Patent Publication Number: US-2013245846-A1

Title: Control system for photovoltaic power plant

Description:
This application is a continuation of application Ser. No. 13/104,471, filed May 10, 2011, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF INVENTION 
     Technical Field 
     The subject matter described herein relates to control systems for solar power plants, such as photovoltaic (“PV”) power plants. 
     BACKGROUND 
     Solar power plants, typically include multiple power generation sites, each of which may include one or more solar power generation devices. The electrical power generated at the multiple power generation sites is typically transmitted to a desired location or to a power grid at one or more locations. 
     A power generation site for a photovoltaic (“PV”) solar power plant may include a power conversion station having multiple PV solar panels connected to an inverter. The PV panels generate electrical power as direct current (“DC”) electricity. The inverter receives electricity generated by the PV solar panels as DC electricity. The DC electricity generated by the PV solar panels may be amplified or otherwise modified before it is transmitted to the inverter. The inverter then converts the DC electricity to alternating current (“AC”) electricity, and transmits the AC electricity to a point of common connection (commonly referred to as a “point of intersection” or “POI”) with other power generation sites in the electrical generating system. 
     A PV power plant typically includes numerous plant devices, such as PV panels, electrical combiner boxes, electrical inverters, trackers used to adjust PV panels, sensors, and other devices that are used in the generation of solar power. One important aspect of PV power plant is how plant devices are controlled and monitored. Conventional control system architectures for electrical generating systems typically either embed control and data aggregation functions into individual power generation devices (such as inverters), or provide this function through the use of other power electronics at the plant site, such as in the D-VAR intelligent grid systems manufactured by American Superconductor. Conventional control system architectures monitor the AC power that is provided from each power generation site, but typically do not monitor the DC power prior to its conversion to AC power. 
     It is desirable to have a control system for a solar power plant that provides plant-level control functions (such as control of plant devices), data acquisition functions, and interconnections between the various elements and facilities of the electrical generating system from a centralized location, which is cost effective, of relatively simple design, and scalable. It is also desirable to monitor and regulate the amount of DC power that is generated by a collection of PV panels prior to its conversion to AC power. 
     Embodiments described herein include various control systems for PV power plants, with a centralized control of various aspects of the electrical generating systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electrical generating system in accordance with embodiments described herein; 
         FIG. 2  is a block diagram of power generation sites of an electrical generating system in accordance with embodiments described herein; 
         FIG. 3  is a block diagram of a plant-level control system in accordance with embodiments described herein; 
         FIG. 4  is a block diagram showing one manner in which a plant-level control function may be provided using the plant-level control system described herein; 
         FIG. 5  is a block diagram of a supervisory control system in accordance with embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and which illustrate specific embodiments of the invention. These embodiments are described in sufficient detail to enable those of ordinary skill in the art to make and use them. It is also understood that structural, logical, or procedural changes may be made to the specific embodiments disclosed herein without departing from the spirit or scope of the invention. 
     This disclosure describes various control systems for photovoltaic (“PV”) electrical generating systems. Described control systems provide a flexible platform for functions including real time power control for plant devices at multiple power generation sites, supervisory control functions for plant operation and for non-time-critical control capability, and data acquisition functions for monitoring plant performance and for supporting operations and maintenance. The described control systems separate plant-level control functions, supervisory control functions, and/or data aggregation functions into their own separate physical and logical entities. 
     Described embodiments of control systems for PV power plants include a plant-level control system and a plant supervisory system, among other elements. Described embodiments include control hardware and software, data acquisition hardware and software, networking elements, interfaces for users, interfaces for other instruments such as sensors, or other devices such as a remote terminal unit, and interfaces to other systems. Described embodiments also include remote access and access security functions. 
       FIG. 1  shows a block diagram of a PV power plant  100 . Plant  100  includes at least one substation  101 , an operations and maintenance station  102 , and one or more power generation sites  103 . Although only a single substation  101  is shown in  FIG. 1 , it should be understood that plant  100  may include multiple substations  101 , each being having multiple associated power generation sites  103 . 
     Each power generation site  103  in plant  100  will typically include multiple plant devices (described further below with regard to  FIG. 2 ). Such plant devices may include PV panels used to generate and transfer electricity, electrical combiner boxes for interconnecting the outputs of a plurality of PV panels, inverters for inverting DC electrical signals to AC electrical signals, trackers or other mounting mechanisms for controlling the configuration of the PV panels, sensors for detecting various conditions, as well as other equipment and components used in the generation, operation, and/or control of solar power. For example, in a PV power plant, each power generation site  103  may include multiple PV panels that generate DC electricity from solar radiation, solar trackers associated with mounted PV panels which direct the PV panels to face incident light, combiner boxes to combine the DC electricity received from multiple PV panels, and inverters to convert DC electricity (for example, received from one or more combiner boxes) to AC electricity for further transmission. 
     Plant  100  includes a point of common power connection among the power generation sites  103 , referred to as a point of intersection (“POI”)  112 . Electrical power generated at the one or more power generation sites  103  is output as AC power onto an electrical power grid  120  (or to another desired location) through POI  112 . Accordingly, POI  112  represents a single output for plant  100 , and the power characteristics measured at POI  112  are the characteristics of the electrical power that is input to grid  120 . 
