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Power Systems & Energy Course: Power Quality Issues with Large-Scale Renewable Plants Jason MacDowell Power Quality – Renewable Plant Perspective • Power quality is a two way street! • Plant effect on grid • Grid effect on plant • Interconnection requirements are typically imposed at the PCC (Point of Common Coupling) • Impacts at the PCC are a function of: • Diversity of the individual WTGs/PV arrays • Characteristics of the collector system • Characteristics of the transmission grid © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-2 Power Quality Basics Undervoltages: Notch – less than one cycle duration Dip – duration of a few cycles Sag – duration of seconds to minutes Interruption – reduction to zero for seconds to minutes Overvoltages: Spike – less than one cycle duration Swell – duration of cycles or more Waveform Distortion: Harmonics – steady-state distortion affecting every cycle integral multiples of operating frequency Interharmonics – non-integral multiples of operating frequency Noise – random high-frequency signals Frequency Shift – variations in frequency through time Flicker – cyclic or random variations in voltage magnitude Unbalance – phase asymmetry © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-3 Flicker Flicker is caused by repetitive variations in voltage – can be irritant to utility customers House Pumps Sump Pumps Air-Conditioning Equipment Domestic Refrigerators Oil Burners Arc Furnace Flashing Signs Arc-W elders Manual Spot-W elders Sews Group Elevators Single Elevator Heights Y-delta Changes on Elevator-MotorGenerator Sets X-Ray Equipment Reciprocating Pumps Compressors Automatic Spot-W elders % Voltage Fluctuation Border Line of Irritation Border Line of Visibility 1 10 Fluctuation per Hour 20 30 10 Fluctuation per Minute 20 30 60 10 15 Function per Second Measurement of flicker has been standardized by IEC, using “flickermeter” algorithm (IEC 61000-4-15) © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-4 Flicker in Wind Plant Applications • Sources of flicker: – – – – Wind turbulence, gusting Drive-train oscillations Blades passing tower Induction generator close-in inrush • Diversity reduces impact on a large wind plant • DFAG and full-conversion reduces impact • GE WTG voltage regulation capability virtually eliminates issue • For induction generators, dynamic compensation could be required to meet tight grid specs © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-5 Flicker in Large PV Plants • Cloud shadow passage potentially can cause significant voltage variation – Frequency of variations usually not sufficient to be technically considered “flicker” unless variations are very large – Very large voltage variations would be unacceptable for other reasons • Output of PV plants becomes smoother as plant rating increases – Finite size of cumulus clouds that cause the most intermittent shadowing – Output of very large plants approaches (1 - %overcast)*Pclear_sky • Plant level voltage regulation can readily mitigate voltage variation © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-6 Non-Repetitive Voltage Change • Some grid codes limit step voltage changes – Typically 1% - 2% for frequent events – Some codes relax limits to 3% for infrequent events • Causes: – – – – Capacitor/reactor bank switching Transformer energization Feeder switching Interconnecting HV cable switching • Solutions: – Limit size of individually switched banks (Qmax = DV x MVASC) – Controlled transformer energization – Resistor preinsertion – Point-on-wave switching – Compensation of HV cables © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-7 Voltage Transients Affecting Grid • Transients are sub-cycle overvoltages • In wind plant, usual cause is capacitor switching – Ringing oscillations, usually at several hundreds of Hz – Severity depends on point-on-wave of energization • Solutions – Synchronized switching -switch closes near voltage zero – Ideally, eliminates transient – Experience has been that switchgear must operate often to “keep in training” – Impedance preinsertion – Resistor preinsertion (breakers) – Lossy inductor (circuit switchers) © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-8 What are Harmonics? • Non-fundamental-frequency AC currents and voltages superimposed on the system – Frequencies are multiples of normal system frequency – Interharmonics are non-integral multiples • Results in repeated distortion of the sine wave • System impacts: – – – – Capacitor overload Excess