Development of Power Conditioner Using Digital Controls for Generating Solar

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Development of Power Conditioner Using Digital Controls for Generating Solar Power Tadashi Ryu Power electronics has been in the limelight in recent years in conjunction with the promotion of energy conservation. Power electronics is a technology for controlling electric power consumption using semiconductors, such as field effect transistors (FETs). A lot of progress has been made particularly to increase efficiency and extend the capacity of inverters to output alternating current. Such effor
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  40  Oki Technical ReviewApri 2009/Issue 214 Vol.76 No.1 Development of Power ConditionerUsing Digital Controls forGenerating Solar Power Tadashi Ryu Power electronics has been in the limelight in recentyears in conjunction with the promotion of energyconservation. Power electronics is a technology forcontrolling electric power consumption usingsemiconductors, such as field effect transistors (FETs). Alot of progress has been made particularly to increaseefficiency and extend the capacity of inverters to outputalternating current. Such efforts have been contributingto energy conservation in the form of various products,ranging from household appliances, such as refrigeratorsand air conditioners, to hybrid cars.OKI Information Systems (hereinafter referred to as“OIS”) has been involved with the development of powerelectronics and power conditioners for generating solarpower through the application of digital controls, inpartnership with OKI Power Tech (hereinafter referred toas “OPT”). Power conditioners are equipments thatgenerate solar power by converting direct current,supplied by solar cell modules, into alternating current.These devices play a very important role in the promotionof higher efficiency (energy conservation) solar powergenerating systems.This paper describes the current status of digitallycontrolled power supplies, the functions of powerconditioners for generating solar power and elementaltechnologies, as well as the status of the developmentsand future activities. Current status of digitally controlled powersupplies Power supplies in use at the present time primarilyuse analog controllers. In recent years, however,research and development has concentrated on digitallycontrolled power supplies that use microcomputers anddigital signal processors (DSPs) *1) . Digitally controlledpower supplies control the overall system that involvesvoltage control, current control and remotecommunication controls using microcomputers andDSPs. The efficiency of the whole system can beincreased utilizing controls that respond to the currentconditions, using information derived from such controls.OIS, in partnership with OPT, started thecommercialization of a DC-DC converter power supply in2004, as an evaluation board for digitally controlledpower supplies that use DSPs manufactured by TexasInstruments. The external appearance of a DC-DCconverter power supply is shown in Photo 1 . Such unitswere supplied to customers who expressed a desire toconduct trials on digitally controlled power supplies and afavorable outcome resulted.Commercialization of a DC-AC inverter power supplyfor communication base stations was completed in 2007.The external appearance of a DC-AC inverter powersupply is shown in Photo 2 . This product is capable ofsustaining high conversion efficiency from light to heavyload conditions, with the use of load-adaptive control.Details pertaining to the load-adaptive control areprovided in a later section.Since 2008 OIS has been developing powerconditioners to generate solar power, as products intendedfor green energy initiatives, in partnership with OPT.The hardware for these products has been developedby OPT, since OPT possesses a wealth of experience inthe development of power supplies and the software hasbeen developed by OIS, since OIS has an abundantexperience with digital controls. *1)DSP is an abbreviation for “digital signal processor”. Photo 1 DC-DC converter power supplyPhoto 2 DC-AC inverter power supply  Special Edition on the Environment: Aiming for Low Carbon Society Oki Technical Review 41 Apri 2009/Issue 214 Vol.76 No.1 Power conditioners for generating solarpower Power conditioners for generating solar power(hereinafter referred to as “PV *2) power conditioners”)feature functions for converting direct current electricpower, supplied by solar cell modules (hereinafterreferred to as “PV modules”) that output electric powerdependent on irradiance and temperature, into alternatingcurrent electric power. Ordinary DC-AC inverter powersupplies are controlled to output the amount of electricpower required by the load, whereas PV powerconditioners require controls to ensure efficient output ofelectric power supplied by PV modules.PV power conditioners are configured either as apower grid interconnecting system or a self sustainedoperation system. A power grid interconnecting system isconfigured to connect the output of a PV powerconditioner to commercial AC mains, which allows for thesale of unconsumed excessive amounts of electric powerto the electric power company by supplying it to thepower grid. A configuration that does not offer anyconnection to the power grid is called