Abstract:
A DC-to-AC power converter is disclosed which provides current regulated three-phase AC outputs and very high conversion efficiencies. The converter sinks power from an external DC current source and steers that current directly into two phases of a three-phase load by using complementary semiconductor switches in disparate half-bridges of a six-pole bridge. The steering switch selection rotates every 60° to direct current into the two phases with the largest voltage differential at any given time. The remaining half-bridge acts as a high-frequency, bi-directional current source to balance the three-phase load currents. This topology and control method significantly reduces power conversion losses. The converter may also include an additional power processing stage that is used to convert an external DC voltage source to a DC current source before the three-phase conversion takes place. Prior art converters first convert “soft” DC sources to voltage sources and then to AC current sources. The invention eliminates the need for large PWM filter inductors and DC bus capacitors used in prior art converters. In addition, 90% of the power being converted is directly processed by steering current into the AC three-phase load without high frequency chopping or switching. The invention is optimized for photovoltaic, utility-grid-interactive applications.

Description:
BRIEF SUMMARY OF THE INVENTION 
       [0001]    A DC-to-AC power converter is disclosed which provides current regulated three-phase AC outputs and very high conversion efficiencies. The converter sinks power from an external DC current source and steers that current directly into two phases of a three-phase load by using complementary semiconductor switches in disparate half-bridges of a six-pole bridge. The steering switch selection rotates every 60° to direct current into the two phases with the largest voltage differential at any given time. The remaining half-bridge acts as a high-frequency, bi-directional current source to balance the three-phase load currents. This topology and control method significantly reduces power conversion losses. The converter may also include an additional power processing stage that is used to convert an external DC voltage source to a DC current source before the three-phase conversion takes place. Prior art converters first convert “soft” DC sources to voltage sources and then to AC current sources. The invention eliminates the need for large PWM filter inductors and DC bus capacitors used in prior art converters. In addition, 90% of the power being converted is directly processed by steering current into the AC three-phase load without high frequency chopping or switching. The invention is optimized for photovoltaic, utility-grid-interactive applications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  illustrates the power topology and control methodology for a DC-to-AC power converter, based on the invention, which converts power from a photovoltaic generator to power which is sourced into a 3-phase electric utility grid. 
           [0003]      FIG. 2  illustrates the power topology and control methodology for a DC-to-AC power converter, based on the invention, which converts power from a DC current source to power which is sourced into a 3-phase electric utility grid. 
           [0004]      FIG. 3  illustrates the on/off timing of the semiconductor switches which make up the three phase bridge shown in both  FIG. 1  and  FIG. 2 . 
           [0005]      FIG. 4A  is for reference and illustrates how the maximum voltage between phases of a balanced 3-phase system oscillates between 75% and 86.6% of the peak-to-peak voltage. 
           [0006]      FIG. 4B  illustrates the portion of the DC current injected into the utility grid by selectively steering the DC current into the two utility phases with the highest instantaneous differential voltage. 
           [0007]      FIG. 4C  illustrates the portion of the DC current or “makeup” current injected into the utility grid to balance the three phase system where this waveform is shaped by pulse width modulation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]      FIG. 1  illustrates the preferred embodiment of the invention. The invention is a DC-to-AC polyphase power converter. Photovoltaic source  400  is connected at power converter input terminals  401  and  402 . Energy storage capacitor  416  converts “soft” photovoltaic source  400  into a “hard” voltage source with respect to the boost circuit PWM switching frequencies. Inductor  411 , IGBT  409  and rectifier  413  are described as a typical non-isolated boost circuit. An additional boost circuit comprising inductor  412 , IGBT  410  and rectifier  414  is connected in parallel. The conduction times of IGBT  409  and IGBT  410  are out of phase so that the operation of the composite boost circuit is interleaved to produce a more constant current at the composite boost circuit inputs and outputs. The conduction times or duty cycle of IGBT  409  and  410  are substantially equal, although phase-shifted at the PWM switching frequency, at any given regulation point. The method of interleaving two boost circuits is known. In operation, voltage sensor  405  senses the voltage across input terminals  401  and  402  to produce signal  406  (V IN ). Reference voltage  404  (V REF ) is compared to signal  406  (V IN ). Both signals  404  and  406  are scaled volt per volt. The difference between signals  404  and  406  is amplified and processed to create signal  407  (V ERROR ). PWM circuit  408  sets the duty cycle, or conduction time ratio, for IGBT switches  409  and  410  proportional to the magnitude of signal  407  (V ERROR ). As such, the voltage of photovoltaic source  400  is servo-regulated to the voltage commanded by reference voltage  404  (V REF ). 
