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Power QualityPQ characteristics of wind turbines

References

ECE 566: Grid Integration of Wind EnergySystems

S. Suryanarayanan

Associate ProfessorECE Dept.

Suryanarayanan ECE 566 Lecture/Week 9


References

Reminders and notifications1 Homework 4 is assigned2 Project: Information on team membership and topic is DUE



ReferencesA brief introduction to PQ

Power Quality (PQ)

What is power quality? [1]Definition?

PQ means different things to different entities: utility;manufacturer; and customerIt may be defined as the faithfulness of a voltage waveformat the point of common coupling to a perfectly sinusoidalwave oscillating at the given power frequency withoutinterruptions.




Power Quality (PQ)

What is power quality? [1]Definition?PQ means different things to different entities: utility;manufacturer; and customer

It may be defined as the faithfulness of a voltage waveformat the point of common coupling to a perfectly sinusoidalwave oscillating at the given power frequency withoutinterruptions.




Power Quality (PQ)

What is power quality? [1]Definition?PQ means different things to different entities: utility;manufacturer; and customerIt may be defined as the faithfulness of a voltage waveformat the point of common coupling to a perfectly sinusoidalwave oscillating at the given power frequency withoutinterruptions.




Power Quality

What is power quality? [1]Lack of power quality is an expensive propositionE.g.: compromised supply reliability; malfunctioningequipment; overheating.Interface issueAny switching in the grid will introduce harmonics,transients, or disruptions to the said voltage waveformAccentuated by the proliferation of switching elementsinterconnected to the grid




Classification of power quality [1]Based on magnitude, frequency, and continuity attributesCommon PQ phenomena:

1 Transients: impulsive and oscillatory2 Long-duration voltage variations: over-voltage,

under-voltage, and sustained interruptions3 Short-duration voltage variations: swell, sag, and

momentary interruptions4 Voltage imbalances5 Distortions: harmonics, DC offset, interharmonics,

notching, and noise6 Voltage fluctuations: Flicker7 Power frequency variations




Some definitions [1]

TransientsTransients are the result of a sudden change in the steadystate operation conditions of a system that may produce anundesirable and momentary response. Also called spikesor surges.Impulsive transients are sudden, non-power frequencychanges in the steady state operation condition (V or I)which is unidirectional in polarity and is characterized byvery fast rise and decay times [1].Oscillatory transients are sudden, non-power frequencychanges in the steady state operation condition (V or I)which is bidirectional in polarity and is characterized byrelatively slower rise and decay times [1].




Transients [1]

Figure : Impulsive transient following a lightning strike. Figure from [2]




Transients [1]

Figure : Oscillatory transient following a cap switching. Figure from [3]





Long-duration voltage variations

Typically refers to RMS deviations at power frequencieswhich last for at least 1 minute.Over-voltage refers to the increase in Vrms ≥ 110% atpower freq. for longer than 60 seconds [1].Under-voltages refers to the decrease in Vrms ≤ 90% atpower freq. for longer than 60 seconds [1].






Not caused by faults in the system; rather, caused by loadvariations and switching operationsOver-voltages are produced by switching OFF a large loador switching ON a cap bank or incorrect transformer tapsettings, especially on weak grids with inadequate voltageregulation controlsUnder-voltages are produced by events opposite toover-voltages causing ones. Colloquially termedbrownout.






When a supply voltage magnitude value is fixed at zero fora time that exceeds 60 seconds, that long-duration voltagevariation is termed as a sustained interruption.Such interruptions usually require manual intervention forrepairUtilities use this for quantifying reliability for reportingpurposesMetrics include: SAIFI, SAIDI, CAIFI, CAIDI, ASAI





Short-duration voltage variations

Typically refers to RMS deviations at power frequencieswhich last for no more than 1 minute.Designated as instantaneous, momentary, or temporary[1]Most usually caused by faults in the systemIncludes sags, swells, and interruptions






