Power Quality Aspects In A Wind Power Plant: Preprint
A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy LaboratoryInnovation for Our Energy FuturePower Quality Aspects in aWind Power PlantPreprintE. Muljadi and C.P. ButterfieldNational Renewable Energy LaboratoryJ. ChaconSouthern California EdisonH. RomanowitzOak Creek Energy Systems, Inc.To be presented at the IEEE Power Engineering SocietyGeneral MeetingMontreal, Quebec, CanadaJune 18–22, 2006NREL is operated by Midwest Research Institute BattelleContract No. DE-AC36-99-GO10337Conference PaperNREL/CP-500-39183January 2006
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POWER QUALITY ASPECTS IN A WIND POWER PLANTE. Muljadi1C.P. Butterfield1National Renewable Energy Laboratory1617 Cole Blvd.Golden, CO 8040112)J. Chacon2Southern California EdisonP.O. Box 800Rosemead, CA 917703)H. Romanowitz3.Oak Creek Energy Systems Inc.14633 Willow Springs Rd.Mojave, CA 93501power systems, renewable energy, reactive powercompensation, self-excitation, harmonics.Abstract— Like conventional power plants, windpower plants must provide the power qualityrequired to ensure the stability and reliability of thepower system it is connected to and to satisfy thecustomers connected to the same grid. When windenergy development began, wind power plants werevery small, ranging in size from under one megawattto tens megawatts with less than 100 turbines in eachplant. Thus, the impact of wind power plant on thegrid was very small, and any disturbance within orcreated by the plant was considered to be in the noiselevel.In the past 30 years, the size of wind turbines andthe size of wind power plants have increasedsignificantly. Notably, in Tehachapi, California, theamount of wind power generation has surpassed theinfrastructure for which it was designed. At the sametime, the lack of rules, standards, and regulationsduring early wind development has proven to be anincreasing threat to the stability and power quality ofthe grid connected to a wind power plant.Fortunately, many new wind power plants areequipped with state of the art technology, whichenables them to provide good service while producingclean power for the grid. The advances in powerelectronics have allowed many power systemapplications to become more flexible and toaccomplish smoother regulation. Applications suchas reactive power compensation, static transferswitches, energy storage, and variable-speedgenerations are commonly found in modern windpower plants.Although many operational aspects affect windpower plant operation, this paper, focuses on powerquality. Because a wind power plant is connected tothe grid, it is very important to understand thesources of disturbances that affect the power quality.In general, the voltage and frequency must be kept asstable as possible.The voltage and currentdistortions created by harmonics will also bediscussed in this paper as will self-excitation, whichmay occur in a wind power plant due to loss of line.I. INTRODUCTIONIn the past 30 years, the size of wind power plants hasincreased significantly. Notably, in Tehachapi,California, the amount of wind power generation hassurpassed the capability of the infrastructure for which itwas designed. The infrastructure was built to supportsmall, scattered wind generation. Similarly, becausewind plants were so small in the past, the rulesgoverning wind generation were more relaxed toencourage development. For example, in the past, windturbines were only required to have capacitorcompensation at each turbine to satisfy the no-loadreactive power generation. But as the amount of windgeneration increases, the lack of rules, standards, andregulations during early wind development has proven tobe an increasing threat to the stability and power qualityof the interconnected grid.Although many operational aspects affect windpower plant operation, in this paper, we focus on powerquality. Because a wind power plant is connected to thegrid, it is very important to understand the sources ofdisturbances that affect its power quality. In general, thevoltage and frequency must be kept as stable as possible;therefore, voltage and current distortions created byharmonics will also be discussed in this paper. Selfexcitation, which may occur in a wind power plant dueto loss of line, will also be presented.Section II describes the voltage and frequencyvariations, and Section III discusses single and multipleturbines representation. Section IV presents harmonicsand self-excitation, and Section V presents the summary.We used Power Systems Simulation for Engineers(PSSETM) from Siemens Power Technologies Inc., andVisual Simulation (Vissim) from Visual Solution Inc.II. VOLTAGE AND FREQUENCY VARIATIONSA. OverviewThis section describes the interaction between thewind power plant, reactive power compensation, and thepower system network. The Tehachapi power systemnetwork used in this study is based on a power systemdiagram from 1999 [1]. Thus, the simulation resultsIndex Terms—wind turbine, wind farm, wind powerplant, power quality, wind energy, aggregation,1
