the prime mover (e.g. the rotational shaft speed of a gas turbine engine) driving the generator and the grid frequency will increase back to a desired system frequency (F), which can correspond to the control frequency. 
     In order to distribute the additional demand placed on the grid system during a load imbalance event, power plant control systems operate under a conventional response plan. For example, each electrical generator will respond to a percentage drop in the control frequency by increasing its output a fixed amount. This is commonly referred to a “droop response.” Droop response can be described as a change in design speed for a 100% governor action. For a 4% droop response, a generator will increase power output 25% for each 1% drop in the control frequency. Thus, a larger or more robust droop response level comprises a smaller droop response percentage as compared to, for example, a typical 4% droop response. Likewise, a smaller or less robust droop response level comprises a larger droop response percentage as compared to, for example, a typical 4% droop response. Droop response is typically regulated by the North American Electric Reliability Corporation (NERC) so that all power plants respond to a load imbalance in the same manner. 
     Examples of controlling power production in power grids are described in U.S. Pat. No. 5,555,719 to Rowen et al.; U.S. Pub. No. 2009/0112374 to Kirchhof et al.; and U.S. Pub. No. 2014/0060065 to Sweet et al. 
     Overview 
     The present inventors have recognized, among other things, that a problem to be solved can include ineffective or inefficient droop responses placed on various power plants within a grid system and various electrical generators within a power plant. For example, each power plant in a grid system and each generator within a power plant is typically expected to provide the same droop response during a load imbalance event. Uniform droop responses can give rise to ineffectiveness at the power plant level and at the individual generator level due to, for example, operational, electrical, administrative, productive, mechanical, economic and financial differences between power plants and generators. Furthermore, inefficiencies can result at the grid level wherein inadequate or ineffective droop responses can result in an overshoot where excess capacity is produced or can even result in load shedding situations. Extreme or uncontrolled load shedding can result in rolling or full blackout conditions or lead to control scheme oscillations. 
     The present subject matter can help provide a solution to this problem, such as by increasing droop response effectiveness by allowing power plants to react differently to a load imbalance event with different droop responses based on one or more various power-plant-specific traits, such as the percentage of total grid demand provided by a power plant, the maintenance history or schedule of a power plant, the location of the power plant relative to end users of the power, and the power generation type of the power plant. Droop response effectiveness can be increased by allowing the grid and power plants to take advantage of differences in the aforementioned power-plant-specific traits. Power-plant-specific droop responses can reduce overshoot thereby reducing oscillations, and load shedding situations. 
     In an example, a method of controlling an imbalance response in a power grid that can comprise a first power plant and a second power plant, the method can comprise: monitoring operation of the first power plant while operating at a first level to provide a first power output, monitoring operation of the second power plant while operating at a second level to provide a second power output, monitoring load demand from the power grid operating at a steady state condition, detecting a load imbalance on the power grid that causes a deviation from the steady state condition, and issuing incongruent load imbalance instructions to the first power plant and the second power plant to provide a load imbalance response to change the first power output and the second power output to reduce the deviation from the steady state condition depending on a power-plant-specific trait of each of the first power plant and the second power plant. 
     In another example, a method of controlling operation of a first power plant in response to changing power grid conditions can comprise: receiving a power-plant-specific power assignment from an operator of a grid system, monitoring an operating frequency of the power grid relative to a control frequency, operating at least one power generator of the first power plant at the control frequency to provide a local power output to meet the power-plant-specific power assignment under steady state conditions, detecting a load imbalance from the power grid wherein the operating frequency and the control frequency are different, and operating the at least one power generator to provide a power-plant-specific imbalance response wherein the local power output is adjusted based on a power-plant-specific-trait of the first power plant relative to a second power plant working with the operator. 
     In an additional example, a control system for operating a power plant can comprise: a power plant controller for controlling at least one power generator at a facility, the power plant controller can comprise: a power generator interface for providing control input signals to the at least one power generator to control output of at least one electrical generator, a grid interface for receiving a control frequency at which a power grid is to be operated, and a current operating frequency of the power grid, and memory having stored therein power-plant-specific data for the power plant relative to other power plants of the power grid, wherein the power plant controller is configured to adjust a droop response of the power plant incongruently relative to the other power plants based on the power-plant-specific data in response to the current operating frequency deviating from the control frequency. 
     This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a power system including multiple electrical generator units within multiple power plants providing output to a distributed grid network. 
         FIG. 2  is a diagram illustrating a first power plant and a second power plant having power-plant-specific traits comprising demand percentage and maintenance state. 
         FIG. 3  is a diagram illustrating a third power plant and a fourth power plant having power-plant-specific traits comprising distance from end users and generation type. 
         FIGS. 4A and 4B  are graphs illustrating conventional frequency or droop response and an incongruent frequency or droop response of the present application, respectively. 
         FIGS. 5A and 5B  are graphs illustrating conventional load response and an incongruent load response of the present application, respectively. 
         FIG. 6  is a schematic diagram illustrating components of controllers for operating the power system and power plants of  FIG. 1 . 
         FIG. 7  is a line diagram illustrating steps of a method for providing incongruent load imbalance responses for power plants of a power grid. 
     
    
    
     In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic diagram of power system  10  illustrating power plant  12 A, power plant  12 B and power plant  12 C providing electrical power to distributed grid network (DGN) or “grid”  14 , which can include controller  15 . First power plant  12 A can include first generator unit  16 A, second generator unit  16 B, heat recovery steam generator (HRSG)  18 , and controller  19 . First generator unit  16 A can comprise first gas turbine  20 A, first electrical generator  22 A and first engine controller  24 A, such as a Distributed Control Systems (DCS) device. Second generator unit  16 B can comprise second gas turbine  20 B, second electrical generator  22 B and second engine controller  24 B, such as a DCS. HRSG  18  can be operatively coupled to steam turbine  26 , which can be connected to electrical generator  28 . DGN  14  can be configured to deliver power from electrical generators  22 A,  22 B and  28  to end users  30 , which can include residential housing units  32  and factory  34 , for example. 
     The present application is directed to systems and methods for controlling power delivery from power plants  12 A,  12 B and  12 C to DGN  14  during load imbalance situations, whether comprising a short term transitory imbalance or a long term new output level. A short term load imbalance can occur such as when another power plant, such as one of power plants  12 B or  12 C goes offline, particularly in a sudden fashion, or when factory  34  goes online, particularly in a sudden fashion. For example, controller  19  can cooperate with controller  15  to operate generator units  16 A and  16 B to more effectively provide power to end users  30  during the load imbalance based on a power-plant-specific trait of power plant  12 A relative to power-plant-specific traits of power plants  12 B and  12 C. In various scenarios, system effectiveness can be achieved by operating power plant  12 A most operationally efficient (also herein referred to as a “contemporaneous efficiency state”), including both productive and economical efficiencies, relative to operational efficiencies of power plants  12 B and  12 C based on the power-plant-specific traits. In various applications, power system  10  can be operated most effectively by operating power plant  12 A with a different droop response than power plants  12 B and  12 C, which can be operated at a typical droop response such as 4%, in response to a load imbalance in system  10 . While an embodiment of the disclosure has been described with turbines  20 A,  20 B,  26  connected individually to generators  22 A,  22 B,  28 , it will be appreciated that the scope of the disclosure is not so limited, and shall include other arrangements of turbines and generators, such as to couple all turbines to a single generator, or to couple the gas turbines  20 A,  20 B to a single generator, etc., for example. 
