(12) United States Patent
`Dent
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,937,822 B2
`Jan. 20, 2015
`
`US00893 7822B2
`
`(54) SOLAR ENERGY CONVERSION AND
`UTILIZATION SYSTEM
`
`(*) Notice:
`
`(76) Inventor: Paul Wilkinson Dent, Pittsboro, NC
`US)
`(
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 854 days.
`(21) Appl. No.: 13/103,070
`(22) Filed:
`May 8, 2011
`
`(65)
`
`(2006.01)
`(2007.01)
`(2007.01)
`28:8
`(
`.01)
`(2006.01)
`
`Prior Publication Data
`US 2012/0281444 A1
`Nov. 8, 2012
`(51) Int. Cl
`H02M 7/537
`H02M 7/5387
`H02M I/32
`He's
`/
`HO2H 3/16
`/
`(52) U.S. Cl.
`CPC. H02.J.3/005 (2013.01); H02H 3/16 (2013.01);
`H02M 7/53871 (2013.01); H02M 1/32
`(2013.01)
`USPC - - - - - - - - - - - grrr. 363/55:363/40: 363/131
`(58) Fist of Classists starsh 37, 39 4155 56.05
`363/97, 98, 131-134, 165
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`ck
`4, 1978 Akamatsu ....................... 363/37
`4,084.220 A
`4,320,449 A * 3/1982 Carroll .......
`... 363 135
`7,082,040 B2 * 7/2006 Raddi et al. ..................... 363, 17
`7,474,016 B2 *
`1/2009 Wang
`307/45
`2013/0070494 A1
`3/2013 Rotzoll ........................... 363.80
`* cited by examiner
`
`Primary Examiner — Adolf Berhane
`Assistant Examiner — Nusrat Quddus
`(74) Attorney, Agent, or Firm — Coats & Bennett, PLLC
`
`ABSTRACT
`(57)
`A 1, 2 or 3-phase DC to AC converter system for reducing the
`cost of Solar energy installations achieves cost reduction by
`eliminating low-frequency power transformers. One DC
`input polarity is selectively Switched to an output terminal
`1nout Olarity 1S Select1VeV SW1tched to an Outout termina
`when the instantaneous AC output from a second outputter
`minal is desired to be of the opposite polarity, while the other
`DC input polarity is used to form an approximation to a
`segment of a sine wave of the desired polarity at the second
`output terminal. A common-mode AC signal is thereby cre
`ated on the balanced DC input lines at a frequency which is a
`multiple of 1, 2 or 3 times the AC output frequency and which
`is useful for detecting ground faults in the DC circuit.
`
`20 Claims, 30 Drawing Sheets
`
`O yelts RC
`in it
`is
`in
`(100) W
`
`Top level block diagram of the inventive lead converter
`XXXX3XXXXXYYXXX
`
`
`
`
`
`FLOATING DG-DC CONVERTER (110)
`
`1.20a
`
`Izod
`i
`Polarity reversing Switches e.g. full H-bridges
`
`
`
`2 volts a - olitput
`3rolinded conductor (i.e. Neutral
`(i.50)
`iot Leg
`
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`1.
`SOLAR ENERGY CONVERSION AND
`UTILIZATION SYSTEM
`
`BACKGROUND
`
`10
`
`15
`
`25
`
`30
`
`The present invention relates to DC-to-AC converters for
`electric power systems. Lower cost, high-power, efficient,
`DC-to-AC converters are of interest for solar energy econom
`ics. In the prior art, DC-AC inverters are the second highest
`cost item next to the photovoltaic panels. For high efficiency
`and low heat dissipation, commutation of DC to produce AC
`preferably uses solid state switches that are either fully on or
`fully off, and do not dwell more than a microsecond or so in
`an intermediate state. Therefore it is more complicated to
`produce a sine wave that takes on all values between the
`negative peak and the positive peak. On the other hand, pro
`ducing a square wave which Switches between the positive
`peak and the negative peak produces a form of AC that is not
`suitable for all loads.
`Various manufacturers provide prior art DC-AC convert
`ers, that fall into one of a few broad classes and operating
`modes. The class of “modified sine wave' converters main
`tains both the same rms and the same peak Voltage as a sine
`wave, while still employing only on-off commutation. This is
`done by switching the voltage between the desired positive
`peak, Zero and the negative peak, spending 50% of the rep
`etition period at Zero, therefore achieving both the same peak
`and the same rms values as a true sine wave, and being
`compatible with a greater variety of loads.
