SUPERVISORY CONTROL FOR SCALABLE MICROGRID SYSTEMS

This application is being filed on 22 July 2011, as a PCT International Patent application in the name of Petra Solar, Inc., a U.S. national corporation, applicant for the designation of all countries except the U.S., and Hussam Alatrash, a citizen of Jordan, and Nasser Kutkut, a citizen of the U.S., applicants for the designation of the U.S. only.

BACKGROUND

[001] The vast majority of today's electric power is generated by large- scale, centralized power plants using fossil fuels, hydropower or nuclear power and is transported over long distances to end-users. In these systems, power flows in one direction from the central power stations through the distribution networks to consumers. Yet, the centralized power generation paradigm has many disadvantages including the environmental impact of greenhouse gases and other pollutants, transmission losses and inefficiency, growing security of supply concerns, system sustainability issues, over-consumption, and, the high cost of ongoing upgrades and replacement of transmission and distribution infrastructure.

SUMMARY

[002] Microgrid control may be provided. First, data may be

communicated, by a supervisory control agent over a communications network, with a plurality of energy resources in a microgrid. A transition of the microgrid from a first operation mode to a second operation mode may be controlled in response to the communicated data.

[003] Both the foregoing general description and the following detailed description are examples and explanatory only, and should not be considered to restrict the invention's scope, as described and claimed. Further, features and/or variations may be provided in addition to those set forth herein. For example, embodiments of the invention may be directed to various feature combinations and sub-combinations described in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[004] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various embodiments of the present invention. In the drawings:

[005] FIG. 1 shows a microgrid;

[006] FIG. 2 shows the role of destabilizing behavior in loss-of-mains detection;

[007] FIG. 3 shows an under-frequency event in a high-penetration scenario;

[008] FIG. 4 shows a microgrid in islanded operating mode;

[009] FIG. 5 shows a microgrid in legacy-tie operating mode;

[010] . FIG. 6 shows a microgrid in smart-tie operating mode;

[011] FIG. 7 is a diagram showing a transition from smart-tie to islanded operation;

[012] FIG. 8 is a diagram showing a transition from islanded to smart-tie operation;

[013] FIG. 9 is a diagram showing a transition from Legacy- tie to smart- tie operation;

[014] FIG. 10 is a diagram showing a transition from smart-tie to legacy- tie operation;

[015] FIG. 11 is a diagram showing a transition from islanded to legacy- tie operation;

[016] FIG. 12 is a diagram showing a transition from legacy-tie to islanded operation;

[017] FIG. 13 is a diagram depicting a microgrid in an alternative observer-based legacy-tie mode configuration;

[018] FIG. 14 is a diagram showing loss-of-mains detection using observer-based anti-islanding; [019] FIG. 15 is a diagram showing an example of microgrid hierarchy; and

[020] FIG. 16 shows a supervisory control agent in more detail. DETAILED DESCRIPTION

[021 ] The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar elements. While embodiments of the invention may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description does not limit the invention. Instead, the proper scope of the invention is defined by the appended claims.

[022] Over the past few years, technological innovations, changing economic and regulatory environments, and shifting environmental and social priorities have spurred interest in distributed generation (DG) system. Distributed Generation is a new model for the power system that is based on the integration of small and medium sized generators based on new and renewable energy

technologies, such as solar, wind, and fuel cells, into the utility grid. All these generators are interconnected through a fully interactive intelligent electricity network.

[023] While most DG resources are primarily used to supplement the traditional electric power systems, DG resources can be grouped to supply nearby loads in specific areas with continuous power during disturbances and interruptions of the main utility grid. Such grouping of DG resources with the nearby loads is referred to as a microgrid.

[024] A microgrid is a collection of energy resources that behave in a concerted manner to form a coherent electrical power system (EPS). A microgrid can operate: i) as part of a host EPS; or ii) in isolation to support local loads.

Furthermore, a microgrid may be capable of performing seem-less transitions between these two operating modes. Energy resources within a microgrid may include various combinations of energy sources, energy storage systems, and loads. A microgrid may also have "children" microgrids as energy resources.

