Circuits and methods for lmiting open circuit voltage of photovoltaic strings.

(54) Title: CIRCUITS AND METHODS TO LIMIT THE OPEN CIRCUIT VOLTAGE OF PHOTOVOLTAIC CHAINS.

(54) Title: CIRCUITS AND METHODS FOR LMITING OPEN CIRCUIT VOLTAGE OF PHOTOVOLTAIC STRINGS.

(57) Summary

A photovoltaic string (100) may include an open circuit voltage limiter (120) which conducts current in one direction to provide a limiter voltage less than an open circuit voltage of the photovoltaic chain (100), and conducts the current in the other direction. One or more open circuit voltage limiters (120) may be connected along the photovoltaic chain (100) or along selected groups of solar cells (115) of the photovoltaic chain (100). The limiter voltage may be greater than a maximum power point voltage but less than the PV circuit open circuit voltage (100).

(57) Abstract

A photovoltaic string (100) may inelude an open Circuit voltage limiter (120) that conducts current in one direction to provide a limiter voltage less than an open Circuit voltage of the photovoltaic string (100), and that conducts current in the other direction. One or more open Circuit voltage limiters (120) may be connected across the photovoltaic string (100) or across selected groups of solar cells (115) of the photovoltaic string (100). The limiter voltage may be greater than a maximum power point voltage but less than the open Circuit voltage of the photovoltaic string (100).

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PATENT TITLE No. 354131

Headlines):

D micilio:

D nomination:

Classification:

SUNPOWER CORPORATION

Rio Robles, San José, California, 95134, USA

CIRCUITS AND METHODS TO LIMIT THE OPEN CIRCUIT VOLTAGE OF PHOTOVOLTAIC CHAINS.

CIP: H01L31 / Q5; G®R &amp; 1? MH | 1I

CPC: HOU3VO604; MG¿Há / 2t

H0lb31 / C

KEITH4ÍWSTON; ZACH / ifc. \

H02H9 / 04 '9 / Q4¡J ^ Í2S50 / 00; H02S50 / 10; H01L31 / 042;

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Inventor (s)

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e'n i la Ley ^ teJa Prd¡5ie3aJji • la son £ * »intacgacidu0 _ ito en lolfii á'áT'Tuíprm, 06 / 201V27K

Number:

MX / a / 2016/006838

International:

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Validity:

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The referei patent

In accordance with the ai from the date of presei

Who subscribes to this title (Official Gazette of the Federation 25/01/2006, 06/05 / 2009,06 / 01/2010 Regulations of the Mexican Institute articles 1, 3, 4, 5, section V, subsection 27 / 12/1999, amended on 10/10/2002, 29/0 'Deputy Generals, Coordinator, Departmental Directors · and other subordinates of the Institute 04/08/2004 and 09/13/2007),

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Nutrient:

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lateN »tialLua · Agency of twenty« fies, non-extendable, counted to cl &amp; lelBntoj ^ ij ^ rfflaWer vigorrtbs l ^ rights.

fitfiaecltóg: III y T'MÍs?, «of the Industrial Property Law 10 ^ 996, 12/26/19 ^ 07 ^ 5/1999, 26/01/2004, 06/16/2005, i iAinciso a), 4th and 12th fractions I and III of the f »07/09/2004, 07/28/2004 and 09/07/2007); Industrial Property Law (DOF that delegates powers to the Directors>, Divisional Assistant Directors, Coordinators 2/1999, amended on 02/04/2000, 07/29/2004,

This letter is signed with an advanced electronic signature (FIEL), based on articles 7 BIS 2 of the Industrial Property Law; 3 of its Regulations, and 1 section III. 2 fraction V, 26 BIS and 26 TER of the Agreement establishing the guidelines for the use of the Electronic Payment and Services Portal (PASE) of the Mexican Institute of Industrial Property, in the procedures indicated.

THE DIVISIONAL DIRECTOR OF PATENTS

NAHANNY CANAL REYES

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Original string;

NAHANNY MARISOL CANAL REYES | 00001000000403252793 | Tax Administration Service | 1695 || MX / 2018/13198 | MX / a / 2016/006838 | Normal patent title with divisional PCT | 1223 | GAGV | Page (s) | vwm8ElbpXWpr5gwEQZ0 AaCTQQ4 =

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qxqrROIzuHIHs5f + GUbgEgCIHqhXbXTqD / TtHAcZnqhqWA0gQkmJdpMv86KmOh8Mwe / MsYuAk0C6 / N + oTU4 // gE + 9a

0vP3 + uZhmDfzMEoP86rXENMH2MOr0u6y4JVLs3C1xQqGfubWpO3w1ZFFMS / lkGSRIUSiZ9o9mfhZlduNT4Yo7ypLVh

Í / ¡J5LtF + T / WBF3zFR + IW85f6KYVRzBHgA + ThnTC8rRegZr63 / g71esA1wq2V3 / k9GKXWTgkxdEL / BUJfNqBCSwkH8

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Arenal No 550 Piso 1, Pueblo Santa María Tepepan. Xochímilco, 16020 Mexico City, (55) 53340700 www.gob.mx/lmpi

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MX / 2018/13198

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CIRCUITS AND METHODS TO LIMIT THE VOLT

OPEN OF PHOTOVOLTAIC CHAINS

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TECHNICAL FIELD

The incorporations of the matter described herein are generally related to solar cells. More particularly, the incorporations of matter are related to photovoltaic strings.

