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PSCAD/EMTDC-Based Modeling and Flicker

Estimation for Wind Turbines

C. Carrillo(1), E. Díaz-Dorado(2) and J. Cidráscarrillo@uvigo.es, ediaz@uvigo.es, jcidras@uvigo.es

Department of Electrical EngineeringUniversidade de Vigo

SPAIN(1) http:// webs.uvigo.es/carrillo, (2) http://webs.uvigo.es/ediaz

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PSCAD/EMTDC-Based Modeling and Flicker Estimation for Wind

Turbines

1. Introduction2. Flicker and wind energy3. Flicker estimation4. Measurements5. Modeling6. Results7. Conclusions

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1. Introduction

Presence of wind energy in the generation share has been continuously increasing during last years.

IN SPAIN:

• During 2008, wind energy sharing was 11%.

• In 22th/jan/2009, new records of energy generation were reached: 11.074 MWh and 234.059 MWh/day.

• Installed capacity at the end of 2007 was 15.131 MW, 30% higher than the previous year.

IN OTHER COUNTRIES:

• At the end of 2007, wind energy sharing was 21,3% in Denmark and 11.7 % in E.U.

• In E.U., installed capacity has grown from 4.753 MW in 1997 to 56.535 MW in 2007.

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1. IntroductionTechnical requirements (grid codes) increase its level of exigencies as a consequence of wind energy growing. One aspect to be taken into account is the impact of wind energy in POWER QUALITY.

Emission limitImmunity level

0

100

0 0,8 time en s

RM

S V

olt

age

en %

Nominal voltage (90%)

Minimum voltage

Sag duration 38 39 40

time in s

volt

• Flicker• Harmonics• Voltage sag ride through

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2. Flicker and wind energy

Power delivered by wind turbines has variations than can provoke flicker due to:

0 100 200 300 400 500 600

5

6

7

win

d sp

eed

in m

/s

0 100 200 300 400 500 600

50

100

time in s

pow

er in

kW

• Random nature of wind Random power• Periodic fluctuations (shadow tower, wind shear, tower

oscillations,…)

10-1

100

101

win

d sp

eed

10-1

100

101

frequency in Hz

pow

er

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2. Flicker and wind energy

Grid codes take into account take into account the possibility of flicker emissions from wind plants. So, certain technical requirements are imposed:

IN SPAIN:

Royal Decree 661/2007: REAL DECRETO 661/2007 installed capacity will be less than a 5% of the short circuit power in the point of connection to the transmission network.

Standard organisations also consider this problem. There are specifics standards regarding to flicker and wind energy.

IEC:

IEC 61400-21 Wind turbines – Part 21: Measurement and assessment of power quality characteristics of grid connected wind turbines

(In Spain: UNE-EN 61400-21)

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3. Flicker estimationIEC 61400-21 proposes a method to estimate flicker emission of wind farms.

Flicker estimation is done by mean of simulation where current measurements from a wind turbine are injected in a virtual network.

Lfic

u0(t)

Rfic

im(t) ufict(t)

virtual networ

kmeasurements

flickermeter voltage

ufict(t) flickermeter

Pstim(t) simulation

current measureme

nts

simulated voltage

flicker

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Flicker estimation is done by using the called “flicker coefficient” c calculated for each wind turbine.

From flicker coefficients, the total flicker Pst,Σ emitted for a wind farm is estimated.

3. Flicker estimation

kk a st

n

Sc , v P

S

• c: is the flicker coefficient• va: is the mean wind speed

• Ψk: is the network impedance angle

• Pst: is the flicker level emitted by one windturbine• Sk: is the network short-circuit power

• Sn: is the nominal apparent power of the windturbine

N 2

st, i k a n,ii 1k

1P c , v S

S

• Pst,Σ: is the whole flicker level• ci: is the flicker coefficient for windturbine “i”• Sn,i: is the nominal apparent power of the windturbine “i”• N: is the number of windturbines

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4. Measurements

Google Earth

Data for simulation have been obtained from the measurements done in the Sotavento Experimental Wind Park (www.sotaventogalicia.com) placed in the Northwest of Spain (Galicia).

Sotavento Wind Park

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4. Measurements

Wind Turbine Model NºPower (kW)

Pitch / Speed

Izar-Bonus 1.3 Mw 1 1300 Variable / Variable

Made AE - 46 4 660 Fixed / Fixed

Neg Micon NM-750 4 750 Fixed / Fixed

Neg Micon NM-900 1 900 Fixed / Fixed

Ecotecnia 44 - 640 4 640 Fixed / Fixed

Made AE-52 1 800 Variable / Variable

Izar-Bonus MK - IV 4 600 Fixed / Fixed

Gamesa G-47 4 660 Variable / Variable

Made AE - 61 1 1320 Fixed / Fixed

The Sotavento Experimental Wind Park has installed 24 wind turbines with a total power of 17,56 MW and an estimated annual energy production of 38.500 MWh.

Sotavento Wind Park

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4. Measurements

Measurements has been done with a long term recorder. Measured variables are:• Voltage (instantaneous and RMS)• Current (instantaneous and RMS)• Power (instantaneous, active and reactive)• Wind Speed (instantaneous)

GEN.

