Serially connected inverters

Abstract

A photovoltaic power generation system, having a photovoltaic panel, which has a direct current (DC) output and a micro-inverter with input terminals and output terminals. The input terminals are adapted for connection to the DC output. The micro-inverter is configured for converting an input DC power received at the input terminals to an output alternating current (AC) power at the output terminals. A bypass current path between the output terminals may be adapted for passing current produced externally to the micro-inverter. The micro-inverter is configured to output an alternating current voltage significantly less than a grid voltage.

Claims

1. A system comprising: an inverter configured to receive direct current (DC) input power and to convert the DC input power to alternating current (AC) output power; a circuit comprising a plurality of switches connected across a pair of output terminals of the inverter, the circuit configured to: enable the inverter to operate in response to the inverter receiving the DC input power and a signal; and enable the plurality of switches to bypass the inverter and disable the AC output power while not receiving the signal.

2. The system of claim 1, wherein the circuit comprises an AC bypass configured to pass an alternating current.

3. The system of claim 1, wherein a first output of the inverter is connected in series to a second output of another inverter.

4. The system of claim 1, wherein the inverter is configured to convert the DC input power to the AC output power based on receiving the signal is received via AC power lines.

5. The system of claim 4, wherein the inverter is configured to stop converting the DC input power to the AC output power based on the signal not being received for at least a predetermined period of time.

6. The system of claim 1, wherein the system is connected, via the inverter, to an electrical grid.

7. The system of claim 1, wherein the inverter is connected to an electrical grid via a bus, the system further comprising at least one switch connected to the bus and configured to disconnect the inverter from the electrical grid.

8. The system of claim 1, further comprising at least one control unit configured to disconnect the inverter from an electrical grid based on one or both of the following conditions existing: a serial voltage of the inverter being less than a voltage of the electrical grid; or a lack of synchronization between the serial voltage and the voltage of the electrical grid.

9. The system of claim 1, further comprising at least one control unit configured to: send the signal to the inverter; and based on a determination to shut down the inverter, stop sending the signal to the inverter.

10. A method comprising: operating an inverter of a power generation system to convert direct current (DC) input power to alternating current (AC) output power in response to receiving the DC input power and a signal; and disabling the inverter in response to receiving the DC input power while not receiving the signal, wherein disabling comprises controlling a plurality of switches connected across a pair of output terminals of the inverter to disable the inverter and disable the AC output power while not receiving the signal.

11. The method of claim 10, wherein the plurality of switches are part of an AC bypass configured to pass an alternating current.

12. The method of claim 10, further comprising connecting a first output of the inverter to a second output of another inverter in series.

13. The method of claim 10, further comprising converting, using the inverter and based on receiving the signal via power lines, the DC input power to the AC output power.

14. The method of claim 13, further comprising stopping converting the DC input power to the AC output power based on the signal not being received for at least a predetermined period of time.

15. An apparatus comprising: a first inverter, the first inverter comprising: input terminals; and output terminals configured to provide part of a serial voltage output that is formed while the input terminals are connected to input power from a power source, wherein the first inverter is configured to convert the input power to output alternating-current (AC) power at the output terminals of the first inverter while maintaining, together with a second inverter, the serial voltage output to be substantially equal to a grid voltage of a grid while the grid is connected across the serial voltage output; and a plurality of bypass switches connected across the output terminals and configured to disconnect, based on an indication associated with shutting down the first inverter, the first inverter from the grid.

16. The apparatus of claim 15, wherein the power source comprises a photovoltaic power source.

17. The apparatus of claim 15, wherein the indication is associated with one or both of: an over voltage condition or an islanding condition.

18. The apparatus of claim 15, further comprising a control unit configured to receive the indication and to cause the plurality of bypass switches to disconnect, based on the indication, the first inverter from the grid.

19. The apparatus of claim 15, wherein the indication is associated with a bypass condition.

20. The apparatus of claim 15, wherein the second inverter is configured to be disconnected from the grid based on the indication being associated with shutting down the second inverter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments are described, by way of example only, with reference to the accompanying drawings, wherein:

(2) FIG. 1 shows a conventional installation of a solar power system.

(3) FIG. 2 illustrates one serial string of DC sources.

(4) FIG. 3 illustrates a power harvesting system.

(5) FIG. 4a illustrates a power harvesting system in accordance with one or more embodiments of the disclosure.

(6) FIG. 4b illustrates a power harvesting system in accordance with one or more embodiments of the disclosure.

(7) FIG. 4c illustrates further details of a bypass in accordance with one or more embodiments of the disclosure.

(8) FIG. 5a illustrates a method of operation of a power harvesting system in accordance with one or more embodiments of the disclosure.

(9) FIG. 5b shows further details of connection and wake-up of a power harvesting system in accordance with one or more embodiments of the disclosure.

(10) FIG. 5c shows further details of operation in accordance with one or more embodiments of the disclosure.

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

DETAILED DESCRIPTION

(12) Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Various aspects are described below with reference to the figures.

(13) A conventional installation of a solar power system 10 is illustrated in FIG. 1. Since the voltage provided by each individual photovoltaic panel 100 is low, several panels 100 are connected in series to form a string 102 of panels 100. For a large installation, in order to achieve higher current, several strings 102 may be connected in parallel. Photovoltaic panels 100 are mounted outdoors, and are connected to a maximum power point tracking (MPPT) module 106 and to an inverter 104. MPPT 106 is typically implemented in the same housing as inverter 104.

(14) Harvested power from the DC sources is delivered to inverter 104, which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which, for residential application, is usually 110V or 220V at 60 Hz or 220V at 50 Hz. AC current from inverter 104 may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from inverter 104 may be directed to store the excess power in batteries.

(15) FIG. 2 illustrates one serial string of DC sources according to conventional art, photovoltaic panels 100, connected to MPPT circuit 106 and inverter 104 to form a power harvesting system 20 connected to load 108. The current versus voltage (IV) characteristics are plotted to the left of each photovoltaic panel 100. For each photovoltaic panel 100, the current decreases as the output voltage increases. At some voltage value the current goes to zero, and in some applications may assume a negative value, meaning that some photovoltaic panels 100 instead of being sources of power become sinks of power. Bypass diodes (not shown) connected in parallel across each photovoltaic panel 100 output are used to prevent any photovoltaic panel 100 from becoming a sink of power. The power output of each photovoltaic panel 100 is equal to the product of current and voltage (P=I*V) and varies depending on the voltage drawn from the panel 100. At a certain current and voltage, the power reaches its maximum (represented by the dot on the IV curve for each graph). It is desirable to operate a panel 100 at this maximum power point (MPP). The purpose of the maximum power point tracking (MPPT) module 106 is to find a suitable “average” maximum power point (MPP) for all panels 100. The maximum power point of the string selected by MPPT module 106 is shown using a dotted line with label MPP. The maximum power point of the string of panels 100 is generally not the maximum power of all panels 100. The dots indicating maximum power point of the individual panels 100 do not fall on the dotted line marked MPP.

(16) FIG. 3 illustrates another power harvesting system 30 according to conventional art, which combines power of multiple photovoltaic panels 100. Each photovoltaic panel 100 has a direct current (DC) output connected to the input of an inverter 104. A bypass diode 310 is connected in parallel across the direct current (DC) output panel 100 for safety requirements. Inverter 104 receives the direct current (DC) output of photovoltaic panel 100 and converts the direct current (DC) to give an alternating current (AC) at the output of inverter 104. Maximum power point tracking (MPPT) module 106 is typically implemented as part of the inverter 104. The outputs of multiple inverters 104 (with inputs attached to multiple photovoltaic panels 100) are connected in parallel to produce an alternating current (AC) output 304. Alternating current (AC) output 304 supplies load 108. Load 108 typically is an alternating current (AC) power grid, alternating current (AC) motor or a battery charging circuit.

(17) Before explaining various aspects in detail, it is to be understood that embodiments are not limited to the details of design and the arrangement of the components set forth in the following description and illustrated in the drawings. Other embodiments are capable of being practiced and 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.

(18) By way of introduction, aspects are directed to serially connected inverters in a grid connected photovoltaic system. In a system with serially connected inverters, as opposed to conventional system 30 which illustrates parallel connected inverters, each inverter is required to output a low voltage, for instance 24 volts AC root mean square (RMS) for ten serially connected inverters. Low output voltage of the micro-inverter is suitable for efficient and low cost micro-inverter topologies. One such topology is discussed in IEEE Transactions on Power Electronics, Vol. 22, No. 5, September 2007, entitled “A Single-Stage Grid Connected Inverter Topology for Solar PV Systems With Maximum Power Point Tracking, this paper proposes a high performance, single-stage inverter topology for grid connected PV systems.

(19) The term “bypass” as used herein refers to an alternate low impedance current path around or through a circuit, equipment or a system component. The bypass is used to continue operation when the bypassed circuit is inoperable or unavailable.

(20) The terms “wake-up” and “shut-down” as used herein refer to processes during, which a photovoltaic system is activated or de-activated respectively. A criterion for “wake-up”, i.e. activation of a photovoltaic panel, for instance, is that a photovoltaic panel is exposed to sufficient light such as at dawn A criterion for “shut-down”, i.e. de-activation of a photovoltaic panel, is that a photovoltaic panel is not exposed to sufficient light, for example at dusk.

(21) Reference is now made to FIG. 4a, which illustrates a power harvesting system 41 according to some embodiments. Photovoltaic inverting modules 410 each have panel 100, bypass diode 310, a control loop 404 and micro-inverter 402. Micro-inverters 402 may have optional synchronization units 408 and current bypass paths 422. Photovoltaic panels 100 have direct current (DC) outputs, which are connected respectively to the input of inverters 402. Bypass diodes 310 may connected in parallel across the direct current (DC) outputs of each panel 100 for safety requirements (e.g. IEC61730-2 solar safety standards). Control loops 404 are configured according to a predetermined criterion, typically to maintain maximum power at the inputs of micro-inverters 402, i.e. from the direct current (DC) outputs of photovoltaic panels 100. Bypass paths 422 are optionally normally-closed relays, which open during operation, and which are connected respectively to the outputs of photovoltaic inverting modules 410. Photovoltaic inverting modules 410 have alternating current (AC) outputs with voltage V.sub.a and current I.sub.a from module 410a; voltage V.sub.b and current I.sub.b from module 410b; voltage V.sub.n and current I.sub.n from module 410n. Outputs of modules 410 are connected in series to give a voltage output V.sub.out, which is applied to a load 406 via switch 414. Switch 414 is preferably controlled by control unit 418. Load 406 typically is an alternating current (AC) power grid, alternating current (AC) motor or a battery charging circuit. Control units 408 typically provide control signals to synchronization units 408 in order to achieve synchronization with load or grid 406. Synchronization units 408 or control unit 418 provide anti-islanding functionality for power harvesting system 41.

(22) Additionally, the outputs of photovoltaic inverting modules 410a-410n are bypassed (i.e. the output of modules 410a-410n are short circuited) by bypass 422 in the event of under voltage production by micro inverter modules 402 or the bypass is opened (i.e. modules 410a-410n are open circuit) in the event of over voltage by micro inverter modules 402 or during a situation of anti-islanding.

(23) Reference is now made to FIG. 4c, which illustrates further details of bypass 422 according to various embodiments. Bypass 422 is controlled by control logic module 460, e.g. a microprocessor 460 controlling micro-inverter 402. Microprocessor 460 has a sensing input connected to the output voltage (V.sub.microinverter) of micro inverter 402. Control logic module 460 has other inputs connected across the bypass path at nodes A and B. Control logic module 460 has two outputs; one output connects to the gate of a metal oxide semi-conductor field effect transistor (MOSFET) Q.sub.1, the other output connects to the gate of MOSFET Q.sub.2. The drain of MOSFET Q.sub.1 is connected to node A and the source of MOSFET Q.sub.1 is connected to the source of MOSFET Q.sub.2, the drain of MOSFET Q.sub.2 is connected to node B. MOSFET Q.sub.1 has a diode with an anode connected to the drain and a cathode connected to the source. MOSFET Q.sub.2 has a diode with an anode connected to the drain and a cathode connected to the source. The bypass current (I.sub.bypass) path is identified between nodes A and B.

(24) A high impedance path is provided between nodes A and B when micro inverter 402 is producing an alternating current (AC) voltage synchronized to grid voltage 406. The high impedance path is provided between nodes A and B when MOSFETs Q.sub.1 and Q.sub.2 are turned off by control logic unit 460. When the high impedance path is provided between nodes A and B currents I.sub.b, I.sub.X, I.sub.in, I.sub.a, I.sub.Y and I.sub.out are equal according to Kirchhoff's current law. A low impedance path is provided between nodes A and B when micro inverter 402 is not producing an AC voltage and another serially-connected micro inverter 402 is producing an AC voltage. A low impedance path is provided between nodes A and B by alternately switching MOSFETs Q.sub.1 and Q.sub.2 on and off alternately via control logic unit 406. When the load 406 is a grid voltage Q.sub.1 and Q.sub.2 are turned alternately on and off according to the frequency of the grid voltage. When the load 406 is a load, Q.sub.1 and Q.sub.2 are turned alternately on and off according to the frequency of synchronized inverters 402a-402n. In the case of low impedance path being provided between nodes A and B in the embodiment according to FIG. 4a; switching MOSFETs Q.sub.1 and Q.sub.2 on and off by control logic unit 460 is achieved via communication signals between central control unit 408 and control units 408a-408n. In the case of low impedance path being provided between nodes A and B in the embodiment according to FIG. 4b; switching MOSFETs Q.sub.1 and Q.sub.2 on and off alternately by control logic unit 460 is achieved via communication signals between control units 408a-408n and information of grid voltage 406 via sensor 416. A low impedance path provided between nodes A and B means that currents I.sub.b, I.sub.bypass and I.sub.out are substantially equal according to Kirchhoff's current law. A low impedance path provided between nodes A and B means that current I.sub.bypass flows alternately from drain to source of Q.sub.2 and the diode of Q.sub.1 for one half cycle and for the other half cycle I.sub.bypass flows alternately through from drain to source of Q.sub.1 and the diode of Q.sub.2.

(25) Reference is now made to FIG. 4b, which illustrates a power harvesting system 42 according to further embodiments. As in power harvesting system 41 photovoltaic inverting modules 410a-410n each has a photovoltaic panel 100, bypass diode 310, control loops 404 and inverters 402 having synchronization units 408 and current bypasses 422. Modules 410a-410n have outputs connected in series to give a voltage output V.sub.out, which is applied to load 406. Sensor 416 preferably senses the live voltage applied to load 406 optionally via electromagnetic pickup on the power line connected to load 406 or directly by having visibility of the grid by virtue of bypasses 422. Sensor unit 412 transfers details of the load voltage (e.g. amplitude, phase, and frequency) to synchronization unit 408a via control line 420. Control signals are optionally sent over power line communications, wireless or over a separate interface.

(26) Although only one control line 420 is shown, optionally multiple or all synchronization units 422 receive synchronization signals from sensor 412.

(27) Reference is now made to FIG. 5a, which shows a flow chart of a method 50 illustrating operation of power harvesting systems 41 and 42 according to various aspects. Method steps include installation (step 500) wake-up (step 501), normal operation (step 503), and shut down (step 505).

(28) 500 Installation and 501 Wake-Up

(29) During installation (step 500), photovoltaic modules 410 are preferably not producing power so as not to be a safety hazard to the installers. Optionally, a “keep-alive” signal is transmitted for instance by control unit 418 over the AC power lines. When the “keep-alive” signal is not received by micro-inverters 402, AC output power is disabled or not produced. Alternatively, if the grid is “visible” to micro-inverters 402, then in the absence of grid voltage, (e.g. switch 414 in FIG. 4a is open) micro-inverters 402 do not produce AC power. Reference is now made to FIG. 5b, which illustrates an installation method 500 according to certain aspects. In step 500a, input terminals of micro-inverters 402 are connected to the output of photovoltaic panels 100. In step 500b, the output terminals of photovoltaic panels 100 are connected serially to give a serial voltage output. After an optional predetermined time delay (step 501a), power inversion is enabled (step 501b). The enabling (step 501b) of power inversion may be performed by synchronization modules 408 when grid voltage is sensed or by control unit 418 when switch 414 is closed.

(30) 503 Operation and 505 Shutdown

(31) Reference is now made again to FIG. 5c, which shows a flow chart of a method 503 for operating serially connected micro-inverter module according to various embodiments. Micro-inverters 402 invert (step 503b) the direct current (DC) power output of photovoltaic panels 100 to alternating current (AC) power at the outputs of micro-inverters 402 while maintaining output voltage equal to the grid voltage. Synchronization (step 503a) between the voltage outputs of micro-inverters 402a-402n and the grid voltage is maintained. Control unit 418 optionally monitors AC synchronization between output voltage V.sub.out and load 406, e.g. grid. Control unit 418 also may provide anti-islanding functionality for power harvesting system 41. If either synchronization and/or voltage of power harvesting system 41 is incompatible with the grid, control unit 418 disconnects power harvesting system from the grid by signaling switch 414. Alternatively, synchronization (step 503a) including maintenance of grid voltage is achieved using synchronization units 422 which can sense the grid by virtue of bypass paths 422. Upon failure of either synchronization (step 503a) or inverting power at grid voltage (step 503b) by any of the serially connected micro-inverter modules 402, then current bypass occurs (step 503d). Current bypass is optionally an active current bypass using active switches as shown in FIG. 4c or preferably a passive current bypass. Shutdown (step 505) occurs for instance at dusk when light levels are two low to maintain the grid voltage at any current level. During shutdown, the photovoltaic system is optionally disconnected from the grid using switch 414 in system 41 or in system 42 each of micro inverter modules 402 stop and present high impedance to the grid.

(32) According to yet further embodiments, the regulation of output voltage of photovoltaic inverting modules 410a-410n is achieved directly by the grid 406. The regulation does not require control unit 418 and switch 414 as shown in FIG. 4a and relies on the fact that grid 406 is almost infinitely greater in terms of potential supply of power by comparison to the AC power produced by photovoltaic inverting modules 410a-410n. The greater power of grid 06 forces photovoltaic inverting modules 410a-410n to adjust to the grid voltage and as such, photovoltaic inverting modules 410a-410n are preferably operated to give as much voltage as possible at their outputs. Typically, photovoltaic inverting modules 410a-410 are capable of sensing grid voltage 406 so as to provide anti-islanding.

(33) The definite articles “a”, “an” is used herein, such as “a photovoltaic panel”, have the meaning of “one or more” that is “one or more photovoltaic panels”.

(34) Although selected embodiments have been shown and described, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention.