advanced power converter for universal and flexible power

8/8/2019 Advanced Power Converter for Universal and Flexible Power

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ADVANCED POWER CONVERTER FORUNIVERSAL AND FLEXIBLE POWERMANAGEMENT IN FUTURE ELECTRICITY

NETWORK

S.PRABAKARAN

M.N.SATHYANARAYANA

FINAL YEAR-EEE

SHRI ANDAL ALAGAR COLLEGE OF ENGINEERING

ABSTRACT

More green power provided byDistributed Generation will enter into theelectricity network in the near future. Inorder to control the power flow and toensure proper and secure operation ofthis future grid, with an increased levelof the renewable power, new power

electronic converters for grid connectionof renewable sources will be needed.These power converters must be able toprovide intelligent power managementas well as ancillary services. This paper presents the overall structure and thecontrol aspects of an advanced powerconverter for universal and flexible power management in the futureelectricity network.

INTRODUCTIONThe establishment of a new paradigm forelectricity networks in Europe in whichthere is large-scale integration ofdistributed energy resources is majorelement of the key strategic objective ofthe EU to secure a supply of energy thatis clean sustainable and economical [1]. The impact of this will be to reducedependence on fossil fuels and reduceclimate change and pollution, which are

of concern to all European citizens. In thenew system, large numbers of small andmedium sized generators and energystorage elements are interconnectedthrough a fully interactive intelligentelectricity network. In order to reach thisgoal, the entire architecture of theelectricity network must be redesigned

and the information and communicationtechnologies (ICT) will be the key factor[2]. New features added by ICT and ICT-based applications such as universaconnectivity, services over internet andweb, distributed intelligence, advancedfault handling, intelligent load sheddingetc will transform the existing electricagrid into a smart one. Among thedifferent architectures of the futureelectricity

systems three conceptual models are ofinterest [1]: Micro-grids, an Internetmodel and Active Networks supported byICT. A possible layout of such an activenetwork is shown in Fig. 1. DistributedGeneration (DG) will increase the numberof power input nodes and will provide abidirectional power flow through thenetwork.


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New power electronics systems willcontrol this power flow and also willprovide flexible DG interfaces to thenetwork. The work presented in thispaper is part of a research project(UNIFLEX-PM) supported by the EuropeanCommunity under the 6th FrameworkProgram with focus on the development

of key enabling technologies to reach theDG objectives. The main objectives inthis project are to research andexperimentally verify new, modularpower conversion architecture foruniversal application in the FutureEuropean Electricity Network.

Fig. 1. Possible architecture of an activenetwork with distributed/on-sitegeneration and fully integrated networkmanagement.The target is to establish technology thatprovides lower cost power electronics tonetwork architects and owners andfacilitates the connection of a broaderrange of distributed generation sources[3].First this paper will make a review of theexisting European grid codes forrenewable systems with focus on MV andHV levels. Then, the general structure ofthe UNIFLEX-PM system as well as thecontrol issues related with its subsystemswill be presented.

INTERCONNECTION REQUIREMENTSFOR RENEWABLE ENERGY SOURCESFew European countries have at thismoment dedicated grid codes addressed

to interconnection requirements ofRenewable Energy Sources (RES) and inmost of the cases these requirementsreflects the penetration of renewablesources into the electrical network or afuture development is prepared withthese demands. Among all kind ofrenewable sources only wind power and

PV installations have specificrequirements. Recently, specificinterconnection requirements forCombined Heat and Power plants withpower ratings of 1.5 MW or more wereissued in Denmark [4] with the maintarget in increasing the contropossibilities of these small units from thesystem operator point of view. Therenewable sources in the Great Britainstransmission grid code can be identified

under different names e.g. DCConverters, Power Park Modules, Nonsynchronous generating units, etc. Theinterconnection requirements in this caseare addressed to power control abilityvoltage quality and fault ride-throughcapabilities [5]. However, for connectionof embedded generators below a certainpower level e.g. 30 MW the DistributionSystem Operators (DSO) in particularegions shall be contacted .DifferentDSOs exist in Germany and the generarules for interconnection at the DS leveare defined in [6]. It can be noticed thatthere are no specific requirements forrenewable energy sources. Moreover inGermany the generating units in theMV/LV distribution systems are usuallynot utilized for the provision of systemservices [6].A document detailing the minimainterconnection requirements for windturbines has been published officially inOctober 2006 in Spain [7]. This documentaddressed just two topics namely faultride-through capabilities and reactivepower/voltage control during the faultand it applies to all operators connectedto the main transmission grid. Howeveraccording with [13] REE is consideringincluding wind plant connected at thedistribution system level. On the othe


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hand in Spain the PV systems haveinterconnection requirements forvoltages up to 1kV defined in [8].However in this case the PV systemsshall not provide system services.Special interconnection requirements forwind power addressed specifically atboth MV and HV levels are issued in

Denmark [9], [10] and Ireland [11], [12].Examining these various grid codes it canbe noticed that the requirements forwind power cover a wide range ofvoltage levels from medium voltage tovery high voltage while the connectiondemands for PV installations have focusonly on the low voltage level particularlyin low voltage distribution networks(domestic applications). Therequirements are also different for these

two renewable systems. PV systems havespecific demands regarding powerquality and safety and protectionfunctions e.g. response to abnormalutility conditions, direct current injectionand grounding. On the other hand, thegrid codes for wind power address issuesthat make the wind farms operate as aconventional power plant into theelectrical network. These requirementshave focus on power controllability,power quality, fault ride-throughcapability and grid support duringnetwork disturbances. According toseveral references e.g. [13] in some ofthe cases these requirements areonerous.All considered grid codes require faultride-throughcapabilities for wind turbines. Voltageprofiles are given specifying the depth ofthe voltage dip and the clearance time aswell [3]. However, in some of the gridcodes the calculation of the voltageduring all types of unsymmetrical faultsis very well defined e.g. Ireland, whileothers do not define clearly thisprocedure. Germany and Spain requiresgrid support during faults by reactivecurrent injection up to 100% from therated current [7]. This demand is difficultto be achieved by some of the wind

turbine concepts e.g. active stall windturbine with directly grid connectedsquirrel-cage induction generatorHowever, the Doubly Fed InductionGenerator-based wind turbines will injectjust the rated current of the partial scalepower converter placed in the rotorcircuit, which is 20%-

30% from the generators rated powerMoving to the wind farm level any windturbine concept can meet thisrequirement if the wind farm isconnected through a Voltage SourceConverter based DC-Link TransmissionSystem to the electrical network. In orderto cope with the network stability issuesall grid codes require the curtailment ofthe produced power and other controsignals available for System Operators

(SO) [3]. However, the ability ofcontrolling the produced power based onthe SO demands is currently providedonly by large wind farms. In future amajor challenge will be to make thisfeature available also to small units. Anincreased level of renewable power intothe electrical network will determineinterconnection requirements for alrenewable sources similar with those forwind power. Therefore, any renewablesystem will have to fulfill the grid codeswithout major changes in the controalgorithms as well as in the hardwarestructure. Thus, a universal and flexiblepower converter is the key element ingridintegration of the renewable sources. The system comprises of a renewableenergy sourceconnected to an AC/DC power converterto a common DC bus-bar. Since the onlyrenewable source able to supportbidirectional power flow is the PEM fuecell, this power converter must be able tosupport just a unidirectional power flowA storage system is also connected tothe DC bus-bar. The structure of thepower converter in this case depends onthe type of the storage element. It canbe an AC to DC conversion stage or just aDC to DC one. The grid connection is


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made through a power converter whichcan also have a modular approach.

UNIFLEX-PM SYSTEM The main target of the UNIFLEX-PMsystem is to provide a universal andflexible power electronic interface forgrid connection of RES including storage

facilities. A possible structure of theUNIFLEX-PM system is shown in Fig. 2.

The power rating of the system is around5 MW (e.g. 1 MW/module) with a ratedvoltage in the PCC of 10-20 kV. Thisvoltage level is very common in most ofthe distribution systems in the Europeancountries. However, using a transformerwith high voltage on the primary side a

connection to the transmission system ispossible.

Renewable Source SystemBasically any renewable source can beused in this system. However, thestructure of the generator side converteras well as its control will be determinedby this source. In wind applications thepower converter and its control aredetermined by the generator type. The

general structure of the UNIFLEX-PMsystem can be used in a full scale powerconverter based wind turbine. Therefore,mainly three generators types can beused in this case namely synchronousgenerator with field winding, permanentmagnet generator and squirrel cageinduction generator. The synchronousgenerator will require a diode bridgerectifier and an additional fully controlled

power converter for the field windingThe permanent magnet generator is usedin direct driven wind turbines and twoconversion stages are required. First anAC/DC power stage based onvoltagesource power converter will assure thevariable speed operation then a DC/DCpower stage will keep the DC-link voltage

fixed on the common DC-bus. Finally, thesquirrel-cage induction generator is usinga fully controlled power converterwithout any additional power stages. Ineach case generators with multiple statorwindings can be used and thus theefficiency is increased at low powerproduction. In most of the biomassCombined Heat and Power plants anelectrical system similar with that fromwind turbines can be used. A PV source

will require a DC to DC conversion stageas well as a fuel cell system.In all cases the control of the generatorside converter has as main goal tooptimize the maximum power extractionand to assure the optimum operationpoint of the generator.

Energy Storage SystemEnergy Storage Systems are in acontinuous development and newimprovements in cost, efficiency, controalgorithms, etc. is added constantlyEach technology has advantages anddrawbacks and choosing a particulatechnology implies several factors e.grenewable energy source and thecorresponding power conversiontopology; grid connection requirementsoverall cost of generation and storageenvironmental and social aspects. Somestorage technologies are constrained byenvironmental or safety considerationsSolenoidal configurations forSuperconducting Magnetic EnergyStorage produces external magnetic fieldand an exclusion zone around the units isrequired. Lead acid batteries requireplanning of handling and transport of theelectrolytewhile Compressed Air Energy Storagerequires a suitable site for the reservoir


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Thus, the energy storage technologymust be chosen in agreement with therenewable energy source and basicallythe following issues must be taken intoaccount:

Functionality of the energy storagesystem versus

time response. In order to compensatefor loadlevelling policies a medium termstorage capacity is needed while powerquality issues will require a very shortterm storage capacity. Area or volume of the storagesystem compared with RES.

Some of the storage technologiesrequire a relative large area or volumetherefore the selection ofthe storage technology must take this

aspect intoaccount. Decoupling of the storage capacityand power rating.Some storage technologies e.g.advanced batteries like REDOX can offerthis feature and can make the entiresystem much more flexible. Relatively long life time ca. 20 yearswhich is typical for example for most ofthe wind turbines. Low Maintenance.The structure of the storage side powerconverter is related to the type of thestorage technology used. This powerconverter shall be able to support abidirectional power flow. Its control hasfocus on optimizing thestorage/extraction of energy in thestorage element.

Grid Interface The grid side converter is the keyelement in respect with theinterconnection of the UNIFLEX-PMsystem to the electrical network. It shallbe able to transfer the renewable powerto the grid, to meet the grid coderequirements and to keep the systemconnected over a wide range ofoperating conditions e.g. faults, islandoperation, etc. Moreover, ancillary

services such as voltage control andblack-start capability must be provided. Aparalleled structure will improve theoverall efficiency of the system and wilalso improve its redundancy. Voltage andcurrent measurement for each phase inthe Point of Common Coupling (PCC)must be available for the converter

control. A hardware protection for powerswitches e.g. high-speed fuses as well asoftware protection can be used. Thecontrol algorithm of the grid interfacemust be able to handle unsymmetricafaults, to inject full reactive currentduring grid faults and to operate both ingrid connected- and islanding mode. Thestructure of this control shall includeadvanced Phase Locked Loop structuresin order to cope with unsymmetrical and

unbalanced voltages, fast current controin each phase and advanced controtechniques for active/reactive power e.ginjection of negative sequence of thevoltage The steady state performancesare given mainly by the switchingfrequency and flicker content of theproduced power. Voltage qualityparameters e.g. harmonic compatibilitylevels, Total Harmonic Distortion, etcshall meet the requirements from EN61000-2-4 standard. Flicker emission canbe improved by using the storagesystem.

Overall Control and EnergyManagement The entire control of the UNIFLEX-PMsystem comprises basically two levels ofcontrol namely the control associatedwith each subsystem and the overalcontrol including the energymanagement. The overall control shalinclude the several modules/functionsPower/frequency control includingprimary control shall provide thebalancing of the instantaneous poweconsumption and the production in thewhole area as well as the secondarycontrol for balancing the powerproduction and demand within the


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regional zones. The power productionshall follow the power setpoint imposedby the Distribution System Operator(DSO) and Transmission System Operator(TSO). Voltage control is controlling thevoltage in the whole area. This controlshall follow the system operatordemands for reactive power. Moreover

the control must be able to regulate thevoltage in islanding mode. Advanced gridmonitoring techniques e.g. detection ofislanding and estimation of the distanceto fault must be included. The detectionof islanding is used to change thepriorities in controlling the frequency andthe voltage in the PCC while theestimation of the distance to fault can beused to change the control strategyduring faults. For example when a fault is

detected near the PCC the grid interfacecannot provide grid support while thedistance to the fault is relatively largethe grid interface can inject 100%reactive current. Short term and mediumterm estimation of the power productionbased on meteorological data e.g. windprediction, solar irradiation, etc. can beused in spot market as well as by theDSO/TSO for balancing the system.Energy management with the maintarget in optimizing the power productionand energy storage based on actual andestimated power production, gridconditions and system operator demandsshall also be included. Optimizing theoperation of the paralleled powermodules based on the actual productioncan be another function provided by thismodule. Circuit breakers must be usedfor overall protection of the system.Communication protocols and dataexchange with the system operatorsmust be provided as well as voltage andcurrent measurement in the PCC.

CONCLUSIONSIn order to facilitate a high penetration ofDG in the future European electricitynetwork advanced power electronicconverters are needed. These convertersshall be able to provide an intelligent

management of the renewable sourcesincluding storage technologies as well assystem services. Currently, all existingsolutions are addressed to particulatechnologies and a universal and flexiblepower converter is needed. This papepresents the structure and the relatedcontrol of such a power converter. This

power converter has a modulararchitecture based on Medium Frequencytransformer isolation modulesincorporating advanced magnetic andinsulating materials. This modulaapproach leads to high reliability and lowcost. Connection at different voltagelevels and powers is made possible byseries/parallel connection of modules This power conversion system caconnect different sources and/or loads

including energy storage with differentcharacteristics and power flowrequirements. Advanced controstrategies to control the local converterenergy storage and energy flow togetherwith global control to manage theinteraction with the grid/loads/storageelements are also included.

REFERENCES[1]. EC New ERA for electricity inEurope. Distributed Generation: KeyIssues, Challenges andProposedSolutions, EUR 20901, 2003ISBN 92-894-6262-0[2]. EC Towards Smart Power NetworksLessonslearned from European research FP5projects, EUR21970, 2005, ISBN 92-79-00554-5;[3]. UNIFLEX-PM Advanced powerconverters foruniversal and flexible powermanagement in futureelectricity network Converterapplications in future European electricitynetwork. Deliverable D2.1, EC Contractno. 019794(SES6), December 2006, p171;


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[4]. EnergiNet Technical regulations ofthermal power station units of 1.5 MW orabove, Regulation TF 3.2.3, July 2006, p.77;[5]. National Grid Electricity Transmissionplc The grid code, Issue 3, Revision 17,September 2006;[6]. VDN Distribution Code 2003. Rules

on access to distribution networks,August 2003[7]. REE Requisitos de respuesta frentea huecos de tension de las instalacionesde produccion de regimen especial, PO12.3, October 2006;[8]. Real Decreto 1663/2000, 29September 2000 -conexin de instalaciones fotovoltaicas ala red debaja tensin;

[9]. EnergiNet Grid connection of windturbines to networks with voltages below100 kV, Regulation TF 3.2.6, May 2004,p. 29;[10]. Energinet - Grid connection of windturbines to networks with voltages above100 kV, Regulation TF 3.2.5, December2004, p. 25;[11]. ESB Networks Distribution Code,version 1.4, February 2005;[12]. CER Wind Farm Transmission GridCodeProvisions, July 2004;[13]. *** - Going mainstream at the gridface. Examining grid codes for wind,Windpower Monthly, September

advanced power converter for universal and flexible power

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