heating Device misoperation Telecommunication interference © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-9 Harmonic Sources in Renewable Plants • Power converters (used in DFAG, full conversion, and controlled rotor resistance induction generators) • PV inverters • Generators • Dynamic compensation equipment – SVCs, STATCOMS • The grid! © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-10 The Grid as a Harmonic Source • Most grids have significant background distortion – Due to nonlinear loads, P-E devices, transformers – Greatest at 3rd, 5th, and 7th harmonic • Good data requires extended-duration monitoring • IEEE-519 sets “recommended practice” for utilities: 69 – 161 kV > 161 kV V THD 1.5% 2.5% 1.0% 1.5% 1.6 1.4 1.2 Voltage (%) Vh Measurements made on an actual utility system 0.8 0.6 0.4 0.2 10 11 12 13 14 15 Harmonic © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-19 Harmonics Injected by Grid Into Plant PCC  Zgrid IPCC VHarm IWTG Harmonics from the grid are a major portion of IPCC © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-20 Resonant Amplification of Grid Harmonics • Wind plant presents a series resonant circuit – Substation transformer + capacitor banks & cable charging – Can amplify grid distortion • Problems caused by excess harmonics in collector system – Capacitor overload – Component heating (WTG, transformers, cable) – Misoperation Vcoll Vh Icap I cap  Vh Z L  ZC Vcoll  Vh  Zc Z L  ZC A lightly damped resonance can cause severe voltage distortion on collector, and high currents in shunt capacitors © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-21 HARMONIC ANALYSIS © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-22 Harmonic Analysis Software Tools Steady-state phasor analysis – one frequency at a time • Time-domain modeling tools are not needed • Some time-domain tools also perform phasor analysis (e.g., EMTP, ATP) Capability to represent frequency-dependent characteristics of network components • XL = 2fL XC =-1/(2fC) • Long-line characteristics of cables and overhead lines • Frequency-dependent damping characteristics are extremely important Must handle a large number of configurations © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-23 Source Characteristics of WTGs/Inverters Modern PV inverters and wind turbines not look like an ideal harmonic current source • Internal filters are in shunt • VSC bridge appears like: – Ideal current source at low frequency (within controller bandwidth) – Ideal voltage source at high frequency (f >> controller bandwidth) – Complex transition of characteristics in between • Shunt impedance of machine Norton equivalent is the preferred representation • Source magnitude spectrum depends on operating point • Equivalent shunt source impedance is a complex function of frequency – does not conform to simple models Characteristics best determined by detailed time-domain simulation with controls modeled © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-24 WTG representation (Internal Distortion) • Doubly-fed asynchronous generator machine • Low distortion energy: – Converter operated with high switching frequency – Converter rating is a fraction of turbine rating • Small distortion filter within the turbine to absorb most of the distortion energy created by the converter Conceptual converter harmonic current distortion flows © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-25 WTG representation and field WTG measurements • Machine presents a shunt impedance to the grid that will absorb distortion currents • Most of measured distortion currents are due to external sources • Harmonic current measurements are severely affected by distortion of external sources Harmonic measurement point Conceptual external source harmonic current distortion flows © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-26 WTG representation – Norton Equivalent • Study assumptions based on physics of the system • Realistic system distortion estimation: – due to WTG – due to external sources (availability of Norton impedances) • Proper measurement interpretations © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-27 Grid Impedance at Harmonic Frequencies • Transmission grid impedance is important to both studies of: – Ambient distortion amplification in the renewable plant – Harmonic current injection by plant into grid • Grid impedance varies with: – – – – Load level Generation commitment Capacitor bank status Line outages Range of impedance for an example transmission bus © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-28 