[025] FIG. 1 shows a microgrid 105. Microgrid 105 may be formed by interconnecting a number of compatible member energy resources (e.g. generation 110, energy storage 115, a child micro grid 120, and loads 125) under the supervision of a supervisory control agent (SCA) 130 as shown in FIG. 1. SCA 130 may be responsible for communicating with the energy resources within microgrid 105 to coordinate operation and transitioning of microgrid 105 between various modes of operation as discussed below. SCA 130 may communicate with the energy resources, for example, through a low-bandwidth communications network (LBCN) 135. The energy resources may also communicate with each other through LBCN 135. SCA 130 may also control the state of a switch 140 to connect microgrid 105 to an EPS 145 and disconnect microgrid 105 from EPS 145.

[026] The energy resources within microgrid 105 may be controlled in a manner that prevents safety hazards, catastrophic failure, or non-acceptable voltage quality in the event of a missing or intermittent operation of LBCN 135. This control may be achieved by embedding the following functional parameters in the energy resources:

i) a minimum acceptable electrical impedance of each energy resource, dependent on the size and capacity of the resource;

ii) predefined profiles for real and reactive power

injection/consumption based on microgrid voltage, frequency, and phase. Such profiles vary based on type and capacity of resource, and the operating mode of the microgrid.

[027] Furthermore, each energy resource may operate in one or more of the following modes:

i) regulator mode: resource power varies in a manner that promotes stability of EPS voltage and frequency; and

ii) passive/destabilizer mode: resource power varies in a manner that has little or negative effect on the stability of EPS voltage and/or frequency. [028] The devices used in modern EPS systems, such as the national grid, are typically dominated by series inductances. This is true of synchronous generators, transmission lines, and transformers. For such a system, devices in regulator mode employ negative-sloped voltage/reactive power and frequency/real power profiles. Destabilizer devices may employ positive-sloped profiles for all or part of the operating range.

[029] Embodiments of the invention may provide a method, hierarchy, and control architecture for supervisory control of microgrids and their respective energy resources and may build safe, reliable, and scalable microgrids. Furthermore, the proposed hierarchy and control architecture may support the host EPS stability and while waiving interconnection requirements that challenge system stability.

[030] Consistent with embodiments of the invention, methods may be provided for: i) ensuring smooth transition from islanded to smart-tie mode and vice versa; ii) ensuring smooth transition from smart-tie to legacy-tie mode and vice versa; and ii) ensuring smooth transition from islanded to legacy-tie mode and vice versa.

[031 ] Moreover embodiments of the invention may provide methods for implementing legacy-tie operation while satisfying the provision for anti-islanding without requiring microgrid member devices to operate in passive/destabilizer mode. In addition, a microgrid hierarchy may be created by arranging microgrids into a pre-defined chain of command and data reporting. The formation of such hierarchy may allow the distribution of intelligence throughout a large microgrid or a larger EPS.

[032] Embodiments of the present invention may provide a system and method for facilitating supervisory control hierarchy for microgrids and their respective energy resources. Energy resources within a micro grid may include, but are not limited to, generation sources, which maybe comprised of fossil fuel generators, photovoltaic (PV) systems, wind turbines, and fuel cells; energy storage resources, such as battery storage systems, and local loads. Embodiments of the present invention may focus on interaction of one or more microgrids with

Alternating Current (AC) power distribution systems. [033] Microglia's (e.g. microgrid 105) may comprise a form of DG. A microgrid may accordingly be subject to a number of interconnection requirements at a point of common coupling (PCC) (e.g. switch 140) to a host EPS (e.g. EPS 145.) Such requirements may entail:

i) Power quality requirements;

ii) Power factor requirements;

iii) Harmonic content limits;

iv) DC current injection limits;

v) Prevention of non-intentional islanding, also known as anti- islanding, and "loss-of-mains" (LoM) protection;

vi) Provisions for ensuring the microgrid does not continue to energize a neighboring section of the host EPS in the case of a loss of electrical interconnection to the rest of that EPS;

vii) This provision is generally only applicable if net average real power is flowing out of the microgrid at the PCC;

viii) Fault protection requirements;

ix) Disconnection/reconnection based line voltage and/or frequency; and

x) Disconnection upon over-current and over- voltage.