BACKGROUND OF THE INVENTION

A solar cell, which is a known device for converting solar radiation to electrical energy, can comprise both P-type and N-type diffusion regions. Solar radiation incident on the solar cell creates electrons and holes that migrate to the regions of diffusion thereby creating voltage differentials between the diffusion regions. The diffusion regions are electrically connected to the corresponding terminals to allow an external electrical circuit to be connected to and receive power from the solar cell. The positive terminal of the solar cell is electrically connected to the P-type diffusion regions, while the negative terminal of the solar cell is electrically connected to the N-type diffusion regions.

Solar cells can l Al A í

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INDUSTRIAL electrically in series to form a photovoltaic chain.

The photovoltaic chain can comprise several solar cells and / or photovoltaic modules, where each photovoltaic module comprises solar cells mounted on the same frame. In a photovoltaic string, the positive terminal of one solar cell is electrically connected to the negative terminal of another solar cell, the positive terminal of the other solar cell is electrically connected to the negative terminal of yet another solar cell, and so on. The output voltage of a photovoltaic string depends on the number of solar cells in the string.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment, a photovoltaic string comprises: (a) a plurality of series connected solar cells; (b) a positive conductor and a negative conductor at opposite ends of the photovoltaic chain, one end of the plurality of the series connected solar cells is connected to the positive conductor and an opposite end of the plurality of the series connected solar cells is connected to the negative conductor; and (c) an open circuit voltage limiter across the plurality of series connected solar cells, the voltage limiter

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open circuit negative conductor Dositive conductor.

and a negative terminal connected to the open circuit voltage limiter has a limiter voltage less than an open circuit voltage of the PV string for the current flowing through the open circuit voltage limiter in one direction and allows flow current through the open circuit voltage limiter in another direction.

In another incorporation, a photovoltaic chain comprises: a plurality of series-connected solar cells and a first open-circuit voltage limiter along a first group of solar cells in the plurality of series-connected solar cells, the first open-circuit voltage limiter has a first limiter voltage less than a maximum voltage of the first group of solar cells for the current flowing through the first open-circuit voltage limiter in one direction and has a first bypass voltage positive for the current flowing through the first open-circuit voltage limiter in another direction. The photovoltaic string may further comprise a second open circuit voltage limiter along a second group of solar cells in the plurality of connected solar cells

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INSTITUTO MEXICANO open has a second limiter voltage less than a maximum voltage of the second group of solar cells for the current flowing through the second open-circuit voltage limiter in one direction and has a second positive bypass voltage for the current that it flows through the second open-circuit voltage limiter in another direction.

In another embodiment, a method of operating a photovoltaic chain comprises providing a first open-circuit voltage limiter along a first group of solar cells connected in series to the photovoltaic chain. A maximum voltage across the first group of series-connected solar cells is limited to a first limiter voltage across the first open-circuit voltage limiter for current flowing in a first direction through the first voltage limiter open circuit. Current is allowed to flow through the first open-circuit voltage limiter in a second direction opposite to the first direction.

These and other features of the present invention will become apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying claims.

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BRIEF DESCRIPTION OF THE FIGURES

A more complete understanding of the material can be derived by referring to the detailed description and claims when considered in conjunction with the following figures, where similar reference numbers refer to similar elements throughout the figures.

FIGURE 1 shows a schematic diagram of a photovoltaic string in accordance with an embodiment of the present invention.

FIGURE 2 shows an ideal IV curve of the open circuit voltage limiter of FIGURE 1 in accordance with an embodiment of the present invention.

FIGURE 3 schematically shows the components of a photovoltaic system with the photovoltaic chain of FIGURE 1 in accordance with an embodiment of the present invention.

FIGURE 4 shows a schematic diagram of a photovoltaic string in accordance with another embodiment of the present invention.

FIGURE 5 shows IV curves for different diodes

Zener in an experiment.

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FIGURE 6 shows example IV curves and energy voltage curves of a photovoltaic string in an experiment.

FIGURE 7 shows a schematic diagram of a photovoltaic string in accordance with another embodiment of the present invention.

FIGURE 8 shows a schematic diagram of a photovoltaic string in accordance with another embodiment of the present invention.

