WO2010078303A2

WO2010078303A2 - Electrical safety shutoff system and devices for photovoltaic modules

Info

Publication number: WO2010078303A2
Application number: PCT/US2009/069658
Authority: WO
Inventors: Michael Gostein; Russell Apfel; Lawrence R. Dunn; William Stueve
Original assignee: Atonometrics, Inc.
Priority date: 2008-12-29
Filing date: 2009-12-29
Publication date: 2010-07-08
Also published as: WO2010078303A3

Abstract

An electrical safety shutoff system and devices for disabling electrical power output from individual photovoltaic modules in a photovoltaic array, including one or more shutoff circuits, each of which can disable electrical power output from an associated module; and at least one enable signal generator that transmits a signal to the shutoff circuits to enable power output; wherein module power output is disabled in the absence of the enable signal.

Description

ATTORNEY DOCKET NO.: ATON003WO

PCT APPLICATION FOR

For

ELECTRICAL SAFETY SHUTOFF SYSTEM AND DEVICES FOR PHOTOVOLTAIC MODULES

By

Michael Gostein (Inventor)

Russell Apfel (Inventor)

Lawrence R. Dunn (Inventor)

William Stueve (Inventor)

Atonometrics, Inc. (Assignee) ELECTRICAL SAFETY SHUTOFF SYSTEM AND DEVICES FOR PHOTOVOLTAIC MODULES

RELATED APPLICATION DATA

[ 001 ] The present application claims priority to U.S. Provisional Patent Application Ser.

No. 61/141,033, filed Dec. 29, 2008, which is incorporated herein by reference.

FIELD OF THE INVENTION

[002] The invention relates to an electrical safety shutoff system and associated devices that can disable the electrical output of individual photovoltaic modules, also known as solar panels, in a photovoltaic array.

BACKGROUND OF THE INVENTION

[003] Photovoltaic (PV) solar energy generation systems use photovoltaic cells ("solar cells") to produce electricity from sunlight. They are typically implemented as arrays of individual panels, referred to as PV modules, wherein each module contains multiple cells. One difficulty with PV systems is that, whenever sunlight is incident on a PV array, the modules will be energized and cannot be turned off. This situation presents certain safety problems.

[004 ] FIGURE 1 illustrates the typical layout of a PV array, with optional elements indicated by dashed lines. Individual modules 100 (of which only one is labeled) are connected in series to form one or more strings 110 having desired output voltage; multiple strings 110 are combined in parallel at one or more string-combiners 120 to aggregate power; and the outputs of the one or more string combiners 120 are fed to one or more inverters 140 which convert the direct current (DC) output of the PV modules 100 to alternating current (AC), which is then provided to a load or utility grid. The series connection of multiple modules 100 is used to achieve high voltages that minimize resistive losses in current-carrying wires comprising the DC power lines 105 which interconnect the system elements. [005] Since the operating voltage of a typical PV array can be hundreds of volts or even up to a thousand volts, electric shock hazards involving the array are a concern. Workers assembling or performing maintenance on a high- voltage PV array must take appropriate precautions. Once an array is constructed, visitors to the site are at potential risk of electric shock if they come into contact with terminals or wiring having defective insulation. Of increasing concern is that in the event of a fire at a PV array site, firefighting personnel are placed at risk by their inability to deactivate the PV system and remove the electrical hazard. The risk of electric shock to firefighters is increased if water or other liquids are sprayed on the PV system, since these may form an electrically conductive path.

[006] Various approaches have been employed to address these problems.

[007 ] One category of approach ("array short-circuit") involves implementing a safety system including a switching device that can electrically short-circuit the positive and negative DC outputs of the PV array to each other in order to bring the array voltage to zero, thereby removing the hazard of electric shock when the switching device is activated. [008] Another category of approach ("array disconnect") involves implementing a switching device that can disconnect the array from the inverter 140 and load, creating an open circuit that brings the array current to zero and removes electrical hazards from the inverter and load portions of the circuit.

[009] A shortcoming of both array short-circuit and array disconnect methods is that they disable only a portion of the PV array. In an array disconnect system, only the portion of the array located between the switching element and the inverter or load is disabled, while hazards persist in the remainder of the array. While an array short-circuit device appears to disable the entire PV system, it is only effective if all electrical interconnects are functioning properly. If one section of the array becomes disconnected due to interconnect failures or wiring disruption, that section will not be shorted and will still present an electrical hazard.

