Abstract:
A method and apparatus for autonomously operating a microgrid power generator. In one embodiment, the method comprises obtaining a first measurement of at least one grid parameter of a microgrid transmission line coupled to a power generator in a microgrid; comparing the first measurement to a turn-on threshold; initiating, when the first measurement is less than the turn-on threshold, power generation by the power generator; obtaining, after initiation of the energy generation, a second measurement of the at least one grid parameter of the microgrid transmission line; comparing the second measurement to a shut-down threshold that is greater than the turn-on threshold; and stopping, when the second measurement exceeds the shut-down threshold, the power generation by the power generator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 62/275,380, entitled “Coordination of Generators in Droop Controlled Microgrids Using Hysteresis” and filed on Jan. 6, 2016, which is herein incorporated in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    Embodiments of the present disclosure relate generally to droop-operated microgrids and, more particularly, to control of generators in a droop-operated microgrid. 
         [0004]    Description of the Related Art 
         [0005]    A conventional microgrid generally comprises at least one energy generator, at least one energy storage device, and at least one energy load. When disconnected from a conventional utility grid, a microgrid can generate power as an intentional island without imposing safety risks on any line workers that may be working on the utility grid. 
         [0006]    Droop control is one technique that may be used for operating energy storage and generation resources in a microgrid that is disconnected from the utility grid. When using droop control, the droop settings of each microgrid resource may be offset from one another in order to coordinate and optimize the use of the different resources. For example, for a microgrid comprising a conventional generator as well as a distributed energy resource (DER) generator and an energy storage device, the generator could be set with a lower frequency set point than the storage device so that it doesn&#39;t turn on unless the DER generator and energy from the storage device are both being fully used. However, such operation typically leads to instability as generators typically have a minimum power they need to run at and the jump in frequency once the generator turns on would lead to the generator being shut off, thereby causing a frequency drop that results in the generator being turned on again and a continuing oscillation. 
         [0007]    Therefore, there is a need in the art for a technique for efficiently coordinating generator operation in a droop-controlled microgrid. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the present invention generally relate to coordinating generator operation in a droop-controlled microgrid as shown in and/or described in connection with at least one of the figures. 
         [0009]    These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  is a block diagram of a power system in accordance with one or more embodiments of the present invention; 
           [0012]      FIG. 2  is a frequency-watt graph comprising a plurality of droop curves in accordance with one or more embodiments of the present invention; 
           [0013]      FIG. 3  is a block diagram of a power conditioner controller in accordance with one or more embodiments of the present invention; 
           [0014]      FIG. 4  is a block diagram of a DER controller in accordance with one or more embodiments of the present invention; 
           [0015]      FIG. 5  is a block diagram of a component controller in accordance with one or more embodiments of the present invention; and 
           [0016]      FIG. 6  is a flow diagram of a method for autonomous control of a microgrid generator in accordance with one or more embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  is a block diagram of a power system  100  in accordance with one or more embodiments of the present invention. This diagram only portrays one variation of the myriad of possible system configurations. The present invention can function in a variety of environments and systems. 
         [0018]    The power system  100  comprises a utility  102  (such as a conventional commercial utility) and a plurality of microgrids  150 - 1 ,  150 - 2 , . . . ,  150 -X (collectively referred to as microgrids  150 ) coupled to the utility  102  via a utility grid  104 . Through their connections to the utility grid  104 , each microgrid  150  as a whole may receive energy from the utility grid  104  or may place energy onto the utility grid  104 . In some communities, coupling energy to a commercial utility grid is strictly controlled by regulation and it is beneficial that the microgrids  150  maintain or strive to maintain a zero energy output policy. Each microgrid  150  is capable of operating without energy supplied from the utility  102  and may cover a neighborhood, a village, a small city, or the like, as the term “microgrid” is not intended to imply a particular system size. 
         [0019]    Although the microgrid  150 - 1  is depicted in detail in  FIG. 1  and described herein, the microgrids  150 - 2  through  150 -X are analogous to the microgrid  150 - 1 . However, the number and/or type of various microgrid components may vary among the microgrids  150 . 