a self sustainedoperation system, which features the ability to operateeven during a power outage. We are developing a hybridsystem that combines the power grid interconnectingsystem and the self sustained operation system. Theconfiguration of the hybrid system is shown in Fig. 1 . Fig. 1 Configuration of hybrid system Elemental technologies for PV power condi-tioner controls The “Maximum Power Point Tracking”, “Power GridInterconnecting Control” and “Power Grid InterconnectingProtective Function”, are the principal technologiesrequired to use the controls of PV power conditioners.Each elemental technology is explained below. (1)Maximum power point tracking An electric power output/voltage characteristic for PVmodules is shown in Fig. 2 . PV modules have anoptimum operating point at which the maximum amountof electric power can be output under certainenvironmental conditions. This optimum operating pointvaries, depending on irradiance and temperature. Theconstant control to sustain electric power supplied by PVmodules to the PV power conditioner at an optimumoperating point is called maximum power point tracking(MPPT *3) ). Fig. 2 Electric power output/voltage characteristic for PVmodules Maximum power point tracking plays an extremelyimportant role in a PV system, since the powergenerating efficiency for the entire system is important,rather than simply the electric power conversionefficiency at the DC-DC converter section or the DC-ACinverter section. (2)Power grid interconnecting control Even though the controls of ordinary DC-AC inverterpower supplies are for the purpose of supplying outputvoltage in an alternating current, with the use of PVpower conditioners interconnected with a power grid,aside from the controls necessary to match the phaseand amplitude of the output voltage with the voltage ofthe power grid, it is also necessary to use additionalcontrols for the purpose of efficiently supplying electricpower provided by PV modules to the power grid.Controls must be executed considering a drop in voltageas well as phase delays due to the reactor in the inverteroutput section. (3)Power grid interconnecting protective function Rapid suspension of operations and protection of thepower grid system are obligatory requirements forequipment connected to power grid systems, accordingto the “Interpretation of Technical Standards on ElectricFacilities” 1) , when any abnormality occurs with respect tovoltage, current and frequency. This applies to PV powerconditioners, which must be equipped with functions toprotect the power grid system. The provisions for settingvalues and setting times vary from one electric powercompany to another, therefore, discussions with theelectric power company are essential prior to connectinga system to a power company's power grid system. *2)PV is an abbreviation for ”photovoltaic“. Power gridPower conditionerPV modules Load DCACDCDCDCDCDCDCDCDC *3)MPPT is an abbreviation for Maximum Power Point Tracking. ElectricpoweroutputFluctuation due toirradiance & temperatureOptimum operating pointOutput voltage  42  Oki Technical ReviewApri 2009/Issue 214 Vol.76 No.1 Aside from the protective functions in relation tovoltage, current and frequency, as described above, PVpower conditioners must be equipped with an islandingdetection capability. An islanding involves the output ofelectric power from PV power conditioners, even whenthe power grid voltage has been suspended, due to apower outage or the like. Equipment connected to apower grid system is required to have a capability fordetecting an islanding, in order to ensure the safety forinspection personnel of power grid systems. Characteristics of PV power conditioners indevelopment The PV power conditioner currently being developedhas the following three features, aside from the improvedbasic functions for PV power conditioner controlsdescribed in previous sections:ãIncorporation of insulation methodãImprovement of power generating efficiencyãCompatibility for networks (1)Incorporation of insulation method Equipment connected to power grid systemsordinarily offer protection against current leakage(grounding current) through the use of a ground-faultinterrupter or the like. PV modules and the power gridsystem are not insulated in non-insulated PV systems,indicating a potential for current leakage overlaid withdirect current components. Therefore, a need exists toinsulate the junction between the grounding detectorcapability and the section that connects to the power gridsystem using an alternating current transformer.An alternating current transformer is required for theinsulation method, but this produces the drawback of alarger system volume. A volume equivalent to that ofequipment adopting the non-insulated method has beenrealized with the PV power conditioner currently beingdeveloped, through the control of multiple compacttransformers. This represents a volume less than two-thirds that of conventional non-insulating-type PV powerconditioners. (2)Improvement of power generating efficiency The power conversion efficiency of a power supplyordinarily represents the value for the output of ratedelectric power. The power conversion efficiency duringlower electric power output is lower than that of the ratedelectric power output. The PV power conditioner currentlybeing developed is equipped with