Abstract:
A power delivery rate from a renewable power source to a load is managed by determining, by processing circuitry, a change in a power generation rate, determining, by the processing circuitry, whether the change in the power generation rate exceeds a limit, and then, adjusting, by control circuitry, a power transfer rate to or from a power storage device, such that the adjusting is sufficient to prevent the power delivery rate from exceeding the limit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/773,504, filed May 4, 2010, the entire contents of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to managing power delivery, and more particularly to managing power generated from renewable resources. 
       BACKGROUND 
       [0003]    Renewable resources, such as wind, wave, and solar energy, are an attractive alternative to the use of fossil fuels in generating power due to their renewable nature and clean operation. However, unlike thermal power plants (e.g., coal-fired or natural gas fired plants), generally, the amount of wave, wind, or solar energy available at any given time can not be controlled or reliably predicted. Further, due to the inherent variability of these renewable energy sources, (e.g., wind gusts and/or directional changes, weather conditions, etc.), the instantaneous power output of an associated power generator (e.g., a wind turbine) may vary significantly from one second to the next. 
       SUMMARY 
       [0004]    In a first aspect, a power delivery rate from a renewable power source to a load is managed by determining, by processing circuitry, a change in a power generation rate, determining, by the processing circuitry, whether the change in the power generation rate exceeds a limit, and then, adjusting, by control circuitry, a power transfer rate to or from a power storage device, such that the adjusting is sufficient to prevent the power delivery rate from exceeding the limit. Other implementations of this aspect include corresponding systems, apparatus, and computer programs, configured to perform the actions of the methods, encoded on computer storage devices. 
         [0005]    In another aspect, a control system for a bidirectional power device coupled to a dynamic power source converting renewable energy into electrical power includes: a storage device having stored thereon machine-readable instructions specifying a ramp rate control operation; a set of I/O ports configured to receive information regarding the bidirectional power device and the dynamic power source; a processor coupled to the set of I/O ports and the storage device and configured to execute the machine-readable instructions to perform operations including: determining a change in a power generation rate; determining whether the change in the power generation rate exceeds a limit; and then, adjusting a power transfer rate to or from the bidirectional power device, wherein the adjusting is sufficient to prevent the power delivery rate from exceeding the limit. 
         [0006]    These and other embodiments can each optionally include one or more of the following features. Managing the power delivery rate can include determining, by the processing circuitry, a present state-of-charge of the power storage device, and adjusting the limit, by the processing circuitry, based on the present state-of-charge of the power storage device. The limit can include a ramp rate limit associated with increases in the power generation rate and adjusting the limit can include: setting the ramp rate limit to a minimum value if the present state-of-charge is less than a minimum state-of-charge; setting the ramp rate to a maximum value if the present state-of-charge is greater than a maximum state-of-charge; and setting the ramp rate to a value between the minimum value and the maximum value if the present state-of-charge is neither less than the minimum state-of-charge nor greater than the maximum state-of-charge. The limit can include a ramp rate limit associated with decreases in the power generation rate and adjusting the limit can include: setting the ramp rate limit to a minimum value if the present state-of-charge is greater than a maximum state-of-charge; setting the ramp rate to a maximum value if the present state-of-charge is less than a minimum state-of-charge; and setting the ramp rate to a value between the minimum value and the maximum value if the present state-of-charge is neither less than the minimum state-of-charge nor greater than the maximum state-of-charge. The limit can include a first ramp rate associated with increases in the power generation rate and a second ramp rate associated with decreases in the power generation rate, and adjusting the limit can include: setting the first ramp rate to a maximum value and the second ramp rate to a minimum value if the present state-of-charge exceeds a maximum state-of-charge. Adjusting the power transfer rate to or from the power storage device can include: increasing the power transfer rate from the power storage device to match a decrease in the power generation rate in excess of the second ramp rate; and setting the power transfer rate to the power storage device to match the increase in the power generation rate in excess of the first ramp rate. The limit can include a first ramp rate associated with increases in the power generation rate and a second ramp rate associated with decreases in the power generation rate, and adjusting the limit can include: setting the first ramp rate to a minimum value and the second ramp rate to a maximum value if the present state-of-charge falls below a minimum state-of-charge. Adjusting the power transfer rate to or from the power storage device can include: increasing the power transfer rate to the power storage device to match an increase in the power generation rate in excess of the first ramp rate; and setting the power transfer rate from the power storage device to match the decrease in the power generation rate in excess of the second ramp rate. 
         [0007]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  illustrates a wind farm including a power management system. 
           [0009]      FIG. 2  illustrates an exemplar power management system. 
           [0010]      FIG. 3  illustrates an exemplar control system for a power management system. 
           [0011]      FIG. 4  illustrates a graphical user interface. 
           [0012]      FIG. 5  illustrates an exemplar ramp rate bias control function. 
           [0013]      FIG. 6  illustrates an exemplar photovoltaic park including a power management system. 
           [0014]      FIG. 7  illustrates an exemplar wave park including a power management system. 
       
