Abstract:
A photovoltaic system, method and apparatus are disclosed. In an exemplary embodiment, the system includes a photovoltaic array, a distribution system that distributes power within a premises of a demand-side energy consumer, an inverter coupled to the distribution system that is configured to convert DC power from the photovoltaic array to AC power and apply the AC power to the distribution system, a damping portion configured to damp high frequency voltages derived from the inverter, and trapping circuitry coupled to the damping portion that is configured to reduce a level of low frequency current traveling through the damping portion.

Description:
PRIORITY 
     The present application claims priority to provisional patent application No. 60/939,807, filed May 23, 2007, entitled: C OMMON  M ODE  F ILTER FOR  S OLAR  I NVERTER , which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to apparatus and methods for converting solar energy to electrical energy, and more specifically to apparatus and methods for more efficient conversion of solar energy to electrical energy. 
     BACKGROUND OF THE INVENTION 
     In photovoltaic (PV) energy systems, inverters are used to convert DC electrical energy that is generated by photovoltaic panels to AC electrical energy that is compatible with the AC distribution systems that are built into the infrastructure of residential, commercial, industrial premises, and large ground mounted solar farms. 
     Typically an inverter is hard grounded (e.g., on the negative rail of the inverter DC input) and an isolation transformer is utilized in connection with the inverter to galvanically isolate the inverter from the AC power system of the premises and to provide a voltage ratio change. Transformers, however, add additional inefficiencies, complexity, weight and substantial cost to inverters. 
     For inverter applications large enough to warrant a dedicated connection to the utility (e.g., a utility transformer connecting the inverter to the utility&#39;s own medium voltage supply), it is often possible to remove the integrally provided transformer from the product. Although purchased as “transformerless,” the operation of such inverters is still predicated on the presence of an isolating transformer to allow for the traditionally hard-grounded PV array configuration. Although inverters have been marketed as being “transformerless” because, when sold, a transformer is not integrated with these inverters, the power distribution system (e.g., 480/277 VAC) of the premises is still isolated from the inverter by a utility transformer. As a consequence, these “transformerless” inverters still rely on a transformer for isolation and are typically limited to installations where medium voltage (e.g., 4160 to 13 KV) transformers are present at the premises. 
     There is not presently a large PV inverter (e.g., an inverter larger than 10 kW) intended to operate independent of any isolating transformer. Presently available equipment also does not allow inversion directly into 480VAC three-phase, but rather into much lower voltages that are ratio-changed to more useful voltages such as 480 Volts by the integrally provided transformer. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims. 
     In one embodiment the invention may be characterized as a photovoltaic system that includes a photovoltaic array, a distribution system that distributes power within a premises of a demand-side energy consumer, an inverter coupled to the distribution system that is configured to convert DC power from the photovoltaic array to AC power and apply the AC power to the distribution system, a damping portion configured to damp high frequency voltages derived from the inverter, and trapping circuitry coupled to the damping portion that is configured to reduce a level of low frequency current traveling through the damping portion. 
     In another embodiment the invention may be characterized as a power conversion device. The power conversion device in this embodiment comprises inputs adapted to receive DC power, an inverter portion configured to convert the DC power to AC power, outputs adapted to apply the AC power to a distribution system of a premises of a demand-side energy consumer, a damping portion configured to damp high frequency voltages derived from the inverter so as to enable the inverter portion to couple with the distribution system without an isolation transformer, and a trapping portion that is configured to reduce a level of low frequency current traveling through the damping circuit. 
     In yet another embodiment the invention may be characterized as a method for applying power to a distribution system of a premises of a demand-side energy consumer. The method in this embodiment includes generating DC power from a renewable source of energy, converting the DC power to AC power, applying the AC power directly to the distribution system of the premises, and filtering high frequency voltages so as to reduce an amount of high frequency voltages propagating to the distribution system and the PV array. 
     As previously stated, the above-described embodiments and implementations are for illustration purposes only. Numerous other embodiments, implementations, and details of the invention are easily recognized by those of skill in the art from the following descriptions and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings wherein: 
         FIG. 1  is a block diagram depicting an exemplary embodiment of a photovoltaic system; 
         FIG. 2  is a schematic representation of an exemplary embodiment of the inverter described with reference to  FIG. 1 ; 
         FIG. 3  is a schematic representation of an embodiment that may be utilized in connection with the embodiments described with reference to  FIGS. 1 and 2 ; 
         FIG. 4  is a schematic representation of another embodiment that may be utilized in connection with the embodiments described with reference to  FIGS. 1 and 2 ; 
         FIG. 5  is a block diagram depicting an exemplary inverter-system architecture; 
         FIG. 6  is a flowchart depicting an exemplary method that may be carried out in connection with the embodiments discussed with reference to  FIGS. 1-5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring first to  FIG. 1 , shown is a block diagram an exemplary embodiment of the present invention. As shown, in this embodiment a first set  102  and a second set  104  of photovoltaic panels are arranged to create a bipolar panel array  106  that is coupled to an inverter  108 , which is disposed between the panel array  106  and a distribution system  110 . As shown, the distribution system  110  in this embodiment is coupled to the secondary side of a wye-configured medium-voltage to 480/277 three-phase transformer  112  that is grounded at its star point. 