presented in this paper do not necessarily reflect thecurrent situation. The current power system network hasbeen improved to accommodate more wind powergeneration.Because the loads and the wind power plants' outputfluctuate during the day, the use of reactive powercompensation is ideal for maintaining normal voltagelevels in the power system network. Reactive powercompensation can minimize reactive power imbalancesthat can affect the surrounding power system.In this section, we will show how the contribution ofwind power plants affects the power distributionnetwork and how the power distribution network andreactive power compensation interact when the windchanges.About 24 wind power plants are included in thesimulation. The wind power plants represent 1- to 70MW wind power plants. The wind power plants areconnected to the rest of the transmission network. Bus1, which is Antelope Substation, is used as the gatewayto the much larger network outside the area and istreated as an infinite bus. Each wind power plantrepresents the following characteristics:1. The characteristics of the wind resource (turbulencelevel, average speed, air density, etc.).2. Diversity of the wind speeds with respect to thelocations of each wind power plant in the Tehachapiarea. For a more detailed discussion on simulation ofwind power plant aggregation, see reference [2].3. The characteristics of the wind turbine (Cp-TSRcharacteristics) and induction generator4.The P-Q (real and reactive power) electricalcharacteristic of the individual wind power plants.When the work presented in this paper was developed,no wind turbine or wind power plant models werecommercially available, thus, the model presented herewas developed at NREL. Today many wind turbinemodels are commercially available and can bedownloaded from the GE website, the Siemens PowerTechnologies International website, and other softwarevendors.MVAR. The generations at Kern River and Bailey are24 MW and 20 MW, respectively. The simulation wasfed by non-uniform wind speeds to simulate the entirearea. A total simulation time of 6000 seconds (100minutes) is used to cover the spectrum of possible windfluctuations. For a wind power plant power system, IECStandard 61400-21 stated that the 10-minute average ofvoltage fluctuation should be within 5% of its nominalvalue [3].A typical reactive power compensator may beimplemented by using a fixed capacitor, a switchedcapacitor, or static compensator [4-5]. The simplestform of reactive power compensation is the static VARcompensator (SVC). By choosing the correct size ofinductor and capacitor, the SVC can be operated togenerate reactive power, which varies from -Qlolim to Qhilim. For example, with a 100-MVAR capacitor and a200-MVAR inductor, a range of 100 MVAR can beachieved and adjusted continuously. If negative reactivepower is not required, a combination of a 100-MVARReactive power from CalCement toAntelope (no-compensation).1.0 p.u.- 26 MVARVoltage at CalCement0.905 p.u.45006000Time (seconds)Figure 1. Reactive power from bus 22 to bus 1, andvoltage at bus 22 without reactive power compensation.Reactive power from CalCement toAntelope (with-compensation).B. Voltage VariationThis section presents the voltage variations caused bythe wind speed changes and a comparison between anuncompensated and a compensated system. The termcompensated refers to the use of static reactive powercompensation to improve the voltage characteristics ofthe wind power plant. The CalCement substation (bus22) has 18 MVAR capacitor compensation installed.Other buses in the area are compensated to a total of77.3 MVAR. The total load in the area (includingsurrounding small towns) is about 259 MW and 46.4- 18 MVAR1.0 p.u.0.95 p.u.Voltage at CalCement45006000Time (seconds)Figure 2. Reactive power from bus 22 to bus 1, andvoltage at bus 22 with reactive power compensation.2