    
    
       [0015]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0016]    Rapid increases in power output can be managed to some degree by manipulating the wind turbine and/or its controls (e.g., yawing or tilting the plane of rotation, varying the blade pitch, using a passive/active stall mechanism, controlling the output of a variable-speed generator, etc.). However, intentionally reducing power output despite the availability of wind energy (i.e., curtailment) decreases the overall energy efficiency of the system. Similarly, preemptively reducing the power output of a wind turbine generator so that a sudden decrease in wind energy appears less abrupt also decreases the overall energy efficiency of the system. Such reductions in power output may be necessary to avoid exceeding a ramp rate limit for delivering power to a load (e.g., a utility grid) and/or for accommodating a power generation schedule based on expected demand. 
         [0017]      FIG. 1  illustrates a wind farm  100  including a power management system (PMS)  110 . As described in more detail below, PMS  110  provides energy storage and management to automatically buffer the output of wind turbine generators (WTGs)  120  to distribution network  160  (e.g., a utility grid). In particular, PMS  110  is operable to minimize or eliminate curtailment, smooth overall power output, limit power ramps, and buffer large wind speed excursions (i.e., wind gusts). In instances where frequent wind gusts cause WTGs  120  to trip or go off-line (i.e. a fault event), PMS  110  is further operable to compensate for the sudden disruption in power output by supplementing the power output to avoid or mitigate a ramp rate violation associated with the negative power ramp. 
         [0018]    The exemplar wind farm configuration illustrated in  FIG. 1  shows PMS  110  coupled to substation  130  through radial feeder  140  of substation main bus  150  (e.g., a 34.5 KV or medium voltage electrical network). WTGs  120  are also coupled to substation main bus  150  through corresponding radial feeders  141 ,  142 . Substation  130  couples PMS  110  and WTGs  120  to distribution network  160  (e.g., a high voltage electrical network) via protective relays  131 ,  134 , AC switchgears  132 ,  135 , and step-up power transformer  133 . Protective relays  131 ,  134  and AC switchgears  132 ,  135  provide a first level of protection from excessively high voltage or current conditions. In some implementations, substation  130  may also include multiple step-up transformers, breakers, relays, current transducers (CT), potential transducers (PT), communication equipment, etc. 
         [0019]    In general, PMS  110  monitors the instantaneous power output from each WTG  120  and adjusts the amount of power delivered to distribution network  160  by storing or supplying power such that the net amount of power delivered to network  160  remains within predetermined limits. In addition, PMS  110  is operable to condition the generated power so as to reduce the variability typically associated with wind generated power (i.e., smoothing). In some implementations, PMS  110  provides a second level of protection to the wind farm components, and/or distribution network  160  and components coupled to the transfer network. For example, in a first implementation, PMS  110  is configured to monitor the quality and characteristics of power being distributed on network  160  and responsive to detecting an out-of-limit condition (e.g., overvoltage, fault, voltage sag, etc.), PMS  110  attempts to compensate by adjusting the transfer of power to distribution network  160 . These and other features are described in further detail below. 
         [0020]    Referring now to  FIG. 2 , an exemplar implementation of PMS  110  includes intertie skid  210  and control system  200  coupled to inverter/charger  220  for controlling the transfer of power to and from battery bank  230  responsive to the control algorithms executed by the control system. Control system  200  is also coupled to protective relays  240  and AC switchgear  250  to monitor fault conditions and alarms. Control system  200  coordinates the operation of the system components, including inverter/charger  220  and battery bank  230 , monitors the operating environment, provides diagnostic capabilities, and manages the overall system operation in response to setup parameters entered via a status and control interface or human-machine interface (HMI). In some implementations, control system  200  includes remote operation terminals for receiving user programmable parameters related to the wind farm power output and for displaying information related to various monitored parameters. The programmable parameters include, for example, limits and targets associated with power smoothing, power storage, target state-of-charge and corresponding limits, etc. 
         [0021]    Intertie skid  210  includes a 34.5 KV to 480/277 V substation transformer  211 , a high voltage fused switch  212 , and a low voltage switchboard  213  and serves to couple the rest of PMS  110  to substation  130  via substation main bus  150 . The 34.5 KV power is provided to intertie skid  210  from a fuse cutout  214  attached to substation main bus  150 . Three #4/0 35 KV shielded cables  215  are protected by the fuse element in fuse cutout  214  and are terminated in a high voltage (HV) fused switch  212 . Fused switch  212  includes station type lightning arrestors on the incoming feed. The fuses in fused switch  212  are sized to protect transformer  211 . The primary of transformer  211  is fed by three #1/0 35 KV shielded cables  217 . The secondary of transformer  211  is connected to low voltage switchboard  213  via fifteen 750 kcmil 600 V cables (5 per phase) and a 300 Amp trip (100% rated) main breaker. 
         [0022]    As illustrated in  FIG. 3 , control system  200  includes supervisory control and data acquisition (SCADA) system  310 , user interface PC (UI-PC)  330 , real-time control processor (RT-PC)  340 , and various controllers and sensors. UI-PC  330  provides a primary user interface to accept user requests, provide warning or error indications, and to receive user programmable control parameters. RT-PC  340  coordinates the remaining elements of PMS  110 . Various control elements are responsible for controlling and monitoring specific system sub-functions. The various control elements are connected via Ethernet network  305 . Each link is monitored for correct operation via the use of semaphores which include “deadman” timers. If a link becomes impaired or fails, the system takes appropriate action, including, for example, shutting down PMS  110  if the control operation is compromised. 
         [0023]    RT-PC  340  controls inverter/charger  220  using the parameters received from the user via UI-PC  330 , data from inverter/charger  220 , and data from the other components, including, for example, current transducers, potential transducers, curtailment signals, etc. A curtailment signal represents a request from the utility operator to curtail power output from the wind farm via a curtailment interface  341  and/or serial interface  342 . For example, curtailment interface  341  is coupled to a 4-20 mA current loop interface to receive a curtailment request. The detected current level at the interface is proportional to the total power output from the wind farm such that a 20 mA signal represents a request for full power output and 4 mA represents a request for full curtailment. RT-PC  340  also receives an Inter-Range Instrumentation Group (IRIG) signal via serial interface  342 . The IRIG signal provides a reliable time reference. 
         [0024]    RT-PC  340  also includes input-output (I/O) modules  343  (e.g., I/O FPGA cards) for receiving currents and potentials from corresponding transducers via optically-isolated signal conditioners (OISC)  344 . I/O modules  343  are coupled together to allow data to be transmitted and received between the modules, and thus, allow them to perform as a single unit. I/O modules  343  are also coupled directly to inverter/charger  220  via fiber optic Ethernet interface  345 . 
         [0025]    Battery bank  230  includes multiple dry cell battery packs connected in a parallel/series configuration to create a single battery bank having a predetermined nominal voltage and Amp-Hour capacity. For example, in some implementations the battery bank includes 72 12-Volt battery packs connected in series to create a battery bank having a nominal voltage of 864 Volts. Each battery pack includes 15 12-Volt dry cell batteries connected in parallel. The batteries are connected in such a way as to ensure that each battery in each battery pack receives a similar or equal voltage at a positive terminal of the respective battery relative to a single reference point. In some implementations, connections are made using precision cabling to provide a uniform DC environment. For example, U.S. patent application Ser. No. 11/549,013, incorporated herein by reference, describes batteries connected in parallel via respective and distinct conductive paths, each conductive path having an under-load resistance differing from an under-load resistance of each other path by less than about 1 milli-ohm. 
