Photovoltaic panel circuitry

Abstract

Circuits integrated or integrable with a photovoltaic panel to provide built-in functionality to the photovoltaic panel including safety features such as arc detection and elimination, ground fault detection and elimination, reverse current protection, monitoring of the performance of the photovoltaic panel, transmission of the monitored parameters and theft prevention of the photovoltaic panel. The circuits may avoid power conversion, for instance DC/DC power conversion, may avoid performing maximum power tracking to include a minimum number of components and thereby increase overall reliability.

Description

BACKGROUND

1. Technical Field

Aspects of the present disclosure relate to distributed power systems, particularly a circuit for integrating with or attaching to a photovoltaic panel.

2. Description of Related Art

A conventional photovoltaic distributed power harvesting system multiple photovoltaic panels are interconnected and connected to an inverter. Various environmental and operational conditions impact the power output of the photovoltaic panels. For example, the solar energy incident, ambient temperature and other factors impact the power extracted from each photovoltaic panel. Dependent on the number and type of panels used, the extracted power may vary widely in the voltage and current from panel to panel. Changes in temperature, solar irradiance and shading, either from near objects such as trees or far objects such as clouds, can cause power losses. Owners and even professional installers may find it difficult to verify the correct operation of the system. With time, many more factors, such as aging, dust and dirt collection and panel degradation affect the performance of the solar photovoltaic distributed power system.

Data collected at the inverter may not be sufficient to provide proper monitoring of the operation of the system. Moreover, when the system experiences power loss, it is desirable to ascertain whether it is due to environmental conditions or from malfunctions and/or poor maintenance of the components of the solar power distributed power system. Furthermore, it is desirable to easily locate any particular solar panel that may be responsible for power loss. However, information collection from each panel requires a means of communication to a central data gathering system. It is desirable to control data transmission, to avoid transmission collisions, and ascertain each sender of data. Such a requirement can be most easily accomplished using a duplex transmission method. However, a duplex transmission method requires additional transmission lines and complicates the system. On the other hand, one-way transmission may be prone to collisions and makes it difficult to compare data transmitted from the various sources. Due to the wide variability of power output of such systems, and the wide range of environmental conditions that affect the power output, the output parameters from the overall system may not be sufficient to verify whether the solar array is operating at peak power production. Local disturbances, such as faulty installation, improper maintenance, reliability issues and obstructions might cause local power losses which may be difficult to detect from overall monitoring parameters.

Electric arcing can have detrimental effects on electric power distribution systems and electronic equipment. Arcing may occur in switches, circuit breakers, relay contacts, fuses and poor cable terminations. When a circuit is switched off or a bad connection occurs in a connector, an arc discharge may form across the contacts of the connector. An arc discharge is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through a medium such as air which is normally non-conducting. At the beginning of a disconnection, the separation distance between the two contacts is very small. As a result, the voltage across the air gap between the contacts produces a very large electrical field in terms of volts per millimeter. This large electrical field causes the ignition of an electrical arc between the two sides of the disconnection. If a circuit has enough current and voltage to sustain an arc, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire. The zero crossing of alternating current (AC) power systems may cause an arc not to reignite. A direct current system may be more prone to arcing than AC systems because of the absence of zero crossing in DC power systems.

In Photovoltaic Power Systems and The National Electrical Code, Suggested Practices: Article 690-18 requires that a mechanism be provided to disable portions of the PV array or the entire PV array. Ground-fault detection, interruption, and array disablement devices might, depending on the particular design, accomplish the following actions; sense ground-fault currents exceeding a specified value, interrupt or significantly reduce the fault currents, open the circuit between the array and the load, short the array or sub-array

According to the IEE wiring regulations (BS 7671:2008) a residual current device (RCD) class II device on the direct current (DC) photovoltaic side for disconnection because of ground-fault current is referred to in regulation 712.412.

The use of photovoltaic panel based power generation systems are attractive from an environmental point of view. However, the cost of photovoltaic panels and their relative ease of theft, might limit their adoption for use in power generation systems.

Thus there is a need for and it would be advantageous to have circuitry integrable or integrated with a photovoltaic panel which provides features including: monitoring of the photovoltaic panel, ground-fault detection and elimination, arc detection and elimination, theft prevention and a safety mode of operation while maintaining a minimal number of components in the circuit to decrease cost and increase reliability.

BRIEF SUMMARY

Various circuits are disclosed which are integrated or integrable with a photovoltaic panel to provide built-in functionality to the photovoltaic panel including safety features such as arc detection and elimination, ground fault detection and elimination, reverse current protection, monitoring of the performance of the photovoltaic panel, transmission of the monitored parameters and theft prevention of the photovoltaic panel. The circuits may avoid power conversion, for instance DC/DC power conversion, may avoid performing maximum power tracking to include a minimum number of components and thereby increase overall reliability.

