Abstract:
An automatic transfer switch configured for connection to a non-traditional, full-time or intermittent power source, such as a wind turbine or solar panel, selectively connects the non-traditional power source as a primary power source and a utility-derived power source as a secondary power source. The intermittent power source includes an energy storage device, such as a bank of DC batteries, to supplement power delivery during periods of low energy production. The power is provided to an AC load via a DC-to-AC inverter. The transfer switch includes an input to monitor the voltage level on the energy storage device and will switch from the primary power source to the utility power source when the voltage level on the energy storage device drops below a preset level. Thus, a loading condition that exceeds the rating of the inverter will not fault the inverter or limit the power available to the loads.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application Ser. No. 61/477,399, filed Apr. 20, 2011, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to a transfer switch for automatically switching between a primary power source and a secondary power source. More specifically, a transfer switch for automatically switching between an alternative energy source and the utility grid is disclosed. 
         [0003]    Historically, it has been known to provide a transfer switch to automatically transfer a load from the utility grid to a backup generator in the event the utility grid was not operating properly. For example, the voltage present on the utility grid is monitored, and if the voltage drops below a preset value, the transfer switch switches to a backup power source, such as a generator. Further, the automatic transfer switch may provide a signal to the generator to start the engine and wait until the generator is operating normally before connecting the load to the generator. 
         [0004]    With the development of alternative energy sources and their associated power conversion equipment, it may be desirable to provide a power source other than the utility grid as the primary power source. An alternative energy source, such as a photovoltaic array or a wind turbine, may be selected. However, the power supply from such alternative sources is variable, dependent on the amount of sunlight or wind present at a given time. As a result, alternative energy sources are often utilized in combination with an energy storage device. Because the energy storage device stores and supplies a DC voltage, an inverter is provided which converts the DC voltage to an AC voltage suitable for connection to the load. 
         [0005]    However, a traditional automatic transfer switch is not well suited for an application in which the primary energy source is an alternative energy source and the secondary energy source is the utility grid. As previously discussed, a typical automatic transfer switch monitors the voltage level of the primary energy source and switches to the secondary source when the voltage level drops below a minimum value. However, an inverter supplying the AC voltage will attempt to regulate the output voltage to maintain the nominal AC voltage, for example 120 VAC. If the alternative energy source is not producing sufficient power to supply the load and the battery has been drained, the inverter will experience a fault condition and shut down. Manual intervention is required to reset the inverter, defeating the purpose of an automatic transfer switch. Further, due to the variable energy production, it is expected that more frequent switching from an alternative energy source to the utility grid would be required than when the utility grid is the primary energy source. 
         [0006]    Thus, it would be desirable to provide an automatic transfer switch suitable for use with an alternative energy source as the primary energy source and the utility grid as a secondary source. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    The subject matter disclosed herein relates to an automatic transfer switch that is configured for connection to non-traditional, full-time or intermittent power sources such as wind turbines, water turbines, and solar panels. The power output of these non-traditional power sources may be used to charge banks of DC batteries, to directly power a load, or a combination thereof. The power is provided to an AC load via a DC-to-AC inverter. The transfer switch is configured such that the inverter-derived power is the primary source, and the utility-derived power is the backup source. The transfer switch operates with the assumption that both sources are available under normal operation. 
         [0008]    The transfer switch will switch from the inverter to the utility when the non-traditional power source cannot maintain sufficient output power to satisfy the needs of the load. Because the DC batteries supplement power when the power source is not capable of producing all of the power supplied to the load, the amount of energy remaining in the DC batteries provides an indication whether the non-traditional power source can supply sufficient output power for the load. As the energy level in the DC battery decreases, the DC voltage present on the terminals of the battery typically decreases as well. By detecting when the battery bank DC voltage drops below a preset level, the transfer switch can switch power sources before the inverter reaches a fault condition and shuts down. The transfer switch will also switch from the inverter to the utility when the load current exceeds the rating of the inverter but before the inverter reaches a fault condition and shuts down. Thus, a temporary loading condition that exceeds the rating of the inverter will not cause either a fault in the inverter or restrict the power available to the loads. 
         [0009]    According to one embodiment of the present invention, a switch for selectively connecting a load to an alternative energy source and an energy storage device as a primary power source and a utility supply as a secondary power source includes a first input configured to receive a signal corresponding to a level of energy in the energy storage device. At least one actuator selectively connects either the primary power source or the secondary power source to the load, and an electrical circuit generates a control signal for the actuator to selectively connect either the primary power source or the secondary power source to the load as a function of the level of energy in the energy storage device. The electrical circuit may include analog electrical devices, and the control signal generated by the electrical circuit may initiate a transition from the primary power source to the secondary power source if the level of energy in the energy storage device is below a predetermined threshold. 
