Abstract:
A solar photovoltaic plant is disclosed where a number of distributed DC-to-DC converters are used in conjunction with a central DC-to-AC converter. Each DC-to-DC converter is dedicated to a portion of the photovoltaic array and tracks the maximum power point voltage thereof. The DC-to-DC converters also boost the photovoltaic voltage and regulate a DC output current for transmission to the central DC-to-AC converter. Five distinct advantages are had over the prior art. First, efficiencies in intra-field power collection are greatly improved by transferring power at higher DC voltages. Second, the number of independent photovoltaic maximum power point trackers in the power plant can be increased, in a cost effective manner, to optimize the overall photovoltaic array energy harvest. Third, each DC-to-DC converter output “looks” like a current source at the input of the DC-to-AC converter and therefore can be easily paralleled. Fourth, the current source nature of the DC-to-DC converter outputs enables the DC-to-AC converter to operate with a minimum, fixed DC bus voltage to provide maximum DC-to-AC power conversion efficiencies. And fifth, each distributed DC-to-DC converter can isolate a faulted portion of the photovoltaic array while the remainder of the array continues producing power.

Description:
This application claims priority of Provisional Application No. 61/148,770 
    
    
     BACKGROUND OF THE INVENTION 
     In large scale, prior art photovoltaic power plants, a great number of solar photovoltaic modules are connected in series and then in parallel to facilitate DC power collection at a central location where it can then be converted to AC power by a DC-to-AC power converter or inverter. 
     Typically, the sizing of conductors in any electrical installation is based on how much current a conductor can handle and remain at a safe temperature. In photovoltaic power plants, the value of the photovoltaic energy is high and conductors are oversized with respect to ampacity in order to limit the overall resistive wiring losses of the system. In a properly designed photovoltaic power plant, the incremental cost to increase the size of a conductor to save one watt of resistive losses should be equal to the cost of installing an additional watt of photovoltaic generating capacity. As such, the cost of collecting power from acres of solar panels is a significant portion of the overall power plant cost. If a megawatt power plant can be built for $5/Watt AC, then a 1% reduction in copper conductor losses could save $50,000 in system costs. 
     If the maximum power point voltage of each solar module could be individually tracked, the overall energy harvest from the photovoltaic power plant would be maximized. This extreme approach would not be cost effective. However, as the number of maximum power point trackers in a system is increased, the annual energy harvest will be increased as well. In large power plants, sections of the total array will have different wind exposures, different local soil reflectivity, different cloud cover, different soiling, different shadowing, different aging characteristics and different “factory” module characteristics. All of these factors will affect the maximum power point voltage of any group of modules. If one large maximum-power-point-tracking DC-to-AC inverter converts the entire array power, this power converter will operate at an average PV operating point. The portions of the array that are statistically weaker or stronger will not operate at their maximum power point voltages and array harvest will be compromised. A number of tradeoffs need to be considered, however, for any system design between complexity (and therefore reliability), power conversion inefficiencies of the maximum power point trackers, system costs and array harvest enhancements. 
       FIG. 2  illustrates a typical, prior art photovoltaic power plant. Photovoltaic sub-array  20  is a collection of series and parallel connected photovoltaic modules. Conductors  27  and  28  carry the combined current of sub-array  20  in conduit  29  to DC-to-AC converter inputs  5  and  6 . This circuit path from a large subarray to the DC-to-AC converter is commonly referred to as a home run. Conductors  27  and  28  are indicated in  FIG. 2  as resistors to represent the total resistance of the conductors for this home run. In a similar way, photovoltaic subarray  30  is connected to DC-to-AC converter inputs  7  and  8  via conductors  37  and  38  in conduit  39 .  FIG. 2  only shows two home runs, for clarity, but the number is variable depending on the system design and photovoltaic array layout. In large power plants, the distance traversed by a given home run can be substantial. To achieve efficient DC power collection in any power plant, it is desirable to make the operating voltage of subarrays  20  and  30  as high as practical. Higher voltage translates to lower current for a given power level and therefore smaller conductor cross sectional area resulting in lower conductor and conduit costs. Typically, the maximum voltage is limited by the photovoltaic module voltage rating from active elements to frame or external insulating surfaces. 