According to features of the present invention, there is provided a circuit for a photovoltaic panel. The circuit may include an input terminal attachable to the photovoltaic panel, an output terminal and a controller. A switch may be operatively connected between the input terminal and the output terminal and a control terminal operatively connected to the controller. The switch when closed may provide a low impedance direct current path for direct current producible by the photovoltaic panel to the output terminal. The circuit may include multiple input terminals and multiple output terminals, high voltage input and output terminals and low voltage input and output terminals which may or may not be at ground potential. The circuit may further include an output bypass circuit connectible across the output terminals. The bypass circuit may be operable to bypass current around the switch and around the photovoltaic panel. The circuit may avoid power, voltage and current conversion between the input terminal and the output terminal. The circuit may further include at least one sensor operatively attached to the controller. The sensor may be configured to measure at least one parameter such as current through the input terminal, voltage at the input terminal, current through the output terminal or voltage at the input terminal. A transmitter may be operatively attached to the controller. The transmitter may be operable to transmit the at least one parameter. The circuit may further include a permanent attachment to the photovoltaic panel.

The circuit may include at least two modules or at least three modules operatively connected to or integrated with the controller selected from a theft detection module, an arc elimination module, a ground fault detection module and/or a safety module. The theft detection module may be operable to detect a potential theft of the photovoltaic panel by configuring the controller to activate the switch and to disconnect the photovoltaic panel from the output terminal(s) responsive to the potential theft detection.

The arc elimination module may be operable to detect an arc within or in the vicinity of the photovoltaic panel or the circuit. The controller may be configured to activate the switch and to disconnect the photovoltaic panel from the output terminal responsive to a detection of the arc. The ground fault detection module may be operable to detect a ground fault within the circuit or the photovoltaic panel. The controller may be configured to activate the switch and to disconnect the photovoltaic panel from the output terminal responsive to a detection of the ground fault. For the safety module, the controller may be configured to activate the switch to select either a safe operating mode to produce a safe limited output power on the output terminal or a normal operating mode to produce a substantially maximum output power from the photovoltaic panel.

The circuit may further include a monitoring module operable to monitor the performance of the photovoltaic panel. The monitoring module may be operable to detect at least one condition of over current, over voltage or over temperature. The controller may be configured to activate the switch responsive to the at least one condition.

According to features of the present invention, a circuit for a photovoltaic panel is provided. The circuit includes input terminals attachable to the photovoltaic panel, output terminals and a controller. A switch may be operatively connected between an input terminal and an output terminal. The switch may include a control terminal operatively connected to the controller. The switch may include a single pole switch with a first pole connected to at least one of the input terminals, a second pole connected to at least one of the output terminals and a control terminal operatively connected to the controller. The circuit may further include an input bypass circuit connectible across the input terminals. The bypass circuit is operable to bypass current around the photovoltaic panel. The circuit may further include an output bypass circuit connectible across the output terminals. The bypass circuit may be operable to bypass current around the switch and around the photovoltaic panel. The switch when closed may provide a low impedance path for direct current between the photovoltaic panel to the output terminal.

The circuit may avoid power conversion between the input terminal and the output terminal. The circuit may also include a direct current (DC) to DC power converter to perform power conversion between the input terminal and the output terminal. The power converter may be a buck circuit, a boost circuit, a buck plus boost circuit, Cuk converter, or a buck-boost circuit.

The circuit may include at least two modules or at least three modules may be operatively connected or integrated with the controller including a monitoring module, a theft detection module, an arc elimination module and/or a ground fault detection module. The monitoring module may be operable to monitor the performance of the photovoltaic panel. The monitoring module may be operable to detect at least one condition such as over rated current, under rated current, over rated voltage, under rated voltage over rated temperature or under rated temperature. The controller may be configured to activate the switch responsive to the at least one condition. The monitoring module may be operable to monitor performance of the circuit. The theft detection module may be operable to detect a potential theft of the photovoltaic panel. The controller may be configured to activate the switch and to disconnect the photovoltaic panel from the output terminal responsive to the potential theft detection. The arc elimination module may be operable to detect arcing within or in the vicinity of the photovoltaic panel. The controller is configured to activate the switch and to disconnect the photovoltaic panel from the output terminal responsive to an arc detection. The ground fault detection module may be operable to detect a ground fault within the junction box or in the vicinity of the photovoltaic panel. The controller is configured to activate the switch and to disconnect the photovoltaic panel from the output terminal responsive to a ground fault detection.

The circuit may further include a safety module operatively connected to the controller. The controller may be configured to activate the switch to select either a safe operating mode to produce a safe working output power on the output terminal or a normal operating mode to produce a substantially maximum output power.