         [0020]    The microgrid  150 - 1  comprises a plurality of microgrid members  152 - 1 ,  152 - 2 , . . . ,  152 -M (collectively referred to as microgrid members  152 ) each coupled to a local grid  132  which in turn is coupled to the utility grid  104  via an island interconnect device (IID)  134 . The local grid  132  may be a trunk of the utility grid  104  or it may be a specifically designed local grid for the microgrid  150 - 1 . 
         [0021]    The IID  134  determines when to disconnect/connect the microgrid  150 - 1  from/to the utility grid  104  and performs the disconnection/connection. Generally, the IID  134  comprises a disconnect component (e.g., a disconnect relay) along with a CPU and an islanding module and monitors the utility grid  104  for failures or disturbances, determines when to disconnect from/connect to the utility grid  104 , and drives the disconnect component accordingly. For example, the IID  134  may detect a fluctuation, disturbance or outage with respect to the utility grid  104  and, as a result, disconnect the microgrid  150 - 1  from the utility grid  104 . The IID  134  may also disconnect the microgrid  150 - 1  from the utility grid  104  when the microgrid  150 - 1  is either overproducing energy or overloading the utility grid  104 . Once disconnected from the utility grid  104 , the microgrid  150 - 1  can continue to generate power as an intentional island without imposing safety risks on any line workers that may be working on the utility grid  104 . In some embodiments, the IID  134  may receive instructions from another component or system for disconnecting from/connecting to the utility grid  104 . 
         [0022]    The microgrid member  152 - 1  comprises a building  116  (e.g., a residence, commercial building, or the like) coupled to a load center  126  which may be within or outside of the building  116 . The load center  126  is coupled to the local grid  132  via a utility meter  120  and a local IID  122 , and is further coupled to a distributed energy resource (DER)  106 , a generator  130 , and one or more loads  118  for coupling power among these components. Although the microgrid member  152 - 1  is depicted in detail in  FIG. 1  and described herein, the microgrid members  152 - 2  through  152 -M are analogous to the microgrid member  152 - 1 . However, the number and/or types of various microgrid member components may vary among the microgrid members  152 . 
         [0023]    The local IID  122  determines when to disconnect/connect the microgrid member  152 - 1  from/to the local grid  132  and performs the disconnection/connection. For example, the local IID  122  may detect a grid fluctuation, disturbance or outage and, as a result, disconnect the microgrid member  152 - 1  from the local grid  132 . The IID  122  may also disconnect the microgrid member  152 - 1  from the local grid  132  when the microgrid member  152 - 1  is either overproducing energy or overloading the local grid  132 . Once disconnected from the local grid  132 , the microgrid member  152 - 1  can continue to generate power as an intentional island without imposing safety risks on any line workers that may be working on the local grid  132 . The local IID  122  comprises a disconnect component (e.g., a disconnect relay) for physically disconnecting from/connecting to the local grid  132 . The local IID  122  may additionally comprise a CPU and an islanding module for monitoring grid health, detecting grid failures and disturbances, determining when to disconnect from/connect to the local grid  132 , and driving the disconnect component accordingly. In some embodiments, the local IID  122  may receive instructions from another component or system for disconnecting from/connecting to the local grid  132 . 
         [0024]    The meter  120  measures the ingress and egress of energy for the microgrid member  152 - 1 ; in some embodiments, the meter  120  comprises the IID  122  or a portion thereof. The meter  120  generally measures real power flow (kWh), reactive power flow (kVAR), grid frequency, and grid voltage (referred to herein as the measured parameters). In certain embodiments these measured parameters may be communicated to a microgrid monitoring system (not shown) that monitors each of the microgrid members  152 . 
         [0025]    The generator  130  is a fuel-based power generator, such as a diesel generator, that automatically increases or curtails energy output depending on the needs of the microgrid member  152 - 1 . The generator  130  comprises one or more components for measuring grid parameters, such as grid frequency and/or grid voltage, and a component controller  128  described in detail further below with respect to  FIG. 5 ; in some embodiments, one or more of the components for measuring the grid parameters may be part of the controller  128 . The component controller  128  may optimize the operation of the generator  130  with respect to the microgrid member  152 - 1  and/or the microgrid  150 - 1  (e.g., by generating control instructions for the generator  130 ); implement control instructions for operating the generator  130  (e.g., instructions received from another component or system); obtain data pertaining to the generator  130  (e.g., performance data, operational data, or the like) which may further be communicated to another component or system; or perform similar functions. 
