Critical clearing time and wind power in small isolated power systems considering inertia emulation
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Article Critical Clearing Time and Wind Power in Small Isolated Power Systems Considering Inertia Emulation Elías Jesús Medina-Domínguez 1,*,:and José F. Medina-Padrón 2,: Received: 5 August 2015 ; Accepted: 2 November 2015 ; Published: 6 November 2015 Academic Editor: Frede Blaabjerg 1Renewable Energies Deparment, Research and Development Division, Canary Islands Institute of Technology (ITC), C/Playa de Pozo Izquierdo s/n, Santa Lucía (Gran Canaria) 35119, Spain 2University Institute of Intelligent Systems and Numeric Applications in Engineering (SIANI), University of Las Palmas de Gran Canaria (ULPGC), Edificio Central del Parque Científico y Tecnológico, Campus Universitario de Tafira, Las Palmas de Gran Canaria 35017, Spain; [email protected] *Correspondence: [email protected]g; Tel.: +34-928-727-560 :These authors contributed equally to this work. Abstract: The stability and security of small and isolated power systems can be compromised when large amounts of wind power enter them. Wind power integration depends on such factors as power generation capacity, conventional generation technology or grid topology. Another issue that can be considered is critical clearing time (CCT). In this paper, wind power and CCT are studied in a small isolated power system. Two types of wind turbines are considered: a squirrel cage induction generator (SCIG) and a full converter. Moreover, the full converter wind turbine’s inertia emulation capability is considered, and its impact on CCT is discussed. Voltage is taken into account because of its importance in power systems of this kind. The study focuses on the small, isolated Lanzarote-Fuerteventura power system, which is expected to be in operation by 2020. Keywords: wind power; isolated power system; transient stability; full converter wind turbine; inertia emulation 1. Introduction The unprecedented worldwide development of wind power over recent decades reached around 318,644 GW by the end of 2014, and it is estimated that global wind power capacity will continue to increase [1]. Integrating wind power into power systems is an increasingly common challenge. Large amounts of wind power can be introduced into continental power systems, such as the European grid managed by the members of the European Network of Transmission System Operators for Electricity (ENTSO-E). On the other hand, small, isolated power systems, such as those found on small islands, can experience problems related to stability, and this can influence wind integration [2]. One issue that deserves consideration in wind power integration is the critical clearing time (CCT), particularly in small, isolated power systems. CCT can be seen as a measure of the transient stability of a power system [3,4] and can be defined as the maximum allowed duration of a three-phase short circuit before loss of system stability. As a part of the Spanish Grid Codes, a set of protection coordination criteria and a methodology leading to CCT evaluation were developed by the Spanish Transmission System Operator (TSO) Red Eléctrica de España (REE) [5,6]. The CCT calculation methodology implies performing dynamic stability simulations by applying a short circuit in the nodes of the network and varying the fault time until a set of criteria are fulfilled [7,8]. The Energies 2015,8, 12669–12684; doi:10.3390/en81112334 www.mdpi.com/journal/energies