         [0009]    In  FIG. 1 , perturb-and-observe circuit  422  sets an initial (nominal, expected) value of  404  (V REF ). For a given reference voltage  404  (V REF ) and set of environmental conditions for photovoltaic source  400 , a resultant value of signal  421  (I BOOST ), as provided by current sensor  420 , is had and the initial value is logged. Next, the value of signal  404  (V REF ) is incrementally stepped and the resultant amplitude of signal  421  (I BOOST ) is logged and compared to the previous logged value. If signal  421  (I BOOST ) was increased, signal  404  (V REF ) is incrementally stepped in the same direction. If decreased, signal  404  (V REF ) is incrementally stepped in the opposite direction. The maximum power point of photovoltaic source  400  is captured when the maximum amplitude of signal  421  (I BOOST ) is determined by this iterative, perturb-and-observe algorithm. The perturb-and-observe algorithm is ongoing to dynamically track the maximum power point of photovoltaic source  400  as environmental conditions change. 
         [0010]    In  FIG. 1 , output terminals  101 ,  201  and  301  of the power converter are connected to an electric utility grid via a dedicated distribution transformer represented by ideal voltage sources  100 ,  200  and  300 . Elements  1 T,  1 B,  2 T,  2 B,  3 T,  3 B are IGBT/anti-parallel diode pairs arranged as a typical six-pole, three-phase bridge. For brevity, each IGBT/anti-parallel diode pair will be simply referred to as an IGBT. Each half-bridge section drives one phase of the electric utility grid. For example, IGBT IT and  1 B connect 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 . The T or B portion the six IGBT designators refer to the placement of the IGBT switch in the six-pole bridge configuration as either a top (T) switch of bottom (B) switch respectively. In  FIG. 1 , 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. 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  424  using multiplier circuits  103 ,  203  and  303  respectively. The signals at the multiplier circuit outputs,  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  424  amplitude. 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 regulated, for a portion of each cycle, 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. 
         [0011]    In  FIG. 1 , 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 to IGBTs  1 T and  1 B are switched in complementary fashion, each substantially 50% conduction-time and 50% off-time. To source current into phase  100  when the voltage of phase  100  is positive, the conduction-time to off-time ratio is increased for gate IT and decreased for gate  1 B. To sink current from phase  100  when the voltage on phase  100  is negative, the conduction-time to off-time ratio is decreased for gate IT and increased for gate  1 B. As such, the current is regulated in a servo loop to replicate current reference  104  (I REF1 ) but only when utility voltage  100  is in a voltage window centered around zero from 50% of the negative peak voltage to 50% of the positive peak voltage. When utility voltage  100  is greater than 50% of the positive peak voltage, signal  1 T ON goes high, the output of logic gate  110  goes high, driving IGBT  1 T on, and logic gate  109  goes low, driving IGBT  1 B off. During this time, commands from PWM block  108  are overridden by logic gates  109  and  110 . In a similar way, when utility voltage  100  is more negative than 50% of the negative peak voltage, signal  1 B ON goes high, the output of logic gate  109  goes high driving IGBT  1 B on and logic gate  110  goes low driving IGBT  1 T off. When the PWM servo loop is overridden, one IGBT in each of 2-phases will be steering current into (positive) and out of (negative) the utility grid.  FIG. 3  illustrates the timing and essence of this approach. 
         [0012]      FIG. 1  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  402 . For the AC current loop implementation, signal  424  is the multiplicand, which programs the amplitude of current references  104  (I REF3 ),  204  (I REF2 ) and  304  (I REF3 ). Signal  424  is a low-pass-filtered analog of the composite boost circuit output current. At any given time, only one of the three AC current servo loop is active and function to make up the current necessary to balance all three phase currents. In operation, the current through current sensor  420  will have a large DC component, a small high frequency ripple component from the boost circuits and a small 360 Hz component. If the 3-phase current balance is perfect, the 360 Hz component will be zero. The average magnitude of signal  421  (I BOOST ) will be proportional to the RMS value of any AC phase current. Other methods may also be used to derive current references  104  (I REF1 ),  204  (I REF2 ) and  304  (I REF3 ). 
         [0013]      FIG. 1  shows a typical six-pole, three-phase bridge topology operating as the DC-to-AC converter polyphase AC current source. Other 3-phase topologies that perform the same function could be used as well. 
         [0014]    In  FIG. 1  the circuit is described as supplying power to the utility grid at unity power factor. If some mix of real and reactive power is desired, the current steering and AC current regulator PWM timing are still synchronized with the AC line voltages but are shifted out of phase by a number of degrees, plus or minus, to produce a reactive power component. 
         [0015]      FIG. 2  shows an alternate DC-to-AC power converter embodiment where the DC-to-AC inverter does not have a boost circuit as in  FIG. 1  and where the input is from DC current source  500  instead of photovoltaic source  400  shown in  FIG. 1 . In  FIG. 2 , DC current source  500  connects at terminals  501  and  502 . Inductor  504  is used to average the PWM current ripple. Diode  503  is used to freewheel the current through inductor  504 . The remainder of the circuit function remains the same as that of the circuit disclosed in  FIG. 1 . 