         [0026]    The loads  118  consume energy obtained via the load center  126  and may be located inside of the building  116  or outside of the building  116 . Some of the loads  118  may be “smart loads” that comprise the component controller  128  for optimizing the utilization of energy (e.g., disconnecting/connecting the smart load  118  when the grid is overloaded/underloaded, modulating operation of smart loads  118 , such as HVAC, pumps, and the like, as needed); implementing control instructions for the load  118  (e.g., instructions received from another component or system); obtaining data pertaining to the loads  118  (e.g., performance data, operational data, and the like) which may further be communicated to another component or system; or performing similar functions. 
         [0027]    One or more of the loads  118  may be an energy storage component that stores energy received via the load center  126 , such as a hot water heater, an electric car, or the like. Such energy storage loads  118  may further deliver stored energy to other loads  118  and/or the local grid  132  as needed, where the energy storage and delivery is controlled by the corresponding component controller  128 . 
         [0028]    The DER  106  comprises power conditioners  110 - 1  . . .  110 -N,  110 -N+1 coupled in parallel to a bus  124  that is further coupled to the load center  126 . Generally the power conditioners  110  are bi-directional power conditioners and those power conditioners  110  in a first subset of power conditioners  110  are coupled to DC energy sources  112  (for example, renewable energy sources such as wind, solar, hydro, and the like) while the power conditioners  110  in a second subset of power conditioners  110  are coupled to energy storage devices  114  (e.g., batteries, flywheels, compressed air storage, hot water heaters, electric cars, or the like). The combination of a DC energy source  112  and a corresponding power conditioner  110  may be referred to herein as a DER generator. In embodiments where the power conditioners  110  are DC-AC inverters, a power conditioner  110  and a corresponding energy storage device  114  may together be referred to herein as an AC battery  180 . 
         [0029]    In the embodiment depicted in  FIG. 1 , the power conditioners  110 - 1  . . .  110 -N are respectively coupled to DC energy sources  112 - 1  . . .  112 -N (e.g., renewable energy sources such as wind, solar, hydro, and the like) for receiving DC power and generating commercial power grid compliant AC power that is coupled to the bus  124 . As further depicted in  FIG. 1 , the power conditioner  110 -N+1 is coupled to an energy storage device  114  to form an AC battery  180 . The power conditioner  110  of the AC battery  180  can convert AC power from the bus  124  to energy that is stored in the energy storage device  114 , and can further convert energy from the energy storage device  114  to commercial power grid compliant AC power that is coupled to the bus  124 . Although only a single AC battery  180  is depicted in  FIG. 1 , other embodiments may comprise fewer or more AC batteries  180 . 
         [0030]    In one or more embodiments, each DC source  112  is a photovoltaic (PV) module. In some alternative embodiments, multiple DC sources  112  are coupled to a single power conditioner  110  (e.g., a single, centralized power conditioner). In certain embodiments, the power conditioners  110  are DC-DC converters that generate DC power and couple the generated power to a DC bus (i.e., the bus  124  is a DC bus in such embodiments); in such embodiments, the power conditioner  110 -N+1 also receives power from the DC bus and converts the received power to energy that is then stored in the energy storage device  114 . 
         [0031]    The DER  106  comprises a DER controller  108  that is coupled to the bus  124  and communicates with the power conditioners  110  (e.g., via power line communications (PLC) and/or other types of wired and/or wireless techniques). The DER controller  108  may send command and control signals to one or more of the power conditioners  110  and/or receive data (e.g., status information, data related to power conversion, and the like) from one or more of the power conditioners  110 . In some embodiments, the DER controller  108  is further coupled, by wireless and/or wired techniques, to a master controller or gateway (not shown) via a communication network (e.g., the Internet) for communicating data to/receiving data from the master controller (e.g., system performance information and the like). 