     In  FIG. 2 , DC-to-AC converter inputs  5  and  7  are connected to fuses  9  and  10  respectively then electrically paralleled to one side of capacitor  4 . DC-to-AC converter inputs  6  and  8  are electrically paralleled to the remaining side of capacitor  4 . The current and voltage characteristic of subarray  20  or  30  is that of an imperfect voltage source or an imperfect current source, depending on the operating point of the subarray. As such, the power source “seen” at the DC-to-AC converter inputs is “soft” with limited voltage and limited current. Capacitor  4  serves to convert this soft source to a low impedance voltage source capable of delivering high peak currents which are orders of magnitude greater than what either subarray could deliver. DC-to-AC converter output terminals  1 ,  2  and  3  are connected to a polyphase electric utility grid. The utility grid is modeled as AC voltage sources  11 ,  12  and  13 . For each phase, the DC-to-AC converter regulates sinusoidal current into the utility grid in phase with the utility voltage at each output terminal  1 ,  2  and  3  to source power into the grid at unity power factor. The sinusoidal current sources within the DC-to-AC converter  1 T/ 1 B,  2 T/ 2 B and  3 T/ 3 B are modeled as controllable current sources capable of sourcing regulated half-sinewaves of current into the positive half-sinewave of utility voltage and sinking regulated half-sinewaves of current out of the negative half-sinewave of utility voltage for each phase. 
     To summarize, the most salient points of this discussion and how they relate to the invention,  FIG. 2  illustrates DC sources  20  and  30  converted to a combined DC voltage source by capacitor  4  in turn converted to a polyphase AC current source to transfer power into a polyphase AC voltage source, the electric utility grid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified electrical schematic illustrating the basic form of the invention with a number of distributed DC-to-DC power converters each having current source outputs and each feeding a central DC-to-AC power converter. 
         FIG. 2  is a simplified electrical schematic illustrating a prior art photovoltaic power system without distributed DC-to-DC power converters. 
         FIG. 3  graphically illustrates the DC-to-DC power conversion approach per the invention with respect to how the DC-to-DC converters transform a typical, imperfect photovoltaic source into a classic current source. 
         FIG. 4  is a simplified electrical schematic illustrating the preferred embodiment of the DC-to-DC converter electrical topology and control method. 
         FIG. 5  is a simplified electrical schematic illustrating an alternate DC-to-DC converter embodiment wherein the DC-to-DC converter inputs and outputs are galvanically isolated. 
         FIG. 6  is a simplified electrical schematic illustrating the preferred embodiment of the DC-to-AC converter electrical topology and control method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates the basic form of the invention. Photovoltaic sub-array  20  is a collection of series and parallel connected photovoltaic modules connected at input terminals  21  and  22  of DC-to-DC converter # 1  and across capacitor  23 . The current and voltage characteristic of subarray  20  is that of an imperfect voltage source or an imperfect current source, depending on the operating point of subarray  20 . As such, the power source “seen” at the DC-to-AC converter inputs is “soft” with limited voltage and limited current. Capacitor  23  serves to convert this soft source to a low impedance voltage source capable of delivering high peak currents which are orders of magnitude greater than what either subarray could deliver. DC-to-DC converter # 1  converts this voltage source to a DC current source. Conductors  27  and  28  in conduit  29  carry the output current of DC-to-DC converter # 1 . Conductors  27  and  28  are indicated in  FIG. 1  as resistors, each representing the total resistance of each conductor from output terminals  25  and  26  of DC-to-DC converter # 1  to the input terminals  5  and  6  of the DC-to-AC converter, respectively. In a similar way, photovoltaic subarray  30  is connected to DC-to-DC converter #N with inputs  31  and  32  and with outputs  35  and  36  connected to DC-to AC converter inputs  7  and  8  respectively via conductors  37  and  38  in conduit  39 .  FIG. 1  only shows two DC-to-DC converters, for clarity, but the number is variable depending on the system design and photovoltaic array layout. In large power plants, the distance traversed between a DC-to-DC converter and the DC-to-AC converter can be substantial. To achieve efficient DC power collection in any power plant, it is desirable to make the transmission voltage between DC-to-DC converters and the DC-to-AC converter as high as practical. Higher voltage translates to lower current for a given power level and therefore smaller conductor cross sectional area resulting in lower conductor and conduit costs. 
     In  FIG. 1 , DC-to-AC converter inputs  5  and  7  are connected to fuses  9  and  10  respectively then electrically paralleled at one side of capacitor  4 . DC-to-AC converter inputs  6  and  8  are electrically paralleled to the remaining side of capacitor  4 . DC-to-AC converter output terminals  1 ,  2  and  3  are connected to a polyphase electric utility grid. The utility grid is modeled as AC voltage sources  11 ,  12  and  13 . For each phase, the DC-to-AC converter regulates sinusoidal current into the utility grid in phase with the utility voltage at each output terminal  1 ,  2  and  3  to source power into the grid at unity power factor. The sinusoidal current sources in the DC-to-AC converter  1 T/ 1 B,  2 T/ 2 B and  3 T/ 3 B are modeled as controllable current sources capable of sourcing regulated half-sinewaves of current into the positive half-sinewave of utility voltage and sinking regulated half-sinewaves of current out of the negative half-sinewave of utility voltage. 
     To summarize, the most salient points of this discussion and how they relate to the invention,  FIG. 1  illustrates photovoltaic sources  20  and  30  each converted to independent DC current sources, in turn paralleled and converted to a common voltage source by capacitor  4 , then in turn converted to a polyphase AC current source to transfer power into a polyphase AC voltage source, the electric utility grid. The invention is a method of using a plurality of distributed DC-to-DC converters, each having a current source output, connected in parallel at the input of a central DC-to-AC inverter. 
       FIG. 3  graphically illustrates the invention DC-to-DC conversion approach and value thereof. Photovoltaic subarray  40  is a collection of series and parallel connected photovoltaic modules illustrated by the standard schematic symbol. Current source  41 , shunt diode  42 , shunt resistor  43  and series resistor  44  comprise a simplified equivalent circuit of subarray  40 . Shunt diode  42  will have one junction drop per series connected photovoltaic cell. Curve  45  is the characteristic current/voltage operating point of subarray  40  for an arbitrary set of environmental conditions. Operating point  46  is the maximum power point of subarray  40 . The desired function of the DC-to-DC converter is to dynamically seek the maximum power point voltage  46  of subarray  40  and source a current into an output voltage across terminals  57  and  58  greater than the input voltage across terminals  51  and  52 . To accomplish this, capacitor  53  converts subarray  40  from a soft power source with limited voltage and limited current into a low impedance voltage source. Switch mode power conversion  54  is performed to regulate a DC output current  55  into whatever voltage exists across output terminals  57  and  58 . 
     In the example illustrated in  FIG. 3 , 100 kW is being converted and conversion losses are assumed negligible. The input voltage, V IN  of 500V, and input current, I IN  of 200 A, correspond to the maximum power point  46  on subarray  40  current/voltage curve  45 . The output voltage, V OUT  of 2000V, is specifically regulated as a core part of this invention by the external DC-to-AC converter. The output current, I OUT  of 50 A, is the 100 kW power level divided by the output voltage, V OUT . For the example shown, the relative cross-sectional area requirement for conductors from a subarray (prior art reference) or DC-to-DC converter to the DC-to-AC converter without and with the invention is 16 to 1.The magnitude of the output current was reduced by 4 to 1 so that the resistive losses, proportional to the square of the current (W Loss =I 2 R), are reduced 16 to 1. 
     Referring again to  FIG. 3 , it should be noted that the DC-to-DC conversion process also converts or transforms a “sloppy” DC source with voltages that can vary in a 2 to 1 ratio, from open circuit to maximum power point voltages over the range of typical environmental conditions, to an output with a fixed voltage and with current that varies with the level of power being converted. The output of the DC-to-DC converter now looks electrically like a perfect photovoltaic source. 
