Abstract:
A method and apparatus for determining a corrected monitoring voltage, at least a portion of the method being performed by a computing system comprising at least one processor. The method comprises generating power at a first location; monitoring the generated power by measuring a first voltage proximate the first location; measuring a second voltage proximate a second location, the first and the second locations electrically coupled; and determining, based on the measured second voltage, a corrected monitoring voltage to compensate the measured first voltage for a distance between the first and the second locations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. provisional patent application Ser. No. 61/206,891, filed Feb. 5, 2009, which is herein incorporated in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present disclosure generally relate to power systems and, more particularly, to a method and apparatus for determining a corrected monitoring voltage. 
         [0004]    2. Description of the Related Art 
         [0005]    Solar panels have historically been deployed in mostly remote applications, such as remote cabins in the wilderness or satellites, where commercial power was not available. Due to the high cost of installation, solar panels were not an economical choice for generating power unless no other power options were available. However, the worldwide growth of energy demand is leading to a durable increase in energy cost. In addition, it is now well established that the fossil energy reserves currently being used to generate electricity are rapidly being depleted. These growing impediments to conventional commercial power generation make solar panels a more attractive option to pursue. 
         [0006]    Solar panels, or photovoltaic (PV) modules, convert energy from sunlight received into direct current (DC). The PV modules cannot store the electrical energy they produce, so the energy must either be dispersed to an energy storage system, such as a battery or pumped hydroelectricity storage, or dispersed by a load. One option to use the energy produced is to employ one or more inverters to convert the DC current into an alternating current (AC) and couple the AC current to the commercial power grid. The power produced by such a distributed generation (DG) system can then be sold to the commercial power company. 
         [0007]    In order to mitigate potential safety hazards, a DG coupled to a commercial power grid must be operated in accordance with relevant regulatory requirements, such as IEEE-1547. As part of meeting the IEEE-1547 requirements, an inverter within a DG must be deactivated under certain circumstances, including line frequency or line voltage operating outside of pre-defined limits. The IEEE-1547 standard specifies that such voltage requirements must be met at a Point of Common Coupling (PCC) between the commercial power system and the DG (i.e., a point of demarcation between the public utility service and the DG). 
         [0008]    For installations where an inverter within a DG is located a significant distance from the PCC, an output voltage measured at the inverter may be higher than a voltage measured at the PCC due a voltage drop along the line from the inverter to the PCC. In some circumstances, the measured voltage at the inverter may exceed the required voltage range although the voltage at the PCC remains within the required range, resulting in the inverter unnecessarily shutting down and thereby reducing energy production. Additionally, as the inverter ceases power production and the voltage at the inverter is reduced to acceptable levels, the inverter once again activates and begins producing power, resulting in a continued oscillation that negatively impacts power production. 
         [0009]    Therefore, there is a need for a method and apparatus for correcting a monitoring voltage measured at an inverter. 
       SUMMARY OF THE INVENTION 
       [0010]    Embodiments of the present invention generally relate to a method and apparatus for determining a corrected monitoring voltage, at least a portion of the method being performed by a computing system comprising at least one processor. The method comprises generating power at a first location; monitoring the generated power by measuring a first voltage proximate the first location; measuring a second voltage proximate a second location, the first and the second locations electrically coupled; and determining, based on the measured second voltage, a corrected monitoring voltage to compensate the measured first voltage for a distance between the first and the second locations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  is a block diagram of a system for distributed generation (DG) in accordance with one or more embodiments of the present invention; 
           [0013]      FIG. 2  is a block diagram of a control module in accordance with one or more embodiments of the present invention; 
           [0014]      FIG. 3  is a block diagram of an inverter in accordance with one or more embodiments of the present invention; and 
           [0015]      FIG. 4  is a flow diagram of a method for determining a corrected monitoring voltage in accordance with one or more embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a block diagram of a system  100  for distributed generation (DG) in accordance with one or more embodiments of the present invention. This diagram only portrays one variation of the myriad of possible system configurations. The present invention can function in a variety of distributed power generation environments and systems. 
         [0017]    The system  100  comprises a plurality of inverters  102   1 ,  102   2  . . .  102   n , collectively referred to as inverters  102 , a plurality of PV modules  104   1 ,  104   2  . . .  104   n , collectively referred to as PV modules  104 , an AC bus  106 , and a load center  108 . 