         [0026]    Battery bank  230  is monitored by programmable automation controller (PAC)  320 . PAC  320  includes multiple I/O modules  321  coupled to the outputs of signal conditioning boards  323 . Signal conditioning boards  323  provide optical isolation for multiple battery sense points in battery bank  230 . For example, each battery pack (i.e., parallel string of batteries) includes a voltage sensor  322  coupled in parallel with the battery pack. The output of each voltage sensor  322  is coupled to a corresponding one of eight signal conditioning boards  323 , each board having nine or more differential input channels and one or more outputs. PAC  320  monitors battery bank  320  gathering battery data and sending it to RT-PC  340  periodically (e.g., once per second). In some implementations, PAC  320  includes a compact chassis housing a single-board computer, multiple FPGA-based data acquisition modules, serial interfaces, and Ethernet interfaces (e.g., a National Instruments Corp. CompactRIO system). 
         [0027]    Inverter/Charger  220  includes a three-phase sinusoidal pulse-width modulated inverter operating in current-controlled mode to generate three-phase sinusoidal output currents with low total harmonic distortion (THD). Insulated-gate bipolar transistor (IGBT) modules  221  are used as switching devices and are coupled to battery bank  230  via an LF/CF-filter  222  to reduce the ripple current in the DC-source. Inverter/Charger  220  enables the bidirectional transfer of power between battery bank  230  and distribution network  160  via intertie skid  210  and substation  130 . For example, depending upon the wind farm grid status, battery status, and the operating parameters, inverter/charger  230  transfers power between a 480 VAC three phase interface with intertie skid  210  and battery bank  230 . Inverter/Charger control signals are received from Embedded Control and Acquisition Device (ECAD)  350  which is coupled to RT-PC  340  via an Ethernet link. ECAD  350  receives input commands, including, for example, target levels for active (P) and reactive (Q) power, wind farm grid status information, from control points in the grid and intertie skid. ECAD  350  is configured to respond directly to grid disturbances requiring immediate action without any intervention from other components within control system  200  to minimize response time. 
         [0028]    Referring now to  FIG. 4 , an exemplary setup/administration screen  400  of UI-PC  330  is illustrated. This screen enables the user to configure system parameters including, for example, the target percent of storage capacity to use in smoothing  410  and curtailment  420  operations and the maximum rates at which the power flow to the grid is allowed to change during smoothing  430  and excursion  440  control operations. 
         [0029]    The wind smoothing parameters define the operating limits for smoothing operations including threshold rates of change and a percent of storage capacity to use. For example, if 10% is selected for “% Storage,” battery bank  230  will be charged or discharged 5% around the nominal operating point (i.e., target state-of-charge) to provide smoothing operations. Further, if the “Smoothing Rate” parameter is set to 0.1 MW/min, the system will not attempt to smooth transitions which occur below this rate. The ramp control parameters define the maximum rate at which the net power output to distribution network  160  is allowed to change in any direction. In some implementations, a single value sets both positive (increasing output) and negative (decreasing output) ramp rate limits. As described in more detail below, the wind smoothing and ramp control algorithms in some implementations include control mechanisms to keep the batteries in the normal operating range, limiting the smoothing and excursion operations as the battery state of charge nears normal operating limits (including, for example, maximum charge capacity and/or maximum depletion). 
         [0030]    In addition to smoothing and ramp rate parameters, screen  400 , in some implementations, enables the user to configure a curtailment capture parameter  420  to set the percent storage capacity to be used for storing power that would otherwise be curtailed by the wind turbine generators. For example, setting the capture parameter  420  to 10% reserves 10% of battery bank capacity to store energy in response to curtailment requests from a utility operator or utility grid control system. Power that would otherwise be curtailed is stored by PMS  110  within predetermined operating parameters (i.e., maximum capacity and current battery state-of-charge). 
         [0031]      FIG. 5  illustrates an exemplar implementation of PMS  110  operating under a first set of conditions. For example, PMS  110  is configured to buffer wind power variability by providing a steady output of power at rates changing no more than a maximum allowable ramp rate for excursion control (e.g., sudden ramps in power due to, for example, wind gusts) and a smoothing ramp rate during smoothing control, thus improving output reliability while enabling more effective grid management and creating more easily dispatchable power. In this implementation, the algorithms implemented by control system  200  are based on parameters that represent the second to second power output of the wind farm. Other time scales may be used for power sources having more or less variability or for coarser control of power output. 
         [0032]    The WTG parameter represents the total wind turbine output (WTG.sub.1+WTG.sub.2+ . . . +WTG.sub.n) at time t (seconds). The upward-ramp-rate (UpRR) and the downward-ramp-rate (DownRR) parameters represent the maximum allowable rate of change in power output (e.g., KW/sec) from wind farm  100 . In some implementations, the UpRR and DownRR values are fixed (e.g., for excursion mitigation without smoothing or for constant smoothing). In other implementations, such as the present example, the UpRR and DownRR values are variable between a minimum (e.g., zero, a smoothing limit (SmthRR), a percentage of the maximum, etc.) and a maximum (e.g., a maximum input rate of PMS  110 , an Excursion ramp rate limit (ExcRR), a percentage thereof, etc.) and depend on the available capacity of PMS  110 . The XP parameter represents the amount of power required (in or out) from PMS  110  to mitigate UpRR or DownRR violations. The SystemOut parameter represents the sum of WTG and XP at time t (WTG.sub.t+XP.sub.t). 
         [0033]    The DeltaP parameter represents the difference between WTG at time t and SystemOut at time t−1 (WTG.sub.t−SystemOut.sub.t−1). DeltaP can also be understood to represent the potential net change in SystemOut assuming PMS  110  stopped contributing at time t (i.e., DeltaP.sub.t=WTG.sub.t−WTG.sub.t−1−XP.sub.t−1). A negative DeltaP indicates a potential decrease in system output and a positive DeltaP indicates a potential increase in system output. If the potential increase/decrease in system output would not violate either UpRR or DownRR, no contribution by XP is necessary at time t. However, if |DeltaP| is greater than UpRR or |DownRR|, PMS  110  will contribute by absorbing or providing the difference in magnitude to avoid or mitigate ramp rate violations and/or to smooth power output, depending on available system capacity. 
         [0034]    In some implementations, the maximum amount of power absorbed or supplied by PMS  110  is gradually reduced as battery bank  230  approaches a maximum state of charge or minimum state of charge. In such a case, XP is limited to the lesser of the scaled maximum output/input and the required contribution to avoid the ramp rate violation (i.e., |XP|=MIN(|ScaledPowerLimit|, |DeltaP−Up/DownRR|)). Such an approach may be useful, for example, to mitigate the ramp rate violations over a longer period of time than would otherwise be possible due to capacity limitations and/or to extend the useful life of PMS  110 . 
         [0035]    The XP_Energy parameter represents the amount of energy required to be transferred to/from PMS  110  at time t to absorb/supply XP. In some implementations, XP_Energy is determined using trapezoidal integration to find the area under the curve: [XP.sub.t−1+XP.sub.t]/2*( 1/3600). Finally, the SOC.sub.t parameter represents the state of charge of battery bank  230  at time t (SOC.sub.t−1−(XP_Energy/1000*SystemSize), where SystemSize represents the capacity of battery bank  230  in MWh). 
         [0036]    The following pseudo-code illustrates an example algorithm for controlling the amount of power (XP) absorbed or supplied by PMS  110 . Other pseudo-code, languages, operations, orders of operations, and/or numbers may be used. 
         [0000]    
       
