I-v measurement system for photovoltaic modules

Abstract

An apparatus for measuring electrical characteristics of solar panels (photovoltaic modules) wherein the apparatus measures current versus voltage (“I-V”) relationships for both illuminated (“light I-V”) and/or non-illuminated (“dark I-V”) conditions; optionally provides single, dual, or four-quadrant source/sink capability; and measures one or more devices under test (DUTs). The apparatus comprises one or more source measurement units (SMUs), wherein each SMU is connected to one DUT, and optionally includes a positive high-voltage programmable power supply and/or a negative high-voltage programmable power supply. Additionally, the apparatus includes a controller which controls the functions of the SMUs, the high-voltage supplies, and other components of the apparatus, wherein the controller communicates with the SMUs over a signal bus. Finally, the apparatus may include a computer to provide a user interface, communicate with the controller to initiate measurements and record results, analyze resulting data to determine measured parameters, and/or store the measured data.

What is claimed is: 
     
         1 . An apparatus for measuring current versus voltage (“I-V”) of photovoltaic (“PV”) modules, comprising:
 at least two source measurement units (“SMUs”), each of which is associated with one PV module device under test (“DUT”); and 
 a controller which controls functions of said at least two SMUs, each of said at least two SMUs further comprising:
 an output terminal electrically connected to one terminal of said associated DUT and a return terminal electrically connected to a second terminal of said associated DUT; 
 a high-voltage amplifier which provides an output voltage to said output terminal of said SMU, wherein said high-voltage amplifier:
 sources current from a positive supply terminal of said SMU via said output terminal of said SMU to said DUT; and/or 
 sinks current from said DUT via said output terminal of said SMU to a negative supply terminal of said SMU; 
 
 one or more voltage measurement circuits measuring the remote voltage at the DUT associated with said SMU; and 
 one or more current measurement circuits measuring the current flowing between said SMU and said associated DUT via said output terminal and said return terminal; 
 
 and 
 wherein said positive supply terminal of said at least two SMUs are electrically connected, and said negative supply terminal of said at least two SMUs are electrically connected. 
 
     
     
         2 . The apparatus of  claim 1 , further comprising a high-voltage power supply coupled to said at least two SMUs via at least one of: said positive supply terminal and said negative supply terminal. 
     
     
         3 . The apparatus of  claim 2 , wherein said controller selectively disables the output voltage of said high-voltage power supply during I-V measurement to minimize internal power dissipation in said high-voltage amplifier. 
     
     
         4 . The apparatus of  claim 2 , wherein said high-voltage power supply has programmable output voltages, and wherein said controller adjusts said programmable output voltages during I-V measurement to minimize internal power dissipation in said high-voltage amplifier. 
     
     
         5 . The apparatus of  claim 2 , wherein I-V data are measured in two or more I-V quadrants. 
     
     
         6 . The apparatus of  claim 2 , wherein the apparatus measures a dark I-V curve of one or more of said DUTs. 
     
     
         7 . The apparatus of  claim 2 , wherein at least one of said high-voltage power supply comprises multiple power supplies connected in series and/or parallel, and the series/parallel configuration of said power supplies is modified by operation of one or more jumpers, switches, relays, or semiconductor devices in order to change the current/voltage limits of said high-voltage power supplies formed by the combination of said multiple power supplies. 
     
     
         8 . The apparatus of  claim 1 , wherein said negative supply terminal of said at least two SMUs are electrically connected to a floating ground return point via at least one bypass diode. 
     
     
         9 . The apparatus of  claim 1 , wherein said high-voltage amplifier operates at a fixed output voltage or current, wherein said fixed output voltage or current is programmed by said controller. 
     
     
         10 . The apparatus of  claim 1 , wherein the apparatus measures a light I-V curve of one or more of said DUTs. 
     
     
         11 . The apparatus of  claim 1 , wherein the apparatus maintains one or more of said DUTs at a fixed point along an I-V curve for an indefinite period, sourcing or sinking power to/from said DUT as required to maintain said fixed point. 
     
     
         12 . The apparatus of  claim 11 , wherein said fixed point is one of a specified voltage, a specified current, the short-circuit current of said DUT, the open-circuit voltage of said DUT, or the maximum power point of said DUT. 
     
     
         13 . The apparatus of  claim 12 , wherein said fixed point is maintained by repeatedly following the steps of:
 measuring a portion of said I-V curve of said DUT between predetermined limits;   identifying said fixed point within said portion of said I-V curve;   setting each of said at least two SMUs corresponding to said DUT to the voltage or current value corresponding to said fixed point; and   waiting for a predetermined time.   
     
     
         14 . The apparatus of  claim 1 , wherein the voltage at said output terminal of each of said at least two SMUs is 100 V or greater. 
     
     
         15 . The apparatus of  claim 1 , wherein the current provided by said at least two SMUs is 10 A or greater. 
     
     
         16 . The apparatus of  claim 1 , wherein said one or more current measurement circuits comprise at least two current sense resistors used to measure current in different ranges, wherein said current sense resistors are selectively placed in series with said associated DUT using at least one switch, relay, or semiconductor device. 
     