     As will be discussed below in greater detail, if the load demand upon DGN  14  is decreasing, and requires a reduction in power generation, the power output of the less effective power plant can be decreased more during the transition period (the time it takes for system  10  to adjust to the load imbalance situation whether comprising a short term transitory imbalance or a long term new output level) than the power output of the more effective power plant. Likewise, if the load demand upon DGN  14  is increasing, and requires an increase in power generation, the power output of the more effective power plant can be increased more during the transition period than the power output of the less effective power plant. The power plant effectiveness can be based on the power-plant-specific traits discussed herein. For example, controller  19  can operate power plant  12 A at a higher droop response percentage (e.g., 5%) than the typical droop response percentage (e.g., 4%) that power plants  12 B and  12 C are expected to operate at if power plant  12 A is, at the time of the load imbalance, operating to provide a greater percentage of the demand for DGN  14 . That is, first power plant  12 A will be less responsive and provide less power to DGN  14 , thereby achieving greater system effectiveness by reducing a likelihood of control system overshoot, which can result in undesirable control oscillation(s). 
     As described in greater detail below, controller  19  can operate power plant  12 A with different droop responses based on a variety of power-plant-specific traits such as, percentage of total grid demand being supplied by the power plant, the distance of the power plant from consumers of power or customers of a grid system, the type of power generators being used at the power plant and the associated responsiveness of the power generators, and the online capacity of the power plant, e.g. the percentage of power generators at the power plant not down for maintenance. 
     As is known in the art, gas turbines  20 A and  20 B operate by compressing air with a compressor, and burning fuel within the compressed air to generate high energy gases that pass through a turbine that produces rotational shaft power to drive an electrical generator. Gas turbine  20 A can include compressor  36 A, combustor  38 A, turbine  40 A, turbine shaft  42 A and output shaft  44 A. Gas turbine  20 B can include compressor  36 B, combustor  38 B, turbine  40 B, turbine shaft  42 B and output shaft  44 B. In some non-limiting examples of embodiments of the present application, gas turbines  20 A and  20 B are constructed in the same manner, e.g., are the same model or have the same capacity. 
     Engine controllers  24 A and  24 B can control the amount of fuel that is delivered to combustors  38 A and  38 B, thereby controlling the power output of gas turbines  20 A and  20 B and thus influence the rotational speed of turbine shafts  42 A and  42 B. Engine controllers  24 A and  24 B can operate the output of gas turbines  20 A and  20 B such that the speed of turbine shafts  42 A and  42 B operate at a control frequency of system  10  under steady state operating conditions. 
     Exhaust gas EA and EB of gas turbines  20 A and  20 B, respectively, can be directed to HRSG  18 . HRSG  18  can utilize the hot exhaust gas EA and EB to produce gas G, such as steam, for driving turbine  26 . 
     Electrical output of generators  22 A and  22 B and electrical generator  28  can be provided to DGN  14 . Interface of generator units  16 A and  16 B with DGN  14  can be controlled by controller  19 , which can interface directly with engine controllers  24 A and  24 B. 
     Grid  14  can operate under a frequency control regime. During steady state operation, power plants  12 A,  12 B and  12 C provide power to grid  14  at a control frequency, such as 60 Hertz. End users  30  can also operate at various levels, thereby creating a total load demand upon the DGN  14  that can change. Thus, controller  15  can distribute the total load demand amongst power plants  12 A,  12 B and  12 C, which can then operate to provide their assigned share of the load demand, operating with a bias toward the control frequency. Each of power plants  12 A,  12 B and  12 C can internally determine how to generate their share of the total load demand. For example, power plant  12 C can operate some or all of the total number of wind turbines in power plant  12 C. Also, power plant  12 A can determine to operate gas turbines  20 A and  20 B to each equally divide the share of power that they produce as part of power plant  12 A. Thus, under steady state operating conditions, end users  30  place a total load demand on grid  14 , and controller  15  allocates the total load demand to power plants  12 A,  12 B and  12 C. 
     End users  30 , or consumers or customers, typically operate within a reasonably predictable operating band for any point in time such that small changes in the total power demand do not produce significant changes in the operation of power plants  12 A,  12 B and  12 C. That is, for example, controller  15  can be programmed to estimate total demand from end users  30  based on seasonal, weather, economic, demographic and historical usage data to within a known operating band. However, sometimes load imbalances can be produced if the total load demand rapidly changes, either upward or downward. Also, the share of the total load demand on each of power plants  12 A,  12 B and  12 C can rapidly change in the event one of power plants  12 A,  12 B and  12 C goes offline, or has a temporary change in power output. In either of these load spike scenarios, controller  15  typically requests each of power plants  12 A,  12 B and  12 C respond in an appropriate manner such that additional loading is shared either equally or proportionally. Regardless, controller  15  expects each of power plants  12 A,  12 B and  12 C to react in a particular manner in response to a load imbalance. For example, in the event of an unexpected load increase, controller  15  can expect a typical 4% droop response from each of power plants  12 A,  12 B and  12 C, assuming each is capable of such response. For example, power plant  12 C may not be capable of such a response given wind conditions. 
     In other embodiments, a load imbalance may result when controller  15  determines that the operating point for the predictable operating band should be reset to a higher or lower output level. For example, controller  15  may request lower collective output from power plants  12 A,  12 B and  12 C during night time as compared to day time due to lower demand. As such, a load imbalance may occur within DGN  14  during a load down (or converse, load up) event. 
     Aspects of the present application are directed to each power plant  12 A,  12 B and  12 C reacting incongruently, e.g, asymmetrically or differently, to an imbalance response called for by grid  14  via controller  15 , for example. Controller  15  can coordinate different droop responses from power plants  12 A,  12 B and  12 C, or controllers  19  can, with information pre-provided by controller  15 , individually manage how to provide the imbalance response desired by controller  15 . For example, if controller  15  desires a total 4% droop response from DGN  14 , controller  15  can request (or power plants  12 A,  12 B and  12 C can individually determine) that power plant  12 A provide a 3% droop response, power plant  12 B provide a 4% droop response, and power plant  12 C provide a 5% droop response, with the droop response assignments being determined based on the power-plant-specific traits identified above, so that controller  15  and DGN  14  still receive the desired imbalance response, e.g., the 4% droop response. The power-plant-specific droop responses can also result in the total droop response being above or below the desired droop response of controller  15 . The incongruent droop responses can be determined based on the power-plant-specific traits described herein. 
       FIG. 2  is a diagram illustrating first power plant  46 A and second power plant  46 B having power-plant-specific traits comprising demand percentage and maintenance state. Power plants  46 A and  46 B are in communication with grid controller  15  via power plant controllers  19 A and  19 B, respectively. Power plants  46 A and  46 B are configured to provide power to end users  30  via DGN  14 . 
     Power plant  46 A can be configured as a 3-on-1 combined cycle plant where gas turbines  48 A,  48 B and  48 C provide exhaust to a single steam turbine (not illustrated in  FIG. 2 ) similar to turbine  26  ( FIG. 1 ). Power plant  46 B can be configured as a 2-on-1 combined cycle plant where gas turbines  48 D and  48 E provide exhaust to a single steam turbine (not illustrated in  FIG. 2 ) similar to turbine  26  ( FIG. 1 ). Note that although power plants  46 A and  46 B are configured as having multiple gas turbines and associated electrical generators, power plants can include only a single electric generator, whether turbine powered or powered by another source. The various droop responses, incongruent load imbalance responses and power-plant-specific traits discussed herein are applicable to power plants having one or at least one electrical generators. 