         
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
               
                 PMS Power Transfer Control Logic 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 XP = 
                 power required from XP system at time t (positive = sourcing; negative = 
               
               
                   
                 absorbing) to maintain UpRR or DownRR 
               
               
                 DeltaP = 
                 Difference of total system output from time t − 1 and total turbine output at time t 
               
               
                   
                 (positive indicates a potential net increase in total system output if XP contribution = 
               
               
                   
                 0; negative indicates a potential net decrease in total system output if XP 
               
               
                   
                 contribution = 0) 
               
               
                 UpRR = 
                 Up ramp rate limit (function of SOC t−1 ) 
               
               
                 DownRR = 
                 Down ramp rate limit (function of SOC t−1 ) 
               
               
                 SystemMax = 
                 maximum power input/output for XP system 
               
               
                 Power_In = 
                 Scaling factor for maximum power input 
               
               
                 Power_Out = 
                 Scaling factor for maximum power output 
               
               
                 SOCt = 
                 State of Charge at time t 
               
               
                 SOC_Max = 
                 Maximum allowable state of charge 
               
               
                 SOC_Min = 
                 Minimum allowable state of charge 
               
               
                 Rech_DB = 
                 value used to set the upper limit to begin scaling power input 
               
               
                 Disch_DB = 
                 value used to set the lower limit to begin scaling power output 
               
               
                 Rech_Exp = 
                 exponent used to define curve for allowable power input after SOC t  exceeds 
               
               
                   