FIGURE 9 shows a schematic diagram of a photovoltaic string in accordance with another embodiment of the present invention.

FIGURE 10 shows an open circuit voltage limiter in accordance with another embodiment of the present invention.

FIGURE 11 shows an open circuit voltage limiter in accordance with another embodiment of the present invention.

FIGURE 12 shows an open circuit voltage limiter in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

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In the present disclosure, numerous details are provided, such as examples of electrical circuits, components, and methods to provide a thorough understanding of the embodiments of the invention. Those of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, the known details have not been shown or described to avoid confusing aspects of the invention.

Photovoltaic strings, which may comprise a plurality of solar cells and / or one or more photovoltaic modules, are characterized by currents of current voltage (IV). An IV curve indicates the amount of output current generated by a photovoltaic string for a given output voltage generated by a photovoltaic string.

During normal operation, a photovoltaic chain is maintained at a maximum energy point (MPP) of curve IV, which is the operating point where the photovoltaic chain generates the maximum output energy. The output current of the PV string at the maximum energy point is the current of the maximum energy point Imp, and the output voltage of the photovoltaic module at the maximum energy point is the voltage of the maximum energy point Vmp.

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At voltages greater than the

INDUSTRIAL maximum Vmp, the output energy of the PV string decreases rapidly and reaches zero at the open circuit voltage Voc. The open circuit voltage Voc is the maximum possible output voltage that can be generated by the PV string. The region between the Vmp voltage and the Voc voltage is rarely used during normal operation, but the electrical characteristics of the PV string in this region have a major impact on the system design specifications. In particular, the electrical components of a photovoltaic system (for example, a photovoltaic inverter, disconnects, cables, junction boxes) must be rated at the maximum possible output voltage, which is the open-circuit voltage Voc. This means that the open-circuit voltage Voc imposes the number of solar cells that can be incorporated into a photovoltaic string of a specified voltage (for example 1000 V).

As a particular example, the Vmp voltage is typically around 20% less than the open circuit voltage Voc. Therefore, if a PV string is specified at 1000V, the PV string typically operates at around 800V. However, there are certain instances in which the PV string is forced

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MEXICAN INSTITUTE generate the open circuit voltage Voc, such^EUfc? ¿8 ^ R¿guá¡M ^ i the inverter MPP (MPPT) tracker -fotQTTnH-air.o is disconnected or before it turns on in the morning. This needs a PV string size specification based on the open circuit voltage

Voc although it results in a size and design of the photovoltaic chain that are not optimal. As will become apparent below, the embodiments of the present invention allow the reduction of the open circuit voltage Voc without impacting the voltage Vmp. This advantageously allows an increase in the number of solar cells in a photovoltaic chain and, therefore, reduces the number of photovoltaic chains in a photovoltaic system while maintaining a constant total power generation.

Reducing the number of photovoltaic chains allows the reduction of the system balance costs (BOS), such as the number of trackers, docks, units, electrical components, etc., of the photovoltaic system.

Referring now to FIGURE 1, a schematic diagram of a photovoltaic string 100 is shown in accordance with an embodiment of the present invention.

In the example of FIGURE 1, the photovoltaic string 100 comprises a plurality of solar cells 115 (that is, 1151, 115-2, 115-3 (...), 115-n). Solar cells 115 are electrically connected in series with a terrffi.

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xyo of a first solar cell 115 being connected to the negative terminal of a second solar cell 115, the positive terminal of the second solar cell 115 being connected to the negative terminal of a third solar cell 115, and so on.

This positive terminal of a 115 end solar cell (for example, a 115-1 solar cell) is connected to the positive output conductor 130 and the negative terminal of the other end 115 solar cell (for example, a solar cell

115-n) is connected to a negative output conductor 131. Output conductors 130 and 131 may be connected to other photovoltaic strings and other components of a photovoltaic system, such as a photovoltaic 200 shown in FIGURE 3, for example .

Groups of solar cells 115 may be mounted on a frame of a photovoltaic module 114. A photovoltaic module 114 comprises several solar cells 115 but only a few are shown in FIGURE 1 for clarity in the illustration. For example, a photovoltaic module 114-1 may comprise solar cells 115-1 to 115-3, a photovoltaic module 114-2 may comprise solar cells 115-4 to 115-6, a photovoltaic module 114-n may comprise solar cells 115-7 to 115-n, and so on. In that example, you can also think that the photovoltaic chain

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OE THE INDUSTRIAL PROPERTY twt electrically connected in series 114, with the positive terminal of one photovoltaic module 114 being connected to the negative terminal of another photovoltaic module 114, and so on. The positive terminal of one end PV module 114 (eg PV module 114-1) is connected to the positive output conductor 130 and the negative terminal of the other end PV module 114 (eg PV module 114-n ) is connected to negative output conductor 131.