     Substation  101 , operations and maintenance station  102 , and power generation sites  103  of plant  100  are interconnected by a plant communications network  560 . Communications network  560  may be a fiber-optic, wired or wireless network, or other suitable type of network for providing communications among various control system components in plant  100 . For example, communications network  560  may be a redundant high speed ring optical fiber local-area network (“LAN”) with private network addressing. Preferably, communications network  560  includes standardized switches with virtual LAN (“VLAN”) support, including support for access by systems of third party service providers. VLAN refers to a LAN network where a group of hosts (typically with a common set of requirements) communicate as if they are attached to the same LAN regardless of their physical location. 
     A network operations center  105  and remote center  106  (for example, an offsite customer control center) may be remotely connected to plant  100 . Network operations center  105  and/or remote center  106  may be connected to a supervisory system  202  of plant  100  using conventional secure internet connections, as described further below. 
     As shown in  FIG. 1 , plant  100  also includes a plant-level control system  201  for providing real time plant-level control functions, such as active power control, voltage regulation, and power factor set point capability, among other control functions. Elements of plant-level control system  201  may be located in substation  101 , as shown in  FIG. 1 . Alternatively, some or all elements of plant-level control system  201  may be located outside of substation  101 , and may be in several different locations (such as other substations) in plant  100 . If plant  100  includes a plurality of substations  101 , plant-level control system  201  may be located in one of the plurality of substations  101 , and connected to other substations  101  and power generation sites  103  through communications network  560 . 
     A supervisory system  202  for providing supervisory control and data acquisition functions, including control and data acquisition capability for plant devices at each power generation site  103 , is provided in operations and maintenance station  102 . Elements of supervisory system  202  may be located in operations and maintenance station  102 , as shown in  FIG. 1 . Alternatively, some or all elements of supervisory system  202  may be located outside of operations and maintenance station  102 , and may be in several different locations (such as in substation  101 , network operations center  105 , or remote center  106 ). 
     Plant-level control system  201  and a supervisory system  202  form an overall control system for plant  100 . The control system for plant  100  is described further below. 
       FIG. 2  is a block diagram showing greater detail of plant devices within power generation sites  103 . It should be understood that the plant devices shown in  FIG. 2  are only exemplary plant devices, and power generation sites  103  may include other plant devices known in the art. 
     Each power generation site  103  shown in  FIG. 2  includes multiple PV panels  225  that serve as power generators. PV panels  225  generate direct-current (“DC”) electricity from solar radiation. Multiple PV panels may be interconnected in series or parallel strings, with each power generation site including one or more strings  226  of PV panels. In  FIG. 2 , a first string  226   a  and a second string  226   b  of PV panels  225  are labeled for clarity, although it should be understood that other strings  226  of PV panels  225  are shown as well. The DC output from multiple PV panels  225  (such as from the PV panels  225  in one or more strings  226 ) is combined at a combiner box  223 . The DC output from one or more combiner boxes  223  is then input into an inverter  222  that inverts the DC electricity into alternating-current (“AC”) electricity. Accordingly, power generation site  103  includes a DC region that includes plant devices and interconnections (e.g., PV panels  225 , combiner boxes  223 , and the input to inverters  222 ) along a power chain prior to inverter  222 , and an AC region that includes plant devices and interconnections (e.g., the output of inverters  222  and power generation site output  451 ) along the power chain after inverter  222 . 
     Each PV panel  225  is also coupled to a mounting mechanism  224 , which may include a tracker that orients one or more PV panels  225  according to present local conditions (e.g., according to the present local environmental conditions and/or the direction of incident sunlight). 
     Each inverter  222  in a power generation site  103  outputs the AC electric power to point of intersection (“POI”)  112  of plant  100  ( FIG. 1 ) through the power generation site output  451 . 
     Each power generation site  103  also includes a monitoring and communication device  450  that includes a storage element  460  (such as a direct-attached storage) and a logic element  461  (such as a programmable logic controller). Monitoring and communication device  450  provides translation of commands (such as commands for plant-level control functions, described further below) that are received over a communications network  560  if the commands are in a different protocol or format than those supported by the plant devices of the power generation site  103 . 
     In addition, monitoring and communication device  450  interfaces with and collects data from integrated monitoring systems of plant devices within power generation site  103 . Monitoring and communication device  450  collects DC power characteristics (e.g., current, voltage, and/or other measurements) of plant devices (such as PV panels  225  and combiner boxes  223 ) and interconnections within the DC region of power generation sites  103 , and collects AC power characteristics (e.g., current, voltage, and/or other measurements) of plant devices (such as inverters  222 ) and interconnections within the AC region. Monitoring and communication device  450  also collects data regarding the overall AC output of the power generation site at output  451 . This data can be stored in the storage element  460  of communication device  450  and provided to plant-level control system  201  and/or supervisory system  202  ( FIG. 5 ) through communications network  560 . 
     The capability at power generation site  103  to monitor DC power characteristics provides for curtailment of DC power that is input into each inverter  222 . For example, as discussed further below with regard to  FIGS. 3 and 4 , plant control functions performed by plant-level control system  201  may require a certain level of AC power to be output from each power generation site  103 . Typically, the AC power that is output from each power generation site  103  is regulated by adjusting the output of power in the AC region (e.g., adjusting the AC power output of the inverter  222 ). Also, each inverter  222  may have a threshold level for one or more input DC power characteristics, such as a maximum amount of input DC current that the inverter  222  can invert to output AC current. If the input DC current to inverter  222  exceeds this threshold, generated DC power may be wasted, and the inverter  222  may be damaged. 
