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Understanding Harmonics

Learn about the causes and effects of harmonics in power quality, and discover mitigation techniques to improve electrical systems. Explore the impact of poor power quality on industries and the importance of meeting power standards. Gain insights into identifying issues such as harmonic distortion, voltage fluctuations, and power outages, and how they can affect equipment and operations. Discover solutions like passive filters, active harmonic conditioners, and transformer-based options to address harmonic problems effectively.

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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

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