         [0010]    According to another aspect of the invention, a second input is configured to receive a signal corresponding to the operating conditions of the secondary power source, and the electrical circuit further generates the control signal for the actuator as a function of the operating conditions of the secondary power source. The control signal generated by the electrical circuit may initiate a transition from the primary power source to the secondary power source if the level of energy in the energy storage device is below a predetermined threshold and if the operating conditions of the secondary power source are within a predetermined range. 
         [0011]    According to still another aspect of the invention, a third input is configured to receive a signal corresponding to a magnitude of current drawn by the load, and the electrical circuit further generates the control signal for the actuator as a function of the current drawn by the load. The control signal generated by the electrical circuit may initiate a transition from the primary power source to the secondary power source if operating conditions of the secondary power source are within a predetermined range and if either the level of energy in the energy storage device is below a predetermined threshold or the magnitude of current drawn by the load is above a predetermined threshold. 
         [0012]    According to another embodiment of the invention, an automatic transfer switch for selectively connecting a first power source and a second power source to a load includes a first input terminal electrically connected to the first power source, a second input terminal electrically connected to the second power source, and an output terminal electrically connected to the load. The first power source includes an alternative energy source and an energy storage system. At least one actuator selectively connects one of the first input terminal and the second input terminal to the output terminal. A first sensor measures a first operating parameter corresponding to an energy level in the energy storage system of the first power source, and a second sensor measures a second operating parameter of the second power source. An electrical circuit provides a control signal to the actuator to connect either the first input terminal or the second input terminal to the output terminal as a function of the first and second operating parameters. 
         [0013]    According to still another embodiment of the invention, a method of selectively connecting a load to either a first power source, which includes an energy storage device, or a second power source, includes the steps of measuring a level of energy in the energy storage device, connecting the load to the first power source if the level of energy in the energy storage device is greater than a predetermined threshold, and connecting the load to the second power source if the level of energy in the energy storage device is less than the predetermined threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         [0014]    Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
           [0015]      FIG. 1  is a block diagram representation of one embodiment of the present invention; 
           [0016]      FIG. 2  is a flowchart illustrating the automatic switching of a transfer switch according to one embodiment of the invention; 
           [0017]      FIG. 3  is an exemplary power distribution panel incorporating one embodiment of the present invention; 
           [0018]      FIG. 4  is the exemplary power distribution panel of  FIG. 3  with a cover removed; and 
           [0019]      FIG. 5  is a block diagram representation of the electrical circuit controlling the transfer switch according to one embodiment of the invention. 
       
    
    
       [0020]    In describing the preferred embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The various features and advantageous details of the subject matter disclosed herein are explained more fully with reference to the non-limiting embodiments described in detail in the following description. 
         [0022]    Referring to  FIG. 1 , a power generation system  10  may include multiple power sources. The power sources may be any suitable combination of power sources according to the requirements of a specific environment or application. A Direct Current (DC) power source  12 , such as a photovoltaic array, generates power having a DC voltage. The DC voltage may be supplied directly to a DC bus  20 , also referred to as a DC distribution system/network or a DC link, or the DC voltage may be supplied to the DC bus  20  via a DC-to-DC power converter  22 . An Alternating Current (AC) power source  14 , such as a wind turbine, water turbine, etc., generates power having an AC voltage. The AC voltage is supplied to the DC bus  20  via an AC-to-DC power converter  24 . Because each of these alternative energy sources generates a variable amount of power, an energy storage device  16 , such as a battery, is also connected to the DC bus  20  via a power converter  26 . The power converter  26  manages bi-directional power flow between the DC bus  20  and the energy storage device  16  such that the energy storage device  16  is charged when the power sources generate energy in excess of the energy required by a load  38  and the energy storage device  16  is discharged, providing the power to or supplementing the power supplied to the load  38 , when the power sources generate less energy than that required by the load  38 . In addition, a connection to utility grid  35  is also available. 
