Power SemiconductorTechnologies for RenewableEnergy Sources

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High power semiconductors are key components for controlling the generation and connection to thenetwork of renewable energy sources such as wind-turbines and photovoltaic cells. For a highest efficiencyof the energy source, it is therefore essential to select the right device for the given conditions. This articlelooks at the performance features for the available high power semiconductors of choice and also takes alook at future device technologies and their expected impact on efficiency. Björn Backlund and MunafRahimo, ABB Switzerland Ltd, Semiconductors, Lenzburg, Switzerland

Renewable energy sources as wind-turbines and photovoltaic cells havereached power levels of several MWswhich have resulted in the need for highpower semiconductor devices foroptimized generation and networkconnection. The state-of-the-art devices ofchoice for these power levels are theIGBTs and IGCTs. Due to the power qualityrequirements, the earlier used solutionswith thyristors in the wind turbines arerarely seen today. During the last 15 years,high power semiconductors have gonethrough a remarkable development.Several new generations of IGBT-dies havelead to a reduction in VCEsat of almost 40 %since the early 1990s, and still a potentialfor further improvement is available. TheBipolar devices have also seen largeimprovements where the introduction ofthe IGCT have had a large impact on theMV-Drive design and higher ratings forthem have recently been introduced or arein development. The thyristors have alsonot been standing still but have moved

from 6500 V, 2600 A to 8500 V, 4000 Adevices based on 150mm silicon now inproduction.The power semiconductors are used for

two main tasks in the chain of renewableenergy sources such as conversion of thepower in the plant, as in wind-turbines,and transmission of the power to the grid.The best solution to determine whatsemiconductors to use for these tasks is tomove top-down by following the pathsystem requirements defining equipmentrequirements which in turn are definingthe power semiconductor requirements.Through this chain the requirements onthe devices are determined regardingitems as required voltage and currentratings, needed degree of controllability,and operating frequency.

Power semiconductors for invertersThe possibilities to achieve the aboverequirements will be looked at with focusat power ratings above 0.5 MW. Forinverter applications, the IGBTs and IGCTs

represent the two main candidates due tothe main features listed in Table 1.As can be seen, both devices have a

distinct set of features making the questionwhich one is the best technology obsolete.What it comes down to is to select thedevice based on application requirementsand own capability to utilize the device toits best. Certain comparisons are thoughhelpful to see what is possible to achievewith the two technologies. One example isthe possible out-put power for a 2-levelinverter as function of the switchingfrequency at a given set of conditions asseen in Figures 1 and 2. Othercomparisons can though have beenselected to promote a certain technologyover another and should not be used tofind out which solution is the best for thegiven task.In practice the choice of components

will be governed by considerations asstandardization by the use of basic buildingblocks for various applications andrequests from customers to use a certain

Table 1: Features for IGCT and IGBT

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comparisons is that SiC and GaN arelimited in voltage, current and componenttypes which means that usefulcomparisons for many systems inrenewable energy are not really possiblesince there are for instance nocomparative GaN and SiC components to

the Silicon-based IGCTs used in 5MW windturbines with full power conversion. Thisoften leads to comparisons for specialcomponents in special applications wherea 1 to 1 comparison is possible thus toooften underestimating the potential ofenergy savings made possible by Silicon or

solution. The fast development of thedevices makes it though necessary to lookcritically at the used solution from time totime to see if it still is the best possibility tofulfill the requirements or if new designswith new devices can improve theequipment performance.Since the operating conditions

determine the preferred semiconductortechnology it is also not possible to givegeneral rules about which component hasthe highest efficiency. This has to bedetermined case by case also consideringthat the different features of the devicetechnologies can have an impact on thecomplete efficiency for the system. It canthough be projected that the efficiency isnot static but will improve with time asnew improved power semiconductors arecontinuously being introduced on themarket.