According to features of the present invention, there is provided a method performable in a photovoltaic solar power harvesting system. The method performs by a circuit integrated or integrable with a photovoltaic panel to form a photovoltaic module. The circuit has input terminals and output terminals. The circuit may include a controller adapted to monitor in parallel multiple types of malfunctions. The controller is adapted to control at least one switch connected between the input terminals and the output terminals to activate the switch and to disconnect thereby the photovoltaic panel from at least one of the output terminals and to bypass the output terminals upon detecting at least one of multiple malfunctions. The malfunctions monitored by the controller may include: an arc, a potential theft, a ground fault or a monitored parameter fault. The detection of the arc may be in the photovoltaic module or in the vicinity of the photovoltaic module. The disconnection of the photovoltaic panel from the at least one output terminal may be responsive to eliminate the arc. The potential theft of the photovoltaic module and the disconnection of the photovoltaic panel from the at least one output terminal may render the photovoltaic module inoperable outside the photovoltaic solar power harvesting system. The detection of a ground fault and in response the disconnection of the photovoltaic panel from the output terminal may eliminate the ground fault. The monitored parameter fault detected may be voltage, current and/or temperature. One or more of the monitored parameters may be out of a previously specified value range, the photovoltaic panel which not behaving according to specification is disconnected and the output terminals are bypassed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1a illustrates a photovoltaic solar power harvesting system, illustrating features of the present invention.

FIG. 1b shows more details of a circuit and a photovoltaic panel shown in FIG. 1a, according to an exemplary feature of the present invention.

FIGS. 1c and 1d show two exemplary switch circuits for a switch shown in FIG. 1b which are operable by a controller.

FIG. 1e shows more details of an active bypass circuit according to an exemplary feature of the present invention.

FIG. 1f shows a timing diagram of operation for the active bypass circuit shown in FIG. 1e.

FIG. 1g shows an example of system level diagram of a controller and modules which may be implemented in the circuit of FIG. 1b.

FIG. 2a shows a method which may be implemented in the circuit of FIG. 1b.

FIG. 2b shows an exemplary method for a circuit which considers the use of an arc detection module with a theft detection module.

FIG. 3 shows a method for arc detection in a power harvesting system shown in FIG. 1a.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The features are described below to explain the present invention by referring to the figures.

Before explaining features of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other features or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

It should be noted, that although the discussion herein relates primarily to photovoltaic systems, the present invention may, by non-limiting example, alternatively be configured using other distributed power systems including (but not limited to) wind turbines, hydro turbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines.

By way of introduction aspects of the present invention are directed to circuitry integrated or integrable with a photovoltaic panel to form a photovoltaic module. The circuitry may include multiple features for monitoring the performance of the photovoltaic panel, detection and elimination of arcs, and/or detection and elimination of ground faults in the photovoltaic module in or in the vicinity of the photovoltaic module or elsewhere in the photovoltaic power harvesting system. The circuitry may also include functionality for theft detection and prevention. The circuitry may also include functionality for providing both a safety mode of operation which features a current limited output and a normal mode of operation for production of solar power.

According to an exemplary feature of the present invention, the circuit is connected or connectible at the input terminals to a photovoltaic panel. The output terminals may be connected to form a string of photovoltaic modules. Multiple photovoltaic modules may be parallel connected to form the photovoltaic solar power harvesting system

The term “vicinity” as used herein in the context or arc and/or ground fault detection may refer to another like photovoltaic module connected in series to form the serial string, another part of the serial string or another string, e.g. a neighboring photovoltaic string connected in parallel.

The term “current bypass” or “bypass” as used herein refers to a low-resistance direct current connection between the two input terminals and/or between two output terminals of the circuit to form an alternative path for direct current and/or power externally applied to the terminals. The bypass provides a current path for string current in the case the photovoltaic panel is disconnected by activation of the switch.

The term “passive” device as used herein, refers to the “passive” device not requiring external power from a source of power to perform a circuit function.

The term “active” device as used herein, refers to the “active” device which requires power from an external source of power to perform a circuit function.

The term “switch” as used herein refers to an active semiconductor switch, e.g. a field effect transistor (FET) in which a controllable and/or variable voltage or current is applied to a control terminal, e.g. gate, of the switch which determines the amount current flowing between the poles of the switch, e.g. source and drain of the FET.

The term “activate” a switch as used herein may refer to opening, closing and/or toggling i.e. alternatively opening and closing the switch.

Reference is also now made to FIG. 1a of a photovoltaic solar power harvesting system 10, illustrating aspects of the present invention. Power harvesting system 10 includes multiple photovoltaic panels 101 connected respectively to multiple circuits 103. Circuit 103 may be housed in a junction box to provide electrical terminations, mechanical support of bus bars a, b and c (not shown) which may be used as an input to circuit 103 from a panel 101. Alternatively, circuit 103 may be integrated with photovoltaic panel 101 without the use of a junction box. Circuit 103 may be attachable and/or re-attachable to panel 101 or may be permanently attachable to panel 101 using for example a thermoset adhesive, e.g. an epoxy adhesive. The electrical outputs of circuits 103 may be connected in series to form a series photovoltaic serial string 107 through which a string current (I_string) may flow. Multiple strings 107 may be connected in parallel and across an input of a load 105. Load 105 may be a direct current (DC) load such as a DC motor, a battery, an input to a DC to DC converter or a DC input to a DC to AC inverter.