Energies 2015,8, 12669–12684 set of criteria to be fulfilled through this trial and the error approach specify that the following is not allowed: (1) loss of system stability; (2) an unacceptable value of load shedding; (3) unacceptable parameters values in the subsequent steady state. The Spanish TSO has established 10% of load as the unacceptable value for Spanish power systems on islands and in other small systems [6]. There are other methods for CCT estimation, such as Lyapunov energy function, artificial neuronal networks or hybrid methods. However, the trial and error method using dynamic simulations provides more accuracy results. In this paper, we use the trial and error method and the above-mentioned criteria to study how using inertia emulation could impact CCT values and eventually the wind power penetration level. A TSO uses CCT for designing protection schemes in power systems. A significant modification of the CCT values can cause problems in protection system behavior that can have an impact on the stability or security of the power system itself. Several papers and reports have investigated wind power and system stability through CCT, and their results show that there is a relationship between them. They suggest a modification of CCT values when wind power is introduced into a power system [9–11]. Other papers have analyzed inertia emulation from wind turbines and have shown a positive impact of the inertia emulation on the frequency response [12–20]. These papers usually look at large power systems and conventional generation based on large thermal or hydro generator units. Generation unit outages or transmission tie line outages are examples of the types of disturbances studied in papers considering inertia emulation. Medium and low voltage networks or feeders are also studied, and the analysis in these papers focuses on the time evolution of frequency. In [21], the power system of an island and wind farms with inertia emulation are considered. The integration of a hydro-pump storage system was analyzed, but short-circuit disturbance was not taken into account. In this paper, the relationship between wind power and CCT is studied in a small, isolated power system with two types of wind turbines: the squirrel cage induction generator (SCIG) and the full converter. Furthermore, the inertia emulation capability of the full converter wind turbine is considered. A comparative analysis of the different results achieved is presented. In order to obtain CCT values, a three-phase short circuit disturbance is simulated. Therefore, inertia emulation performance was studied when a three-phase short circuit takes place. Network voltages are also considered because of their relevance in this type of power system. Thus, a more global analysis of the small and isolated power system can be undertaken. Almost all of the system’s conventional generation is based on small diesel generators. The study has been carried out on the planned Lanzarote-Fuerteventura power system, which is expected to be in operation by 2020. Its generation power capacity, voltage level, inertia constant and grid topology are features that make it a good example of a small, isolated power system. Modeling and analysis are performed using PSSrE v32 software (Siemens-PTI, Schenectady, NY, USA). 2. Methodology CCT values are obtained through a trial and error procedure [22] when using dynamic analysis for several three-phase short circuit events at some Lanzarote-Fuerteventura power system buses. Simulation results were studied to find out what determines the CCT. These short circuit disturbances were simulated at five selected buses in order to know the overall behavior of the power system. The selected buses are: (1) Punta Grande power plant and 12670
Energies 2015,8, 12669–12684 Las Salinas power plant; (2) Haría-Teguise and Jandía as the farthest buses from power plants; and (3) Corralejo, which is a bus in between both power plants. In order to analyze the wind power impact on CCT, three types of wind turbine were studied: SCIG, full converter and full converter with inertia emulation capability. Each wind farm in the power system is equipped with only one kind of wind turbine. Thus, their CCT values can show some differences. In every case, wind power was increased from 0 MW to 150 MW, in steps of 10 MW. This wind power was generated by wind farms according to their rated powers. In order to find CCT values, the CCT definition given in the Introduction section is used. In this way, the unacceptable value of load shedding is equal to or higher than 10% of the load. Photovoltaic power plants were not modeled to avoid their effects on the results. The model of the Lanzarote-Fuerteventura power system for steady-state analysis and dynamic simulation was created using PSSrE v32. 