multiple DC-DCconverter circuits and the number of these DC-DCconverter circuits in operation varies, depending on theelectric power output conditions. This makes it possible tomaintain a high efficiency electric power conversion inconditions ranging from low to high electric poweroutputs. A conceptual diagram of a load-adaptive controlis shown in Fig. 3 . Fig. 3 Concept of load adaptive control Improvements to the electric power generatingefficiency are being made from a structural approach forthe housing of the PV power conditioner currently beingdeveloped. When the internal temperature of the PVpower conditioner rises, a function that restricts output istriggered in order to prevent damage and thedeterioration of the device. Although large amounts ofelectric power can be generated during periods when thelevel of irradiance is high, the internal temperature riseswhen the surrounding temperature increases. Thefunction for restricting the electric power output from thePV power conditioner is then triggered, which results in adeterioration of the electric power generating efficiency.The thermal analysis simulation, shown in Photo 3 , wasused in preparation for the housing design, to enable thedesign of a structure that minimizes a rise in temperature.This allowed us to extend the time period during whichthe maximum amount of electric power can be supplied,while improving the electric power generating efficiency. Photo 3 Thermal analysis simulation (3)Compatibility for networks PV systems are often installed at unmanned locationsand for this reason it is essential for them to beconnected to the internet in order for such PV systems tobe maintained in an efficient manner. Hence, a piece ofequipment, such as a personal computer capable ofconnecting to the internet, is most commonly used withconventional PV power conditioners. Efficiency of product being developedEfficiency(%)Efficiency of conventional productsSignificant energy conservation effects High efficiency throughout low to highelectric power output range Load (W)  Special Edition on the Environment: Aiming for Low Carbon Society Oki Technical Review 43  Apri 2009/Issue 214 Vol.76 No.1 An Ethernet *4) capability is built into the PV powerconditioner currently being developed so that the unit canbe connected to the internet on its own. Since multipleEthernet connections are available on each unit, it ispossible to connect PV power conditioners with eachother, which makes the implementation of parallelsynchronized operations an easy task.The electric power generating efficiency can potentiallybe improved even further through the exchange ofinformation and coordination among PV powerconditioners. Network configurations are shown in Fig. 4 . Fig. 4 Network configurations Future developments Digitally controlled power supplies are consideredmore advantageous than analog controlled powersupplies, for the purpose of demonstrating the beneficialaspects in the current market, since the enhancement ofintelligence for all product systems is being sought. Wehave been developing DC-AC inverter power suppliesand PV power conditioners according to this belief. Thegreen energy market we are about to enter into with ourPV power conditioner, is expected to continue growing inthe future, which allows us to plan the expansion of ourpower conditioner product series. A development roadmapis shown in Fig. 5 . Fig. 5 Development roadmap A power conditioner with a 10 kW rating forcommercial use has been developed. We can conceive ofcreating units that have a larger capacity with several tensof kW or applications not limited to solar power generationbut with other types of electric power generation that usealternative green energy sources, such as wind power orfuel cells. We can also consider applications to fullyindependent systems by loading batteries onto the PVmodules, as shown in Fig. 6 , in order to provide a stablesupply of electric power in the event of a disaster. Fig. 6 Fully independent system In addition we believe that further improvement of theefficiency can be achieved by combining the amorphouspowder core, for which OPT is currently making greatefforts to develop, with digital control. We believe that thiscan be applied not only to our PV power conditioners butalso to DC-DC converter power supplies, DC-AC inverterpower supplies and various other power supplies, in ourendeavors to proactively tackle environmental problems. References 1)Nuclear and Industrial Safety Agency, Ministry ofEconomy, Trade and Industry (Editing): “Interpretationof Technical Standards on Electric Facilities”, 13thedition, Bun-ichi Sogo Shyuppan, published April2008. Authors Tadashi Ryu: OKI Information Systems Co., Ltd.,Electronics Manufacturing Development, Dept.-3,Development Team 2. *4)Ethernet is a registered trademark of Xerox Corporation of the United States. InternetInternetPowerconditionerPowerconditionerPowerconditioner Personalcomputer Conventional productProduct in development Power conditioners forwind power generation DiversificationIndependency Capacity increase Power conditionersfor fuel cellsPower conditioner forsolar power generation,10 kWBattery-loaded powerconditionersLarge-capacity powerconditioners Parallel operationcompatible powerconditioners Solar panelsPowerconditioner Load
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