     By measuring power characteristics in the DC region of power generation site  103  through monitoring and communication device  450 , the DC power that is input to each inverter  222  can be regulated in the DC region. For example, the amount of overall power generated from a first group of PV panels  225  (e.g., a first string  226   a  of PV panels  225 ) that are associated with a first combiner box  223 , and the amount of overall power drawn from a second group of PV panels  225  (e.g., a second string  226   b  of PV panels  225 ) that are associated with a second combiner box  223  can be regulated through controlled relays, switches, or other controlled devices. Regulating the overall power drawn by different groups of PV panels  225  associated with respective combiner boxes  223  can regulate the total DC power load that is input to each respective inverter  222 . 
     In addition, regulating the overall power drawn by different groups of PV panels  225  associated with respective combiner boxes allows for balancing the power produced by the two groups of PV panels  225  during optimal conditions at both groups of panels, and to increase one group in response to the other group&#39;s decrease in power generation, for example due to less than optimal conditions at the other group&#39;s PV panels  225 . 
     Each combiner box  223  may be configured to regulate the DC power generated by the group of PV panels  225  associated therewith. For example, in response to measurements in the DC region, a controlled relay, switch, or other controlled device in a combiner box  223  can be triggered to stop receiving DC current from one or more associated PV panels  225 . This can regulate the amount DC current that is being input into inverter  222  by each associated combiner box  223 , which in turn impacts the amount of AC power that is output by inverter  222 . 
     Power generation site  103  also includes one or more sensors  452  for monitoring environmental conditions or conditions at PV panels  225 . For example, sensors  452  may be configured to monitor horizontal irradiance, plane of array irradiance, wind speed, wind direction, outside air temperature, outside relative humidity, daily and annual rainfall, panel temperature, and/or other conditions related to the PV panels  225 . Sensors  452  are also connected to monitoring and communication device  450  in order to provide data collected by sensors  452  to supervisory system  202  ( FIG. 5 ). 
     As described further below with regard to  FIGS. 3-5 , plant-level control system  201  and supervisory system  202  form a control system for plant  100  ( FIG. 1 ) that includes separate physical or logical entities for control and data aggregation functions. The control system provides advantages over conventional systems, including providing for control of power characteristics of the plant&#39;s output at POI  112  through the broadcast of uniform control functions to different plant devices, as well as through control of the plant devices (e.g., PV panels  225 , electrical combiner boxes  223 , inverters  222 , mounting mechanisms  224 , sensors  452 , etc.) on an individual basis. In addition, plant-level control functions are governed by a master controller  213  ( FIG. 3 ) that can be configured locally by users of plant-level control system  201 , or by users of supervisory system  202 , as described further below. 
       FIG. 3  shows a block diagram of plant-level control system  201 , which is shown as contained within a substation  101 . As discussed above, elements of plant-level control system  201  can be implemented within substation  101  of plant  100  ( FIG. 1 ), or separately from substation  101 . Plant-level control system  201  includes hardware and software sufficient to enable fully automatic and independent operation of plant  100 , as described further below. 
     Plant-level control system  201  includes one or more substation remote terminal units (“RTU”)  210 . Substation remote terminal unit  210  may be a microprocessor-controlled device that is configured to act as a local control hub for the plant-level control system, and that is interfaced to other elements of plant-level control system  201  using standard substation equipment interface protocols, such as DNP3 or IEC6185, or other interface protocols known in the art. Substation RTU  210  also includes a web-based user interface (e.g., with a display, keyboard, etc.) for local operation, configuration, and troubleshooting of plant  100 . Substation RTU  210  provides flexibility for grid operators and others to locally provide commands (e.g., instructions and/or parameters for plant control functions, described further below) to master controller  213  (described below). 
     Plant-level control system  201  also includes a master controller  213 . Master controller  213  may include one or more logic engines including at least one processor. Master controller  213  may be a separate element, or may be built into substation RTU  210 . Master controller  213  provides instructions for real time plant-level control functions to plant devices in power generation sites  103  ( FIG. 2 ) across communications network  560 . Master controller  213  may be configured through substation RTU  210 , or through a supervisory user interface  230  ( FIG. 5 ) of supervisory system  202  (described further below) over communications network  560  or another secure connection. 
     Plant-level control system  201  also includes a POI monitoring device  211 , which may be interfaced with master controller  213  and/or to substation RTU  210  via known protocols, such as DNP3 or IEC61850, or other protocols known in the art. POI monitoring device  211  includes sensors and other elements configured to detect various power characteristics, such as voltage level, reactive voltage, power level, and other power characteristics known in the art, at POI  112 . 
     Plant-level control system  201  is designed to regulate the real and reactive power output of plant  100  as a single large generator. As described above with regard to  FIGS. 1 and 2 , plant  100  includes one or more PV panels  225  ( FIG. 2 ) in each of multiple power generation sites  103 . Each PV panel  225  generates DC electrical power based on its present capability (for example, based on the current local environmental conditions at the PV panel  225 ). Plant-level control system  201  coordinates plant-level control functions for plant devices in each power generation site  103  in order to regulate the overall output of plant  100 , for example by regulating power characteristics in the DC region of each power generation site  103  (e.g., regulating the output of strings  226  of PV panels  225  or combiner boxes  223 ), by regulating power characteristics in the AC region of each power generation site  103  (e.g., regulating the output of inverters  222  or power generation site outputs  451 ), or both. By coordinating the plant-level control functions, plant-level control system  201  provides typical large power plant features—such as active power control, voltage regulation, and/or volt-ampere reactive (“VAR”) regulation, described further below—to plant  100 . Master controller  213  is capable of providing plant-level control functions for all of plant  100 . 