3. Description of the Studied Power System The system studied here is the Lanzarote-Fuerteventura power system 2020. Lanzarote and Fuerteventura are two of the Spanish Canary Islands. The Lanzarote and Fuerteventura power systems are linked by a submarine cable and, therefore, constitute a single power system. At present, each island has only one conventional power plant. On Lanzarote, Punta Grande has a capacity of 232.4 MW, and Las Salinas, on Fuerteventura, has a capacity of 187.0 MW. Both power plants have conventional diesel and gas turbine units. New power generators are not expected to be installed in the foreseeable future, because power demand has decreased over recent years [23]. The units at these power plants are listed in Table 1. Table 1. Conventional generation units. Punta Grande unit name Capacity (MVA) Las Salinas unit name Capacity (MVA) Diesel 1 9.4 Diesel 1 5.4 Diesel 2 9.4 Diesel 2 5.4 Diesel 3 9.4 Diesel 3 6.3 Diesel 4 20 Diesel 4 9.4 Diesel 5 20 Diesel 5 9.4 Diesel 6 30 Diesel 6 30 Diesel 7 22.5 Diesel 7 18 Diesel 8 22.5 Diesel 8 18 Diesel 9 22.5 Diesel 9 18 Diesel 10 22.5 Gas 1 32.42 Diesel 11 22.5 Gas 2 40.93 Gas 1 27.24 Gas Mobile 1 15 Gas 2 40.93 - - Peak power demand in 2013 was 251 MW, and it is expected to reach 384 MW by 2020. At present, the power system is operated at 66 kV in the transmission network and at 20 kV at the distribution level. The transmission network has eight buses. The rated voltage of the submarine cable is 66 kV, with a transmission capacity of 60 MVA. New substations and transmission lines of 132-kV rated voltage have been planned for 2020. In addition, the undersea link will be reinforced by a second submarine cable of 132-kV rated voltage and 130 MVA of capacity. The power system will then have 26 buses at two voltage levels, 66 kV and 132 kV. Currently, the two-island system has five wind farms. Total wind power capacity is 21.86 MW. The most widely-used type of wind turbine is the SCIG with a gear box. 12671
Energies 2015,8, 12669–12684 The installed wind power is expected to have increased by 2020. Wind power could rise to 162 MW [24]. Most of the new wind farms will be full converter wind turbines. 4. System Modeling This section describes the model of the Lanzarote-Fuerteventura 2020 power system that was implemented in PSSrE v32. Conventional units in power plants, wind power energy converters, power substations and lines of the transmission network have been modeled. The distribution system and power demands are represented through loads at corresponding buses. A single-line diagram of the Lanzarote-Fuerteventura 2020 power system is shown in Figure 1. Transmission lines have been modeled using a πmodel with their conductances neglected. The power factor of loads is 0.9. A two-winding transformer model is used for transformers of generation units and for substation transformers. Resistance and reactance are considered. Energies2015,8,page–page 4 4.SystemModeling ThissectiondescribesthemodeloftheLanzarote‐Fuerteventura2020powersystemthatwas implementedinPSS®Ev32. Conventionalunitsinpowerplants,windpowerenergyconverters,powersubstationsandlines ofthetransmissionnetworkhavebeenmodeled.Thedistributionsystemandpowerdemandsare representedthroughloadsatcorrespondingbuses. Asingle‐linediagramoftheLanzarote‐Fuerteventura2020powersystemisshowninFigure1. Transmissionlineshavebeenmodeledusingaπmodelwiththeirconductancesneglected.The powerfactorofloadsis0.9.Atwo‐windingtransformermodelisusedfortransformersofgeneration unitsandforsubstationtransformers.Resistanceandreactanceareconsidered. Figure1.Lanzarote‐Fuerteventurapowersystemexpectedbytheyear2020. 