       FIG. 4  illustrates the preferred embodiment of the DC-to-DC converter electrical topology and control method. Photovoltaic source  60  is a collection of series and parallel connected photovoltaic modules connected to the input terminals  61  and  62  and across capacitor  64 . Inductor  69 , IGBT  70  and rectifier  72  are configured as a typical non-isolated boost circuit. In parallel, there is another, typical non-isolated boost circuit comprising inductor  65 , IGBT  66  and rectifier  68 . Both boost circuits supply current to inductors  74  and  75 . Rectifier  73  used to freewheel the current in inductors  74  and  75  when neither rectifier  68  or  72  is in conduction. Inductors  65  and  69  may also have taps where the IGBT collector connection splits the inductor windings into two sections and were the rectifier connection point remains unchanged. Current sensor  76  senses output current I OUT . Voltage sensor  63  senses photovoltaic source  60  voltage. In operation, a nominal reference voltage  82  (V REF ) is set by perturb-and-observe circuit  84  and is compared to feedback signal  81  (V IN ). Signal  81  (V IN ) is proportional to photovoltaic subarray  60  voltage. Both signals  81  and  82  are scaled volt per volt. The difference between signals  81  and  82  is amplified and processed to create signal  80  (V ERROR ). Signal  80  (V ERROR ) is then applied to PWM converter  79 . PWM converter  79  drives each IGBT gate  67  and  71  with a high frequency pulse train where the gate on-time to gate off-time ratio is a function of signal  80  (V ERROR ). When signal  81  (V IN ) is greater than signal  82  (V REF ), the gate on-time to gate off-time ratio is increased to draw more current from source  60  and capacitor  64 . When signal  81  (V IN ) is less than signal  82  (V REF ), the gate on-time to gate off-time ratio is decreased to draw less current from source  60  and capacitor  64 . This constitutes a servo loop which regulates the voltage of photovoltaic source  60  to the desired reference voltage as programmed by signal  82  (V REF ). PWM converter  79  drives each IGBT gate with a different high frequency pulse train where gate  67  and gate  71  are never driven on at the same time. This interleaved operation of two boost circuits reduces the input and output current switching frequency ripple. After perturb-and-observe circuit  84  has set an initial value of signal  82  (V REF ), the resultant amplitude of signal  83  (I OUT ) is logged, the value of signal  82  (V REF ) is incrementally stepped and the resultant amplitude of signal  83  (I OUT ) is logged and compared to the previous logged value. If signal  83  (I OUT ) was increased, signal  82  (V REF ) is incrementally stepped in the same direction. If decreased, signal  82  (V REF ) is incrementally stepped in the opposite direction. Since the output voltage across DC-to-DC converter terminals  77  and  78  is fixed by the external DC-to-AC converter, the maximum power point of photovoltaic subarray  60  is captured when the maximum amplitude of signal  83  (I OUT ) is determined by this iterative, perturb-and-observe algorithm. The perturb-and-observe algorithm is ongoing to dynamically track the maximum power point of subarray  60  as environmental conditions change. Communication port  85  is a bi-directional serial communications link with the DC-to-AC converter. Control power for the DC-to-DC converter could either be derived from subarray  60  or from an external source. 
       FIG. 5  illustrates an alternate DC-to-DC converter embodiment where the DC-to-DC converter inputs and outputs are electrically or galvanically isolated. The control methodology is essentially the same as in  FIG. 4 . Photovoltaic source  500  is a collection of series and parallel connected photovoltaic modules connected to the input terminals  501  and  502  and across capacitor  504 . Elements  505 ,  507 ,  509  and  511  are IGBT/diode combinations arranged in a typical, known full bridge topology driving high frequency transformer primary winding  513 . High frequency transformer secondary winding  514  is connected to a typical full bridge rectifier comprising diodes  515 ,  516 ,  517  and  518 . The full bridge rectifier supplies current to inductors  520  and  521 . Inductors  520  and  521  may or may not be coupled. Rectifier  519  is used to freewheel inductors  520  and  521  with a lower loss, single junction voltage drop. Current sensor  522  senses output current I OUT . Voltage sensor  503  senses photovoltaic source  500  voltage. In operation, a nominal reference voltage  526  (V REF ) is set by perturb-and-observe circuit  530  and is compared to signal  525  (V IN ). Signal  525  (V IN ) is proportional to photovoltaic subarray  500  voltage. Both signals  525  and  526  are scaled volt per volt. The difference between signals  525  and  526  is amplified and processed to create signal  527  (V ERROR ). Signal  527  (V ERROR ) is then applied to PWM converter  528 . PWM converter  528  drives IGBT gate pairs  506 / 512  and  508 / 510  with high frequency pulse trains where the gate on-time to gate off-time ratio is a function of signal  527  (V ERROR ); when signal  525  (V IN ) is greater than signal  526  (V REF ), the gate on-time to gate off-time ratio is increased to draw more current from source  500  and capacitor  504 , when signal  525  (V IN ) is less than signal  526  (V REF ), the gate on-time to gate off-time ratio is decreased to draw less current from source  500  and capacitor  504 . This constitutes a servo loop which regulates the voltage of photovoltaic source  500  to the desired reference voltage as programmed by signal  526  (V REF ). PWM converter  528  drives each IGBT gate pair with a different high frequency pulse train where gate pairs  506 / 512  and  508 / 510  are never driven on at the same time. After perturb-and-observe circuit  530  has set an initial value of signal  526  (V REF ), the resultant amplitude of  529  (I OUT ) is logged, the value of signal  526  (V REF ) is incrementally stepped and the resultant amplitude of  529  (I OUT ) is logged and compared to the previous logged value. If  529  (I OUT ) was increased, signal  526  (V REF ) is incrementally stepped in the same direction. If decreased, signal  526  (V REF ) is incrementally stepped in the opposite direction. Since the output voltage across DC-to-DC converter terminals  523  and  524  is fixed by the external DC-to-AC converter, the maximum power point of photovoltaic subarray  500  is captured when the maximum amplitude of I OUT is determined by this iterative, perturb-and-observe algorithm. The perturb-and-observe algorithm is ongoing to dynamically track the maximum power point of subarray  500  as environmental conditions change. Communication port  531  is a bi-directional serial communications link with the DC-to-AC converter. Control power for the DC-to-DC converter could either be derived from subarray  500  or from an external source. 