                 Rech_DB 
               
               
                 Disch_Exp = 
                 exponent used to define curve for allowable power output after SOC t  exceeds 
               
               
                   
                 Disch_DB 
               
             
          
           
               
                 IF DeltaP &gt; UpRR THEN 
               
             
          
           
               
                   
                 IF SOC t  &gt; SOC_Max THEN 
               
             
          
           
               
                   
                 Power_In = 0 
               
             
          
           
               
                   
                 ELSEIF SOC t  &lt; Rech_DB THEN 
               
             
          
           
               
                   
                 Power_In = 1 
               
             
          
           
               
                   
                 ELSE 
               
             
          
           
               
                   
                 Power_In = 1− [(SOC t  − Rech_DB)/(SOC_Max − Rech_DB)]{circumflex over ( )}Rech_Exp 
               
             
          
           
               
                   
                 ENDIF 
               
               
                   
                 IF (DeltaP − UpRR) &gt; Power_In*SystemMax THEN 
               
             
          
           
               
                   
                 XP = − Power_In*SystemMax 
               
             
          
           
               
                   
                 ELSE 
               
             
          
           
               
                   
                 XP = −(DeltaP − UpRR) 
               
             
          
           
               
                   
                 ENDIF 
               
             
          
           
               
                 ELSEIF DeltaP &lt; DownRR THEN 
               
             
          
           
               
                   
                 IF SOC t  &lt; SOC_Min THEN 
               
             
          
           
               
                   
                 Power_Out = 0 
               
             
          
           
               
                   
                 ELSEIF SOC t  &gt; Disch_DB THEN 
               
             
          
           
               
                   
                 Power_Out = 1 
               
             
          
           
               
                   
                 ELSE 
               
             
          
           
               
                   
                 Power_Out = 1 − [(Disch_DB − SOC t )/(Disch_DB − SOC_Min)]{circumflex over ( )}Disch_Exp 
               
             
          
           
               
                   
                 ENDIF 
               
               
                   
                 IF (DownRR − DeltaP) &gt; Power_Out*SystemMax THEN 
               
             
          
           
               
                   
                 XP = Power_Out*SystemMax 
               
             
          
           
               
                   
                 ELSE 
               
             
          
           
               
                   
                 XP = (DownRR − DeltaP) 
               
             
          
           
               
                   
                 ENDIF 
               
             
          
           
               
                 ELSE 
               
             
          
           
               
                   
                 XP = 0 
               
             
          
           
               
                 ENDIF 
               
               
                   
               
             
          
         
       
     