     In various examples, power plants  46 A and  46 B can be configured to provide different percentages of the total demand end users  30  place on DGN  14 . For example, power plant  46 A can be configured to provide a greater percentage of the total demand. For discussion purposes, power plant  46 A can provide 60% of the total grid demand of end users  30  while power plant  46 B can provide 40%. Additionally, other power plants can be connected to DGN  14 , such as power plant  46 C of  FIG. 3 , but power plant  46 A can still generate a larger percentage of the total grid demand relative to power plant  46 B. Consider a load imbalance event, during which power plants  46 A and  46 B can be configured to react incongruently, or differently, rather than each acting identically as is typical. For example, in a sudden partial power outage event where a third power plant, such as power plant  46 C of  FIG. 3  goes offline, it would be appreciated that use of the same droop response for the (relatively) larger power plant  46 A and smaller power plant  46 B could result in a reactionary over-production of power, also known as an overshoot condition, because power plant  46 A is configured to provide more power to end users  30  than power plant  46 B, and is likely to be slower in responding. 
     In an embodiment, a likelihood of such an overshoot may be reduced by increasing a droop response percentage of power plant  46 A. By increasing its droop response percentage from the typical 4% to a 5% droop response, power plant  46 A can thereby provide a slower response and reduce the likelihood of an overshoot of power supply, which is inefficient and ineffective. 
     Further, power plant  46 B can maintain a 4% droop response because, for example, increasing the load imbalance response on a power plant providing a small percentage of the total power may overburden the power plant causing inefficiencies and may be ineffective in meeting the load imbalance response. The maintained response of the relatively smaller power plant, which is also likely to be relatively faster in responding, is likely to provide the requested change in power demand that can expedite a return to the nominal control frequency without encountering the overshoot condition. Thus, in some instances, if the total contribution of power plant  46 B is small, it still may be desirable to decrease the droop response percentage to prevent inefficient operation of the larger contributing power plant, even if power plant  46 B is driven to maximum contribution. Larger units tend to be much more efficient when allowed to remain at a steady state load condition. They are also traditionally slower to respond due to momentum and inertia and such other laws of physics. So, in this case, the smaller unit might be driven up even to maximum contribution if it allows overall operations to retain the efficiency and stability of the larger unit and let the smaller units contribute their own and the portion that would be provided by the larger unit in order to provide a more economical event response with all of their tools available. 
     In various examples, grid controller  15  may provide instantaneous droop response instructions to power plant controllers  19 A and  19 B for power plants  46 A and  46 B based on the monitored percentage of total grid demand being placed on each of power plants  46 A,  46 B by end users  30  via DGN  14 . In other examples, controllers  19 A and  19 B may be provided with information from controller  15  to allow controllers  19 A and  19 B to react independently. For example, control  15  can provide controllers  19 A and  19 B with the percentage of total power demand being generated by each power plant on DGN  14 . 
     In various examples, power plants  46 A and  46 B can be configured to provide incongruent, e.g., asymmetric or different, droop responses based on conditions of operating assets within or near each of power plants  46 A and  46 B. For example, power plants  46 A and  46 B may be operating in different maintenance states where one or more of gas turbines  48 A- 48 D may be down for repair. For example, power plant  46 A may be operating with gas turbine  48 C down for repair. In such a scenario, if power plant  46 B goes offline, controller  19  for power plant  46 A may adjust the droop response for gas turbines  48 A and  48 B relative to a droop response that would occur if all three gas turbines  48 A- 48 C were operating. For example, instead of increasing the droop response percentage as discussed above, gas turbines  48 A and  48 B could maintain a 4% droop response to prevent both overshoot and overburdening. 
     For example, power plants operate on maintenance intervals and may shut down annually or every eighteen months for maintenance. This can be due to the wear on the components and portions of the system will be replaced as the maintenance activity; e.g., fuel injectors, thermal barrier coatings and the like can be checked and repaired. The overall efficiency of the unit can decrease while operating with the degraded parts just prior to the maintenance interval. Therefore, we can assume, for example, that power plants  46 A and  46 B are identical co-operational gas turbines, but with annual maintenance outages occurring in October and April, respectively. If we have frequency events happening in December, the unit that was just serviced in October may be more operationally efficient for the owner per MW produced, due to productive efficiency factors. So, it can be advantageous to favor the most-recently-serviced gas turbine during the droop response. 
       FIG. 3  is a diagram illustrating first power plant  46 A and third power plant  46 C having power-plant-specific traits such as distance from end users and generation type. Power plants  46 A and  46 C are in communication with grid controller  15  via power plant controllers  19 A and  19 C, respectively. Power plants  46 A and  46 C are configured to provide power to end users  30  via DGN  14 . 
     Power plants  46 A and  46 C can be located at different geographic locations relative to end users  30 . For example, first power plant  46 A can be located distance D 1  from end users  30  and third power plant  46 C can be located distance D 2  from end users  30 . 
     Power plant  46 A can be configured as a 3-on-1 combined cycle plant where gas turbines  48 A,  48 B and  48 C provide exhaust to a single steam turbine (not illustrated in  FIG. 2 ) similar to turbine  26  ( FIG. 1 ). Power plant  46 C can be configured as a wind farm having wind turbines  50 A,  50 B and  50 C. 
     Incongruent droop response can be determined by determining which of distances D 1  and D 2  is smaller in order to provide a droop response. For example, if a power plant connected to DGN  14  other than power plants  46 A and  46 C, such as power plant  46 B ( FIG. 2 ) goes offline, whichever of power plant  46 A and  46 C that is closer to end users  30  can provide a greater load imbalance response, such as by providing a smaller droop response percentage. Thus, because power plant  46 A is closer to end users  30  such that distance D 1  is smaller than distance D 2 , power plant  46 A can reduce its droop response percentage, such as from the typical 4%, while power plant  46 C can maintain a 4% droop response. Power plant  46 A thereby provides a more robust droop response that can more rapidly address the power deficit. 
     Incongruent load response can also be provided by decreasing the droop response percentage of the more responsive of power plants  46 A and  46 C. For example, power plant  46 A, as described above, can comprise a combined cycle gas turbine power plant, while power plant  46 C can comprise a wind farm or wind power plant. Combined cycle power plants can be more responsive to changing conditions due to greater control over the power generation process. For example, as discussed herein, power output of gas turbines can be increased by providing more fuel to the combustion process on demand. However, wind turbines  50 A- 50 C of power plant  46 C at least in part are dependent on environmental or wind conditions for power production. Thus, power plant  46 C cannot always be as responsive as is desired to changing grid conditions. For example, if a power plant connected to DGN  14  other than power plants  46 A and  46 C, such as power plant  46 B ( FIG. 2 ) goes offline, whichever of power plant  46 A and  46 C has the more responsive power generating type, e.g. the power generating type that can more rapidly increase power output, can provide a greater load imbalance response, such as by providing a smaller droop response percentage. Thus, because power plant  46 A is more responsive, power plant  46 A can reduce its droop response percentage, such as from the typical 4%, while power plant  46 C can maintain a 4% droop response. Power plant  46 A thereby provides a more robust droop response that can more rapidly address the power deficit. In certain scenarios, it may even be that power plant  46 C is causing the imbalance due to rapidly changing wind conditions. 