A central unit 109 may be operationally connected by control line 114 to and located in the vicinity of load 105. Central unit 109 include a transmitter and/or receiving for transmitting and receiving power line communications (PLC) or wireless communications 117 to and from circuits 103. Current and/or voltage sensors 119a, 119b operatively attached to central unit 109 may sense the input of load 105 so as to measure input voltage (VT) and input current (IL) to load 105. Central unit 109 may also be operatively attached to a network 115, e.g. Internet for the purposes of remote monitoring or control of system 10. Central unit 109 may also serve as to send appropriate control signals to circuits 103 based on previously determined operating criteria of power harvesting system 10. Alternatively or in addition, a master circuit 103a in a string 107 may provide independent control within a string 107 and/or may work in conjunction with central unit 109.

Reference is now made to FIG. 1b which shows more details of circuit 103 and photovoltaic panel 101 shown in FIG. 1a, according to an exemplary feature. According to the example, photovoltaic panel 101 includes two sub-strings 11 of serially connected photovoltaic cells which output to bus bars a, b and c which are the input terminals to circuit 103. Circuit 103 may be housed in a junction box to provide electrical terminations, mechanical support of bus bars a, b and c and to provide the input terminals to circuit 103. The input of circuit 103 includes two bypass diodes 120a and 120b with anodes connected respectively to bus bars c and b and cathodes connected respectively to bus bars a and b. A transceiver 126 may also be operatively attached to controller 122. Transceiver 126 may provide power line communications (PLC) at node Y and/or node X. Transceiver 126 may alternatively provide wireless communications. A single pole switch SW1 connects serially between the cathode of diode 120a and node X. The control of switch SW1 is operatively attached to controller 122. Switch SW1 may be opened and closed by controller 122. A bypass circuit 121 is connected across nodes X and Y. Nodes X and Y provide connection of a circuit 103 into serial string 107. An alternative implementation of bypass circuit 121 shown in FIG. 1b, may have bypass diodes 120a and 120b replaced by two bypass circuits 121.

During normal operation of solar power harvesting system 10, panels 101 are irradiated by the Sun, panel 101 current (I_PV) is substantially equal to the string current (I_string), switch SW1 is closed and current (I_B-out) flowing through output bypass circuit 121 is substantially zero. The maximum string current (I_string) is normally limited by the worst performing panel 101 in a photovoltaic string 107 by virtue of Kirchhoff current law.

In a panel 101, if certain photovoltaic cells in sub-string 11 are shaded, the current passing through the shaded cells may be offered an alternative, parallel path through the inactive cells, and the integrity of the shaded cells may be preserved. The purpose of diodes 120a and 120b is to draw the current away from the shaded or damaged cells associated with diodes 120a and 120b in respective sub-strings 11. Bypass diodes 120a and 120b become forward biased when their associated shaded cells in one or more sub-strings 11 become reverse biased. Since the photovoltaic cells in a sub-string 11 and the associated bypass diodes 120a and 120b are in parallel, rather than forcing current through the shaded photovoltaic cells, the bypass diodes 120a and 120b bypass the current away from the shaded cells and maintains the connection to the next sub-string 11.

Controller 122 may be programmed under certain circumstances based on previously determined criteria, for instance based on current and voltage sensed on sensors 124a-124d, to open switch SW1, and thereby disconnect panel 101 from serial photovoltaic string 107. Bypass circuit 121 may be configured to provide a low impedance path such that the output bypass current (I_B-out) of bypass circuit 121 is substantially equal to string 107 current (I_string). Bypass circuit 121 allows disconnection of photovoltaic panel 101 from photovoltaic string 107 while maintaining current flow and power production from the remaining photovoltaic panels 101 of photovoltaic string 107.

Reference is now made to FIGS. 1c and 1d which show two variant switch circuits controllable by controller 122 for switch SW1 shown in FIG. 1b. The first switch circuit switch SWa is a single pole switch or semiconductor switch, e.g. FET, with a diode connected in parallel across the single pole switch. Switch SWa may be connected serially between node X and the cathode of diode 120b with the anode of the diode of switch SWa connected to the cathode of diode 120b and the cathode of the switch diode to node X. When switch SWa is open circuit, current from panel 101 to node X may flow through the diode of switch SWa and any reverse current from node X may be blocked. A similar series arrangement for switch SW1 is shown in FIG. 1d where switch SWa is wired in series with another switch SWb.

Switch SW1 may alternatively or in addition be connected at the low voltage terminal between node Y and the anode of diode 120a. An alternative arrangement for switch SW1 may have switch SWa connected serially between node X and the cathode of diode 120b and to have another switch SWb connected serially between node Y and the anode of diode 120a. In this alternative, the diode of switch SWb has an anode connected to node Y and a cathode connected to the anode of diode 120a. In this alternative, when both switches SWa and SWb are open circuit, current from panel 101 to node X may flow through the diode of switch SWa and any reverse current from node X may be blocked. Similarly, current from node Y to panel 101 may flow through the diode of switch SWb and any reverse current from node Y may be blocked.