         [0037]    Thus, if, without contribution from PMS  110 , the net change in system output from time t−1 to time t would be greater than the up ramp rate limit, PMS  110  absorbs (i.e., negative XP value): (i) nothing if SOC.sub.t is greater than the maximum allowable state of charge (i.e., XP=Power_In*SystemMax=0 since DeltaP−UpRR would be greater than zero) and the ramp rate violation is allowed to occur; (ii) the required amount to prevent a violation, up to the system maximum if SOC.sub.t is less than the set point for scaling down power input; or (iii) the required amount to prevent a violation, up to the scaled system maximum (i.e., Power_In*SystemMax). 
         [0038]    Further, if, without contribution from PMS  110 , the net change in system output from time t−1 to time t would be less than the down ramp rate limit (i.e., exceeding a negative rate of change limit), PMS  110  provides (i.e., positive XP value): (i) nothing if SOC.sub.t is less than the minimum allowable state of charge (i.e., XP=Power_Out*SystemMax=0 since DownRR−DeltaP would be greater than zero); (ii) the required amount to prevent a violation, up to the system maximum if SOC.sub.t is greater than the set point for scaling down power output; or (iii) the required amount to prevent a violation, up to the scaled system maximum (i.e., Power_Out*SystemMax). 
         [0039]    As described above, in some implementations, the values for UpRR and DownRR depend on the state-of-charge (SOC) of the battery bank at time t.  FIG. 5  illustrates an exemplary ramp rate control chart  500  for adjusting UpRR and DownRR according to the current SOC of the battery bank (e.g., battery bank  230  of  FIG. 2 ). In this example, a target SOC value  530  (e.g., 50%) serves as a reference point for the UpRR and DownRR control algorithms. Deadband limits define an area or band where no change is made to the corresponding values (e.g., UpRR  510  and/or DownRR  520 ). Transition limits define the upper and/or lower bounds beyond which the corresponding limit is set to the MinRR or MaxRR value. The ramp rate control algorithms attempt to keep the current SOC within +/−DB of the target SOC by adjusting UpRR and DownRR to increase charging/discharging of the battery bank. 
         [0040]    Referring first to positive rates of change in power output from the renewable energy source, UpRR  510  is assigned a value between a minimum ramp rate (MinRR) 511 (e.g., 0% of the maximum desired ramp rate), a secondary ramp rate (SecRR)  512  (e.g., 10% of the maximum desired ramp rate), and a maximum ramp rate  513  (MaxRR) (e.g., 100% of the maximum desired ramp rate) based on the current SOC value. Setting MinRR, SecRR, and MaxRR to pre-programmed percentages of the maximum desired ramp rate allows the ramp rates to be automatically defined based on a single value (e.g., an excursion ramp rate limit, a desired smoothing ramp rate, etc.). 
         [0041]    In some implementations, the percentage settings for each of the ramp rates (MinRR, SecRR, MaxRR) and/or the ramp rate values themselves may be entered directly, providing more advanced control. Further, in some implementations, target SOC  530 , and the SOC limits associated with the corresponding ramp rate limits (e.g., UpRR and/or DownRR) are individually configured for up ramp rates and for down ramp rates to provide for additional customization. For example, ramp rate controls and/or limits may be implemented to mitigate only one type of ramp rate violation, such as, for example, an up ramp rate. Such implementations may include an additional PMS  110 , battery bank  230 , or alternate power source, for example, to supplement power output during decreases in WTG total power output. 
         [0042]    Referring to UpRR  510  in  FIG. 5 , when the current SOC is exactly equal to target SOC  530 , UpRR is equal to SecRR  512 . In this example, UpRR deadband limits  532  and  533  are −5% and 0% of target SOC  530 , respectively. Therefore, while the current SOC remains within this range, UpRR remains equal to SecRR  512 . Beyond this range, UpRR  510  transitions to MinRR  511  or MaxRR  513  depending on the current SOC. For example, if the current SOC drifts below lower DB limit  532 , UpRR  510  will be set to a value between SecRR  512  and MinRR  511 . As a result, PMS  110  will absorb a larger portion of any positive increases in generated power to increase the current SOC. Once the current SOC drifts below lower transition limit  531 , UpRR  510  is set to MinRR  511 . In this example, MinRR  511  is equal to 0% of the allowable ramp rate limit which allows any positive increase in generated power to be redirected to or absorbed by battery bank  230 , increasing the current SOC and resulting in no net increase in power output to the load. 
         [0043]    If, however, the current SOC drifts beyond the upper DB limit  533  (which is also the target SOC  530  in this example), UpRR  510  will be set to a value between SecRR  512  and MaxRR  513 . As a result, PMS  110  will absorb less charge during any positive increases in generated power to slow the increase in the current SOC. Consequently, greater increases in generated power or up ramp rates will be seen by the load. Once the current SOC drifts past upper transition limit  534 , UpRR  510  is set to MaxRR  513 . In this example, MaxRR  513  is set to 100% of the allowable ramp rate limit. Some implementations include additional upper DB limits  533  and/or transition limits  534 . For example, in some implementations, MaxRR  513  is set to a value between SecRR and 100% of the allowable ramp rate limit when the current SOC drifts past the first upper transition limit  534 . Once the current SOC drifts past a second upper transition limit  534  (not shown), MaxRR is set to 100% of the allowable ramp rate. In this way, the UpRR control algorithm provides for multiple levels of SOC control and/or ramp rate control. 
         [0044]    Referring now to DownRR  520  in  FIG. 5 , when the current SOC is exactly equal to target SOC  530 , DownRR is equal to SecRR  522 . In this example, DownRR deadband limits  537  and  536  are 0% and 4% of target SOC, respectively. Therefore, while the current SOC remains within this range, DownRR remains equal to SecRR  522 . Beyond this range, DownRR  520  transitions to MinRR  521  or MaxRR  523  depending on the current SOC. For example, if the current SOC drifts beyond the upper DB limit  536 , DownRR  520  will be set to a value between SecRR  522  and MinRR  521 . As a result, PMS  110  will provide (i.e., discharge) more and more supplemental power to decrease the current SOC by limiting any negative change in power delivered to the load. Once the current SOC drifts past upper transition limit  535 , DownRR  520  is set to MinRR  520 . In this example, MinRR  520  is equal to 0% of the allowable ramp rate limit which allows any decrease in generated power output to be supplied by battery bank  230 , decreasing the current SOC and resulting in no net decrease in power output to the load. 
         [0045]    If, however, the current SOC drifts below lower DB limit  537 , DownRR  520  will be set to a value between SecRR  522  and MaxRR  523 . Consequently, PMS  110  will allow greater negative ramp rates to be seen by the load as the current SOC continues to decline. Once the current SOC drifts below lower transition limit  538 , DownRR  520  is set to MaxRR  523 . In this example, MaxRR  523  is set to 100% of the allowable ramp rate limit. Some implementations include additional lower DB limits  538  and/or transition limits  538 . For example, in some implementations, MaxRR  523  is set to a value between SecRR and 100% of the allowable ramp rate limit when the current SOC drifts past the first lower transition limit  538 . Once the current SOC drifts past a second transition limit  538  (not shown), MaxRR  523  is set to 100% of the allowable ramp rate. In this way, the DownRR control algorithm provides for multiple levels of SOC control and/or ramp rate control. 
         [0046]    The various combinations of ramp rate limits and SOC limits allow PMS  110  to maximize charge/discharge in the direction that will aggressively push the SOC of battery bank  230  back towards the target SOC while mitigating any ramp rate violations. Further, the UpRR and DownRR control algorithms effectively help maintain system stability and prevent large depth of discharge cycles. Additionally, the probability of violating a ramp rate limit and the severity of any ramp rate violations are greatly reduced assuming PMS  110  is appropriately sized based on the power generation capability of the power source and the associated variability. 
         [0047]    In some implementations, the non-transitioning ramp rate is set to zero before the transitioning ramp rate reaches MaxRR. This provides more aggressive control of SOC by maintaining a constant power output during any change in the total generated output opposite the transitioning direction. For example, if UpRR is transitioning towards MaxRR (i.e., current SOC is increasing), DownRR is set to zero such that any decrease in generated power (e.g., WTG) is immediately supplemented by power from PMS  110  (effectively decreasing SOC). Similarly, if DownRR is transitioning towards MaxRR (i.e., current SOC is decreasing), UpRR is set to zero such that any increase in generated power is transferred to PMS  110  (effectively increasing SOC). 
         [0048]    The following pseudo-code illustrates another example algorithm for up and down ramp rate control. In this example, the upper DB limit for UpRR is given by SOCTgt+DB and the lower DB limit for DownRR is given by SOCTgt−DB. The lower DB limit for UpRR and the upper DB limit for DownRR are both equal to the target SOC. In addition, MaxRR, SecRR, and MinRR limits are applied to both UpRR and DownRR with corresponding sign notations as appropriate. Determinations are made based on the state of charge at time t−1 rather than the current state of charge so that the results for UpRR and DownRR at time t can be fed forward to the PMS Power Transfer Control Logic described above. Other pseudo-code, languages, operations, orders of operations, and/or numbers may be used. 
         [0000]    
       
