Abstract:
An electrical DC-to-AC power conversion apparatus is disclosed that is primarily intended for use with solar photovoltaic sources in electric utility grid-interactive applications. The invention improves the conversion efficiency and lowers the cost of DC-to-AC inverters. The enabling technology is a novel inverter circuit topology, where throughput power, from DC source to AC utility, is processed a maximum of 1½ times instead of 2 times as in prior-art inverters. The AC inverter output configuration can be either single-phase, split-phase or poly-phase.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/248,826, entitled “Monopolar DC to Biopolar to AC Converter,” filed on Feb. 21, 2003, which issued as U.S. Pat. No. 7,064,969 on Jun. 20, 2006. 
    
    
     BACKGROUND OF INVENTION 
     The invention is an electrical power conversion topology and apparatus for converting and delivering power from a mono-polar DC source to an AC load. 
     Photovoltaic (PV) cells produce power over a wide voltage range depending on the amount of sunlight and the temperature of the photovoltaic cell. There are National Electric Code and class-of-equipment restrictions that make PV arrays much more cost effective when sized for a maximum of 600 Vdc. In order to source AC power into the electric utility grid, over the expected range of DC voltages, prior art utility-interactive inverters use two power conversion stages. 
       FIG. 6  shows one common prior art inverter topology. Photovoltaic (PV) array  10  is connected to the inverter at input terminals  11  and  12  across energy storage capacitor  59 . Transistors  51 ,  52 ,  55  and  56  are connected in a typical full-bridge arrangement. For clarity, anti-parallel diodes across each transistor are not shown. The full bridge is driven by a control circuit to regulate sinusoidal current in phase with the electric utility voltage across output terminals  71  and  72 . Current sensor  54  provides feedback to the control circuit. Inductor  53  smoothes the high frequency, pulse width modulated (PWM) waveform created by the switching action of transistors  51 ,  52 ,  55  and  56 . Transformer  60  steps down the utility voltage at terminals  71  and  72  to present a lower voltage to DC-to-AC converter  50  so that power can be delivered from PV array  10  to electric utility grid  70  under all conditions of temperature and irradiation on PV array  10 . Electric utility grid  70  is shown as typical, residential, 120/240 Vac, split-phase configuration with a center earth ground. PV array  10  can be operated grounded or ungrounded. 
     The inverter topology illustrated in  FIG. 6  has a number of limitations. First, all of the power from PV array  10  to electric utility grid  70  must be processed twice, once by DC-to-DC converter  50  and once by transformer  60 . Transformer  60 , from a loss-inventory perspective, is considered an AC-to-AC converter stage. Power is lost in each of these power conversion stages with a negative impact on overall inverter conversion efficiency. Second, transformer  60  operates at the electric utility line frequency and as such is heavy and expensive. 
       FIG. 7  shows another prior art inverter topology. Photovoltaic (PV) array  10  is connected to the inverter at input terminals  11  and  12 . Energy storage capacitor  81 , inductor  82 , current sensor  83 , transistor  84  and diode  85  are arranged as a typical, non-isolated, voltage boost converter. Capacitor  41  is shared by DC-to-DC converter  80  and DC-to-AC converter  50 . Transistors  51 ,  52 ,  55  and  56  are connected in a typical full-bridge arrangement. For clarity, anti-parallel diodes across each transistor are not shown. The full bridge is driven by a control circuit to regulate sinusoidal current in phase with the electric utility voltage across output terminals  71  and  72 . Current sensor  54  provides feedback to the control circuit. Inductors  53  and  57  smooth the high frequency, pulse width modulated (PWM) waveform created by the switching action of transistors  51 ,  52 ,  55  and  56 . Electric utility grid  70  is shown as typical, residential, 120/240 Vac, split-phase configuration with a center earth ground. PV array  10  must be operated ungrounded. 