`Still, there are loads that do not tolerate the modified sine
`wave; for example appliances that present inductive loads,
`such as induction motors, some cellphone and laptop battery
`chargers, fluorescent lamps and tumble dryers, and any
`device with an internal power Supply that uses capacitive
`reactance as a lossless Voltage-dropping means, can malfunc
`tion on modified sine waveforms. Moreover, there is a poten
`tial problem with radio and TV interference due to the high
`level of harmonics of the modified square wave converter.
`Such a waveform is therefore not a candidate for coupling
`Solar-generated power into the utility network or into house
`wiring.
`“True sine wave' is another class of prior art DC-AC con
`verter. Linear amplifiers provide the absolute cleanest AC
`power waveforms, but their inefficiencies cause high heat
`dissipation in converters of several kilowatts capacity. More
`45
`over, linear amplifiers lose efficiency rapidly when operating
`into non-unity power factor loads. Some sine wave inverters
`overcome the problems with linear amplifiers by using digi
`tally-synthesized waveforms, which are multi-step approxi
`mations to a smooth sine wave. One example of a step
`50
`approximation sine wave inverter is the XANTREX
`(formerly Trace) SW4048.
`In U.S. Pat. No. 5,930,128 by current Inventor, a power
`waveform generator was disclosed that involved expressing
`the sinusoidal waveform as a series of numerical samples in a
`number base comprising a plurality of digits; selecting cor
`responding digits from each numerical sample and generating
`therefrom a waveform corresponding to the sequence of each
`digit, then using combining means to form a weighted com
`bination of the digit-corresponding waveforms, the weights
`60
`being chosen in relation to the numerical significance of each
`digit. For example, using a ternary number base, the weight
`ing means would add the digit waveforms in the ratios 1:1/3:
`1/9 and could for example be a transformer with these turns
`ratios.
`U.S. Pat. No. 5,373,433 also describes using series con
`nected, turns-ratio weighted transformer coupling of 3-level
`
`35
`
`40
`
`55
`
`65
`
`2
`waveforms to produce a 27-level step approximation to a sine
`wave. The principle described therein is similar to that used in
`the aforementioned XANTREX SW4048 inverter. The com
`bining means disclosed in the 128 patent for combining
`digit-corresponding waveforms was, in a low-frequency case,
`a series connection of transformers having turns ratios in the
`ratios of corresponding numerical digits, and in a high-fre
`quency case, comprised a set of quarter wave lines having
`characteristic impedances in the ratios of corresponding dig
`its.
`In a device built in accordance with the 128 patent, the
`series-connected transformer is the appropriate version for 60
`HZ, as 4 wavelength lines are impractical at 60 Hz; however,
`the transformers needed for the inventions of the 128 and
`533 patents represent a significant fraction of the total cost
`and weight of medium-power converters, and also account for
`a few percent loss in total efficiency. Therefore, other solu
`tions that avoid the disadvantages and pitfalls of the above
`prior art would be useful, and in particular, a solution avoid
`ing these low-frequency transformers would be a benefit.
`Transformerless inverters are known in the prior art, par
`ticularly for utility-interactive inverters, which use high-fre
`quency switching or pulse width modulation to approximate
`a sinewave. However, a disadvantage that arises in these prior
`art converters is the imposition of the high-frequency switch
`ing waveform on the Solar array, which can capacitively
`couple through the glass cover upon touching it, potentially
`causing RF burn to personnel or damage to the Solar panel, as
`well as causing the Solar array to radiate Substantial radio
`interference. Thus a design is required that can create a more
`benign Voltage fluctuation on the Solar array DC conductors.
`Another categorization of convertorrelates to whether they
`are designed to power loads directly, or whether they are
`designed to feed and sell power back into the electricity grid.
`A load inverter that can power loads directly is said to operate
`in standalone mode, and is also called a “standalone inverter.
`while a grid-tie inverter is said to operate in grid-interactive
`mode and is also called a “grid-interactive inverter'.
`For safety and other reasons, the latter have to meet differ
`ent specifications than the former, especially under fault con
`ditions. In particular, a load inverter should be a constant
`Voltage source, while a grid-tie inverter does not have a con
`stant Voltage output but must adapt to the Voltage of the grid,
`and is a current source. Moreover, a load inverter is always
`used with battery storage, and should maintain efficiency at
`both light and heavy loads and have low no-load power con
`Sumption, so that the battery is not discharged while the
`inverter is idling at night. Grid-tie inverters however do not
`have the same a requirement for no-load power consumption,
`as they do not operate at night.