capacitor and a 100-MVAR inductor provides a range of0 MVAR to 100 MVARThe reactive power compensation is located inside thewind power plant at VarWind, bus 40. Figures 1 and 2compare an uncompensated wind power plant and acompensated wind power plant. Without reactive powercompensation, the voltage variation at CalCement (bus22) drops to 0.905 p.u., and with proper compensation,the voltage drops to 0.95 p.u., which is consideredallowable for a wind power plant. Note, that the reactivepower flows from the Antelope drops significantly (from26 MVAR to 18 MVAR) when the reactive power iscompensated locally. Notice that the voltage tracesfollow the reactive power traces closely and theminimum voltage occurs at the minimum reactive powerflowing from CalCement (or maximum reactive powerflowing from Antelope to CalCement).C. Frequency VariationFrequency variation is affected by the rate of changein real power flow. Thus, there is a difference in thetrend between the voltage-reactive power tracescompared to frequency-real power traces relationship.The reactive power improves the voltage characteristicof the wind power plant and surrounding area.Comparing Figures 3 and 4, the real power flow is notaffected by the reactive power compensation. Althoughthe frequency variation is barely noticeable, it indicatesthat there are differences in the rate-changes in the realpower flow between compensated and uncompensatedsystems. It shows that there is a reduction in thefrequency swings; however, because the frequencyswing is very small to begin with, it is probably notwarranted to claim that the reactive power compensationhelps reduce the frequency variations. Note, that theminimum frequency does not occur at the maximumpower flow but rather the maximum rate of change ofthe power flow.Real powerIII. SINGLE TURBINE AND MULTIPLE TURBINESA. Single Turbine Representation (STR)In this section, we look at a wind power plantrepresented by one group of wind turbines. This is theworst-case assumption because we assume that all thewind turbines in this group are synchronized. Thus, thesame wind fluctuations and tower shadow effects willaffect the output power of the wind power plant and thepower quality at the PCC.24. 01µHzFrequency at CalCement- 21. 6 µHzB. Multiple Turbine Representation (MTR)In this section, we focus on the aggregation impact onthe wind power plant output at the PCC. We use thesame wind turbulence intensity and the same impedanceof the transmission line. We measured the real andreactive power fluctuations, the voltage fluctuations, andat the PCC of a wind power plant. We quantified thedifference in power and voltage fluctuations level if wetreat a wind power plant as a single turbine or asmultiple groups of turbines.The flicker levelmeasurement can be implemented using designspecification in IEC 61000-4-15 [1].Ideally we would like to model every wind turbine onthe wind power plant. Unfortunately, a large windpower plant can have more than 100 wind turbines onsite. Therefore, it is not possible to represent all theturbines simultaneously, because the computing timewould be excessive. To closely represent a real windpower plant without simulating each wind turbine, wemade the following assumptions:A large wind power plant (200 turbines) is dividedinto several groups of wind turbines.45006000Time (seconds)Figure 3. Real power from bus 22 to bus 1, and frequency atbus 22 without reactive power compensation.Real power21. 4 µHz- 21. 6 µHz- 20. 2 µHzFrequency at CalCement45006000Time (seconds)Figure 4. Real power from bus 22 to bus 1, and frequency atbus 22 with reactive power compensation.3
· The wind speed is uniform for each group of windturbines.· The groups are arranged in sequence.· Our interest is in the long-term simulation, thus thestart-up of each turbine is not a major concern. Besides,because the wind speeds are different at each turbinelocation, all the wind turbines do not start at the sametime.· All the turbines in the wind power plant are exposed toof 18.7 m/s and turbulence level of 19.7%. The timeseries wind speed shifts by 1 minute for each group.· The contribution of each group is chosen randomly.For example, a wind power plant with 3 groups ofturbines may be proportioned as 35% from the firstgroup, 25% from the second group, and 40% from thethird group.· This concept of grouping is repeated for differentnumbers of groups, but the total wind turbines in theentire wind power plant is kept the same—200 turbines.