         [0018]    Each inverter  102   1 ,  102   2  . . .  102   n  is coupled to a PV module  104   1 ,  104   2  . . .  104   n , respectively. In some embodiments, a DC-DC converter may be coupled between each PV module  104  and each inverter  102  (e.g., one converter per PV module  104 ). Alternatively, multiple PV modules  104  may be coupled to a single inverter  102  (i.e., a centralized inverter); in some such embodiments, a DC-DC converter may be coupled between the PV modules  104  and the centralized inverter. 
         [0019]    The inverters  102  are coupled to the AC bus  106 , which in turn is coupled to the load center  108 . The load center  108  houses connections between incoming power lines from a commercial power grid distribution system and the AC bus  106 , and represents a Point of Common Coupling (PCC) between the system  100  and the commercial power grid. The inverters  102  convert DC power generated by the PV modules  104  into AC power, and meter out AC current that is in-phase with the AC commercial power grid voltage. The system  100  couples the generated AC power to the commercial power grid via the load center  108 . Additionally or alternatively, the generated power may be coupled to appliances, and/or energy generated may be stored for later use; for example, the generated energy may be stored utilizing batteries, heated water, hydro pumping, H 2 O-to-hydrogen conversion, or the like. In some alternative embodiments, the system  100  may comprise other types of renewable energy generators in addition to or in place of the inverters  102 , such as wind turbines, hydroelectric systems, or the like. 
         [0020]    The system  100  further comprises a control module  110  coupled to the AC bus  106 . The control module  110  is capable of issuing command and control signals to the inverters  102  in order to control the functionality of the inverters  102 . 
         [0021]    In accordance with one or more embodiments of the present invention, each of the inverters  102  applies voltage compensation to locally measured voltages (i.e., voltages measured at the inverter  102 ) when determining a monitored voltage for comparison to relevant voltage regulatory requirements. Such voltage compensation corrects for a voltage drop that occurs along the AC bus  106  between the inverters  102  and the PCC and allows the inverters  102  to determine monitoring voltage levels with respect to the PCC (i.e., corrected monitoring voltage levels) for ensuring compliance with the relevant voltage regulatory requirements. In the event that a corrected monitoring voltage exceeds required limits, the corresponding inverter  102  may be deactivated or, alternatively, AC voltage regulation may be performed. 
         [0022]    In some embodiments, the control module  110  may receive one or more voltage samples (i.e., measurements) indicating a voltage proximate (i.e., at or near) the PCC. The control module  110  may then broadcast these PCC voltage samples V pcc  to one or more inverters  102  for determining the corresponding corrected monitoring voltage as described further below. The PCC voltage samples V pcc  may be obtained by a measurement unit  112  deployed at or near the load center  108 ; in some embodiments, the measurement unit  112  and the control module  110  may be a single integrated unit. The measurement unit  112  may sample the voltage proximate the PCC, for example, utilizing an analog to digital (A/D) converter, and communicate the PCC voltage samples V pcc  to the control module  110  for broadcast to the inverters  102 . The measurement unit  112  may convert the voltage samples to a root mean square (RMS) value prior to transmission to the controller  110 ; alternatively, the controller  110  or the inverters  102  may perform such conversion. One example of such a measurement unit may be found in commonly assigned U.S. patent application Ser. No. 12/657,447 entitled “Method and Apparatus for Characterizing a Circuit Coupled to an AC Line” and filed Jan. 21, 2010, which is herein incorporated in its entirety by reference. 
         [0023]    In some embodiments, the measurement unit  112  may communicate the PCC voltage samples V pcc  to the control module  110  utilizing power line communication (PLC), and the control module  110  may then broadcast the PCC voltage samples V pcc  to the inverters  102  utilizing PLC; alternatively, other wired and/or wireless communication techniques may be utilized. In one or more alternative embodiments, the PCC voltage samples V pcc  may be obtained by the measurement unit  112  and communicated directly (i.e., without the use of the controller  110 ) to one or more inverters  102  utilizing any of the communications techniques previously mentioned. 
         [0024]    In some alternative embodiments, the control module  110  may determine the corrected monitoring voltage for one of more of the inverters  102 , determine whether each corrected monitoring voltage is within required limits, and/or initiate deactivation of one or more inverters  102  for which the corrected monitoring voltage levels exceed required limits. 
         [0025]      FIG. 2  is a block diagram of a control module  110  in accordance with one or more embodiments of the present invention. The control module  110  comprises a communications transceiver  202  coupled to at least one central processing unit (CPU)  204 . The CPU  204  is additionally coupled to support circuits  206 , and a memory  208 . The CPU  204  may comprise one or more conventionally available microprocessors. Alternatively, the CPU  204  may include one or more application specific integrated circuits (ASIC). The support circuits  206  are well known circuits used to promote functionality of the central processing unit. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, network cards, input/output (I/O) circuits, and the like. 