     In other examples, combined cycle gas turbine power plants can be less responsive than simple cycle gas turbine power plants. As such, if power plant  46 A were a combined cycle gas turbine power plant and power plant  46 C were a simple cycle gas turbine power plant, because power plant  46 C is more responsive, power plant  46 C can reduce its droop response percentage, such as from the typical 4%, while power plant  46 A can maintain a 4% droop response. 
     Based on the various power-plant-specific traits described above, such as with respect to  FIGS. 2 and 3 , controller  15  can determine or be programmed to determine which of power plants  46 A,  46 B and  46 C can be incongruently favored or biased during a load imbalance, which may last for long or short term transition periods. Examples of load imbalance can include a sudden, significant demand drop or demand increase from end users  30 , or a sudden output drop from one or more of power plants  12 A,  12 B and  12 C, as is discussed, for example, with reference to  FIGS. 4A and 4B . In response to detecting a load imbalance, controller  15  can issue imbalance response instructions to each of power plants  12 A,  12 B and  12 C. For example, controller  19  for power plant  12 A can receive the imbalance response and take appropriate action, such as to implement a particular droop response. Another example of a load imbalance can comprise a projected long term change in power demand that might require a load up or load down rebalancing of power generation from power plants  12 A,  12 B and  12 C, as is discussed, for example, with reference to  FIGS. 5A and 5B . As such, controller  15  can issue load rebalancing instructions to power plants  12 A,  12 B and  12 C such as in a load down or load up situation where total power to grid  14  is changed for long term durations. 
     Such evaluation or determination can be implemented automatically, such as using one or more processor circuits coupled to one or more memory circuits or other storage devices. A cost, effectiveness or efficiency function can be established accounting for the various factors mentioned above (e.g., location, responsiveness, power contribution, mechanical or financial constraints), such as implemented using one or more of a look-up-table, an analytical expression (e.g., including various parameters or weighting factors), or other scheme. In an example, inputs to the cost, effectiveness or efficiency function can include one or more of a monitored parameter (e.g., frequency, frequency stability, output power, voltage magnitude) from the power grid to which the power plants  12 A,  12 B,  12 C,  46 A,  46 B and  36 C are coupled, or other parameters such as state information concerning the power plants or their associated prime movers. An output of the cost, effectiveness or efficiency function can include a relative value corresponding to an associated power plant, generator units  16 A or  16 B, or an associated prime mover. Such a cost value can be used to establish an operating point for the power plant or associated generator units, such as to operate the associated prime movers in an asymmetric manner to perform load imbalance compensation. 
       FIGS. 4A and 4B  are graphs illustrating a conventional frequency or droop response vs. an incongruent frequency or droop response of the present application, respectively, for temporary load imbalance situations.  FIGS. 4A and 4B  show graph  60  including speed plot  62 , first load plot  64  and second load plot  66 . For example, speed plot  62  can correspond to the operating speeds of generator units  48 A- 48 C of power plant  46 A, and operating speeds of generator units  48 D and  48 E of power plant  46 B, indicated as revolutions per minute (RPM) (which is indicative of the instantaneous grid frequency). Load plots  64  and  66  can correspond to the load (power output) being provided by each of power plants  46 A and  46 B, such as in megawatts (MW), at a given time.  FIGS. 4A and 4B  can provide load adjustment for a frequency change that can occur as a result of a load imbalance, such as a temporary change in demand on grid  14 . 
     For example, load plots  64  and  66  indicate that power plants  46 A and  46 B provide a steady state output of, for example, 150 MW at 3600 RPM, as indicated by segments  64 A and  66 A. Speed plot  62  can operate at 3600 RPM at segment  62 A under steady state operating conditions, such as when grid  14  is operating at the control frequency of 60 Hz. Note, load plots  64  and  66  are described as being the same for simplicity, but do not need to be the same in various examples. During a temporary reduction load imbalance situation, such as a large, short term reduction in power consumption at the factory  34 , the load on power plants  46 A and  46 B can suddenly drop at segments  64 B and  66 B. The reduced load results in an increase of the instantaneous grid frequency relative to the control frequency, as shown by the spike of speed plot  62  at segment  62 B to a level above segment  62 A, indicating that each of power plants  46 A and  46 B are less burdened. In transition zone  68 A of  FIG. 4A , controller  15  can request that power plants  46 A and  46 B operate to adjust the load output of each of power plants  46 A and  46 B until the load returns back to the steady state operating level of 150 MW. Following a load imbalance on grid  14 , power plants  46 A and  46 B will return to the previous steady state operation, such as to return to the control frequency and again each provide 150 MW of output. As shown in  FIG. 4A , controllers  19 A and  19 B can operate power plants  46 A and  46 B equally, or congruently, so that they provide the same load imbalance response as speed returns to the steady state operating condition at segment  62 C. For example, NERC guidelines can provide a droop response instruction, such that power plants  46 A and  46 B react to the load imbalance with a 4% droop response.  FIG. 4A  shows power plants  46 A and  46 B equally sharing the 4% droop response that is provided to grid  14  by power plant  46 A and  46 B. 
     Alternatively, during the temporary reduction load imbalance situation, the power output of the less effective or responsive (as may be determined based on one or more of the previously described power-plant-specific traits) power plant of the two power plants  46 A and  46 B can be reduced more rapidly, as shown in  FIG. 4B . 
     Likewise, the same principle, to bias, favor, or more rapidly increase the power output of the more effective power plant shall apply during an temporary increase load imbalance situation, such as a large, short term increase in power consumption at factory  34 , or a sudden increase in temperature resulting in many housing units  32  increasing their use of air conditioners. The droop responses to the short term load imbalance situations may last for a terminable period of time before the droop responses correct the load imbalance and the frequency of the grid is restored to the control frequency. 
       FIG. 4B  shows transition zone  68 B where controllers  19 A and  19 B can operate power plants  46 A and  46 B incongruently so that they each undergo a different load imbalance response as speed returns to the steady state operating condition at segment  62 C. If desired, and consistent with present NERC guidelines, grid  14  may still receive an effective total 4% droop response from power plants  46 A and  46 B, but the droop response will be incongruently distributed between power plants  46 A and  46 B. However, in some examples and embodiments, power plants  46 A and  46 B can act to provide an effective total droop response other than a droop response suggested by present NERC guidelines. As such, the disclosure of the present application can provide a droop response framework as an alternative to guidelines, such as NERC guidelines, that require a certain minimum droop response threshold. That is, the total droop response provided by power plants  46 A and  46 B may not equal a typical 4% droop response, as would be provided if each power plant were operated with a congruent or symmetric droop response. 
     In an example embodiment, if power plant  46 B is more effective (as determined by the aforementioned power-plant-specific traits such as proximity to users, more responsive in size or power type, or more fully online) than power plant  46 A, power plant  46 B can be operated to provide more of the load during the time period of transition zone  68 B, thus relying less on the relatively ineffective load production from power plant  46 A for the transitory time period. In an extreme example, a single power plant can be used to provide one-hundred percent of the droop response, but this may introduce increased operational costs resulting from inefficient operation of a single power plant at elevated rates. Such increased costs would have to be weighed against potential benefits resulting from extreme bias to the most effective power plant. 
