Harmonic Design Considerations for Wind Farms To Ensure Grid Code Compliance Liam Breathnach Power System Studies Group ESB International
Agenda Introduction Harmonic Theory and Concepts Grid Code Requirements Modelling and Simulation Case Studies Mitigation Measures
Introduction AC Power systems sinusoidal voltages / currents Non-linear devices Power electronic equipment Saturable devices May cause distortion Unwanted effects on equipment and system
Introduction Grid Transformer System Picture Area Picture Area Cable Wind Turbine & Step-up Transformer Grid Voltage
Introduction Examples of Unwanted Effects: Conductors Increased losses Transformer Increased losses and excessive heating Circuit Breakers Nuisance tripping Capacitors Permanent failure
Introduction Doubly Squirrel Fed Cage Induction Generator No Harmonics currents Synchronous Induction generator / Induction with variable Generator resistance with Full Converter No Harmonics currents
Harmonic Theory and Concepts 50 Hz Harmonics occur at multiples of the fundamental + 150 Hz frequency (50Hz) + 250 Hz Distorted waveform can be expressed in terms of it s + 350 Hz component waveforms
Harmonic Theory and Concepts Harmonic currents produced by a wind turbine Fundamental 50Hz MC's PlotXY - Fourier chart(s). Copying date: 26/05/2010 File WF_Harmonics_Paper_rev0p0_BaseCase_TurbineHarmonics.pl4 Variable c:x0016a-x0031a [peak] MC's Initial Time: PlotXY 1.98 - Fourier Final Time: chart(s). 2 Copying date: 27/05/2010 File WF_Harmonics_Paper_rev0p0_BaseCase_TurbineHarmonics.pl4 Fifth Harmonic = 250Hz Variable c:x0016a-x0031a 70 Initial Time: 1.98 Final Time: 2 3000 60 2500 2000 40 Current 1500 30 1000 20 500 10 Harmonics >50Hz 00 0 0 510 10 20 15 30 20 40 25 50 30 harmonic order order Frequency [Hz]
Harmonic Theory and Concepts Voltage Distortion: System Impedance Vh Zh* Ih Impedances varies with frequency X L 2 fl May result in harmonic voltages Total Harmonic Distortion (THD) X C 1 2 fc
Harmonic Theory and Concepts Impedance 1.2 *106 1.0 0.8 0.6 System Harmonic Impedance 0.4 0.2 Resonance 0.0 0 500 1000 1500 2000 2500 (file WF_Harmonics_Paper_rev0p0_Cable_SeriesReactor.pl4; x-var f) v:pcca M C's PlotXY - Fourier chart(s). Copying date: 26/05/2010 File WF_Harmonics_Paper_rev0p0_BaseCase_TurbineHarmonics.pl4 Frequency Variable [Hz] c:x0016a-x00 Initial Time: 1.98 Final Time: 2 70 60 50 Harmonic Current Current 40 30 20 10 0 0 10 20 30 40 50 harmonic order
Harmonic Theory and Concepts System Short Circuit Level System Harmonic Impedance affected by: Short Circuit Level Capacitor Cable/Overhead Banks Line Q A Capacitor banks lower the resonant frequencies I h Connection Cable/ Overhead Line Transformers/ Reactors
Grid Code Requirements Distortion limits outlined in IEC 61000-3-6 High Voltage THD planning level is 3% Limits for individual harmonic magnitudes
Percent [%] Grid Code Requirements Individual Harmonic Planning Limits 2.5 5 th Harmonic, 250Hz, 2% limit 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 40 45 50 Harmonic Order
Modelling & Simulation Build model Represent harmonic sources as current injectors Wind Test Report (IEC 61400-21) Frequency scan analysis Identify potential resonances Processing techniques individual harmonic distortions THD 1.2 *106 1.0 0.8 0.6 0.4 0.2 0.0 0 500 1000 1500 2000 2500 (file WF_Harmonics_Paper_rev0p0_Cable_SeriesReactor.pl4; x-var f) v:pcca
Modelling & Simulation Loads not producing harmonics Damping Phase angles of sources Transformer connections R L Modelling of lines and cables PI equivalent model (multiple sections) System representation C How far back? Capacitor banks Short circuit capacity
Case Studies Short Circuit Capacity High Short Circuit Level Typically lower harmonics Lower Short Circuit Level Weaker system More pronounced resonance peaks
Case Studies - Cable vs Overhead Line 1.2 5 *10 *10 6 6 1.0 4 Overhead 0.8 Line 3 Impedance System Relatively small capacitance 0.6 2 0.4 Cable 1 Impedance 0.2 Voltage Greater capacitance 0.0 M0 C's PlotXY - Fourier chart(s). Copying date: 26/05/2010 File 00 WF_Harmonics_Paper_rev0p0_BaseCase_TurbineHarmonics.pl4 500 1000 1500 2000 2000 Variable 2500 2500 Frequency [Hz] c:x0016a-x0031 Initial Time: 1.98 Final Time: 2 Lowers resonant frequencies (f ile (f ile WF_Harmonics_Paper_rev wf _harmonics_paper_rev0p0_basecase_ohl.pl4; 0p0_basecase_cable.pl4; x-v ar f f )) v :PCCA 70 Frequency [Hz] 60 50 Current 40 30 20 10 0 0 10 20 30 40 50 harmonic order
Case Studies Distance Along Cable 1.2 *10 6 1.0 Point along Cable End of Cable 0.8 Impedance 0.6 0.4 0.2 0.0 0 500 1000 1500 2000 2500 (file wf_harmonics_paper_rev0p0_basecase_cable.pl4; x-var f) v:sect3a v:pcca Cable capacitance distributed along its length Point to connection important Frequency [Hz]
Harmonic Mitigation Active Filters Mitigate multiple harmonics Complex and costly Passive Filters Simple and reliable Relatively inexpensive Cannot mitigate multiple harmonics
Network Impedance p.u. Harmonic Mitigation 25 20 Impedance 15 10 5 0 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 Frequency (Hz) Without Filter With Filter Frequency [Hz]
Summary Wind turbines may be source of harmonics May interact with system impedance Resulting distortion may: Damage equipment Increase losses Mitigation measures May prove costly Identify potential issues early