Symmetrical And Unsymmetrical Fault Currents Of A Wind .
Symmetrical and UnsymmetricalFault Currents of a Wind PowerPlantPreprintV. Gevorgian, M. Singh, and E. MuljadiTo be presented at the IEEE Power and Energy Society GeneralMeetingSan Diego, CaliforniaJuly 22-26, 2012NREL is a national laboratory of the U.S. Department of Energy, Office of EnergyEfficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.Conference PaperNREL/CP-5500-53463December 2011Contract No. DE-AC36-08GO28308
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Symmetrical and Unsymmetrical Fault Currents of a Wind Power PlantV. GevorgianMember IEEEvahan.gevorgian@nrel.govM. SinghMember IEEEmohit.singh@nrel.govE. MuljadiFellow, IEEEeduard.muljadi@nrel.govNational Renewable Energy Laboratory1617 Cole Blvd.Golden, CO 80401, USAoped by the Western Electricity Coordinating Council(WECC) Wind Generator Modeling and Validation WorkGroup (WGMG) [3]. The WGMG recommends the use of asingle-machine equivalent representation of multiple windturbines operating in a single WPP. Based on industry experience, this representation is also considered adequate for positive-sequence transient-stability simulations. The WECCsingle-machine equivalent representation of a WPP is shownin Figure 1. The interconnection transmission lines, transformers, and reactive power compensation are present in thisrepresentation.Abstract – The size of wind power plants (WPPs) keeps gettingbigger and bigger. The number of wind plants in the U.S. hasincreased very rapidly in the past 10 years. It is projected thatin the U.S., the total wind power generation will reach 330 GWby 2030. As the importance of WPPs increases, planning engineers must perform impact studies used to evaluate short-circuitcurrent (SCC) contribution of the plant into the transmissionnetwork under different fault conditions. This information isneeded to size the circuit breakers, to establish the proper system protection, and to choose the transient suppressor in thecircuits within the WPP. This task can be challenging to protection engineers due to the topology differences between differenttypes of wind turbine generators (WTGs) and the conventionalgenerating units.This paper investigates the short-circuit behavior of a WPPfor different types of wind turbines. Both symmetrical faultsand unsymmetrical faults are investigated. Three different software packages are utilized to develop this paper. Time domainsimulations and steady-state calculations are used to perform theanalysis.Index Terms — Fault contribution, induction generator, protection, short circuit, wind power plant, and wind turbine.EI. INTRODUCTIONFigure 1: A single-machine equivalent representation of a WPPnergy and environmental issues have become one of thebiggest challenges facing the world. In response to energyneeds and environmental concerns, renewable energy technologies are considered the future technologies of choice [1],[2]. Renewable energy is harvested from nature, and it isclean and free. However, it is widely accepted that renewableenergy is not a panacea that comes without challenges. Withthe federal government’s aggressive goal of achieving 20%wind energy penetration by 2030, it is necessary to understand the challenges that must be overcome when using renewable energy.In the years to come, there will be more and more windpower plants (WPPs) connected to the grid. With the goal of20% wind penetration by 2030, the WPP’s operation shouldbe well planned. The power system switchgear and powersystem protection for WPPs should be carefully designed tobe compatible with the operation of conventional synchronous generators connected to the same grid. This paper attempts to illustrate the behavior of short-circuit current (SCC)contributions for different types of WTGs.The power system model used for WPP short-circuit behavior simulation was adopted from a modeling guide devel-Organization of the PaperThe organization of this paper is as follows; in section II,the SCC characteristics of different WTG types will be presented for a symmetrical fault. In section III, the characteristics of SCC for unsymmetrical faults will be discussed. Finally, in section V, the conclusion will summarize the paper’sfindings. Detailed dynamic modeling of four different typesof WTGs is simulated in PSCADTM.II. SHORT-CIRCUIT BEHAVIOR UNDER SYMMETRICAL FAULTSA utility-sized wind turbine is larger than non-grid windturbine applications. In the early days, the turbines weresized from 10 kW to 100 kW. Nowadays, wind turbines aresized above 1000 kW (1 MW).A. SCC from a Type 1 WTGThe first generation of utility-sized WTGs were fixedspeed turbines with a squirrel-cage induction generator(SCIG) and is called a Type I generator in wind-related applications. The SCIG generates electricity when it is drivenabove synchronous speed. Normal operating slips for an in-1