     Plant-level control system  201  is also interfaced with supervisory system  202  (described further below), for example, through a supervisory user interface  230  ( FIG. 5 ), through communications network  560  or through another secure connection to substation RTU  210  or master controller  213 . Interfacing plant-level control system  201  with supervisory system  202  allows plant-level control system  201  to provide data (e.g., as alarm signals, status information, metering, operating modes information, and monitoring information to supervisory system  202 ) to supervisory system  202 , and to accept commands for plant-level control functions from supervisory system  202 . To ensure reliable supervisory control capability, master controller  213  includes an internal clock or receives an external clock that is synchronized to a clock of the supervisory user interface  230  ( FIG. 5 ), preferably with ±0.1 second. 
     Commands for plant control functions are provided to master controller  213  from supervisory system  202  through supervisory user interface  230  ( FIG. 5 ), from substation RTU  210 , or automatically through other interfaced equipment (such as automatically generated commands from inverters  222  ( FIG. 2 )). To ensure security, access to the local user interface of substation RTU  210  and/or the supervisory system  202  is secured and password protected. Firmware and software upgrades for the control system are preferably capable of execution either locally through substation RTU  210  or remotely through supervisory system  202 . 
     Master controller  213  is configured to provide instructions to plant devices over communications network  560  using non-proprietary communication schemes, such as DNP3, IEC61850, or other common protocols known in the art. Configuring master controller  213  to communicate using non-proprietary communication schemes minimizes latency by utilizing commercially available hardware that is designed for high speed protection schemes to monitor the transmission interconnect between components of plant  100 . For example, the described configuration allows for coupling of plant devices manufactured by a first OEM to other plant devices manufactured by a different OEM in a direct fashion, thus providing fast and reliable plant level control. Alternatively, as discussed above with regard to  FIG. 2 , each power generation site  103  may include a logic element (such as logic element  461  within monitoring and communication device  450 ) that serves as an intermediary for communicating and translating instructions in different protocols for different plant devices. 
       FIG. 4  is a block diagram showing one manner in which plant-level control system  201  can be used to issue commands for plant control functions to control plant devices within each power generation site  103 . It should be understood that the described process can be applied to various plant control functions described below, as well as to numerous other plant control functions known in the art. 
     As shown in  FIG. 4 , a user (e.g., a grid operator) may provide a command P(x)—such as instructions and/or parameters for plant control functions—to master controller  213  through substation remote terminal unit (“RTU”)  210 . The user may provide the command P(x) through substation RTU  210 , or through supervisory system  202 . Alternatively, supervisory system  202  may provide the command P(x) directly to master controller  213 . If the command P(x) originates in supervisory system  202 , it is sent to master controller  213  over communications network  560 . 
     Command P(x) may be an instruction to activate or deactivate one or more power control functions and/or a parameter for power control functions, such as a power set point, a desired voltage for the point of intersection (“POI”), or other values used for power control functions. 
     Master controller  213  also receives real time measurements M(y,z) of power characteristics and/or other conditions at POI  112  from POI monitoring device  211 . If the measurements M(y,z) at POI  112  deviate from those specified by command P(x), master controller  213  is configured to determine the appropriate action (e.g., if power should be increased or decreased), make any calculations that may be needed (e.g., calculating set point values for the increase or decrease), and generate instructions I( 1 , 2 ,N) for this action. Master controller  213  then provides the generated instructions I( 1 , 2 ,N) to one or more of power generation sites  103  over communications network  560 . 
     Master controller  213  can be configured to make determinations and generate instructions I( 1 , 2 ,N) according to control algorithms that are input to master controller  213  through substation RTU  210  or supervisory system  202 . Instructions I( 1 , 2 ,N) may include individualized instructions for each power generation site  103 , or individualized instructions for multiple plant devices within each power generation site  103 . Alternatively, instructions I( 1 , 2 ,N) may be identical and commonly sent to all power generation sites  103 . 
     Each power generation site  103  that receives instructions (or a component thereof) from master controller  213  may then provide the appropriate instructions to associated plant devices. For example, as described above with regard to  FIG. 2 , power generation site  103  may include a monitor and communication device  450  equipped with a programmable logic element  461  for interpreting instructions received over communications network  560 . Monitor and communication device  450  may then pass the received instruction (or component thereof) to one or more associated inverters  222  ( FIG. 2 ) or other plant devices at the respective power generation site  103 . Alternatively, power generation site  103  may not include a monitor and communication device  450 , and the instructions may be received directly by inverters  222  or other plant devices at power generation site  103  through communications network  560 . 
