Patent Description:
Utility networks provide an electrical power system to utility customers. The distribution of electric power from utility companies to customers utilizes a network of utility lines connected in a grid-like fashion, referred to as an electrical grid. The electrical grid may consist of many independent energy sources energizing the grid in addition to utility companies energizing the grid, with each independent energy source being referred to as a distributed power (DP) generation system. The modern utility network includes the utility power source, consumer loads, and the distributed power generation systems which also supply electrical power to the network. The number and types of distributed power generation systems is growing rapidly and can include photovoltaics, wind, hydro, fuel cells, storage systems such as battery, superconducting flywheel, and capacitor types, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines. These distributed power generation systems are connected to the utility network such that they operate in parallel with the utility power sources.

<CIT> discloses a solar array power generation system including a solar array electrically connected to a control system. The solar array has a plurality of solar modules, each module having at least one DC/DC converter for converting the raw panel output to an optimized high voltage, low current output. Each DC/DC converter requires a signal to enable power output of the solar modules.

<CIT> discloses a signaling system for detecting power line discontinuity in a power distribution system having a local power generator in a feeder line employing a signal generator that provides a local generator control signal with a preset protocol continuously and is located in the power distribution system upstream of said local power generator. A power detector terminates the local generator control signal and a signal detector in said feeder line detects the presence of the signal and triggers the shutdown of the local generator when the signal is not detected by the signal detector for a predefined period of time.

<CIT> discloses a power line communication device for a vehicle that detects a communication signal received through a power line and extracts incoming data composed of a digital signal. The device dulls a waveform of the digital signal of the incoming data by use of a resistor and a capacitor, and thereby converts the incoming data into an analog signal. Thereafter, the device converts the analog signal into a digital signal by use of an inverter circuit based on a given threshold level, and thereby subjects the incoming data to waveform shaping.

<CIT>, published as <CIT>, discloses solar power generation plants comprising one or more parallel strings of photovoltaic (PV) modules that feed into a low-voltage system via inverted rectifiers. A switching element is associated with each PV module at the output end, the switching element being switchable by means of an enable signal in such a way that the associated PV module is dead when there is no enable signal while being activated when an enable signal is supplied.

<CIT> discloses a device has a number of photovoltaic solar modules whose rated power fluctuates depending on parameters such as solar intensity, module temperature, solar technology and aging, and D. converters connected in parallel on the output side and to a central D. converter that converts the intermediate D. voltage from the converters into a sinusoidal voltage of defined frequency. Each solar module is electrically connected to an individual D. voltage converter that transforms the module output D. voltage into a significantly higher intermediate D. voltage, so that the solar modules are decoupled by their individual D. voltage converters.

A conventional installation of a solar distributed power system <NUM>, including multiple solar panels <NUM>, is illustrated in <FIG>. Since the voltage provided by each individual solar panel <NUM> is low, several panels <NUM> are connected in series to form a string <NUM> of panels <NUM>. For a large installation, when higher current is required, several strings <NUM> may be connected in parallel to form overall system <NUM>. The interconnected solar panels <NUM> are mounted outdoors, and connected to a maximum power point tracking (MPPT) module <NUM> and then to an inverter <NUM>. MPPT <NUM> is typically implemented as part of inverter <NUM> as shown in <FIG>. The harvested power from DC sources <NUM> is delivered to inverter <NUM>, which converts the direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at <NUM>, or 220V at <NUM>. The AC current from inverter <NUM> may then be used for operating electric appliances or fed to the power grid.

As noted above, each solar panel <NUM> supplies relatively very low voltage and current. A problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current i<NUM>. Furthermore, a power inverter, such as inverter <NUM>, which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of <NUM> (which is the peak voltage). Hence, in many applications, the power sources, such as solar panels <NUM>, are combined in order to reach the correct voltage or current. A large number of panels <NUM> are connected into a string <NUM> and strings <NUM> are connected in parallel to power inverter <NUM>. Panels <NUM> are connected in series in order to reach the minimal voltage required for inverter <NUM>. Multiple strings <NUM> are connected in parallel into an array to supply higher current, so as to enable higher power output.