4.1.ConventionalGeneration DieselgeneratorsarerepresentedbyGENSALmodels,andgasturbinegeneratorsaremodeled withGENROE.BothmodelsbelongtothePSS®Elibrary. TheturbinegovernormodelusedfordieselunitsisDEGOV1(Woodwarddieselgovernor).The GAST2Aturbinegovernormodelisusedforgasturbineunits. PSS®Emodelsusedforexcitationcontrolindieselunitsandgasturbineunitsaresimplified excitationsystem(SEXS)andESDCA1(IEEETypeDC1Aexcitationsystem),respectively. Figure 1. Lanzarote-Fuerteventura power system expected by the year 2020. 4.1. Conventional Generation Diesel generators are represented by GENSAL models, and gas turbine generators are modeled with GENROE. Both models belong to the PSSrE library. The turbine governor model used for diesel units is DEGOV1 (Woodward diesel governor). The GAST2A turbine governor model is used for gas turbine units. PSSrE models used for excitation control in diesel units and gas turbine units are simplified excitation system (SEXS) and ESDCA1 (IEEE Type DC1A excitation system), respectively. 12672
Energies 2015,8, 12669–12684 4.2. Wind Conversion Energy System 4.2.1. Squirrel Cage Induction Generator (SCIG) Wind Turbine A SCIG wind turbine, or so-called Type 1, is connected to the grid through a transformer. The generator operates at a nearly fixed speed, related to the grid frequency. The wind turbine generates active power when the shaft speed is higher than the grid frequency. The generator consumes reactive power to create its magnetic field. Thereby, a capacitor bank is used for power factor correction. Normally, in this wind turbine type, there is a soft-starter to limit the higher starting currents [25]. A diagram of this wind turbine is depicted in Figure 2. Energies2015,8,page–page 5 4.2.WindConversionEnergySystem 4.2.1.SquirrelCageInductionGenerator(SCIG)WindTurbine ASCIGwindturbine,orso‐calledType1,isconnectedtothegridthroughatransformer.The generatoroperatesatanearlyfixedspeed,relatedtothegridfrequency.Thewindturbinegenerates activepowerwhentheshaftspeedishigherthanthegridfrequency.Thegeneratorconsumesreactive powertocreateitsmagneticfield.Thereby,acapacitorbankisusedforpowerfactorcorrection. Normally,inthiswindturbinetype,thereisasoft‐startertolimitthehigherstartingcurrents[25].A diagramofthiswindturbineisdepictedinFigure2. Figure2.Diagramofthesquirrelcageinductiongenerator(SCIG)windturbine. ThemodelusedforSCIGwindturbinesisthewell‐knownPSS®EWT1model[26].Thismodel includestheWT1Ggeneratormodel,theWT12TwindturbinemodelandtheWT12A pseudo‐governormodel. TheWT1Gmodelisamodificationofthestandardinductionmachinemodelandconsidersrotor fluxdynamics.TheWT12Tmodelisbasedonatwo‐massrepresentationofthewindturbinedrive shaft.Itincludesrotorblades,theshaftandthemachinewiththegearbox.Thismodeldetermines speeddeviation.TheWT12Amodelcalculatesthemechanicaltorqueoftheblades’shaftby processingrotorspeeddeviationandactivepoweratgeneratorterminals.Aconnectivitydiagramof thesemodelscanbeseenin[26]. 4.2.2.FullConverterWindTurbine Inafullconverterwindturbine,ageneratorisconnectedtothegridthroughapowerconverter. Thisallowsvariablespeedintheturbineshaft.Theconverterrectifiesthevariable‐frequencyAC powerfromthegeneratorintoDCandtheninvertstheDCpowertoACatthegridfrequency.Figure 3showsthistypeofwindturbine.Thegeneratorcanbeawoundrotorsynchronousgenerator (WRSG),apermanentmagnetsynchronousgenerator(PMSG)oranSCIG[25].Inthispaper,the WRSGisconsidered. Figure3.Diagramofafullconverterwindturbine. Figure 2. Diagram of the squirrel cage induction generator (SCIG) wind turbine. The model used for SCIG wind turbines is the well-known PSSrE WT1 model [26]. This model includes the WT1G generator model, the WT12T wind turbine model and the WT12A pseudo-governor model. The WT1G model is a modification of the standard induction machine model and considers rotor flux dynamics. The WT12T model is based on a two-mass representation of the wind turbine drive shaft. It includes rotor blades, the shaft and the machine with the gear box. This model determines speed deviation. The WT12A model calculates the mechanical torque of the blades’ shaft by processing rotor speed deviation and active power at generator terminals. A connectivity diagram of these models can be seen in [26]. 