     The inverter topology illustrated in  FIG. 7  has a number of limitations. Again, all of the power from PV array  10  to electric utility grid  70  must be processed twice, once by DC-to-DC converter  80  and once by DC-to-AC converter  50 . Power is lost in each of these power conversion stages with a negative impact on overall inverter conversion efficiency. Second, when the inverter is operating, there will be large AC common mode voltages, at the utility grid frequency and at the PWM switching frequency, on PV array  10  with respect to earth. The array becomes a radio transmitter. Also, additional conversion losses are had by charging and discharging the parasitic PV-array-to-earth-ground capacitance. In most U.S. jurisdictions, this inverter topology must be used with an external isolation transformer to meet regulatory code requirements. 
     Other prior-art inverter use a high frequency, double conversion topology which uses a high-frequency, transformer isolated DC-to-DC, voltage boosting converter first stage and a full-bridge DC-to-AC second stage. This topology significantly reduces the inverter weight and cost, a problem with the  FIG. 6  topology and mitigates the problem of AC common mode voltage on the PV array, a problem with the  FIG. 7  topology. This approach, however, yields the lowest conversion efficiencies because there are too many semiconductor losses. In terms of this discussion, the DC-to-DC stage used in these topologies is, more precisely, a DC-to-High Frequency AC-to-DC converter. Designs based on these topologies are complex, have high component parts counts and, as such, are less robust. 
     In all prior art topologies discussed, 100% of the throughput power is processed twice and power is lost in each conversion stage. The invention is an improvement over the prior art because the DC-to-AC conversion for the entire PV power converter can be done with 1½ conversion steps, instead of 2, for systems with grounded PV arrays and with effectively less than 1½ conversion steps for systems with ungrounded PV arrays. This translates to at least 25% less complexity, cost and conversion losses over the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the preferred embodiment of the invention in a system with an ungrounded PV array and a 120/240 Vac utility grid connection. 
         FIG. 2  illustrates a second embodiment where the PV array and DC-to-DC converter connections are swapped in a system with an ungrounded PV array and a 120/240 Vac utility grid connection. 
         FIG. 3  illustrates an embodiment of the invention with a specific DC-to-DC converter type in a system with an ungrounded PV array and a 120/240 Vac utility grid connection. 
         FIG. 4  illustrates an embodiment of the invention as part of a system using an earth-grounded array and a 120/240 Vac utility grid connection. 
         FIG. 5  illustrates an embodiment of the invention as part of a system using an earth-grounded array and a 120 Vac utility grid connection. 
         FIG. 6  shows a first common prior art inverter topology. 
         FIG. 7  shows a second common prior art inverter topology. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A number of slightly different DC-to-AC inverter topologies will be disclosed, all with the common characteristic that less than 1½ conversion stages are used or, stated differently, that at least ½ of the power from a DC source is converted only once in the DC-to-AC conversion process. The topologies are variations of the central idea of the invention configured to facilitate different options for the PV array grounding and the utility grid configuration. 