         [0023]    A transfer switch  40  is provided to selectively connect the electrical load  38  to the power sources. The transfer switch  40  includes a first input terminal  42  and a second input terminal  44 , each input terminal being configured to establish an electrical connection to one of the power sources. The transfer switch  40  also includes an output terminal  46  configured to establish an electrical connection to the load  38 .  FIG. 1  is a line drawing and illustrates each of the input terminals,  42  and  44 , and the output terminal  46  by a single line. Nevertheless, it is contemplated that each of the terminals may include a single connection or multiple connections, for example two terminals, to selectively transfer connection between a single power lead or between multiple power leads according to the application requirements. According to still another embodiment, one power lead from each source may be selectively connected to the load while another lead from each source may be tied to a common connection. 
         [0024]    According to one embodiment of the invention, the first input terminal  42  is connected to an alternative energy source, such as a wind turbine, water turbine, or photovoltaic array, and the second input terminal  44  is connected to the utility grid  35 . Referring again to  FIG. 1 , one or more AC energy sources  14 , one or more DC energy sources  12 , an energy storage device  16 , or any combination thereof may be joined to the DC bus  20 . The DC bus  20  is connected to the transfer switch  40  via an inverter  30 , which converts the DC voltage present on the DC bus  20  to an AC voltage suitable to be supplied to the load  38 . The output of the inverter  30  is connected to the first input terminal  42  of the transfer switch  40 . 
         [0025]    A switch  51  selectively connects either the first input terminal  42  or the second input terminal  44  to the output terminal  46 . Although the switch  51  is illustrated as a single pole, double throw switch, any suitable configuration, such as without limitation a double pole, single throw switch or two single pole, single throw switches, may be used to connect the terminals. The first input terminal  42  is electrically connected to a first input  52  on the switch  51 . The second input terminal  44  is electrically connected to a second input  54  on the switch  51 . An output  56  from the switch  51  is electrically connected to the output terminal  46 . The contact  53  of the switch  51  may be either mechanically or electronically controlled to selectively connect one of the inputs,  52  or  54 , to the output  56  of the switch  51 . Mechanical operation may be provided by a lever that is accessible by an operator to toggle the contact  53  of the switch  51  between the two inputs,  52  and  54 . Electrical operation may be provided by an actuator, such as a solenoid, or a pair of actuators that are selectively energized to draw the contact  53  of the switch  51  between the two inputs,  52  and  54 . 
         [0026]    The transfer switch  40  further includes a control circuit  50  which generates at least one control signal  55  to selectively position the contact  53  between either the first input  52  or the second input  54  of the switch  51 . The control circuit  50  receives input signals from multiple sensors which monitor the operating performance of the power sources connected to the transfer switch  40 . According to one embodiment of the invention, as shown in  FIGS. 1 and 5 , a first voltage sensor  62  monitors the amplitude of the AC input voltage from the inverter  30 . A second voltage sensor  64  monitors the amplitude of the AC input voltage from the utility grid  35 . A current sensor  66  monitors the amplitude of the current drawn by the load  38 . A third voltage sensor  68  monitors the amplitude of the DC voltage present at the energy storage device  16 . According to alternate embodiments of the invention, it is contemplated that voltage and/or current sensors may be positioned in alternate locations of the power system  10  and in alternate combinations to monitor operating performance of the power sources. 
         [0027]    Referring also to  FIG. 2 , at step  110 , the control circuit  50  reads the values from each of the sensors, indicating the operating status of each of the power sources. At step  120 , the control circuit  50  checks that the voltage level at the energy storage device  16  is at an acceptable level. As the energy storage device  16  is discharged, the amplitude of the DC voltage begins to drop. Discharge of the energy storage device indicates that the alternative energy source is not generating sufficient energy to satisfy the power demand of the load  38 . If the charge remaining in the energy storage device drops below a minimum level, it may no longer be possible to fully supplement the alternative energy source to satisfy the demand of the load  38 . At this operating point, the inverter  30  would attempt to maintain the commanded magnitude of the AC output voltage; however, with insufficient power at the input of the inverter  30 , the inverter could enter a faulted state, requiring a manual reset. In order to prevent the inverter  30  from entering the faulted state, the control circuit  50  may be preconfigured with a threshold value below which it is undesirable to allow the DC voltage at the energy storage device  16  to drop. 