         [0026]    The memory  208  may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory  208  is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory  208  generally stores the operating system  214  of the control module  110 . The operating system  214  may be one of a number of commercially available operating systems such as, but not limited to, SOLARIS from SUN Microsystems, Inc., AIX from IBM Inc., HP-UX from Hewlett Packard Corporation, LINUX from Red Hat Software, Windows 2000 from Microsoft Corporation, and the like. 
         [0027]    The memory  208  may store various forms of application software, such as inverter control software  210  for operably controlling the inverters  102 . The communications transceiver  202  communicably couples the control module  110  to the inverters  102  to facilitate command and control of the inverters  102 . The communications transceiver  202  may utilize wireless or wired communication techniques for such communication. 
         [0028]      FIG. 3  is a block diagram of an inverter  102  in accordance with one or more embodiments of the present invention. The inverter  102  comprises a power conversion module  302 , a conversion control module  304 , a voltage monitoring module  306 , an AC current sampler  308 , and an AC voltage sampler  310 . 
         [0029]    The power conversion module  302  is coupled to the PV module  104  and acts to convert DC current from the PV module  104  to AC output current. The conversion control module  304  is coupled to the AC voltage sampler  310  for receiving an AC voltage reference signal from the commercial power grid, and to the power conversion module  302  for providing operative control and driving the power conversion module  302  to inject the generated AC output current in phase with the grid as required by the relevant standards. 
         [0030]    The voltage monitoring module  306  is coupled to the conversion control module  304 , the AC current sampler  308 , and the AC voltage sampler  310 . The AC current sampler  308  is coupled to an output terminal of the power conversion module  302 , and the AC voltage sampler  310  is coupled across both output terminals of the power conversion module  302 . The AC current sampler  308  and the AC voltage sampler  310  obtain samples (i.e., measurements) of the AC inverter current and AC inverter voltage, respectively, at the output of the power conversion module  302  and provide such inverter output current and voltage samples to the voltage monitoring module  306 . The AC current sampler  308  and the AC voltage sampler  310  may each comprise an A/D converter for obtaining the inverter output current and voltage samples, respectively. In some other embodiments, rather than being directly measured, the inverter output current may be estimated based on DC input voltage and current to the inverter  102  and the AC voltage output from the inverter  102 . 
         [0031]    The voltage monitoring module  306  may be comprised of hardware, software, or a combination thereof, and comprises at least one CPU  314  coupled to support circuits  316 , memory  318 , and a communications transceiver  324 . The communications transceiver  324  is further coupled to at least one of the output lines from the power conversion module  302  for communicating via PLC, for example, with the control module  110  and/or the measurement unit  112 . In alternative embodiments, the communications transceiver  324  may utilize wireless and/or other wired communications techniques for such communication. 
         [0032]    The CPU  314  may comprise one or more conventionally available microprocessors. Alternatively, the CPU  314  may include one or more application specific integrated circuits (ASIC). The support circuits  316  are well known circuits used to promote functionality of the central processing unit. Such circuits include, but are not limited to, a cache, power supplies, clock circuits, buses, network cards, input/output (I/O) circuits, and the like. 
         [0033]    The memory  318  may comprise random access memory, read only memory, removable disk memory, flash memory, and various combinations of these types of memory. The memory  318  is sometimes referred to as main memory and may, in part, be used as cache memory or buffer memory. The memory  318  generally stores the operating system (OS)  320  of the voltage monitoring module  306 . The OS  320  may be one of a number of commercially available OSs such as, but not limited to, Linux, Real-Time Operating System (RTOS), and the like. 
         [0034]    The memory  318  may store various forms of application software, such as voltage monitoring module (VMM) software  322  for determining a corrected monitoring voltage corresponding to the inverter  102 . 
         [0035]    To determine the corrected monitoring voltage, the voltage monitoring module  306  determines a correction coefficient K v  based on an inverter output voltage sample (i.e., a measurement of the inverter output voltage) received from the AC voltage sampler  310 , and a PCC voltage sample that indicates a measurement of a voltage proximate the PCC. In some embodiments, the PCC voltage sample is an RMS value obtained by the measurement unit  112  and communicated from the control module  110  as previously described. The voltage monitoring module  306  determines the correction coefficient, K v , as follows: 
         [0000]        K   v   =V   pcc   /V   meas   (i) 
         [0036]    where V pcc  is the PCC voltage sample and V meas  is the inverter output voltage sample. 
         [0037]    Additionally, when K v  is computed, the voltage monitoring module  306  computes an output power of the inverter, P meas . In some embodiments, the voltage monitoring module  306  utilizes the inverter output voltage sample V meas  and an inverter output current sample received from the AC current sampler  308 , as well as phase angle, to determine the output power of the inverter, P meas . Alternatively, the voltage monitoring module  306  may calculate P meas  based on DC voltage and DC current pertaining to the inverter  102  (for example, DC voltage and DC current samples obtained by the conversion control module  304 ) and a conversion efficiency of the inverter  102 . 
         [0038]    In some embodiments, the inverter  102  is pre-set with an initial K v =1 and determines a new K v  upon receiving a valid V pcc  measurement message. Such a message may be validated utilizing conventional communication techniques, such as addressing and checksums (e.g., cyclic redundancy check, or CRC). If the new K v  is within an acceptable correction coefficient range, the voltage monitoring module  306  utilizes the new K v  to determine a corrected monitoring voltage; if the new K v  is not within the acceptable correction coefficient range, K v  remains at its pre-set value until a next V pcc  is obtained. In some embodiments, the acceptable correction coefficient range is 0.95&lt;K v &lt;1.05. 
         [0039]    The voltage monitoring module  306  utilizes K v  for computing the corrected monitoring voltage V corr  upon obtaining inverter output current and voltage samples I inv  and V inv , respectively (e.g., inverter output current and voltage samples obtained subsequent to the samples utilized to compute K v ). The voltage monitoring module  306  determines the corrected monitoring voltage V corr  as follows: 
         [0000]        V   corr   =V   inv *(1−(( P   inv *(1 −K   v ))/ P   meas ))  (ii) 
         [0040]    where K v  is the correction coefficient, V inv  and P inv  are the inverter output voltage and power, respectively, and P meas  is the inverter output power determined at the time K v  was computed. P inv  is computed based on inverter output current and voltage samples obtained by the AC current sampler  308  and the AC voltage sampler  310 , respectively. 
         [0041]    The voltage monitoring module  306  periodically computes V corr  based on one or more of an updated inverter output power measurement P inv  an updated inverter output voltage measurement V inv , or an updated K v  (e.g., K v  may be updated upon receiving a new valid V PCC  measurement message). In some embodiments, V corr  as well as all corresponding voltage, current, power, and control parameters are determined at least once every line cycle (e.g., every 16.6667 milliseconds). If a new valid V pcc  measurement message is not received within an aging time window, K v  is reset to “1” until a valid V pcc  message is received. In some embodiments, a linear aging function may be utilized; alternatively, a nonlinear aging function, or a combination of a linear and a nonlinear aging functions may be utilized. 
         [0042]    In the event that the inverter output power P inv , moves outside of a valid output power range for the current K v , K v  is reset to “1” until a new valid V pcc  measurement message is received and a new K v  determined. The valid output power range may be some percentage of the rated total inverter power, for example, within the range of 5% to 20% of the inverter power rating. In some embodiments, K v  may be reset to “1” in the event that the inverter output power P inv  exceeds a maximum power deviation. 
         [0043]    The corrected monitoring voltage V corr  provides a more accurate estimate of the voltage at the PCC than the inverter output voltage alone for determining compliance with regulatory requirements pertaining to voltage levels at the PCC. In some embodiments, the voltage monitoring module  306  determines whether the corrected monitoring voltage V corr  is within a required voltage range with respect to the regulatory requirements; in the event the corrected monitoring voltage V corr  exceeds the required voltage range, the voltage monitoring module  306  provides a deactivation signal to the conversion control module  304  to deactivate the power conversion module  302  or, alternatively, AC voltage regulation may be performed. In some alternative embodiments, one or more of determining the corrected monitoring voltage V corr , determining compliance with regulatory requirements, and/or deactivation of one or more inverters as a result of one or more corrected monitoring voltage levels exceeding a required voltage range may be performed by the control module  110 . 
         [0044]      FIG. 4  is a flow diagram of a method  400  for determining a corrected monitoring voltage in accordance with one or more embodiments of the present invention. In some embodiments, such as the embodiment described below, AC current from a DG system comprising at least one DC-AC inverter is coupled to a commercial power grid at a PCC. Although the embodiment below is described with respect to a single inverter, each inverter of the DG system may utilize the method  400 . In alternative embodiments, a controller for the DG system may utilize the method  400  for determining one or more corrected monitoring voltages and/or driving the corresponding inverters accordingly. 
