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Harmonics and PFC

Harmonics and PFC. The love story By Pol Nisenblat. THE FUTURE OF POWER QUALITY. DEFINITIONS. PROBLEMS. “DETUNED” SOLUTIONS. TUNED SOLUTIONS. HOW TO CHOOSE. OPEN DISCUSSION. ELECTRICAL POWER SOURCE.

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Harmonics and PFC

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  1. Harmonics and PFC The love story By Pol Nisenblat

  2. THE FUTURE OF POWER QUALITY DEFINITIONS PROBLEMS “DETUNED” SOLUTIONS TUNED SOLUTIONS HOW TO CHOOSE OPEN DISCUSSION

  3. ELECTRICAL POWER SOURCE • It is worldwide common practice to assume that utilities generate a near perfect sine wave voltage • Typical electrical energy source is introduced by “voltage” source

  4. DISTRIBUTION TRANSFORMER • Typical Internal Impedance is only 2-7% of the fully loaded Secondary

  5. IDEAL ENERGY SOURCE • Simplified power control by periodical switching (chopping) • Nonlinear load operation generatescurrent waveform distortion

  6. HARMONICS - DEFINITIONS • Harmonics are integral multiples of some fundamental frequency that, when added together, result in a distorted waveform + f(x) = sin(x) sin(5x) f(x) = 5 sin(5x) f(x) = sin(x) + = 5

  7. HARMONICS - DEFINITIONS • Harmonic order are MULTIPLES of the fundamental frequency. • Typical Harmonics are the 3rd, 5th, and 7th • Where H3 = 150 Hz, H5 = 250 Hz, H7 = 300 Hz (at 50Hz world) • In fact, any waveform may be constructed from a sine wave and some number of its harmonics like:

  8. HARMONICS – MOST IMPORTANT DEFINITIONS • Harmonics are originated at the load side! • Harmonics are created in the current! • Harmonic sources are - current sources

  9. HARMONIC SOURCE • High internal impedance • The harmonic current is being PUSHED towards lowest external impedance path

  10. HARMONIC CURRENT FLOW • Typical harmonic current flow is towards distribution transformer 0.01 Ohm   1 Ohm

  11. HARMONICS – PROBLEMS? • I2r losses + “skin” effect • Voltage distortions • Iron-core losses • Cables/transformers overheat • Upstream pollutions • But, the real problems are yet to come

  12. HARMONIC CURRENT WITH PFC • Where the H5 current should go now?

  13. PARALLEL RESONANCE Parallel resonance =Infinity impedance

  14. PARALLEL RESONANCE • High impedance at the resonance frequency • The resonance frequency changes with changed number of capacitor groups (N)

  15. PARALLEL RESONANCE

  16. PARALLEL RESONANCE • Harmonic current turns back to the loads • Harmonic VOLTAGE raises dramatically • Harmonic current circulates (ping pong) between capacitors and distribution transformer V

  17. PARALLEL RESONANCE – WHAT TO DO? • Xl1 reactor in series to the capacitor moving parallel resonance frequency downstream • The new resonance frequency at:Xc=Xl+Xl1 • Locate parallel resonance frequency below lowest dominant harmonic

  18. kVAsc hr = kVAC PARALLEL RESONANCE FREQUENCY –”RULE OF THUMB” • Where hr is the harmonic number of the parallel resonance • Unfortunately, not accurate enough

  19. HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? • For those who love formulas:

  20. HOW TO CALCULATE PARALLEL RESONANCE FREQUENCY? • Unfortunately, Xl (Xtr) value which is actually the total distribution network inductance at the frequency of interest - is unknown • But, we do know that the parallel resonance frequency would always be locateddownstream to the serial resonance frequency between Xl1-Xc

  21. “DETUNED” SOLUTION

  22. WHAT FREQUENCIES ARE SAFE? • Even” harmonics are normally not present • “Triple” harmonics are canceled on DELTA connected loads with balanced 3rd harmonic • On balanced loads and 3 phase DELTA capacitors cases, tuning in H3.5-H4.2 ranges are most popular

  23. DETUNED SOLUTION - DEFINITIONS • Tuned frequency is defined by serial resonance point • Serial connected reactors are defined by % rather then Henries • Reactor’s % defined as impedance at the fundamental frequency with respect to the capacitor’s impedance at the same frequency

  24. 7% “DETUNED” SOLUTION

  25. TUNED SOLUTIONS - FILTERS • Passive filters are “tuned” to just below the harmonic frequency • Care should be taken – not to overload! Low impedance path at 5th harmonic

  26. TOO HIGH HARMONIC LEVELS? • Disconnect ALL PFC capacitors and check again • Significant difference? – Use detuned solution • No difference? – Use tuned solution • 2nd and 4th harmonics too high? - Check voltage converters • Weak network? 14% detuned solutions are preferred • Balanced 3rd harmonic in current? – Use 5.67-7% solutions • Unbalanced compensation? – Use 14% solutions only • Voltage harmonics without current sources? • Don’t use filters! Speak to utility.

  27. Questions? Contact: Asaf Laifer Alaifer@elspec-ltd.com 972-4-6174127

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