4.2.2. Full Converter Wind Turbine In a full converter wind turbine, a generator is connected to the grid through a power converter. This allows variable speed in the turbine shaft. The converter rectifies the variable-frequency AC power from the generator into DC and then inverts the DC power to AC at the grid frequency. Figure 3shows this type of wind turbine. The generator can be a wound rotor synchronous generator (WRSG), a permanent magnet synchronous generator (PMSG) or an SCIG [25]. In this paper, the WRSG is considered. Energies2015,8,page–page 5 4.2.WindConversionEnergySystem 4.2.1.SquirrelCageInductionGenerator(SCIG)WindTurbine ASCIGwindturbine,orso‐calledType1,isconnectedtothegridthroughatransformer.The generatoroperatesatanearlyfixedspeed,relatedtothegridfrequency.Thewindturbinegenerates activepowerwhentheshaftspeedishigherthanthegridfrequency.Thegeneratorconsumesreactive powertocreateitsmagneticfield.Thereby,acapacitorbankisusedforpowerfactorcorrection. Normally,inthiswindturbinetype,thereisasoft‐startertolimitthehigherstartingcurrents[25].A diagramofthiswindturbineisdepictedinFigure2. Figure2.Diagramofthesquirrelcageinductiongenerator(SCIG)windturbine. ThemodelusedforSCIGwindturbinesisthewell‐knownPSS®EWT1model[26].Thismodel includestheWT1Ggeneratormodel,theWT12TwindturbinemodelandtheWT12A pseudo‐governormodel. TheWT1Gmodelisamodificationofthestandardinductionmachinemodelandconsidersrotor fluxdynamics.TheWT12Tmodelisbasedonatwo‐massrepresentationofthewindturbinedrive shaft.Itincludesrotorblades,theshaftandthemachinewiththegearbox.Thismodeldetermines speeddeviation.TheWT12Amodelcalculatesthemechanicaltorqueoftheblades’shaftby processingrotorspeeddeviationandactivepoweratgeneratorterminals.Aconnectivitydiagramof thesemodelscanbeseenin[26]. 4.2.2.FullConverterWindTurbine Inafullconverterwindturbine,ageneratorisconnectedtothegridthroughapowerconverter. Thisallowsvariablespeedintheturbineshaft.Theconverterrectifiesthevariable‐frequencyAC powerfromthegeneratorintoDCandtheninvertstheDCpowertoACatthegridfrequency.Figure 3showsthistypeofwindturbine.Thegeneratorcanbeawoundrotorsynchronousgenerator (WRSG),apermanentmagnetsynchronousgenerator(PMSG)oranSCIG[25].Inthispaper,the WRSGisconsidered. Figure3.Diagramofafullconverterwindturbine. Figure 3. Diagram of a full converter wind turbine. 12673
Energies 2015,8, 12669–12684 Model characteristics of the considered wind turbine are described in [27,28]. The dynamic behavior of the wind turbine is related to the converter control system for the simulation times normally used. Therefore, modeling the converter control system is enough for wind turbine representation in almost all cases. Figure 4shows a diagram for the model of the wind turbine. Energies2015,8,page–page 6 Modelcharacteristicsoftheconsideredwindturbinearedescribedin[27,28]. Thedynamicbehaviorofthewindturbineisrelatedtotheconvertercontrolsystemforthe simulationtimesnormallyused.Therefore,modelingtheconvertercontrolsystemisenoughfor windturbinerepresentationinalmostallcases.Figure4showsadiagramforthemodelofthe windturbine. Figure4.Simplifieddiagramofthefullconverterwindturbinemodel. Thepowerconverterallowssupportinggridvoltagebygeneratingorconsumingreactive power.Reactivepowercontrolisavailableforalloftheactivepoweroperationrange.Thiscanbe seeninFigure5. Figure5.P‐Qcharacteristicofthefullconverterwindturbine. Afullconverterwindturbinewithfaultride‐throughcapabilitycanremaininoperationwhen avoltagediporanovervoltageoccurs.Thewindturbineisalsoabletoinjectorconsumeadditional reactivepowertoparticipateinnetworkvoltagerestoration. Thereferenceoftotalcurrentrelatedtoreactivepower,isobtainedbyEquation(1): , ∆ (1) P(pu) Q(pu) 1.0 Q generated Q consumed 0 -0.75 0.75 Figure 4. Simplified diagram of the full converter wind turbine model. The power converter allows supporting grid voltage by generating or consuming reactive power. Reactive power control is available for all of the active power operation range. This can be seen in Figure 5. Energies2015,8,page–page 6 Modelcharacteristicsoftheconsideredwindturbinearedescribedin[27,28]. Thedynamicbehaviorofthewindturbineisrelatedtotheconvertercontrolsystemforthe simulationtimesnormallyused.Therefore,modelingtheconvertercontrolsystemisenoughfor windturbinerepresentationinalmostallcases.Figure4showsadiagramforthemodelofthe windturbine. Figure4.Simplifieddiagramofthefullconverterwindturbinemodel. Thepowerconverterallowssupportinggridvoltagebygeneratingorconsumingreactive power.Reactivepowercontrolisavailableforalloftheactivepoweroperationrange.Thiscanbe seeninFigure5. Figure5.P‐Qcharacteristicofthefullconverterwindturbine. Afullconverterwindturbinewithfaultride‐throughcapabilitycanremaininoperationwhen avoltagediporanovervoltageoccurs.Thewindturbineisalsoabletoinjectorconsumeadditional reactivepowertoparticipateinnetworkvoltagerestoration. Thereferenceoftotalcurrentrelatedtoreactivepower,isobtainedbyEquation(1): , ∆ (1) P(pu) Q(pu) 1.0 Q generated Q consumed 0 -0.75 0.75 Figure 5. P-Qcharacteristic of the full converter wind turbine. A full converter wind turbine with fault ride-through capability can remain in operation when a voltage dip or an overvoltage occurs. The wind turbine is also able to inject or consume additional reactive power to participate in network voltage restoration. 12674
Energies 2015,8, 12669–12684 The reference of total current related to reactive power IQ,ref is obtained by Equation (1): IQ,ref “Qref U`∆IQ(1) where Qref is the generated or consumed reactive power of the wind turbine before fault, Uis the voltage at the wind turbine terminals and ∆IQis the additional reactive current determined as shown in Figure 6. Energies2015,8,page–page 7 whereisthegeneratedorconsumedreactivepowerofthewindturbinebeforefault,isthe voltageatthewindturbineterminalsand∆istheadditionalreactivecurrentdeterminedasshown inFigure6. Figure6.Additionalcurrentrelatedtothereactivepowercharacteristic. 4.2.3.InertiaEmulation Akindofactivepowercontrolthatsomewindturbinescanprovideisinertiaemulation,also calledsyntheticinertiaorsimulatedinertia.Awindturbinewithinertiaemulationcapabilityisable toincreasethegeneratedactivepowerfromkineticenergystoredintherotatingmass,bymeansof thepowerconvertercontrol.Ifthereisalowsystemfrequencyvalue,thewindturbineinjectsextra activepowerintothenetwork,emulatingconventionalsynchronousgenerators’inertia[12,29]. Becausekineticenergyisusedtoincreasetheactivepower,thespeedofthewindturbine decreases.Inordernottoreducewindturbinespeedexcessively,emulationinertiacanonlybe providedduringafewseconds.Accordingtosomemanufacturers,windturbinescanprovidethis extraactivepowerwithinthefirst10s[30–32].Afterthepowerincreases,aperiodoftimeisneeded torestorethespeedtoanacceptablevalue.Thisrecoveryperiodisabouttwicethepowerincrease time[16,33]. Dependingonthewindturbine,extraactivepowercanvaryfrom4%to10%ofthewindturbine ratedpower[18,34].ExtraactivepowercanbeobtainedbyEquation(2)usedin[30]. 1 2 ω 1 2 ω (2) whereistheextraactivepowerduration,isthetotalmomentofinertiaofthewindturbine, ωistheinitialrotorangularspeedandωistherotorangularspeedattime. Inthispaper,theinertiaemulationcontrolmodelforthefullconverterwindturbineconsidered isbasedon[16].ThiscanbeseeninFigure7.Theoutputvaluedependsontheactualvalueof thesystemfrequency,asshowninFigure8. Figure 6. Additional current related to the reactive power characteristic. 