     In either the case of  FIG. 4A  or  FIG. 4B , output of power plants  46 A and  46 B can be returned to congruent or equal operation, as shown by segments  64 C and  66 C. 
       FIGS. 5A and 5B  are graphs illustrating conventional load response vs. an incongruent load response of the present application, respectively, for long term readjustment of total load requested by controller  15 .  FIGS. 5A and 5B  show graph  70  including total load plot  72 , first load plot  74  and second load plot  76 . Total load plot  72  can correspond to the operating loads of power plants  46 A and  46 B, indicated as megawatts (MW). Load plots  74  and  76  can correspond to the load being provided by each of power plants  46 A and  46 B, such as in megawatts (MW), at a given time, respectively. Note, load plots  64  and  66  are described as being the same for simplicity, but do not need to be the same in various examples. Load plot  72  is offset on the Y axis to improve visibility by avoiding overlap with load plots  74  and  76 .  FIGS. 5A and 5B  can illustrate a load adjustment, or load down imbalance response, for a load change that can occur as a result of a load imbalance, such as a change in demand on grid  14 . For example, load on grid  14  can suddenly drop when factory  34  goes offline resulting in a long term change in power demand. Additionally, weather or other conditions can cause controller  15  to adjust the baseline operating output of power plants  12 A,  12 B and  12 C to account for environmental temperature increases or nighttime operating conditions that can necessitate longer term adjustment of power output versus as compared to a short term droop response. In a load down imbalance response, output of the less effective (as determined by the aforementioned power-plant-specific traits such as proximity to users, more responsive in size or power type, or more fully online) gas turbine can be more rapidly reduced, as shown in  FIG. 5B . Likewise, controllers  19 A and  19 B can operate power plants  46 A and  46 B to respond to a load up imbalance response by favoring the more effective power plant, based on one or more power-plant-specific traits. 
     For example, load plots  74  and  76  indicate that power plants  46 A and  46 B provide a steady state output of, for example, 200 MW, as indicated by segments  74 A and  76 A. Total load plot  72  shows a corresponding 400 MW output at segment  72 A under steady state operating conditions. The load requirement of the grid  14  can suddenly drop at time T 1  during a load imbalance situation. Accordingly, the demand on power plants  46 A and  46 B can also drop, such that segments  74 B and  76 B decline in transition zone  78 A. Total load plot  72  correspondingly drops at segment  72 B. In transition zone  78 A of  FIG. 5A , controllers  19 A and  19 B can operate power plants  46 A and  46 B to adjust the load output of each of power plants  46 A and  46 B until the total load drops to the new demand of 360 MW. As shown in  FIG. 5A , controllers  19 A and  19 B can operate power plants  46 A and  46 B equally, or congruently, so that they undergo the same transition, indicated by segments  74 B and  76 B, as output is adjusted to meet the subsequent new steady state operating condition at segment  72 C.  FIG. 5A  shows power plants  46 A and  46 B equally sharing the 40 MW drop by reducing the output of each equally 20 MW, as shown by segments  74 C and  76 C. 
       FIG. 5B  shows transition zone  78 B where controller  15  can operate power plants  46 A and  46 B incongruently so that they undergo different load reductions to transition to the new steady state operating condition at segment  72 C. For example, if power plant  46 B is more effective than power plant  46 A, power plant  46 B can be operated to provide more of the load during time period of transition zone  78 B, thus relying less on the relatively ineffective load production from power plant  46 A for a transitory period of time. As discussed above, the load up or load down imbalance response for each power plant can be different to achieve an operational benefit that can be weighed against any operational cost. 
     After controller  15  for grid  14  has accounted for any load imbalance on grid  14 , power plants  46 A and  46 B will operate at the new steady state operation, such as by providing 360 MW of output. In either the case of  FIG. 5A  or  FIG. 5B , output of power plants  46 A and  46 B can be returned to congruent or equal operation following the transition period  78 A,  78 B. 
       FIG. 6  is a schematic diagram illustrating components of controller  15  for operating power system  10  and power plant controller  19  for operating generator units  16 A and  16 B of  FIG. 1 . Controller  15  can include circuit  80 , power supply  82 , memory  84 , processor  86 , input device  88 , output device  90  and communication interface  92 . Controller  15  can be in communication with grid  14 , which can provide power to end users  30 . Controller  15  can also be in communication with power plant controller  19 , which can be in communication with one or more gas turbine engine controllers, such as engine controllers  24 A and  24 B. Engine controllers  24 A and  24 B can be in communication with gas turbines  20 A and  20 B, respectively, to control operation of each turbine. For example, engine controller  24 A can be configured to control the combustion process in combustor  38 A, which can alter the power output of gas turbine  20 A to influence the speed of turbine shaft  42 A and the flow of exhaust gas EA to HRSG  18  ( FIG. 1 ). To that end, engine controller  24 A can be configured to operate one or more fuel injectors  94 , variable vanes  96  and exhaust gas valve  98  for gas turbine  20 A. Engine controller  24 B can also control similar parameters and components of gas turbine  20 B, but description and illustration is omitted with reference to  FIG. 6  for brevity. 
     Power plant controller  19  and engine controllers  24 A and  24 B can also include various computer system components that facilitate receiving and issuing electronic instructions, storing instructions, data and information, communicating with other devices, display devices, input devices, output devices and the like. For example, power plant controller  19  can include power supply  100 , memory  102 , processor  104 , control circuit  106  and the like. Power plant controllers  19 A,  19 B and  19 C can be configured similarly to controller  19 . 
     Circuit  80  can comprise any suitable computer architecture such as microprocessors, chips and the like that allow memory  84 , processor  86 , input device  88 , output device  90  and communication interface  92  to operate together. Power supply  82  and power supply  100  can comprise any suitable method for providing electrical power to controller  15  and controller  19 , respectively, such as AC or DC power supplies. Memory  84  and memory  102  can comprise any suitable memory devices, such as random access memory, read only memory, flash memory, magnetic memory and optical memory. Input device  88  can comprise a keyboard, mouse, pointer, touchscreen and other suitable devices for providing a user input or other input to circuit  80  or memory  84 . Output device  90  can comprise a display monitor, a viewing screen, a touch screen, a printer, a projector, an audio speaker and the like. Communication interface  92  can comprise devices for allowing circuit  80  and controller  15  to receive information from and transmit information to other computing devices, such as a modem, a router, an I/O interface, a bus, a local area network, a wide area network, the internet and the like. 
     Controller  15  can be configured to operate grid  14  and, as such, can be referred to the “home office” for power system  10 . Grid  14  can comprise power plants  12 A,  12 B and  12 C, as well as power plants  46 A,  46 B and  46 C, high voltage transmission lines that carry power from distant sources to demand centers, and distribution lines that connect end users  30 . As mentioned, power grids can be configured to operate at a control frequency where all power input into the grid from disparate sources in input at the same frequency to facilitate integration of the power. In an example, grid  14  can operate at a control frequency of 60 Hertz (Hz). 
     Controller  15  can determine the demand being placed on grid  14 , such as by monitoring the consumption of end users  30 . Controller  15  can coordinate generation of power from power plants  12 A,  12 B and  12 C ( FIG. 1 ), as well as power plants  46 A,  46 B and  46 C. That is, controller  15  can assign or instruct each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C how much power output they should contribute to grid  14 , and such assignment may be dynamic and reactive based upon the capabilities and availability of any of the power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C. Controller  15  can ensure that the total power generated by power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C meets the power demand of end users  30 . If power demand of end users  30  exceeds or is less than power supplied by power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C, controller  15  can dictate response strategies for each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C. For example, in the event of a power demand increase that exceeds the predicted operating band, controller  15  can ensure that each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C responds so that only one or less than all of the power plants is prevented from bearing the burden of generating power for the deficiency. Thus, controller  15  can interface with a power plant controller for each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C, like controller  19  for power plant  12 A. 
