Eff'Ects of Small Embedded Generation on Power Quality

7/28/2019 Eff'Ects of Small Embedded Generation on Power Quality

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EFF' ECTSOF SMALL EM BEDDED GENERATION ON POWER QUALITYIEE Colloquiumon "Issues in Power Quality"

Warwick - 28th November 1995N Jenkins and G Strbac

Manchester Centre for Electrical Energy, UMIST, Manchester, UKAbstractThi s short paper considers the impact of embedded generators on the power quality ofdistribution networks. The various types of embedded generation are discussed and theirimpact on the network reviewed. The potential for improvement of network power qualityusing embedded generators is addressed and the barriers to implementation brieflyconsidered.IntroductionIn the early days of public electricity supply small local generators were used to supply localloads. However, for the last forty years the electricity supply system in the UK has beendeveloped using large central generating stations transmitting power to loads via the NationalGrid; often over considerable distances. There are now indications that, in somecircUmstances, the use of small-scale local generation embedded in distribution systems maybe becoming attractive once again.A number of reasons for this resurgence in interest in embedded generation may beidentified. Technical advances in small reciprocating engines and gas turbines haveencouraged the development of small-scale combined heat and power (CHP) plant.Considerable research effort is being expended in the investigation of fuel-cells; oneapplicationof which is small-scale CHP. Techniques to extract and utilise land-fill gas havebeen developed and the generationof electricity from waste, using a variety of technologies,is now well established. Wind turbine and photovoltaic technologies have made rapidprogress over the last ten years to allow the exploitation of the distributed renewable energyresource..

These technical advances have been accompanied by institutional and administrative changeswhich are probably even more significant for the development of embedded generation. TheUK Governmenthas set a target of 5000 MW of CHP and 1500 MW of new renewableschemes to be in service by the year 2000 but a number of the CHP schemes will be toolarge to be considered as embedded. The support mechanism for renewable energy, theNFFO (Non Fossil Fuel Obligation) scheme, appears to be working effectively although therehave been some recent reports to indicate that the rate of installation of CHP schemeshasdropped due to adverse changes in the commercial environment.Thereisalso a growing awareness that real and reactive power flows through the distributionsystem need not be unidirectional, ie from the high voltage networks towards the customer,

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but canbe bidirei=tionaldepending onthelevels of embedded generation and local loads.Thischange is illustrated in Figures 1 and 2. Bidirectional power flow poses obvious technicalchallenges, as most distribution networks were not configure4 for significant penetrations ofembedded generation, but also raises comercial difficulties. The commercial framework ofdistribution system charges was developed in an environment of unidirectional power flowsand rather simple passive loads and m y not prove adequate for embedded generationschemes, especially those which havean abilitytoboth improve or reduce the power qualityof the network El].Efects of Embedded Generationon Power Q d t yEmbedded generation may havea very considemble effecton the quality of power receivedby customers. The impact depends on the relative sizeof the generator with respect to thenetwork fault level and X/R ratio, and the design and characteristics of the generator andprime mover installed. The effects may include:

Steacky state voltage excursionsTransient voltage variations (flicker)Phase voltage unbalanceVoltage waveform distortion (harmonics)Engine driven schemes, eg CHP or land-fill gas, can use either synchronous or inductiongenerators. Clearly the frequency of the dstribution system is fixed by that of the NationalGrid but the generators and customer loads will have a considerable act on the localvoltage. A synchronous machine can be configured to attempt to contrlocal busbar but only at the costof reactive power flows, and hence losses, in the network.Thisproblem is compounded if there are multiple generatorsona circuit. This is illustratedin Figure 3. Multiple operation of synchronous generatorsona netwoa new problem. However, in this case there are a number of complicatgenerators may beownedand operated by organisations without a directquality of the distribution network. The real power output of thedetermined by the demand for heat or electricity of the premises in whiand not by conditionson the distribution network. At present the mais the connection agreement which cannoteasily be modified and, for small gesort of overall real time control of reactive power and hence voltage isprohibitively expensive. In general the owner of the generator will be charged for reactivepower and lasses which are incurred in the distribution network but receive no benefit forvoltage control. Thus it is common for the excitation system of the synchronouto be arranged to give operationeither at a fixed, high, power factor or atreactive power flow.Induction generators have no directmeansof controlling reactive power flow but may beequipped with local power factor correction capacitors to reduce the demand for reactivepower [4]. Decause of the contra-flow of real and reactive power the voltage change at thegenerator busbar is determined by theX/R ratio of the system andsocareful choice of powerfactor correction can result in small voltage changes but, again, at the expense of reactivepower flows. Induction generators will draw a large energising current on connecticanbecontrolled very effectively using anti-parallel thyristor soft-startequipmentgenerators have a low negative phase sequence impedan~. herefore, in the presence of



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network voltage unbalance they will draw negative phase sequence current and so reduce thevoltage unbalance, Figure5.Some forms of renewable energy schemes, eg wind turbines, use induction generators toprovide damping in response to a varying torque input. The torque input to a wind turbinegenerator varies with the site wind conditions but also has a cyclic component at thefrequency at which the blades pass in front of the tower. These torque variations result incyclic real and reactive power flows which then may cause voltage variations on thedistribution system. There is some evidence to suggest that, under certain conditions, therotors of some wind turbines fall into synchronism for short periods and so lead to largerpower flows than would beexpected. However, it should be emphasised that no practicalproblems have been reported due to thi s effect.There is emerging use of power electronic convertors to connect embedded generators to thenetwork. The use of thyristor based naturally commutating convertors leads to the wellhown problems of low power factor and the injection of large low order harmonic currents.T hi s technology is now being superseded by the use of IGBT or GTO based network sideinvertors. These can be operated at any desired power factor and, if a suitable switchingfrequency is chosen, do not produce low order harmonics. In principle they can be used asactive filters and so improve the system power quality. They may also be used to providesupport to the network under abnormal conditions and ameliorate the effect of voltage sags.Although embedded generation using advanced power electronic network side convertorshasthe potential to improve distribution system power quality, at thi s time, it is not clear howaction of thi s kind may be rewarded and hence encouraged.ConclusionsEmbedded generation is now emerging as a significant sourceof electricity in theUK as wellas in a number of European countries. Thi s development has been stimulated by bothtechnical advances and changes in the commercial and institutional environment. In a numberof technologies proven equipment is now available and it is the non-technical issues whichwill determine the rate at which embedded generation develops. Embedded generation doeshave an impact on power quality and, under the present arrangements, the operators of suchplant have obligations to remain within particular limits eg voltage and harmonics. However,thi s ignores the possible improvements which embedded generators can make to the powerquality of distribution networks by their ability to control dynamically both real and reactivepower injections. Revised institutional and technical approaches are required if the operatorsof embedded generation are to be encouraged to improve actively the power quality ofdistribution networks.

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References

1) Strbac G and Jenkins N, "Wind energy in a competitive supply environment", paperpresented at the European Wind Energy Association Conference on Economics of WindEnergy, Helsinki, 5th-7th September 1995.2) J enki ns N and Vaudin A, "The electrical systems of five wind farms", IMechEProceedings A, V ol209, August 1995.



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Eff'Ects of Small Embedded Generation on Power Quality

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