     
         17 . The apparatus of  claim 16 , wherein said controller actuates said switch, relay, or semiconductor device for selecting said current sense resistors only when the current and voltage at said switch, relay, or semiconductor device are within acceptable limits. 
     
     
         18 . The apparatus of  claim 16 , wherein a measurement offset of at least one of said one or more current measurement circuits is automatically calibrated when said at least one of one or more current measurement circuit is deselected by said switch, relay, or semiconductor device, such that the current in said at least one of one or more current measurement circuit is known to be zero. 
     
     
         19 . The apparatus of  claim 1 , wherein each of said at least two SMUs comprises a control element which communicates with said controller and controls functions of said SMU, wherein said control element executes an I-V measurement sequence, performs maximum power point tracking, and/or maintains another fixed point along an I-V curve and wherein said control element is a microcontroller or field-programmable gate array. 
     
     
         20 . The apparatus of  claim 1 , wherein said high-voltage amplifier comprises:
 a program input;   an operational amplifier;   a pair of transistors for producing a positive or negative drive current;   a positive and/or negative supply terminal;   an output terminal;   output transistors which when enabled by said positive or negative drive current:
 source current from said positive supply terminal to said output terminal; and/or 
 sink current from said output terminal to said negative supply terminal; and 
   a current limit function which protects said output transistors;   wherein said operational amplifier compares said program input with the voltage at said output terminal scaled by a gain factor and produces an error signal which drives said pair of transistors for producing a positive or negative drive current;   and wherein said current limit function limits the output current from said output transistors by providing an alternate path for said drive current when said output current exceeds a threshold value, and wherein said threshold value is reduced as the voltage difference across said output transistors is increased.   
     
     
         21 . The apparatus of  claim 20 , wherein said output transistors are either bipolar junction transistors (“BJT”), Darlington BJT transistors, MOSFET transistors, or insulated gate bipolar transistors (“IGBT”). 
     
     
         22 . The apparatus of  claim 20 , wherein said high-voltage amplifier further comprises a bypass diode which allows current to flow from said output transistor to a floating ground return point when no negative power supply is connected to said negative supply terminal. 
     
     
         23 . The apparatus of  claim 20 , wherein said high-voltage amplifier further comprises one or more blocking diodes in series with at least one of: said positive supply terminal and said negative supply terminal. 
     
     
         24 . The apparatus of  claim 20 , wherein said current limit function comprises at least one sense resistor, a voltage reference, and a gain element, wherein the gain element provides an alternate path for said drive current when the voltage drop across said sense resistor exceeds said voltage reference. 
     
     
         25 . The apparatus of  claim 24 , wherein said voltage reference is a precision voltage reference and wherein said gain element is a comparator or operational amplifier. 
     
     
         26 . The apparatus of  claim 20 , wherein said current limit function reduces said output current as a function of voltage drop across said output transistors in a stepwise linear fashion including two or more breakpoints. 
     
     
         27 . The apparatus of  claim 26 , wherein the current allowed by said current limit function optimally uses the safe operating area of said output transistors or approximates a curve of constant power dissipation. 
     
     
         28 . The apparatus of  claim 1 , wherein each of said at least two SMUs further comprise a precise fixed or programmable current compliance function which limits the output current from said high-voltage amplifier, wherein said current compliance function comprises a compliance amplifier which compares an output of at least one of said of one or more current measurement circuits to at least one precise fixed or programmable reference value and provides a limit signal to said high-voltage amplifier when said reference value is exceeded. 
     
     
         29 . The apparatus of  claim 28 , wherein said current compliance function automatically limits output current to a value that protects at least one of said of one or more current measurement circuits when said at least one of one or more current measurement circuit is engaged. 
     
     
         30 . The apparatus of  claim 1 , wherein said controller progressively engages said of one or more current measurement circuit, beginning with a higher current range and proceeding to lower current ranges, in order to verify that actual current is within acceptable limits for said lower current ranges. 
     
     
         31 . The apparatus of  claim 1 , further comprising a circuit for detecting an AC mains power line cycle of the apparatus, and wherein an output signal of said circuit is used as a trigger signal to initiate and/or conclude data acquisition from one or more of said at least two SMUs, such that said data acquisition occurs substantially synchronously with said power line cycle in order to suppress noise substantially synchronous with said power line cycle. 
     
     
         32 . The apparatus of  claim 1 , further comprising a master chassis and an expansion chassis, wherein said controller is contained within said master chassis and one or more of said at least two SMUs are contained within said expansion chassis, and wherein said master chassis and said expansion chassis are connected. 
     
     
         33 . The apparatus of  claim 1 , wherein said controller includes non-volatile memory which is used to store program information and/or calibration information. 
     
     
         34 . The apparatus of  claim 1 , further comprising a computer which communicates with said controller, provides a user interface, initiates measurements and/or records results, analyzes resulting data to determine measured parameters, stores calibration and/or configuration data, and/or stores resulting data. 
     
     
         35 . The apparatus of  claim 34 , further comprising multiple controllers, wherein each of said multiple controllers controls one or more of said at least two SMUs, wherein said computer performs at least one of:
 communicating with said multiple controllers; and   managing and collecting data from said at least two SMUs.