Interruptions are events during which the |V | ≤ 10% for nomore than 60 secondsCaused by system faults, equipment and controlfailures/malfunctionsInterruptions may be preceded by a voltage sag due to thecausal event (fault). Then, breakers may isolate a part ofthe system leading to interceptionsAutomatic reclosers are set to operate in 0.5 secondswhen the fault is non-permanentInterruptions due to control or equipmentfailure/malfunction may have irregular duration




Momentary interruption [1]

Figure : 3-φVrms for momentary interruption due to a fault andrecloser action. Figure from [1]






A voltage sag (or dip) is a condition when .10 ≤ Vrms ≤ .90puCauses: system faults; motor starting; energization ofheavy loads leading to heavy current drawSags are quantified using CBEMA or ITIC [4]Sags lasting less than 1

2 cycles are classified as transients,since it is difficult to measure their RMSSags lasting longer than 60 seconds are grouped underunder-voltages, and attributed to causes other than faults





Short-duration voltage variationsA voltage swell is a condition when 1.10 ≤ Vrms ≤ 1.80 pufor 0.033 ≤ tdur ≤ 60sCauses: system faults, typically unsymmetricalSwells are quantified using CBEMA or ITIC [4]




Voltage sags [1]

Figure : CBEMA and ITIC curves. Figure from [4]





Voltage unbalance

Defined as the ratio of the maximum deviation from theaverage 3φ voltage (or current) to the average 3φ voltage(or current), expressed in %The main source of voltage unbalances (≤ 2%) is 1φ loadson a 3φ circuitAlso caused by blown fuses in one phase of the threephasesSevere imbalance (≥ 5%) is caused by single-phasingconditions, i.e., when one phase of the connecting supplyto a 3φ motor or transformer is cut-off [5].





Waveform distortionsDC offsetHarmonicsInterharmonicsNotchingNoise





Waveform distortionsDC offset

When a DC bias (i.e., average value 6== 0) is present in anAC waveform, a DC offset is said to occurCauses: geomagnetic disturbance or asymmetry of powerelectronic converters (i.e., 1

2 -wave rectifiers)Can lead to transformer saturation during normal operationCan lead to increased heating and loss of transformer lifeCan also cause electrolytic erosion of grounding & andother connectors





Waveform distortionsHarmonics

VTHD =

√(V 2

2 + V 23 + V 2

4 + ...)

V1(1)

Harmonics are sinusoidal components of voltages (orcurrents) with frequencies that are integer multiples of thepower frequency (i.e., V2 has a frequency of 2× 60)Presence of harmonics distorts the ideal sinusoid; Can bestudied by Fourier decomposition of the time-seriesCaused by inherent non-linearity in the loads, switchingdevices, etc.May impact loads by heating, increased losses,malfunctions, etc.




Voltage waveform distortions [1]

Figure : An FFT based decomposition of a distorted waveform.Figure from [6]





Waveform distortionsInter-harmonics

Are frequency components of voltages (or currents) withfrequencies that are non-integer multiples of the powerfrequency (i.e., V2.5 has a frequency of 2.5× 60)Causes: static frequency converters, cyclo-converters,induction furnaces, arcing devicesImpacts: can excite severe resonant modes, affect powerline carrier signals, & introduce flickering.





Waveform distortionsNotching

Periodic voltage disturbances caused by normal operationof power electronic devices when current is commutatedfrom one phase to anotherDuring the current commutation, there is a momentaryshort circuit between the 2 associated phases. This causesthe voltage to drop closer to zero as allowed by the systemimpedances. This manifests as a notch.Due to its continuous nature, it can be characterizedthrough the harmonic spectrum of the affected voltageThe associated freq. components can be pretty high andnot easily captured by harmonic analysis equipment




Voltage waveform distortions [1]

Figure : A waveform distorted by notches. Figure from [7]





Waveform distortionsNoise

When unwanted electrical signals with broadband spectralcontent lower than 200 kHz distorts the system voltage (orcurrent), then it is called noiseCause: power electronic devices, control circuits, arcingeqpt., solid-state rectifiers, switching power supplies, etc.Problem accentuated by improper grounding circuit. Thiseliminates a safe and easy path for high freq. componentsto leave the circuitImpact reduced by using filters, isolation transformers, lineconditioners, etc.