<FIG> illustrates one serial string <NUM> of DC sources, e.g., solar panels 101a -101d, connected to MPPT circuit <NUM> and inverter <NUM>. The current versus voltage (IV) characteristics is plotted (110a - 110d) to the left of each DC source <NUM>. For each DC power source <NUM>, the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications the voltage value may assume a negative value, meaning that the source becomes a sink. Bypass diodes (not shown) are used to prevent the source from becoming a sink. The power output of each source <NUM>, which is equal to the product of current and voltage (P=i*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point (MPP). The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources.

In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of system <NUM> using MPPT module <NUM>. MPPT module <NUM> receives the current extracted from all of solar panels <NUM> together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module <NUM> maintains a current that yields the maximum average power from system <NUM>.

However, since power sources 101a - 101d are connected in series to single MPPT <NUM>, MPPT <NUM> selects a maximum power point which is some average of the maximum power points of the individual serially connected sources <NUM>. In practice, it is very likely that MPPT <NUM> would operate at an I-V point that is optimum for only a few or none of sources <NUM>. In the example of Figure IB, the selected point is the maximum power point for source 101b, but is off the maximum power point for sources 101a, 101c and 101d. Consequently, the arrangement is not operated at best achievable efficiency.

The present applicant has disclosed in co-pending <CIT> entitled "Distributed Power Harvesting Systems Using DC Power Sources", the use of an electrical power converter, e.g. DC-to-DC converter, attached to the output of each power source, e.g. photovoltaic panel. The electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level.

The term "signaling" or "signaling mechanism" as used herein refers to either a signal modulated on an electromagnetic carrier signal or a simple unmodulated signal such as an on/off signal "keep alive" signal or "dry contact" signal. For a modulated signal, the modulation method may be by any such method known in the art by way of example, frequency modulation (FM) transmission, amplitude modulation (AM) , FSK (frequency shift keying) modulation, PSK (phase shift keying) modulation, various QAM (Quadrature amplitude modulation) constellations, or any other method of modulation.

The term "power module" as used herein includes power converters such as a DC-DC power converter but also includes modules adapted to control the power passing through the module or a portion of the power, whether by switching or other means.

According to an aspect of the present invention, there is provided a system as defined in claim <NUM>.

The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate various features of the illustrated embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not necessarily drawn to scale.

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

It should be noted, that although the discussion herein relates primarily to wake- up and shutdown methods in photovoltaic systems and more particularly to those systems previously disclosed in <CIT>, the present invention may, by non-limiting example, alternatively be configured as well using conventional photovoltaic distributed power systems and other distributed power systems including (but not limited to) wind turbines, hydroturbines, 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, it is important to note that aspects of the present invention have important safety benefits. While installing or performing maintenance on photovoltaic systems according to certain aspects of the present invention, installers are protected from danger of shock or electrocution since systems according to embodiments of the present invention do not output potentially dangerous high voltage and/or currents when an operational inverter is not connected during installation and maintenance procedures.

Before explaining embodiments 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 scope of protection is solely defined by the claims.

Reference is now made to <FIG> which illustrates a distributed power harvesting circuit <NUM> which is not covered by the scope of the claims, based on the disclosure in<CIT>. Circuit <NUM> enables connection of multiple distributed power sources, for example solar panels 101a - 101d, to a single power supply. Series string <NUM> of solar panels <NUM> may be coupled to an inverter <NUM> or multiple connected strings <NUM> of solar panels <NUM> may be connected to a single inverter <NUM>. In configuration <NUM>, each solar panel 101a - 101d is connected individually to a separate power converter circuit or a module 205a - 205d. Each solar panel <NUM> together with its associated power converter circuit <NUM> forms a power generating element <NUM>. (Only one such power generating element <NUM> is marked in <FIG>. ) Each converter 205a - 205d adapts optimally to the power characteristics of the connected solar panel 101a - 101d and transfers the power efficiently from input to output of converter <NUM>. Converters 205a - 205d are typically microprocessor controlled switching converters, e.g. buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters 205a - 205d may also contain a number of component converters, for example a serial connection of a buck and a boost converter. Each converter 205a - 205d includes a control loop <NUM>, e.g. MPPT loop that receives a feedback signal, not from the converter's output current or voltage, but rather from the converter's input coming from solar panel <NUM>. The MPPT loop of converter <NUM> locks the input voltage and current from each solar panel 101a - 101d at its optimal power point, by varying one or more duty cycles of the switching conversion typically by pulse width modulation (PWM) in such a way that maximum power is extracted from each attached panel 101a - 101d. The controller of converter <NUM> dynamically tracks the maximum power point at the converter input. Feedback loop <NUM> is closed on the input power in order to track maximum input power rather than closing a feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters.

As a result of having a separate MPPT circuit in each converter 205a - 205d, and consequently for each solar panel 101a - 101d, each string <NUM> may have a different number or different specification, size and/or model of panels 101a - 101d connected in series. System <NUM> of <FIG> continuously performs MPPT on the output of each solar panel 101a - 101d to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels 101a - <NUM> d. As a result, the MPPT circuit within the converters 205a - 205d harvests the maximum possible power from each panel 101a - 101d and transfers this power as output regardless of the parameters effecting other solar panels 101a - 101d.

The outputs of converters 205a - 205d are series connected into a single DC output that forms the input to inverter <NUM>. Inverter <NUM> converts the series connected DC output of converters 205a - 205d into an AC power supply, Inverter <NUM>, regulates the voltage at the input of inverter <NUM>. In this example, an independent control loop <NUM> holds the voltage input to inverter <NUM> at a set value, say <NUM> volts. The current at the input of inverter <NUM> is typically fixed by the power available and generated by photovoltaic panels <NUM>.

information regarding wakeup or shut-down may be conveyed from inverter <NUM> to converters <NUM>. The information may be transmitted using any of the methods well known to those experienced in the art. According to certain embodiments, a modulation method is used, by way of example, frequency modulation (FM) transmission, amplitude modulation (AM), FSK (frequency shift keying) modulation, PSK (phase shift keying) modulation, various QAM (Quadrature amplitude modulation) constellations, or any other method of modulation. Alternatively and not forming part of the invention, inverter <NUM>, while converting power from its input to its output, actively creates a frequency ripple in serial string <NUM>. During normal operation, the <NUM> (or <NUM> in USA) ripple is detectable in serial string <NUM> since the capacitors of inverter <NUM> do not entirely block the alternating current (AC), and an additional signaling mechanism is not required to produce the <NUM>/<NUM> signal in serial string <NUM>. Alternatively or in addition, and not forming part of the invention, one or more switching frequencies of inverter <NUM>, typically <NUM> or <NUM> may be detectable as leakage or provided intentionally to serial string <NUM>.

Reference is now made to <FIG> which supports the present invention but is not part of the invention per-se. In <FIG>, converter <NUM> is shown in more detail. Integrated with power converter <NUM> is a detector/receiver <NUM>, according to a feature of the present invention which is configured to receive, optionally amplify and detect the signal, e.g. at <NUM>/<NUM> originating in inverter <NUM>.

Controller <NUM> preferably either polls a signal input <NUM> from receiver/detector <NUM> or uses signal input <NUM> as an interrupt so that only when detector/receiver <NUM> detects the <NUM>/<NUM> signal, is module <NUM> in a normal operating mode converting power from its input to its output. Receiver <NUM> is alternatively configured to detect the <NUM>/<NUM> inverter switching frequency and provides an enabling signal to controller on signal input <NUM> while inverter <NUM> is operating.

Reference is now made to <FIG> which illustrates an exemplary DC-to-DC converter <NUM>, according to a feature of the present invention. DC-to-DC converters are used to either step down or step up a DC voltage input to a higher or a lower DC voltage output, depending on the requirements of the output circuit. However, in the embodiment of <FIG> the DC-DC converter <NUM> is used as a power converter, i.e., transferring the input power to output power, the input voltage varying according to the MPPT at the input, while the output current is dictated by the constant input voltage to inverter <NUM>, <NUM>. That is, the input voltage and current may vary at any time and the output voltage and current may vary at any time, depending on the operating condition of DC power sources <NUM>.