Wind Meter

meter

time in s

pow

er in

kW

win

d sp

eed

in m

/s

Long term measurements

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4. Measurements

Mean spectrum of Wind Speed and Power have been analyzed to identify power oscillations (tower shadow, wind shear,...) in the power delivered by wind turbines.

frequency in Hz frequency in Hz

wind speed spectrum power spectrum

Power Oscillations

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4. Measurements

Main oscillations in power are identified. Most of them are related to the rotation speed of rotor (or low speed shat in drive train).

Power Oscillations

2,7 5,2 8,3 10,8 15,2

2,7 46,4 261,7 475,8 644,4

0,28 1p-l 0,37 0,21 0,42 0,91 1,650,42 1p-h 0,07 0,40 0,75 1,12 1,820,85 3p-l 0,33 0,70 0,65 1,02 1,801,27 3p-h 0,01 0,57 1,93 3,41 7,231,70 6p-l 0,06 0,09 0,09 0,19 0,512,54 6p-h 0,03 0,06 0,23 0,35 0,812,55 9p-l 0,03 0,06 0,23 0,35 0,813,82 9p-h 0,01 0,03 0,14 0,25 0,614,25 15p-l 0,01 0,03 0,09 0,11 0,216,36 15p-h 0,00 0,01 0,05 0,06 0,100,66 Tower 0,07 0,13 0,67 1,17 3,38

Oscillation Power in kWOscillation

Frequency (Hz)

Mean Wind Speed (m/ s)

Mean Power (kW)

frequency 1p: is related to rotation speed of the rotor

-l: low speed of generator

-h: high speed of generator

Results for a fixed speed and fixed pitch wind turbine

Nominal Power (kW) 660

Rotor diameter (m) 46

Tower Heigh (m) 45,25Turbine Rotation Low

Speed (rpm)17

Turbine Rotation High Speed (rpm)

25,5

Gearbox ratio 59,500

GeneratorAsynch. generator with two speeds

Pitch Fixed

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4. Measurements

Main oscillations in power are identified in different wind turbine topologies installed in the Sotavento Experimental Wind Park.

Frequencies 1P and 3P are the most important.

Power Oscillations

Summary of Results

Low Wind Speed

Medium Wind Speed

High Wind Speed

1300 FS/VP 5,11% 1,11% - 3P660 FS/FP 1,50% 0,74% 1,12% 3P750 FS/FP 4,23% 1,32% - 1P and 3P640 FS/FP 2,96% 1,65% - 1P

800VS/VP

SYNC+AC/AC0,77% 0,07% - 1P

600 FS/FP 28,36% 2,17% - 1P and 3P660 VS/VP (DFIG) 3,12% 0,21% - 1P and 20Hz

FS or VS: fixed or variable speed; FP or VP: Fixed or variable pitch

Wind Turbine Topology

Nominal Power (kW)

Main Power Oscillation

Components

Peak Value of Power oscillations

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5. Modeling

PSCAD have been used for simulation. Elements to be modeled were:• Aerodynamic model, turbine behavior.• Drive train• Generator• Network• Power electronics (AC/AC converter)

Wind Turbines

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5. Modeling

AG

SG

AG

Fixed Speed

Variable Speed. DFIG

Variable Speed. Sync. Gen. + AC/AC Converter

Wind turbines topologies installed in Sotavento have been modeled.

Wind Turbines

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5. ModelingAerodynamic model

Wind Speed

Rotor Speed

Pitch

Mechanical Torque

Power Oscillations Calculation

Wind Speed

TotalMechanical

Torque

Oscillating Torque

To estimate mechanical torque in the generator shaft, the following components have been considered:

• Rotor aerodynamic behavior• Oscillating power components

time in s

frequency in Hz

pow

er in

Wpo

wer

in W

P simulatedP measuredP oscillations

POWER SPECTRUM

TIME SERIES

P simulatedP measuredP oscillations

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6. ResultsComparison against measurements

• Power simulated• Power measured• Estimated Oscillating Power Components

Measurements are compared with results from simulation. In following graphics a comparison is done between:

Results for a fixed speed and fixed pitch wind turbine

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6. ResultsComparison against measurements

Results for a fixed speed and variable pitch wind turbine

time in s

frequency in Hz

pow

er in

Wpo

wer

in W

P simulatedP measuredP oscillations

POWER SPECTRUM

TIME SERIES

P simulatedP measuredP oscillations

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6. ResultsComparison against measurements

Results for a variable speed and variable pitch wind turbine (synch+ AC/AC)

time in s

frequency in Hz

pow

er in

Wpo

wer

in W

P simulatedP measuredP oscillations

POWER SPECTRUM

TIME SERIES

P simulatedP measuredP oscillations

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6. ResultsComparison against measurements

Results for a variable speed and variable pitch wind turbine (DFIG)

time in s

frequency in Hz

pow

er in

Wpo

wer

in W

P simulatedP measuredP oscillations

POWER SPECTRUM

TIME SERIES

P simulatedP measuredP oscillations

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6. ResultsFlicker estimation

From simulation data a flicker estimation at HV level with different short circuit capacity values have been analyzed:• Scc Min. and Max. are the minimum and maximum values of short circuit power in the wind farm at 132 kV level.• Scc 20x is the short circuit power obtained by multiplying the power of wind farm by 20.