         
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                   
               
               
                 Up and Down Ramp Rate Control Logic 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SOCTgt = Target SOC 
               
               
                 SOCt − 1 = SOC at previous second or t − 1 
               
               
                 DB = deadband limit 
               
               
                 UpRR = ramp rate limit applied when the power output from the wind farm is increasing 
               
               
                 DownRR = ramp rate limit applied when the power output from the wind farm is decreasing 
               
               
                 MaxRR = ramp rate applied if SOC passes outside DB limit 
               
               
                 SecRR = ramp rate used if SOC within DB limit 
               
               
                 DroopGain = gain used when SOC is between SOCTgt and DB; equal to (MaxRR − SecRR)/DB 
               
               
                 IF SOC t−1  &lt; (SOC Tgt  − DB) THEN 
               
             
          
           
               
                   
                 DownRR = −MaxRR 
               
               
                   
                 UpRR = 0 
               
             
          
           
               
                 ELSEIF (SOC Tgt  − DB) &lt; SOC t−1  &lt; SOC Tgt  THEN 
               
             
          
           
               
                   
                 DownRR = −(SecRR + (SOC Tgt  − SOC)*DroopGain) 
               
               
                   
                 UpRR = SecRR 
               
             
          
           
               
                 ELSEIF SOC Tgt  &lt; SOC t−1  &lt; (SOC Tgt  + DB) THEN 
               
             
          
           
               
                   
                 DownRR = −SecRR 
               
               
                   
                 UpRR = SecRR + (SOC t−1  − SOC Tgt )*DroopGain 
               
             
          
           
               
                 ELSEIF (SOC Tgt  + DB) &lt; SOC t−1  THEN 
               
             
          
           
               
                   
                 DownRR = 0 
               
               
                   
                 UpRR = MaxRR 
               
             
          
           
               
                 ENDIF 
               
               
                   
               
             
          
         
       
     
         [0049]    Additional or fewer ramp rate limits are used in different implementations, depending on the intended purpose and configuration of PMS  110  and/or the renewable power source serviced by PMS  110 . For example, in at least one implementation, MinRR corresponds to 5% of an excursion limit (ExcRR), SecRR corresponds to 10% of ExcRR, and MaxRR corresponds to 70% of ExcRR. UpRR and DownRR are stepped up or down to equal the the appropriate ramp rate limit based on the SOC at time t−1. The table below provides an exemplary algorithm for assigning UpRR and DownRR based on the SOC at time t−1, the target SOC, and deadband limits +/−DB1 and +/−DB2. 
         [0000]    
       
         
               
               
               
             
           
               
                   
               
               
                 SOC Region 
                 DownRR 
                 UpRR 
               
               
                   
               
             
             
               
                 SOC t−1  &lt; SOC Tgt  − DB2 
                 −ExcRR 
                 0 
               
               
                 SOC Tgt  − DB2 ≦ SOC &lt; SOC Tgt  − DB1 
                 −SmthRR 
                 MinRR 
               
               
                 SOC Tgt  − DB1 ≦ SOC ≦ SOC Tgt  + DB1 
                 −SecRR 
                 SecRR 
               
               
                 SOC Tgt  + DB1 &lt; SOC ≦ SOC Tgt  + DB2 
                 −MinRR 
                 SmthRR 
               
               
                 SOC t−1  &gt; SOC Tgt  + DB2 
                 0 
                 ExcRR 
               
               
                   
               
             
          
         
       
     
