Presentation on theme: "Zbigniew Leonowicz, PhD"— Presentation transcript:
1Zbigniew Leonowicz, PhD Power-Electronic Systems for the Grid Integration of Renewable Energy SourcesBased on: J.M. Carrasco, J.T Bialasiewicz, et al:Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006.Zbigniew Leonowicz, PhD
2OutlineNew trends in power electronics for the integration of wind and photovoltaicReview of the appropriate storage-system technologyFuture trends in renewable energy systems based on reliability and maturity
3IntroductionIncreasing number of renewable energy sources and distributed generatorsNew strategies for the operation and management of the electricity gridImprove the power-supply reliability and qualityLiberalization of the grids leads to new management structures
4Power-electronics technology Plays an important role in distributed generationIntegration of renewable energy sources into the electrical gridFast evolution, due to:development of fast semiconductor switchesintroduction of real-time controllers
5Outline (detailed)Current technology and future trends in variable-speed wind turbinesPower-conditioning systems used in grid-connected photovoltaic (PV)Research and development trends in energy-storage systems
6Wind turbine technology Wind-turbine market has been growing at over 30% a yearImportant role in electricity generationGermany and Spain
7New technologies - wind turbines Variable-speed technology – 5% increased efficiencyEasy control of active and reactive power flowsRotor acts as a flywheel (storing energy)No flicker problemsHigher cost (power electronics cost 7%)
13Full converter Energy Transfer Control of the active and reactive powers total-harmonic-distortion controlEnergy storageCONTROL of Vdcdriver controlling the torque generator, using a vector controlstrategy
14Rectifier and chopper step-up chopper is used to adapt the rectifier voltage to the dc-link voltage of the inverter.
15Semiconductor-Device Technology Power semiconductor devices with better electrical characteristics and lower pricesInsulated Gate Bipolar Transistor (IGBT) is main component for power electronics
17Comparison between IGCT and IGBT IGBTs have higher switching frequency than IGCTsIGCTs are made like disk devices – high electromagnetic emission, cooling problemsIGBTs are built like modular devices - lifetime of the device 10 x IGCTIGCTs have a lower ON-state voltage drop- losses 2x lower
18Grid-Connection Standards for Wind Farms Voltage Fault Ride-Through Capability of Wind Turbinesturbines should stay connected and contribute to the grid in case of a disturbance such as a voltage dip.Wind farms should generate like conventional power plants, supplying active and reactive powers for frequency and voltage recovery, immediately after the fault occurred.
20Power-Quality Requirements for Grid-Connected Wind Turbines - flicker + interharmonicsDraft IEC standard for “power-quality requirements for Grid Connected Wind Turbines”
21IEC Standard IEC-61400-21 Flicker analysis Switching operations. Voltage and current transientsHarmonic analysis (FFT) - rectangular windows of eight cycles of fundamental frequency. THD up to 50th harmonic
22Other StandardsHigh-frequency (HF) harmonics and interharmonics IEC and IECTo obtain a correct magnitude of the frequency components, define window width, according to the IECswitching frequency of the inverter is not constantmethods for summing harmonics and interharmonics in the IECCan be not multiple of 50 Hz
23Transmission Technology for the Future Offshore installation.
24HVAC Disadvantages: High distributed capacitance of cables Limited length
25HVDCMore economic > 100 km and power MW 1) Sending and receiving end frequencies are independent. 2) Transmission distance using dc is not affected by cable charging current. 3) Offshore installation is isolated from mainland disturbances 4) Power flow is fully defined and controllable. 5) Cable power losses are low. 6) Power-transmission capability per cable is higher.
32Direct-Drive Technology for Wind Turbines Reduced sizeLower installation and maintenance costFlexible control methodQuick response to wind fluctuations and load variationAxial flux machines
33Future Energy-Storage Technologies in Wind Farms Zinc bromine batteryHigh energy density relative to lead-acid batteries • 100% depth of discharge capability • High cycle life of >2000 cycles at • No shelf life • Scalable capacities from 10kWh to over 500kWh systems • The ability to store energy from any electricity generating sourceFor wind-power application, the flow (zincbromine) battery system offers the lowest cost per energy storedand delivered. The zinc–bromine battery is very different inconcept and design from the more traditional batteries suchas the lead–acid battery. The battery is based on the reactionbetween two commonly available chemicals: zinc and bromine.The zinc–bromine battery offers two to three times higherenergy density (75–85 W・ h per kilogram) along with the sizeand weight savings over the present lead/acid batteries. Thepower characteristics of the battery can be modified for selectedapplications. Moreover, zinc–bromine battery suffers no loss ofperformance after repeated cycling. It has a great potential forrenewable energy applications
34Hydrogen as a vehicle fuel Electrical energy can be produced and delivered to the grid from hydrogen by a fuel cell or a hydrogen combustion generator.The fuel cell produces power through a chemical reaction and energy is released from the hydrogen when it reacts with the oxygen in the air.
35Variable-speed wind turbine with hydrogen storage system
36PV Photovoltaic Technology PV systems as an alternative energy resourceComplementary Energy-resource in hybrid systemsNecessary:high reliabilityreasonable costuser-friendly design
37PV-module connections The standardsEN , IEEE1547,U.S. National Electrical Code (NEC) 690IEC61727power quality, detection of islanding operation, groundingstructure and the features of the present and future PV modules.Islanding refers to the condition of a distributed generation (DG) generator continuing to power a location even though power from the electric utility is no longer present. Consider for example a building that has solar panels that feed power back to the electrical grid; in case of a power blackout, if the solar panels continue to power the building, the building becomes an "island" with power surrounded by a "sea" of unpowered buildings.Islanding can be dangerous to utility workers, who may not realize that the building is still powered even though there's no power from the grid. For that reason, distributed generators must detect islanding and immediately stop producing power.In intentional islanding, the customer disconnects the building from the grid, and forces the distributed generator to power the building.EN European standard regulating harmonic currents - a brief summary. The IEC standard imposes limits on the harmonic currents drawn from the mains supply. This standard requires that electrical appliances be type tested to ensure that they meet the requirements in the standard. It is applicable to electrical and electronic equipment having an input current up to and including 16A per phase and intended to be connected to public low-voltage distribution systems i.e. supply voltages nominally 230V ac or 415V ac 3 phase. The standard defines four classes of waveform according to the different types of equipment. For example, one of the Classes (Class B) applies to portable tools, whereas the typical switched mode waveform is generally in another Class (Class D). Each Class has different harmonic limits up to the 40th, which must not be exceeded. Some Classes have dynamic limits which are set according to the power drawn by the device. The scope of the EN standard includes products such as lighting equipment, portable tools, all electronic equipment, consumer products and appliances and industrial equipment. This standard does not cover equipment which has a nominal supply voltage less than 220V ac. No limits have been specified for professional equipment above 1kW. Although these requirements cover only products to be sold within EEC countries, a similar IEEE document exists for the USA and Japan is also considering similar legislation.Title Photovoltaic (PV) Systems - Characteristics of the Utility InterfaceSystèmes photovoltaïques (PV) – Caractéristiques de l'interface de raccordement au réseauInternational Electrotechnical CommissionPublication Date: Dec 1, 2004Scope:Scope and objectThis International Standard applies to utility-interconnected photovoltaic (PV) power systems operating in parallel with the utility and utilizing static (solid-state) non-islanding inverters for the conversion of DC to AC. This document describes specific recommendations for systems rated at 10 kVA or less, such as may be utilized on individual residences single or three phase. This standard applies to interconnection with the low-voltage utility distribution system.The object of this standard is to lay down requirements for interconnection of PV systems to the utility distribution system.NOTE 1 An inverter with type certification meeting the standards as detailed in this standard should be deemed acceptable for installation without any futher testing.This standard does not deal with EMC or protection mechanisms against islanding.NOTE 2 Interface requirements may vary when storage systems are incorporated or when control signals for PV system operation are supplied by the utility.
39IslandingPV Generator Converter AC-DC Local Loads Grid
40Market Considerations PV Solar-electric-energy growth consistently 20%–25% per annum over the past 20 years1) an increasing efficiency of solar cells2) manufacturing-technology improvements3) economies of scale
41PV growth2001, 350 MW of solar equipment was sold 2003, 574 MW of PV was installed.In 2004 increased to 927 MWSignificant financial incentives in Japan, Germany, Italy and Francetriggered a huge growth in demandIn 2008, Spain installed 45% of all photovoltaics, 2500 MW in 2008 to an drop to 375 MW in 2009
42PerspectivesWorld solar photovoltaic (PV) installations were gigawatts peak (GWp) in 2007, and 5.95 gigawatts in 2008The three leading countries (Germany, Japan and the US) represent nearly 89% of the total worldwide PV installed capacity.2012 are and 12.3GW- 18.8GW expected
44EfficiencyMarket leader in solar panel efficiency (measured by energy conversion ratio) is SunPower, (San Jose USA) %market average of 12-18%.Efficiency of 42% achieved at the University of Delaware in conjunction with DuPont (concentration) in 2007.The highest efficiency achieved without concentration is by Sharp Corporation at 35.8% using a proprietary triple-junction manufacturing technology in 2009.
45Design of PV-Converters IGBT technologyInverters must be able to detect an islanding situation and take appropriate measures in order to protect persons and equipmentPV cells - connected to the gridPV cells - isolated power supplies
47Converter topologies Central inverters Module-oriented or module-integrated invertersString invertersThe central converters connect inparallel and/or in series on the dc side. One converter is used forthe entire PV plant (often divided into several units organizedin master–slave mode). The nominal power of this topology isup to several megawatts. The module-oriented converters withseveral modules usually connect in series on the dc side andin parallel on the ac side. The nominal power ratings of suchPV power plants are up to several megawatts. In addition, inthe module-integrated converter topology, one converter per PVmodule and a parallel connection on the ac side are used. In thistopology, a central measure for main supervision is necessary.
48Multistring converter Integration of PV strings of different technologies and orientations
49Review of PV Converters S. B. Kjaer, J. K. Pedersen, F.Blaabjerg „A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules”, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 5, SEPTEMBER/OCTOBER 2005Demands Defined by the Grid- standards (slide 37) EN standard (applied in Europe) allows higher current harmonicsthe corresponding IEEE and IEC standards.
50limiting the injection is to avoid saturation of the distribution transformerslimits are rather small (0.5% and 1.0% of rated output current),and such small values can be difficult to measure preciselywith the exciting circuits inside the inverters. This can bemitigated with improved measuring circuits or by including aline-frequency transformer between the inverter and the grid.Some inverters use a transformer embedded in a high-frequencydc–dc converter for galvanic isolation between the PV modulesand the grid. This does not, however, solve the problem withdc injection, but makes the grounding of the PV moduleseasier.
51IslandingIslanding is the continued operation of the inverter when the grid has been removed on purpose, by accident, or by damageDetection schemes - active and passive.The passive methods -monitor grid parameters.The active schemes introduce a disturbance into the grid and monitor the effect.. In other words, the grid has been removedfrom the inverter, which then only supplies local loads.
52Grounding & ground faults The NEC 690 standard - system grounded and monitored for ground faultsOther Electricity Boards only demand equipment ground of the PV modules in the case of absent galvanic isolationEquipment ground is the case when frames and other metallic parts are connected to ground.
53Power injected into grid Decoupling is necessaryp –instantaneousP - average
54Demands Defined by the Photovoltaic Module The inverters must guarantee that the PV module(s) is operatedat the MPP, which is the operating condition where the mostenergy is captured. This is accomplished with an MPP tracker(MPPT).maximum power point tracker (or MPPT) is a high efficiency DC to DC converter that presents an optimal electrical load to a solar panel or array and produces a voltage suitable for the load.PV cells have a single operating point where the values of the current (I) and Voltage (V) of the cell result in a maximum power output. These values correspond to a particular resistance, which is equal to V/I as specified by Ohm's Law. A PV cell has an exponential relationship between current and voltage, and the maximum power point (MPP) occurs at the knee of the curve, where the resistance is equal to the negative of the differential resistance (V/I = -dV/dI). Maximum power point trackers utilize some type of control circuit or logic to search for this point and thus to allow the converter circuit to extract the maximum power available from a cell.new technologies like thin-layer silicon, amorphous-silicon, and hoto Electro Chemical (PEC)Voltage in the range from 23 to 38 V at a power generation of approximate 160 W, and their open-circuit voltage is below 45 V.New technolgies - voltage range around V at several hundred amperes per square meter cell
55Maximum Power Point Tracker EX.: ripple voltage should be below 8.5% of the MPP voltage in order to reach a utilization ratio of 98%u is the amplitude of the voltage ripple, PMPP and UMPPare the power and voltage at the MPP, alpha and beta are the coefficientsdescribing a second-order Taylor approximation of thecurrent, and the utilization ratio KPV is given as the average generatedpower divided by the theoretical MPP power.
56Cost Cost effectiveness using similar circuits as in single-phase power-factor-correction (PFC) circuitsvariable-speed drives (VSDs)
57High efficiency wide range of input voltage and input power very wide ranges as functionsof solar irradiation and ambient temperature.
58Meteorological data.(a) Irradiation distribution for a reference year.(b) Solar energy distribution for a reference year.Total time ofirradiation equals 4686 h per year.Total potential energy is equal to 1150 kWh=(m2 year) 130 W/m2
59Reliability long operational lifetime most PV module manufacturer offer a warranty of 25 years on 80% of initial efficiencyThe main limiting components inside the inverters are the electrolytic capacitors used for power decoupling between the PV module and the single-phase gridHowever, the equation assumes a constant temperature,which can be approximated when the inverter is placedindoors and neglecting the power loss inside the capacitor, butcertainly not when the inverter is integrated with the PV module,as for the ac module. In the case of a varying temperature a meanvalue of (8) must be applied to determine the lifetime
60Topologies of PV inverters Centralized InvertersString InvertersMulti-string InvertersAC modules & AC cell technology
61Centralized Inverters PV modules as series connections (a string)series connections then connected in parallel, through string diodesDisadvantages !high-voltage dc cables betweenthe PV modules and the inverter, power losses due to a centralizedMPPT, mismatch losses between the PV modules, lossesin the string diodes, and a nonflexible design where the benefitsof mass production could not be reached. The grid-connectedstage was usually line commutated by means of thyristors,involving many current harmonics and poor power quality.The large amount of harmonics was the occasion of new invertertopologies and system layouts, in order to cope with theemerging standards which also covered power quality.
62String Inverters Reduced version of the centralized inverter single string of PV modules is connected to the inverterno losses on string diodesseparate MPPTsincreases the overall efficiencyThe input voltagemay be high enough to avoid voltage amplification. This requiresroughly 16 PV modules in series for European systems.The total open-circuit voltage for 16 PV modules may reach asmuch as 720 V, which calls for a 1000-V MOSFET/IGBT inorder to allow for a 75% voltage de-rating of the semiconductors.The normal operation voltage is, however, as low as 450510 V. The possibility of using fewer PV modules in seriesalso exists, if a dc–dc converter or line-frequency transformeris used for voltage amplification.
63AC module inverter and PV module as one electrical device No mismatch losses between PV modulesOptimal adjustment of MPPThigh voltage-amplification necessaryThe ac module depicted in Fig. 3(d) is the integration of theinverter and PV module into one electrical device . It removesthe mismatch losses between PV modules since there is only onePV module, as well as supports optimal adjustment between thePV module and the inverter and, hence, the individual MPPT. Itincludes the possibility of an easy enlarging of the system, dueto the modular structure. The opportunity to become a “plugand-play” device, which can be used by persons without anyknowledge of electrical installations, is also an inherent feature.On the other hand, the necessary high voltage-amplification mayreduce the overall efficiency and increase the price per watt,because of more complex circuit topologies. On the other hand,the ac module is intended to be mass produced, which leads tolow manufacturing cost and low retail prices.The present solutions use self-commutated dc–ac inverters,by means of IGBTs or MOSFETs, involving high power qualityin compliance with the standards.
65Multi-string Inverters FlexibleEvery string can be controlled individually.The multi-string inverter depicted in Fig. 3(c) is the furtherdevelopment of the string inverter, where several strings are interfacedwith their own dc–dc converter to a common dc–ac inverter, . This is beneficial, compared with the centralizedsystem, since every string can be controlled individually. Thus,the operator may start his/her own PV power plant with a fewmodules. Further enlargements are easily achieved since a newstring with dc–dc converter can be plugged into the existing platform.A flexible design with high efficiency is hereby achieved.
66AC cell One large PV cell connected to a dc–ac inverter Very low voltageNew converterconceptsvery low voltage, V and 100Wper squaremeter, up to an appropriate level for the grid, and at the sametime reach a high efficiency. For the same reason, entirely newconverter concepts are required.
67Classification of Inverter Topologies Single-stage inverterDual stage inverterMulti-string inverterThe inverter of Fig. 4(a) is a single-stage inverter, which musthandle all tasks itself, i.e., MPPT, grid current control and, perhaps,voltage amplification. This is the typical configuration fora centralized inverter, with all the drawbacks associated with it.The inverter must be designed to handle a peak power of twicethe nominal power, according to (1).Dual-stage inverter. The dc–dc converter isnow performing the MPPT (and perhaps voltage amplification).Dependent on the control of the dc–ac inverter, the output fromthe dc–dc converters is either a pure dc voltage (and the dc–dcconverter is only designed to handle the nominal power), or theoutput current of the dc–dc converter is modulated to follow arectified sine wave (the dc–dc converter should now handle apeak power of twice the nominal power). The dc–ac inverteris in the former solution controlling the grid current by meansof pulsewidth modulation (PWM) or bang-bang operation. Inthe latter, the dc–ac inverter is switching at line frequency, “unfolding”the rectified current to a full-wave sine, and the dc–dcconverter takes care of the current control. A high efficiency canbe reached for the latter solution if the nominal power is low. Onthe other hand, it is advisable to operate the grid-connected inverterin PWM mode if the nominal power is high.multi-string inverter.The only task for each dc–dc converter is MPPT and perhapsvoltage amplification. The dc–dc converters are connected to thedc link of a common dc–ac inverter, which takes care of the gridcurrent control. This is beneficial since better control of eachPV module/string is achieved and that common dc–ac invertermay be based on standard VSD technology.
68Power Decoupling Capacitors Power decoupling is normally achieved by means of an electrolyticcapacitor. As stated earlier, this component is the mainlimiting factor of the lifetime. Thus, it should be kept as small aspossible and preferably substituted with film capacitors. The capacitoris either placed in parallel with the PV modules or in thedc link between the inverter stages; this is illustrated in Fig. 5.The size of the decoupling capacitor can be expressed as(9)Where Ppv is the nominal power of the PV modules, Uc is themean voltage across the capacitor, and uc is the amplitude ofthe ripple. Equation (9) is based on the fact that the current fromthe PV modules is a pure dc, and that the current drawn fromthe grid-connected inverter follows a waveform sinus squared,assuming that is constant.CPV = 2,4 miliFCDC =33microF
69Transformers and Types of Interconnections Component to avoid (line transformers= high size, weight, price)High-frequency transformersGrounding,The transformer is a paradox within PV inverters. As statedpreviously, system grounding of the PV modules is not requiredas long as the maximum output voltage is below 50 V. On theother hand, it is hard to achieve high-efficiency voltage amplificationwithout a transformer, when the input voltage is in therange from 23 to 45 V. Third, the transformer is superfluouswhen the input voltage becomes sufficiently high. A normalfull-bridge inverter cannot be used as grid interface, when boththe input and the output of the inverter are be grounded. In addition,the large area of PV modules includes a capacitance of0.1 nF 10 nF per module to ground . This can also causesevere oscillations between the PV modules and (stray) inductancesin the circuit.
70Types of Grid Interfaces Inverters operating in current-source modeFig. 8 shows four, out of many, possible grid-connectedinverters. The topologies of Fig. 8(a) and (b) are line-frequency-commutated current-source inverters (CSIs). Thecurrent into the stage is already modulated/controlled to followa rectified sinusoidal waveform and the task for the circuit issimply to re-create the sine wave and inject it into the grid. Thecircuits apply zero-voltage switching (ZVS) and zero-currentswitching (ZCS), thus, only conduction losses of the semiconductorsremain.Since the current is modulated by another stage, the otherstage must be designed for a peak power of twice the nominalpower, according to (1) and power decoupling must be achievedwith a capacitor in parallel with the PV module(s). The converterfeeding the circuit of Fig. 8(a) can be a push–pull with asingle secondary transformer winding, and a flyback with twosecondary windings for the circuit of Fig. 8(b).Line-commutated CSI switching at twice the line frequency
71Voltage-Source Inverters standard full-bridge three-level VSIstandard full-bridge three-levelVSI, which can create a sinusoidal grid current by applyingthe positive/negative dc-link or zero voltage, to the grid plusgrid inductor. The voltage across the grid and inductor is oftenpulsewidth modulated, but hysteresis (bang-bang) current controlcan also be applied. A variant of the topology in Fig. 8(c) isthe half-bridge two-level VSI, which can only create two distinctvoltages across and requires double dc-link voltage and doubleswitching frequency in order to obtain the same performance asthe full bridge.