         [0016]      FIG. 3  illustrates the timing of semiconductor switches  1 T,  1 B,  2 T,  2 B,  3 T and  3 B from  FIG. 1 . These designators reference a given phase and whether the switch is a top switch (T) or bottom switch (B). Complementary switches are not commanded on concurrently but may overlap when changing states without damage since the supply to these switches is a current source. V 100 , V 200  and V 300  correspond to the phase voltages  100 ,  200  and  300  on  FIG. 1 . When the amplitude of any phase voltage exceeds 50% of its peak value, plus or minus, the top or bottom switch, respectively, for that phase is gated on. When the amplitude of any phase voltage is less than 50% of its peak value, plus or minus, the complementary top and bottom switches for that phase are alternately gated on and off so that the ratio of top switch conduction-time to bottom switch conduction-time tracks the phase voltage sign for that 60° portion of the waveform. At voltage zero cross, this duty cycle ratio is unity to produce a net average current. If the phase voltage is positive, the net conduction-time for the top switch is greater. If the phase voltage is negative, the net conduction-time for the bottom switch is greater. Therefore, at any given time, two of the three half-bridges are controlled to directly steer current into the AC load and the remaining half-bridge functions as a high frequency, bi-directional, switched current source. The current steering is controlled to close each top switch in each half-bridge in rotation with a conduction-time of 120° per switch and controlled to close each bottom switch in each half-bridge in rotation with a conduction-time of 120° per switch. The top switch and bottom switch turn-on times for a given half-bridge are out of phase by 180° so that the three-phase bridge always has a combination of one top switch and one bottom switch in full conduction on disparate half-bridges for 60°. The remaining half-bridge with no switches in continuous conduction during this 60° conduction time of the other two half-bridges is controlled to operate as a high frequency bi-directional current source where the direction and magnitude of the current into the AC load connected to this phase can be controlled by the conduction-time ratio of the top and bottom switches and where the half-bridge section dedicated to function as the high frequency bi-directional current source changes in rotation every 60°. 
         [0017]    The invention leverages the characteristic of three-phase systems wherein there is always a phase-to-phase voltage difference between two of three phases between 0.75 and 0.866 of the peak-to-peak voltage. When the available boost current is steered in phase with this voltage, power transfer into the utility grid is had. The two phases that share this relationship change every 60°. The phase that is not in conduction for a given 60° period is used to “make up” the required current needed to balance the three phase system. The net make-up power is approximately 10% of the power being converted. In  FIG. 4A  the maximum phases-to-phase voltage differences are shown by the dotted lines at 30° increments. If the DC source ( 400  in  FIG. 1 ) is floating with respect to the AC utility load (voltage sources  100 ,  200  and  300  in  FIG. 1 ) or if the electric utility connection is via delta or ungrounded wye configured transformer windings, then these peak-to-peak voltages can be transposed as shown in  FIG. 4B . Waveform  601  is the portion of boost circuit current feeding the rotating 3-phase steering function. In  FIG. 4C , waveform  602  is the portion of boost circuit current feeding the rotating 3-phase PWM function. The total current out of the boost circuit in  FIG. 1  and into the 6-pole bridge is the sum of waveforms  601  and  602  and is substantially a DC level that varies based on the energy available from the photovoltaic source ( 400  in  FIG. 1 ). 
         [0018]    In another DC-to-AC power converter embodiment, the rotating current steering function is used without the rotating PWM function. At the utility point of connection, a typical prior-art converter is connected in parallel to supply the small amount of power needed to balance the net three-phase current into the utility grid. 
         [0019]    Some possible applications for the invention are renewable energy converters, motor drives, uninterruptible power supplies. 
         [0020]    In  FIG. 2 , the DC-to-AC converter disclosed may also be used as an AC-to-DC converter by connecting a DC load in place of current source  500 , inverting AC voltage references  102  (V AC1 ),  202  (V AC2 ) and  302  (V AC3 ), adding diodes in series with all IGBTs (opposing the anti-parallel diodes) and removing freewheeling diode  503 . Instead of sourcing current into voltage sources  100 ,  200  and  300 , power will be sourced from the utility grid and supplied to the DC load. 
         [0021]    This invention is a novel power converter topology and associated regulation method where a DC current source is connected directly to the input of a six-pole bridge as opposed to the prior art where a soft DC source would supply DC bus energy storage capacitors at the input of the six-pole bridge. With prior-art converters, each half-bridge section of the six-pole bridge is pulse-width-modulated at high frequencies and then filtered with three large line filter inductors to integrate the pulse modulation and enable current regulation into an AC load. With the invention, the bulk of the power converted by the six-pole bridge is done at low frequency for a substantial reduction in power conversion losses. In addition, two groups of major power components are eliminated, the line filter inductors and the DC bus capacitors, thereby reducing the cost of the power converter. Also, the invention provides a higher degree of fault tolerance for the six-pole bridge where complementary switches in the same half-bridge can be allowed to cross conduct.