     In response to the respective instructions, integrated controls within the plant devices may adjust the plant device output in order to alter the overall output of the power generation site  103 . For example, an inverter  222  ( FIG. 2 ) may be configured to adjust an electrical power output level or voltage level according to the received instructions. Alternatively, inverters  222  may include integrated circuitry that is configured to generate and transmit appropriate instructions to one or more connected plant devices in the DC region of power generation site  103 , such as combiner boxes  223 , mounting mechanisms  224 , and/or PV panels  225  ( FIG. 2 ), to adjust their respective properties, and therein adjust the output of inverters  222  within the power generation site  103 . 
     Examples of control functions that may be provided by the plant-level control system  201  include set-point control of the power output at POI  112 , set-point control of the DC current input to one or more inverters  222 , ramp-up and ramp-down control of the power output at POI  112 , voltage regulation of the voltage at POI  112 , power factor regulation of the power factor at POI  112 , and frequency response control of the frequency at POI  112 , to name but a few. These functions are described further below. 
     In set point control, plant-level control system  201  regulates the maximum active power output of plant  100 , as measured at the POI  112 . Plant-level control system  201  activates set-point control according to an active power instruction from a user, for example, through substation RTU  210  or supervisory system  202  via supervisory user interface  230  ( FIG. 5 ). The user similarly may provide a set point parameter. When set point control is activated, plant-level control system  201  will regulate the maximum active power that plant  100  produces at POI  112  based on the provided set point parameter. 
     When set point control is activated, if the set point parameter is higher than the measured power output of POI  112 , set point control will have no effect on the power generation sites  103 . If the set point parameter is lower than the measured power output of the POI  112 , master controller  213  generates instructions to reduce the output of plant  100  to be within the specified limit, for example by reducing the corresponding output from plant devices at one or more of power generation sites  103 . For example, master controller  213  may generate instructions to instruct inverters  222  ( FIG. 2 ) within one or more power generation sites  103  to reduce the inverters&#39;  222  respective overall AC power outputs. Alternatively, master controller  213  may generate instructions to one or more DC plant devices (e.g., combiner boxes  223  or mounting mechanisms  224  for PV panels  225 ) to reduce the DC plant devices&#39; output (such as through triggering relays in the combiner boxes  223  or reconfiguring mounting mechanisms  224  to redirect the PV panels  225 ). Plant-level control system  201  continues to monitor the measured power output of POI  112  in real time through POI monitoring device  211 . If additional adjustments are needed to reduce the power output of POI  112  in response to the set point parameter, master controller  213  provides additional instructions to power generation sites  103 . 
     DC current input regulation for one or more individual inverters  222  ( FIG. 2 ) prevents damage from the inverters  222  receiving excess loads of DC power, such as from one or more combiner boxes  223 . Plant-level control system can be configured to regulate DC current input for all inverters  222  within plant  100 , either uniformly or according to inverter-specific parameters, or can be configured to regulate DC current input for only select inverters  222 . 
     Plant-level control system  201  activates DC current input regulation for one or more inverters  222  according to an active power instruction from a user, for example, through substation RTU  210  or supervisory system  202  via supervisory user interface  230  ( FIG. 5 ). Alternatively, plant-level control system  201  can be configured to activate DC current input regulation based on a command that is automatically-generated by the inverter  222 . Similarly, a threshold DC current input parameter may be provided by the user, or may be provided automatically by inverter  222 . 
     When DC current input regulation is activated, if the threshold DC current input parameter is higher than the measured DC current that is being input to the regulated inverter  222 , the regulation will have no effect on the power generation sites  103 . If the threshold DC current input parameter is lower than the measured DC current that is being input to the regulated inverter  222 , master controller  213  generates instructions to reduce the output of one or more DC plant devices associated with the regulated inverter  222 . For example, master controller  213  may generate instructions to be provided to a combiner box  223  associated with the inverter  222 , in order to trigger one or more controlled relays within the combiner box  223  that will stop the combiner box from receiving and outputting DC current from one or more associated PV panels  225 , or one or more strings  226  of PV panels  225 . The instructions from master controller  213  may be provided to the associated combiner box  223  directly over plant network  560 , through monitoring and communication device  450 , and/or through inverter  222 . 
     Alternatively, master controller  213  can be configured to generate instructions to be provided to mounting mechanisms  224  (e.g., trackers) that control the configuration of one or more PV panels  225  associated with the regulated inverter  222 , in order to adjust the configuration of the PV panels  225  and reduce the overall DC power that is being generated. For example, the mounting mechanisms  224  could be controlled to redirect PV panels  225  so that less light is radiant on the PV panels  225 , causing the PV panels  225  to generate a lower DC power output. The instructions from master controller  213  may be provided to the mounting mechanisms  224  directly over plant network  560 , through monitoring and communication device  450 , and/or through inverter  222 . 
     Plant-level control system  201  continues to monitor the measured DC input current of the regulated inverter  222  in real time through monitoring and communication device  450 . If additional adjustments are needed to reduce the DC current input of the regulated inverter  222  in response to the threshold DC current input parameter, master controller  213  provides additional instructions. 
     Ramp-up and ramp-down control ensure that the output of plant  100  ( FIG. 1 ) does not increase or decrease, respectively, faster than specified by ramp-up and ramp-down rate parameters. Instructions to activate ramp-up control, ramp-down control, or both, and the ramp-up and ramp-down rate parameters, are entered by a user through substation RTU  210  or supervisory system  202  via supervisory user interface  230  ( FIG. 5 ) and provided to master controller  213 . The plant output is measured at the POI  112  by POI monitoring device  211 , and these measurements are provided to master controller  213 . Master controller  213  is configured to determine the change between present measurements at POI  112  and previous measurements at POI  112 . 