     Although not shown in  FIG. 4  or  5  for clarity, contactors could be connected to automatically break any combination of current carrying DC-to-DC converter input and output conductors to isolate a faulted photovoltaic source or the entire DC-to-DC converter. Photovoltaic source ground fault currents could be sensed by measuring the differential current in both input conductors or by other know means. Photovoltaic source insulation resistance or leakage could be sensed and determined by any known means. A faulted DC-to-DC converter could be determined by self-diagnostics. 
     In  FIG. 5 , photovoltaic source  500  may be grounded to earth at the positive pole, terminal  501 , the negative pole, terminal  502 . Photovoltaic source  500  may also be configured as a grounded bipolar source where the photovoltaic source is split into positive and negative monopoles with a common, center earth ground. 
       FIG. 6  illustrates the preferred embodiment of the DC-to-AC converter electrical topology and control method. The outputs of any number of DC-to-DC converters, as disclosed in  FIG. 4  or  5 , are connected in parallel at input terminals  401  and  400  and (electrically) across capacitor  412 . Elements  110 ,  112 ,  210 ,  212 ,  310  and  312  are IGBT/diode pairs arranged as a typical six-pole, three-phase bridge. Designators  109 ,  111 ,  209 ,  211 ,  309  and  311  refer to the associated IGBT gate drives, respectively. Each half bridge section drives one phase of the electric utility grid. For example, IGBT/diode  110  and  112  connect through inductor  113  to output terminal  101  and utility grid phase  100 . Current sensor  105  measures the current out of this half bridge section and into utility grid phase  100 . In  FIG. 6 , all reference designators starting with 1, 2 and 3 are associated with utility phases  100 ,  200  and  300  respectively. The remaining half-bridge sections associated with utility phases  200  and  300  are connected in a similar manner. The electric utility grid is modeled as three ideal AC voltage sources. In operation, voltage sensor  405  senses the voltage across input terminals  401  and  400  to produce signal  406  (V IN ). A reference voltage  402  (V DC REF ) is set to be slightly higher than the peak-to-peak voltage of the electric utility grid (utility voltage sensing is not shown for clarity). Reference voltage  402  (V DC REF ) will be automatically adjusted within the range of utility high-line and low-line voltages as the utility voltages changes. Reference voltage  402  (V DC REF ) is compared to signal  406  (V IN ). Both signals  402  and  406  are scaled volt per volt. The difference between signals  402  and  406  is amplified and processed to create signal  403  (V ERROR ). Signals  102  (V AC1 ),  202  (V AC2 ) and  302  (V AC3 ) are synthesized, low distortion, fixed amplitude sinewaves synchronized with utility grid voltages  100 ,  200  and  300  respectively. Point  430  is an arbitrary circuit ground reference for the control system. Signals  102  (V AC1 ),  202  (V AC2 ) and  302  (V AC3 ) are multiplied by signal  403  (V ERROR ) by multiplier circuits  103 ,  203  and  303  respectively. The signals at the multiplier circuit outputs, signals  104  (I REF1 ),  204  (I REF2 ) and  304  (I REF3 ) are identical to signals  102  (V AC1 ),  202  (V AC2 ) and  302  (V AC3 ) except changed in amplitude as a linear function of signal  403  (V ERROR ). Signals  104  (I REF1 ),  204  (I REF2 ) and  304  (I REF3 ) are the sinusoidal references or models for the desired current to be injected into phases  100 ,  200  and  300  of the utility grid. The current in each phase is independently regulated to its reference value by comparing the reference  104  (I REF1 ),  204  (I REF2 ) and  304  (I REF3 ) to the actual phase current value (feedback), signals  106  (I AC1 ),  206  (I AC2 ) and  306  (I AC3 ) respectively to generate error signals  107  (I ERROR1 ),  207  (I ERROR2 ) and  307  (I ERROR3 ) respectively. Error signals  107  (I ERROR1 ),  207  (I ERROR2 ) and  307  (I ERROR3 ) drive PWM circuits  108 ,  208  and  308 , respectively. 