         [0032]    In certain embodiments, the DER controller  108  comprises the local IID  122  or a portion of the local IID  122 . For example, the DER controller  108  may comprise an islanding module for monitoring grid health, detecting grid failures and disturbances, determining when to disconnect from/connect to the local grid  132 , and driving a disconnect component accordingly, where the disconnect component may be part of the DER controller  108  or, alternatively, separate from the DER controller  108 . In some embodiments, the DER controller  108  may coordinate with the local IID  122 , e.g., using power line communications. 
         [0033]    Each of the power conditioners  110  is a droop-controlled power conditioner such that when the microgrid member  152 - 1  is disconnected from the local grid  132  and/or the utility grid  104  (e.g., using the IID  122  and/or the IID  134 ), the power conditioners  110  employ a droop control technique for parallel operation without the need for any common control circuitry or communication between the power conditioners  110 . Each of the power conditioners  110  comprises a power conditioner controller  140  (described in detail further below with respect to  FIG. 3 ) having a droop control module for implementing the droop control techniques, thereby allowing the power conditioners  110  to share the load in a safe and stable manner. 
         [0034]    Although the microgrid member  152 - 1  is depicted as having a single DER  106  in  FIG. 1 , in other embodiments the microgrid member  152 - 1  may have additional DERs. In one or more alternative embodiments, the DER  106  is absent from the microgrid member  152 - 1  and the microgrid member energy storage loads  118  and generator  130  employ the droop control techniques described herein. 
         [0035]    When the microgrid member  152 - 1 , or the entire microgrid  150 - 1 , is disconnected from the local grid  132  and/or the utility grid  104 , the microgrid member storage components (i.e., the AC battery  180 , energy storage loads  118 ) and generation components (i.e., the DER generators, the generator  130 ) employ a droop technique for operation without the need for any common control circuitry or communication between the power components, where the droop settings for the components are offset from one another to coordinate and optimize the use of the different components. The microgrid member storage and generation components may employ a standard or classic volt-VAR-frequency-watt droop technique, where real power is a function of frequency only and reactive power is a function of voltage only, or any other type of droop technique which cross-couples watts, VARs, volts, and frequency using rotational transformation. 
         [0036]    In accordance with one or more embodiments of the present invention, the droop curve for the generator  130  is a hysteretic droop curve having a shut-down frequency that is higher than the turn-on frequency, where the difference between the shut-down frequency and the turn-on frequency is greater than the expected frequency jump that occurs when the generator  130  is turned on (i.e., the frequency jump that results from the minimum operating power of the generator  130 ). The hysteretic droop curve allows the generator  130  to autonomously enable and disable energy generation based on one or more measured grid parameters, such as frequency, as described herein. One embodiment of the generator&#39;s hysteretic droop curve is described below with respect to  FIG. 2 . 
         [0037]      FIG. 2  is a frequency-watt graph  200  comprising a plurality of droop curves in accordance with one or more embodiments of the present invention. The frequency-watt graph  200  is based on a standard or classic volt-VAR-frequency-watt droop technique, where real power is a function of frequency only and reactive power is a function of voltage only. In such a droop technique, for frequency-watt control a simple Cartesian coordinate plane can be used to show the relationship between real power and frequency as depicted by the frequency-watt graph  200 . Although the frequency-watt graph  200  pertains to those embodiments where a classic volt-VAR-frequency-watt droop technique is employed, in one or more other embodiments any other type of droop control which cross-couples watts, VARs, volts, and frequency using rotational transformation may be employed; for example, in other embodiments voltage and frequency may vary based on a complex relationship. 
         [0038]    The frequency-watt graph  200  comprises a horizontal axis  202  and a vertical axis  204 . Moving upward along the vertical axis  204  represents an increase in frequency. Moving to the left of the vertical axis  204  represents no generation and a decrease in loading, while moving to the right of the vertical axis  204  represents an increase in generation (and also an increase in loading). 