duction generator are between 0% and -1%. The simplifiedsingle-phase equivalent circuit of a squirrel-cage inductionmachine is shown in Figure 2 [4].The circuit in Figure 2 is referred to the stator side whereRS and Rr are stator and rotor resistances, Lsσ and Lrσ are stator and rotor leakage inductances, Lm is magnetizing reactance, and s is rotor slip. The example single-line connectiondiagram of a Type I generator is shown in Figure 3. In thecase of a fault, the inertia of the wind rotor drives the generator after the voltage drops at the generator terminals, the pitchcontroller must be deployed to avoid a run-away problem.The rotor flux may not change instantaneously right after thevoltage drop due to a fault. Therefore, voltage is produced atthe generator terminals causing fault current flow into thefault until the rotor flux decays to zero. This process takes afew electrical cycles. The fault current produced by an induction generator must be considered when selecting the ratingfor circuit breakers and fuses. The fault current is limited bygenerator impedance (and can be calculated from parametersin Figure 2) and impedance of the system from the short circuit to the generator terminals.23;415;As shown in Figure 4, the fault current is driven by the decaying flux trapped in the rotor winding as represented by theright portion of equation (1). The larger the leakage inductances (σ), the smaller is the fault current amplitude. The faultcurrent dies out after the flux driving the fault current depleted to zero. Note that the DC and AC transient components ofthe SCC flowing out of the stator windings induce fault currents in the rotor winding and vice versa until the magneticflux is depleted.Min Isc-peakMax Isc-peakFigure 4: SCC from a Type I WTGFigure 2: Equivalent circuit of a Type 1 generatorThe current calculated from equation (4) is shown in Figure4 using parameters for a typical 2-MW induction generatorwhen and pre-fault voltage of 0.7 p.u. The current reaches themaximum value at π (first half a period). Therefore, it may bea good approximation to calculate the maximum (peak) current by substituting/2 into (1). The resulting equationfor peak current will be: 16It was demonstrated experimentally in [6] that equation (6)gives satisfactory accuracy for peak current assessment.Figure 3: Connection diagram for a Type 1 WTGThe initial value of fault current fed in by the inductiongenerator is close to the locked rotor-inrush current. Assuming a three-phase symmetrical fault, an analytical solution canbe found to estimate the current contribution of the generator.The SCC of an induction generator can be calculated as [5]: 2sin1sin1C. SCC from a Type 2 WTGThe variable slip generator is essentially a wound-rotor induction generator with a variable resistor connected in seriesto the rotor winding (for Type 2 WTGs, refer to Figure 5 andFigure 6).Where α is the voltage phase angle for a given phase, σ is′′the leakage factor,is stator transient reac′tance, andand ′ are stator and rotor time constants representing the damping of the DC component in stator and rotorwindings. The transient stator and rotor inductancesandcan be determined from the equation (2).2
from damaging the power converter. Additional dynamicbraking on the DC bus is also used to limit the DC bus voltage.E. Type 4 WTGsA typical connection diagram for a Type 4 WTG is shownin Figure 8. The SCC contribution for a three-phase fault islimited to its rated current or a little above its rated current. Itis common to design a power converter for a Type 4 windturbine with an overload capability of 10% above rated. Notethat in any fault condition, the generator stays connected tothe power converter and is buffered from the faulted lines onthe grid.Figure 5: Equivalent circuit for a Type 2 generatorThe modified rotor time constant can be calculated by addingthe effect of the external resistor Rext, where Rext is the valueof external resistance that happens to be in the circuit at thetime of the fault. So, adding the external resistors increasesthe overall rotor resistance.Figure 8: PMSG direct-drive WTG diagramFigure 6: Connection diagram for a Type 2 WTGF. SCC Comparison for Symmetrical FaultsThe SCC for different types of wind turbines are not thesame. For each turbine type, the peak value of the magnitudeof the SCC is affected by the transient reactance, the pre-faultvoltage, the effective rotor resistances, and other circumstances at the instant the fault occurs.As shown in Figure 9, the Type 1 WTG has the largest SCCand the shortest settling time. The Type 2 WTG has an additional rotor resistance that is activated above rated wind speedto limit the output power of the generator. Below rated windspeed, the SCC behavior of the Type 2 WTG is similar to theType 1 WTG. Above rated wind speed, the SCC behavior ofType 2 WTG is affected by the external rotor resistance. Thesettling time of the SCC of Type 2 WTG is lower than thesettling time of the SCC of Type 1 WTG.The SCC behavior of the Type 3 WTG is affected by thecrowbar and the dynamic braking actions. For a very nearfault, the crowbar may be fully deployed and thus, short circuiting the rotor winding, and the SCC behavior resemblesthe Type 1 WTG; however, if the crowbar and the dynamicbraking can maintain the operation of the rotor side powerconverter, the