         [0028]    If the DC voltage level of the energy storage device  16  is greater than the threshold value, the control circuit  50  proceeds to step  130  and checks the amplitude of the current being drawn by the load  38 . The control circuit  50  may be preconfigured with a maximum current value above which it is undesirable to operate the inverter  30 , since inverters commonly have a short-term current rating as well as a long-term current rating. By monitoring the amplitude of the current drawn by the load  38 , the transfer switch  40  may disconnect the load  38  from the alternative energy source and reconnect the load  38  to the utility grid  35  if, for example, the current drawn exceeds the long-term current rating of the inverter  30  but before the current exceeds the short-term current rating of the inverter  30 , causing the inverter  30  to enter a faulted state. If both the DC voltage level at the energy storage device  16  and the current drawn by the load  38  are within the desired operating limits, the control circuit  50  generates a control signal  55  to connect the alternative energy source to the load  38 , as shown at step  150 . 
         [0029]    If the DC voltage level of the energy storage device  16  is less than the threshold level, the control circuit  50  proceeds to step  140  and checks the magnitude of the AC voltage being supplied by the utility grid  35 . Similarly, if the DC voltage level of the energy storage device  16  is above the threshold level but the current drawn by the load  38  exceeds its setpoint, the control circuit  50  again proceeds to step  140  and checks the magnitude of the AC voltage being supplied by the utility grid  35 . The control circuit  50  may be preconfigured with a minimum and/or a maximum value for the AC voltage from the utility grid  35 . If the AC voltage from the utility grid  35  is operating at an acceptable level, the control circuit  50  generates a control signal  55  to connect the utility grid  35  to the load  38 , as shown at step  160 . Verifying that the utility grid  35  is operating within acceptable parameters prior to connecting the load  38  to the utility grid  35  reduces the potential for damage to the electrical systems or devices which constitute the load  38 . 
         [0030]    If neither power source is operating within desired parameters, the control circuit  50  generates a control signal  55  to connect the alternative energy source to the load  38 , as shown at step  150 . For example, if either the DC voltage level of the energy storage device  16  is less than the minimum voltage desired for operation of the inverter or the current drawn by the load exceeds the maximum current draw desired for operation of the inverter yet the utility grid  35  is not within acceptable operating parameters, continuing to provide power from the alternative energy source may extend the duration of time that power may be supplied to the load  38 . The inverter  30  will attempt to maintain the commanded output voltage and supply the demanded current as long as the energy from the primary source allows and as long as the current does not exceed the short term overload of the inverter  30 . Thus, if the utility grid  38  is not available, keeping the primary source connected to the load  38  may allow power to be supplied during a temporary dip in voltage or spike in current. However, if the utility grid  35  remains unavailable and the primary source is outside of its desired operating parameters for too long, the inverter  30  will necessarily fault to prevent damage to the inverter  30 . 
         [0031]    The steps illustrated in  FIG. 2  are performed on a periodic or continuous basis. Referring also to  FIG. 5 , the components of the control circuit may be implemented either in a digital or analog manner. Each of the sensors monitoring performance of either the primary or secondary supply provides an analog signal to the control circuit  50 . Comparators implemented, for example, using operational amplifiers compare the signal to a reference value to determine whether each supply is operating within an acceptable range. The outputs of the operational amplifiers may drive discrete logic gates and solenoid driver circuits,  150  or  160 . The solenoid driver circuits,  150  or  160 , activate solenoids,  170  or  180 , to selectively position the switch  51  and to connect either the primary source or the secondary source to the load  38 . Thus, an analog circuit may continuously monitor the operating conditions of each of the sources and select the proper source accordingly. Optionally, a portion of the control circuit  50  may be implemented digitally, for example in a field programmable gate array (FPGA), application specific integrated circuit (ASIC), or within a microprocessor. A digital circuit may, therefore, periodically monitor the operating parameters according to the execution of instructions within a device. 
         [0032]    Regardless of implementation, it is desirable that a transfer between primary and secondary sources occurs quickly enough to avoid an observable disruption of power at the load  38 . Because the transfer switch  40  is configured to be connected to a variable or intermittent energy source as the primary energy source and to the utility grid as a secondary energy source, it is contemplated that the transfer switch  40  will need to switch between the primary and secondary sources more frequently than a conventional transfer switch. Further, such transfers may occur under normal operating conditions rather than as a backup system solely in the event of a failure of the first power source. It is desirable that, when necessary, the transfer switch  40  disconnects the primary power source and reconnects the secondary power source quickly to minimize or eliminate noticeable indications of the switch over, for example a flickering of lights or temporary interruption of the picture on a television. As a result, implementing the control circuit  50  using the analog circuit devices may provide improved responsiveness and fast switching capabilities. 
         [0033]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention