Consulting-Specifying Engineer - February 2009 - (Page 14)

M/E/P Roundtable The use of 12-pulse insulated-gate bipolar transistors and harmonic filtering can make the VFD more generatorfriendly. Rapidly shifting voltage pulses in a pulse width modulation VFD drive can interact with the capacitance and inductance of conductors feeding motors. Increased voltage peaks as high as 1,500 free site analysis software for determining the calculated levels of current and voltage harmonics as it pertains to the standard. Lane: Power quality meters can be used to determine the magnitude of harmonics in an existing facility. Calculations can be used for proposed new systems generators should be periodically turned on, similar to fire drills, to ensure proper operation during an emergency. If needed, the rating of the generator may have to be increased to handle critical loads supplied by VFDs, like pumps in water treatment plants or computer loads in airports. Peeran: Switchgear and switchboards are usually provided with multifunction electronic instruments which display kW, kVA, pf, current, and voltage. These instruments also have the capability to measure harmonic currents, voltages, and percentage THD. Any unexplained changes in the harmonic measurements are indications of potential problems. C S E : How are harmonic prob lems resolved after they are diagnosed? Swamy: There are many harmonic mitigation techniques available. Simple solutions like adding a 3% input inductor or using a dc link inductor are most effective in reducing harmonics. In standby generator or water treatment plant applications, depending on the recommendations listed in IEEE 519-1992, a more aggressive approach may be needed. It is best to avoid capacitor-based harmonic filters since they cause more problems than they solve. A few important reasons are as follows: • Passive techniques involving capacitors are associated with circulating current that causes unnecessary power loss resulting in poor efficiency. • In capacitor-based passive filters, the circulating current creates high voltage at the VFD input terminals that result in higher-than-normal average dc bus voltage under light load conditions. This can deteriorate the performance of VFDs and compromise the mean time between failures of many components in the VFD. • Passive filters are also associated with the possibility of causing network resonance. If they are installed, you should monitor resonance conditions and avoid them. Simple solutions like adding a 3% input inductor or using a dc link inductor are most effective in reducing harmonics. - Mahesh Swamy V at a nominal 480 V motor terminal can result from these shifting voltage pulses. As you increase the feeder length to the motor, the voltage peaks will also increase. This occurrence can strain and consequently erode the stator winding insulation in the motor. As a result, the life of the motor fed by the VFD can be significantly reduced. To mitigate this effect, many engineers will insert a 3% or a 5% impedance load reactor on the load side of a VFD for situations where the motor lead length is in excess of 110 ft. Swamy: Hospitals, airports, and banks that rely on standby generators during utility power outage are most susceptible to nonlinear load. Depending on the make of alarm systems, fire alarms or burglar alarms have been found to be affected by distorted voltage waveforms caused by higher than normal current harmonics existing in a weak ac system. CSE : What preventive checks or tests should building owners perform to determine if they have a harmonics problem? Walcott: A site harmonic analysis conducted by the utility or an independent power quality consultant will go a long way at identifying potential harmonic problems. The building owner should become familiar with the IEEE 519 guidelines as it pertains to the site specific requirements. The guidelines detail the utility for connection between the customer and the utility. As a point of reference, various drive manufactures have to determine the potential level of harmonic distortion. The harmonic profile of the proposed equipment and the impedance of the components of the electrical distribution system must be identified. IEEE 519 can then be referenced with the output of these calculations. When documenting the distortion levels of an existing facility, an engineer or technician should gather power quality measurements at the secondary side of serving transformers and standby generators. In addition, measurements should be gathered at the input to sensitive electronic equipment and at the input conductors for significant electronics loads. The current waveform should appear to be a sine wave for an individual linear load without any waveform distortion. The current waveform for an individual nonlinear load will appear to be distorted. The distortion level of an individual nonlinear load will be dependent on a number of factors including load current, source impedance, and system voltage distortion. The waveform measured at the point of common coupling will be a combination of all of the individual loads, both linear and nonlinear, within the system. Swamy: If the building is using power factor correction capacitors as part of an old system and recently have switched over to VFDs, then the power factor correction (pfc) capacitor units should be removed. If unable to be removed due to city codes, then the pfc units should be detuned and the fuses periodically checked to maintain conformity. Standby 14 Consulting-Specifying Engineer • FEBRUARY 2009

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Consulting-Specifying Engineer - February 2009

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