In one embodiment, the PV string 100 includes an open circuit voltage limiter 120 electrically connected along the PV string 100. In the example of FIGURE 1, the limiter 120 includes a negative terminal 121 connected to the positive output conductor 130 of the photovoltaic chain 100, and a positive terminal 122 connected to the negative output conductor 131 of the photovoltaic chain 100.

In one embodiment, the open circuit voltage limiter 120 is configured to limit the open circuit voltage Voc of the PV array 100 to a limiter voltage Vlimit that is greater than the maximum power point voltage Vmp but less than the voltage open circuit Voc. In other words,

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Open circuit voltage limiter 120 can comprise an electrical device, component, or circuit which passes current in one direction beyond a specified voltage and which passes current in the other direction beyond a different voltage. In one embodiment, the open-circuit voltage limiter 120 exhibits diode-like asymmetric characteristics in both direct and reverse current directions. With the polarity of the open circuit voltage limiter 120 shown in FIGURE 1, an arrow 123 shows a direct current direction through the limiter 120, and an arrow

124 shows a reverse current direction through limiter 120.

In one embodiment, limiter 120 allows a reverse current to flow from negative terminal 121 to positive terminal 22 as shown by arrow 124 when the voltage across limiter 120 exceeds the voltage of limiter Vlimit. Limiter 120 maintains the voltage of the limiter Vlimit for voltages across limiter 120 that exceed the voltage of the limiter Vlimit. This condition can occur during the open circuit of the photovoltaic string 100, such as when the output conductor 130 or the output conductor 131 is not connected to another

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the chain f otovoltaié ^ ™; ^ chain fntnvnltaira 1Ω0 will generate possible, which is the circuit voltage, that is, when open. In this case, the maximum open circuit output voltage Voc. When this happens, the limiter 120 will limit the voltage along the PV string 100 to the voltage of the limiter Vlimit, which, as explained, is less than the open circuit voltage Voc.

This advantageously reduces the maximum possible output voltage of the PV string 100, allowing more solar cells 115 to be added to the PV string 100 without increasing the maximum possible output voltage.

Limiter 120 allows direct current to flow from positive terminal 122 to negative terminal 121 as shown by arrow 123 when the voltage across limiter 120 exceeds a bypass voltage Vbypass. The bypass voltage Vbypass is a very low voltage compared to the limiter voltage Vlimit. For example, the bypass voltage may be as low as 0.4V or the forward voltage drop of a Schottky or pn junction diode.

In order to extract the maximum energy from a photovoltaic system, the impacts of electrical mismatches must be minimized. This electrical mismatch can arise from factors such as inconsistencies in performance or shading that can cause operating voltage.

<img file="MX354131B_D0014.tif" />

of an inflicted photovoltaic module is reduced ^ m / ^ Qñxn

OE THE INDUSTRIAL PKuPIEOAD accommodate the photovoltaic string current. In the worst case scenario, the inflicted photovoltaic module (or the solar cell strings therein) can be forced into reverse bias, which can result in severe heating, performance degradation, and reliability issues. In order to mitigate these effects, the limiter 120 allows the direct current to pass through it beyond the Vbypass voltage such that when the PV string 100 enters reverse bias, the limiter 120 diverts current and limits the power consumption of the Photovoltaic module inflicted 114.

Bypass diodes are commonly used in photovoltaic modules to protect the individual sub-chains within a photovoltaic module or the entire photovoltaic module during the electrical mismatch conditions mentioned above. For example, Schottky and pn junction diodes have been used as bypass diodes. Schottky diodes typically have lower forward voltages than pn junction diodes, such that they dissipate less energy and operate at lower temperatures when in direct bias, that is, bypass mode; however, they have higher leakage currents and have limited reverse breakdown voltage capabilities. On the other hand the diodíss'L'de

L> '¿LA l · hi INI represent very low leakage currents and can be designed to break at much higher voltages, making them suitable as bypass diodes along photovoltaic chains containing more cells. However, bypass diodes generally do not carry current and do not have a high enough voltage in the reverse direction, making them unsuitable for use as an open circuit voltage limiter.

FIGURE 2 shows an ideal IV curve of the open circuit voltage limiter 120 in accordance with an embodiment of the present invention. Curve IV of the

FIGURE 2 shows the current through the limiter 120 as a function of the voltage across the limiter 120. As shown in FIGURE 2, with a direct current, the limiter 120 does not conduct current until after a positive bypass voltage Vbypass, which can be a couple of volts, for example 0.4V. With reference to reverse current, limiter 120 ideally does not conduct current until the magnitude of the voltage across limiter 120 exceeds the magnitude of the limiter voltage

Vlimit. As shown in FIGURE 2, the limiter 120 limits the magnitude of the voltage across the limiter 120 to the voltage of the limiter Vlimit with the current going in the reverse direction this is the current

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INDUSBUAÍ negative terminal 121 to positive terminal 122 as shown by arrow 124 (see FIGURE 1). The limiter voltage Vlimit is a negative voltage in the example of the

FIGURE 2 because current is going through limiter 120 in the reverse direction. Limiter 120 limits the voltage across limiter 120 to bypass voltage Vbypass with current going in the direct direction, that is, current flowing from positive terminal 122 to negative terminal 121 as shown by arrow 123 (see FIGURE 1).