     The illustrated arrangement of the components depicted in  FIG. 1  is logical and not meant to be an actual hardware diagram; thus, additional components can be added or combined with the components that are depicted in an actual implementation. It should also be recognized that the components, in light of the disclosure herein, may be readily implemented by one of ordinary skill in the art. 
     As an example, the inverter  108  is depicted as coupling directly to the array  106 , but this is certainly not required. In some embodiments, for example, a PV interface is interposed between the array  106  and the inverter  108 . In these embodiments, the PV interface generally operates to enable the inverter  108 , which is designed to operate at lower voltages, to be utilized in connection with the PV array  106  that operates at least a portion of the time (e.g., while unloaded) at a voltage that exceeds the designed operating voltage of the inverter  108 . U.S. application Ser. No. 11/967,933, entitled Photovoltaic Inverter Interface Device, System and Method, which is incorporated herein by reference discloses exemplary PV interfaces that may be utilized in connection with one or more embodiments of the present invention. 
     In general, the photovoltaic array  106  converts solar energy to DC electrical power, which is converted to AC power (e.g., three-phase power) by the inverter  108 . As shown, the AC power output by the inverter  108  is applied to the distribution system  110 , which in many embodiments is the three phase distribution system of a demand-side energy consumer (e.g., a commercial entity, industrial entity, or collection of residential units). In other embodiments, however, it is contemplated that the distribution system  110  is a portion of a utility distribution system. The transformer  112  in the exemplary embodiment is configured to couple the distribution system  110  to medium voltage power provided by an electrical utility company. 
     In some embodiments, the cells in the array  106  include crystalline (e.g., monocrystalline or polycrystalline) silicon that operates in an open load state at 1200 Volts and operates in a loaded state between 660 and 960 Volts. And in other embodiments the array includes cells comprising amorphous silicon that operates in an open load state at 1400 Volts and a loaded state around 900 Volts. One of ordinary skill in the art will appreciate, however, that the photovoltaic array  102  may include a variety of different type photovoltaic cells that are disposed in a variety of different configurations. For example, the photovoltaic cells may be arranged in parallel, in series or a combination thereof. 
     As depicted, the first set of panels  102  and the second set of panels  104  are tied together, and as shown, in some embodiments, the first and second sets of panels  102 ,  104  are tied together outside of the inverter  108 . In these embodiments, it is contemplated that a line (not shown) sized for relatively low currents will couple, via a fuse, the tie point  114  between the first and second sets of panels  102 ,  104  to ground during nighttime. This connection would be made by a relay and therefore reference the array in the conventional manner when it is not producing power. 
     This embodiment has the benefit of utilizing one lower gauge, and hence less costly, wire between the tie point  114  of the first and second panels  102 ,  104  and ground. U.S. application Ser. No. 12/022,147, entitled System and Method for Ground Fault Detection and Interruption, which is incorporated herein be reference, discloses, among other technical advancements that may be utilized in connection with embodiments of the present invention, techniques for tying panels (e.g., the first and second sets of panels  102 ,  104 ) together. 
     In other embodiments, however, the first and second sets of panels  102 ,  104  are tied together within the inverter  108  via a normally open switch that is closed during operation of the inverter  108 . 
     Beneficially, the embodiment depicted in  FIG. 1 , unlike typical approaches, does not include a separate isolation transformer between the inverter  108  and the distribution system  110 , and as a consequence, efficiency of the system is increased while the cost, size, and weight of the inverter  108  are decreased. 
     Moreover, multiple inverters may be placed side by side without an isolation transformer. And beneficially, the number of inverters that may be placed side by side is virtually unlimited, and the inverters may be placed in a convenient location (e.g., away from an entry point of utility power to a building). 
     Referring next to  FIG. 2 , shown is an exemplary embodiment of the inverter  108  depicted in  FIG. 1 . As shown, the inverter in this embodiment includes a ground fault portion, disconnect contactor, surge protection, a common mode choke, an inverter portion, line reactors, AC contactor, and EMI filter aligned in series. In addition, a drive board is shown coupled between the inverter and a control computer, and the control computer is coupled to an inverter interface that includes a front door interface, a data reporting computer, and Ethernet interface and a wireless interface. 
     As depicted, in this embodiment the common mode choke is located on the DC side of the converter, but this is not required, and in other embodiments, the common mode choke may be positioned on the AC side of the inverter. It has been found that capacitance of the solar array to ground may cause undesirable instability of the inverter. The addition of filtering components (e.g., common mode choke and other filter components) prevents this undesirable behavior. 