72VSI Half-bridge diode-clamped three-level VSI three-level VSI, is one of many different multilevelVSIs, which can create 3, 5, 7 distinct voltages across the gridand inductorThis is beneficial since the switching frequencyof each transistor can be reduced and, in the mean time, goodpower quality is ensured.The command signals for the transistors in the CSI and thereference for the grid-current waveform are mostly based onmeasured grid voltage or zero-crossing detection. This may resultin severe problems with power quality and unnecessary faultsituations. According to , the main reasons for these problemsare the background (voltage) harmonics and poor design.The harmonics may initiate series resonance with the capacitorsplaced around in the grid (e.g., in refrigerators), due to positivefeedback of the inverter current or a noisy signal from thezero-crossing detection. A solution for this problem is to use aphase-locked loop (PLL) for establishing a current waveformreference of high quality.
73AC Modules 100-W single-transistor flyback-type HF-link inverter 100 W, out 230 V, in 48 V, 96%, pf=0,955The circuit is made up around a single-transistor flybackconverter, with a center-tapped transformer. The two outputsfrom the transformer are connected to the grid, one at a time,through two MOSFETs, two diodes, and a common filter circuit. The flyback converter can, in this way, produce botha positive and a negative output current.
74AC modules 105-W combined flyback and buck–boost inverter 105 W, out 85V, in 35V, THD <5%The next topology in Fig. 10 is a 105-W combined flybackand buck–boost inverter . The need for a large decouplingcapacitor is avoided by adding a buck–boost converter to theflyback converter. The leakage inductance included in the transformerresults in a voltage spike across the transistor denotedSDC in Fig. 10, during turn-off. A dissipative RCD clamp wouldnormally be used to remove the overvoltage; see the previoustopology.However, theRCD clamp circuit interacts heavily withthe buck–boost circuit, causing the inverter to malfunction. Thesolution is the modified Shimizu topology presented in the nextsection . Finally, the energy-storing capacitor mustcarry the entire load current, which increases the demands forits current-ripple capabilities.
75AC modules Modified Shimizu Inverter (160W, 230, 28V, 87%) Note that the polarity of the PV module is reversedThe inverter in Fig. 11 is an enhanced version of the previoustopology, rated for 160 W. The main improvement within thisinverter is the replacement of the single-transistor flyback converterwith a two-transistor flyback converter, to overcome problemswith overvoltage.
76AC modules 160-W buck–boost inverter in 100V out 160V The topology in Fig. 12 is a 160-W buck–boost inverter .Again, a small amount of energy is stored in the leakage inductance.This energy is now recovered by the body diodes oftransistors and . On the other hand, the diodeis blocking for the energy recovery, and no further informationis given in  about the type of applied clamp circuit.
77AC modules150-W flyback dc–dc converter with a line-frequency dc–ac unfolding inverterin 44V, out 120VThe topology in Fig. 13 is a 150-W flyback dc–dc convertertogether with a line-frequency dc–ac unfolding inverter . In, the same topology is applied for a 100-W inverter, exceptthat the grid filter is removed from the dc link to the grid side.The line-frequency dc–ac inverter is in both cases equipped withthyristors, which can be troublesome to turn on, since they requirea current in their control terminal to turn on.
78AC modules 100-W flyback dc–dc converter with a PWM dc–ac inverter 30V – 210 VThe inverter in Fig. 14 is a 100-W flyback dc–dc converter togetherwith aPWMdc–ac inverter , . The output stage isnow made up of four transistors, which are switched at high frequency.The grid current is modulated by alternately connectingthe positive or the negative dc-link voltage (the constant voltageacross ) to the inductor Lgrid in DxTswitch s, and zero voltagein ( is the duty cycle and is the switchingperiod).
79AC modules110-W series-resonant dc–dc converter with an HF inverter toward the grid30-230V , 87%The inverter in Fig. 15 is based on a 110-W series-resonantdc–dc converter with an HF inverter toward the grid , and250Win . The series-resonant converter is the first resonantconverter visited here. The inverter toward the grid is modifiedin such a way that is cannot operate as a rectifier, seen from thegrid side. Adding two additional diodes does this. The advantageof this solution is that no in-rush current flows when the inverteris attached to the grid for the first time.
80AC modules dual-stage topology Mastervolt Soladin 120 in 24-40V, out 230V, 91%, pf=0,99The commercially available Mastervolt Soladin 120 inverter is a “plug-and-play” inverter, based on the topology inFig. 16. The nominal input power is 90 W at 20–40 V, but theopportunity to operate at peak 120 W exists. The Soladin 120inverter is a dual-stage topology without inherent power decoupling.The capacitor in parallel with the PV module is, therefore,rather larger (2x 1000 mF at 50 V), since it must work as anenergy buffer. According to the work in Section II-B, this resultsin a small-signal amplitude in the range from 1.8 to 3.0 V, whichcorresponds to a PV utilization factor from to at fullgeneration.
81String Inverters Single-stage Dual-stage The string and multi-string systems are the combinationof one or several PV strings with a g.the inverters should be of the single- or dual-stage type withor without an embedded HF transformer. Next follow someclassical solutions for the string and multi-string inverters.rid-connected inverter
82String Invertera transformerless half-bridge diode-clamped three-level inverterThe inverter in Fig. 17 is a transformer-less half-bridge diode-clampedthree-level inverter , . Turning S1 and S2 oncan create a positive output voltage, turning S2 and S3 on createszero voltage, and finally, turning S3 and S4 on creates anegative voltage. Each of the two PV strings is connected to theground/neutral of the grid, thus, the capacitive earth currents arereduced, and the inverter can easily fulfill the NEC 690 standard.The inverter can be further extended to five levels by addingmore transistors, diodes, and PV strings. However, this requiresthat the outer strings (e.g., the strings placed at locations #0 and#4 in Fig. 17, not illustrated) must be carefully sized since theyare loaded differently than strings #1 and #2. Another serious drawback is that string #1 is only loaded during positive gridvoltage, and vice versa for string #2. This requires the decouplingcapacitors to be enlarged with a factor of approximately Pi, compared to Section IV-B. This is not an advantage for thecost or the lifetime.
