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 modem 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, super-conducting 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.

One common problem faced by modern utility networks is the occurrence of islanding. Islanding is the condition where a distributed power generation system is severed from the utility network, but continues to supply power to portions of the utility network after the utility power supply is disconnected from those portions of the network. All photovoltaic systems must have anti islanding detection in order to comply with safety regulations. Otherwise the photovoltaic installation may shock or electrocute repairmen after the grid is shut down from the photovoltaic installation generating power as an island downstream. The island condition complicates the orderly reconnection of the utility network and poses a hazard also to equipment. Thus, it is important for an island condition to be detected and eliminated.

Several techniques have been proposed to guard against islanding. For example, one method involves the monitoring of auxiliary contacts on all circuit breakers of the utility system between its main source of generation and DP systems. The auxiliary contacts are monitored for a change of state which represents an open circuit breaker on the utility source. The utility circuit breaker is typically monitored and tripped by external protective relays. When a loss of utility is detected by the change in state of the auxiliary contact of a circuit breaker, a transferred trip scheme is employed to open the interconnection between the utility and the distributed power system. A transferred trip scheme uses the auxiliary contacts of the utility source being monitored. The auxiliary contacts are connected in parallel with other devices which can trigger the trip of the local interconnection breaker. When the auxiliary contacts change state, a trip is induced on the local interconnection breaker. This prevents an island condition from occurring. The drawback of such a method is that often the point of utility isolation (the point at which the utility circuit breaker opens) is of such a distance from the local distributed power system that running a contact status signal back to the local distributed power system control system is not practical.

Anti-islanding schemes presently used or proposed include passive schemes and active schemes. Passive schemes are based on local monitoring of the grid signals, such as under or over voltage, under or over frequency, rate of change of frequency, phase jump, or system harmonics, for example. Active schemes are based on active signal injection with monitoring of the resulting grid signals, such as impedance measurement for example, or active signal injection with active controls, such as active frequency shifting or active voltage shifting for example. With active schemes, some distortion may occur in the output current waveform, thereby resulting in a tradeoff between islanding detection time and waveform distortion, with faster detection typically resulting in higher total harmonic distortion.

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>. 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 <FIG>, 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, coupled 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. This system may be used also to address the anti-islanding issue.

The term "leakage" as used herein refers to electrical power which is radiated or conducted into an electrical signal line typically at low levels and typically because of insufficient isolation.

<CIT> discloses solar array converters that may include safety mechanisms that limit the output voltage to a safe level until a minimum load is detected. Output voltage is then ramped up in response to detecting this minimum load. Said document also discloses having power converters connected to the solar arrays waiting for a release signal from the inverter before providing power to the inverter.

The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below.

The present invention provides systems as defined in independent claims <NUM> and <NUM>. Some optional features of the present invention are defined in the dependent claims.

According to an aspect of the present invention, there is provided in a distributed power system multiple DC power sources and multiple power modules which include inputs coupled respectively to the DC power sources. The power modules each include outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. A protection mechanism in the power modules shuts down the power modules and ceases the power input to the inverter when the inverter stops producing the output power. Typically, the inverter is connected to the electrical grid. A monitoring mechanism is attached to the electrical grid which monitors one or more electrical parameters of the electrical grid. A shutdown mechanism is attached to the monitoring mechanism which when one or more of the electrical parameters is out of predetermined specification, the inverter stops the production of the output power or disconnects from the grid. A switch is preferably disposed between the serial string and the inverter. The switch is activated by the shutdown mechanism and the protection mechanism senses a change in current flowing through the serial string when the switch is activated. When the switch is connected serially with the serial string, the protection mechanism senses that current less than a previously specified minimal threshold current in the serial string; or when the switch is connected in parallel with the serial string the protection mechanism senses a current greater than a previously specified maximal threshold current in the string. Alternatively a signal-providing mechanism is attached to the inverter which provides a signal based on the shutdown mechanism. Multiple receivers are attached respectively to the power modules. The receivers receive the signal and multiple enabling mechanisms, which are attached respectively to the receivers, enable the respective power modules to supply the input power to the inverter based on the presence of the signal or absence thereof. When the signal is a keep-alive signal, the enabling mechanisms enable the respective power modules to supply the input power to the inverter based on the presence of the keep-alive signal. When the signal is a shut-down signal, the enabling mechanism disables the respective power modules and stops supply of the input power to the inverter based on the presence of the shut-down signal. The signal in the serial string is optionally from the electrical grid and detected at the frequency of the electrical grid or detected at a higher frequency up converted from the frequency of the electrical grid. The signal in the serial string is optionally from the inverter or the output power therefrom, and detected at a switching frequency of the inverter. The signal is optionally superimposed on the power input to the inverter from the serial string. The signal may be wirelessly transmitted by the signal-providing mechanism, and the receiver in each of the power modules, receives the wirelessly transmitted signal.

According to another aspect of the present invention, there is provided a protection method in a distributed power system including DC power sources and multiple power modules each of which include inputs coupled to the DC power sources. The power modules each include outputs coupled in series to form a serial string. An inverter is coupled to the serial string. The inverter converts power input from the string and produces output power. When the inverter stops production of the output power, each of the power modules is shut down and thereby the power input to the inverter is ceased. When the inverter is connected to and supplies the output power to the electrical grid, one or more electrical parameters of the grid are monitored. When the one or more electrical parameters of the grid are out of a predetermined specification, the inverter is shut down and thereby production of the output power is stopped or the inverter is disconnected from the grid. When the inverter is shut down, a switch disposed between the serial string and the inverter is activated. When the switch is activated a change in current flowing through the serial string is sensed. Alternatively a signal is provided based on the shutdown mechanism. Multiple receivers are attached respectively to the power modules. The receivers receive the signals which enable the respective power modules to supply the input power to the inverter based on the presence of the signal or absence thereof. When the signal is a keep-alive signal, the respective power modules supply the input power to the inverter based on the presence of the keep-alive signal. When the signal is a shut-down signal, the respective power modules stop supply of the input power to the inverter based on the presence of the shut-down signal. The signal may be based on current in the serial string from the electrical grid and detected at the frequency of the electrical grid or detected at a higher frequency up converted from the frequency of the electrical grid. The signal in the serial string is optionally from the inverter or the output power therefrom, and detected at a switching frequency of the inverter. The signal is optionally actively superimposed on the power input to the inverter from the serial string. The signal may be wirelessly transmitted, and the receiver in each of the power modules, receives the wirelessly transmitted signal.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

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 anti-islanding 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 high voltage such as when solar panels are exposed to sunlight. Similarly, firefighters, even after they shut down the main electrical switch to a burning building can safely break into the burning building or hose the roof of the building with water without fear of high voltage DC conduction through the water, since high voltage direct current feeding the inverter is safely turned off.

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.

Referring now to the drawings, <FIG> illustrates a distributed power harvesting circuit <NUM>, previously disclosed in <CIT>. Circuit <NUM> enables connection of multiple distributed power sources, for example solar panels 201a - 201d, 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 201a - 201d is connected individually to a separate power conditioner, here a 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 201a - 201d 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 201a - 201d 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 201a - 201d. 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 201a - 201d, each string <NUM> may have a different number or different specification, size and/or model of panels 201a - 201d connected in series. System <NUM> of <FIG> continuously performs MPPT on the output of each solar panel 201a - 201d to react to changes in temperature, solar radiance, shading or other performance factors that effect one or more of solar panels 201a - 201d. As a result, the MPPT circuit within the converters 205a - 205d harvests the maximum possible power from each panel 201a - 201d and transfers this power as output regardless of the parameters effecting other solar panels 201a - 201d.

The outputs of converters 201a - 201d 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 201a - 201d into an AC power supply. Inverter <NUM>, may be set to regulate 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>.

In order to legally be allowed to connect to the grid in each country, inverter <NUM>,<NUM> is preferably designed to comply with local electrical regulations. Electrical regulations typically dictate, among other things, the minimal and maximal voltages of the grid e.g. <NUM>-<NUM> root mean squares voltage V, and a range of permitted frequency, e. g <NUM>-<NUM>. Whenever the grid deviates from allowed values inverter <NUM>,<NUM> is required to disconnect from the grid. Disconnection from the grid is typically performed using software controlling inverter <NUM>, <NUM> and control circuitry which constantly monitors grid parameters, e.g. voltage, frequency.

In system <NUM>, solar panels <NUM> are directly connected (e.g. in series-parallel) to inverter <NUM>. When an islanding condition is detected, inverter <NUM> is disconnected from the grid. Hence, inverter <NUM> stops drawing current and therefore panels <NUM> output a relatively high open circuit voltage typically <NUM>% higher than the normal operating voltage. An open circuit voltage <NUM>% higher than nominal working voltage is typically safe, (less than the allowed 600VDC in the USA and 1000VDC in Europe) which are typical ratings for inverters <NUM> designed to be able to handle the higher open circuit voltage.

In system <NUM>, there are power converters <NUM> which "push" power to the output of converters <NUM>. Under an islanding condition which has been detected by inverter <NUM>, inverter <NUM> is shut down and current is not flowing between converters <NUM> and inverter <NUM>. Consequently, in system <NUM>, the open circuit voltage at the input to inverter <NUM>, reaches dangerous voltages, higher than the open circuit maximum voltage ratings of inverters <NUM>, <NUM>.

Reference is now made to <FIG> which illustrates an exemplary DC-to-DC converter <NUM> previously disclosed in co-pending <CIT>. 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> may be 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, A/Ds, 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>.

Reference is now made to <FIG> which illustrates a system <NUM> for protection during an islanding condition, in accordance with embodiments of the present invention. For simplicity, a single string <NUM> is shown of distributed power sources, e. g solar panels 201a-201d connected to respective power converters 405a-d. Serial string <NUM> is input to inverter <NUM> through wires <NUM> and <NUM>. The output of inverter <NUM> is connected to and supplies electrical power to the electrical grid. Inverter <NUM>, typically includes a monitoring, and detection mechanism <NUM> which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring and detection mechanism <NUM> typically causes inverter <NUM> to be shut down or inverter <NUM> is disconnected from the grid so that output power is no longer supplied by inverter <NUM> to the grid. At the same time, a signal <NUM> is transmitted to a switch mechanism <NUM> which may be located at the input of inverter <NUM> before input capacitor <NUM>. Switch mechanism <NUM> is optionally packaged with inverter <NUM> or may be integrated with inverter <NUM> and packaged separately. In this example, signal <NUM> activates switch mechanism <NUM> so that when switch <NUM> is activated, the current flowing through serial string <NUM> and wires <NUM>, <NUM> varies abruptly.

Reference is now also made to <FIG> which illustrates in more detail converter <NUM>. Converter <NUM> is equipped with a current sensing mechanism <NUM> which upon sensing a variation in current through serial string <NUM> signals controller <NUM> to shut down and stop converting power. Typically, current sensing mechanism <NUM> includes an analog/digital converter which continuously feeds data to controller <NUM>. Controller <NUM> detects a shutdown in current and decides to shut down the converters <NUM> accordingly.

Reference is now also made to <FIG> which illustrate schematically switch mechanism <NUM> in more detail. <FIG> illustrates switch mechanism <NUM> in a serial configuration in which switch <NUM> is connected in series with the serial string <NUM> and <FIG> illustrates a parallel configuration in which switch <NUM> is connected in parallel with serial string <NUM>. In the serial configuration (<FIG>) switch <NUM> is closed during normal operation of inverter <NUM>. When an island condition is detected, serial switch <NUM> opens during shut down of inverter <NUM>. Current sensing mechanism <NUM> upon sensing zero current signals controller <NUM> that output current is less than a previously specified minimum value and controller <NUM> shuts down power conversion in converter <NUM>. In the parallel configuration (<FIG>), switch <NUM> is open during normal operation of inverter <NUM>. When an island condition is detected, parallel switch <NUM> closes during shut down of inverter <NUM>. With all the current of serial string <NUM> flowing through the switch <NUM> at minimal load, the current increases to above a previously specified maximum current. Current sensing mechanism <NUM> upon sensing a current maximum signals controller <NUM> that output current is above maximal previously specified value and controller <NUM> shuts down power conversion. Switch mechanism <NUM> in different embodiments may be embodied by a mechanical switch or a solid state switch with current and voltage ratings appropriate to the present application. Switch mechanism <NUM> is preferably selected by one skilled in the art of power electronics so that arcing across its open terminals is avoided while practicing some embodiments of the present invention.

Reference is now made <FIG> which illustrates a method, according to an embodiment of the present invention. In decision block <NUM>, output power from inverter <NUM>, <NUM> is constantly monitored. If output power is stopped, power converters <NUM> are shut down.

Reference is now made to <FIG>, illustrating a system <NUM> according to other embodiments of the present invention for protection during an islanding condition. For simplicity, a single string <NUM> is shown of distributed power sources, e. g solar panels 201a-201d connected to respective power converters 505a-d. Serial string <NUM> is input to inverter <NUM> through wires <NUM> and <NUM>. The output of inverter <NUM> is connected to and supplies electrical power to the electrical grid. Inverter <NUM>, typically includes a monitoring and detection mechanism <NUM> which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring/detection mechanism <NUM> typically shuts down inverter <NUM> or disconnects from the grid, so that output power is no longer supplied by inverter <NUM> to the grid. During normal operation, a line communications transmitter <NUM> superimposes a keep-alive signal, for instance between <NUM> kilohertz to <NUM> Megahertz on direct current (DC) input lines <NUM> and <NUM> attached to serial string <NUM>.

Reference is now also made to <FIG> which illustrates converter <NUM> in more detail. The keep-alive signal is constantly monitored and detected by a line communications receiver <NUM>. Only while receiver <NUM> senses the keep-alive signal does receiver <NUM> provide an enable signal to controller <NUM>. When controller <NUM> doesn't receive an enabling signal from receiver <NUM>, controller <NUM> shuts down power conversion of converter <NUM>.

Alternatively, instead of a "keep-alive" signal, a stop signal <NUM> which is first generated by monitoring and detection mechanism <NUM> when an islanding condition is detected, is transmitted to receiver <NUM>. The stop signal is transmitted over line communications by superimposing a varying (e.g. <NUM> to <NUM> ) signal over the power lines of serial string <NUM>. Receiver <NUM> receives the stop signal and relays the stop signal to controller <NUM> using, e.g., a single disable bit. Controller <NUM> on receiving a disable signal, stops converting power to the output of converter <NUM>. Typically, when converters <NUM> are disabled they go into a bypass mode which allows current from other converters <NUM> to pass through. Hence, the stop signal may be continued until all power stops being supplied on string <NUM> by all of converters <NUM>.

It should be noted that one skilled in the art would realize that although in system <NUM>, converters <NUM> are shown to have feedback loop <NUM>, as in controller <NUM> of system <NUM>, embodiments of the present invention as illustrated in system <NUM> using switch mechanism <NUM> and/or in system <NUM> using line communications, to the serial string may be applied to and find benefit in other distributed power systems using converters without feedback loops <NUM> as applied to prior art system <NUM>. Similarly, conventional inverters <NUM> may be used instead of inverter <NUM> with communications transmitter <NUM> added to inverter <NUM> either by the inverter manufacturer or as a retrofit. For example, <FIG> illustrates a system according to an embodiment of the invention applied as a retrofit to a prior art system, such as the system of <FIG>. In this example, detection mechanism <NUM> and switch mechanism <NUM> are installed between the grid and the conventional inverter <NUM>. Of course, detection mechanism <NUM> and switch mechanism <NUM> may be incorporated into the inverter, e.g., for original installation, rather than a retrofit. Also, other implementations described herein may be used instead of detection mechanism <NUM> and switch mechanism <NUM>. Advantages of incorporation of monitoring and detection mechanism <NUM> and one of switch mechanism <NUM> or communications transmitter <NUM> into system <NUM> is beneficial during installation, maintenance, and firefighting.

Reference in now made to <FIG> which illustrates system <NUM>, according to another embodiment of the present invention for protection during an islanding condition. For simplicity, a single string <NUM> is shown of distributed power sources, e. g solar panels 201a-201d connected to respective power converters 605a-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. Inverter <NUM>, typically includes a monitoring and detection mechanism <NUM> which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring and detection mechanism <NUM> typically shuts down inverter <NUM> so that output power is no longer supplied by inverter <NUM> to the grid. During normal operation, a <NUM> (or <NUM>. in USA) ripple current is detectable between lines <NUM>, <NUM> and in serial string <NUM> since capacitors of inverter <NUM> do not block entirely the alternating current (AC), or the <NUM>/<NUM> is intentionally leaked into serial string <NUM> through lines <NUM>, <NUM>.