· Eventually we will compare the impact of windturbine distribution by comparing the flicker and thevoltage fluctuations based on groupings (one group only,two groups, four groups, and eight groups).020 040 03 00T im e (sec)5 00Wind Speed (m/s)111213141516T im e (se c )17181920181920W in d Farm Real P o wer (1 6 gro up s)Real P o wer (M W )WP16GP (MW) at PCCP at PCC40302010010111213141516T im e (sec)17Figure 6. Real power output of a wind farm in a WP1Gand in a WP16G.Let us consider the output of STR and MTR and placethe two graphs next to each other for a bettercomparison. Figure 6 shows variation of real power forboth representations taken at the point ofinterconnection. The time scale is changed to make aneasier observation of the nature of power fluctuationswithin a short time frame. In these particular traces, thetrace of tower shadow is very visible. Tower shadoweffect is the effect of power fluctuations due to powerproduction deficit every time a blade passes the turbinetower. Usually the tower shadow has a frequency 3 perrevolution. This effect is commonly known as 3 peffects. Besides the tower shadow, the power variation isalso caused by the wind speed variations with time. Forthe STR, the power fluctuation reflects the powerfluctuation of a single turbine. It is amplified by thenumber of turbines within the wind power plant. For theMTR, the power fluctuation is the collective behavior ofseveral groups of wind turbines with each group fed by adifferent time series as illustrated in Figure 5.The label WP1G is a single-group representation andWP16G is a 16-group representation. Comparing the twographs, it is obvious that there is some smoothing effectin the power fluctuations if we consider that the windpower plant consists of sixteen different groups of windturbines. Figure 7 shows the voltage fluctuations as thewind speed varies with two different representations.The STR obviously shows very large variations of thevoltage at the point of interconnection as the wind speedw310 0205010w1306020040010w4w2WP1G10W in d Sp eed (gro up 1 )30Re a l P o we r50C. Comparison between STR and MTRTo start, consider the time series of wind speed shown inFigure 5. In an STR, the wind speed is applied to asingle turbine and the output of the single wind turbine ismultiplied by the number of the turbines within the windpower plant. In an MTR, the time series of the windspeed is subdivided into several sections and eachsubdivision is applied to a different group of turbines.For example, for the figure shown, the time series ofwind speed is divided into four different files with thestarting time (t 0) at w1, w2, w3 and w4. Thisassumption is an approximation of the time it takes forthe wind speed to travel from one group of turbines toanother group of turbines down wind. Although thisassumption is not perfect, by assuming that the windspeed has a characteristic of frozen turbulence, and thatthe turbulence does not change as it passes a windturbine, we can more closely simulate the real situation.40W in d Fa r m P & Q6060 0Figure 5. Wind speed variation applied to the wind turbines.4
Per phase voltage (in p.u.)1.10P er phase volt age at P CCWP1G1.05InfiniteBus1.00Line FeederTransformer.95Caps.9001.10Per phase voltage (in p.u.)Vph at Infinit e Bus and at P CC (in p.u.)To local loads100200300400T ime (sec)500600Figure 8. The physical diagram of the system under investigation.requires reactive power from the grid to operatenormally. The grid dictates the voltage and frequency ofthe induction generator.Although self-excitation does not occur duringnormal grid-connected operation, it can occur during offgrid operation. For example, if a wind turbine operatingin normal mode becomes disconnected from the powerline due to a sudden fault or disturbance in the linefeeder, the capacitors connected to the inductiongenerator will provide reactive power compensation.However, the voltage and the frequency are determinedby the balancing of the systems.One disadvantage to self-excitation is the safetyaspect. Because the generator is still generating voltage,it may compromise the safety of the personnel inspectingor repairing the line or generator. Another disadvantageis that the generator’s operating voltage and frequencyare determined by the balance between the system’s realpower and the reactive power. Thus, if sensitiveequipment is connected to the generator during selfexcitation, the equipment may be subjected toover/under voltage and over/under frequency operation.In spite of the disadvantages of operating the inductiongenerator in self-excit
We used Power Systems Simulation for Engineers (PSSE. TM) from Siemens Power Technologies Inc., and Visual Simulation (Vissim) from Visual Solution Inc. II. VOLTAGE AND FREQUENCY VARIATIONS. A. Overview This section describes the interaction between the wind power plant, reactive power compensation, and the power system network.
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