         [0039]    The frequency-watt graph  200  further comprises a plurality of droop curves pertaining to the microgrid member resources described above. A storage droop curve  206  pertains to those microgrid member components that store energy (the AC battery  180  and one or more of the smart loads  118  that store energy and can deliver it back); a DER generator droop curve  208  corresponds to the DER generators (i.e., the DC sources  112 /power conditioners  110 ); and a generator droop curve  210  corresponds to the generator  130 . Although a single storage droop curve  206  is depicted in  FIG. 2 , in some embodiments different storage droop curves  206  may be employed for one or more of the microgrid member components that store energy. For example, different droop curves may be used for different battery chemistries depending on their round-trip efficiency and relative costs. Generally, when there are multiple storage droop curves  206  they fall between the DER generator droop curve  208  and the generator droop curve  210 . 
         [0040]    For the storage droop curve  206 , the balance between energy storage and loading occurs along the vertical axis  204  at the nominal grid frequency, for example 60 Hz, as depicted in  FIG. 2 . Moving left of the vertical axis  204 , as less and less load and thus less and less power is being drawn, charging of the storage components increases; as the loading of the local grid  132  is decreased, its increases. Moving right of the vertical axis  204 , as loading increases, energy is drawn from the energy storage components. Additionally, the DER generators begin generation and increase their generation to a maximum point. 
         [0041]    As shown by the droop curves  206 ,  208 , and  210 , the droop settings for the different types of microgrid member components are offset from one another to coordinate and optimize the use of the different resources. In particular, the DER generators are set with a higher frequency target than the storage components to prevent the DER generation from being curtailed unless the storage component charging has been maximized, thereby preventing available renewable resource energy from being wasted while there is capacity to store it. Similarly, the generator  130  is set with a lower frequency set point than the storage components to prevent the generator  130  from turning on until both the DER generators and the storage components are being fully utilized. 
         [0042]    As depicted in  FIG. 2 , the generator droop curve  210  is a hysteretic droop curve having a higher shut-down frequency than its turn-on frequency. The difference between the shut-down frequency and the turn-on frequency is greater than the expected generator turn-on frequency jump resulting from the generator&#39;s minimum operating power in order to prevent oscillatory behavior in the generator  130 . For example, one or more loads may be turned on (e.g., a hairdryer, a vacuum, and the like) such that sufficient power cannot be supplied by the DER generators and the storage resources. The generator  130  thus turns on and the power line experiences a frequency jump, where the magnitude of the frequency jump is based, in part, on the generator&#39;s minimum operating power. As a result of the hysteresis band  212  for the generator droop curve, the increased frequency remains within the hysteresis band  212  and the generator  130  remains on (e.g., on its minimum load). As more load begins being drawn, for example one or more additional appliances are turned on, the generator throttling begins and occurs over the throttling range of the generator. 
         [0043]    The magnitude of the expected frequency jump can be calculated based on the minimum power of the generator and the aggregate droop response of the system, which can be measured or inferred based on knowing the droop gains of all the participating DERs. For example, for a generator with a minimum power of 1 kW on a system having two other DERs, one with a frequency-watt droop gain of 0.1 Hz/kW and another with a gain of 0.5 Hz/kW, the aggregate droop gain is 1/(1/0.1+1/0.5)=0.083 Hz/kW. In such a system, a jump of 0.083 Hz in frequency is expected when the generator turns on, and the hysteresis may be set to 0.2 Hz to allow sufficient margin. In general, as the system gets larger and larger the required hysteresis would become smaller and smaller since the generator turning on would have less effect on the overall system. 
         [0044]      FIG. 3  is a block diagram of a power conditioner controller  140  in accordance with one or more embodiments of the present invention. The power conditioner controller  140  comprises a transceiver  314 , support circuits  304  and a memory  306 , each coupled to a central processing unit (CPU)  302 . The CPU  302  may comprise one or more conventionally available microprocessors or microcontrollers; alternatively, the CPU  302  may include one or more application specific integrated circuits (ASICs). The power conditioner controller  140  may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. In one or more embodiments, the CPU  302  may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein. 
         [0045]    The transceiver  314  may be coupled to the power conditioner&#39;s output lines for communicating with the DER controller  108  and/or other power conditioners  110  using power line communications (PLC). Additionally or alternatively, the transceiver  214  may communicate with the DER controller  108  and/or other power conditioners  110  using other type of wired communication techniques and/or wireless techniques. 
         [0046]    The support circuits  304  are well known circuits used to promote functionality of the CPU  302 . Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. 