83String Inverter two-level VSI, interfacing two PV strings The inverter in Fig. 18 is a two-level VSI, interfacing two PVstrings , . This inverter can only produce a two-leveloutput voltage, thus, the switching frequency must be double theprevious one in order to obtain the same size of the grid inductor.The main difference between this and the former topology isthe generation control circuit (GCC), made by transistorsS2 and S3 and inductor Lpv , which can load each PV string independently.Actually, one of the PV strings can even be removedand sinusoidal current can still be injected into the grid.The GCC is an advantage since an individual MPPT can be appliedto each string. Further enlargement is easily achieved byadding another PV string plus a transistor, a capacitor, and aninductor. The drawback of this topology and the topology inis their buck characteristic, for which reason the minimuminput voltage always must be larger than the maximumgrid voltage. For example, the maximum grid voltage is equalto 230Vx root(2)=360V, and the minimum voltage across a PVmodule is 2,3 V-3 V (MPP voltage minus the 100-Hz rippleacross the PV strings). Hence, two strings, each of minimum 18modules, are required for the former topology and two stringsof minimum nine modules for the latter topology.
84SMA Sunny Boy 5000TLthree PV strings, each of 2200 W at V, with own MPPTThe commercially available inverter (SMA Sunny Boy5000TL , ) in Fig. 19 is designed for three PV strings,each of 2200 W at V, and each with their ownMPPT. The circuits interfacing the PV strings are standardboost converters, which is beneficial since the HF current rippleat the input terminals of the converters is easily filtered by afilm capacitor. The grid-connected dc–ac inverter is a two-levelVSI. When this is pointed out, it becomes obvious that the PVstrings cannot be system grounded, thus, this inverter is notallowed in the U.S. due to the NEC 690 standard.
85PowerLynx Powerlink PV 4.5 kW three PV strings, each V, 1500 WThe dc–dc converters are based on current-source full-bridge inverters with embedded HF transformerand rectifier. The PV strings are easily system grounded and noproblem with the NEC 690 standard exists, since this inverterincludes galvanic isolation between the PV string and the grid.Once again, the current-source input stage is beneficial since itreduces the requirement for the filter capacitor in parallel withthe PV strings. Furthermore, the diodes included in the rectifiersare current commutated which involves low reverse recovery ofthe diodes and low voltage stress. The grid-connected dc–ac inverteris a three-? level VSI.
86Evaluation and Discussion component ratingsrelative costlifetimeefficiencyratings of the semiconductors are based on the averageor rms currents and the peak voltages they have to withstandThe relative cost is computed on the basis of the calculatedratings, a component survey at different vendorsThe lifetime is evaluated by the size of the de-coupling capacitors,and the amount of current they have to carry. A highcurrent involves high power loss in the capacitors, which resultsin hot spots inside the capacitors, and an increased temperatureis the main factor of the lifetime.The efficiency for each inverter has been computed at six differentoperating points, based on “average” components fromthe component survey. According to the definition of the Europeanefficiency, the individual efficiencies are weighted andsummed up according to
87Results Dual-stage CSI = large electrolytic decoupling capacitor VSI = small decoupling electrolytic capacitor.Dual-stage CSIs like the circuits in Fig. 8(a) and (b) sufferfrom a large electrolytic decoupling capacitor, whereas decouplingfor the VSI can be achieved with a small electrolytic capacitor.This is beneficial when lifetime is the issue, since, as already alreadystated, the electrolytic capacitor is the main limiting singlecomponent within the inverters.
88Results - Efficiency Low efficiency=87% C=68 mF 160V High efficiency=93%C=2,2 mF 45VOnly two circuits are different from the others when examiningthe European efficiency; these are the inverters in Figs. 11and 16. The inverter in Fig. 11 has a low efficiency, which iscaused by the high voltage ratings for the semiconductors onthe PV side, and in the mean time, high current also flows in thecircuit. The push–pull inverter in Fig. 16 has a higher efficiencythan the other inverters. This is mainly due to a low conductionloss in the PV-side converter, where only two transistors are carryingthe current. On the other hand, the voltage stress for thetwo transistors is double that of the other inverters (except theone in Fig. 11). This is also seen in the ratings of the semiconductorsfor this inverter, which are higher than the others. If oneshould select an inverter topology based on this comparison, thepush–pull inverter in Fig. 16 would be a preferable choice, sinceit offers high efficiency and relatively low price, but attentionshould be paid to the decoupling capacitor, which is the weakestpoint.
89Discussion - String Inverters The dual-grounded multilevel inverters p.82 – good solution but quite large capacitors 2x640mF 810V -> half-period loadingbipolar PWM switching toward the grid p.83 & 84 (no grounding possible, large ground currents) – 2x1200 mF 375 Vcurrent-fed fullbridge dc–dc converters with embedded HF transformers, for each PV string – p.85 – 3x 310 mF 400VThe dual-grounded multilevel HBDC inverters can be a goodsolution, but attention should be paid to the decoupling capacitors,which in the case of the inverter in Fig. 17 must be ratherlarge since they are only loaded in half of the grid period. A solutioncould be to include some kind of balancing circuit, likethe balancing GCC in Fig. 18.Two of the reviewed topologies, (see Figs. 18 and 19) usebipolar PWM switching toward the grid. This is beneficial forthe GCC inverter in Fig. 18, but not for the topology in Fig. 19due to the requirement for a high dc-link voltage and two decouplingcapacitors in series to create a midpoint. Besides this, theinverter in Fig. 19 cannot be system grounded which is a requirementfrom the NEC 690 standard, but common-mode electricalnoise at the terminals of the PV module can also generate largeground currents, due to the capacitances from the PV modulesto ground.The last topology visited here is based on current-fed fullbridgedc–dc converters with embedded HF transformers, foreach PV string. This requires more components than the threeprevious inverters, but their ratings are lower and the benefits ofmass production could be easily achieved. Both commerciallyavailable inverters show good efficiency and grid performance.
90Resume – PV InvertersLarge centralized single-stage inverters should be avoidedPreferable location for the capacitor is in the dc link where the voltage is high and a large fluctuation can be allowed without compromising the utilization factorHFTs should be applied for voltage amplification in the AC module and AC cell conceptsLine-frequency CSI are suitable for low power, e.g., for ac module applications.High-frequency VSI is also suitable for both low- and high-power systems, like the ac module, the string, and the multistring invertersThis review has covered some of the standards that invertersfor PV and grid applications must fulfill, which focus on powerquality,injectionofdccurrentsintothegrid,detectionofislandingoperation, and system grounding. The demands stated by thePV modules have also been reviewed; in particular, the roleof power decoupling between the modules and the grid hasbeen investigated. An important result is that the amplitudeof the ripple across a PV module should not exceed 3.0 V inorder to have a utilization efficiency of 98% at full generation.Finally, the basic demands defined by the operator have alsobeen addressed, such as low cost, high efficiency, and longlifetime.The next part of the review was a historical summary of thesolutions used in the past, where large areas of PV modules wereconnected to the grid by means of centralized inverters. This includedmany shortcomings for which reason the string invertersemerged. A natural development was to add more strings, eachwith an individual dc–dc converter and MPPT, to the commondc–ac inverter, thus, the multi-string inverters were brought tolight. This is believed to be one of the solutions for the future.Another trend seen in this field is the development of the acmodule, where each PV module is interfaced to the grid withits own dc–ac inverter.The historical review was followed with a classification ofthe inverters: number of power processing stages, type of powerdecoupling between the PV module and the grid, transformersand types of interconnections between the stage, and types ofgrid interfaces. The conclusions from the classifications are asfollows.1) Large centralized single-stage inverters should beavoided, except if the input voltage is sufficiently high toavoid further amplification. The dual-stage inverter is thesolution for ac modules and ac cells, since they requirevoltage amplification. Last, if several strings are to beconnected to the grid, the multi-sting concept seems tobe the obvious choice.2) Nothing is gained by moving the decoupling capacitorfrom the input of the inverter to the dc link, when PVmodules are connected in series to reach a high voltagefor the inverter. On the other hand, in the case of the acmodule and the ac cell, the preferable location for thecapacitor is in the dc link where the voltage is high and alarge fluctuation can be allowed without compromisingthe utilization factor. Electrolytic capacitors should bereplaced with film capacitors in order to increase thereliability, but this also involves a higher price, especiallyfor high-power inverters, where a large capacitance isrequired. On the other hand, a high reliability can bea major sales parameter.3) HFTs should be applied for voltage amplification in theac module and ac cell concepts. It is also beneficialto include an HFT in larger systems in order to avoidresonance between the PV modules and inductances inthe current main paths. The resonance can, however,also be mitigated with inverter topologies that supportgrounding on both input and the output terminals. Thedual grounding scheme is also a requirement in the U.S.for PV open-circuit voltages larger then 50 V, but notin Europe and Japan.4) Line-frequency CSIs are suitable for low power, e.g., forac module applications. On the other hand, a high-frequencyVSI is also suitable for both low- and high-powersystems, like the ac module, the string, and the multistringinverters.
91Converter topologies (general) PV inverters with dc/dc converter (with or without isolation)PV inverters without dc/dc converter (with or without isolation)Isolation is acquired using a transformer that can be placed on either the grid or low frequency (LF) side or on the HF sideConvert_interface p. 10The isolation used in both categories is acquired using atransformer that can be placed on either the grid or lowfrequency(LF) side or on the HF side. The line-frequencytransformer is an important component in the system due toits size, weight, and price. The HF transformer is more compact,but special attention must be paid to reduce losses ,. The use of a transformer leads to the necessary isolation(requirement in U.S.), and modern inverters tend to use anHF transformer. However, PV inverters with a dc/dc converterwithout isolation are usually implemented in some countrieswhere grid-isolation is not mandatory.
92HF dc/dc converter full-bridge single-inductor push–pull double-inductor push–pullBasic designs focused on solutions for HF dc/dc convertertopologies with isolation such as full-bridge or single-inductorpush–pull permit to reduce the transformer ratio providinga higher efficiency together with a smoother input current.However, a transformer with tap point is required. In addition,a double-inductor push–pull is implemented in other kind ofapplications (equivalent with two interleaved boost convertersleading to a lower ripple in the input current), but extra inductoris needed . A full-bridge converter is usually usedat power levels above 750 W due to its good transformerutilization .
93Another classification number of cascade power processing stages-single-stage-- dual-stage-----multi-stageThere is no any standard PV inverter topologysingle-stage inverter must handle all tasks such asmaximum-power-point-tracking (MPPT) control, grid-currentcontrol, and voltage amplification. This configuration, which isuseful for a centralized inverter, has some drawbacks because itmust be designed to achieve a peak power of twice the nominalpower.In thiscase, the dc/dc converter performs the MPPT (and perhaps voltageamplification), and the dc/ac inverter is dedicated to controlthe grid current by means of pulsewidth modulation (PWM),space vector modulation (SVM), or bang–bang operationmultistage inverters can be used, as mentioned above.In this case, the task for each dc/dc converter is MPPT and,normally, the increase of the dc voltage. The dc/dc convertersare connected to the dc link of a common dc/ac inverter, whichtakes care for the grid-current control. This is beneficial sincea better control of each PV module/string is achieved, and thatcommon dc/ac inverter may be based on a standard variablespeed-drive (VSD) technology.Severaluseful proposed topologies have been presented, and somegood studies regarding current PV inverters have been done, . The current control scheme is mainly used in PVinverter applications . In these converters, the current intothe stage is modulated/controlled to follow a rectified sinusoidalwaveform, and the task for the circuit is simply to recreate thesine wave and inject it into the grid. The circuits apply zerovoltageswitching (ZVS) and zero-current switching (ZCS).Thus, only conduction losses of the semiconductors remain.If the converter has several stages, power decoupling must beachieved with a capacitor in parallel with the PV module(s).The current control scheme is employed more frequently becausea high-power factor can be obtained with simple controlcircuits, and transient current suppression is possible whendisturbances such as voltage changes occur in the utility powersystem. In the current control scheme, operation as an isolatedpower source is difficult, but there are no problems with gridinterconnection operation.PV automatic-control (AC) module inverters used to be dualstageinverters with an embedded HF transformer. Classicalsolutions can be applied to develop these converters: flybackconverters (single or two transistors), flyback with a buck–boostconverter, resonant converters, etc. For string or multistringsystems, the inverters used to be single or dual-stage inverterswith an embedded HF transformer. However, new solutions tryto eliminate the transformer using multilevel topologies.A very common ac/dc topology is the half-bridge two-levelVSI, which can create two different voltage levels and requiresdouble dc-link voltage and double switching frequency in orderto obtain the same performance as the full bridge. In thisinverter, the switching frequency must be double the previousone in order to obtain the same size of the grid inductor. Avariant of this topology is the standard full-bridge three-levelVSI, which can create a sinusoidal grid current by applying thepositive/negative dc-link or zero voltage, to the grid plus gridinductor . This inverter can create three different voltagesacross the grid and inductor, the switching frequency of eachtransistor is reduced, and good power quality is ensured. Thevoltage across the grid and inductor is usually pulsewidthmodulated but hysteresis (bang-bang) current control can alsobe applied.Other multilevel topologies can be taken into account andin  cascade multilevel inverters are studied. Seven basicthree-level cells can be used to achieve fifteen levels in theoutput signals without using an output transformer. This isbeneficial for the power system and results in an improvementin the THD performance of the output signals. However,other problems such as commutation and conduction lossesappear .