[0010] Another system for electrical disconnection uses thermally activated switches that can short-circuit portions of the array if an over-temperature condition is detected. However, this system only responds to thermal triggers. Furthermore, with this system it is difficult for personnel to know with certainty when the PV array, or any portion of it, has been disabled. [ 0011 ] A significant potential hazard in a PV array is electrical arcing, particularly because PV systems use DC rather than AC electricity. In general, arc faults in an electrical system can be of several types. Series arcs occur when normal current flow is interrupted at failed or improper interconnections, while parallel arcs occur when a portion of the circuit is short-circuited due to failed electrical isolation. Ground faults are a special case of parallel arcs. Any of these faults may create hazards to personnel, damage the electrical system, or start a fire. In particular, a series arc occurring at a poor, intermittent, or corroded connection or at a broken wire will have increased resistance which causes heat dissipation and increased temperatures. This often accelerates deterioration until a failure occurs, sometimes violently. [ 0012 ] Analogous to other electrical systems, arc detection circuitry can be implemented for PV arrays, as indicated by the optional fault detection and interrupt 130 in Figure 1. For example, Haeberlin and Real outline an approach to arc detection in "Arc Detector for Remote Detection of Dangerous Arcs on the DC Side of PV Plants" in the proceedings of the 22nd European Photovoltaic Solar Energy Conference, Milano, Italy, September 2007. They describe a system which, upon detecting a series arc in a PV array, triggers an array disconnect switch, possibly integrated within an inverter, thereby removing the load and extinguishing the arc. The system may also separately detect parallel arcs and subsequently trigger an array short-circuit switch to bring the voltage to zero and extinguish the arc.

[0013] A shortcoming of this type of system is that it requires the ability to distinguish between series and parallel arcs, which require different countermeasures. Furthermore, applying the wrong countermeasure can worsen the problem. In addition, either array short-circuit or array disconnect may result in disabling only a portion of the system, as already discussed. [0014 ] More recently, various products have been introduced which include the capability to individually disconnect each module 100 in a PV array by activating a switch incorporated into the module 100 via a control signal. For example, this feature has been incorporated within such products as micro-inverters, power optimizers, and monitoring systems, designed to be installed on individual PV modules 100. Such systems permit individual modules 100 to be disconnected, thus limiting any electrical hazard. However, if systems rely on processing of a control signal by a controller device within the module 100 in order to activate the disconnect switch, their reliability may not be adequate for safety purposes. [0015] In view of the problems discussed in the foregoing, there exists a need for an improved method to disable the electrical output of an array of PV modules 100.

BRIEF SUMMARY OF THE INVENTION

[0016] The invention provides a method, system, and associated devices that can be used to disable the electrical output of an array of PV modules 100. It is an object of the invention to provide a system that can confine electrical power within individual PV modules 100 and eliminate hazards at wiring and interconnections, and to do so with greater reliability and lower cost than prior approaches.

[ 0017 ] It is an advantage of the disclosed subject matter to reduce or eliminate electrical shock hazards related to PV modules 100 and arrays.

[ 0018 ] Another advantage of the disclosed subject matter is to disable a particular module, sub-array, array, or entire system in response to user control, an electrical anomaly, or other emergency.

[0019] Yet another advantage of the disclosed subject matter is to reduce or eliminate electrical shock hazards independent of malfunctioning electrical interconnects. [ 0020 ] An additional advantage of the disclosed subject matter is to reduce or eliminate hazards attributable to arcing faults.

[0021] FIGURE 2 depicts an overview of a shutoff system according to the disclosed subject matter. Labels on some repeated elements are omitted for clarity, and dashed lines indicate optional elements or combinations of elements. The system consists of individual "shutoff circuits" 300, each of which can disable the electrical output of a single module 100; and one or more "enable signal generators" 400, each of which transmits signals to the shutoff circuits 300 to enable electrical power output from their associated modules 100. The shutoff circuits 300 are designed such that, in the absence of an enable signal 310, the shutoff circuits 300 revert to a safe state in which the module 100 power is disabled. Preferably, the shutoff circuits 300 are integrated into the assemblies or the junction boxes of their associated modules 100. The enable signal 310 is transmitted via the DC power lines 105 of the PV array, such that no additional wiring to the modules is required beyond the normal interconnections. For simplicity Figure 2 depicts only one string 110, but it should be understood that the system could contain a plurality of strings 110.