[034] Research has shown that some interconnection requirements pose stability and quality challenges to the host EPS operation in the event of high penetration of DG. The provision for anti-islanding is particularly problematic. This is because conventional methods include two problematic components, namely: i) limits for operating voltage and frequency and destabilizing behavior. Regarding limits for operating voltage and frequency, interconnection standards require DG to automatically cease to export power whenever observed line voltage and/or frequency fall outside preset limits. FIG. 2 shows the role of destabilizing behavior in loss-of-mains detection. Regarding destabilizing behavior, interconnection standards require DG to cease to energize a closely-matched neighboring load in the case of non-intentional islanding. In response to this requirement, conventional DG employ an active control algorithm by which real and/or reactive power output of the DG is modulated in a manner that promotes instability in the host EPS. Destabilizing behavior is intended to push an island outside the voltage and/or frequency limits, leading to a halt of power generation.

[035] As a result of such provisions, DG is unable to ride-through temporary grid faults when it may be especially valuable to it to stay online and contribute to system recovery. A particularly troubling scenario is an under- frequency condition due to scarcity of available energy. Sudden mass-tripping of DG resources in such a scenario will create a forceful transient that will exacerbate the situation. FIG. 3 shows an under- frequency event in a high-penetration scenario.

[036] Moreover, destabilizing behavior is required to be active as long as the DG exports net active power, even during normal interconnection to a healthy EPS. This may result in long-term stability issues for an EPS with large penetration of DG.

[037] Consistent with embodiments of the invention, in order for microgrid 105 to support host EPS 145 stability, an operating mode (e.g. "smart-tie" mode) that may waive interconnection requirements that challenge system stability may be defined. The smart-tie mode may be useful for allowing:

i) Fault ride-through;

ii) Low- voltage or short-circuit ride-through;

iii) Under/over frequency ride-through; and

iv) Voltage and frequency regulation support.

[038] Consistent with embodiments of the invention a number of operating modes may be defined: i) islanded operating mode; ii) legacy-tie operating mode; and iii) smart-tie operating mode.

Islanded Operating Mode

[039] FIG. 4 shows microgrid 105 in the islanded operating mode. In the islanded operating mode, an intentional island is formed in isolation from host EPS 145 (e.g. by opening switch 140.) The voltage and frequency of microgrid 105 may be collectively regulated by member energy resources (e.g. generation 110, energy storage 115, and loads 125). During the islanded mode of operation, microgrid 105 does not exchange real or reactive power with host EPS 145. Therefore, no host EPS interconnection requirements apply. The stages of a supervisory and control method initiating and maintaining the islanded mode may include: i) SCA 130 disconnecting microgrid 105 from host EPS 145 by opening switch 140;

ii) The microgrid member devices (e.g. energy resources) may be encouraged to operate in regulator mode;

iii) The energy resources may collectively regulate microgrid voltage within tolerable limits;

iv) The energy resources may share and exchange real and reactive power based on a predefined priority schedule, without any communications between them; and

v) SCA 130 may influences power distribution priority among the energy resources by issuing commands over LBCN 135.

Legacy-tie Operating Mode

[040] FIG. 5 shows microgrid 105 in the legacy-tie operating mode. In the legacy-tie operating mode, microgrid 105 maybe operated in parallel to host EPS 145. During the legacy-tie mode of operation, microgrid 105 may exchange real and/or reactive power with host EPS 145. Therefore, host EPS interconnection requirements may apply at the PCC (e.g. switch 140). SCA 130 may be responsible for monitoring power exchange at the PCC, and ensuring the collective compliance of microgrid 105 to applicable interconnection requirements. SCA 130 may send commands over the LBCN 135 to ensure collective compliance to these

requirements. The sequence of supervisory and control events initiating and maintaining legacy-tie mode may include:

i) SCA 130 connects microgrid 105 to host EPS 145 (e.g. by closing switch

140)

ii) If net real power is exported to host EPS 145 from microgrid 105, a minimum set of microgrid member devices are required to operate in

passive/destabilize mode (this may be done in order to avoid the formation of a non- intentional island by supporting a neighboring matched load;

iii) The energy resources share and exchange real and reactive power based on predefined priority schedule, without any communications between them; and iv) SCA 130 may influence real and reactive power injection and

consumption by various energy resources in microgrid 105 by issuing commands over LBCN 135.

Smart-tie Operating Mode

[041] FIG. 6 shows microgrid 105 in the smart-tie operating mode. In the smart-tie operating mode, microgrid 105 may be operated in parallel to host EPS 145. During smart-tie mode of operation, microgrid 105 may be granted a partial or complete waiver of host EPS interconnection requirements at the PCC. This waiver may be temporary or permanent. For example, the partial or complete waiver may be granted for a finite period of time, or scheduled based on line or ambient condition. The sequence of supervisory and control events initiating and

maintaining smart-tie mode may include:

i) SCA 130 connects microgrid 105 to host EPS 145 by closing switch 140; ii) Microgrid 105 member devices (e.g. energy resources) are encouraged to operate in regulator mode;

iii) The energy resources share and exchange real and reactive power based on predefined priority schedule, without any communications between them; and iv) SCA 130 influences real and reactive power injection and consumption by various energy resources in microgrid 105 by issuing commands over the LBCN 135.

Mode Transitions

[042] Delivering a microgrid system may require smooth mode transitions between islanded operating mode, legacy-tie operating mode, and smart-tie operating mode. The smoothness (or turbulence) of such mode transitions may depend on multiple factors including: i) the operating mode of microgrid member energy resources; ii) effectiveness and speed of SCA 130 and LBCN 135; iii) the level of power exchange at the PCC at the moment of transition; and iv) the state of host EPS 145.

Islanded To Smart-Tie Mode Transition and Back [043] The transition from islanded to smart-tie operating mode and vice versa takes place under, for example, a couple of scenarios. First, microgrid 105 may transition into islanded mode as a protection mechanism, or in order to isolate local loads from host EPS 145 disturbances. This may be a planned event, or a sudden reaction to a system disturbance. Second, microgrid 105 may transition back into smart-tie when appropriate for system operation. The objective may be to import or export energy, support host EPS 145, and/or rely on its support for local loads.

[044] Achieving a smooth transition between islanded and smart-tie modes may not require member energy resources to change operating mode. Described below is the smart-tie to islanded operation and the islanded to smart-tie operation.

[045] FIG. 7 shows a transition from smart-tie to islanded operation as switch 140 goes from closed to open. Transitioning from smart-tie to islanded operation involves a number of supervisory and control events including, for example, the following. First, the current exchanged at the PCC (e.g. switch 140) may be reduced. If the transition is a planned event based on a schedule or external command, SCA 130 may reduce current exchanged at the PCC. This may be achieved by issuing commands to member energy resources of microgrid 105 over LBCN 135. Next, SCA 130 may interrupt electrical connection to host EPS 145 by opening switch 140. Then the voltage and frequency within microgrid 105 may be collectively regulated by member energy resources. The transition (from smart-tie to islanded) may appear as a mere power transient to member energy resources of microgrid 105. The power quality within the transition may largely be a function of the strength of this transient. A more forceful and turbulent transition is expected if a large amount of current is interchanged at the PCC at the instant of the transition.

[046] FIG. 8 shows the transition from islanded operation to smart-tie operation. Transitioning from islanded to smart-tie operation involves a number of supervisory and control events including, for example, the following. First, SCA 130 may synchronize the voltage, frequency, and phase shift of microgrid 105 to that of host EPS 145 by issuing commands over LBCN 135. Next, SCA 130 may establish electrical connection (e.g. by closing switch 140) between host EPS 145 and microgrid 105 when microgrid 105 and host EPS 145 are deemed acceptably synchronized. This transition may appear as a mere power transient to member energy resources of microgrid 105. Current exchanged at the PCC just after the transition reflects the degree of synchronization. Upon completing the transition, SCA 130 may reconfigure the member energy resources of microgrid 105 in order to regulate current flow into host EPS 145.

Smart-Tie To Legacy-Tie Mode And Back

[047] The transition from smart-tie to legacy-tie operating mode and vice versa takes place under a number of scenarios. First, microgrid 105 may transition into smart-tie mode whenever requested or given permission by host EPS 145. It may also transition into smart-tie mode if net average power is being imported from host EPS 145, thereby waiving the requirement for anti-islanding at the PCC.

Second, microgrid 105 maybe required to transition back to legacy-tie operation if no waiver exists for anti-islanding at the PCC, and net average power is being exported to host EPS 145. Third, when operating as a member resource of a larger parent microgrid, a microgrid may transition between smart-tie and legacy-tie operation based on commands received from a parent SCA. The microgrid may transition into smart-tie when encouraged to operate as a regulator, and transition back to legacy-tie when required to operate as a passive/destabilizer element.