Wide band-gap materialsAnother interesting item is thedevelopment of new wide band-gapsemiconductor materials in addition to thedominating silicon starting material. Thesalient features of Silicon compared withthe most developed candidates for newsemiconductor materials are listed in Table2. One important aspect of the high powersemiconductor development is its impacton efficiency and energy saving, or in otherwords how “green” it is. Renewable energysources are today almost exclusivelyequipped with power electronics andtherefore it makes a difference what powersemiconductor are used also due to thelarge impact of secondary effects ascooling capacity.One major issue for efficiency

Figure 1: Comparison in current rating for astandard package equipped with SPT dies

Figure 2: Comparison in current rating forstandard IGCTs

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showing a decrease in inverter size atequal performance where it isquestionable if the small size is ofimportance. Aspects as EMI and insulationfatigue due to very short switching timesalso need to be included in thecomparisons. This since what may lookmost promising on equipment level maybe a solution that is sub-optimal on systemlevel.Due to cost, reliability and availability

only Silicon is an option for bulk powerapplications. Other materials are currentlyonly for niche markets where the possible

efficiency increase is important enough tocompensate for costs, reliability risk, etc.Although a comparison is very difficult to

make at higher power levels we havemade an attempt and in Figure 3 acomparison on module level shows thecurrent capability of a standard modulesize using SPT+ IGBT dies with either aSPT+-diode, extrapolated data for a SiCdiode or the new BIGT with IGBT anddiode integrated on one die. Based oncomparisons like this one, it is alsopossible to calculate losses and efficiencyfor the different solutions at one set of

Table 2: Features for Silicon, Silicon Carbide and Gallium Nitride based devices

conditions in the same way as discussed inthe comparison between IGBT and IGCT. Due to the fast changing landscape of

wide band-gap materials and devices, alsonot forgetting that Silicon-based devicesare continuously being improved, it isexpected that especially applications belowabout 0.5MW will see substantial changesduring the coming years. As a result of this,and also the development on Silicon-based devices for higher power levels, wewill see a gradual improvement inefficiency thus reducing the power lostbetween generation and consumptionhaving a positive effect both in economicalas well an environmental terms.

Bringing the renewable power intothe grid Renewable energy sources are quite oftenremotely located without a sufficientinfrastructure to feed the electrical energyinto the grid. For a complete study ofpower electronics for renewable energysources we must therefore also look at thepossibilities to transmit the energy in anefficient way. For hydro power stations as the three

Gorges dam in China and Rio Madeira inBrazil, HVDC solutions have been chosento transmit the power. At these systemswith transmission lengths of above2000km the total losses, including thelosses in the converter stations, can bereduced with 50% compared to astandard AC-transmission. Thiscorresponds to savings per project of up toseveral TWh yearly. This is done simply byusing large area high voltage mm thyristorswhere current systems are equipped with100 - 125mm thyristors with 150mmdevices recently being introduced for usein UHVDC-systems with voltage levels upto 800kV.Also for other transmission systems the

Figure 3: Comparison in current rating for astandard package equipped with BIGT and SPT+dies vs. SiC carbide diode contribution to anIGBT module (top: inverter mode, bottom:rectifier mode)

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losses and costs can be largely reduced bythe use of HVDC transmission techniques,which is especially apparent for off-shorewind parks where power in the range of300 - 500MW will be transmitted throughthe sea to sub-stations on land. The HVDCLightTM system (Figure 4) is based on IGBTtechnology with a special design thatensures that the module remains shortedin case of a failure enabling a continuationof operation if redundancy is built into thesystem. Starting at the tender power levelof 3MW back in 1997 these systems hasgradually grown larger and it is a merequestion of time until voltage sourceconverter based HVDC-systems with theuse of the latest power semiconductortechnologies will brake the GW-barrier.Small scale renewable energy with a

large number of units spread over a largearea also create issues for the grid stabilitywhich can be solved with differentmeasures normally referred to as smartgrids. Although power electronics will playan important part in these systems, weleave them out of the discussion heresince they are not directly connected toefficiency of renewable energy sources.

ConclusionsRenewable energy sources as wind

turbines and photo-voltaic cells havegrown rapidly in size and power in recentyears. The requirements on them fornetwork compatibility have also increasedsince their impact on the grid is far fromnegligible. Due to a steady developmenton the high power semiconductor side,devices are available to meet therequirements on controllability andefficiency and new devices and devicematerials are on the way enabling furtherimprovements. To utilize the possibilitiesto their optimum the device choiceshould only be made when therequirements and operating conditions forthe high power semiconductors areknown. To use a device just because it ispopular among other users may notmean that it is the best choice for everycase since the best device is determinedby the particular circumstances for theactual project.

LiteratureBjörn Backlund: “Comparison of High

Power Semiconductor Technologies forRenewable Energy Sources”, PEESpecial Session “Power Electronics forEfficient Inverters in Renewable EnergyApplications”, PCIM Europe 2010, May4, Room Paris

Figure 4: Sea cable for an HVDC LightTM system islaid out for connection of an off-shore windpark to the main land grid