[0022] Each shutoff circuit 300 contains at least a switch element 330 and a signal detector 320. The switch element 330 is arranged such that in its normal state, the module 100 power output is disabled. The signal detector 320 detects the presence of the enable signal 310 and, if the enable signal 310 is present, causes the switch element 330 to change to a state that enables module 100 power output. The shutoff circuit 300 may be implemented as a combination of discrete devices or integrated substantially into a single device or integrated circuit. [0023] In one embodiment of the shutoff circuit 300, which is denoted "circuit interrupter," a normally open switch element 330 is placed in series with the PV generating capacity 102 of the module 100, such that in the default state, the circuit is interrupted. The signal detector 320 causes the switch 330 to close to complete the circuit when an enable signal 310 is detected. In another embodiment of the shutoff circuit 301, denoted "circuit shorter," (NOTE: 300 refers to the shutoff circuit of the circuit interrupter embodiment whereas 301 refers to the shutoff circuit of the circuit shorter embodiment; however, on the figures, 300 and 301 refer to the same diagram element and for clarity 301 has been omitted in some figures) a normally closed switch element 331 (NOTE: 330 refers to the switch of the circuit interrupter embodiment whereas 331 refers to the switch of the circuit shorter embodiment; however, on the figures, 330 and 331 refer to the same diagram element and for clarity 331 has been omitted in some figures) is placed in parallel with the PV generating capacity 102 such that in the default state, the circuit is shorted. A signal detector 321 (NOTE: 320 refers to the signal detector of the circuit interrupter embodiment whereas 321 refers to the signal detector of the circuit shorter embodiment; however, on the figures, 320 and 321 refer to the same diagram element and for clarity 321 has been omitted in some figures) opens the switch 331 when the enable signal 310 is detected. Hereafter the shutoff circuit, signal detector, and switch of the circuit interrupter versus circuit shorter embodiments (300, 320, 330) and (301, 321, 331), respectively, are understood to be interchangeable where the context does not distinguish between one and the other. [0024 ] In one embodiment, the shutoff circuit 300 is passive, in the sense of requiring no independent power source and containing no logic elements. The signal detector 300 changes the state of the switch element 330 using only energy derived from the enable signal 310. [0025] In another embodiment, the shutoff circuit 300 is powered by its associated module 100, and uses this power to amplify the signal detection and activate the switch element 330.

[0026] In still another embodiment, the shutoff circuit 300 is powered by its associated module and also contains a controller 360 (not shown) that can control the switch element 330. In this case, for example, the controller 360 (not shown) could cause the module 100 power output to be disabled even when the enable signal 310 is present.

[0027 ] In yet another embodiment, the shutoff circuit 300 includes both a controller 360

(not shown) and sensing elements (370 (not shown), 371 (not shown)) with which the controller 360 (not shown) can detect arc faults or ground faults in its associated module 100, and the controller 360 can cause the switch element 330 to disable module 100 power output in order to protect against detected faults.

[0028] Each enable signal generator 400 generates an enable signal 310 that, when detected by the shutoff circuits 300, will enable module 100 power output. The enable signal 310 may be, for example, a high-frequency AC current or voltage. In one embodiment, the enable signal 310 is transmitted continuously in order to maintain module 100 power output. In another embodiment, the enable signal 310 is transmitted at regular intervals, and module 100 power output is disabled if the enable signal 310 is not detected by the shutoff circuit 300 within a predetermined time. In one embodiment, the enable signal 310 may be modulated in order to encode information, which may be received by a controller 360 (not shown) within a shutoff circuit 300 or by another device. Such information could include, for example, instructions to enable or disable the power output of a particular module 100.

[0029] For small PV arrays, only a single enable signal generator 400 is required. For larger arrays, multiple enable signal generators 400 may be used. These may be combined in a master-slave relationship.

[ 0030 ] The enable signal generator 400 includes a power supply 430 (not shown) for generating the enable signal 310. In one embodiment, the power supply 430 (not shown) derives power from a power source external to the PV array, such as an electric grid. In another embodiment, power is derived directly from the PV array. In either case, the enable signal generator 400 may include an energy storage device 432 (not shown) such as a battery, to facilitate starting the signal generation without the external power source.

[ 0031 ] The enable signal generator 400 may contain a disconnect switch 444 (not shown) to remove the enable signal 310 from the PV array, thus shutting off the array. It may also shut off the array in response to control signals from other equipment, such as inverters 140, fault detection systems 130, or other devices.

[ 0032 ] Enable signal generators 400 may be integrated with other components of the PV array, such as inverters 140 and/or string combiners (120, 121).

[0033] These and other aspects of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the claimed subject matter, but rather to provide a short overview of some of the subject matter's functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following FIGURES and detailed description. It is intended that all such additional systems, methods, features and advantages that are included within this description, be within the scope of the accompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[ 0034 ] The novel features believed characteristic of the invention will be set forth in the claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: [0035] FIGURE 1 depicts the typical layout of a photovoltaic array, including multiple parallel strings of series-connected PV modules, according to the prior art. Dashed lines indicate optional elements or combinations of elements.

[0036] FIGURE 2 depicts a schematic diagram of a PV array incorporating a shutoff system according to the disclosed subject matter. Only one of a potential plurality of module strings is depicted. Dashed lines indicate optional elements or combinations of elements. [0037 ] FIGURE 3 depicts a schematic diagram of a PV array incorporating a safety shutoff system according to the disclosed subject matter, in which multiple enable signal generators are used to enable modules in multiple PV sub-arrays. Dashed lines indicate optional elements or combinations of elements.

[ 0038 ] FIGURE 4 depicts a comparison of voltages along a PV module string with circuits enabled versus disabled when using shutoff circuits in the circuit interrupter embodiment.

[0039] FIGURE 5 depicts the functional elements of a shutoff circuit in the circuit interrupter embodiment. Dashed lines indicate optional elements or combinations of elements. [0040] FIGURE 6 depicts an electrical schematic of a simple exemplary implementation of a shutoff circuit in the circuit interrupter embodiment.