Reference is now also made to <FIG> which illustrates converter <NUM> in more detail. The <NUM>/<NUM> leakage is constantly monitored and detected by a receiver <NUM>. Only while receiver <NUM> senses the leakage from the grid does receiver <NUM> provide an enable signal to controller <NUM>. When controller <NUM> doesn't receive an enabling signal from receiver <NUM>, controller <NUM> shuts down power conversion of converters <NUM>.

Alternatively or in addition, one or more switching frequencies of inverter <NUM>, typically <NUM> or <NUM>. may be detected as leakage or provided intentionally to serial string <NUM> along lines <NUM>,<NUM>. Receiver <NUM> is configured to detect the <NUM>/<NUM> inverter switching frequency and provides an enabling signal to controller while inverter <NUM> is operating.

Reference is now made to <FIG>, showing a simplified block diagram according to an embodiment of the present invention for up conversion of <NUM>/<NUM>. into a higher frequency in order to enable faster detection in receiver <NUM> of leakage from the grid. The <NUM> Hertz or <NUM> Hertz signal is AC coupled by capacitor <NUM> to remove the direct current component in serial string <NUM> and lines <NUM> and <NUM>. The <NUM>/<NUM>. signal is optionally amplified and rectified by a full wave rectifier <NUM> so that a <NUM> or <NUM> unipolar DC ripple is achieved. The <NUM>/<NUM> unipolar signal is split. One portion of the <NUM>/<NUM>. unipolar ripple is converted to a square wave, such as in a comparator/digitize circuit <NUM>. A second portion of the <NUM>/<NUM> unipolar ripple undergoes a known phase shift, e. g of <NUM>. in a phase shifter <NUM> and output to a second comparator/digitizing circuit <NUM>. The two outputs of two digitizing circuits <NUM>,<NUM> undergo an exclusive OR in a XOR circuit <NUM> which outputs a signal at a much higher frequency, e.g. <NUM>.

Reference is now made to <FIG>, illustrating a system <NUM> according to other embodiments of the present invention for protection during an islanding condition. For simplicity, a single string <NUM> is shown of distributed power sources, e. g solar panels 201a-201d connected to respective power converters 705a-d. Serial string <NUM> is input to inverter <NUM> through wires <NUM> and <NUM>. The output of inverter <NUM> is connected to and supplies electrical power to the electrical grid. Inverter <NUM>, typically includes a monitoring and detection mechanism <NUM> which monitors one or more parameters of the electrical grid such as voltage and/or frequency. If one or more of the grid parameters is out of specification indicating an islanding condition, monitoring, and detection mechanism <NUM> typically shuts down inverter <NUM> or disconnects inverter <NUM> from the grid so that output power is no longer supplied by inverter <NUM> to the grid. During normal operation, a wireless transmitter <NUM> transmits wirelessly a signal, for instance between <NUM> Megahertz-10Gigahertz.

Reference is now also made to <FIG> which illustrates converter <NUM> in more detail. The wireless signal is received and constantly monitored by a wireless receiver <NUM> Only while receiver <NUM> senses the wireless signal does receiver <NUM> provide an enable signal to controller <NUM>. When controller <NUM> doesn't receive an enabling signal from receiver <NUM>, controller <NUM> shuts down power conversion of converter <NUM>.

Claim 1:
A system comprising:
a plurality of direct-current-to-direct-current, DC/DC, power converters (<NUM>, <NUM>) having output terminals coupled in series to form a serial string (<NUM>, <NUM>) having power lines (<NUM>, <NUM>);
wherein each of the plurality of power converters (<NUM>, <NUM>) comprises a controller (<NUM>) that is configured to convert input power to direct-current output power on the power converter output terminals, and a receiver (<NUM>, <NUM>);
characterized in that each receiver (<NUM>, <NUM>) is configured to generate a disable signal in response to receiving a stop signal;
wherein each controller (<NUM>) is configured, in response to receiving the disable signal from the corresponding receiver (<NUM>, <NUM>), to stop the corresponding power converter (<NUM>, <NUM>) providing power at the converter output terminals and cause the corresponding converter to enter into a bypass mode which allows current from other power converters in the serial string to pass through the power converter from one of its output terminals to the other of its output terminals.