     In  FIG. 6 , each phase is driven from its respective PWM circuit in a similar way. For example when zero current is being regulated into phase  100  of the utility grid, gate drives  111  and  109  are switched in complementary fashion each substantially 50% on-time and 50% off-time. Inductor  113  averages the resultant high frequency pulses for a net current of zero. When it is desired to source current into phase  100  when the voltage of phase  100  is positive, the on-time to off-time ratio is increased on gate  111  and decreased on gate  109 . To sink current from phase  100  when the voltage on phase  100  is negative, the on-time to off-time ratio is decreased on gate  111  and increased on gate  109 . As such, the current in each three phases is regulated in a servo loop to replicated the current references  104  (I REF1 ),  204  (I REF2 ) and  304  (I REF3 ). 
       FIG. 6  shows three AC current regulator servo loops, one for each phase and one DC voltage regulator servo loop that regulates the input voltage of the DC-to-AC converter across terminals  401  and  400 . As previously discussed in this description of  FIG. 6 , signal  403  (V ERROR ) is the difference between the desired, regulated voltage at the DC-to-AC converter input  402  (V DC REF ) and actual feedback voltage  406  (V IN ). When  406  (V IN ) is higher than  402  (V DC REF ),  403  (V ERROR ) is increased, therefore increasing the amplitude of all three AC phase currents and therefore increasing the power into the utility grid and therefore reducing the DC-to-AC input voltage and  406  (V IN ) to the desired reference value  402  (V DC REF ). This can be accomplished because the outputs of all DC-to-DC converters connected to the input of the DC-to-AC converter are seen as current sources by the DC-to-AC converter. As the collective power available from all connected DC-to-DC converters changes so will signal  403  (V ERROR ) to regulate the DC-to-AC converter input voltage and all DC-to-DC converter output voltages to a bounded value (V DC REF ). 
       FIG. 6  shows a typical six-pole, three-phase bridge operating as the DC-to-AC polyphase current source converter. Other topologies, such as multi-level neutral clamp topologies could be used as well. While the DC-to-AC polyphase current source converter power topology is not part of this invention, the control method is. 
       FIG. 6  also shows a bi-directional serial communication interface  420  for connection to all DC-to-DC converters in the field. This communication link will be used for operational commands, status/fault reporting, diagnostics, data acquisition and other communication and data sharing functions. 
     Both DC-to-DC converter configurations disclosed and the DC-to-AC converter disclosed have some kind of microcontroller, microprocessor, digital signal processor or discrete logic control platform. Other ancillary circuits, component parts and functions such as, but not limited to, power supplies, sensors, contactors and switches are not shown, for clarity, in the figures provided. 
     A narrative description of what the invention is follows. The invention is a novel design for a solar photovoltaic power plant comprising a photovoltaic array, a plurality of distributed DC-to-DC converters, one central DC-to-AC inverter and the novel way in which these components interact. Each DC-to-DC converter transforms a portion of the total photovoltaic array to a current source capable of delivering current to the DC-to-AC converter at a voltage higher than that of the photovoltaic source. This transmission voltage between the DC-to-DC converters and the DC-to-AC converter is set by the DC-to-AC converter. Specifically, two novel DC-to-DC (more exactly, photovoltaic-to-DC current) converter topologies are disclosed, both operated by the same novel control method. In addition, a novel DC-to-AC converter control method is disclosed which works in concert with the DC-to-DC converter topologies and control method. The DC-to-AC converter power topology is not novel.