Consulting-Specifying Engineer - February 2009 - (Page 11)

PARTICIPANTS M/E/P Roundtable Harmonics & VFDs: prevention, analysis, resolution Our roundtable explores the harmonics issue from prevention to problem resolution. BY MICHAEL IVANOVICH, Editor-in-Chief, PATRICK LYNCH, Associate Editor CSE: How can variable frequency drives (VFDs) cause harmonic problems in facilities, and what are the symptoms? Keith Lane: VFDs can reduce the starting inrush current and kVA to half of that of an across-the-line starter. The reduction of inrush current in an electrical distribution system has a number of advantages. Many utilities require some form of reduced voltage starting for all motors over 25 hp. On the downside, VFDs utilize silicon controlled rectifiers to chop up the ac waveform and will provide a nonlinear waveform. This nonlinear waveform will cause voltage distortion across the reactor of the generator or the utility transformer and can cause unacceptable transient performance. This will adversely affect the performance of the entire electrical system. The combination of linear and nonlinear loads fed from a common power source can have a significant effect on the electrical distribution system. In situations where mostly linear loads are connected, a pure sinusoidal waveform current can flood the system with fundamental current. Significant linear load in the system will negate the effect of the nonlinear load. A system with primarily nonlinear loads will display a more distorted waveform at the power source. IEEE 519 limits the amount of total harmonic distortion (THD) at the point of common coupling. Mahesh Swamy: All voltage-source-inverterbased VFDs have an ac to dc rectifier unit with a large dc capacitor to smooth the voltage ripple. The dc bus capacitor draws charging current only when it is discharged into the motor load. The charging current flows into the capacitor when the input rectifier is forward biased. This occurs when the instantaneous input voltage is higher than the dc voltage across the dc bus capacitor. The pulsed current drawn by the dc bus capacitor is rich in harmonics because it is discontinuous. The voltage harmonics generated by VFDs are due to the flat-topping effect caused by a weak ac source charging the dc bus capacitor without any intervening impedance. The distorted voltage waveform gives rise to voltage harmonics, which is of more importance than current harmonics. The reason is simple. Voltage is shared by all loads and it affects all loads connected in an electrical system. Current distortion has a local effect and pertains to only the circuit that is feeding the nonlinear load. Nonsinusoidal currents that draw from the ac source cause undue stress on power delivery equipment that results in poor overall efficiency. The HVAC industry, especially hospitals and airports that use generators for backup power, is very vulnerable. Generators have high impedance and can lose regulation due to the harmonic voltage drop across the stator. In addition, the excess heating in the windings can force derating of the generator, and in severe conditions can trip the generator. Syed Peeran: By its nature, the VFD is a nonlinear load in the electrical system in the sense that it draws a nonsinusoidal current even if the applied voltage is perfectly sinusoidal. Modern VFDs are designed to minimize this nonlinearity by providing multipulse (12-, 18-, and 24-pulse) line converters, active front-end converters, or passive harmonic filters and line reactors at the input. Despite this, harmonic currents flow in electrical systems subjecting other equipment Keith Lane, PE, RCDD, LEEP AP Partner/Principal Lane Coburn & Assocs. Seattle Syed Peeran, PE, PhD Senior Engineer Camp Dresser & McKee Inc. Cambridge, Mass. Mahesh Swamy, PE, PhD Chief Engineer R&D Yaskawa Electric America Inc. Waukegan, Ill.. Wayne Walcott Product Line Application Support Manager MTE Corp. Menomonee Falls, Wis. Consulting-Specifying Engineer • FEBRUARY 2009 11

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Consulting-Specifying Engineer - February 2009 Contents Viewpoint News M/E Roundtable Codes & Standards Electrical Modernization: A Tale of Two Hospitals Retrofitting Office Lighting Controls Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - February 2009

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