     Circuit  80  can communicate with, that is, read from and write to, a memory device such as memory  84 . Memory  84  can include various computer readable instructions for implementing operation of grid  14 . Thus, memory  84  can include instructions for monitoring demand on and power being supplied to grid  14 . Circuit  80  can be connected to various sensors to perform such functions. Memory  84  can also include information that can assist controller  15  in providing instruction to power plant controller  19  and controllers  19 A,  19 B and  19 C. For example, memory  84  can include power-plant-specific information for each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C, such as the type, size (capacity), age, maintenance history, location, the location within the geography covered by grid  14 , and proximity to end users  30  of each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C. Memory  84  can also include instructions for determining the percentage of each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C contribution to the total power supply. 
     Controller  19  can be configured to operate power plant  12 A. As mentioned, power plants  12 B and  12 C can be configured to operate with similar controllers as controller  19 , but illustration and description is omitted. Likewise, controllers  19 A,  19 B and  19 C can be configured to operate similarly as controller  19 , including the inclusion of memory  102 . Memory  102  can include various computer readable instructions for implementing operation of power plant  12 A. Thus, memory  102  can include instructions for monitoring a power generation assignment from controller  15 , instructions for power generation for each of engine controllers  24 A and  24 B, droop responses and imbalance responses for each of generator units  16 A and  16 B and the like. Memory  102  can also include information that can assist controller  19  in responding to imbalance requests from controller  15 , such as they type, size (capacity), age, maintenance history and location for each of gas turbines  20 A and  20 B. 
     Additionally, memory  102  can include operational efficiency and effectiveness information, such as productive and economical effectiveness information for each of generator units  16 A and  16 B, including gas turbines  20 A and  20 B. For example, memory  102  can include the electrical production efficiency of each of turbines  20 A and  20 B such as, for example, is illustrated in  FIG. 2 . Memory  102  can include economical information such as maintenance and economical history for each of gas turbines  20 A and  20 B such as, for example, is illustrated in  FIG. 3 , which can also include time since last service, repair, overhaul, refurbishment status, etc. Memory  102  can also include information relating to operational efficiency and effectiveness of power plant  12 A including the financial efficiency of each of gas turbines  20 A and  20 B, such as various contractual obligations for operators of power plant  12 A and manufacturers of and service providers for gas turbines  20 A and  20 B. For example, operators of power plant  12 A can have a Long Term Service Agreement (LTSA) for each of gas turbines  20 A and  20 B. The LTSA can sometimes require the manufacturer to provide, without fee to the power plant operator, routine maintenance, including parts and labor. There can, however, be restrictions placed on the operation of the gas turbines. For example, if the gas turbines are operated above an effective economical hours limit, an actual economical hours limit, above a threshold number of starts and stops, or operated above a temperature threshold for an hours limit (effective economical hours can be calculated, for example, from actual economical hours and number of hours operated above the temperature threshold), the fee arrangement can include charges to the power plant operator. For example, the power plant operator can be required under the LTSA to make higher payments, additional payments, penalty payments or the like. Memory  102  can also include power-plant-specific information for each of power plants  12 A,  12 B and  12 C, as well as power plants  46 A,  46 B and  46 C, such as the type, size (capacity), age, maintenance history, location, the location within the geography covered by grid  14 , and proximity to end users  30  of each of power plants  12 A,  12 B,  12 C,  46 A,  46 B and  46 C. 
     Controller  19  can receive notifications of changes in steady state operation of power system  10  from controller  15 . Controller  19  can also directly monitor operation of grid  14  to detect power demand and load imbalances, using sensors or other components and systems. In either configuration, controller  15  can be indirectly or directly monitoring power demand and detecting load imbalance conditions. 
     In response to steady state operating conditions or load imbalance conditions, controller  19  can issue instructions to, and receive inputs from engine controllers  24 A and  24 B of gas turbines  20 A and  20 B. For example, controller  19  can issue start and stop command signals to engine controllers  24 A and  24 B. Engine controllers  24 A and  24 B can activate an electric or pneumatic starter motor to rotate turbine shaft  42 A, and operate fuel injectors  94  to provide fuel to combustors  38 A and  38 B, as well as operate an ignitor to begin the combustion process. Engine controllers  24 A and  24 B can increase or decrease the power output by controlling the combustion process, such as by providing more or less fuel to combustors  38 A and  38 B with injectors  94  and, if desired, adjustment of variable vanes  96  that can be located in compressors  36 A and  36 B. Increased or decreased power output of gas turbine  20 A and  20 B can correspond to increased or decreased speed of shafts  42 A and  42 B, respectively. 
     Controller  19  can also issue instructions to engine controllers  24 A and  24 B for operating gas turbines  20 A and  20 B in response to a load imbalance on grid  14 . Controller  15  for power system  10  can, in response to determining a load imbalance, issue instructions or power generation assignments to power plants  12 A,  12 B and  12 C. The load imbalance instructions can require that each power plant increase or decrease power generation for a fixed or variable length of time. Thus, controller  19  can issue power generation instructions to engine controllers  24 A and  24 B, and engine controllers  24 A and  24 B can issue operating instructions to gas turbines  20 A and  20 B to produce the assigned power generation. These instructions can include increasing or decreasing the power output by controlling the combustion process within combustors  38 A and  38 B with injectors  94  and variable vanes  96 , thereby also resulting in a change in the speed of shafts  42 A and  42 B. As discussed herein, power plant controllers  19 ,  19 A,  19 B and  19 C can use power-plant-specific data stored in memory  102  or obtained elsewhere, such as from controller  15 , to incongruently operate power plant  12 A relative to power plants  12 B,  12 C,  46 A,  46 B and  46 C during a load imbalance response to provide operation that increases the operational benefit of power plant  12 A or the home office of grid  14 . The operational benefit can be in the form of, for example, a decrease in maintenance fees due to avoidance of penalty charged or a decrease in fuel consumption resulting from more efficient total mechanical operation of gas turbines  20 A and  20 B. 
       FIG. 7  is a line diagram illustrating steps of method  110  for providing incongruent load imbalance responses for plants  12 A,  12 B and  12 C. Method  110  can also be used for operating power plants  46 A,  46 B and  46 C in addition to or alternatively to power plants  12 A,  12 B and  12 C, though description is provided with reference to power plants  12 A,  12 B and  12 C for simplicity. At step  112 , a power grid, such as power grid  14 , can operate in a steady state condition. That is, each of power plants  12 A,  12 B and  12 C can operate their respective power generation equipment at a predicted, assigned output to meet expected demand from end users  30  that typically varies within a known band that can be readily accommodated by power plants  12 A,  12 B and  12 C without load rebalancing. At step  112 , each controller  19  for power plants  12 A,  12 B and  12 C can control and monitor the operation of generator units  16 A and  16 B. Likewise, controller  15  can monitor the input of each of power plants  12 A,  12 B and  12 C into grid  14 . 