[0041] FIGURE 7 depicts an electrical schematic of a second exemplary implementation of a shutoff circuit, in which power from the associated PV module is used to amplify detection of the enable signal.

[0042] FIGURE 8 depicts the functional elements of a shutoff circuit device in the circuit shorter embodiment. Dashed lines indicate optional elements or combinations of elements.

[0043] FIGURE 9 depicts a comparison of voltages along a PV module string with circuits enabled versus disabled when using shutoff circuits in the circuit shorter embodiment. [0044 ] FIGURE 10 depicts functional elements of an enable signal generator. Dashed lines indicate optional elements or combinations of elements.

[0045] In the figures, like elements should be understood to represent like elements, even though reference labels are omitted on some instances of a repeated element, for simplicity.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0046] Although described with particular reference to disabling power output from photovoltaic modules, those with skill in the arts will recognize that the disclosed embodiments have relevance to a wide variety of areas in addition to those specific examples described below. [0047 ] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

Overview

[0048] Figure 2 depicts a PV array incorporating a module-level shutoff system according to the disclosed subject matter. Dashed lines indicate optional elements and combinations of elements. The figure depicts only a single string 110, but it should be understood that multiple strings 110 could be present and combined at a string combiner (120, 121), which is not shown.

[0049] As shown schematically in Figure 2, a shutoff circuit 300 is associated with each

PV module 100. Preferably, it is incorporated within the module 100 assembly, such as within the junction box. The shutoff circuit 100 comprises at least a switch element 330 and a signal detector 320. In one embodiment, an enable signal generator 400 couples an enable signal 310 onto the DC power lines 105 interconnecting the modules 100. The enable signal 310 may be, for example, a high-frequency AC voltage or current. In an alternative embodiment the enable signal 310 could be delivered by a separate wired or wireless communication medium; however, this would increase the cost of implementation. The shutoff circuits 300 are configured such that the electrical output of each module 100 is enabled only when the enable signal 310 is present. In the absence of the enable signal 310, the shutoff circuits 300 revert to a safe state in which electrical output from the modules 100 is disabled.

[ 0050 ] In one embodiment, the enable signal 310 is transmitted continuously in order to maintain module 100 power output. In another embodiment, the enable signal 310 is transmitted at regular intervals, and module 100 power output is disabled if the enable signal 310 is not detected by the shutoff circuit 300 within a pre-determined time. In one embodiment, the enable signal 310 may be modulated in order to encode information, which may be received by a controller 360 (not shown) within a shutoff circuit 300 or by another device. Such information could include, for example, instructions to enable or disable the power output of a particular module 100.

[ 0051 ] The enable signal generator 400 may be controlled from a control panel 410, and the control panel 410 may include a manual shutoff switch 444 (not shown) that stops the enable signal 310 and therefore disables the modules 100. The enable signal generator 400 may also respond to control signals from other equipment, such as signals from an inverter 140, fault detection equipment 130, or other equipment. Similarly, the enable signal generator 400 may respond to signals from fire alarms or other safety systems. Response to these control signals allows automatic shutoff of the PV array.

[ 0052 ] The enable signal generator 400 may be a separate piece of equipment, or, as illustrated by dashed lines in Figure 2, may be integrated with an inverter 140 or other equipment.

[0053] Figure 2 depicts a particular embodiment (denoted "circuit interrupter") in which a normally open switch element 330 is in series with the PV generating capacity 102 and the switch element 330 is closed only when the enable signal 310 is detected. In another embodiment (denoted "circuit shorter") a normally closed switch element 331 is in parallel with the PV generating capacity 102 and the switch element 331 is opened only when the enable signal 310 is detected.

[ 0054 ] As depicted in Figure 2, an optional PV bypass diode 340 may be included allowing current flowing in a string 110 to bypass a disconnected module 100. [0055] In one embodiment, a PV array may contain multiple enable signal generators

400, which may be operated independently or in a master-slave relationship. FIGURE 3 depicts a PV array with multiple sub-arrays 125, wherein each sub-array has a slave enable signal generator 401 which in turn requires signals from master enable signal generator 402 in order to output its own signal, such that in the absence of a signal from the master 402 each sub-array 125 will be disabled. The various enable signal generators (400, 401, 402) may be integrated with other PV array equipment. For example, as indicated by dashed lines, the master 402 could optionally be integrated with an inverter 141 and the slaves 401 could optionally be integrated with string combiners 121. The master enable signal may be delivered to the slaves 401 via the DC power lines 106, or may be delivered via a separate wired or wireless communication link 404.