         [0047]    The memory  306  may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory  306  is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory  306  generally stores the operating system (OS)  308 , if necessary, of the power conditioner controller  140  that can be supported by the CPU capabilities. In some embodiments, the OS  308  may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like. 
         [0048]    The memory  306  stores various forms of application software, such as a power conditioner control module  310  for controlling, when executed, power conversion by the power conditioner  110 , and a droop control module  312  for employing, when executed, droop control techniques as described herein. The memory  306  additionally stores a database  314  for storing data related to the operation of the power conditioner  110  and/or the present invention, such as one or more droop curves described herein. 
         [0049]      FIG. 4  is a block diagram of a DER controller  108  in accordance with one or more embodiments of the present invention. The DER controller  108  comprises a transceiver  414 , support circuits  404  and a memory  406 , each coupled to a central processing unit (CPU)  402 . The CPU  402  may comprise one or more conventionally available microprocessors or microcontrollers; alternatively, the CPU  402  may include one or more application specific integrated circuits (ASICs). The DER controller  108  may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. In one or more embodiments, the CPU  402  may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein. 
         [0050]    The DER controller  108  generally communicates, via the transceiver  414 , with the power conditioners  110  using power line communications (PLC), although additionally or alternatively the transceiver  414  may communicate with the power conditioners  110  using other types of wired and/or wireless communication techniques. In some embodiments, the DER controller  108  may further communicate via the transceiver  414  with other controllers within the microgrid and/or with a master controller (not shown). 
         [0051]    The support circuits  404  are well known circuits used to promote functionality of the CPU  402 . Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. 
         [0052]    The memory  406  may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory  406  is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory  406  generally stores the operating system (OS)  408 , if necessary, of the power conditioner controller  140  that can be supported by the CPU capabilities. In some embodiments, the OS  408  may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like. 
         [0053]    The memory  406  stores various forms of application software, such as a DER control module  410  for controlling operations pertaining to the DER  106  (e.g., collecting performance data for the power conditioners  110 , generating control instructions for the power conditioners  110 , and the like). The memory  406  additionally stores a database  412  for storing data related to the operation of the DER  106 . 
         [0054]      FIG. 5  is a block diagram of a component controller  128  in accordance with one or more embodiments of the present invention. The component controller  128  comprises support circuits  504  and a memory  506 , each coupled to a central processing unit (CPU)  502 . The CPU  502  may comprise one or more conventionally available microprocessors or microcontrollers; alternatively, the CPU  502  may include one or more application specific integrated circuits (ASICs). The component controller  128  may be implemented using a general purpose computer that, when executing particular software, becomes a specific purpose computer for performing various embodiments of the present invention. In one or more embodiments, the CPU  502  may be a microcontroller comprising internal memory for storing controller firmware that, when executed, provides the controller functionality described herein. 
         [0055]    The support circuits  504  are well known circuits used to promote functionality of the CPU  502 . Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, input/output (I/O) circuits, and the like. 
         [0056]    The memory  506  may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory  506  is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory  506  generally stores the operating system (OS)  508 , if necessary, of the component controller  128  that can be supported by the CPU capabilities. In some embodiments, the OS  508  may be one of a number of commercially available operating systems such as, but not limited to, LINUX, Real-Time Operating System (RTOS), and the like. 
         [0057]    The memory  506  stores various forms of application software, such as a component control module  510  for controlling, when executed, one or more functions of the corresponding component, and a droop control module  512  for employing, when executed, droop control techniques as described herein. In one or more embodiments, the droop control module  512  may be the same as the droop control module  312 . In certain other embodiments, the component controller  128  may be used in place of the power conditioner controller  140 . 
         [0058]    The memory  506  additionally stores a database  512 , for example for storing data related to the operation of the corresponding component, such as one or more of the droop curves described herein. 
         [0059]    When a microgrid member  152  is disconnected from the local grid  132  and/or the utility grid  104 , the power conditioner controllers  140  and the component controllers  128  facilitate automatic control of the corresponding components. For example, the power conditioner control module  310  and the droop control module  312 , when executed, facilitate automatic control of the corresponding power conditioner  110 ; e.g., the power conditioner control module  310  may monitor the power line frequency and/or voltage at the corresponding power conditioner  110  to ensure that the frequency and/or voltage stay within designated parameters as driven by the droop control module  312 . In one or more embodiments, the droop control module  512  may be the same as the droop control module  312 . 
