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Understanding Harmonics

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  1. Understanding Harmonics Richard Molloy Technology Sales Manager, Power Quality

  2. Agenda • Introduction • Definition of ‘Power Quality’ • Identification of power quality problems • Harmonics – causes and effects • Mitigation techniques • Conclusion

  3. The cost of poor power quality • Cost of power quality problems to European industry & commerce is estimated at €10 billion per annum • Expenditure on preventative measures is less than 5% of this

  4. Source – Leonardo Power Quality Initiative Definition of Power Quality • ‘A supply that is always available, always within voltage and frequency tolerance, with a pure, noise free, sinusoidal wave shape’

  5. How good is good enough? • No definitive answer – entirely dependant on compatibility of equipment and supply

  6. Power standards • Power standards are defined by the electricity regulator OFGEM • Standard EN 50 160 • ‘Voltage characteristics of electricity supplied by public distribution systems’

  7. Long term interruptions 10 to 50 Short term interruptions 30 to 1000 Dips 30 to 1000 Short-term over-voltage <1.5kV Steady state voltage 230V +/- 10% for 95% of time Voltage unbalance <2% for 95% of time EN 50 160

  8. Total harmonic distortion </= 8% for 95% of time Transient over-voltages Majority <6kV Frequency 50Hz +/- 1% for 99.5% of time Frequency 50Hz +/- 2% for 100% of time EN 50 160

  9. Identification of problems • Harmonic distortion • Voltage sags (‘dips’, ‘brownouts’) • Voltage swells (‘surges’) • Outages (‘power cuts’, ‘blackouts’) • Transient voltage surges (‘spikes’) • Earthing (‘grounding’) • Poor power factor

  10. Harmonics

  11. Definition • Waveforms with frequencies that are multiples of the fundamental frequency (50Hz UK & Europe, 60Hz North America)

  12. Waveforms - Fundamental Fundamental Wave, 50Hz

  13. Waveforms – Fundamental and 2nd Harmonic Fundamental Wave, 50Hz 2nd Harmonic, 100Hz

  14. Waveforms - Fundamental, 2nd and 3rd harmonic Fundamental Wave, 50Hz 2nd Harmonic, 100Hz 3rd Harmonic, 150 Hz

  15. Fundamental + 2nd harmonic

  16. Fundamental + 3rd harmonic

  17. All wave-shapes can be reduced to a sine wave plus harmonics • Even a square wave • Square wave equation

  18. Switched mode power supply current waveform

  19. Harmonic spectrum of SMPS

  20. Causes of harmonics • Harmonic currents are caused by the use of non-linear loads: • Switched mode power supplies • HF fluorescent ballasts • Compact fluorescent lamps • Inverters • Variable frequency drives • UPS systems

  21. Effects of harmonics • Erroneous operation of control systems • Excessive heating in rotating machines • Overloading of transformers • Overloading of switchgear and cables • Nuisance tripping of circuit breakers

  22. Effects of harmonics • Overloading of capacitors • Damage to sensitive electronic equipment • Excessive currents in neutral conductor

  23. Effects of Triple-N harmonics • Triple-N harmonics are odd multiples of 3 times fundamental frequency, i.e., 3rd, 9th, 15th etc. • They are all in phase and sum in the neutral conductor • Switched Mode Power Supplies (SMPS) produce a lot of 3rd harmonic - this is especially problematic in commercial buildings due to the vast number of computers, office equipment etc.

  24. Effects of Triple-N harmonics

  25. Effects of Triple-N harmonics • A 3-phase star connected system with a balanced linear load has no current flowing in the neutral • Where a lot of 3rd (or other triple-N) harmonics are present, neutral currents can be considerably in excess of phase currents • This causes overheating of neutral conductors. Note these may only be 50% rated in older buildings • Neutrals do not normally have over-current protection

  26. Limits on Harmonic Distortion • Harmonic currents flowing back to the supply cause harmonic voltage distortion due to the supply impedance • Governed by Engineering Recommendation G5/4 • Title : ‘Limits for Harmonics in the U.K. Electricity Supply System’. • Guidance ONLY

  27. Mitigation measures • Neutral up-sizing • Passive filters • Active harmonic conditioners • Transformer based solutions

  28. Neutral up-sizing • All neutrals in the system, including switchgear etc., must be rated for the neutral current as well as phase currents • A 4 or 5 core 3 phase cable is rated for current flowing in the phase conductors only. Current in the neutral can cause overheating of the cable • Above 7th harmonic (350 Hz), skin effect should be considered • Cables should be de-rated in accordance with IEC 60364-5-523 / BS 7671 (Appendix 4)

  29. Passive filters • Capacitor and reactor combination • Tuned to specific frequency • Requires higher voltage capacitors • Designed for a fixed system requirement

  30. Harmonic production IL IH

  31. Harmonics and capacitors IL IH IC

  32. Effects of Resonance

  33. Avoiding resonance with PFC capacitors • Calculate the Resonant Frequency

  34. Adding reactors

  35. Effect of adding reactors Current flowing into supply in A Series Reactor Tuned to the frequency shown below

  36. Single Frequency Filter Double Tuned Filter 2nd Order High Pass Filter Filters |z| f (Hz) |z| f (Hz) |z| f (Hz)

  37. Harmonics In Practice Sub-Station

  38. When others add to your system Sub-Station

  39. Active harmonic conditioner • Harmonic current compensation, 2nd to 25th • Harmonic neutral current compensation • Global or selective harmonic current compensation • Site adjustable compensation parameters

  40. Active harmonic conditioner AHC

  41. AHC points of connection INCOMING SUPPLY SUB BOARD 1 SUB BOARD 2 DIS BOARD DIS BOARD

  42. AHC points of connection INCOMING SUPPLY SUB BOARD 1 SUB BOARD 2 AHC GLOBAL DIS BOARD DIS BOARD

  43. AHC points of connection INCOMING SUPPLY SUB BOARD 1 SUB BOARD 2 AHC GLOBAL DIS BOARD AHC PARTIAL DIS BOARD

  44. AHC points of connection INCOMING SUPPLY SUB BOARD 1 SUB BOARD 2 AHC GLOBAL DIS BOARD AHC PARTIAL AHC LOCAL DIS BOARD

  45. AHC advantages • Continued guaranteed effective harmonic compensation • Easy to use and install • Auto configures • NOT susceptible to harmonic overload • Expandable • Compatible with electric generators • Connected anywhere

  46. Transformer based solutions • 3rd Harmonic rejection transformers • Phase shifting transformers • Isolation or harmonic suppression transformers

  47. Conclusions

  48. Conclusions • As more electronic equipment is used in industry and commerce, harmonics have become a major power quality problem – more harmonics are generated, and more equipment is adversely affected by these harmonics • A combination of good design practice and effective harmonic mitigation measures is required

  49. Conclusions • The power quality required will be dependant upon the equipment to be operated at any given location • A holistic approach to power quality is required – one solution is unlikely to address all the problems – a combination of equipment will be required to achieve the quality required.

  50. Power quality measurement • Most power quality problems can be measured or monitored – if you suspect a problem, we can conduct a PQ survey to identify: • Harmonic distortion • Transient voltage disturbance • Power factor • Load survey • Unbalance • Flicker