Reference is now made to FIG. 1e which shows more details of an active bypass circuit 121 according to an exemplary feature. Bypass circuit 121 includes switches SW2 and SW3 (operatively attached to a controller 130) and a charging circuit 141. Switches SW2 and SW3 in the example are implemented using metal oxide semiconductor field effect transistors (MOSFETs). Alternative solid state switches, e.g. bipolar transistors may be used for switches SW2 and SW3. The drain (D) of switch SW2 connects to node X. The source (S) of switch SW2 connects to the source (S) of switch SW3. An integral diode of switch SW2 has an anode connected to the source (S) of switch SW2 and a cathode connected to the drain (D) of switch SW2. The drain (D) of switch SW3 connects to node Y. Switch SW3 may have an integral diode with an anode connected to the source (S) of switch SW3 and a cathode connected to the drain (D) of switch SW3. Controller 130 connects to and senses node Z where the source of switch SW2 connects to the source (S) of switch SW3. Controller 130 connects to and senses node X and also connects to and senses node Y the drain (D) of switch SW3. Controller 130 also provides the direct current (DC) voltage (V_logic) required by buffer drivers B1 and B2. Buffer drivers B1 and B2 ensure sufficient power is available to turn switches SW2 and SW3 on and off. The outputs of buffer drivers B1 and B2 are connected to the gates (G) of switches SW2 and SW3 respectively. Buffer drivers B1 and B2 receive their respective logic inputs from controller 130. Charging circuit 141 has an input which connects to node Y and to node Z. Connected to node Z is the anode of a zener diode Z1. The cathode of zener diode Z1 connects to node Y. Zener diode Z1 may be alternatively implemented as a transient voltage suppression (TVS) diode. A charge storage device, e.g. capacitor C1 has one end connected to the cathode of diode rectifier DR1 and the other end of charge storage device C1 connected to node Z. The anode of diode rectifier DR1 connects to node Y. Charge storage C1 device may be a capacitor, a battery or any device known in the art for storing electrical charge. The end of capacitor C1 connected to the cathode of diode rectifier DR1 provides the DC voltage (V_logic) to controller 130 and buffer drivers B1 and B2.

During the normal operation of power harvesting system 10 during which panels 101 are irradiated, the output of a circuit 103 need not be bypassed by bypass circuit 121. Bypass circuit 121 does not bypass by virtue of switches SW2 and SW3 both being off (open). Switches SW2 and SW3 both being off means substantially no current between respective drains and sources of switches SW2 and SW3 because the respective gates (G) of switches SW2 and SW3 are not been driven by buffer drivers B1 and B2.

By virtue of the analog inputs of controller 130 to the source (S) and drain (D) of switches SW2 and SW3 respectively and the source (S) of switch SW3, controller 130 is able to sense if an open circuit or a reverse voltage polarity exists across nodes X and Y. The open circuit sensed on nodes X and Y may indicate that switch SW1 is open and/or a sub-string 11 is open circuit. The reverse polarity across nodes X and Y may indicate that a panel 101 is shaded or faulty or that the panel 101 is operating as a sink of current rather than as a source of current.

The open circuit and/or the reverse polarity across nodes X and Y may cause bypass circuit 121 to operate in a bypass mode of operation. The bypass mode of operation of bypass circuit 121 may be when a panel 101 is partially shaded. The bypass mode of operation of circuit 121 may also be just before the normal operation when it still too dark to obtain a significant power output from panels 101, circuit 121 may have no power to work.

Reference is now made to FIG. 1f which shows a timing diagram for circuit 121 operation. As soon as sufficient light irradiates panels 101 and current flows in photovoltaic string 107, zener diode Z1 has voltage drop VZ1 which charges capacitor C1 so as to provide V_logic to controller 130. When capacitor C1 is being charged during time T1, the voltage drop of the output across nodes X and Y is the voltage (VZ1) of zener Z1 plus the voltage across the integral diode of switch SW2. When V_logic is sufficient, all the active circuitry in controller 130 starts to work which closes switches SW2 and SW3 for a time period T2. Time period T2 may be much greater than time period T1. Switches SW2 and SW3 being closed (during time T2) gives a voltage drop across nodes X and Y. Therefore, with the longer time period T2 and the voltage drop across nodes X and Y, overall, less power may be lost by bypass circuit 121. Controller 130 continues to work until the voltage (V_logic) of charge storage device C1 drops below a minimal voltage and once again charge storage device C1 has voltage drop VZ1 from zener Z1 which charges capacitor C1 so as to provide V_logic. Once sufficient power is generated from panels 101, controller 130 can get a voltage supply from a panel 101 at nodes X and Y. Controller 130 may also further receive an external enable in order to work in synchronization with all the other bypass circuits 121 in a photovoltaic string 107.