4.2.3. Inertia Emulation A kind of active power control that some wind turbines can provide is inertia emulation, also called synthetic inertia or simulated inertia. A wind turbine with inertia emulation capability is able to increase the generated active power from kinetic energy stored in the rotating mass, by means of the power converter control. If there is a low system frequency value, the wind turbine injects extra active power into the network, emulating conventional synchronous generators’ inertia [12,29]. Because kinetic energy is used to increase the active power, the speed of the wind turbine decreases. In order not to reduce wind turbine speed excessively, emulation inertia can only be provided during a few seconds. According to some manufacturers, wind turbines can provide this extra active power within the first 10 s [30–32]. After the power increases, a period of time is needed to restore the speed to an acceptable value. This recovery period is about twice the power increase time [16,33]. Depending on the wind turbine, extra active power can vary from 4% to 10% of the wind turbine rated power [18,34]. Extra active power Pext can be obtained by Equation (2) used in [30]. Pextt“1 2JWTω2 r0´1 2JWTω2 rt (2) where tis the extra active power duration, JWT is the total moment of inertia of the wind turbine, ωr0 is the initial rotor angular speed and ωrt is the rotor angular speed at time t. In this paper, the inertia emulation control model for the full converter wind turbine considered is based on [16]. This can be seen in Figure 7. The output value Pext depends on the actual value of the system frequency, as shown in Figure 8. 12675
Energies 2015,8, 12669–12684 Energies2015,8,page–page 8 Figure7.Basicinertiaemulationmodelblockdiagram. Figure8.Graphbehavioroftheinertiaemulationconsidered. is10%ofratedpower;fDeadbandis49.85Hz;andfminis49.25Hz.Therecoveryperiod implementedinthemodelistwicethepowerincreasetime.Inertiaemulationisavailablefrom4%of ratedpower,anditstimeresponseiswithin800ms. 4.3.ProtectionsRelays Protectionrelayshavebeenmodeledforboththeconventionalunitsandthewindturbine generators.Theprotectionrelaysconsideredare:undervoltageandovervoltage,underfrequency andoverfrequency,overcurrentandoverspeed.Settingsfortheseprotectionrelaysaregivenin Table2. Table2.Protectionrelaysettingsforconventionalunitsandwindturbines. Generator type Parameter Under voltage Overvoltage Under frequency Over frequency Over current 1 Over current 2 Over speed Conventional generator Value 0.75 pu 1.12 pu 47 Hz 52.2 Hz 1.25 pu 3 pu 1.08 pu Delay (s) 0.8 1 1.25 2 6.5 0.5 Instant Wind turbine Value 0.8 pu 1.12 pu 47 Hz 51 Hz 1.25 pu 3 pu 1.08 pu (Only SCIG WT) Delay (s) 5 0.3 1.3 0.1 6.5 0.5 Instant (Only SCIG WT) 4.4.LoadSheddingScheme Aloadsheddingschemehasbeenimplementedinthepowersystemmodel.Loadshedding relaysareassociatedwithloadbuses.Threestepshavebeenincludedintheloadsheddingscheme. Thesettings’frequencyvaluesforfirst,secondandthirdstepsare49.0,48.9and48.8Hz. Figure 7. Basic inertia emulation model block diagram. Energies2015,8,page–page 8 Figure7.Basicinertiaemulationmodelblockdiagram. Figure8.Graphbehavioroftheinertiaemulationconsidered. is10%ofratedpower;fDeadbandis49.85Hz;andfminis49.25Hz.Therecoveryperiod implementedinthemodelistwicethepowerincreasetime.Inertiaemulationisavailablefrom4%of ratedpower,anditstimeresponseiswithin800ms. 4.3.ProtectionsRelays Protectionrelayshavebeenmodeledforboththeconventionalunitsandthewindturbine generators.Theprotectionrelaysconsideredare:undervoltageandovervoltage,underfrequency andoverfrequency,overcurrentandoverspeed.Settingsfortheseprotectionrelaysaregivenin Table2. Table2.Protectionrelaysettingsforconventionalunitsandwindturbines. Generator type Parameter Under voltage Overvoltage Under frequency Over frequency Over current 1 Over current 2 Over speed Conventional generator Value 0.75 pu 1.12 pu 47 Hz 52.2 Hz 1.25 pu 3 pu 1.08 pu Delay (s) 0.8 1 1.25 2 6.5 0.5 Instant Wind turbine Value 0.8 pu 1.12 pu 47 Hz 51 Hz 1.25 pu 3 pu 1.08 pu (Only SCIG WT) Delay (s) 5 0.3 1.3 0.1 6.5 0.5 Instant (Only SCIG WT) 4.4.LoadSheddingScheme Aloadsheddingschemehasbeenimplementedinthepowersystemmodel.Loadshedding