SCC behavior is very close to a Type 4 WTG.For almost all of the SCC for Type 3 WTGs, only a smallamount of SCC current is passed through the power converterbecause of the current limit of the power semiconductors.The Type 4 WTG has a full power converter between thegenerator and the grid, thus, the SCC is very well regulatedwith SCC maintained at 1.1 p.u. rated current.D. SCC from a Type 3 WTGA Type 3 WTG is implemented by a doubly-fed inductiongenerator (DFIG). It is a variable speed WTG where the rotorspeed is allowed to vary within a slip range of 30%. Thus,the power converter can be sized to about 30% of rated power. The DFIG equivalent circuit is similar to one for a regular induction generator except for additional rotor voltage,representing voltage produced by a power converter. Undernormal operation, this voltage is actually from a currentcontrolled power converter with the ability to control the realand reactive power output instantaneously and independently.Figure 7: Connection diagram for a Type 3 WTGThe typical connection diagram for a DFIG (Type 3) WTGis shown in Figure 7. In an ideal situation, the power converter connected to the rotor winding should be able to withstand the currents induced by the DC and AC componentsflowing in the stator winding. However, the components ofthe power converter (IGBT, diode, and capacitor, etc) aredesigned to handle only normal currents and normal DC busvoltage. A crowbar system is usually used for protecting thepower electronics converter from overvoltage and thermalbreakdown during short-circuit faults. A crowbar is usuallyimplemented to allow the insertion of additional resistanceinto the rotor winding to divert the SCC in the rotor windingG. Summary of the Symmetrical Fault SCCTo summarize, the SCC for a symmetrical faults can be approximated by the values listed in Table I [7]. Both the maximum and the minimum values are shown.3
III. UNSYMMETRICAL FAULTSTABLE IThe nature of the fault produces a different response fordifferent wind turbine types. In this section, the observationof the short-circuit behavior for unsymmetrical faults on different types of WTGs will be presented.Note that operating an induction generator under an unbalanced condition creates torque pulsation and unbalanced currents. If this condition persists for a long period of time, itmay excite other parts of the wind turbine, and the unbalanced currents may create unequal heating in the three-phasewindings, thus, shorten the life of the winding insulation.Unlike in a symmetrical three-phase fault, the positivesequence voltage source continues to drive the fault current during the fault. As shown in Figure 11 and Figure 12, the remaining un-faulted (normal) phases continue to maintain the air-gapflux. The initial conditions of the fault currents are different foreach phase. The three line currents usually show a different DCoffset, which eventually settles out over time.MAXIMUM AND MINIMUM POSSIBLE VALUE OF THE SCCWTGMaxISC PEAKMinISC PEAKType 12Type 22V sX2'S2V sX'S2V s2VsXX S'' 2S (9 Rr' ) 2Type 322VsX S'1.1IRATEDType 41.1IRATED0For a Type 1 WTG, the maximum SCC is based on the assumption that the DC offset is at the worst condition, and theminimum SCC is calculated by assuming that the DC offset iszero. For a Type 2 WTG, the maximum value is computed when 0 Ω. The minimum value is computed when the slipreaches 10% above synchronous speed. And for a Type 3 WTG,the maximum value is computed when the crowbar shorts therotor winding and the minimum value is computed when thepower converter can follow the commanded current (i.e., in casethe fault occurs far away from the point of interconnection (POI),the remaining terminal voltage is sufficiently high enough to letthe power converter operate normally and supply the commanded currents). Note that for a symmetrical fault, the actual faultcurrent for each phase is different from the other phases due tothe fact that the time of the fault occurs at a different phase anglefor different phases, thus affecting the DC offset. For a Type 4WTG, the stator current can always be controlled because of thenature of power converter, which is based on a current-controlledvoltage source converter.A time domain simulation is performed in PSCAD, and thesteady-state calculations are performed using Mathcad andCyme software for a symmetrical fault. The results are tabulated in Table II. The calculated results from different software platforms are very close to the approximation listed inTable I. Note, that only Type 1 and Type 4 are listed. TheType 2 and Type 3 WTGs will respond differently because ofthe existence of the external rotor resistance in a Type 2 WTGand the activation of the crowbar circuit in a Type 3 WTG,which will respond non-linearly to the fault. The SCC for aType 2 and Type 3 WTG, as indicated in Table I, will havethe size difference between the SCC of the Type 1 and Type 4WTGs.A. Single Line-to-Ground (SLG) FaultsThe single line-to-ground fault is the most likely to occur ina)Type 1 WTG.b)Type 2 WTG.c)Type 3 WTG.d)Type 4 WTG.TABLE IIISC PEAK COMPARISON FOR DIFFERENT SOFTWARE PLATFORMSWTGTable IPSCADCymeMathcadTypeMinMax13.4 p.u.6.3p.u.5 p.u.5.5 p.u.3.8 p.u.40 p.u.1.1p.u.1.1 p.u.1.1 p.u.1.1 p.u.Figure 9: SCC of a symmetrical fault for four types of WTGs4