         [0050]    In other implementations, the ramp rates are individually assigned a value and transition regions are defined to smooth the ramp rate transition from a first value to the next. In addition, some implementations include logic and/or routines for handling certain types of events. For example, frequent and/or severe wind gusts may cause one or more wind turbine generators to trip or go offline to avoid component damage. This event is recognized as a fault event to which PMS  110  responds by providing sufficient power to maintain the current operation (e.g., smoothing and/or ramp rate control). A determination may be made that normal operation will resume momentarily based on information, such as, for example, average sustained wind speeds, frequency of wind gusts, expected changes in weather, and other meteorological data). Based on the determination, the normal ramp rate control algorithm may be suspended allowing the current SOC to drop below the deadband limit without decreasing the power provided. In some implementations, the target SOC is adjusted temporarily according to the weather conditions. 
         [0051]    In some implementations, PMS  110  is configured to generate a curtailment signal based on the current SOC of battery bank  230 . For example, in addition to limiting the amount of power absorbed when SOC t-1 &gt;SOC Tgt +DB2, PMS  110  generates a curtailment signal which when received by WTGs  120  causes the WTGs to implement curtailment measures, such as, e.g., yawing or tilting the plane of rotation, varying the blade pitch, etc., further reducing the probability of an UpRR violation. This may be useful, for example, for re-enabling or maintaining power smoothing operations during periods of frequent excursions. 
         [0052]    As described above, in addition to ramp control and smoothing operations, PMS  110  also provides the ability to capture curtailed wind power in order to increase operating efficiency and overall wind farm capacity. For example, during low demand periods (typically late at night and/or early in the morning), the utility may constrain the output of the wind turbine generators to balance the grid supply with demand. Depending on the value of the curtailment signal and the strength of the wind, the operating efficiency of the wind farm can be significantly reduced during curtailment periods. PMS  110  is operable to absorb the excess capacity without modifying any curtailment mechanisms that may already be in place. 
         [0053]    For example, WTGs  120  and PMS  110  are each configured to detect when the curtailment signal value decreases below the wind farm&#39;s potential output. Responsive to the detection, WTGs  120  immediately adjust to reduce the net output of the wind farm to a value below or equal to the curtailment value. Once PMS  110  determines the net output of the wind farm is equal to the curtailment value, it begins to absorb power from the wind farm at a user programmable rate (e.g., 600 kW/min or 10 kW/sec) slightly reducing the total output of the wind farm. If additional wind energy is available, WTGs  120  increase net power output until the curtailment level is reached once again. During this time the power absorbed by PMS  110  remains constant. The process repeats as long as there is excess wind power to be gathered and the curtailment signal value is less than the wind farm&#39;s potential output (based on current wind speeds). If, during the curtailment period, the wind power suddenly decreases below the curtailment signal value, PMS  110  stops absorbing power and immediately begins supplying power to maintain a net output having a rate of change less than or equal to the maximum ramp rate (e.g., −ExcRR). 
         [0054]    Each time the process is repeated, the amount of power absorbed by PMS  110  (PAbsorbed) increases and the excess amount of available wind power (PAvailable) decreases. The “potential wind power” (PPotential) is equal to the power that could be generated by the wind farm if there were no curtailment restrictions and no power was absorbed by PMS  110 . PLimit represents the curtailment signal value. Thus, PAvailable=PPotential−PLimit−PAbsorbed. Once PAvailable is equal to zero, no additional power is available for PMS  110  to absorb. If PAvailable becomes less than zero, PMS  110  stops absorbing power and immediately begins supplying power to maintain a net output having a rate of change less than or equal to the maximum ramp rate. 
         [0055]    User programmable system parameters set the percentage of the storage capacity to be dedicated to capture curtailed wind power during certain periods of the day, week, year, etc., and the percentage of the storage capacity to be dedicated for smoothing and excursion control. When the storage capacity allocated for curtailment is full, PMS  110  will continue smoothing and excursion control. PMS  110  will release the energy stored during curtailment at the first available opportunity at the maximum allowable rate. The opportunity to release energy to the grid when not in curtailment (i.e., PLimit=PPotential) is determined by comparing PPotential with the total capacity of the wind farm. 
         [0056]      FIGS. 6 and 7  illustrate exemplary implementations power management systems  610 ,  710  (e.g., PMS  110  described above) for providing excursion, smoothing, and curtailment control/operations for photovoltaic (PV) parks including PV panels  621  in PV array  620  and for wave power parks including power generators  721  in wave power array  720 , respectively. 
         [0057]    In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiments of the invention. It will be apparent however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. The particular embodiments described are not provided to limit the invention but to illustrate embodiments of the invention. The scope of the invention is not to be determined by the specific examples provided above but only by the claims below. In other instances, well-known circuits, structures, devices, and operations have been shown in block diagram form or without detail in order to avoid obscuring the understanding of the description. Where considered appropriate, reference numerals or terminal portions of reference numerals have been repeated among the figures to indicate corresponding or analogous elements, which may optionally have similar characteristics. 
         [0058]    Certain operations may be performed by hardware components, or may be embodied in machine-executable instructions, that may be used to cause, or at least result in, a circuit or hardware programmed with the instructions performing the operations. The circuit may include a general-purpose or special-purpose processor, or logic circuit, to name just a few examples. The operations may also optionally be performed by a combination of hardware and software. 
         [0059]    One or more embodiments include an article of manufacture that includes a tangible machine-accessible and/or machine-readable medium having stored thereon instructions, that if executed by a machine (e.g., an execution unit) causes the machine to perform the operations described herein. The tangible medium may include one or more solid materials. The medium may include, a mechanism that provides, for example stores, information in a form that is accessible by the machine. For example, the medium may optionally include recordable mediums, such as, for example, floppy diskette, optical storage medium, optical disk, CD-ROM, magnetic disk, magneto-optical disk, read only memory (ROM), programmable ROM (PROM), erasable-and-programmable ROM (EPROM), electrically-erasable-and-programmable ROM (EEPROM), random access memory (RAM), static-RAM (SRAM), dynamic-RAM (DRAM), Flash memory, and combinations thereof. Still other embodiments pertain to a computer system, embedded system, or other electronic device having an execution unit configured to perform one or more of the operations disclosed herein. 
         [0060]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, solar and/or geothermal energy may be used instead of or in addition to wind energy to provide renewable energy. Further, the capacity, measurement resolution, response time, and limits described above are merely exemplar values. Accordingly, other embodiments are within the scope of the following claims.