     At steps  114 A,  114 B and  116 C, controllers  19  for power plants  12 A,  12 B and  12 C can receive their assigned load demand from controller  15  and issue corresponding instructions, e.g., power output command signals, respectively. For example, controller  19  for power plant  12 A can issue instructions for operation of gas turbines  20 A and  20 B such that engine controllers  24 A and  24 B can issue appropriate fuel, air and speed instructions to gas turbines  20 A and  20 B to achieve the desired electrical output from generators  22 A and  22 B, respectively. Thus, at step  116 , power plants  12 A,  12 B and  12 C can provide the assigned power output from controller  15  to grid  14 . 
     At step  118 , controller  15  and controller  19  for power plant  12 A can monitor grid  14 . Controllers  19  for power plants  12 B and  12 C can also monitor grid  14 , but illustration and description is omitted for simplicity. Controller  15  for power system  10  can read the total load demand on grid  14  from end users  30 . Controller  15  can reference information, such as information stored in memory  84  including power-plant-specific traits of power plants  12 A,  12 B and  12 C, to evaluate the capacity, effectiveness, efficiency and location of power plants  12 A,  12 B and  12 C to determine how to divide the total load demand between power plants  12 A,  12 B and  12 C to provide steady state operating instructions to controllers  19  for power plants  12 A,  12 B and  12 C. 
     At step  118 , controller  15  and controller  19  can continue to monitor steady state operation of power system  10 , monitoring output of power plants  12 A,  12 B and  12 C and demand from end users  30 . At step  120 , controller  15  and controller  19  can detect a load imbalance on grid  14 . As discussed, examples of load imbalance can include a sudden, significant demand drop or demand increase from end users  30 , or a sudden output drop from one or more of power plants  12 A,  12 B and  12 C. Other examples of load imbalance can include long term load readjustments for grid  14 . In response to detecting a load imbalance, controller  15  can issue imbalance response instructions to each of power plants  12 A,  12 B and  12 C. As described herein, controller  15  can issue incongruent droop responses specific to each of power plants  12 A,  12 B and  12 C based on power-plant-specific trait data stored in memory  84  and memory  102 . For example, controller  19  for power plant  12 A can receive the imbalance response and take appropriate action. In other examples, controller  15  can issue load rebalancing instructions to power plants  12 A,  12 B and  12 C such as in a load down or load up situation where total power to grid  14  is changed for long term durations. Likewise, controller  19  can determine an imbalance response based on information stored in memory  102 , such as power-plant-specific trait data regarding operation of power plant  12 A relative to power plants  12 B and  12 C. 
     At step  122 , one or both of controller  15  and controller  19  can implement a power-plant-specific load imbalance response. In one embodiment, controller  15  issues incongruent imbalance response instructions to each of power plants  12 A,  12 B and  12 C that is most operationally effective for grid  14 . That is, different imbalance response instructions can be issued to each of power plants  12 A,  12 B and  12 C based on power-plant-specific traits determined by controller  15 . For example, controller  19  can determine how much of the total grid demand is being provided by power plant  12 A, a maintenance state of power plant  12 A, how close power plant  12 A is to end users  30 , and the type of power being generated by power plant  12 A. Controller  19  can additionally determine those power-plant-specific traits for power plants  12 B and  12 C. The effectiveness determination can be evaluated based on instantaneous, real-time operating conditions of power plants  12 A,  12 B and  12 C. That is, for example, demand percentages, maintenance states, locations and generation types can be considered, such as is discussed with reference to  FIGS. 2 and 3 . Other non-real-time factors can be considered, such as engine model and power plant type, etc. Additionally or alternatively, each controller  19  for power plants  12 A,  12 B and  12 C can determine an appropriate power-plant-specific action to meet that imbalance response that is most economically efficient for grid  14 . That is, different imbalance response instructions can be executed by each of power plants  12 A,  12 B and  12 C based on power-plant-specific traits determined by each of controllers  19  for each respective power plant. 
     The remainder of  FIG. 7  is discussed with reference to a droop response to a sudden short term output drop by a power plant, but the imbalance responses discussed can apply to load rebalancing instructions for long term readjustment of grid  14 . 
     At step  124 A, controller  19  can execute a total grid demand percentage droop response. At step  124 B, controller  19  can execute a maintenance condition droop response. At step  124 C, controller  19  can execute a location-based droop response. At step  124 D, controller  19  can execute a power generation type droop response. For any droop response, at step  126 , controller  19  can provide a response to the load imbalance indicated by the shift of the instantaneous grid frequency away from the control frequency. In other examples, controller  19  can provide a response to a load imbalance resulting from a controlled load up or load down situation for longer term adjustments of total power production for grid  14 . 
     In a total grid demand percentage droop response at step  124 A, controller  19  can operate whichever of power plants  12 A,  12 B and  12 C that is most effective at preventing oscillations in addressing the load imbalance. For example, if power plant  12 A is configured to contribute a greater percentage of the total grid demand to grid  14 , power plant  12 A can be operated with an increased droop response percentage to provide a smaller than typical droop response in order to reduce overshoot. 
     In a maintenance condition droop response at step  124 B, controller  19  can operate power plants  12 A,  12 B and  12 C to more effectively utilize available electrical generator resources. For example, controller  19  can provide a different droop response depending on the number of electrical generators that are online, e.g., not down for maintenance. If all electrical generators are online, then the droop response may be provided consistent with the power-plant-specific traits described herein, such as a maintenance condition droop response at step  124 B, a location-based droop response at step  124 C or a power generation type droop response at step  124 D. If one or more electrical generators are offline, then the droop response percentage may be maintained at the typical level, rather than being increased to reduce overshoot as described above. 
     In a location-based droop response at step  124 C, controller  19  can operate whichever of power plants  12 A,  12 B and  12 C that is most effective at transmitting power to end users  30 . For example, power plants that are closer to end users  30  can reduce their droop response percentage compared to the typical level in order to provide more power to address the load imbalance. 
     In a power generation type droop response at step  124 D, controller  19  can operate whichever of power plants  12 A,  12 B and  12 C that is most effective at responding to the load imbalance in a timely manner. For example, power plants that utilize gas turbines as prime movers are very responsive, e.g., quick to increase electrical output, can reduce their droop response percentage compared to the typical level in order to provide more power to address the load imbalance. 
     At step  126 , an incongruent turbine droop response can be implemented. For example, instructions from controller  19  can be issued to engine controller  24 A in response to the actual, instantaneous frequency of grid  14  deviating from the control frequency. Likewise, controllers  19  of power plants  12 B and  12 C can operate to provide droop responses that are either consistent with the typical droop response level determined by the home office, or can implement their own power-plant-specific droop response as described herein. 
     In any event, grid  14  will receive a total droop response that may be above, at, or below the typical drop response, but which will provide grid  14  with a more effective allocation of resources that can prevent overshoot (overcompensation), reduce oscillations, and prevent grid  14  from implementing other load imbalance responses, such as load shedding or rolling blackouts which undesirably cause some or all of end users  30  to lose power. 
     The systems and methods discussed in the present application can be useful in increasing operational benefit of electrical power producers, either at the grid level or the power plant level. Utilizing the power-plant-specific traits and droop responses described herein, operational benefits can be achieved that include providing more responsive droop responses that more quickly provide additional power to the grid to address load imbalances and prevent outages, or providing less robust droop responses that provide adequate power to the grid to address load imbalances without causing overshoot. The power-plant-specific droop responses can be implemented in short term and long term imbalance situations. Short term load imbalance situations can include “droop responses” that occur as a result of sudden changes in power demand from the grid at a steady state operating condition, and long term load imbalance situations can include “load changes” that occur as a result of a planned transition period from a first steady state operating condition to a second different steady state operating condition. 