Converter <NUM> is connected to a corresponding DC power source <NUM> at input terminals <NUM> and <NUM>. The converted power of the DC power source <NUM> is output to the circuit through output terminals <NUM>, <NUM>. Between the input terminals <NUM>, <NUM> and the output terminals <NUM>, <NUM>, the converter circuit includes input and output capacitors <NUM>, <NUM>, backflow prevention diodes <NUM>, <NUM> and a power conversion circuit including a controller <NUM> and an inductor <NUM>.

Diode <NUM> is in series with output <NUM> with a polarity such that current does not backflow into the converter <NUM>. Diode <NUM> is coupled between the positive output lead <NUM> through inductor <NUM> which acts a short for DC current and the negative input lead <NUM> with such polarity to prevent a current from the output <NUM> to backflow into solar panel <NUM>.

A potential difference exists between wires <NUM> and <NUM> due to the electron-hole pairs produced in the solar cells of panel <NUM>. Converter <NUM> maintains maximum power output by extracting current from the solar panel <NUM> at its peak power point by continuously monitoring the current and voltage provided by panel <NUM> and using a maximum power point tracking algorithm. Controller <NUM> includes an MPPT circuit or algorithm for performing the peak power tracking. Peak power tracking and pulse width modulation (PWM) are performed together to achieve the desired input voltage and current. The MPPT in controller <NUM> may be any conventional MPPT, such as, e.g., perturb and observe (P&O), incremental conductance, etc. However, notably the MPPT is performed on panel <NUM> directly, i.e., at the input to converter <NUM>, rather than at the output of converter <NUM>. The generated power is then transferred to the output terminals <NUM> and <NUM>. The outputs of multiple converters <NUM> are connected in series, such that the positive lead <NUM> of one converter <NUM> is connected to the negative lead <NUM> of the next converter <NUM>.

In <FIG>, converter <NUM> is shown as a buck plus boost converter. The term "buck plus boost" as used herein is a buck converter directly followed by a boost converter as shown in <FIG>, which may also appear in the literature as "cascaded buck-boost converter". If the voltage is to be lowered, the boost portion is substantially shorted. If the voltage is to be raised, the buck portion is substantially shorted. The term "buck plus boost" differs from buck/boost topology which is a classic topology that may be used when voltage is to be raised or lowered, and sometimes appears in the literature as "cascaded buck-boost". The efficiency of "buck/boost" topology is inherently lower then a buck or a boost. Additionally, for given requirements, a buck-boost converter will need bigger passive components then a buck plus boost converter in order to function. Therefore, the buck plus boost topology of <FIG> has a higher efficiency than the buck/boost topology. However, the circuit of <FIG> continuously decides whether it is bucking or boosting. In some situations when the desired output voltage is similar to the input voltage, then both the buck and boost portions may be operational.

The controller <NUM> may include a pulse width modulator, PWM, or a digital pulse width modulator, DPWM, to be used with the buck and boost converter circuits. Controller <NUM> controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. In some circumstances both the buck and boost portions may operate together. That is, the input voltage and current are selected independently of the selection of output current and voltage. Moreover, the selection of either input or output values may change at any given moment depending on the operation of the DC power sources. Therefore, in the embodiment of <FIG>, converter <NUM> is constructed so that at any given time a selected value of input voltage and current may be up converted or down converted depending on the output requirement.

In one implementation, an integrated circuit (IC) <NUM> may be used that incorporates some of the functionality of converter <NUM>. IC <NUM> is optionally a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC <NUM> may be designed for a high mean time between failures (MTBF) of more than <NUM> years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in <FIG>, the buck plus boost portion of the converter <NUM> is implemented as the IC <NUM>. Practical considerations may lead to other segmentations of the system. For example, in one aspect of the invention, the IC <NUM> may include two ICs, one analog IC which handles the high currents and voltages in the system, and one simple low-voltage digital IC which includes the control logic. The analog IC may be implemented using power FETs which may alternatively be implemented in discrete components, FET drivers, ADs, and the like. The digital IC may form controller <NUM>.

