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Variable Frequency Drives Harmonics Overview

Variable Frequency Drives Harmonics Overview. Harmonics Overview. What causes harmonics ? Non-linear load devices create harmonics when they convert ac to dc, dc to dc, dc to ac, and ac to ac. This can produce high levels of harmonic voltage and current distortion. . Harmonics Overview.

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Variable Frequency Drives Harmonics Overview

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  1. Variable Frequency DrivesHarmonics Overview

  2. Harmonics Overview What causes harmonics ? Non-linear load devices create harmonics when they convert ac to dc, dc to dc, dc to ac, and ac to ac. This can produce high levels of harmonic voltage and current distortion.

  3. Harmonics Overview • Harmonics operate at a frequencythat is a multiple of the fundamental • Harmonics operate at a magnitude that is inversely proportional to the magnitude of the fundamental • In other words... the higher the harmonic order, the less effect it has on the waveform. • In most cases, harmonic analysis is limited to the 13th harmonic.

  4. Repeated 60 times every second 0 166 msec 1 cycle Harmonics Overview • Frequency is defined as the number of times a given waveform repeats itself every second. • Example: the typical household line current impresses 115 volts AC, 60 times per second.

  5. Fundamental = 60Hz 5th Harmonic = 300Hz Harmonics Overview • Harmonic currents operate at the same time as the fundamental, but at faster rates • Harmonic currents are additive, producing a “distorted” sine wave.

  6. Harmonics Overview • Harmonic Currents are currents that flow at the same time as a fundamental current, 60Hz frequency, but at multiples of the fundamental. • For a typical 6 pulse inverter, these multiples are: • Fundamental = 60 Hz • 5th Harmonic = 60 x 5 = 300 Hz • 7th Harmonic = 60 x 7 = 420 Hz • 11th Harmonic = 60 x 11 = 660 Hz • 13th Harmonic = 60 x 13 = 780 Hz • 17th Harmonic = 60 x 17 = 1,020 Hz

  7. Repeated 300 times every second 0 30 msec 1 cycle Harmonics Overview • A harmonic current operates in “harmony” with the fundamental but faster; i.e. the 5th harmonic of the 60 Hz fundamental repeats itself 300 (60 x 5) times per second.

  8. Harmonics Overview Harmonics have several harmful side effects: • Can interfere with electronic equipment and people • through radiated (RFI) interference through the air • emitted (EMI) interference over power and control wires. • Can cause an increase in supplied power (KVA) over what is really needed (kW or HP) • Entire AC supply line must be sized accordingly • End user transformers, switchgear, wires, contactors, and fuses have to be bigger. • The power company generator • Can cause Poor Power

  9. Harmonics Overview Poor Power. . . Reduces the Load Carrying Capacity of Your Electrical System! • Transformers & wiring are sized by current capacity. If additional current is required due to low power factor, capacity for work producing current is displaced. Everything in your building is oversized to make up for poor power factor.

  10. Harmonics – DC Capacitance Link • A DC capacitance link provides voltage to the inverter between the peaks of the mains voltage waveform. Current only flows from the mains into the DC link when the mains voltage exceeds that of the capacitor. Current only flows when the mains voltage is near its peak, as shown. • The larger the DC link capacitor, the shorter the period of current flow, and the higher the peak current. Single or Three V + Phase Power C C Current - D C Link Rectifier Inverter

  11. Harmonic - Current Wave Form Ideal current wave but not attainable with a 6-pulse drive What the current wave form looks like with a 6-pulse drive Approaching a perfect current wave

  12. Typical Drive Input Current Waveform MM4 45kW Line Current Waveform (low impedance mains) I RMS = 75A Power = 12kW (x3) If current is sinusoidal Only 52A is required for the Same power.

  13. SED2 Typical Waveform SED2 11kW Line Current Waveform I RMS = 53A Power = 12kW (x3) If current is sinusoidal Only52Ais required for the same power.

  14. DC Link Design Results 15HP Typical VFD Line Current Harmonics Spectrum 15 HP SED2 Line Current Harmonics Spectrum Meter deflection

  15. Harmonics Competitive Analysis How do other VFD manufacturers address Harmonics? • Some build in DC link chokes • This adds cost, weight and current losses • Some offer line reactors • Rarely built-in to VFD - often offered as a separate option • Commonly mounted in bypass cabinet, or supplied in its own enclosure • Adds cost, weight and can cause nuisance tripping of drive on undervoltage • Some build special 12 and 18 pulse drives • Extremely expense and not practical for most application under 100HP • Some just ignore the problem

  16. Harmonics - The SED2 Advantage • SED2 is specifically designed for HVAC applications • Fans and pumps with variable torque load characteristics • Sophisticated Siemens control technology enables the SED2 to significantly reduce current harmonics by dramatically reducing the size (capacitance) of the DC link. • In lieu of large (high uF) electrolytic capacitors used in most VFDs, SED2 uses much lower value (low uF) plastic capacitors. • Typical 15Hp VFD: DC link capacitance of 1000uF • SED2 15HP VFD: DC link capacitance of 20uF

  17. Siemens SED2 VFD Advantages • Low harmonics design • Small enclosure size • Many standard features • Reduces operating cost • Low power losses/high efficiency • Minimal effect on customer power • Wide power range – from ½ to 125 HP

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