`A complete alternative energy installation may thus com
`prise a number of functions, including load inverters, grid-tie
`inverters, load management for manually or automatically
`transferring load between the utility and alternative energy
`Supplies, storage batteries, battery chargers, circuit breakers,
`Surge protectors and other safety devices to protect equipment
`and wiring and eliminate the risk of electrical mishaps under
`conceivable fault conditions. Other than the inverters and the
`array these additional components are known as “balance-of
`system’ components.
`For high power grid-tie installations, typically 20 kW and
`above, 3-phase inverters are preferable in order to keep the
`gauge and cost of wiring down and to assist in maintaining
`balance between the three phases of the electricity grid. For
`converters over 100 Kw, 3-phase is often mandated by the
`utility company. Three phase inverters using pulse width
`modulation are known from the art of solid state Motor
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`Drives, but they are not suitable for grid-interactive use for
`many reasons, and Motor Drives do not need or have ground
`leak detection on the DC bus, which is internal.
`The total cost of balance-of-system components required
`in an installation can be significant; therefore it is an objective
`of this disclosure to describe novel designs of inverters, safety
`devices and automatic load management devices that provide
`a more efficient and cost effective complete installation, and
`which achieve cost reductions in the electronics to comple
`ment the currently falling cost of photovoltaic panels.
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`Such as a delta-Sigma modulator, pulse-width modulator or
`similar, such that the current delivered to the utility approxi
`mates a sinusoidal current with low harmonic content. The
`waveform generator timing is controlled Such that the power
`delivered to the utility substantially equals the solar power
`available from the array, and such that the delivered current is
`substantially in-phase with the utility voltage. Output relays
`connects the inverter to the utility only when certain condi
`tions such as utility Voltage limits, array Voltage limits and
`utility frequency limits are satisfied. When the relays are not
`connecting the inverter to the electric utility, energy originat
`ing from the photovoltaic array may be used for other pur
`poses, such as charging a battery for operating a load inverter.
`The utility connection may be made via a two-pole ACGFCI
`breaker Such that any ground fault on either Solar array ter
`minal throws the breaker and isolates the array, preventing
`electrical mishap. Thus the second inverter configuration may
`be used alone in grid-interactive mode, or may be used
`together with the first inverter implementation and a
`rechargeable battery in order also to provide utility backup in
`the event of a power outage. Such a bimodal installation may
`be programmed to prioritize battery charging, and when fully
`charged, excess Solar array power is then fed to the electric
`utility via the grid-tie inverter.
`A three-phase grid-tie inverter according to the second
`implementation is also described. The three phase inverter
`comprises Switches to connect the negative of the DC source,
`to one of the set of three phase output terminals for the whole
`of the period during which it is instantaneously negative
`relative to a mean Voltage or to a ground, neutral or reference
`potential, while the other terminals of the set of AC output
`terminals are zero or relatively positive compared to the
`mean, ground, neutral or reference potential terminal, alter
`nating in a rotating sequence with connecting the positive of
`the DC source to one the set of three phase AC output termi
`nals for the whole of the period during which it is instanta
`neously positive relative to the mean, ground, neutral or ref
`erence potential and the other terminals of the set of AC
`output terminals are Zero or relatively negative to the mean,
`ground, neutral or reference potential. The AC output volt
`ages from each of the set of AC output terminals has a unique
`phase, which, In the case of two AC output terminals may be
`Zero and 90 degrees, or, for three AC output terminals, may be
`0, 120 and 240 degrees, while in a single phase case the
`unique phase is simply 0 degrees.
`A common characteristic of the inventive converters is that
`a common-mode AC signal is created in-phase on both the
`positive and negative DC input terminals and on all DC con
`ductors. The AC signal is of the same frequency as the AC
`output in the single phase case, three times the AC output
`frequency in the three-phase case, and two times the AC
`output frequency in a quadriphase (0 and 90) case. A ground
`leakage fault in the DC circuit is thus detectable by detecting
`an AC leakage current at this characteristic frequency, which
`is technically much easier than detecting a DC leakage cur
`rent.
`A Smart load management center is also described which
`can selectively power each load or branch circuit either from
`the load converter, when solar or battery power or other alter
`native energy source is sufficient, or from the utility Supply.