In the exemplary circuit <NUM> shown, the buck converter includes input capacitor <NUM>, transistors <NUM> and <NUM>, diode <NUM> positioned in parallel to transistor <NUM>, and inductor <NUM>. Transistors <NUM>, <NUM> each have a parasitic body diode <NUM>, <NUM>. The boost converter includes inductor <NUM>, which is shared with the buck converter, transistors <NUM> and <NUM> a diode <NUM> positioned in parallel to transistor <NUM>, and output capacitor <NUM>. Transistors <NUM>, <NUM> each have a parasitic body diode <NUM>, <NUM>.

System <NUM> includes converters <NUM> which are connected in series and carry the current from string <NUM>. If a failure in one of the serially connected converters <NUM> causes an open circuit in failed converter <NUM>, current ceases to flow through the entire string <NUM> of converters <NUM>, thereby causing system <NUM> to stop functioning. Aspects of the present invention provide a converter circuit <NUM> in which electrical components have one or more bypass routes associated with them that carry the current in case of an electrical component failing within one of converters <NUM>. For example, each switching transistor of either the buck or the boost portion of the converter has its own diode bypass. Also, upon failure of inductor <NUM> , the current bypasses the failed inductor <NUM> through parasitic diodes <NUM>,<NUM>.

In <FIG>, detector/receiver block <NUM> is shown which is configured to provide an enable signal <NUM> to microcontroller <NUM>.

Reference in now made to <FIG>, which illustrate system <NUM>, according to an embodiment of the present invention. For simplicity, a single string <NUM> is shown of distributed power sources, e.g. solar panels 101a-101d connected to respective power modules 405a-d. Serial string <NUM> is input to conventional inverter <NUM> through wires <NUM> and <NUM>. The output of inverter <NUM> is connected to and supplies electrical power to the electrical grid. At the input of inverter <NUM>, is connected a signaling mechanism <NUM> which superimposes a signal on serial string <NUM> through wires <NUM> and <NUM> when inverter <NUM> is converting power to the grid.

Reference is now also made to <FIG> which illustrates in more detail signaling mechanism <NUM>. Signaling mechanism <NUM> includes a relay <NUM> which is normally open and controlled by a microcontroller <NUM>. Relay <NUM> is switched at a given rate, e.g. <NUM>, and the signal is superimposed by action of relay <NUM> onto serial string <NUM> over wires <NUM> and <NUM>. Microcontroller <NUM> typically provides the control of the signal, e.g. <NUM>, during normal operation of distributed power system <NUM>. Microcontroller <NUM> is typically connected to one or more sensors in order to monitor the operation of inverter <NUM>. In the example of <FIG>, microcontroller <NUM> monitors over- voltage of the input DC voltage to inverter <NUM>. The example shown in <FIG> includes an input DC voltage tap <NUM> connected to an analog to digital converter (A/D) <NUM>, the output of which is provided to microcontroller <NUM>. The tap <NUM> may be, e.g., a Hall-effect sensors, series connected resistor across which the voltage drop is measured, etc. In one embodiment, an over-voltage condition as measured by microcontroller <NUM>, results in microcontroller <NUM> stopping the signaling through relay <NUM> and/or opening one or more protective relays <NUM>, <NUM> in series with the input DC voltage to inverter <NUM>. Note that one switch <NUM> or <NUM> may be enough for performing the required action, and two switches in series are shown solely for the purpose of illustration that double protection might be required by some regulatory bodies. A power management block <NUM> taps voltage for powering microcontroller <NUM> and any other active electronics components (not shown) in block <NUM>.

Reference is now made to <FIG> which illustrates in more detail certain aspects of power module <NUM>. Integrated with power module <NUM> is detector/receiver <NUM> which is configured to receive, optionally amplify and detect the signal, e.g. at <NUM>, produced by signal mechanism <NUM>. Controller <NUM> preferably either polls signal input <NUM> or uses signal input <NUM> as an interrupt so that only when detector/receiver <NUM> detects the <NUM> signal, is module <NUM> operating in a normal operating mode. Power module <NUM> is shown to include a bypass diode <NUM>. Optionally, power module <NUM> may include a conventional DC/DC switching converter with a control loop based on output power, Power module <NUM> includes at least one switch <NUM> controlled by controller <NUM> which functions to stop normal operation of power from the input of module <NUM> to the output of <NUM> when signal input <NUM> is absent indicating that inverter <NUM> is not transferring power to the electrical grid.