     As discussed further herein, it has also been found that eliminating the isolation transformer that is normally interposed between an inverter and a distribution system (e.g., distribution system  110 ) may lead to high frequency (e.g., 18 kHz) common mode voltages propagating to the photovoltaic system. These high frequency voltages may vary in frequency depending upon the switching frequency of the inverter, but they may create adverse consequences relative to other inverters that may be coupled to the distribution system or other loads in the distribution system. 
     In addition, it has been found that when the high frequency voltages are filtered, 180 Hz voltages that originate from pulse width modulation saturation may also propagate through the high frequency filter, which creates substantial energy losses and heat generation. Moreover, 60 Hz voltage fluctuations due, for example, to asymmetrical loading, may propagate through the distribution system. 
     As a consequence, the common mode choke in the exemplary embodiment is implemented in connection with damping circuitry to remove the high frequency voltages (e.g., 18 kHz) in connection with low frequency (e.g., 180 Hz) traps to prevent the flow of the 180 Hz voltages through the damping circuitry. 
     Referring to  FIG. 3 , for example, shown is a schematic representation of a common-mode choke  302  used in connection with damper networks  304  and a parallel resonant tank circuits  306  that operate as traps for 180 Hz voltages that may be used in connection with the embodiments described with reference to  FIGS. 1 and 2 . In operation, the damper networks  304  filter the high frequency voltages (e.g., 18 kHz) and the parallel resonance tank circuits  306  operate to create a high impedance for 180 Hz frequencies. As a consequence, the damper circuits  304  remove potentially harmful high frequency currents and the parallel resonance tank circuits  306  prevent 180 Hz voltages from creating substantial energy losses by preventing 180 Hz current from flowing through the damper networks  304 . 
     Although not required, the capacitor in each of the damper networks  304  may be realized by a 60 microfarad capacitor and the resistor in each of the networks  304  may be implemented with a 50 Ohm resistor. In addition, the capacitor in the parallel resonance tank circuits  306  may be implemented with a 40 microfarad capacitor and the inductor may be realized by a 19.5 milliHenry inductor. The combination of the parallel inductor and capacitor act as an open circuit at 180 Hz allowing high frequencies to be attenuated but not dissipating excessive power at 180 Hz. 
     Referring to  FIG. 4 , shown is a schematic representation of an alternative to the embodiment described with reference to  FIG. 3 . As shown, this embodiment is substantially the same as the embodiment depicted in  FIG. 3 , except the parallel resonant tank circuits  306  described with reference to  FIG. 3  have been replaced by a single parallel resonant tank circuit  406  that also operates as a trap for 180 Hz voltages. In operation, the damper networks  404  filter the high frequency voltages (e.g., 18 kHz) and the parallel resonance tank circuit  406  operates to create a high impedance for 180 Hz frequencies using fewer components than the resonance tanks circuits  306 . 
     Referring next to  FIG. 5 , shown is an exemplary embodiment of an inverter-system architecture. As shown, in this embodiment N inverters are configured to operate in connection with a single distribution system without requiring the use of isolation transformers. Beneficially, a common mode choke (e.g., the common mode chokes discussed with reference to  FIGS. 2 ,  3 , and  4 ) in connection with damping networks (e.g., damping networks  304 ,  404 ) enable multiple inverters to be utilized with the same distribution system without corresponding isolation transformers; thus allowing a multiple-inverter system to be realized without the need for installing expensive, bulky and heavy isolation transformers. 
     Referring next to  FIG. 6 , shown is a flowchart depicting an exemplary method that may be carried out in connection with the embodiments described with reference to  FIGS. 1-5 . As shown, DC power is initially generated (e.g., from a renewable source of energy such as solar energy captured by the array  106 )(Blocks  602 ,  604 ), and the DC power is converted to AC power (e.g., by an inverter described with reference to FIGS.  1 - 5 )(Block  606 ) before the AC power is applied to a distribution system (e.g., a distribution system for a large residential, commercial, or industrial facility)(Block  608 ). In many embodiments, the AC power is directly applied to the distribution system (e.g., without interposing an isolation transformer between the inverter and the distribution system). 
     As depicted in  FIG. 6 , in many implementations, high frequency components that may be generated by the inverter are filtered so as to reduce or prevent the high frequency components that would ordinarily propagate to the source of the DC power (e.g., array  106 ) or to other devices (e.g., other inverters or electronic devices) that may be coupled to the distribution system (Block  610 ). In addition, in some embodiments it has been found that it is advantageous to reduce the flow of low frequency currents that may otherwise flow through the filter(s) for the high frequency voltages (Block  612 ). As previously discussed for example, 180 Hz voltages that originate from pulse width modulation saturation may propagate through the high frequency filter, and create substantial energy losses and heat generation. 
     Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention.