Claims:1. A device for switching between photovoltaic (PV) mode and grid mode of power supply to a connected load, the device comprising;
a MOSFET configured to get periodically ON and enable a short circuit current to flow through photovoltaic panels when the power supply is in grid mode;
a sensor configured to measure the short circuit current flowing through the photovoltaic panels;
a controller configured to receive the measured short circuit current and compare the measured current with a preset threshold current;
wherein the controller is further configured to switch power supply to PV mode if the measured current is more than the threshold current.
2. The device of claim 1, wherein the power supply in the grid mode is continued if the measured current is less than the threshold current.
3. The device of claim 1, wherein the controller switches power supply from the photovoltaic panels to ON before switching OFF the grid mode.
4. The device of claim 1, wherein the controller is a DSP(Digital Signal Processor).
5. The device of claim 1, wherein the switching of power supply between different modes is enabled by the controller by actuating a set of relays configured in respective power lines.
6. The device of claim 1, wherein the connected load is a pump operated by a three phase motor.
7. The device of claim 1, wherein the three phase motor gets power supply through a 3 phase inverter.
8. A method for switching between photovoltaic (PV) mode and grid mode of power supply to a connected load, the device comprising steps of;
Enabling periodically a short circuit current to flow through photovoltaic panels when the power supply is in grid mode;
measuring the short circuit current flowing through the photovoltaic panels;
comparing the measured current with a preset threshold current; and
switching power supply to PV mode if the measured current is more than the threshold current.
9. The method of claim 8, wherein after it is found that the measured current is more than the threshold current, the power supply is simultaneously maintained from PV mode and the grid mode for some time before switching off the grid mode to enable seamless transfer from the PV mode to the grid mode.
10. The method of claim 8, wherein power supply in the grid mode is continued if the measured current is less than the threshold current.
, Description:TECHNICAL FIELD
[0001] The present disclosure generally relates to low voltage power distribution systems. In particular it pertains to a method and system for switching between two alternate sources of power.
BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] In view of concerns regarding environmental effects of fossil fuels, alternate sources of electric power such as wind power energy, solar energy etc. are increasingly being used to meet power requirements for various applications such as to run pumps. Of various such unconventional sources solar power generated through Photovoltaic Power solar panels (and accordingly also referred to as Photovoltaic Power or PV power and the term used interchangeably hereinafter) is more reliable and preferred especially in countries such as India where sunshine is available in abundance throughout the year.
[0004] However, since the PV power depends on intensity of solar irradiations, any reduction in solar irradiations for reasons such as shading or cloud conditions causes reduction in the power output from solar panels. In such situations, power supply has to be switched to an alternate mode such as grid to continue running of the application. To have minimum disruptions in the application being run, it is preferable that power supply source is switched to the alternate mode the moment PV power supply is not adequate. And in order to use PV power to a maximum extent, it is also necessary that power supply is switched back to the PV power supply the moment it is able to meet the load. The above requirement calls for automatic switching arrangement that is able to seamlessly changeover from one source to other as and when situation demands.
[0005] While switching from PV mode to Grid mode is possible as it can be based on output from the PV panel, shifting back from grid mode to solar mode is the challenge as it involves determining output from the PV panel and ascertaining if it is adequate.
[0006] Various prior art references exist that disclose one or other method of seamless and automatic switching between alternate power supply sources. Most of them do not directly feed PV power to the load and use a battery bank in between. Certain others disclose methods based on sensing photovoltaic output voltage. However, variation in the photovoltaic output voltage of the solar panel is not drastic during the day and thus cannot be a true measure of power output and its adequacy to run the connected load. It is the PV current which truly represents the power output as PV current is directly proportional to solar intensity.
[0007] There is therefore a need in the art to provide a system and method for auto-transfer from grid to PV mode based on current output from the PV panels.
[0008] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0009] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0010] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0011] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0012] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

OBJECTS OF THE INVENTION
[0013] A general object of the present disclosure is to provide a system for auto-transfer from grid to PV mode.
[0014] An object of the present disclosure is to provide a methodology for auto-transfer from grid mode to PV mode based on current output from the PV panels.
[0015] Yet another object of the present disclosure is to provide a system to measure the PV current value to decide the suitable conditions for switching from grid to PV mode.
[0016] Still another object of the present disclosure is to provide a system of sequential switching in a particular order so that load is uninterrupted while switching from grid mode to PV mode.

SUMMARY
[0017] Aspects of the present disclosure pertain to a method and device for automatic switching between different power sources. In particular the system and method of the present disclosure pertain to switching between grid mode and PV mode
[0018] In an aspect, the disclosed method provides for switching from grid to PV based on current output from the PV panels. In an embodiment, the present method provides for periodic sampling of current output from the PV panels. If sampled value exceeds a preset threshold value it is decided that enough solar power is available to make the switch to solar mode.
[0019] In an aspect, the short circuit PV current helps in deciding whether the solar intensity is enough to provide adequate PV power and changeover to PV mode can be made.
[0020] In an exemplary embodiment the periodicity for sampling can by suitable design of circuit, be as low as a few minutes and exemplary period of sampling can be one minute. Thus the disclosed method can take care of short duration transient reduction of power output from PV due to clouds and help to maximize power drawn from PV panels and minimize dependence of the grid power.
[0021] In an aspect, the disclosed device for automatic switching between different power sources can comprise a controller, a MOSFET switch and a current sensor, wherein when the device is in grid mode, the MOSFET switch can get periodically switch ON for a fixed period and allow a short circuit current from the PV panels to flow through a circuit configured with the current sensor. The controller can be configured to receive the sensed short circuit current and compare the sensed current with a threshold current. In case at the end of the ON period, the sensed current is more than the threshold current, the controller can suitably actuate corresponding relays to switch power supply from the grid mode to the PV mode. Alternatively if at the end of the ON period, the sensed current is not more than the threshold current, the controller can continue to allow the power to be drawn from the grid.
[0022] In an aspect, the disclosed device for automatic switching between different power sources can accomplish switching in a seamless manner by switching multiple relays in a controlled order.
[0023] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components

BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0025] FIG.1 illustrates an exemplary schematic diagram of a 3-phase solar pump driven by PV power and grid power sources in accordance to an embodiment of the present disclosure.
[0026] FIG.2 illustrates an exemplary method flowchart for the disclosed method for switching from grid mode to PV mode in accordance to an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0027] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0028] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0029] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0030] Embodiments of the present disclosure pertain to a method and device for automatic switching between different power sources. It is to be appreciated that though the embodiments of the present disclosure have been explained with reference to Photovoltaic power as one of the two sources besides grid power, the disclosed device and method can be applied to other systems that depend on other sources such as wind power, hydro power etc. to name a few, for which continued stable supply is not assured. Further the embodiments of the present disclosure have been explained with reference to a pump, however any other application can be benefitted by the disclosed method and device without any limitation.
[0031] In an aspect, the disclosed method provides for switching from grid to PV based on current output from the PV panels. In an embodiment, the present method provides for periodic sampling of current output from the PV panels. If sampled value exceeds a preset threshold value it is decided that enough solar power is available to make the switch to solar mode. In an aspect, the short circuit PV current helps in deciding whether the solar intensity is enough to provide adequate PV power and changeover to PV mode can be made
[0032] In an aspect, the disclosed device for automatic switching between different power sources can comprise a controller, a MOSFET switch and a current sensor, wherein when the device is in grid mode, the MOSFET switch can get periodically switch ON for a fixed period and allow a short circuit current from the PV panels to flow through a circuit configured with the current sensor. The controller can be configured to receive the sensed short circuit current and compare the sensed current with a threshold current. In case at the end of the ON period, the sensed current is more than the threshold current, the controller can suitably actuate corresponding relays to switch power supply from the grid mode to the PV mode. Alternatively if at the end of the ON period, the sensed current is not more than the threshold current, the controller can continue to allow the power to be drawn from the grid.
[0033] FIG.1 illustrates an exemplary schematic diagram100 of a 3-phase solar pump driven by PV power and grid power sources in accordance to an embodiment of the present disclosure. The power supply arrangement for the solar pump can incorporate solar panels such as PV_panel102 for supply of PV power, and electrical grid 110 for grid mode operation when the PV power is not adequate. Switching from one mode to other can be facilitated by PV_relays 106 and Relay_grid 114. PV_panel 102 can be connected through a circuit breaker MCB_PV 104 for protection from damage by overload or short circuit. There can also be a pre-charging section incorporating Relay_grid1 120 placed parallel to Relay_grid1 120 to pre-charge a DC link capacitorin order to avoid inrush current.
[0034] In an embodiment, the two PV_relays 106 in the solar section, and the Relay_grid 114 in the grid section can be automatically operated to connect and disconnect the 3 phase inverter 122 from the solar section and the grid for switching from one mode to other by a controller 124. In an aspect the controller 124 can be a Digital Signal Processor (DSP) configured to perform Maximum Power Point Tracking (MPPT) i.e. to get maximum power from the PV panels 102, wherein the MPPT is done based on current output from the PV panels.
[0035] In an embodiment, the disclosed device for switching from PV mode to the grid mode can further comprise a MOSFET switch 108 and a current sensor 110, wherein when the device is in grid mode, the MOSFET switch 108 can get periodically switched ON for a fixed period and allow a short circuit current IPV from the PV panels 102 to flow through a circuit configured with the current sensor 110. The controller 124 can be configured to receive the sensed short circuit current IPV and compare the sensed current with a threshold current IPV_threshold. In case at the end of the ON period, the sensed current is more than the threshold current i.e. IPV >IPV_threshold, the controller 124 can suitably actuate corresponding relays to switch power supply from the grid mode to the PV mode. Alternatively if at the end of the ON period, the sensed current IPV is not more than the threshold current IPV_threshold, the controller 124 can continue to allow the power to be drawn from the grid.
[0036] In an exemplary embodiment, the periodicity for sampling can be a few minutes such as five minutes and the period of sampling can be one minute. Thus the disclosed method can take care of short duration transient reduction of power output from PV due to clouds and help to maximize power drawn from PV panels and minimize dependence of the grid power.
[0037] In an aspect, the automatic switching between different power sources can be accomplished in a seamless manner by switching multiple relays in a controlled particular order. For example when it is observed by the controller that IPV >IPV_threshold confirming that enough solar power is available, the PV_relay 106 is put ON soon after the MOSFET switch 108 gets OFF but before the Grid_relay 114 gets OFF keeping the PV_relay 106 and the Grid_relay 114 in ON condition simultaneously for some time. Thereafter, the Grid_relay 114 gets OFF thus maintaining a seamless power supply. During these switchingsRelay_grid120 remains OFF as it gets ON only for initial dc link capacitor charging purpose when switching to the grid mode.
[0038] On the other hand if it is detected that IPV