FIGURE 3 schematically shows the components of the photovoltaic system 200 in accordance with an embodiment of the present invention. The components of the photovoltaic system shown in the FIGURE example include a combiner box 112, a plurality of photovoltaic panels 114, and a photovoltaic inverter 110. A photovoltaic system may include a plurality of inverters and combiner boxes but only one of each is shown in FIGURE 3 for clarity of illustration. Also, photovoltaic system 200 may include one, two, or more photovoltaic strings 100. A combiner box 112 provides junctions where photovoltaic modules 114 can be combined in parallel and / or

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Open circuit 120 can be installed along a photovoltaic string 100 in combiner box 112. The outputs of photovoltaic strings 100 are electrically connected to photovoltaic inverter 110, which converts direct current (DC) generated by solar cells 115 to alternating current (AC) suitable to be sent to an electrical network of a utility company, for example. Only some of the solar cells 115 are labeled in FIGURE 3 for clarity in the illustration.

FIGURE 4 shows a schematic diagram of a photovoltaic string 100A in accordance with an embodiment of the present invention. PV string 100A is the same as PV string 100 except for the use of an open circuit voltage limiter 120A. The components of the photovoltaic chain 100A are otherwise the same as explained with reference to FIGURE 1.

In the FIGURE 4 example, the open circuit voltage limiter 120A is an embodiment of the open circuit voltage limiter 120. The open circuit voltage limiter 120A has the same IV curve and electrical characteristics, including the negative terminal

121 and positive terminal 122. In the example in FIGURE 4, á 1

INSTITUTO AitXICAN · l A ^ IL · 'V of the prop: e> -a »' C ·» ^ ··¹ 'the open circuit voltage limiter 120A cCTttjOTencre ^ HLiá- * Zener diode 140. The cathode of the Zener diode Γ4Ό¹¹ esriá¹ 'connect to the negative terminal 121 and the anode of the Zener diode 140 is connected to the positive terminal 122. This connects the cathode of the Zener diode 140 electrically to the positive output conductor 130 of the PV string 100A, and the anode of the Zener diode 140 to the conductor of negative output 131 of the photovoltaic chain 100A.

In general, a Zener diode is a particular type of diode that is specifically designed to operate in reverse break mode. This allows you to conduct current in both forward and reverse bias directions. Zener voltage is the voltage at which the Zener diode begins to pass current in the reverse direction. The Zener voltage is negative and is typically much higher in absolute magnitude than the Zener diode forward voltage. Because Zener diodes have direct bias characteristics that are very similar to standard diodes, they can also be used in bypass applications. In the example in FIGURE 4, this allows the Zener diode

140 be used as a voltage limiter and as a bypass diode to protect against an electrical mismatch.

FIGURE 5 shows IV curves for different diodes

Zener in an experiment. In the example in FIGURE 5, IV curve 502 is for a Zener diode with

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18V, IV 503 curve is for a Zener diode with a voltage

17V Zener, IV 504 curve is for a Zener diode with a Zener voltage of 15V, IV 505 curve is for a Zener diode with a Zener voltage of 14V, IV 506 curve is for a Zener diode with a Zener voltage of 12V, and the IV 507 curve is for a Zener diode with a Zener voltage of 10 V. All of the Zener diodes mentioned above have axial packages and are rated for 5 watts except for the 12V Zener diode, which has a snap mount package and is rated for 50W. The resistive slope of the 12V Zener diode (see 506) is significantly steeper than other devices due to the low resistance of the package. FIGURE 5 also shows an IV 502 curve of a Schottky diode for comparison. As shown in FIGURE 5, Zener diodes begin to conduct current in the negative voltage direction once the Zener voltage has been exceeded. The forward bias characteristics of direct diodes are similar to a conventional Schottky or pn junction diode. The Schottky diode does not conduct current in the negative voltage direction (see 501) until a much higher reverse voltage is reached (eg 40 V).

By connecting the Zener 140 diode anode to the

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140 to the positive output conductor 130 as in FIGURE 4, the PV string 100A is prevented from operating at voltages greater than the Zener voltage, which is the limiter voltage Vlimit in this case. At Zener voltage, the diode

Zener 140 begins to pass current, effectively bypassing PV string 100A and setting 1st PV string 100A at Zener voltage. In one embodiment, the Zener voltage is selected to be between the nominal maximum power point voltage Vmp and the open circuit voltage Voc of the PV string 100A, as shown in EQ. 1, advantageously allowing the

<td>voltage</td><td>of</td><td>maximum output</td><td>possible</td><td>of</td><td>the</td><td>chain</td><td>photovoltaic</td>

<td colspan="2">100A be</td><td>limited to</td><td>voltage</td><td>of the</td><td colspan="2">limiter</td><td>Vlimit without</td>

<td>impact</td><td>the</td><td colspan="2">energy generation</td><td>of</td><td>the</td><td>chain</td><td>photovoltaic</td>

100A. The 120A open circuit voltage limiter with its Zener 140 diode thus modifies the characteristics of the curve

IV of the PV string 100A between the maximum power point voltage Vmp and the open circuit voltage Voc.

FIGURE 6 shows IV curves (upper graph) and energy-voltage curves (lower graph) of a 100A photovoltaic string with a single low concentration photovoltaic receiver module (LCPV) having 24 solar cells in one experiment. In the example in FIGURE 6, curve IV

523 It is for a Zener 140 diode with a vol

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INDUSTRIAL curve IV 524 is for a Zener 140 diode with a Zener voltage of 15V, curve IV 525 is for a Zener diode 140 with a Zener voltage of 14V, and curve IV 526 is for a diode

Zener 140 with a Zener voltage of 12V. Curve IV 520 is for a 100A photovoltaic string without the 120A limiter and is presented as a base. As shown in curves IV of FIGURE 6, the addition of the limiter 120A comprising the Zener diode 140 modifies curve IV of the photovoltaic chain 100A, allowing the reduction of the maximum possible voltage generation of the photovoltaic chain 100A of the Open circuit voltage from base to Zener voltage from Zener diode 140.

FIGURE 6 also shows example energy-voltage curves (bottom graph) of the 100A photovoltaic string with the 24 cell low concentration photovoltaic receiver (LCPV) module. The 533 power-voltage curve is for the diode

Zener 140 with the Zener voltage of 17V, the power curve voltage 534 is for the Zener 140 diode with the Zener voltage of 15V, the power-voltage curve 535 is for the Zener diode

140 with the Zener voltage of 14V, and the energy-voltage curve

536 It is for Zener 140 diode with 12V Zener voltage.

The 530 power-voltage curve is for the base configuration without any 120A limiter. In curves IV of FIGURE 6, a larger portion of curve IV is found

<img file="MX354131B_D0017.tif" />

INSTITUTO MLXKONO Di ÍD> INDUSTRIAL PROTJTDAD open circuit voltage Voc of the photovoltaic chain

100A converges to the maximum power point voltage Vlimit as the Zener voltage is progressively decreased from the open circuit voltage Voc of the 100A PV string. In the case of the Zener diode with the voltage

12V Zener (see 536), Zener voltage drops below

Vmp, resulting in reduced chain power generation.

In general, there is a limited value in operating between the maximum power point voltage

Vmp and open circuit voltage Voc because the power generation is significantly less than at the maximum power point. The open circuit voltage limitation described herein, however, reduces the open circuit voltage Voc (and therefore increases the fill factor) of the PV string and allows further optimization of the system design based at a lower peak voltage. If the Zener voltage is exceeded, the diode

Zener 140 acts as a load t receives current that would otherwise be flowing from the PV string

100A to output conductors 130 and 131.

Zener voltage can be adjusted to almost any value from a few volts to several hundred volts, and as

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PROPERTY On this basis, Zener diodes can be used for the voltage across the solar cell chains of various lengths. One factor to consider when using Zener diodes as an open-circuit voltage limiter is the dissipation of energy that occurs in Zener breakdown mode. The energy is directly proportional to the Zener voltage, and this can become prohibitively high if the diverted current is high. As the voltage limitation is increased and the Zener voltage is reduced from the open circuit voltage Voc to the maximum power point voltage Vmp, the power dissipation by the Zener diode increases. The compensation must be balanced in the circuit design, and the thermal load can be appropriately managed by using heat diffusers, heat sinks, and other passive or active heat management solutions.

In the example of FIGURE 4, the Zener diode 140 is connected along a complete photovoltaic chain. In another embodiment, a Zener diode may be connected across only one or a few solar cells, reducing the Zener voltage necessary for the limiting effect. In this case, multiple Zener diodes can be connected within the laminate of a photovoltaic module, or specific solar cells or groups of solar cells can be selected for regulation, ιΜΜΐ

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For example, in a linear LCPV receiver containing linear series of solar cells, Zener diodes can be connected across pairs of solar cells through interconnects. This approach allows more flexibility in design since the limiting total voltage of the PV string can be adjusted not only by means of the Zener voltage but also by means of the number of diodes

Zener. This approach can be enhanced by connecting a bypass diode at the chain level in parallel to address the electrical mismatch. In yet another addition, the Zener diode can be implemented at the system level, limiting the voltage of a number of modules.