During the bypass mode, controller 130 is able to sense on nodes X and Y if a panel 101 is functioning again and so controller 130 removes the bypass. The bypass across nodes X and Y is removed by turning switches SW2 and SW3 off.

Reference now made to FIG. 1g which shows an example of a system level diagram of a controller 122 which may be implemented in a circuit 103. Controller 122 includes a processor 16 which may be operatively attached to transceiver 126, switch SW1, sensors 124a-124d and storage 18. Storage 18 may include software modules and/or additional circuitry may provide functionality such as: for monitoring performance of the photovoltaic panel 160, ground fault detection 166, safety/normal mode operation 169, arc detection and elimination 162, theft detection and prevention 164. Circuit 103 may be configured to avoid power conversion, e.g. DC to DC conversion during normal power production. Circuit 103 may be configured to avoid maximum power point tracking of photovoltaic panel 101. In some configurations, switch SW1 may be a single switch, e.g. FET and therefore extra components, e.g. FET switches may be avoided.

160 Monitoring Performance and Control of Photovoltaic panel 101 and Circuit 103

Monitoring performance of photovoltaic panels has been disclosed by the present inventors in US patent publication 2008/0147335. Monitoring may include monitoring input power at the input terminals (bus bars a,b,c) of circuit 103 and/or output power at output terminals nodes X and Y of circuit 103 by sensing current and voltage using sensors 124a-124d of circuit 103. Temperature sensors (not shown) may also be included in circuit 103 for measuring ambient temperature, temperature on the circuit board of circuit 103 and/or temperature of the photovoltaic panel 101. Monitoring results may be periodically or randomly transmitted to central unit 109 by communications over DC lines to inverter 105 or by wireless communication. Based on the monitoring results, if one or more sensed parameters are found out of rated specification, controller 122 may be programmed to activate, e.g. open switch SW1 and to disconnect photovoltaic panel 101 from photovoltaic string 107. Bypass circuit 121 autonomously bypasses string current around SW1 and photovoltaic panel 101.

DC power cables connecting load 105 to photovoltaic panel 101 and/or circuits 103 may provide a communication channel between central unit 109 and photovoltaic panels 101 As previously disclosed by the present inventors in co-pending patent application GB1100463.7, lengths of cables connecting load 105 to panels 101 or circuits 103 may be long and may contain one or several wire cores. The topography of a distributed power generation system to a large extent dictates the installation and placement of cable runs. Physical proximity of wires not having an electrical association may increase the chances of the wires in the cables being subject to the effects of noise if those wires are to be considered for signaling by DC power line communications. Crosstalk is a type of noise which refers to a phenomenon by which a signal transmitted on a cable, circuit or channel of a transmission system creates an undesired effect in another cable, circuit or channel Crosstalk may be usually caused by undesired capacitive, inductive, or conductive coupling from one cable, circuit or channel, to another. Crosstalk may also corrupt the data being transmitted. Known methods of preventing the undesirable effects of crosstalk may be to utilize the shielding of cables, junction boxes, panels, inverters, loads or using twisted pair cables. Additionally, filtering techniques such as matched filters, decoupling capacitors or chokes may be used to prevent the undesirable effects of crosstalk. However, these ways of preventing the undesirable effects of crosstalk may be unavailable or impractical in a power generation system and/or may be prohibitively expensive in terms of additional materials and/or components required.

Within photovoltaic installation 10, a wire at positive potential and a wire at negative potential electrically associated therewith may be physically proximate thereto only at a point of connection to a piece of equipment. However, elsewhere in photovoltaic field 10, the wires may be separated and not be within the same cable run. In a photovoltaic power generation system, with power line communication over DC cables, it may be desirable to send a control signal or receive a monitoring signal between central unit 109 and circuit 103. Crosstalk may cause the other circuits 103 in power generation system 10 to inadvertently receive the control signal which is of course undesirable.

A method is disclosed, whereby signaling between a photovoltaic module 101/103 and a load 105 provides an association between the photovoltaic module 101/103 and the load 105. In an initial mode of operation, an initial code may be modulated to produce an initial signal. The initial signal may be transmitted by central unit 109 along DC line from load 105 to circuit 103. The initial signal may be received by circuit 103. The operating mode may be then changed to a normal mode of operation, and during the normal mode of operation a control signal may be transmitted central unit 109 along DC line from load 105 to circuit 103. A control code may be demodulated and received from the control signal. The control code may be compared with the initial code producing a comparison. The control command of the control signal may be validated as a valid control command associated with load 105 with the control command only acted upon when the comparison is a positive comparison.

166 Ground Fault Detection

As previously disclosed by the present inventors in co-pending application GB1020862.7, a device may be adapted for disconnecting at least one string carrying direct current power in multiple interconnected strings. Similarly, circuit 103 may include a differential current sensor adapted to measure a differential current by comparing respective currents in the positive lines (terminating at node X) and negative line (terminating at node Y). The differential current may be indicative of a ground fault in circuit 103 and/or photovoltaic panel 101. If a potential ground fault is detected, then SW1 and/or a similar switch in the negative line may be activated, e.g. opened. Bypass circuit 121 may autonomously bypass string current around SW1 and photovoltaic panel 101.