MITIGATING HARMONICS © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-29 Mitigating Grid Harmonic Amplification • Avoid resonances – Selection of transformer impedances and capacitor bank ratings – This option is limited by the wide range of tunings possible – Avoid certain operating configurations – E.g., start up WTGs on one feeder before switching in another feeder, on startup – Detune shunt capacitors – Add inductor in series with capacitor – Capacitor cannot resonate at a frequency higher than the frequency defined by the L-C combination – Typical to select L-C to resonant at 4.8th harmonic • Filter out harmonics – Complex design – Necessary if wind plant is an objectionable harmonic source © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-30 Harmonic Filter Topologies 10 10 10 Impedance Single-Tuned Filter 100 Zn 10 0.1 0.01 10 10 10 High-Pass Filters 100 10 fn Fre que ncy 10 100 10 fn Fre que ncy 10 10 Impedance Zn 10 100 10 © 2016act General Electric International, Inc All rightsbanks reserved Not distribution without permission Filters like shunt capacitor at forfundamental frequency 11-31 Practical Filter Design Considerations • Initial tuning tolerance – Tuning taps on inductors – Variable inductors • Operational variations – Temperature variations (capacitors) – Component failures (capacitors) – System frequency variation – Component aging Filter design must be sufficiently robust to meet design objectives for expected variations © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-32 Final Remarks • Modern WTG/PV converters tend to cause low distortion at PCC: – High frequency PWM converter switching – Distortion filter • Harmonic system studies should be based on realistic converter modeling guidelines • Wind/Solar plant equipment can create resonances (Shunt Caps), particularly to grid distortion If shunt capacitors are required, consider de-tuning • If filters are needed, design must be sufficiently robust to meet design objectives for expected variations © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11-33 [...]... without permission 11-12 Harmonic Performance Specifications IEEE 519 specifies the following limits for “dispersed generation”: Harmonic Order h < 11 12 - 17 18 - 23 23 - 35 35 - 50 Odd 2.0% 1.0% 0 .75 % 0.3% 0.15% Even 0.5% 0.25% 0.1 87% 0. 075 % 0.0 37% THD THD  2.5% 50  Ih 2 h2 • Voltage specifications might also be made (individual, and RSS) • Need to consider impact on wind plant equipment as well:... nonlinear loads, P-E devices, transformers – Greatest at 3rd, 5th, and 7th harmonic • Good data requires extended-duration monitoring • IEEE-519 sets “recommended practice” for utilities: 69 – 161 kV > 161 kV V THD 1.5% 2.5% 1.0% 1.5% 1.6 1.4 1.2 Voltage (%) Vh Measurements made on an actual utility system 1 0.8 0.6 0.4 0.2 0 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Harmonic © 2016 General Electric International,... amplification is reduced • In a realistic system, harmonic voltages and currents can be greatly amplified © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11- 17 Resonance Example 0.15 15 Per-unit harmonic voltage 0.1 Current amplification factor 10 Vf If 0.05 0 5 0 200 400 f 600 0 0 200 400 600 f Typical wind plant parameters, 1% harmonic current injection from...WTG/Inverter Harmonics • PV inverters, and power converters used in most WTGs use PWM technology – Harmonics are clustered around multiples of the switching frequency – High frequency; are easily filtered – Random phase superposition self-cancels... to external sources (availability of Norton impedances) • Proper measurement interpretations © 2016 General Electric International, Inc All rights reserved Not for distribution without permission 11- 27 Grid Impedance at Harmonic Frequencies • Transmission grid impedance is important to both studies of: – Ambient distortion amplification in the renewable plant – Harmonic current injection by plant into ... “dispersed generation”: Harmonic Order h < 11 12 - 17 18 - 23 23 - 35 35 - 50 Odd 2.0% 1.0% 0 .75 % 0.3% 0.15% Even 0.5% 0.25% 0.1 87% 0. 075 % 0.0 37% THD THD  2.5% 50  Ih h2 • Voltage specifications.. .Power Quality – Renewable Plant Perspective • Power quality is a two way street! • Plant effect on grid • Grid effect on plant... Electric International, Inc All rights reserved Not for distribution without permission 11-2 Power Quality Basics Undervoltages: Notch – less than one cycle duration Dip – duration of a few cycles

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