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  1. 1. K.R.S.Kashyap, B.Dheeraj Reddy / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.247-251247 | P a g eAncillary Services And Stability Analysis Of DistributedGeneration SystemK.R.S.Kashyap, B.Dheeraj ReddySchool of Electrical Engineering, Vellore Institute of Technology, Vellore, IndiaSchool of Electrical Engineering, Vellore Institute of Technology, Vellore, IndiaABSTRACTThis paper considers a distributedgeneration system interconnected to the AC gridthrough power electronic interfaces and toprovide energy service and system ancillaryservices (in particular voltage regulation andpartial compensation or elimination of somepower quality disturbances, such as waveformdistortion etc.,). In addition to the above, theimpact of Wind Energy Systems (WES) on DGnetwork and the stability of the whole systemwas analyzed. This method is proposed toimprove the power quality and stability of DGsystem. The experimental system was establishedusing MATLAB software and their results arepresented and discussed.Keywords-Ancillary services, Distributed EnergyResources (DER), Distributed Generation (DG),FACTS(Flexible AC Transmission Systems), MicroGrid, Stability.1.0 INTRODUCTIONElectricity networks are in the era of majortransition from stable passive distribution networkswith unidirectional electricity transportation toactive distribution with bidirectional network [1].The extent of penetration of DG systems with thegrid is increasing rapidly and thus its role is gainingprominence in electrical industry. The majoradvantage of DG systems is the inclusion ofrenewable/non-conventional energy sources and itsintegration into low voltage distribution networks.DG technologies include Solar Photo-Voltaic, windturbines, fuel cells, and micro turbines which reducethe emissions when compared with conventionalplants, conserves natural resources and also improvepower quality. It can supply power to local loadsthus reducing the investment of conventional powerstations also reduce the peak demand forecasted.With proper control over the distributed generationunits, voltage sags are negated, reactive power flowis controlled. Thus DG units with control measuresfor parameters are termed as Micro Grid (MG).From the reliability point of view, micro sourcesmust be equipped with Power Electronic Interfaces(PEI) and latest sophisticated controls so as toprovide the required flexibility to ensure operationas single aggregated system and to maintain qualityof power within the permissible limits and energyoutput [1]. Ancillary services are necessary tosupport necessary to support the transmission ofelectric power to maintain reliable operations of theinterconnected systems. Unlike conventional powerstations, micro grids are located close to the load,thus they are more suitable for providing ancillaryservices.The impact of Distributed Generation(DG) units on the voltage stability, power qualityand reactive power margins have becomesignificant. It is well known fact that Reactivepower is very difficult to transport. At highloadings, relative losses of reactive power ontransmission lines are often significantly greaterthan relative real power losses. Reactive powerconsumption or losses can increase significantlywith the distance of power to be transported. Themajor advantage of DG is that they provide reactivepower capability locally. In fact, DG has betterregulation than Flexible AC Transmission Systems(FACTS) devices [2] and the same has beendiscussed later in this paper. Voltage stability is theability of a power system to maintain steady statevoltage in the system under normal operatingcondition and also after the occurrence of adisturbance in the electrical power network. Themain factor contributing to voltage instability isusually the voltage drops that limit the capacity oftransmission networks to transfer power betweenbuses. Insufficient reactive power margin to supplythe load is a major cause for voltage instabilityproblems. A voltage decrease may lead to increasein power consumption and causes more drops involtage and finally a collapse due to this cumulativeeffect. The desired voltages can be obtained eitherby directly controlling the voltage or by controllingthe reactive power flow which in turn will affect thevoltage drop.The advantages offered by DG can beobserved in the case of interconnected grids wherea fault arising in the synchronizing network (thenetwork connecting a generating station to the grid)might affect the whole system. To overcome thisproblem, the synchronizing network should bemade complicated that a fault in the network willhave no affect on the grid.
  2. 2. K.R.S.Kashyap, B.Dheeraj Reddy / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.247-251248 | P a g e2.0 SYSTEM AND ITS CONTROLMicro Grid can operate either in grid connectedmode or in islanded connected mode. This studydeals with grid connected mode of operation. Maincomponents of Micro grid include distributedenergy resources, power electronic interface,controls and battery system for energy balance.From the grid point of view, the main advantage ofmicro grid is that it is treated entity within thepower system. This ascertains its easycontrollability and in compliance with grid ruleswithout hampering reliability and security of powersystem utility. The structure of the micro grid isshown in figure 1.3.0 SYSTEM DESCRIPTIONDG system is assumed to be a combinationof AC and DC sources. The AC system is convertedto DC using active power factor correctiontechnique (boost type PWM). DC-DC converter isused for voltage regulation. Boost converter is usedsince electromagnetic interference (EMI) is less andso reduced filtering requirements. The outputcurrent is reduced. Seven level inverter is used. Byusing seven level inverter, the output waveform isimproved and thus reducing its harmonic content.To obtain sinusoidal wave, LC filter is used. Thecontrol system is designed to reduce the time lagand surges thereby improving the communicationbetween DG systems for optimal operation.3.1. Voltage ControlIn this study DG system comprises of DCand AC systems. The AC distributed energyresources are converted to DC. However, powerconverters draw pulsating input current from theline, this not only reduces the power factor but alsoinjects significant amount of harmonics. There areharmonic norms such as IEC 1000-3-2 [3]introduced for improving power quality. Activepower factor correction (APC) method is used.Boost converter is used since EMI is less soreduced filtering requirements. The DC- DCconverter is used to regulate the voltage obtainedfrom DG system. Though traditionally capacitorbanks and voltage regulators are used, the reactivepower supplied from capacitor banks drops assquare of voltage [4]. This may lead to voltagecollapse. But, a micro grid can perform smoothvoltage regulation locally in response to controllersettings. PWM techniques are used to increase theefficiency and control in voltage regulation.Reference voltage is obtained from the requiredload voltage and accordingly pulses are generatedand control over voltage profile is obtained. Itspower factor control also, in effect, results involtage control. Thus the deployment of capacitorscan be avoided at the consumer end if the power isprovided by DG unit.3.2. Frequency and Reactive Power ControlThe voltage obtained is now fed to voltagesource inverter. Technical recommendations likeIEEE 1547 prescribe that DER’s should beautomatically disconnected from MV and LVdistribution network in case of tripping of circuitbreaker (CB). This is mandatory feature of inverter.Here, we use seven level inverter. By using sevenlevel inverter, improving the output waveform thusreducing its harmonic content and hence the size offiltering requirements and electromagneticinterference (EMI) will be reduced [5]. Outputcurrent with better harmonic profile, less stress onelectronic components owing to reduced voltage,switching losses that are less than conventional twolevel inverters, all of which make them lighter,cheaper and compact. The output of inverter used isshown in figure 2.
  3. 3. K.R.S.Kashyap, B.Dheeraj Reddy / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.247-251249 | P a g eFig.2. Inverter OutputTo maximize the efficiency of power electronicconverters, pulse width modulation is used. UsingPWM technique and suitable topology, the outputhas been modified as shown in figure 3.Fig.3. Inverter output using PWMThe maximum permissible value is limitedto 10MVA as per IEEE recommendations. Supplyof reactive powers from micro sources in responseto local voltage signals is also more economicduring distribution voltage sags. There are manyalgorithms used to control reactive power, directpower control (DPC) method [6] is regularly used.Moreover, local supplies of real and reactive powersignificantly reduce feeder losses. Since reactivepower must be provided closer to the loads Microgrid is reliable source for reactive power supplythan FACTS devices. The frequency can also becontrolled using signals given to power electroniccomponents used in inverter. Thus, reactive powerand frequency can be maintained according to theload requirements. Micro grid with large numbermicro sources can suffer from reactive poweroscillations without proper voltage control.Therefore these control systems must communicateamong themselves for reliable operation.4.0 LOAD FLOW ANALYSISThe main purpose of installing DG will beobserved in islanding mode, where DG alone isresponsible for supplying the entire load. With theload isolated from the grid the DG has to take careof the complete load. But the installed capacity ofthe DG will not be sufficient to supply the entireload. In that case, important and sensitive loads areselected and are supplied power through DG andthe remaining loads are disconnected from thesystem. The voltage profile has to be maintainedsame as that of the grid while supplying the load.Every DG unit has a voltage limit up to which itcan maintain good stability but beyond certain loadthe stability is reduced and the continuity in thepower supply is lost. To ensure continuity, asuitable load flow analysis algorithm is required tocalculate the system voltage and stability. Thisanalysis would help in calculating the systemvoltage and power and determine the maximumvoltage that the DG or grid could withstand withoutcollapsing.5.0 MATHEMATICAL MODEL OFSTABILITY INDEXFig.4. Single Line Diagram of the ModelFor a distribution line model, given in fig.4. thequadratic equation which is mostly used for thecalculation of the line sending end voltages in loadflow analysis [7,8] can be written in general formasand from this equation, line receiving end activeand reactivepower can be written as]/]/From the above equations the real and reactivepower will exist only if(1)(2)Summing the eqs. (1) and (2) gives,(3)It is clearly seen that the value of the equationdecreases with the increase of the transferred powerand impedance of the line, and it can be used as abus stability index for a distribution networks as=(4)
  4. 4. K.R.S.Kashyap, B.Dheeraj Reddy / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.247-251250 | P a g e6.0 RESULTS OF THE ANALYSISIn this paper, to assess the steady stabilityof the system with Wind Turbine InductionGenerator (WTIG), the WTIG was connected tobus 12 of the IEEE 12 bus network, and buses 11and 12 were isolated from the remaining network.The analysis is first done without interconnectionof DG in the system with gradual increase in theload. The stability indices for base load without theconnection of DG are given in table1.Table1: Stability indices for base load without DGBus No. Stability Index1 0.997712 0.989163 0.977964 0.861035 0.851916 0.733927 0.649338 0.649059 0.6480010 0.6325811 0.6187012 0.61834The load is increased in steps and results are shownin table 2. With increase in the load, the currentflowing through distribution lines increases andresults in increase of losses and voltage drop.Table2: Stability indices for maximum load withoutDGBus No Stability Index1 0.999562 0.972703 0.929314 0.667585 0.639356 0.407057 0.280388 0.280389 0.2789110 0.2605311 0.2446612 0.24450The WTIG is connected to bus 12 of the networkand operated both in the grid connected andislanded modes. In islanding mode buses 11 and 12are isolated. The results for the connection are asshown.Fig.5. Bus voltages with and without DGFig.6. Reactive power loading (pu) vs. voltage atbus 12 without DGThe variation of the stability of the WTIG withrespect to the wind speed is as shown in fig.7.Fig.7. Real power loading (pu) vs. stability index
  5. 5. K.R.S.Kashyap, B.Dheeraj Reddy / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.247-251251 | P a g eThe maximum loading capacity of the systemwithout DG is found to be 1.8 times the base load.7.0 CONCLUSIONEnergy services and system Ancillaryservices (voltage regulation and partial eliminationof power quality disturbances) were provided andhave been presented to explore a wide perspective.Methods for controlling parameters were discussedand presented with a view for further improvement.At distribution level, capacitor banks could result inhigh harmonic voltages and currents due toharmonic resonance. DG system synchronized togrid provides better regulation and stability for thecomplete network. Reactive power supply throughDG system provides better regulation than FACTSdevices. The concept of load shedding can beapplied to a DG system which could help indisconnection of unimportant loads duringislanding mode. Thus, the peak load reduces andthe load factor will improve. Storage in batteries, atlow voltage level can prevent over voltages andoverloads due to solar power or small generationunits. The DG system is stable if the stability indexas in equation (4) is greater than or equal to zero.The control system in one DG unit has tocommunicate with other units to meet the systemslocal real and reactive power demand.8.0 ACKNOWLEDGMENTThis work has been supported by VIT University.The authors would like to thank all the concerned.REFERENCES[1] S.CHOUDHURY, S.P.CHOUDARYAND P.CROSSLEY, “Micro Grids andActive Distribution Networks”.Renewable Energy Series 2009.[2] N. G. Hingorani, L. Gyugyi,"Understanding FACTS; Concepts andTechnology of Flexible AC TransmissionSystems," IEEE Press book, 2000.[3] Standard IEC 1000-3-2 imposes limits tolow frequency harmonic currentsgenerated by electronic equipment.[4] MATH H.J. BOLLEN AND FAINANHASSAN, “Integration of DistributedGeneration in the Power System”.[5] A.K JHA, B.G FERNADES ANDA.KISHORE, “A Single Phase SingleStage AC/DC Converter with High InputPower Factor and Tight Output VoltageRegulation,” Progress in electromagneticsResearch symposium 2006, Cambridge,USA.[6] J. ELOY GARCI, S.AMALTES AND J.LRODRIGUEZ-AMENDO, “Direct powercontrol of voltage source inverters withUnbalanced grid voltages,” IET PowerElectronics-20070042.[7] Eminoglu U, Hocaoglu MH, “A newpower flow method for radial distributionsystems including voltage dependent loadmodels”. Electric power systems research.2005;76:106-114.[8] M.Chakravorty, and D.Das, “Voltagestability analysis of radial distributionnetworks,” Electric Power and EnergySystem, Vol.23, pp.129-135, 2001.