Voltage fluctuations

These are systematic variations of the voltage envelope ora series of voltage changes, the magnitude of which doesnot normally exceed the ranges specified by standardssuch as the ANSI C84.1 (.9 ≤ |V | ≤ 1.1 pu)Flicker is a visual manifestation of an electromagneticphenomenon, i.e., the voltage fluctuationLoads can exhibit continuous and rapid variations in theload current magnitudes (i.e., in arc furnaces) that cancause voltage fluctuationsFlicker exists in the low frequency voltage modulation ofthe voltage at the system frequency




Voltage fluctuation [1]

Figure : Voltage fluctuation characteristic of an arc furnace load.Figure from [8]






Flicker is measured subjective to the sensitivity of thehuman eyeIEC 61000-4-15 is the standard that defines the methodsand specifications of instrumentation for measuring flickerThe measurement method simulates the lamp/eye/brainfunction and produces a fundamental metric calledshort-term flicker sensation (Pst)Pst is normalized to 1.0 to represent the level of voltagefluctuation required to cause noticeable flicker to 50% of asample observers






Plt is the long-term flicker sensationUsed for verifying compliance with compatibility limitsestablished by standards bodies and used in utilitycontractsObtained as an average of Pst samples




Voltage fluctuation [1]

Figure : Pst and Plt plots. Figures from [9]





Power frequency variationsDefined as the deviation of the power system fundamentalfrequency from the specified nominal valuePower system freq. is directly related to the rotationalspeed of the generators in the systemAny variation in this value is largely due to imbalancesbetween the demand and supplyIn interconnected systems, the accepted limits for systemfrequency are tightIn isolated systems, freq. deviations are expected to bemore profound. Here, the governor responses to abruptload changes may not be adequate to regulate within thetight bandwidth reqd. by freq. sensitive eqpt.



ReferencesIntroduction

Introduction [10]

The needWhen wind power is injected into the grid, it will alter thepower qualityThese changes to PQ must comply with requirements elsethey may cause loss of supply, $, and comfortPQ is an interface issue—hence, wind turbines/farmsconnecting to grid is where the PQ should be assessed




Introduction [10]

The needPQ standards for the grid exist: IEEE 519, etc.Wind turbines are not manufactured to site specifications,rather to manufacturer specificationsNeed for consistency in assessment of PQ characteristicsof such a challenging set up led to the development of theIEC 61400-21




Introduction [10]

The needHistory of IEC 61400-21 (‘Measurement and assessmentof power quality characteristics of grid connected windturbines’)

Work began in 1996First publication in 2001Second (revised) edition including recent experiences,technical needs, and harmonizing with grid codes, waspublished in 2008

IEC 61400-21 describes procedures for determining thePQ characteristics of wind turbinesWe will look at the application rather than the proceduresthemselves in our scope




Introduction [10]

PQ chars. of wind turbines as described by IEC 61400-21Rated dataEmission of voltage fluctuations and flickerEmission of current harmonics, interharmonics, and higherfreq. componentsResponse to voltage dipsActive power capabilities and controlReactive power capabilities and controlGrid protection and reconnection times




PQ chars. of wind turbines as described by IEC61400-21 [10]

Rated data [11]Rated data is used for normalizing purposes in IEC 61400-21[10]

Active power at the wind turbine terminals, Pn

Reactive power at the wind turbine terminals, Qn

Apparent power at the wind turbine terminals, Sn

Rated voltage (phase-to-phase) at the wind turbineterminals, Un

Rated current at the wind turbine terminals, In





Emission of voltage fluctuations and flicker

Flicker is the visual manifestation of voltage fluctuationsThis can be measured using a flickermeterInput to this instrument is the voltage; output is the severityof the flicker (i.e., Pst)So long as a Pst < 1.0, customers are generallyfrustrated/affectedSince voltage fluctuations are caused by changes in loador generation, wind turbines are especially credited withcausing themIEC 61400-21 differentiates between continuous andswitching operations for flicker quantification