a power system. The magnetic flux in the air gap, althoughsmaller than normal and unbalanced, is maintained by theremaining un-faulted lines. Thus, the short circuit in SLGfaults will continue to flow until the circuit breaker removesthe fault from the circuit.Figure 10 shows the sequence circuits of the WPP shown inFigure 1. The sequence circuits are arranged to compute theSLG fault currents. Although present in the actual simulation, the cable capacitance and the capacitor compensationshown in Figure 1 are not drawn in Figure 12 to avoid clutterand to simplify the drawing. We represent the transformerwinding connections in the zero sequence equivalent circuitas an on-off switch indicating the availability of the zero sequence current path. Since the low side of the pad-mountedtransformer is connected in delta, there is no sequence currentflowing out of the WTG.We also placed a switch at the negative sequence equivalent circuit for the WTG to indicate that there is no negativesequence current contribution from a Type 4 WTG because itis controlled to provide symmetrical currents regardless of theterminal voltage.In Figure 13, the SCC for a Type 1 WTG is shown both forthe three-phase currents and the corresponding sequencecomponents. The changes in positive sequence and the sudden appearance of the negative sequence are also shown. Theabsence of the zero sequence current is a consequence ofwinding connections.Figure 13: SCC for a single line-to-ground in a Type 1 WPPMain : Graphs1.801.60Type 4I2-I2oI6 I6-I6o1.401.201.00yOtherTypesFigure 11: SCC for SLG for a Type 3 WTGCURRENT (kA)I2 -20I3c0.4011.051.11.151.21.251.31.351.40.20TIME e 14: SCC for a single line-to-ground in a Type 4 WPPa) At the point of interconnectionb) At the wind generator terminalsFigure 12: SCC for LLG fault of a Type 2 WTGIn Figure 14, the SCC for a Type 4 WTG shows the faultcurrents in its sequence current components. At the POI(Figure 14a), there exist both the zero sequence and the negative sequence currents because of the substation transformerwinding connection (YgYg) and collector system capacitancesrespectively. As shown in Figure 14b, at the generator terminals however, there is a pad-mounted transformer (YgΔ)that will block the zero sequence component, and the Type 4WTG produces a positive sequence componen
This paper investigates the short-circuit behavior of a WPP for different types of s. Both symmetrical faults wind turbine and unsymmetrical faults investigated. Three different sofare t-ware packages utilizedare to develop this paper. Time domain simulations and steady-state calculations are used to perform the analysis.
The symmetrical components of the fault current are I a1 I a2 I a0 1/3I a V a1 V a2 V a0 ZfIa1 (ii) Line to line fault: line to line fault at any point in a power system then the currents and voltages at the fault can be expressed as- I a 0, I b -I c V b-V c I b Zf (iii) Double line to ground fault: For this condition the .
Symmetrical Components and Unsymmetrical Faults . Example 1: Example 1: Last Assignment Watch the 20 min video on how to navigate Powerworld when conducting fault analysis at the link below, then calculate the 3-phase and single-phase fault currents at each of the 6
Mar 01, 2018 · symmetrical components [1]. Even some fault location algorithms employ symmetrical components, especially the negative-sequence components due to their immunity to system load flow and mutual coupling effects and due to their better homogeneity in negative-sequence impedance networks [2]. Traditionally, the core use of symmetrical components has
CDS G3 Fault List (Numerical Order) Fault codes may be classified as sticky or not sticky: Type of fault Method to clear Not sticky Clears immediately after the fault is resolved Sticky Requires a key cycle (off and on) after the fault is resolved to clear. CDS G3 Fault Tables 90-8M0086113 SEPTEMBER 2013 Page 2G-3
In this paper, the calculation of double line to ground fault on 66/33 kV transmission line is emphasized. Figure 3. Double-Line-to-Ground Fault 2.4 Three-Phase Fault In this case, falling tower, failure of equipment or . ground fault, fault current on phase a is I a 0. Fault voltages at phase b and c are: V b .
Capacitors 5 – 6 Fault Finding & Testing Diodes,Varistors, EMC capacitors & Recifiers 7 – 10 Fault Finding & Testing Rotors 11 – 12 Fault Finding & Testing Stators 13 – 14 Fault Finding & Testing DC Welders 15 – 20 Fault Finding & Testing 3 Phase Alternators 21 – 26 Fault Finding & Testing
illustrated in Figure 3. This is a fault occurring with phase A to ground fault at 8 km measured from the sending bus as depicted in Figure 1. The fault signals generated using ATP/EMTP are interfaced to the MATLAB for the fault detection algorithm. (a) Sending end (b) Receiving end Figure 3. Example of ATP/EMTP simulated fault signals for AG fault
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