[0056] FIGURE 4 further depicts the operation of the shutoff system, in a circuit interrupter embodiment. The figure compares the voltages along an exemplary string 110' when the shutoff circuits 300 are enabled (left side - Figure 4A) versus disabled (right side - Figure 4B). The exemplary string 110' consists of 12 modules 100', wherein the modules 100' have a max power point voltage of 25 V and an open circuit voltage of 35 V, and where the negative terminal of the string 110' is at ground potential (0 V). When the enable signal 310 is present (left side - Figure 4A) signal detectors 320 cause the normally open switch elements to close (330') and the modules 100' are enabled. Voltages add from the negative to the positive end of the string 110' and voltages up to 300 V are present. In contrast, when the enable signal is not present (right side - Figure 4B), the switch elements are in their open state (330") and modules 110' are disabled. Current is arrested and cannot flow along the string 110', the wiring between the modules 100' is essentially disconnected, and the maximum voltage difference between any two points is limited to the open circuit voltage of 35 V. Furthermore, voltage hazards are confined to within the module 100' construction. Therefore, when the modules 100' are disabled the hazard potential is greatly reduced. In addition, both series and parallel arcs will be interrupted in most circumstances.

[0057 ] A shutoff system according to the disclosed subj ect matter will automatically protect against hazards caused by breaking of wiring or opening of connectors during operation of the PV array. If an open circuit develops within a module string 110 or along any interconnecting wiring, the enable signal 310 will be blocked and therefore the modules 100 beyond the open circuit point will be shut off.

Circuit Interrupter

FUNCTIONAL ELEMENTS - NON-POWERED

[ 0058 ] FIGURE 5 depicts the functional elements of the shutoff circuit 300 in the circuit interrupter embodiment. Optional elements are shown with dashed lines. [0059] The shutoff circuit 300 is connected to the PV generating capacity 102 of its associated module 100 through the "PV + In" and "PV - In" terminals (305, 306) and is connected to the PV array via the "PV + Out" and "PV - Out" terminals (307, 308). Note that the polarity refers to the relative voltage and not to the direction of positive current flow through the shutoff circuit 300, which is from "-" to "+".

[0060] A normally-open switch element 330 is in series with either the "+" or "-" leg of the circuit. The switch element 330 may be, for example, a mechanical relay or a solid-state device such as a transistor. In particular, a field-effect transistor (FET) may be advantageous since it can be switched with low current to its control input. The switch element 330 should be designed to withstand the inductive voltages that may be created when it opens and disrupts the string 110 current. Therefore, the switch element 330 may preferentially be designed to open slowly enough to limit inductive voltages to acceptable levels, and/or may include a bypass element such as a diode to suppress transient voltage spikes.

[0061] A signal detector 320 is placed in series with either the "+" or "-" leg of the circuit. The signal detector 320 detects the enable signal 310 and causes the switch element 330 to close when the enable signal 310 is present. The signal detector 320 may be implemented, for example, as an inductive or capacitive filter or resonant circuit, or in another manner. In one embodiment, passive (unamp lifted) detection of the enable signal 310 causes the circuit to drive the control gate of switch element 330 to enable module 100 power output.

[0062] A PV bypass element 340, such as a diode, may be included to allow the module

100 to be electrically bypassed when the switch 330 is open or when the module 100 is nonfunctioning, e.g. due to shading. In this case the signal detector 320 must be positioned within the portion of the circuit that will remain in series with the rest of the array when the PV bypass 340 is activated, to ensure that the enable signal 310 can be sensed. Note that the PV bypass 340 could also be implemented as a switch element with a control terminal. A signal bypass 345, such as a capacitor, may be included in parallel with the PV bypass 340 to permit passage of the enable signal 310.

FUNCTIONAL ELEMENTS - POWERED [0063] The elements described so far constitute an embodiment in which control of the switch element 330 is powered only by the enable signal 310. In other embodiments, the shutoff circuit 300 includes a power supply 350 that draws power from the associated PV module 100 in order to operate active components. For example, the power supply 350 could consist of a resistive divider, a zener diode, or a voltage regulator integrated circuit, together with a filter capacitor.

[0064 ] In one such embodiment, a driver circuit 325 is used to amplify the output of the signal detector 320 and operate the switch element 330. For example, the driver 325 may include an operational amplifier.

[0065] In another embodiment, a controller 360, such as a microcontroller, may be included. The controller 360 may analyze the enable signal 310 received by the signal detector 320 and may either enable or disable the switch element 330 via the driver 325 according to internal logic. In one embodiment, the enable signal 310 may be modulated in order to encode information, which may be received by the controller 360. Such information could include, for example, instructions to enable or disable the power output of a particular module 100. [0066] The controller 360 may also contain a communication mechanism, such as a wireless link, allowing the shutoff circuit 300 to be either enabled or disabled in response to a remote signal. In one embodiment, the wireless communication link could be used to deliver the enable signal 310, instead of delivering it via the DC power line.

[0067 ] In yet another embodiment, current sense elements (370, 371) are included in series with the "+" and "-" legs of the circuit, and the sensed currents are analyzed by the controller 360. The current sense elements (370, 371) permit the detection of ground faults or arc faults within the module 100, through analysis of the sensed signals using logic within the controller 360. The controller 360 may therefore cause the module 100 to shut off due to a locally detected fault. Similarly, voltage sense elements (not shown) could be included in addition to or instead of the current sense elements (370, 371).