     If ramp-up control is activated, master controller  213  determines whether the electrical power at POI  112  is increasing at a greater rate than that specified by the ramp-up rate parameter (for example, due to large variations in the level of incident sunlight on PV panels  225  in  FIG. 2 ). If the power at POI  112  is increasing at a rate that is less than or equal to the rate specified by the ramp-up rate parameter, ramp-up control has no effect on plant  100  ( FIG. 1 ). If the power at POI  112  is increasing at a greater rate than specified by the ramp-up rate parameter, master controller  213  generates instructions to reduce the output of plant  100  to correspond with the ramp-up rate specified by the ramp-up rate parameter, for example by reducing the corresponding output from one or more power generation sites  103 . 
     Similarly, if ramp-down control is activated, master controller  213  determines whether the electrical power at POI  112  is decreasing at a greater rate than that specified by the ramp-down rate parameter. If the power at POI  112  is decreasing at a rate that is less than or equal to the rate specified by the ramp-down rate parameter, ramp-down control has no effect on the operation of plant  100 . If the power at POI  112  is decreasing at a greater rate than specified by the ramp-down rate parameter, master controller  213  generates instructions to increase the output of plant  100  to correspond with the ramp-down rate specified by the ramp-down rate parameter, for example by increasing the corresponding output from one or more power generation sites  103 . Because present environmental conditions (e.g., cloud cover for PV power plants) can greatly reduce the capability of multiple PV panels  225  ( FIG. 2 ) to generate electricity, and thus greatly affect the overall power available from plant  100 , plant-level control system  201  may be configured to utilize techniques such as auxiliary power storage or other known methods to ensure that when ramp-down control is activated, the overall output does not decrease faster than the specified ramp-down parameter. Such techniques may be controlled by substation RTU  210  or supervisory system  202  via supervisory user interface  230 . 
     Voltage regulation controls the plant voltage at the POI  112  to a specified level. For example, when voltage regulation is activated, a desired POI voltage parameter may be entered by a user through substation RTU  210  or supervisory system  202  via supervisory user interface  230  ( FIG. 5 ). POI monitoring device  211  determines the voltage at the POI  112 , and provides this voltage to master controller  213 . Master controller  213  then determines appropriate voltage or voltage-ampere reactive (“VAR”) set points based on the desired POI voltage parameter. VAR is a value typically used to measure reactive power in an AC electrical power system. Master controller  213  sends instructions including the set points to the plant devices in order to achieve closed loop voltage feedback control. It should be understood that the set points may be uniform set points for each power generation site  103 , or may be individualized for each power generation site  103  or for plant devices in each power generation site  103 . 
     Power generation sites  103  receive the set points and adjust their respective reactive power levels accordingly. POI monitoring device  211  continues to provide measurements of the voltage at POI  112  to master controller  213 . If the voltage at POI  112  is still determined to deviate from the desired POI voltage parameter, master controller  213  provides additional instructions to plant devices (e.g., inverters  222 , combiner boxes  223 , or other plant devices involved in power generation) at power generation sites  103 . Switched capacitor banks (not shown) may also be activated by master controller  213  to compensate for net inductive losses that may occur at power generation sites  103  as a result of the voltage regulation. 
     Power factor regulation controls the power factor at the POI  112  according to a specified power factor parameter. The “power factor” of an AC electrical system is the ratio of real power (i.e., the actual power flowing to the load) to apparent power (i.e., current multiplied by voltage). Power factor regulation is provided in a manner similar to voltage regulation. When a user provides an instruction to activate power factor regulation and a power factor parameter (for example, through substation RTU  210  or supervisory system  202  via supervisory user interface  230  (FIG.  5 )), POI monitoring device  211  detects the ratio of real power to apparent power at POI  112 , and provides this ratio to master controller  213 . Master controller  213  is configured to determine whether the measured ratio is within tolerable limits of the power factor parameter provided by the user. If so, power factor regulation has no effect on plant  100 . If not, master controller  213  provides appropriate instructions to plant devices (e.g., inverters  222 , combiner boxes  223 , or other plant devices involved in power generation) at power generation sites  103  to achieve power factor regulation. 
     Plant-level control system  201  may also provide for frequency regulation of plant  100 , including regulation of frequency power droop at POI  112 . For example, plant-level control system  201  may activate down-power regulation in one or more inverters  222  to compensate when the output frequency at POI  112  is determined to be higher than nominal. Similarly, plant-level control system  201  may activate up-power regulation in one or more inverters  222  to compensate when output frequency at POI  112  is lower than nominal. Instructions to activate frequency regulation, or parameters to define when frequency regulation should be automatically activated, may be provided by a user, for example, through substation RTU  210  or supervisory system  202  via supervisory user interface  230  ( FIG. 5 ). 
     While plant-level control system  201  may also be configured to provide low voltage and high voltage “ride-through” capability (using parameters and/or commands provided through substation RTU  210  or supervisory system  202  via supervisory user interface  230  (FIG.  5 )), these capabilities are preferably provided by the individual plant devices themselves. Low voltage ride-through (“LVRT”) refers to a plant&#39;s ability to remain on line and “ride through” a low voltage condition at one of the inverters, which may be caused, for example, by an inadvertent grounding of the system or by a sudden connection of a large load. Similarly, high voltage ride-through (“HVRT”) refers to the plant&#39;s ability to handle high voltage conditions at one of the inverters. Both LVRT and HVRT capability require very fast response from the plant. Accordingly, this capability is preferably provided at an individual plant device, such as through circuitry integrated in each inverter  222  ( FIG. 2 ), rather than by plant-level control system  201 . 