Reference is now made to <FIG> which illustrates a simplified method for safe operation of system <NUM>, according to an aspect of the present invention. In step <NUM>, active control circuits, e.g. microcontroller <NUM>, are turned on. Module <NUM>, <NUM> begins operation (step <NUM>) in a safety mode. In safety mode, output voltage from module <NUM> is limited, for instance output voltage is limited to <NUM> volts and output current is limited to <NUM> mA so that a person can touch the wires of serial string <NUM>, <NUM> without any danger of electrocution.

Controller <NUM> maintains safety mode operation (step <NUM>) until a communications signal, e.g. <NUM>, is received (decision box <NUM>) by receiver/detector <NUM> from signaling block <NUM>. When the communications signal is received (decision block <NUM>) indicating inverter <NUM> or <NUM> is connected and converting power, safety mode (step <NUM>) of operation ends. When the communications signal is received (decision block <NUM>), module <NUM> preferably enters a normal operation mode (step <NUM>), typically with maximum power point tracking. The normal operation of transferring power is maintained as long as the communications signal, e.g. <NUM> is received from signal mechanism <NUM>, and no other warning condition is present. If the communications signal is not detected, or another warning condition is present, the normal mode (step <NUM>) is typically ended and power conversion of modules <NUM> is typically turned off. If in decision box <NUM>, the communications signal is not detected, or another warning condition is present, the normal mode (step <NUM>) is typically ended and power conversion of modules <NUM> is typically turned off.

Reference is now made to <FIG>, which illustrates a method <NUM> for wake-up and shutdown of module <NUM>, according to embodiments of the present invention. Method <NUM> is applicable to both systems <NUM> and <NUM>. In step <NUM>, active control circuits, e.g. microcontroller <NUM>, are turned on. Active control circuits are typically turned on (step <NUM>) in the early morning when there is sufficient light to power the active control circuits typically with voltage of DC voltage source <NUM> reaching three volts. In decision block <NUM>, when voltage output - or power output - from DC voltage source <NUM> is sufficiently high and stable (e.g. voltage input to module <NUM> is ten volts for a period of <NUM> seconds), then module <NUM>,<NUM> begins operation (step <NUM>) in a safety mode. In safety mode, output voltage from module <NUM> is limited, for instance output voltage is limited to <NUM> volts so that a person can touch the wires of serial string <NUM>,<NUM> without any danger of electrocution. Note also, that in this case even if <NUM> modules are connected in series, the maximum output voltage of the string doesn't exceed 50V - which means the string voltage is still safe. Referring back to <FIG>, the controller <NUM> may alternate the switches (e.g. switches <NUM> & <NUM> of buck converter) at a low duty-cycle in order to maintain a low output voltage.

Referring back to <FIG>, controller <NUM> maintains safety mode operation (step <NUM>) until a communications signal, e.g. <NUM>, is received by receiver/detector <NUM> from signaling block <NUM>. When the communications signal is received (decision block <NUM>) indicating inverter <NUM> or <NUM> is connected and converting power, safety mode (step <NUM>) of operation ends. When the communications signal is received (decision block <NUM>), module <NUM> preferably enters a voltage control mode (step <NUM>) and voltage output between wires <NUM>,<NUM> is slowly ramped up. Voltage continues to ramp up, typically as high as +60V until module <NUM>,<NUM> detects that current is being drawn (step <NUM>). When sufficient current is drawn (step <NUM>), module <NUM>, <NUM> begins normal operation, (step <NUM>) e.g. for module <NUM>, the normal mode is the maximum power point (MPP) tracking mode of converting DC power from its input to its output by maintain maximum power at its input. The normal operation of transferring power is maintained as long as the communications signal, e.g. <NUM> is received from signal mechanism <NUM>, and no other warning condition is present. If the communications signal is not detected, or another warning condition is present, the normal mode (step <NUM>) is typically ended and power conversion of modules <NUM> is typically turned off. Exemplary warning conditions in decision box <NUM>, which cause module <NUM>,<NUM> to end normal mode (step <NUM>) and to stop transferring power to its output include: (i) input voltage less than predetermined value, e.g. about <NUM> volts for <NUM> seconds, (ii) rapid change in output voltage, for instance greater than <NUM>% in <NUM> milliseconds, (iii) reception of signal requesting to stop producing power, (iv) not receiving a signal to produce power (in the case where recurring "allow production" signals are required for the converter to function), or (v) output exceeds over voltage threshold caused for instance when multiple modules <NUM> in string <NUM> are converting power (step <NUM>) and one of modules <NUM> of string <NUM> shuts down, then the other modules <NUM> of string <NUM> have a raise of output voltage.