`The Smart load center may be end-user configured to priori
`tize which loads are preferentially powered from the alterna
`tive energy source, and may be configured to permit deeper
`battery charge-discharge cycles in utility back-up mode only
`during the hopefully very infrequent utility outages, than
`when utility power is available, thus maximizing the life of
`the battery before replacement is needed. The smart load
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`SUMMARY
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`New methods and apparatus for the conversion of DC
`power to AC power are disclosed herein, together with smart
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`energy management technology to provide an efficient
`green energy installation. In a first implementation, a novel
`DC to AC convertor comprises a waveform generator based
`on expressing the desired output Voltage waveform as a series
`of numerical samples in a number base, each sample being
`expressed as a plurality of digits in the number base; one or
`more high-frequency, bidirectional, isolating DC to DC con
`verters to convert input DC power at a first voltage to a series
`of relatively floating DC output Voltages, the floating Voltages
`being in the same ratio to one another as powers of the number
`base; switches connected to each of the floating DC supplies
`and controlled according to a corresponding digit of a
`numerical sample to generate a corresponding floating output
`Voltage waveform, the floating output waveforms then being
`directly connected in series to form the desired power output
`waveform having a desired repetition frequency thereby
`eliminating the need for further low frequency transformers
`or other weighting means. In a preferred embodiment, one of
`the DC voltages may be chosen to equal the input source
`voltage so that no isolating DC-DC convertor is required for
`that supply. Preferably it is the supply from which the greatest
`average power is drawn, which is typically the highest Supply
`Voltage corresponding to the most significant digit of the
`given number base. The DC source, which may be a solar
`array-charged battery, is configured to allow its positive and
`negative terminals to be alternately interchanged so that the
`positive and negative terminals are alternately connected to
`the grounded neutral conductor, or reference potential termi
`nal, which facilitates detection of ground leakage faults in the
`DC circuit using an AC ground leak detector. While one of the
`DC input conductors is connected to the ground, neutral or
`reference potential terminal, the other DC input is routed to
`and processed by circuits which produce the desired output
`waveform, Such as a sine wave, that is output from at least one
`live or “hot” AC output terminal.
`A second implementation couples power produced by a
`Solar array into the electricity grid. In a single-phase, grid
`interactive inverter according to the second implementation, a
`Switch connects the negative terminal of the Solar array to line
`neutral and thus to ground, or else to a reference potential
`terminal, when the required output to the utility grid hotleg is
`instantaneously positive, and a second Switch, which is inhib
`ited from operating at the same time as the first Switch, con
`nects the Solar array positive terminal to line neutral and thus
`to the ground or reference potential terminal when the
`required output to the utility grid hot leg is instantaneously
`negative. Third and fourth switches alternately connect or
`disconnect the Solar array terminal not connected to line
`neutral to or from Smoothing and inverter hash filters Supply
`ing at least one AC output terminal that is connected to the
`utility hot leg, the pattern of connects and disconnects being
`determined by a high frequency digital waveform generator
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`center also has provision to shed load in a prioritized order
`during a prolonged outage when limited Solar energy is being
`received.
`A Solar combiner is normally a simple junction box for
`connecting the outputs of several Solar panels or strings of
`panels in parallel. When strings are connected in parallel, it is
`desirable to use blocking diodes such that a shaded panel does
`not rob current from the total. Many combiners ignore the
`potential for differential shading and omit these diodes. An
`advantageous Solar combiner is described that selectively
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`transfers the output current of each string to a first priority
`load, such as battery charging, a second priority load, such as
`a grid-tie inverter, or disconnects both loads. The Solar com
`biner described may thus be controlled to progressively
`increase or decrease a battery charging current while divert
`ing current not used for battery charging to a grid tie inverter
`or other diversion load. By disconnecting all strings from both
`battery charging and the diversion load, the Solar combiner
`also provides a local and/or remote-controlled DC disconnect
`function. In residential installations, DC wiring from a Solar
`array is required by the NEC to be enclosed in metallic con
`duit up to the first DC disconnect. By providing a remote DC
`disconnect function, the combiner described can be located
`right at the Solar array, for example attached to the inside of
`the attic roof behind a roof-mounted array. By this means, the
`internal wiring need not be in metallic conduit. The solar
`combiner communicates with the inventive load convertor,
`which monitors battery state, in order to provide a battery
`charge controller function.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 shows a top-level block diagram of a DC-AC load
`converter
`FIG. 2 shows abidirectional DC-DC converter
`FIG.3 shows a floating H-bridge for commutating a float
`ing DC Supply
`FIG. 4 shows the waveforms of four ternary digits over a
`repetition cycle
`FIG. 5 shows waveforms of the 120 volt H-bridge and on
`the DC supply
`FIG. 6 shows a common mode hash filter
`FIG. 7 shows details of the transient response of the com
`mon mode hash filter
`FIG. 8 shows an RFI filter used to suppress high frequen
`cies on the output
`FIG.9A-9B shows the difference between a correct and an
`erroneous RFI filter
`FIG. 10 shows more detail of a load inverter design accord
`ing to the invention
`FIG.11 shows the startup circuit for limiting inrush current
`FIG. 12 shows the output sinusoidal waveform from an
`inventive converter.