Continuous operation—Flicker coefficientFlicker coefft. is a normalized measure of the max. flickeremitted (99 percentile) from a wind turbine in continuousoperation

c(ψk , νk ) = Pst × Sk

Sn(2)

ψk is network impedance phase angleνk is the annual wind speed averagePst is the flicker emission from the wind turbineSk is the short crkt. apparent power of grid






Continuous operation—Flicker coefficientFlicker coefft. of a wind turbine is given as the 99percentile for specified values of:ψk =

{30◦,50◦,70◦,85◦} and

νk ={

6,7.5,8.5,10}

m/sVariable speed machines are commonly expected to havelower flicker coefficientsFixed-speed machines may have high value (pitchcontrolled) to average values (stall controlled).






Switching operationSome switching events that may cause voltage fluctuations:turbine start-up at cut-in speedTurbine start-up at rated speedPole switching in fixed-speed generators

Acceptance depends on:grid voltagenumber of times of occurrences: max. number of switchingoperations within a 10 minute period (N10) and a 2 hourperiod (N120) must be specified.N10 and N120 depend on control system settings






Switching operation—Flicker step factorFlicker step factor is a normalized measure of the flickeremitted due to a single switching operation of a windturbine

kf (ψk ) =1

130× Sk

Sn× Pst × T 1.31

p (3)

kf (ψk ) is the flicker step factorTp is the duration of the voltage fluctuation






Switching operation—Flicker step factorFlicker step factor of a wind turbine is given for specifiedvalues of:ψk =

{30◦,50◦,70◦,85◦} and for specified switching

actionsVariable speed machines are commonly expected to havelower flicker step factorFixed-speed machines may have average value (pitchcontrolled) to high values (stall controlled).






Switching operation—Voltage change factorVoltage change factor is a normalized measure of thevoltage change due to a single switching operation of awind turbine

ku(ψk ) =√

3× Sk

Sn× Umax − Umin

Un(4)

ku(ψk ) is the voltage change factorUmax & Umin are the max. & min. RMS (phase-to-neutral) voltages due to switching






Switching operation—Voltage change factorVoltage change factor of a wind turbine is given forspecified values of:ψk =

{30◦,50◦,70◦,85◦} and for specified switching

actionsVariable speed machines are commonly expected to havelower flicker step factorFixed-speed machines may have average value (pitchcontrolled) to high values (stall controlled)max(ku) = ki , where ki is the ratio between the max.inrush current and the rated currentHowever, ku is function of ψkSuryanarayanan ECE 566 Lecture/Week 9




Current harmonics, interharmonics, and higher freq.components

These are to be stated for the operation of a wind turbinewithin the active power bins ofPn =

{0,10,20, ...,90,100

}%

THD (current) is derived from the individual harmonics





Current harmonics, interharmonics, and higher freq.components

Interharmonics are specified up to 2kHz according toAnnex-A of IEC 61000-4-7,2002Higher freq. comp. specified for 2 ≤ f ≤ 9kHz according toAnnex-B of IEC 61000-4-7,2002All freq. comp. for stated for wind turbine operating at zeroreactive power draw/injectionIEC 61400-21 does not require specification of harmonicsat start-up or switching operations





Response to voltage dips

Voltage dips or sags are due to faults in the system andmodern turbines are required to stay online to ride throughthe fault (LVRT, ZVRT requirement)IEC 61400-21 requires wind turbines to be tested towithstand certain sags stating the response by thetime-series of the following:

PnQnUnIn





Response to voltage dips

Tests are carried out for the turbines operating at low(0.1Pn −−0.3Pn) and high power (0.9Pn) outputsTests include symmetrical 3φ sags at

{0.9,0.5,0.2

}pu

voltage with duration of{

0.5,0.5,0.2}

s respectivelySame holds true for two-phase sagsTest verifies response of wind turbine to voltage sag





Active power capabilities and controlIEC 61400-21 considers max. measured power, ramp ratelimits, and set-point controlMax. measured power is stated for