     The preferred embodiment of the invention is shown in  FIG. 1 . PV array  10  is connected to inverter input terminals  11  and  12 , across energy storage capacitor  42  and across the input of DC-to-DC converter  20 . The output of DC-to-DC converter  20  is connected to energy storage capacitor  41 . Capacitors  41  and  42  comprise the “bipolar energy storage element” referred to in the claims. Transistors  51 ,  52 ,  55  and  56  are connected in a typical full-bridge arrangement. For clarity, anti-parallel diodes across each transistor are not shown. The full bridge is driven by a control circuit to regulate sinusoidal current in phase with the electric utility voltage across output terminals  71  and  72 . Current sensor  54  provides feedback to a control circuit. Inductor  53  and  57  smooth the high frequency, pulse width modulated (PWM) waveform created by the switching action of transistors  51 ,  52 ,  55  and  56 . A 60 Hz sinusoidal current is sourced into utility grid lines  74  and  75 . This regulation methodology is known and is not part of this disclosure. Utility grid configuration  70  is a typical, residential, split-phase, 120/240 Vac service with earth-grounded center-tap  76 . PV array  10  and DC-to-DC converter  20  have no earth-ground reference. As such the voltage “seen” by DC-to-AC converter  50  is the voltage across the series combination of energy storage capacitors  41  and  42 . The voltage across capacitor  42  is always greater than the voltage across capacitor  41 . For example for a PV system designed to work at ambient temperatures of between 0° F. (−18° C.) and 115° F. (46° C.), PV array  10  voltage across capacitor  42  would be 443 Vdc and 326 Vdc respectively. The minimum required voltage across both capacitors required for DC-to-AC converter  50  to source undistorted current into a nominal 120/240 Vac utility grid is about 380 Vdc. Therefore, on the coldest day all of the throughput power, from PV array  10  to utility grid  70  is processed in a single, very high efficiency power conversion by DC-to-AC converter  50  alone and DC-to-DC converter  20  does not operate. On the hottest day DC-to-DC converter  20  regulates 54 Vdc (380 Vdc minus 326 Vdc) across capacitor  41 . On the hottest day, 14% of the power is processed twice, once by DC-to-DC converter  20  and a second time by DC-to-AC converter  50 . The other 86% of the power is processed by DC-to-AC converter  50  alone. The “makeup” voltage supplied by and regulated by DC-to-DC converter  20  will be a function of the PV array voltage and the utility grid voltage. Higher AC utility grid voltages will require more of a contribution from DC-to-DC converter  20 . The voltage across PV array  10  will be regulated by an iterative perturb-and-observe algorithm to track the maximum power point of PV array  10  under all conditions. These regulation and control methodologies are known. The invention is a novel power conversion topology using known control and regulation methods. 
       FIG. 2  illustrates a variant of the topology disclosed in  FIG. 1 . There are two differences. First, the location of PV array  10  and DC-to-DC converter  20  are exchanged. Second, capacitor  43  has been added. These two differences have no effect on the inverter performance or function described in  FIG. 1 . With the inclusion of capacitor  43 , an AC ground reference is established for PV array  10  and the bipolar energy storage element formed by capacitors  41  and  42 . In some inverter designs the addition of capacitor  43  will reduce electromagnetic radiation. 
       FIG. 3  illustrates one possible circuit configuration for DC-to-DC power converter  20  in  FIG. 1 . Transistor  21 , inductor  23  and diode  24  are configured as a typical, non-isolated flyback converter. Current sensor  22  provides feedback to a control circuit. The flyback, DC-to-DC power converter topology and regulation methods thereof are known. Diode  25  is used to bypass capacitor  41  when no additional “makeup” voltage is required at the output of DC-to-DC flyback converter  20 . 
       FIG. 4  illustrates a slightly different version of the topology shown in  FIG. 3  where one side of PV array  10  is connected to earth ground  76 . The circuit function is the same as in  FIG. 3  except that DC-to-DC converter  20  must always supply ½ of the power processed by DC-to-AC converter  50 . Also, because the DC supply to DC-to-AC converter  50  is bipolar and the utility grid connection  70  is split-phase, two regulator circuits are required, one for each 120 Vac circuit  72  and  74 . As such, additional current sensor  58  is required. 
       FIG. 5  is the same as  FIG. 4  except that the inverter has one less half-bridge section and supplies power to a single-phase 120 Vac utility grid  70 . 
     The embodiments of this invention are illustrated in the figures using IGBT type semiconductor switching devices. The invention is specific arrangements of switching devices and other components that connect to form novel power circuit topologies based on a central concept. The switching device type does not define the topology. As such, Field Effect Transistors (FETs), Bipolar Junction Transistors (BJTS) or any substantially similar semiconductor switching device type could be substituted for any of the IGBT devices illustrated in  FIGS. 1 through 5 .