[048] Transitions between smart-tie and legacy-tie operation may essentially be mode transitions of microgrid member energy resource devices, while microgrid 105 continues to be electrically connected to host EPS 145. The degree to which such transitions are challenging to achieve may depend on the timing accuracy required.

Legacy-Tie To Smart-Tie Operation

[049] FIG. 9 shows a transition from legacy-tie operation to smart-tie operation. Transitioning from legacy-tie to smart-tie operation involves a number of supervisory and control events including, for example, the following. First, SCA 130 may send commands to member energy resource devices of microgrid 105 to encourage operation in regulator mode. Next, member energy resource devices of microgrid 105 may begin to operate in regulator mode for a short "grace period" before the intended transition instant. This interval may be equal to the time limit specified by interconnection standard for detection and response to a loss-of-mains (TLoM) event.

Smart-Tie To Legacy-Tie Operation

[050] FIG 10 shows transition from smart-tie to legacy-tie operation.

Transitioning from smart-tie to legacy-tie operation involves a number of supervisory and control events including, for example, the following. First SCA 130 may send command to member energy resource devices of microgrid 105 that require operation in passive/destabilize mode. Next, member energy resource devices of microgrid 105 may continue to operate in regulator mode for a short "grace period" after the intended instant of transition to legacy-tie operation. This interval may be equal to the time limit for loss-of-mains detection and response less that required to achieve this task, TLoM-tdetect. Detection time, tdetect, may be considerably shorter than TLoM, depending on the quality of the detection algorithms.

[051 ] In both cases, planned transitions can be achieved with a high timing accuracy if member energy resource devices of microgrid 105 share a common time- reference. Achieving a high timing accuracy for unplanned transitions, however, may place strong requirements for LBCN 135 in terms of bandwidth and reliability. System architecture should be designed to avoid the need for such transitions whenever feasible.

Islanded To Legacy-Tie Mode And Back

[052] The transition from islanded to legacy-tie operating mode and vice versa takes place under a number of scenarios. First, transitions between islanded and legacy-tie operation may be important in the current regulatory framework. This may be because no provision exists yet for temporary or permanent waiver of the anti-islanding requirement at the PCC of microgrid 105.

Second, microgrid 105 may transition into islanded operation in order to maintain power supply to local loads, particularly critical or emergency loads. Third, microgrid 105 may transition back to legacy-tie operation whenever appropriate for system operation. The objective may be to import or export energy, and/or rely on host EPS 145 support for local loads. [053] Transitions between islanded and legacy-tie operation may be challenging because these two modes are very different in nature. Timing of the change of state of electrical connection to host EPS 145 should be tightly controlled relative to the transitioning of member energy resource devices of microgrid 105 between regulator and passive/destabilizer modes.

Islanded To Legacy-Tie Operation

[054] FIG 11 shows a transition from islanded to legacy-tie operation.

Transitioning from islanded to legacy-tie operation may involve a number of supervisory and control events including, for example, the following. First, SCA 130 may send commands to member energy resource devices of microgrid 105 in order to synchronize local voltage, frequency, and phase to that of host EPS 145. Next, SCA 130 may establish electrical connection (e.g. close switch 140) between microgrid 105 and host EPS 145 when synchronization is deemed acceptable. Then SCA 130 may send commands to member energy resource devices of microgrid 105 to command their transition into passive/destabilizer mode in order to satisfy the provision for anti-islanding at the PCC. The transition to passive/destabilizer mode should occur after the electrical connection is established with host EPS 145 in order to avoid loss of local voltage and frequency regulation. Moreover, the transition into passive/destabilizer mode may be delayed if net average power is imported from host EPS 145 after the transition event.