Open circuit voltage limiter 120 can be incorporated into a PV string 100 in a variety of configurations. FIGURE 7 shows a schematic diagram of a PV string 100B in accordance with an embodiment of the present invention. The PV string 100B is a particular embodiment of the PV string 100 of FIGURE 1 where multiple open circuit voltage limiters 120 (i.e., 1201, 120-2, and 120-3) are employed to limit the maximum possible voltage along particular groups of solar cells 115 to cover the entire chain ϊ Μ ΡI ^ 3 ^: 3¾

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100B photovoltaic. Having limiters of^INV ^¹i^!t¹aj '^' '' -^{:: i}of 'open circuit 120 along ca vis 11

100B, the voltage requirement of the Vlimit limiter, and therefore power dissipation, of any open circuit 120 voltage limiter can be reduced.

In the example of FIGURE 7, an open circuit voltage limiter 120-1 limits the maximum possible voltage across solar cells 115-1, 115-2, 115-3, and 115-4; an open circuit voltage limiter 120-2 limits the maximum possible voltage across solar cells 115-5 and 115-6; and an open circuit voltage limiter 120-3 limits the maximum possible voltage across solar cells 115-7, 115-8, and 115-n. The negative terminal 121 of the open circuit voltage limiter 120-1 is connected to the positive output conductor 130, the negative terminal 121 of the open circuit voltage limiter

120-2 is connected to the positive terminal 122 of the open circuit voltage limiter 120-1, the negative terminal

121 The 120-3 open circuit voltage limiter is connected to the positive terminal 122 of the 120-2 open circuit voltage limiter, and the positive terminal 122 of the 120-3 open circuit voltage limiter is connected to the negative output conductor 131 Limiter voltage

Total vlimit throughout the entire 100B photovoltaic chain has been distributed

<img file="MX354131B_D0019.tif" />

MEXICAN INSTITUTE J ct i> PROP! T · ς.

. , INDUSTRIAL 120 open circuit voltage limiters, allowing individual 120 open circuit voltage limiters to have a reduced limiter voltage Vlimit and therefore less power dissipation in voltage limiting mode, compared to have a single open-circuit voltage limiter 120 that covers the entirety of the PV string 100B.

FIGURE 8 shows a schematic diagram of a 100C photovoltaic string in accordance with an embodiment of the present invention. The 100C photovoltaic chain is a particular addition to the 100 photovoltaic chain of the

FIGURE 1 wherein one or more open circuit voltage limiters 120 are employed along some, but not all, of the solar cell groups in the PV string 100C. This allows more design options in that the open-circuit voltage Voc across the entire PV string 100C can be limited by the number of open-circuit voltage limiters 120 employed and the number of solar cells 115 limited by the 120 individual open circuit voltage limiters.

In the example in FIGURE 8, the open circuit voltage limiter 120-1 limits the maximum possible voltage across the solar cells 115-1, 115-2, and 115-3, and the open circuit voltage limiter

DíU i * P,^z .,. ·: ··· 'i í \ I / iL' _i * - * · * maximum possible voltage across the solar cells 1157, 115-8 and 115-n. The maximum possible voltage across the 115-4, 115-5, and 115-6 solar cells is not limited. However, the total maximum possible voltage across the PV string 100C, that is, between output conductors 130 and 131, is reduced by limiters 120-1 and

120-2.

Providing an open circuit voltage limitation along the entire photovoltaic chain, as in chains 100, 100A, and 100B, advantageously allows direct current to be conducted through one or more limiters 120, thereby providing forms bypass protection in the event of an electrical mismatch. The PV string 100C in FIGURE 8 does not have bypass protection but may include a bypass diode as in the PV string 100D in FIGURE 9. The 100D photovoltaic chain is a particular addition to the photovoltaic chain

100C with the addition of a 251 bypass diode. The bypass diode

251 Provides protection against electrical mismatch by allowing direct current conduction to deflect the 100D PV string. Bypass diode 251 may comprise a Schottky or pn junction diode. The 100D photovoltaic chain is otherwise the same as the 100C photovoltaic chain

<img file="MX354131B_D0020.tif" />

As can be appreciated, an open circuit voltage limiter 120 can be implemented as an electrical circuit and using devices other than diodes

Zener. Other devices that may be employed include metal oxide varistors (MOVs), transistors (MOSFETs, or

BJT), opposite parallel diodes with specifically tuned direct voltages, thyristors, etc. by themselves and / or in combination with other devices to form an electrical circuit, for example, an electrical circuit comprising transistors. Basically any device or circuit that is capable of diverting current beyond a specific threshold voltage in one bias direction and a different threshold voltage in the other bias direction can be used to achieve the functionality of the open circuit.