169 Safety/Normal Mode Operation

During normal mode operation of circuit 103, electrical power produced by photovoltaic panel 101 is provided to string 107. Maximum power point tracking may be provided at the input of load 105 for the interconnected strings so that in absence of shading or component failure most or all of photovoltaic panels contribute to the harvested power at or near the maximum power point. In conventional solar power harvesting systems, potential electric shock hazard may exist on the output terminals of the photovoltaic module 101/103.

Consequently, during installation of a conventional system, photovoltaic panels may be covered to avoid light absorption by the photovoltaic panels and to prevent electrocution during installation.

A safety mode of operation may be provided by activating or toggling switch SW1, which may be a portion of a buck and/or boost converter in circuit 103 attached to a photovoltaic panel 101. Toggling switch SW1 at a known duty cycle may be used to force photovoltaic panel 101 far away from its maximum power point and the power output to string 107 may be forced to be very low avoiding other safety means such as covering photovoltaic panels during installation.

During the safety mode of operation, photovoltaic module 101/103 may be connected or disconnected and while being irradiated by the sun. Therefore, during the routine maintenance or installation of the power harvesting system 10, controller 122 of circuit 103 may be configured to open and close switch SW1 to produce a safe working output power on output terminals of the circuit 103. The safe working output power may be according to a predetermined duty cycle of switch SW1 opening and closing.

During the normal operation of the power harvesting system 10 when power harvesting system 10 is irradiated, it may be that photovoltaic module 101/103 is disconnected from a string 107 as a result of a malfunction or theft. In the case of theft it may well be desirable that a safe working output power on output terminals of the circuit 103 is produced so that a thief is not electrocuted for example.

164 Theft Detection

A number methods and/or devices for detection and/or theft prevention of photovoltaic panels are disclosed by the present applicant(s) in United States Patent Application 20100301991.

The use of codes is discussed above as a mechanism to avoid cross talk in monitoring and control signals carried over DC lines to central unit 109. Codes may be additionally used as a mechanism for theft detection and prevention. A first code is written in memory associated with load 105 and a second code is stored in the memory 18 located and operatively attached to circuit 103. The second code may be based on the first code or the second code may be a copy or a hash of the first code. The writing of the first code and/or the storing of the second code may be performed during installation of the power harvesting system. After the first code is read and stored in the first memory, and the second code is read and stored in memory 18, during the electrical power generation, the first code is compared with the second code or its hash. If the comparison is correct, (for instance the codes correspond) then power transfer from circuit 105 to string 107 is allowed, and switch SW1 is closed. Otherwise, if the codes do not match then switch SW1 is opened by controller 122. If circuit 105 is permanently attached or highly integrated with photovoltaic panel 101 then it will be difficult for the thief to benefit from the theft. Other methods for theft detection and/or protection as disclosed in international application PCT/IB2010/052413 may similar be used in conjunction with the present disclosure.

162 Arc Detection

Electric arcing can have detrimental effects on electric power distribution systems and electronic equipment. Arcing may occur in switches, circuit breakers, relay contacts, fuses and poor cable terminations. When a circuit is switched off or a bad connection occurs in a connector, an arc discharge may form across the contacts of the connector. An arc discharge is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through a medium such as air which is normally non-conducting. At the beginning of a disconnection, the separation distance between the two contacts is very small. As a result, the voltage across the air gap between the contacts produces a very large electrical field in terms of volts per millimeter. This large electrical field causes the ignition of an electrical arc between the two sides of the disconnection. If a circuit has enough current and voltage to sustain an arc, the arc can cause damage to equipment such as melting of conductors, destruction of insulation, and fire.

FIG. 3 shows a method 301 for arc detection in system 10 shown in FIG. 1a. In step 303 an initial mode of operation for system 10 is initiated. The initial mode may be when system 10 is first installed, when after installation on a daily basis panels 101 are illuminated at dawn or after a routine maintenance of system 10 where panels 101 may have been replaced or cables reconnected etc. The initial mode may also be initiated at various times during the day and times of the month. The initial mode initiated at various times during the day and times of the month may be performed in respect to the fact that the orientation of the sun varies throughout the year. The initial mode may take into account other factors such as temperature, cloud cover or accumulated dust deposition on the surfaces of a panels 101 for example.

In the initial mode, a baseline noise voltage or current may be measured (step 305) for a string 107 or a group of interconnected strings 107 as shown in system 10 and the overall noise voltage or current for system measured at load 105 via sensors 119a and 119b. The initial mode initiated at various times during the day and times of the month may be stored in a look up table in central unit 109 and/or master circuit 103a or in each circuit 103. As a result of the baseline noise voltage or current measured in step 305 a noise voltage or current threshold 309 may be set in step 307. Threshold 309 may be an adaptive or a constant value which may be measured in frequency range between 10 kilo-Hertz (kHz) to 400 kHz. Once the threshold 309 value has been set for system 10, normal operation of system 10 is initiated in step 311. If the threshold value 309 is exceeded for a predefined time, indicating potential arcing, a panel 101 may be disconnected (step 205) from a string 107 using switch SW1 in the circuit 103 associated with the panel 101. Otherwise normal operation of system 10 continues in step 311.