`FIG. 13 shows the output spectrum from the converter
`before RFI filtering
`FIG. 14 shows the converter output noise spectrum after
`RFI filtering
`FIG. 15 shows the principle of a single-phase grid intertie
`inverter.
`FIG. 16 shows the principle of a 3-phase grid intertie
`inverter
`FIG. 17 shows a complete solar installation for grid-inter
`active use only.
`FIG. 18 shows a complete Solar installation using a stan
`dalone inverter.
`FIG. 19 shows a bimodal solar electric installation
`FIG. 20 Equivalent circuit of a photovoltaic (solar) cell
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`FIG.21 shows the DC-DC converter waveforms for reduc
`ing standby current
`FIG.22 shows the outline schematic of a smart load center
`FIG. 23 Shows three phase waveforms
`FIG.24 shows another arrangement of a 3-phase grid inter
`tie inverter
`FIG. 25 shows a modified doublet and core flux wave
`forms.
`FIG. 26 shows a vector diagram of a generator back-feed
`ing the grid
`FIG. 26a shows the vector diagram for rectifier mode
`FIG.27 shows a remote-controlled solar combiner and DC
`disconnect
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`DETAILED DESCRIPTION
`
`Referring to FIG. 1, a DC-to-AC conversion apparatus
`according to a first implementation comprises an input (100)
`for a floating DC power source, for example 120 volts DC
`from ten 12-volt rechargeable batteries connected in series; a
`pair of output terminals (150) for the AC output, one of which
`may be connected to the grounded conductor or neutral of the
`AC load; a bidirectional DC-DC converter (110) for convert
`ing the DC input to a number offloating DC output Voltages,
`and a set of reversing switches 120a-120d controlled by opto
`isolated driver circuit 200. While one of a pair of single phase
`power conductors is normally a grounded neutral conductor,
`the inventive converter may alternatively provide a floating
`output relative to an arbitrary reference potential terminal.
`The bidirectional property of the DC-DC convertor 110
`implies that power may instantaneously flow in either direc
`tion at any pair of input or output terminals. If the current
`flows out of a positive terminal, the direction of power flow is
`“out', while if current flows into a positive terminal, the
`direction of power flow is “in”. The DC-DC convertor is of a
`Substantially lossless, Switching type, implying that all power
`that flows in must come out, although the convertor may
`optionally contain energy storage capacitors such there may
`be an instantaneous imbalance between input and output
`power according as the capacitors are accumulating or releas
`ing energy. In this example, it is assumed that the inverter
`output waveform to be generated is represented as a sequence
`of numerical samples, each numerical sample value being
`expressed with four digits in the ternary number system, e.g.
`(T4, T3, T2, T1), wherein each digit Ti (i=1 . . . 4) can only
`take on one of the three values-1, 0 or +1. In correspondence
`with the place significance of the different ternary digits, a
`number of floating DC Supplies are generated with ratios
`1:1/3:1/9:1/27. Assuming a 120-volt DC input source. The
`floating DC supplies generated by converter 100 are therefore
`40v, 13.33v and 4.44 volts which are respectively /3rd, /6th
`and /27th of the nominally 120 volt floating DC power source.
`The sum of the DC input and all outputs of the DC-DC
`converter is 120+40+13.33+4.44=177.77 volts. This is the
`peak voltage that could be generated at the AC output (150),
`and corresponds to a useful sine wave output voltage of 125.7
`Volts rms. If necessary, all the Voltages can be scaled to
`produce other output voltages, for example 100v, 115, 120V,
`125 V, 220V etc., while still maintaining the power-of-3 ratios
`between the floating Supply Voltages. Other Voltage outputs
`or waveforms (within the maximum available peak Voltage of
`all DC Supplies added together) may alternatively be gener
`ated by choosing the appropriate sequence of ternary digits.
`For example, the invention could be used to produce an output
`waveform for driving a vibration table for mechanical testing
`purposes, the waveform being either non-repetitive, or having
`a desired repetition frequency.
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`Other number systems than ternary could be used; for
`example, the binary number system could be use