         [0045]    The method  400  begins at step  402  and proceeds to step  404 . At step  404 , a correction coefficient K v  of an inverter has an initial value of “1”. In some embodiments, the inverter may be preset with K v =1, for example, at a factory during manufacturing. The method  400  proceeds to step  406 , where a voltage sample (i.e., measurement) indicating a voltage proximate the PCC (“V pcc ”), is obtained. The PCC voltage sample V pcc  may be received by the inverter as part of a validated message transmitted to the inverter; for example, the message may be broadcasted from a control module coupled to the DG system and validated utilizing conventional communication techniques, such as addressing and checksums (e.g., cyclic redundancy check, or CRC). In some embodiments, a data logger (e.g., the measurement unit  112  previously described) may be coupled proximate the PCC, for example, at a load center coupling the DG system to the commercial power grid. The data logger may sample (i.e., measure) the voltage proximate the PCC, convert the voltage sample to an RMS value, and communicate the resulting PCC voltage sample V pcc  to the controller for broadcast to the one or more inverters of the DG system. In some alternative embodiments, the PCC voltage sample V pcc  may be converted to an RMS value at the controller or the inverter. In some other alternative embodiments, the data logger may directly communicate the PCC voltage sample V pcc  to the inverters utilizing wireless and/or wired communications techniques. 
         [0046]    The method  400  proceeds to step  407 , where a voltage sample of an AC output voltage of the inverter (V meas ) is obtained, for example, by an AC voltage sampler of the inverter. At step  408 , a new K v  is determined as follows: 
         [0000]        K   v   =V   pcc   /V   meas   (iii) 
         [0047]    Additionally, at the time K v  is computed, an output power of the inverter (P meas ) is determined based on V meas  and a sample of the AC output current from the inverter obtained, for example, by an AC current sampler of the inverter. 
         [0048]    At step  410 , a determination is made whether K v  is within an acceptable correction coefficient range; in some embodiments, the acceptable correction coefficient range is 0.95&lt;K v &lt;1.05. If it is determined that K v  is not within the acceptable correction coefficient range, the method  400  returns to step  404 . If it is determined that K v  is within the acceptable correction coefficient range, the method  400  proceeds to step  411 . 
         [0049]    At step  411 , inverter output current and voltage samples are obtained (V inv  and I inv , respectively) and utilized to determine the inverter output power (P inv ). At step  412 , a corrected monitoring voltage is determined as follows: 
         [0000]        V   corr   =V   inv *(1−(( P   inv *(1 −K   v ))/ P   meas ))  (iv) 
         [0050]    At step  414 , a determination is made whether the corrected monitoring voltage is within required regulatory limits. In some embodiments, the inverter may compare the corrected monitoring voltage to the regulatory limits to make the determination; alternatively, the corrected monitoring voltage may be communicated, for example to the controller or the measurement unit, for determining compliance with the regulatory limits. If the result of such determination is no, the method  400  proceeds to step  422 , where the inverter is deactivated; alternatively, AC voltage regulation may be performed. In some embodiments where the inverter is deactivated, upon determining that the corrected monitoring voltage exceeds required limits, the inverter may cease power production; alternatively, the inverter may receive a control signal from the controller or the measurement unit causing the inverter to cease power product. The method  400  then proceeds to step  424  where it ends. If the result of the determination at step  414  is yes, the method  400  proceeds to step  416 . 
         [0051]    At step  416 , a determination is made whether a new valid V pcc  measurement message has been received. In some embodiments, V pcc  may be determined and communicated to the inverter at least once every line cycle (e.g., every 16.6667 milliseconds). If a new valid V pcc  measurement message has been received, the method  400  returns to step  407 . If a new valid V pcc  measurement has not been received, the method  400  proceeds to step  418 . At step  418 , a determination is made whether an aging time window for K v  has been exceeded. In some embodiments, a linear aging function may be applied to K v  such that K v  reaches a value of 1 at the end of the aging time window; alternatively, a nonlinear aging function, or a combination of linear and nonlinear aging functions, may be utilized. If the aging time window has been exceeded, the method  400  returns to step  404 . If the aging time window has not been exceeded, the method  400  proceeds to step  419 . 
         [0052]    At step  419 , inverter output current and voltage samples (V inv  and I inv , respectively) are obtained and utilized to determine a new inverter output power P inv . At step  420 , a determination is made whether the inverter output power P inv  is within a valid output power range for the current K. The valid output power range may be some percentage of the rated total inverter power, for example, within the range of 5% to 20% of the inverter power rating. If the inverter output power P inv  is within the valid output power range, the method  400  returns to step  412 . If the inverter output power exceeds the output power range, K v  is reset to 1 and the method  400  returns to step  404 . 
         [0053]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.