[0068] Note that, in embodiments containing a controller 360, individual modules 100 can be disabled according to logic within their associated controllers 360, without shutting down the entire array or string 110, provided that PV bypass elements (e.g. 340) are included. [0069] When the PV module 100 is used to power the driver 325 that controls the switch element 330, there is the possibility for oscillatory behavior. For example, if a PV module 100 is shaded, it may be driven into reverse bias by excessive current flowing from the remainder of the string 110, thereby removing power to the active components of the module's associated shutoff circuit 300. As a result, the driver 325 will not operate and the switch 330 will revert to the normally open position. However, the reverse bias condition would then be lifted, the PV power generation capacity would be restored, and the driver 325 would be able to close the switch 330, restarting the cycle. In one embodiment, the effect of such oscillations is reduced by incorporating circuitry that lowers the frequency of restart events by decreasing the speed of the circuit response. In another embodiment, the effect of such oscillations is reduced by using the controller 360 to manage restart events, for example by introducing time delays, pro-actively disabling module power output in response to detected under-voltage conditions, or through other methods.

EXEMPLAR Y IMP LEME NTA TIONS

[0070] FIGURE 6 depicts a circuit schematic for a simple exemplary implementation of the shutoff circuit 300 in a circuit interrupter embodiment containing only the switch element 330 and the signal detector 320.

[0071] Ql is an enhancement-mode FET which constitutes the switch element 330. Ll,

Cl, Rl, and Dl form the signal detector 320. The enable signal 310 is a high-frequency AC current imposed on the DC power line (105, 106) interconnecting the modules 100. The enable signal 310 generates an AC voltage across Ll, which is rectified by Dl and charges Cl, creating a DC voltage at the gate of Ql. This turns on Ql, permitting current to flow out of the module 100. If the enable signal 310 is removed, Cl discharges through Rl and Ql is turned off when its gate voltage falls below the threshold.

[ 0072 ] The size of inductor Ll and the frequency and magnitude of the enable signal 310 must be chosen to develop sufficient voltage to turn on FET Ql . Typically 5-10 V are required for this type of device. For example, this may be achieved with an inductor of ~1 mH and an enable signal 310 of -100 kHz and ~10 niA. Using a higher frequency and/or a greater current magnitude would permit the use of a smaller and therefore less expensive inductor, while lower frequencies or current magnitudes require a larger and more expensive inductor. Therefore, the enable signal 310 frequency is preferably on the order of 100 kHz or higher. [0073] The time constant of the device is controlled by R 1 , C 1 , and L 1. In particular, by choosing values appropriately, the device can be designed such that the switching speed of Ql is limited by Ll . When the enable signal 310 is removed, Ql will begin to turn off, but the subsequent reactive voltage developed across Ll may charge Cl and therefore slow down the switching of Ql. This can be used to prevent the device from turning off too quickly. [0074 ] FIGURE 7 depicts an exemplary implementation of a shutoff circuit 300 in a circuit interrupter embodiment that includes a power supply 350 and powered components. The switch element 330 and signal detector 320 are similar to those of Figure 6, however here operational amplifier Ul serves as a driver for the gate of Ql, amplifying the detected enable signal 310. This permits choosing smaller values of the inductor Ll or of the frequency or magnitude of the enable signal 310. Ul is powered from the PV module 100; its positive rail is powered from the "PV + In" input 305, while its negative rail is powered from the simple power supply 350 formed by zener diode D3, resistors R5 and R9, and capacitor C3. [0075] The circuit implementations of Figure 6 and Figure 7 are meant only to be exemplary. Other implementations may be used to achieve the functions of the shutoff circuit 300 as outlined with reference to Figure 5.

Circuit Shorter

[0076] FIGURE 8 depicts the functional elements of a shutoff circuit 301 in a circuit shorter embodiment. Dashed lines indicate optional elements. In contrast to the circuit interrupter embodiment, the switch element 331 is now a normally-closed switch placed in parallel with the PV generating capacity, and the switch 331 is opened when the enable signal 310 is detected by signal detector 321. Other elements are substantially the same as discussed in reference to Figure 5. The normally-closed switch element 331 could be implemented, for example, as a mechanical relay or a solid-state device such as a transistor. In particular, it could be implemented as a depletion-mode FET. [0077 ] In one embodiment, the normally-closed switch element 331 brings the voltage across the input terminals (305, 306) to zero when it is closed. In this case, module 100 power is not available to power the functions of controller 360 or driver 326 while module 100 is in its disabled state. Therefore, signal detector 321 must derive enough energy from the enable signal 310 to drive switch 331 to its open state in order to enable module 100 power output. In an alternative embodiment, these limitations are lifted by, for example, placing a voltage limiting device (such as a diode) in series with switch element 331, in order to prevent the module 100 voltage from falling all the way to zero and therefore permitting power supply 350 to function when the module 100 is in its disabled state. The voltage should be kept low to minimize power dissipation in the voltage limiting device.