Flicker estimations for each of the above situations have been done.

Scc Min 4755 Scc Max 5023 Scc 20x 20 x 17.5 MVA

Shorcircuit Power at 132 kV (MVA)

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6. ResultsFlicker estimation

Short circuit values at 132 kV level during Scc simulation

Pst calculated for a single WTG

Wind Speed during simulation

Flicker at short-circuit power Scc MinFlicker at short-circuit power Scc MaxFlicker at short-circuit power Scc 20x

Results for a fixed speed and fixed pitch wind turbine (640 kW)

Scc WT (MVA) 6Scc WT (º) 71,56

Mean Wind Speed (m/s) 9,2344Pst WT Sim. 0,027209

Flicker Coefficient 0,25508Pst Scc Max 0,00018Pst Scc Min 0,00017

Pst Scc 20x 0,00249

Flicker coefficient (IEC 61400-21)

Pst Scc Max, Pst Min and Pst Scc 20x values have been calculated supposing a wind farm formed for N equal WTG with a total power of 17.5 MW (approx.)

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6. ResultsFlicker estimation

Results for a fixed speed and fixed pitch wind turbine (600 kW)

Results for a fixed speed and variable pitch wind turbine (1300 kW)

Scc WT (MVA) 6Scc WT (º) 71,56

Mean Wind Speed (m/s) 8,1795Pst WT Sim. 0,026456Flicker Coefficient 0,26456Pst Scc Max 0,00018Pst Scc Min 0,00017Pst Scc 20x 0,00247

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 7Pst WT Sim. 0,022304Flicker Coefficient 0,22304Pst Scc Max 0,00022Pst Scc Min 0,00021Pst Scc 20x 0,00307

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6. ResultsFlicker estimation

Results for a fixed speed and fixed pitch wind turbine (660 kW)

Results for a fixed speed and variable pitch wind turbine (750 kW)

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 8,38Pst WT Sim. 0,012706Flicker Coefficient 0,25027Pst Scc Max 0,00018Pst Scc Min 0,00017Pst Scc 20x 0,00248

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 10,76Pst WT Sim. 0,022567Flicker Coefficient 0,39116

Pst Scc Max 0,00030Pst Scc Min 0,00028

Pst Scc 20x 0,00414

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6. ResultsFlicker estimation

Results for a fixed speed and fixed pitch wind turbine (1320 kW)

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 10,76Pst WT Sim. 0,037043Flicker Coefficient 0,36482Pst Scc Max 0,00037Pst Scc Min 0,00035Pst Scc 20x 0,00511

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6. ResultsFlicker estimation

Results for a variable speed and variable pitch wind turbine (DFIG, 660 kW)

Results for a variable speed and variable pitch wind turbine (Sync, 800 kW)

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 10,3526Pst WT Sim. 0,09Flicker Coefficient 1,77273Pst Scc Max 0,00125Pst Scc Min 0,00119Pst Scc 20x 0,01755

Scc WT (MVA) (MVA) 13Scc WT (º) 71,56

Mean Wind Speed (m/s) 10,3616Pst WT Sim. 0,044174Flicker Coefficient 0,71783Pst Scc Max 0,00055Pst Scc Min 0,00052Pst Scc 20x 0,00774

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6. ResultsFlicker estimation

Results for a fixed speed and fixed pitch wind turbine (600kW)

Results for a fixed speed and variable pitch wind turbine (1300 kW)

WTG nominal power (MW) 0,60Scc WT (MVA) (MVA) 6,00

Scc WT (º) 71,56Mean Wind Speed (m/s) 8,1795

Pst WT 0,0265Pst WT without oscill. 0,0119

Pst error 55,1%

WTG nominal power (MW) 1,30Scc WT (MVA) (MVA) 13,00

Scc WT (º) 71,56Mean Wind Speed (m/s) 7,0000

Pst WT 0,0223Pst WT without oscill. 0,0119

Pst error 46,8%

The effect of including the power oscillation components is demonstrated in the examples shown

bellow.

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7. Conclusions

- In this paper, a method to evaluate flicker based in IEC 61400-21 is presented. The current measurement values used in this standard have been replaced for a complete WTG model. - Oscillating power components (shadow tower, wind shear...) has been included in order to obtain results that are more realistic.-WTG models have been validated by comparing the simulation results with the measured data.

The proposed method allows calculating of flicker emission in single WTG or wind farms at different working conditions.

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PSCAD/EMTDC-Based Modeling and Flicker Estimation for Wind

TurbinesC. Carrillo(1), E. Díaz-Dorado(2) and J. Cidrás

carrillo@uvigo.es, ediaz@uvigo.es, jcidras@uvigo.esDepartment of Electrical Engineering

Universidade de VigoSPAIN

(1) http:// webs.uvigo.es/carrillo, (2) http://webs.uvigo.es/ediaz

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