Reference is now made to <FIG>, which illustrates a method <NUM> performed by signaling block <NUM> attached at the input of inverter <NUM>. In step <NUM>, inverter <NUM> is off or inverter <NUM> is on standby, and not converting power to its output. In decision box <NUM>, start conditions for turning on inverter <NUM>,<NUM> are determined. Typically, as a safety requirement, inverter <NUM> delays operation (converting power to its output) until after at least <NUM> minutes of connection to a functioning AC-grid at its output. This safety requirement may be achieved using microcontroller <NUM> and at least one of relays <NUM> and <NUM> in signaling block <NUM>. In inverter <NUM>, a minimum voltage is required at the input to inverter <NUM> (e.g. if the safety output voltage of each module is 2V, and the minimal-length string allowed contains <NUM> modules, the inverter will wait until at least 10V are present at its DC input) and only thereafter does inverter <NUM> begin to charge its input, typically to a specified standard input of 400V.

In step <NUM>, communications signal, e.g. <NUM>, is superimposed on serial string <NUM>,<NUM> from signaling mechanism <NUM> when at least a <NUM> Watt load is attached to the output of inverter <NUM>. In decision box <NUM>, when the specified input voltage is reached, e.g. 400V for inverter <NUM>, inverter <NUM> is turned on or inverter <NUM> is attached to serial string <NUM> by mechanism <NUM>. In decision box <NUM>, if a time out occurs before the minimum specified input voltage is reached of inverter <NUM>,<NUM> then inverter is returned to the off or standby state (step <NUM>). Otherwise inverter <NUM>,<NUM> is connected or turned on in step <NUM>. Inverter <NUM>, <NUM> remains on and connected unless a warning condition (decision box <NUM>) occurs. Possible warning conditions include, (i) disconnection from the electrical grid, (ii) electrical grid stops producing power (islanding), (iii) less than <NUM> Watts transferred in the last minute, (iv) input voltage to inverter <NUM>,<NUM> is over the maximum limit, and (v) input power is over the maximum limit. If a warning condition occurs (decision box <NUM>) communications signal is turned off (step <NUM>).

The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the server arts. It is intended that the specification and examples be considered as exemplary only, with a true scope of the invention being indicated by the following claims.

Claim 1:
A system comprising:
a plurality of power modules (<NUM>), wherein each power module (<NUM>) includes a DC-to-DC converter (<NUM>) and a power input connected to a DC power source (<NUM>), wherein each power module (<NUM>) includes a power output,
wherein the plurality of power modules are coupled at their power outputs to form a serial string having a direct current output;
wherein each power module (<NUM>) is configured to operate in a safety mode if no signal superimposed onto the power output is received, and configured to end the safety mode of operation when the signal is received, and wherein each power module (<NUM>) is arranged, during the safety mode, to limit voltage at the power output to a voltage less than a predetermined value and greater than zero to prevent any danger of electrocution when touching wires of the serial string;
an inverter (<NUM>) comprising a power input coupled to the direct current output of the serial string and a power output connectable to an electric grid; and
a signaling mechanism (<NUM>) connected between the power input of the inverter and the direct current output of the serial string, wherein the signaling mechanism is configured to superimpose the signal onto the serial string, and turn off the signal if a warning condition occurs, wherein the signaling mechanism is configured to determine the warning condition.