     It should be understood that plant-level control system  201  provides other plant control functions in similar manner to those described above. Provided plant control functions may include controlled startup/shutdown of plant  100 , static capacitor/reactor coordination, implementation of a system protection scheme to counteract undesirable conditions that may cause serious stress to plant  100 , or other control functions for power plants and other electrical generating systems that are known in the art. 
       FIG. 5  shows a block diagram of supervisory system  202 . Supervisory system  202  provides real time monitoring, alarm processing, historical data archiving, and supervisory control of plant  100 , among other functions. As discussed above, elements of supervisory system  202  can be implemented within operations and maintenance station  102  of plant  100  ( FIG. 1 ), or separately from operations and maintenance station  102 . Supervisory system  202  includes hardware and software sufficient to enable fully automatic supervisory control and data acquisition for plant  100 , as described further below. 
     Supervisory system  202  is connected to plant-level control system  201  ( FIG. 3 ) at least through communications network  560 . In addition, and as described further below, supervisory system  202  is also accessible remotely, such as through network operations center  105  and/or a remote center  106  (which may serve as an offsite customer control center) of plant  100  ( FIG. 1 ). 
     Supervisory system  202  includes supervisory user interface  230 . Supervisory user interface  230  is a self-contained web-based user interface that can be used by onsite personnel (for example, at substation RTU  210  or at operations and maintenance station  102  in  FIG. 1 ), as well as at offsite customer control personnel (such as at remote center  106 ) and oversight personnel (such as at network operations center  105 ) through a remote connection (discussed further below). Supervisory user interface  230  provides for monitoring, both locally and remotely, elements of plant  100  ( FIG. 1 ), including key operation and performance indicators of the plant  100 , status and operating conditions of individual components (including individual plant devices), system operation (e.g., starting and stopping of inverters  222  (FIG.  2 )), and troubleshooting and diagnostic capabilities for the overall system and for individual components. Supervisory user interface  230  also provides for management of all alarms generated by components of plant  100 . 
     Supervisory user interface  230  also provides web-based operational control of aspects of the plant-level control system  201  ( FIG. 3 ). For example, supervisory user interface  230  may be configured to receive external commands and thus provide remote operational control of plant-level control functions executed by master controller  213  ( FIG. 3 ) through communications network  560 . Supervisory user interface  230  also provides for configuration of master controller  213  through communications network  560 . Additionally, supervisory user interface  230  provides for operation of master controller  213  and/or individual plant devices through communications network  560 , such as control and/or start-up and shutdown sequencing of individual inverters  222  ( FIG. 2 ). 
     Supervisory user interface  230  provides authorized users both local and/or remote access to a graphical user interface through a web browser client. Supervisory user interface  230  provides the user with the ability to monitor key components of plant  100  ( FIG. 1 ), including plant output at POI  112 , output and status of each power generation site  103 , connectivity of communications network  560 , and the status and output of individual plant devices (such as inverters  222 , combiner boxes  223 , mounting mechanisms  224 , PV panels  225 , and sensors  452  ( FIG. 2 )). 
     Supervisory user interface  230  may be the primary device for communicating with and providing commands to plant-level control system  201 , and is preferably capable of providing a complete visualization of the present capability of plant  100  ( FIG. 1 ), such as voltage, power factor, and power level of plant  100  as measured at POI  112 . Supervisory user interface  230  is also preferably configured to set plant-level control system  201  into different operating modes, including normal operational start up, shut down, in-service and out-of-service, as well as be able to adjust parameters for plant level control functions. 
     Supervisory user interface  230  is also configured to provide access to historical data saved on historical data server  232  (described further below), and reports generated from this historical data, locally at substation  101  ( FIG. 1 ) or operations and maintenance station  102 , and remotely (such as at network operations center  105  or remote center  106 ). In addition, supervisory user interface  230  can provide real time or historical access, either locally or remotely, to one or more key performance indicators of plant  100 , such as overall electrical power output and availability. 
     Supervisory system  202  also includes a real time data server  231 . Real time data server  231  is configured to aggregate real time data, such as operational and other data, from several or all plant devices within plant  100  ( FIG. 1 ) over communications network  560 . Data stored on real time data server  231  is accessible to a user through supervisory user interface  230 . Real time data server  231  communicates to each of the plant devices through communications network  560 , and updates the data at a regular interval (e.g., once every second). Real time data server may also aggregate real time data from POI monitoring device  211  and/or master controller  213  ( FIG. 3 ) through communications network  560 . 
     Plant devices that provide data to real time data server  231  preferably include one or more storage elements, such as a direct-attached storage (“DAS”) element. As discussed above with regard to  FIG. 2 , monitoring and control element  450  in each power generation site  103  may include a storage element  460  to store data from plant devices within the power generation site  103 . Alternatively, individual plant devices (such as in PV panels  225 , inverters  222 , sensors  452 , etc.) may include their own storage elements. 