     VARIOUS NOTES &amp; EXAMPLES 
     Example 1 can include or use subject matter such as a method of controlling an imbalance response in a power grid comprising a first power plant and a second power plant, the method can comprise: monitoring operation of the first power plant while operating at a first level to provide a first power output, monitoring operation of the second power plant while operating at a second level to provide a second power output, monitoring load demand from the power grid operating at a steady state condition, detecting a load imbalance on the power grid that causes a deviation from the steady state condition, and issuing incongruent load imbalance instructions to the first power plant and the second power plant to provide a load imbalance response to change the first power output and the second power output to reduce the deviation from the steady state condition depending on a power-plant-specific trait of each of the first power plant and the second power plant. 
     Example 2 can include, or can optionally be combined with the subject matter of Example 1, to optionally include a steady state condition that can comprise a control frequency, the first power plant and the second power plant being configured to operate at the control frequency in the steady state condition, and the load imbalance that can comprise a deviation from the control frequency. 
     Example 3 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 or 2 to optionally include the load imbalance response comprising adjusting at least one of the first power output and the second power output to reduce the deviation. 
     Example 4 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 3 to optionally include the load imbalance response comprising changing a droop response of at least one of the first power plant and the second power plant to accommodate the load imbalance. 
     Example 5 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 4 to optionally include the power-plant-specific trait comprising a total grid demand percentage of each power plant, and the first power plant has a first grid demand percentage and the second power plant has a second grid demand percentage. 
     Example 6 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 5 to optionally include the load imbalance response comprising: increasing a droop response percentage for a power plant having a larger of the first grid demand percentage and the second grid demand percentage in a power outage condition. 
     Example 7 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 6 to optionally include the load imbalance response comprising: maintaining a droop response percentage for a power plant having a smaller of the first grid demand percentage and the second grid demand percentage in the power outage condition. 
     Example 8 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 7 to optionally include the load imbalance response comprising: increasing a droop response percentage for a power plant having a smaller of the first grid demand percentage and the second grid demand percentage in a power outage condition. 
     Example 9 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 8 to optionally include the power-plant-specific trait comprising a location of each power plant, and the first power plant having a first location located a first distance from power consumers of the power grid and the second power plant having a second location located a second distance from the power consumers of the power grid. 
     Example 10 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 9 to optionally include the load imbalance response comprising increasing a droop response percentage of a power plant having a larger of the first distance and the second distance in a power outage condition. 
     Example 11 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 10 to optionally include the power-plant-specific trait comprising a capacity type of each power plant, and the first power plant having a first capacity type with a first responsiveness and the second power plant having a second capacity type with a second responsiveness. 
     Example 12 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 11 to optionally include the load imbalance response comprising decreasing a droop response percentage for a power plant having a larger of the first responsiveness and the second responsiveness in a power outage condition. 
     Example 13 can include, or can optionally be combined with the subject matter of one or any combination of Examples 1 through 12 to optionally include the power-plant-specific trait comprising a repair state of each power plant, the load imbalance response comprising: comparing the repair state of the first power plant and the repair state of the second power plant, and reducing a droop response percentage for the power plant having the repair state that indicates a reduced capacity to respond to the imbalance. 
     Example 14 can include or use subject matter such as a method of controlling operation of a first power plant in response to changing power grid conditions can comprise: receiving a power-plant-specific power assignment from an operator of a grid system, monitoring an operating frequency of the power grid relative to a control frequency, operating at least one power generator of the first power plant at the control frequency to provide a local power output to meet the power-plant-specific power assignment under steady state conditions, detecting a load imbalance from the power grid wherein the operating frequency and the control frequency are different, and operating the at least one power generator to provide a power-plant-specific imbalance response wherein the local power output is adjusted based on a power-plant-specific-trait of the first power plant relative to a second power plant working with the operator. 
     Example 15 can include, or can optionally be combined with the subject matter of Example 14, to optionally include the power-plant-specific trait comprising a percentage of a total power demand of the grid system contributed by the local power output. 
     Example 16 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 or 15 to optionally include the power-plant-specific imbalance response comprising increasing a droop response percentage of the first power plant when a percentage of the total power demand of the first power plant is greater than a percentage of the total power demand of the second power plant. 
     Example 17 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 16 to optionally include the power-plant-specific trait comprising a distance of the first power plant from power consumers of the grid system. 
     Example 18 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 17 to optionally include the plant-specific imbalance response comprising increasing a droop response percentage of the first power plant when a distance of the first power plant from the power consumers is greater than a distance of the second power plant from the power consumers. 
     Example 19 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 18 to optionally include the power-plant-specific trait comprising a responsiveness of the at least one power generator of the first power plant. 
     Example 20 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 19 to optionally include the power-plant-specific imbalance response comprising decreasing a droop response percentage of the first power plant when a responsiveness of the at least one power generator of the first power plant is greater than a responsiveness of a generator of the second power plant. 
     Example 21 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 20 to optionally include the power-plant-specific trait comprising a repair state of the first power plant. 
     Example 22 can include, or can optionally be combined with the subject matter of one or any combination of Examples 14 through 21 to optionally include the power-plant-specific imbalance response comprising reducing a droop response percentage of the first power plant when the first power plant has less capacity down for repair than the second power plant. 
     Example 23 can include or use subject matter such as a control system for operating a power plant can comprise: a power plant controller for controlling at least one power generator at a facility, the power plant controller can comprise: a power generator interface for providing control input signals to the at least one power generator to control output of at least one electrical generator, a grid interface for receiving a control frequency at which a power grid is to be operated, and a current operating frequency of the power grid, and memory having stored therein power-plant-specific data for the power plant relative to other power plants of the power grid, wherein the power plant controller is configured to adjust a droop response of the power plant incongruently relative to the other power plants based on the power-plant-specific data in response to the current operating frequency deviating from the control frequency. 
     Example 24 can include, or can optionally be combined with the subject matter of Example 1, to optionally include the power-plant-specific data being provided by a power system controller for the power grid. 
     Example 25 can include, or can optionally be combined with the subject matter of one or any combination of Examples 23 or 24 to optionally include the power-plant-specific data being generated by the power plant controller. 
     Example 26 can include, or can optionally be combined with the subject matter of one or any combination of Examples 23 through 25 to optionally include the power-plant-specific data comprising location data of the power plant relative to power consumers of the power grid. 
     Example 27 can include, or can optionally be combined with the subject matter of one or any combination of Examples 23 through 26 to optionally include the power-plant-specific data comprising a percentage of total grid power demand supplied by the power plant. 
     Example 28 can include, or can optionally be combined with the subject matter of one or any combination of Examples 23 through 27 to optionally include the power-plant-specific data comprising a responsiveness of the at least one electrical generator of the power plant. 
     Each of these non-limiting examples can stand on its own, or can be combined in various permutations or combinations with one or more of the other examples. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, in an example, the code can be tangibly stored on one or more volatile, non-transitory, or non-volatile tangible computer-readable media, such as during execution or at other times. Examples of these tangible computer-readable media can include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAMs), read only memories (ROMs), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.