[055] Next, in order to satisfy the anti-islanding requirement, microgrid 105 should be anti-islanding ready inside a short "grace period." The grace period time limit is equal to TLoM - tdetect. The time required for microgrid 105 to become anti-islanding ready is that required for an acceptable set of member devices to transition into passive/destabilizer mode. This time is measured from the instant that net active power is exported to host EPS 145 in legacy-tie mode. An "acceptable set" of member devices can be defined as the minimum set whose engagement in destabilization for anti-islanding is sufficient for the successful detection of non- intentional islanding. An "acceptable set" may vary depending on the specific characteristics of devices within microgrid 105, and the amount of average power exported to host EPS 145. The use of a common time-reference for microgrid devices (e.g. member energy resource devices of microgrid 105) is a useful tool for reducing the time required for microgrid 105 to become anti-islanding ready.

Legacy-Tie To Islanded Operation

[056] FIG 12 shows a transition from legacy-tie to islanded operation. Transitioning from legacy-tie to islanded operation may involve a number of supervisory and control events including, for example, the following. First, if the transition is a planned event based on a schedule or external command, SCA 130 may reduce current exchanged at the PCC. This may be achieved by issuing commands to member energy resource devices of microgrid 105 over LBCN 135. Next, SCA 130 may interrupt electrical connection to host EPS 145. Then SCA 130 may send commands to member energy resource devices of microgrid 105 to encourage operation in regulator mode. It is important that an acceptable set of member devices makes transitions to regulator behavior in a timely fashion. This transition should happen before the member energy resource devices of microgrid 105 destabilize local voltage and frequency. For planned transitions into islanding mode, an acceptable set of devices transition into regulator mode before the electrical connection is interrupted.

[057] If net power is exported to host EPS 145 during this transition, the interruption of electrical connection should happen in a time shorter than the maximum allowed for detecting and reacting to an islanding condition, TLoM. In the event of unplanned transitions into islanding mode, devices are required to transition to regulator mode after the electrical connection to host EPS 145 is severed. A fast and reliable communication network is desirable in order to allow the transition to happen before local voltage and frequency regulation is lost. After a transient period, voltage and frequency within microgrid 105 may be collectively Regulated By Member Resources. Alternative Implementation Of Legacy-Tie Operation

[058] The provision for anti-islanding in legacy-tie presents particular challenges to systems design and mode transitions. Conventional schemes operate the microgrid in as a destabilizer element, and require timely and reliable communications to facilitate smooth transitions in and out of legacy-tie. In contrast, power quality and protection requirements can be achieved with minimum system requirements. Power quality standards can be achieved in a slow manner by issuing commands over LBCN 135, without requiring member energy resource device to operate in the problematic passive/destabilizer mode. The abnormal voltage and frequency protection requirement can be achieved by SCA 130 without coordination with microgrid member energy resource devices.

[059] In this light, the ability to satisfy the provision for anti-islanding without requiring member energy resource devices of microgrid 105 to operate in passive/destabilizer mode is highly valuable. The ability to fully implement legacy- tie operation while allowing member energy resource devices of microgrid 105 to operate in regulator mode greatly simplifies transitions into and out of legacy-tie mode. Observer-based anti-islanding schemes are proposed here to achieve that goal.

[060] FIG 13 shows microgrid 105 in an alternative observer-based legacy- tie mode configuration including. An observer 1305 may perform observer-based anti-islanding techniques. The function of observer 1305 may be performed by SCA 130. Observer-based anti-islanding techniques may entail the monitoring of one or more quantities at the PCC and using that information to detect islanding, even if line voltage and frequency remain within acceptable limits. Observer-based anti- islanding techniques may include island detection based on, but not limited to: i) the rate of change of frequency at the PCC; ii) synchrophasor measurement data; iii) response of real and/or reactive power to voltage and frequency at the PCC; and iv) impedance estimation. FIG 14 shows loss-of-mains detection using observer-based anti-islanding. Scalability and Hierarchy [061] FIG 15 shows an example of a microgrid hierarchy. As shown in FIG. 15, a microgrid hierarchy may be created by arranging microgrids into a predefined chain of command and data reporting. The formation of such hierarchy may allow the distribution of intelligence throughout a large microgrid or a larger EPS. One objective served by the microgrid hierarchy is to avoid the reliance on a centralized energy management controller that would: i) require massive data collection and communications resources; ii) perform massive operations of data processing and decision making; and iii) present a single point of failure. Another objective served by the microgrid hierarchy is to allow the sectioning of the system, and the formation of intentional islands at different levels of the hierarchy.