         [0060]    By using such localized droop control, each component can autonomously optimize its operation with respect to the microgrid member  152 /overall microgrid  150 . For example, for the generator  130 , the component controller  128  may optimize the generation of power; for smart loads  118 , the component controller  128  may optimize the consumption of energy (e.g., by controlling the energy consumed by individual loads either through throttling the flow or turning on and turning off various loads at certain times); and for smart loads  118  that are energy storage devices, the component controller  128  may optimize the energy flow into and out of the storage devices. In some embodiments, the droop control module  512  may be the same as the droop control module  312  previously described. 
         [0061]      FIG. 6  is a flow diagram of a method  600  for autonomous control of a microgrid generator in accordance with one or more embodiments of the present invention. In some embodiments, the method  600  is an implementation of the droop control module  512  previously described. In some embodiments, a computer readable medium comprises a program that, when executed by a processor, performs at least a portion of the method  600  that is described in detail below. 
         [0062]    In some embodiments, such as the embodiment described below, the microgrid generator is a conventional generator that is part of a microgrid, such as the generator  130  of the microgrid member  152 - 1 /microgrid  150 - 1  previously described, that is operating in an islanded mode. The components of the microgrid are electrically interconnected by an AC transmission grid, which may be referred to as a local grid, a microgrid grid, or a microgrid transmission line. 
         [0063]    The method  600  begins at step  602  and proceeds to step  604 . At step  604 , the frequency of the microgrid transmission line measured at (or proximate) the generator is obtained. Generally, the generator comprises one or more components for periodically measuring the grid frequency, such as a phase lock loop (PLL), although in some alternative embodiments the frequency may be periodically measured by a component external to the generator. In some other embodiments, one or more other parameters may be measured as part of providing autonomous control of the generator, such as the grid voltage at or proximate the generator. Although the hysteretic method described herein may be applied to any form of droop, whether frequency-watt, voltage-var, or cross-coupled droop (where frequency and voltage both have an effect on watts and var), applying the technique to voltage-var forms of droop may be more complex as the minimum VAR of a generator isn&#39;t always clearly known. 
         [0064]    The method  600  proceeds to step  606 , where a determination is made whether the grid frequency is below a generator turn-on threshold. In some embodiments, the turn-on threshold may be set at a lower value than that of other components within the microgrid, such as storage assets, DER generators, and the like, such that the generator doesn&#39;t turn on until both DER generators and storage assets are both being fully used. 
         [0065]    If the result of the determination at step  606  is no, that the grid frequency is not less than the turn-on threshold, the method  600  returns to step  604 . If the result of the determination at step  606  is yes, that the grid frequency is less than the turn-on threshold, the method  600  proceeds to step  608 . 
         [0066]    At step  608 , the generator turns on to begin power generation. At step  610 , the grid frequency continues being obtained (e.g., measured directly or measured values are obtained). At step  612 , a determination is made whether the grid frequency has exceeded a generator shut-down threshold. The shut-down threshold is such that the difference between the shut-down frequency and the turn-on frequency is greater than the expected frequency jump that occurs when the generator is turned on (i.e., the frequency jump that results from the minimum operating power of the generator). In some embodiments, the turn-on and shut-down thresholds may be set at 59.2 Hz and 60.3 Hz, respectively. If the result of the determination at step  612  that no, that the grid frequency is not greater than the shut-down frequency, the method  600  returns to step  610 . If the result of the determination at step  612  is yes, that the grid frequency is greater than the shut-down frequency, the method  600  proceeds to step  614 . 
         [0067]    At step  614 , power generation by the generator is shut down. The method  600  proceeds step  616 , where a determination is made whether to continue operation. If the result of the determination at step  616  is yes, to continue, the method  600  returns to step  604 . If the result of the determination at step  616  is no, to not continue operation, the method  600  proceeds to step  618  where it ends. 
         [0068]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof and the scope thereof is determined by the claims that follow.