[0078] FIGURE 9 compares the voltages along an exemplary PV module string 110' incorporating shutoff circuits 301 in the circuit shorter embodiment, when the shutoff circuits 301 are enabled (left side - Figure 9A) versus disabled (right side - Figure 9B). As discussed in reference to Figure 4, for illustration an exemplary string of 12 modules 100' is shown wherein the modules 100' operate at a max power point of 25 V and wherein the negative terminal of the string 110' is at ground potential (0 V). When the modules 100' are enabled (left side - Figure 9A), the switch elements are in an open state (331'), voltages add from the negative to the positive end of the string, and voltages up to 300 V are present. When the modules 100' are disabled (right side - Figure 9B), switch elements are in their closed state (331"), each module 100' is shorted and the voltage at all points of the string 110' is near zero. Most electrical hazards are eliminated, and both series and parallel arcs will be interrupted in most circumstances.

Enable Signal Generator

[0079] In one embodiment, the enable signal generator 400 imposes the enable signal

310 on the DC power lines (105, 106) interconnecting modules 100 in the array, in order to enable module 100 power output.

[0080] FIGURE 10 depicts the functional elements of the enable signal generator 400, with optional elements indicated by dashed lines. [0081] Power from the PV array is fed through the enable signal generator 400 via the four +/- in/out terminals (421, 422, 423, 424).

[ 0082 ] A signal generator 440 generates the enable signal 310, which is applied to either the "+" or "-" leg of the circuit via a driver 442 and signal coupling elements 446. For example, the signal may be a high-frequency AC voltage or current. Preferably, the frequency is on the order of 100 kHz or higher. In one embodiment, the enable signal 310 is generated continuously, while in another embodiment, it is generated at regular intervals. In another embodiment, the enable signal 310 may be modulated in order to encode information to be transmitted. [0083] The signal coupling element 446 may be implemented as, for example, a bypass capacitor, a transformer, or a semiconductor device such as a transistor.

[ 0084 ] A switch 444, positioned either locally or remotely, provides for the interruption of the enable signal 310 in order to disable the PV modules 100. The placement of the switch 444 between driver 442 and signal coupling 446 indicated in Figure 10 is only exemplary. Other placements of switch 444 could also serve to disable the generation or application of enable signal 310.

[0085] Filter elements 450 on one or both legs of the circuit may be used to block high- frequency signals and thus prevent the enable signal 310 from interfering with other equipment installed on the PV array, such as inverters 140, as well as to prevent high-frequency signals from such other equipment from reaching the modules and enabling them spuriously. [0086] A controller element 460, such as a microcontroller, may be included. The controller 460 may implement the signal generator 440 function in software. In addition, it may process control signals received from other equipment, such as inverters 140, fault detectors 130, or other safety systems, in order to automatically shut down the PV array under certain conditions. The controller 460 also may be used in a slaved enable signal generator 401 to respond to signals from a master 402.

[ 0087 ] A power supply 430 is included within the enable signal generator 400 in order to operate the controller 460, signal generator 440, and driver 442. This power supply 430 may derive power from the PV array itself and/or from an external source, such as a utility power grid to which the array is connected, or any other external power source. With the shutoff system of disclosed subject matter in place, a power source is required in order to start the PV array, since the initial default state of the modules is "off until the enable signal 310 is generated. Therefore, the enable signal generator 400 may include an energy storage device 432, such as a rechargeable battery, in order to start the PV array in the absence of an external power source. [ 0088 ] In an alternative embodiment, the enable signal generator 400 uses a wireless transmission device to deliver the enable signal 310 to the shutoff circuits 300, rather than imposing an enable signal 310 on the DC power lines.

Combination with a Monitoring Device

[0089] The safety shutoff system and devices disclosed herein could be implemented in conjunction with a module-level monitoring system and devices such as disclosed in U.S. Provisional Patent Application number 61/102,933, "Photovoltaic Module Performance Monitoring System, Method, and Storage Medium" and Patent Cooperation Treaty application PCT/US09/59716 "Photovoltaic Module Performance Monitoring System And Devices," in order to realize certain benefits, including sharing of components and enabling of additional features due to synergistic operation.

Embodiment for Modules with AC Output

[0090] The preceding discussion focuses on modules having DC electrical output. It will be recognized that the disclosed subject matter may also be applied to photovoltaic modules which include micro-inverters producing AC outputs.

[0091] Although particularly described with reference to a small number of modules, module strings, shutoff circuits, enable signal generators, string combiners, inverters, and the like, this disclosure is intended to include any number of these components. [0092] Further, although example circuits and schematics to implement the elements of the disclosed subject matter have been provided, one skilled in the art, using this disclosure, could develop additional hardware and/or software to practice the disclosed subject matter and each is intended to be included herein.

[0093] In addition to the above described embodiments, those skilled in the art will appreciate that this disclosure has application in a variety of arts and situations and this disclosure is intended to include the same.