Reference is now made to FIG. 2a which shows a method 251 applicable to system 122 shown in FIG. 1g. In decision 253 a number of malfunctions may be detected which including arc detection 162, theft detection 164, ground fault detection 166, or a monitored parameter fault detection. It is possible in decision 253 to have various combinations of detection together; for example, arc detection 162 along with theft detection 164 or arc detection 162 with theft detection 164 and ground fault detection 166. A detection of a malfunction may cause switch SW1 to open to disconnect panel 101 from string 107 and the output terminals of circuit 103 output may be autonomously bypassed by bypass circuit 121 (step 255).

Reference is made to FIG. 2b which shows an exemplary method 201 for circuit 103. In decision 203, if arcing is detected in the vicinity of a panel 101, panel 101 may be disconnected from a string 107 by opening switch SW1 in circuit 103. Panel 101 may be then bypassed using bypass 121. In decision 209, methods for arc detection may be applied to verify if arcing has been eliminated by bypassing circuit 103. If in decision block 209, arcing has not been eliminated, panel 101 may be re-connected in step 211 and another panel 101 may be selected in the string 107 and disconnected from string 107. Testing for arc elimination continues in decision 209. In decision 209 it may well be that if an arc is not eliminated, a whole string 107 may be disconnected by opening switches SW1 in string 107 and another string 107 may be checked to see if arcing may be taking place there instead.

A similar method to that shown in method 201 may also be applied to ground fault detection 166.

The indefinite articles “a”, “an” is used herein, such as “a switch”, “a module” have the meaning of “one or more” that is “one or more switches” or “one or more modules”.

Although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features. Instead, it is to be appreciated that changes may be made to these features without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.

Claims

1. A circuit comprising: a first node, a second node, and a third node;a first switch connected between the first node and the second node;a second switch connected between the second node and the third node;a first diode connected between the second node and the third node;a charge storage device connected to the second node; anda second diode with an anode connected to the third node and a cathode connected to the charge storage device.

2. The circuit of claim 1, wherein the first diode is a zener diode.

3. The circuit of claim 1, wherein the first diode is a transient voltage suppression (TVS) diode.

4. The circuit of claim 1, wherein the charge storage device includes a capacitor.

5. The circuit of claim 1, further comprising a third diode connected between the first node and the second node, and a fourth diode connected between the second node and the third node.

6. The circuit of claim 5, wherein the third diode is an integral diode of the first switch, and the fourth diode is an integral diode of the second switch.

7. The circuit of claim 1, wherein the first switch and the second switch are metal oxide semiconductor field effect transistors.

8. The circuit of claim 1, wherein a source of the first switch is connected to a source of the second switch.

9. The circuit of claim 1, further comprising a controller connected to the third node.

10. The circuit of claim 9, further comprising a first buffer driver and a second buffer driver, wherein the first buffer driver and the second buffer are connected to the controller, and wherein the first buffer driver is connected to a gate of the first switch, and the second buffer driver is connected to a gate of the second switch.

11. The circuit of claim 1, further comprising a third switch connected to the first node.

12. The circuit of claim 11, wherein the third switch is connected between the first node and a first photovoltaic panel.

13. The circuit of claim 12, wherein at least one of the first node or the third node is connected to a series string, wherein the series string includes a second photovoltaic panel.

14. A method comprising: connecting a first switch between a first node and a second node;connecting a second switch between the second node and a third node;connecting a first diode between the second node and the third node;connecting a charge storage device to the second node; andconnecting an anode of a second diode to the third node and a cathode of the second diode connected to the charge storage device.

15. The method of claim 14, wherein the first diode is a zener diode or a transient voltage suppression (TVS) diode.

16. The method of claim 14, wherein the charge storage device includes a capacitor.

17. The method of claim 14, further comprising connecting a third diode between the first node and the second node, and connecting a fourth diode between the second node and the third node.

18. The method of claim 17, wherein the third diode is an integral diode of the first switch, and the fourth diode is an integral diode of the second switch.

19. The method of claim 14, further comprising connecting a source of the first switch to a source of the second switch.

20. A system comprising: a series string of photovoltaic panels; anda circuit connected to the series string of photovoltaic panels, the circuit comprising: a first node, a second node, and a third node;a first switch connected between the first node and the second node;a second switch connected between the second node and the third node;a first diode connected between the second node and the third node;a charge storage device connected to the second node; anda second diode with an anode connected to the third node and a cathode connected to the charge storage device,wherein at least one of the first node or the third node is connected to at least one photovoltaic panel of the series string of photovoltaic panels.