94Future very efficient PV cells roofing PV systems PV modules in high building structures
95Future trendsPV systems without transformers - minimize the cost of the total systemcost reduction per inverter watt -make PV-generated power more attractiveAC modules implement MPPT for PV modules improving the total system efficiency„ plug and play systems”The increasing interest and steadily growing number ofinvestors in solar energy stimulated research that resulted inthe development of very efficient PV cells, leading to universalimplementations in isolated locations . Due to theimprovement of roofing PV systems, residential neighborhoodsare becoming a target of solar panels, and some current projectsinvolve installation and setup of PV modules in high buildingstructures .PV systems without transformers would be the most suitableoption in order to minimize the cost of the total system. On theother hand, the cost of the grid-connected inverter is becomingmore visible in the total system price. A cost reduction perinverter watt is, therefore, important to make PV-generatedpower more attractive. Therefore, it seems that centralizedconverters would be a good option for PV systems. However,1012 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006Fig. 13. Typical compensation system for renewable energy applications based on flywheel energy storage.problems associated with the centralized control appear, and itcan be difficult to use this type of systems.An increasing interest is being focused on ac modules thatimplement MPPT for PV modules improving the total systemefficiency. The future of this type of topologies is todevelop “plug and play systems” that are easy to install fornonexpert users. This means that new ac modules may seethe light in the future, and they would be the future trendin this type of technology. The inverters must guarantee thatthe PV module is operated at the maximum power point(MPP) owing to use MPPT control increasing the PV systemsefficiency. The operation around the MPP without toomuch fluctuation will reduce the ripple at the terminals of thePV module.Therefore,
96Research MPPT control THD improvements reduction of current or voltage ripplestandards are becoming more and more strictTherefore, the control topics such as improvements ofMPPT control, THD improvements, and reduction of currentor voltage ripples will be the focus of researchers in theyears to come . These topics have been deeply studiedduring the last years, but some improvements still can bedone using new topologies such as multilevel converters. Inparticular, multilevel cascade converters seem to be a goodsolution to increase the voltage in the converter in orderto eliminate the HF transformer. A possible drawback ofthis topology is control complexity and increased number ofsolid-state devices (transistors and diodes). It should be noticedthat the increase of commutation and conduction losseshas to be taken into account while selecting PWM or SVMalgorithms.Finally, it is important to remember that standards, regardingthe connection of PV systems to the grid, are actually becomingmore and more strict. Therefore, the future PV technology willhave to fulfil them, minimizing simultaneously the cost of thesystem as much as possible. In addition, the incorporation ofnew technologies, packaging techniques, control schemes, andan extensive testing regimen must be developed. Testing is notonly the part of each phase of development but also the part ofvalidation of the final product
98Energy Storage Systems Improvement of QualitySupport the Grid during InterruptionFlywheels – spinning mass energy(commercial application with active filters)In order to improve the quality of the generated power,as well as to support critical loads during mains’ power interruption,several energy-storage technologies have been investigated,developed, proved, and implemented in renewableenergy systems. However, flywheels are very commonly useddue to the simplicity of storing kinetic energy in a spinningmass. For approximately 20 years, it has been a primary technologyused to limit power interruptions in motor/generatorsets where steel wheels increase the rotating inertia providingshort power interruptions protection and smoothing of deliveredpower. One of the first commercial uses of flywheels in conjunctionwith active filtering to improve frequency distortionon a high-voltage power-system line is described in the lit.
99Flywheel-energy-storage low-speed flywheels (< 6000 r/min) with steel rotors and conventional bearingsmodern high-speed flywheel systems (to r/min) advanced composite wheels ultralow friction bearing assemblies, such as magnetic bearings
100Applications of flywheels Most applications of flywheels in the area of renewableenergy delivery are based on a typical configuration wherean electrical machine (i.e., high-speed synchronous machineor induction machine) drives a flywheel, and its electricalpart is connected to the grid via a back-to-back converter, asshown in Fig. 13. Such configuration requires an adequatecontrol strategy to improve power smoothing –. Thebasic operation could be summarized as follows. When thereis excess in the generated power with respect to the demandedpower, the difference is stored in the flywheel that is drivenby the electrical machine operating as a motor. On the otherhand, when a perturbation or a fluctuation in delivered poweris detected in the loads, the electrical machine is driven bythe flywheel and operates as a generator supplying neededextra energy. A typical control algorithm is a direct vectorcontrol with rotor-flux orientation and sensorless control usinga model-reference-adaptive-system (MRAS) observer.
101ResearchExperimental alternatives for wind farms =flywheel connected to the dc linkControl strategy = regulate the dc voltage against the input power surges/sags or sudden changes in the load demandSimilar approach applied to PV systems, wave energyD-static synchronous compensator (STATCOM)Frequency control using distributed flywheelsExperimental alternatives for wind farms include flywheelcompensation systems connected to the dc link, which are thesame as the systems used for power smoothing for a singleor a group of wind turbines . Usually, a control strategyis applied to regulate the dc voltage against the input powersurges/sags or sudden changes in the load demand. A similarconfiguration can be applied to solar cells . Anotherrenewable energy resource where power oscillations need tobe smoothed is wave energy. In , a D-static synchronouscompensator (STATCOM) is proposed, as an alternative to flywheels,to accomplish the output power smoothing on a waveenergyconverter where several operating conditions should betaken into account. Recent proposals on using flywheels toregulate the system frequency include the disposal of a matrixof several flywheels to compensate the difference between thenetwork’s load and the power generated .Recently, there has been research where integrated flywheelsystems can be encountered. Those systems use the same steelrotor of the electrical machine as energy-storage element .Two of the main advantages of a system like that are its highpowerdensity and its similarity with a standard electrical machine.It seems that a new trend for energy storage in renewableenergy systems is to combine several storing technologies (aswhat occurs in uninterruptible power system (UPS) application),where a storage system integrates compressed-air system,thermal storage unit, and flywheel energy storage .
102Hydrogen-storage systems Storabletransportable,highly versatileefficientclean energy carrierfuel cells to produce electricityThis section aims to analyze new trends in hydrogen-storagesystems for high-quality back-up power. The hydrogen-fueleconomy has been rapidly increasing in industrial applicationdue to the advantages of the hydrogen of being storable, transportable,highly versatile, efficient, and clean energy carrierto supplement or replace many of the current fuel options. Itcan be used in fuel cells to produce electricity in a versatileway, for example, in portable applications, stationary use ofenergy, transportation, or high-power generation. The use offuel cells in such applications is justified since they are a veryimportant alternative power source due to their well-known specificcharacteristics such as very low toxic emissions, low noiseand vibrations, modular design, high efficiency (especially withpartial load), easy installation, compatibility with a lot of typesof fuels, and low maintenance cost.
103Hydrogen technology Storage Technologies compressed or liquefied gas by using metal hydrides or carbon nanotubesTechnologiesHydrogen could be stored as compressed or liquefied gas or by using metal hydrides or carbon nanotubes . For a particularapplication, the choice of a storage technology implies atradeoff between the characteristics of available technologies interms of technical, economical, or environmental performance. Applications must also include a discussion of the lifecycleefficiency and cost of the proposed storage system. Thisanalysis should consider the total life of the proposed hydrogenstoragesystem including raw-material requirements, manufacturingand fabrication processes, integration of the system intothe vehicle or off-board configuration, useful service life, andremoval and disposal processes including recycling. Recently,research and development are focused on new materials or technologiesfor hydrogen storage: metal hydrides (reduce the volumetricand pressure requirements for storage, but they are morecomplex than other solutions), chemical hydrides, carbon-basedhydrogen-storage materials, compressed- and liquid-hydrogentanktechnologies, off-board hydrogen-storage systems (a typicalrefueling station will be delivering 200–1500 kg/dayof hydrogen), and new materials and approaches for storinghydrogen on board a vehicle. Applications to identify andinvestigate advanced concepts for material storage that have thepotential to achieve 2010 targets of 2 kWh/kg and 1.5 kWh/L.
104Compressed-Air Energy Storage -CAES Energy storage in compressed airGas turbinesEnergy storage in compressed air is made using a compressorthat stores it in an air reservoir (i.e., an aquifer like the ones usedfor natural-gas storage, natural caverns, or mechanically formedcaverns, etc.). When a grid is operating off peak, the compressorstores air in the air reservoir. During discharge at peak loads,the compressed air is released to a combustor where it is mixedwith oil or gas driving a gas turbine. Such systems are availablefor 100–300 MW and burn about one-third of the premium fuelof a conventional simple cycle combustion turbine.An alternative to CAES is the use of compressed air invessels (called CAS), which operates exactly in the same wayas CAES except that the air is stored in pressure vessels ratherthan underground reservoirs. Such difference makes possiblevariations consisting of the use of pneumatic motor acting ascompressors or driving a dc motor/generator according to theoperation required by the system, i.e., storing energy whenthere is no extra demand of energy or delivering extra power atpeak loads.Recent research is devoted to the maximum-efficiency pointtrackingcontrol  or integrated technologies for powersupplyapplications
105Supercapacitors 350 to 2700 F at of 2 V. modules 200 -to 400 V long life cyclesuitable for short discharge applications <100 kW.Supercapacitors, which are also known as ultracapacitors orelectric double layer capacitors (EDLC), are built up with modulesof single cells connected in series and packed with adjacentmodules connected in parallel. Single cells are available withcapacitance values from 350 to 2700 F and operate in the rangeof 2 V. The module voltage is usually in the range from 200to 400 V. They have a long life cycle and are suitable for shortdischarge applications and are less than 100 kW. New trendsfocused on using ultracapacitors to cover temporary high peakpowerdemands , integration with other energy-storagetechnologies, and development of high-voltage applications.
106Superconducting Magnetic Energy Storage (SMES) energy in a magnetic field without resistive lossesability to release large quantities of power during a fraction of a cycleIn an SMES, a coil of superconducting wire stores electricalenergy in a magnetic field without resistive losses. Also, there isno need for conversion between chemical or mechanical formsof energy.Recent systems are based on both general configurations ofthe coil: solenoidal or toroidal. The second topology has aminimal external magnetic field but the cost of superconductorand coil components is higher than the first topology. Suchdevices require cryogenic refrigerators (to operate in liquidhelium at −269 ◦C) besides the solid-state power electronics.The system operates by injecting a dc current into the superconductingcoil, which stores the energy in magnetic field.When a load must be fed, the current is generated using the energystored in the magnetic field. One of the major advantagesof SMES is the ability to release large quantities of power duringa fraction of a cycle. Typical applications of SMES are correctionsof voltage sags and dips at industrial facilities (1-MWunits) and stabilization of ring networks (2-MW units).New trends in SMES are related to the use of low temperaturesuperconductors (liquid-nitrogen temperature), theuse of secondary batteries, and the integration of STATCOM and several topologies of ac–dc–ac converters withSMES .
107Battery Storage Several types of batteries Discharge rate limited by chemistryBattery StorageThe use of batteries as a system to interchange energy withthe grid is well known. There are several types of batteries usedin renewable energy systems: lead acid, lithium, and nickel.Batteries provide a rapid response for either charge or discharge,although the discharge rate is limited by the chemicalreactions and the type of battery. They act as a constant voltagesource in the power systems. New trends in the use of batteriesfor renewable energy systems focused on the integration withseveral energy sources (wind energy, PV systems, etc.) andalso on the integration with other energy-storage systems complementingthem. Also, there are attempts to optimize batterycells in order to reduce maintenance and to increment its lifetime
108Pumped-Hydroelectric Storage (PHS) variable-speed drivesMW, efficiencies around 75%.Pumped-Hydroelectric Storage (PHS)As batteries, PHS is a mature technology where a swamp ofwater stored at a certain high elevation is used to generate electricenergy by hydroturbines, whenever there is an additionalpower demand in the grid. When no extra generation is needed,the water is pumped back up to recharge the upper reservoir.One limitation of PHS is that they require significant land areaswith suitable topography. There are units with sizes from 30 to350 MW, with efficiencies around 75%.New trends in PHS are focused on the integration withvariable-speed drives (cycloconverters driven doubly fed inductionmachine)  and the use of underground PHS (UPHS),where the lower reservoir is excavated from subterranean rock.Such a system is more flexible and more efficient but requires ahigher capital cost.
109Conclusionspower-electronic technology plays a very important role in the integration of renewable energy sourcesoptimize the energy conversion and transmissioncontrol reactive powerminimize harmonic distortionto achieve at a low cost a high efficiency over a wide power rangeThe new power-electronic technology plays a very importantrole in the integration of renewable energy sources into the grid.It should be possible to develop the power-electronic interfacefor the highest projected turbine rating, to optimize the energyconversion and transmission and control reactive power, tominimize harmonic distortion, to achieve at a low cost a highefficiency over a wide power range, and to have a high reliabilityand tolerance to the failure of a subsystem component.In this paper, the common and future trends for renewableenergy systems have been described. As a current energysource, wind energy is the most advanced technology due toits installed power and the recent improvements of the powerelectronics and control. In addition, the applicable regulationsfavor the increasing number of wind farms due to the attractiveeconomical reliability. On the other hand, the trend of the PVenergy leads to consider that it will be an interesting alternativein the near future when the current problems and disadvantagesof this technology (high cost and low efficiency) aresolved. Finally, for the energy-storage systems (flywheels, hydrogen,compressed air, supercapacitors, superconducting magnetic,and pumped hydroelectric), the future presents severalfronts, and actually, they are in the same development level.These systems are nowadays being studied, and only researchprojects have been developed focusing on the achievement ofmature technologies.
110Conclusions Achieve a high reliability tolerance to the failure of a subsystem component.common and future trends for renewable energy systems have been described.Wind energy is the most advanced technologyRegulations favor the increasing number of wind farms.The trend of the PV energy leads to consider that it will be an interesting alternative in the near futureThe new power-electronic technology plays a very importantrole in the integration of renewable energy sources into the grid.It should be possible to develop the power-electronic interfacefor the highest projected turbine rating, to optimize the energyconversion and transmission and control reactive power, tominimize harmonic distortion, to achieve at a low cost a highefficiency over a wide power range, and to have a high reliabilityand tolerance to the failure of a subsystem component.In this paper, the common and future trends for renewableenergy systems have been described. As a current energysource, wind energy is the most advanced technology due toits installed power and the recent improvements of the powerelectronics and control. In addition, the applicable regulationsfavor the increasing number of wind farms due to the attractiveeconomical reliability. On the other hand, the trend of the PVenergy leads to consider that it will be an interesting alternativein the near future when the current problems and disadvantagesof this technology (high cost and low efficiency) aresolved. Finally, for the energy-storage systems (flywheels, hydrogen,compressed air, supercapacitors, superconducting magnetic,and pumped hydroelectric), the future presents severalfronts, and actually, they are in the same development level.These systems are nowadays being studied, and only researchprojects have been developed focusing on the achievement ofmature technologies.