[062] The microgrid hierarchy of FIG. 15 may be created by assigning parent/child relationships between the SCA's of different microgrids. These relationships may be based on the size of different microgrids as well as their topological and geographical location within the overall system. They may be pre- assigned or dynamically modified in real-time to adapt to various operating conditions.

[063] According to this approach, a child microgrid may be enlisted as a member of a parent microgrid by creating a parent-child relationship between the SCA's of these microgrids. This relationship may allow the parent SCA to treat the child microgrid as a generic DG resource. The parent SCA may then use DG communications protocols and data models to gather data from the child microgrid, and issue commands to supervise its operation. The child SCA may be responsible for collecting data from its member systems, presenting aggregated data to parent SCA, analyzing commands issued by parent SCA, and issuing commands to member systems to ensure the proper response at the PCC to the parent micro-grid.

[064] The operating mode of a child microgrid may be perceived in the parent system as follows: i) microgrid operating in smart-tie mode maybe perceived as a member resource in regulator mode; ii) a microgrid operating in legacy-tie mode may be perceived as a member resource in passive/destabilizer mode; and iii) microgrid operating in islanded mode may be perceived as being absent or shutdown. [065] FIG. 16 shows SCA 130 of FIG. 1 in more detail. As shown in FIG. 2, SCA 130 may include a processing unit 1610 and a memory unit 1615. Memory 1615 may include a software module 1620 and a database 1625. While executing on processing unit 1610, software module 220 may perform processes as described above. SCA 130 may be implemented, for example, using a computing device comprising a small embedded controller integrated circuit.

[066] The elements shown in FIG. 1 may communicate over LBCN 135 ("the network"). The network may comprise, for example, a local area network (LAN) or a wide area network (WAN). Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the

Internet. When a LAN is used as the network, a network interface located at any of the processors may be used to interconnect any of the processors. When the network is implemented in a WAN networking environment, such as the Internet, the processors may typically include an internal or external modem (not shown) or other means for establishing communications over the WAN. Further, in utilizing the network, data sent over the network may be encrypted to insure data security by using known encryption/decryption techniques.

[067] In addition to utilizing a wire line communications system as the network, a wireless communications system, or a combination of wire line and wireless may be utilized as the network in order to, for example, exchange web pages via the Internet, exchange e-mails via the Internet, or for utilizing other communications channels. Wireless can be defined as radio transmission via the airwaves. However, it may be appreciated that various other communication techniques can be used to provide wireless transmission, including infrared line of sight, cellular, microwave, satellite, packet radio, and spread spectrum radio. The processors in the wireless environment can be any mobile terminal, such as the mobile terminals described above. Wireless data may include, but is not limited to, paging, text messaging, e-mail, Internet access and other specialized data applications specifically excluding or including voice transmission. For example, the processors may communicate across a wireless interface such as, for example, a cellular interface (e.g., general packet radio system (GPRS), enhanced data rates for global evolution (EDGE), global system for mobile communications (GSM)), a wireless local area network interface (e.g., WLAN, IEEE 802), a bluetooth interface, another RP communication interface, and/or an optical interface.

[068] Embodiments of the invention may be practiced in an electrical circuit comprising discrete electronic elements, packaged or integrated electronic chips containing logic gates, a circuit utilizing a microprocessor, or on a single chip containing electronic elements or microprocessors. Embodiments of the invention may also be practiced using other technologies capable of performing logical operations such as, for example, AND, OR, and NOT, including but not limited to mechanical, optical, fluidic, and quantum technologies. In addition, embodiments of the invention may be practiced within a general purpose computer or in any other circuits or systems.

[069] Embodiments of the invention, for example, may be implemented as a computer process (method), a computing system, or as an article of manufacture, such as a computer program product or computer readable media. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process. Accordingly, the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.). In other words, embodiments of the present invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. A computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

[070] The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific computer-readable medium examples (a non-exhaustive list), the computer- readable medium may include the following: an electrical connection having one or more wires, , a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory),

[071] Embodiments of the present invention, for example, are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products according to embodiments of the invention. The functions/acts noted in the blocks may occur out of the order as shown in any flowchart. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved.

[072] While the specification includes examples, the invention's scope is indicated by the following claims. Furthermore, while the specification has been described in language specific to structural features and/or methodological acts, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as example for embodiments of the invention.