Consulting-Specifying Engineer - February 2009 - (Page 12)

M/E/P Roundtable to nonsinusoidal voltages. Symptoms of serious harmonic problems are: • Overheating of transformers, motors • Incorrect operation of electronic instruments and various types of sensors and meters • Excessive current in and failure of power factor correcting capacitors can either be immediate, as with fire alarm systems and standby generators, or delayed as in cables and transformers. When reactive power compensation is used with nonlinear loads, resonance conditions can occur that may result in even higher levels of harmonic voltage and current distortion. Most often, the current flowing in the power factor (pf) generator with reduced impedance can reduce the effects caused by the nonlinear load. Peeran: Symptoms should be apparent from day one of energizing the VFDs. The problems may be minor and may go unnoticed. However, as systems grow and as more VFDs, motors, and power factor correcting capacitors are installed, the problems get aggravated and surface in many ways. CSE : What types of facilities and equipment are more susceptible to VFD-related harmonic problems? Peeran: All types of modern facilities are vulnerable to harmonic problems— in particular, facilities with microprocessor-controlled machinery in which there is extensive electronic monitoring and instrumentation. Chemical and pharmaceutical manufacturing plants, where precise control of temperature is required, are vulnerable. Research laboratories that require abatement of power quality problems also suffer from VFDrelated problems. Walcott: Any site that uses sensitive process control or measurement devices will have problems with harmonics. The ac power source in a telecom center, airport, and hospital power room feeds a variety of loads such as diagnostic equipment, computers, HVAC, lights, and phones. These represent a mix of linear and nonlinear loads all powered from the same ac source. If the nonlinear content overrides the linear, harmonics could introduce a significant distortion to the ac voltage powering and affecting every thing connected to it. Lane: VFDs draw loads in a nonlinear fashion, similar to the solid state starter. The VFD will continue to draw its load in a nonlinear manner, as the frequency of the motor can be altered by control devices through the entire operation of the motor. A generator feeding a system with a VFD should have an oversized alternator similar to that of a system feeding a soft start without a bypass contactor. The effect of the harmonic content in the electrical distribution system must be analyzed both under normal utility power and under standby generator power. - Keith Lane • premature failure of circuit breakers. Wayne Walcott: Utility power harmonics are generated when a load draws nonlinear current from a sine wave voltage source. Harmonics are defined from the Fourier series that defines any periodic wave shape. Typically a three-phase, sixpulse VFD will draw nonlinear current that introduces the fifth and seventh harmonics on top of the fundamental frequency. A single phase, four-pulse VFD will produce the third harmonic or “triplen” harmonic. The symptoms of the VFD related harmonics are evident as a distorted line voltage. This harmonic voltage distortion can disrupt or stop operation of sensitive equipment. CSE : How long before symptoms become apparent , and how are they recognized? Swamy: Nonlinear loads generate voltage and current harmonics, which can have adverse effects on equipment that is used to deliver electrical energy. Examples of power delivery equipment include power system transformers, feeders, and circuit breakers. Power delivery equipment is subject to higher heating losses due to harmonic currents consumed by nonlinear loads. Harmonics can have a detrimental effect on emergency or standby power generators, telephones, and other sensitive electrical equipment like fire alarm systems. Depending on the sensitivity of the equipment, the effect correction capacitor is higher than its rating. If fuses are used in series with the capacitors, these will quickly open, rendering the pf correction equipment useless. Walcott: Effects and evidence of harmonic voltage distortion vary greatly and are dependent on the severity and size of the offending load. A key indicator is the type of equipment using the utility power and the site power distribution strategy. A heavily automated industrial site may be disrupted by as little 2% voltage harmonic distortion. Excessive triplen or neutral currents will over heat neutral lines, and disrupt phone systems and office equipment. Lane: Voltage drop and voltage distortion can be a major issue in electrical distribution systems with high harmonic content. This voltage distortion will be much higher when a facility is under standby generator power than when on utility power. Typical generators will have 15% to 20% internal reactive impedance, whereas utility transformers will typically have between 2% to 5% internal reactive impedance. Therefore, the effect of the harmonic content in the electrical distribution system must be analyzed both under normal utility power and under standby generator power. Waveform notching caused by this voltage distortion can be detrimental to solid state timing devices that rely on zero crossing switches. To compensate for the voltage distortion from nonlinear loads, a larger 12 Consulting-Specifying Engineer • FEBRUARY 2009

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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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