Claims

WHAT IS CLAIMED IS:

1. A system for disabling electrical power output from one or more photovoltaic modules in a photovoltaic array, comprising: one or more shutoff circuits, each of which can disable electrical power output from an associated module; and at least one enable signal generator which generates an enable signal and transmits said signal to one or more of said shutoff circuits, wherein said shutoff circuits disable electrical power output from said associated modules in the absence of said enable signal.

2. The system of claim 1 , wherein one or more electrical hazards are confined substantially within said modules in the absence of said enable signal.

3. The system of claim 1, wherein said shutoff circuits are integrated substantially within the assemblies or junction boxes of said associated modules.

4. The system of claim 1, wherein said enable signal is transmitted via power lines interconnecting said modules.

5. The system of claim 1 , wherein said enable signal is transmitted via a wireless communications medium.

6. The system of claim 1 , wherein the electrical output of said modules is DC.

7. The system of claim 1 , wherein said modules incorporate micro-inverters and the electrical output of said modules is AC.

8. The system of claim 1 , wherein said shutoff circuits are integrated substantially into a single device or integrated circuit.

9. The system of claim 1 , wherein said shutoff circuits comprise at least: a switch element, wherein said switch element has a normal state that disables electrical output from said associated module; and a signal detector, wherein said signal detector causes said switch element to change to another state that enables electrical output from said associated module when said signal detector detects said enable signal.

10. The system of claim 0, wherein: said module has a photovoltaic generating capacity; and said switch element is normally open and is placed substantially in series with said photovoltaic generating capacity of said module, such that in the absence of said enable signal said module's electrical output is interrupted.

11. The system of claim 0, wherein: said module has a photovoltaic generating capacity; and said switch element is normally closed and is placed substantially in parallel with said photovoltaic generating capacity of said module, such that in the absence of said enable signal the electrical output of said module is substantially shorted.

12. The system of claim 0, wherein energy necessary to change said switch element from said normal state to said other state is derived from said enable signal.

13. The system of claim 9, wherein said shutoff circuits further comprise a power supply, wherein said power supply derives power from said associated module.

14. The system of claim 13, wherein said shutoff circuits further comprise a driver circuit, wherein said driver circuit changes the state of said switch element using energy derived from said power supply, and wherein said driver circuit is connected to said signal detector.

15. The system of claim 13, wherein said shutoff circuits further comprise a controller, wherein said controller derives power from said power supply and controls said switch element using a driver circuit.

16. The system of claim 15, further comprising sensing elements, wherein said sensing elements sense at least one of a current or voltage of said associated module, and wherein said controller processes one or more signals from said sensing elements to identify one or more fault conditions, and wherein said controller controls said switch element in response to said fault conditions.

17. The system of claim 15, wherein said enable signal is modulated to encode information to be received by said controller.

18. The system of claim 17, wherein said information includes commands to enable or disable electrical output from said associated module.

19. The system of claim 1, wherein said enable signal is a high-frequency AC current or voltage.

20. The system of claim 1 , wherein said enable signal is transmitted continuously.

21. The system of claim 1, wherein said enable signal is transmitted at regular intervals, and wherein electrical power output of said associated module is disabled if said enable signal is not detected by a signal detector within a pre-determined time.

22. The system of claim 1 , additionally comprising at least one master enable signal generator, wherein said master enable signal generator controls at least one of said enable signal generators.

23. The system of claim 22, wherein said master enable signal generators control said enable signal generators via a wireless communications medium.

24. The system of claim 1 , wherein said enable signal generator includes a power supply, wherein said power supply derives power from at least one of: said photovoltaic array; an external power source; an electric utility grid; and an energy storage device contained within said enable signal generator.

25. The system of claim 1 , wherein said enable signal generator includes a disconnect switch which interrupts transmission of said enable signal to said shutoff circuits.

26. The system of claim 1 , wherein said enable signal generator is controlled by an inverter or a fault detection system.

27. The system of claim 1 , wherein said enable signal generator is integrated substantially within a string combiner, an inverter, or a fault detection system.

28. A device for disabling electrical power output from an associated photovoltaic module in a photovoltaic array, comprising: a switch element, wherein said switch element has a normal state that disables output from said associated module; and a signal detector, wherein said signal detector causes said switch element to change to a state which enables output from said associated module when said signal detector receives an enable signal from an external enable signal generator.

29. The device of claim 28, wherein said enable signal is transmitted over one or more power lines connecting said associated module to the photovoltaic array.

30. The device of claim 28, wherein said signal detector causes said switch element to change state using energy derived from said enable signal.

31. The device of claim 28, further comprising a power supply which derives power from said associated module.

32. The device of claim 31 , further comprising a driver circuit powered by said power supply, wherein said driver circuit causes said switch element to change state in response to said signal detector.

33. The device of claim 32, further comprising a controller which controls said driver circuit.

34. The device of claim 33, further comprising sensing elements which sense at least one of a voltage or current of said associated module, wherein said controller processes signals from said sensing elements to identify fault conditions, and wherein said controller controls said switch element in response to said fault conditions.