     Real time data server  231  is configured to process and provide monitoring of different types and quantity of data. For example, real time data server  231  may also be configured to process alarm signals received over communications network  560 , and provide these alarm signals to alarm manager  235  (described further below). 
     Supervisory system  202  also includes a historical data server  232 . Historical data server  232  may be a Structured Query Language (“SQL”) server or similar server that is configured to capture and store all operational data from plant  100  ( FIG. 1 ). Historical data server  232  can also be configured to store different types and quantity of data, such as various alarms and status messages from plant  100  or plant devices. 
     Historical data server  232  runs data collection routines to accumulate data from the real time data server  231  and store the data, at a configurable frequency, into historical data server  232  in a manner accessible for later use, such as in a SQL format. These data collection routines may be defined and modified using the supervisory user interface  230 . The data collection routines may also be configured to derive statistics (such as the average power output by plant  100  over specified intervals). These statistics may be retained on historical data server  232 , and the corresponding real-time data from real time server  231  may be compressed or deleted after the desired statistics are derived, rather than stored on historical data server  232 . Preferably, the data collection routines are regularly performed during a time period where plant activity is at a minimum and thus the server processing load is minimal, such as overnight for a solar power plant. 
     As shown in  FIG. 5 , supervisory system  202  also includes functional elements such as a reporting system  233  including a report processing element  234 , an alarm manager  235 , a remote access security element  236  providing access to a router or modem  250 , a data transfer interface  237 , an external systems data server  238 , a mobile interface  239 , and a tracker system monitoring element  240 . These functional elements may be provided via one or more processors in user interface  230  or elsewhere in supervisory system  202  (for example, in operations and maintenance station  102  ( FIG. 1 )). 
     Reporting system  233  is a web-based system for providing a report generated by data collected by the data collection routines from real time data server  231  and/or historical data server  232 . Reporting system  233  includes a report processing element  234 , which may be a processor system configured to generate a standard report or to accept user specifications in order to configure the reports to the user&#39;s specific needs. For example, if historical data server  232  is a SQL server, third party software tools that are known in the art and widely available may be used to configure the report processing element  234  to generate reports specifically configured to the user&#39;s needs. 
     Alarm manager  235  collects instances of alarms, warnings, and/or diagnostics messages from various plant devices directly through communications network  560  or through real time data server  231 , or both. Alarm manager  235  provides these messages to both local and remote users of supervisory system  202  (for example, through supervisory user interface  230 ) in order to aid troubleshooting and performing of diagnostics. Alarm manager  235  can be configured to classify each alarm into pre-specified configurable priority categories. For example, some alarms can be classified as requiring user acknowledgment. As another example, some alarms can be configured to trigger additional actions, such as remote notification through e-mail or automatic dial-out. 
     Remote access security element  236  provides remote users, such as users at network operations center  105  or remote center  106 , secure access to supervisory system  202  and/or supervisory user interface  230  through a network router or modem connection  250 . Remote access security element  236  includes firewall capability, and provides proxy and/or authentication services for users outside of plant  100  who wish to access to the communications network  560 . Remote access security element  236  may also be configured to maintain an audit trail of authenticated users, their activity while accessing supervisory system  202 , and unsuccessful attempts to access supervisory system  202 . 
     Data transfer interface  237  provides for transfer of a subset of data from the historical data server  232  to a remote database or system, such as to the network operations center  105  or remote center  106  ( FIG. 1 ). Data transfer interface  237  preferably includes capability for configuring and defining the desired subset of data for transfer, and for specifying the time interval for such a transfer. For example, data transfer interface  237  may be configured to automatically create a file of data in a predefined format on a periodic basis, which can be transferred to other centralized locations for further processing. Data transfer interface  237  may also be configured to provide data from real time data server  231  to the supervisory database or system. 
     External systems data server  238  may be a server configured as an object linking and embedding for process control (“OPC”) server. OPC is an industry standard for the communication of real time plant data between control devices from different manufacturers. Accordingly, external systems data server  238  allows external systems to access certain plant data, as permitted by supervisory system  202 . 
     Mobile interface  239  provides access to certain plant data through a mobile device. For example, a user may be permitted, via mobile interface  239 , to view key performance indicators and/or alarms through supervisory user interface  230  on a mobile device. Mobile interface  239  is preferably configured such that it does not provide full plant control or supervisory control functions. In addition, mobile interface  239  preferably includes appropriate security in order to ensure that only authorized users have access to the plant data. 
     Tracker monitoring element  240  collects and makes available information related to the configuration of the mounting mechanisms  224  ( FIG. 2 ) at power generation sites  103 . As discussed above with regard to  FIG. 2 , mounting mechanisms  224 , such as trackers, are used to orient PV panel cells at power generation sites. Tracker monitoring element  240  provides a user of the supervisory user interface  230  access to orientation data and diagnostics for individual mounting mechanisms  224 , which may be collected by real time data server  231  across communications network  560 . 
     The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modification and substitutions to specific processes, architectures, systems, and structures can be made. For example, it should be understood that appropriate components and configurations other than those specifically described in connection with the above embodiments may be used, and that the steps of the processes described above may be performed in a different order than the specific order in which they are described. The described concepts are easily applied to additional types of plant devices, plant control functions, power plants, and electrical generating systems known in the art. Accordingly, the embodiments of the invention are not to be considered as being limited by the foregoing description and drawings, but only by the scope of the appended claims.