{P600,P60,P0.2

},

measured as a{

600,60,0.2}

s average respectivelyThese are relevant data for load-flow studies andprotection setting studiesVariable speed turbines may typically provide forP0.2 = P60 = P600 = Pn, whereasIn fixed-speed turbines (stall or pitch controlled) P0.2 > Pn





Active power capabilities and controlAbility of wind turbine to operate in ramp rate limitationmode or set-point mode is germane to situations in gridswith high penetration of wind energyThis ability is verified by two 10min test periods showingthe time-series of the available and measured P0.2 outputTest on ramp rate limit is conducted for operation at a ramprate of 10% or Prated per minuteTest on set-point control is conducted for operationadjustments from 100% to 20% of Prated with 2minoperation at each set-point





Reactive power capabilities and controlReactive power is tested as the maximum 1min averageinductive and capacitive Q of the wind turbine for{

0,10,20, ...90,100}

% of rated active power outputAbility to control Q is verified by two tests:

Allowing the reactive power set-point to zero and themeasuring the 1min average reactive power for{

0,10,20, ...90,100}

% of ratedOperating at about 50% of rated power, the reactiveset-point is to be changed from 0 to maximum capacitivevalue and after 2min to its maximum inductive value, andthe measured P0.2 shall be known





Grid protection and reconnection timesIEC 61400-21 requires the functionality of grid protection tobe verified by testingActual disconnection levels and times are determined forover-voltages (frequencies) and under-voltages(frequencies)Reconnection time after disconnection of the turbine due togrid failure shall be state for cases where the grid hasfailed for

{10,60,600

}s

Reconnection time is the time from the instant when thegrid service is available again to the instant when theturbine start producing power.



References

R.C. Dugan et al. Electrical power systems quality.McGraw-Hill professional engineering. McGraw-Hill,2003. ISBN: 9780071386227. URL: http://books.google.com/books?id=welSAAAAMAAJ.

Power quality in electrical systems: Impulsive transientsin power systems. URL: http://goo.gl/4rm527(visited on 05/10/2011).

Joshipura. Transient Overvoltages. URL:http://goo.gl/qXEkFc (visited on 01/27/2012).

ITIC curve - Power acceptability curve for informationtechnology. URL: http://goo.gl/MKP2pX (visited on04/04/2011).

Electrotechnick: A magazine on electrical engineering.URL: http://goo.gl/09bnav.


http://books.google.com/books?id=welSAAAAMAAJ

http://books.google.com/books?id=welSAAAAMAAJ

http://goo.gl/4rm527

http://goo.gl/qXEkFc

http://goo.gl/MKP2pX

http://goo.gl/09bnav


References

Artur Litovka. Effect of harmonics on power distributionsystems. Aero Nav Labs. URL: http://aeronavlabs.blog.com/2014/01/23/effect-of-harmonics-on-power-distribution-systems/ (visited on 01/23/2014).

Notching. Progress Energy. URL:http://goo.gl/NDvpzO.

Mark McGranaghan. Getting a handle on lighting flicker.Electrical Construction and Maintenance. URL:http://ecmweb.com/content/getting-handle-lighting-flicker (visited on 11/01/2002).

Flicker in the power master series. Outram research Ltd.URL: http://www.outramresearch.co.uk/pages/product_flicker.shtml.


http://aeronavlabs.blog.com/2014/01/23/effect-of-harmonics-on-power-distribution-systems/




http://goo.gl/NDvpzO

http://ecmweb.com/content/getting-handle-lighting-flicker

http://ecmweb.com/content/getting-handle-lighting-flicker

http://www.outramresearch.co.uk/pages/product_flicker.shtml

http://www.outramresearch.co.uk/pages/product_flicker.shtml


References

T. Ackermann. Wind Power in Power Systems. Wiley,2012. ISBN: 9781119941835. URL: http://books.google.com/books?id=QM60LmgaeeQC.

Power quality measurement procedure, Ver. 4. MeasNet.URL: http://goo.gl/m8K30u (visited on 10/01/2009).


http://books.google.com/books?id=QM60LmgaeeQC

http://books.google.com/books?id=QM60LmgaeeQC

http://goo.gl/m8K30u

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