FIGURE 10 shows an open circuit voltage limiter 120B in accordance with an embodiment of the present invention. The open circuit voltage limiter 120B is a particular embodiment of and operates in the same way as the open circuit voltage limiter 120. Accordingly, like the open circuit voltage limiter 120A of FIGURE 4, the voltage limiter 120B open circuit can be used in

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MEXICAN INSTITUTE photovoltaic chains 100, 100A, 100B, 100C, Ϊ ^ ΗΪΚτ ^ λ? ó ^ gaS-ú photovoltaic chains as a limiter ¿ja, n wnltajp, .dfí, open circuit.

In the example of FIGURE 10, the open circuit voltage limiter 120B and a reverse diode 171. The cathode of the forward diode 172 is connected to the negative terminal 121 and the anode of the direct diode 172 is connected to the positive terminal 122. The cathode of the reverse diode 171 is connected to the positive terminal 122 and the anode of the reverse diode 171 is connected to the negative terminal 121.

Diodes 171 and 172 may comprise Schottky or pn diodes, and therefore conduct current only in one direction. However, in the example of FIGURE 10, diodes 171 and 172 are in a parallel anti-15 configuration with one diode being positioned to flow current in one direction and the other diode being positioned to flow current in the opposite direction. This allows the 120B open circuit voltage limiter to flow current in both directions. Through proper design of the direct characteristics of diodes 171 and 172, the voltage beyond which diodes 171 and 172 conduct current can be optimized.

For example, the forward voltage drop of the forward diode 172 may be designed to be as low

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INSTITUTO MEXICANO, J possible because it is used in bypas mode - ^^ '^ g example, the forward voltage drop of the reverse diode 171 may be designed to provide a limiter voltage Vlimit that is greater than the set point voltage maximum power Vmp but less than open circuit voltage Voc as in EQ. one. During open circuit voltage conditions, current will flow from negative terminal 121, through reverse diode 171, and to positive terminal 122 when the voltage across limiter 120B exceeds the voltage of limiter Vlimit, which is along reverse diode 171. Note that reverse diode 172 will not conduct current during open circuit voltage conditions because it will have reverse bias. In the event of an electrical mismatch, the forward diode 172 will have a forward bias to divert current away from the PV string. The reverse diode 171 will have a reverse bias at that time, and therefore will not conduct current.

FIGURE 11 shows an open circuit voltage limiter 120C in accordance with an embodiment of the present invention. The 120C Open Circuit Voltage Limiter is a particular addition to and works the same way as the Circuit Voltage Limiter

I Μ. Abierto i open 120. Consequently, the limited B ^ iiijlQ / L'G ^ ué> 3 ^ ¿s ^ r Á

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INDUSTRIAL ^T* '2 used in photovoltaic chains 100, 100A, 100B, 100C,

100D, and other photovoltaic strings as an open circuit voltage limiter. The 120C open circuit voltage limiter is the same as the 120B open circuit voltage limiter except for the use of a plurality of direct diodes 172 (i.e., 172-1, 172-2, (...), 172 -n) and a plurality of reverse diodes (171 (i.e. 171-1, 1712, (...), 171-n). Using multiple forward diodes 172 and / or reverse diodes 171 allows optimization of forward voltage drop or current carrying diode capacity in the open circuit voltage limiter 120C.

The 120C open circuit voltage limiter is otherwise the same as the 120B open circuit voltage limiter.

FIGURE 12 shows an open circuit voltage limiter 120D in accordance with an embodiment of the present invention. The 120D open circuit voltage limiter is a particular embodiment of and works in the same way as the 120 open circuit voltage limiter. Accordingly, the 120D limiter can be used in the PV strings 100, 100A, 100B, 100C,

100D, and other photovoltaic strings as an open circuit voltage limiter. The voltage limiter

<img file="MX354131B_D0021.tif" />

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Dt INDUSTRIAL PROPERTY the use of direct diodes 172v and open circuit 120D is the same as that of open circuit 120C except for additional in series with diodes additional diodes in series with reverse diodes 171. In particular, one or more diodes can be placed in series with a forward diode 172 and / or a reverse diode 171 to allow further optimization of the forward voltage drop or current carrying capacity of the diodes in the voltage limiter 120D open circuit. The 120D open circuit voltage limiter is otherwise the same as the open circuit voltage limiter

120C.

Electrical circuits and methods for limiting the open circuit voltage of photovoltaic strings have been disclosed. Although the specific embodiments of the present invention have been provided, it should be understood that these embodiments are for illustrative purposes only and are not limiting. Many additional additions will be apparent to people who are normally well versed in the subject when reading the disclosure.

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<img file="MX354131B_D0023.tif" />

MEXICAN INSTITUTE

OF INDUSTRIAL PROPERTY