relaysareassociatedwithloadbuses.Threestepshavebeenincludedintheloadsheddingscheme. Thesettings’frequencyvaluesforfirst,secondandthirdstepsare49.0,48.9and48.8Hz. Figure 8. Graph behavior of the inertia emulation considered. Pext max is 10% of rated power; fDeadband is 49.85 Hz; and fmin is 49.25 Hz. The recovery period implemented in the model is twice the power increase time. Inertia emulation is available from 4% of rated power, and its time response is within 800 ms. 4.3. Protections Relays Protection relays have been modeled for both the conventional units and the wind turbine generators. The protection relays considered are: under voltage and overvoltage, under frequency and over frequency, over current and over speed. Settings for these protection relays are given in Table 2. Table 2. Protection relay settings for conventional units and wind turbines. Generator type Parameter Under voltage Overvoltage Under frequency Over frequency Over current 1 Over current 2 Over speed Conventional generator Value 0.75 pu 1.12 pu 47 Hz 52.2 Hz 1.25 pu 3 pu 1.08 pu Delay (s) 0.8 1 1.25 2 6.5 0.5 Instant Wind turbine Value 0.8 pu 1.12 pu 47 Hz 51 Hz 1.25 pu 3 pu 1.08 pu (Only SCIG WT) Delay (s) 5 0.3 1.3 0.1 6.5 0.5 Instant (Only SCIG WT) 4.4. Load Shedding Scheme A load shedding scheme has been implemented in the power system model. Load shedding relays are associated with load buses. Three steps have been included in the load shedding scheme. The settings’ frequency values for first, second and third steps are 49.0, 48.9 and 48.8 Hz. 12676
Energies 2015,8, 12669–12684 Tripping of the first and second steps causes a load shedding higher than 10%. The setting time delay for this tripping is 0.45 s. 5. Results and Discussion Simulation results have been analyzed to investigate which of the three criteria mentioned in the Introduction section determines the CCT in the buses of the Lanzarote-Fuerteventura 2020 power system. The answer was found to be load shedding in all cases. Figure 9presents the CCT values obtained for all of the wind turbine types: SCIG, full converter and full converter with inertia emulation capability. It can be seen that there is a progressive decrease in all of them when wind power is injected for the five buses under short circuit. Energies2015,8,page–page 9 Trippingofthefirstandsecondstepscausesaloadsheddinghigherthan10%.Thesettingtime delayforthistrippingis0.45s. 5.ResultsandDiscussion Simulationresultshavebeenanalyzedtoinvestigatewhichofthethreecriteriamentionedinthe IntroductionsectiondeterminestheCCTinthebusesoftheLanzarote‐Fuerteventura2020power system.Theanswerwasfoundtobeloadsheddinginallcases.Figure9presentstheCCTvalues obtainedforallofthewindturbinetypes:SCIG,fullconverterandfullconverterwithinertia emulationcapability.Itcanbeseenthatthereisaprogressivedecreaseinallofthemwhenwind powerisinjectedforthefivebusesundershortcircuit. (a)(b) (c)(d) (e) Figure9.Criticalclearingtime(CCT)valuesobtained:(a)PuntaGrandebus;(b)Haría‐Teguisebus; (c)LasSalinasbus;(d)Jandíabus;(e)Corralejobus. 100 120 140 160 180 200 220 240 260 0 20406080100120140 CriticalClearingTime(ms) WindPowerGenerated(MW) PuntaGrandeBus 150 200 250 300 350 400 450 500 550 0 20406080100120140160 CriticalClearingTime(ms) WindPowerGenerated(MW) Haría‐TeguiseBus 150 170 190 210 230 250 270 290 310 330 0 20406080100120140 CriticalClearingTime(ms) WindPowerGenerated(MW) LasSalinasBus 150 250 350 450 550 650 750 850 0 20406080100120140 CriticalClearingTime(ms) WindPowerGenerated(MW) JandíaBus 150 200 250 300 350 400 450 500 550 600 0 20406080100120140 CriticalClearingTime(ms) WindPowerGenerated (MW) CorralejoBus Full Converter Wind Turbine Full Converter Wind Turbine with Inera Emulaon SCIG Wind Turbine Figure 9. Critical clearing time (CCT) values obtained: (a) Punta Grande bus; (b) Haría-Teguise bus; (c) Las Salinas bus; (d) Jandía bus; (e) Corralejo bus. 12677
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