Consulting-Specifying Engineer - November 2008 - (Page S6)

on-peakperformance >>>>>>>>>>>>>> Sizing standby power for non-linear load applications that voltage distortion in the 15% to 18% range generally results in application problems, while limiting distortion levels to less than 10%, through alternator sizing, may be cost prohibitive and best done through adding harmonic filters to the loads. Moving from the harmonic current of the device to a resulting system voltage distortion is as simple as applying Ohm’s law. Ohm’s law predicts that when current flows through an impedance, a corresponding voltage will result. Though the math of harmonics doesn’t allow the equation to be as simple, the same effect occurs with harmonics. When load-produced harmonic current flows through the source impedance of the alternator, a corresponding voltage distortion is created. To control the distortion to 10%, either the harmonic currents need to be reduced or the alternator’s source impedance needs to be reduced. The source impedance of interest is the alternator’s sub-transient reactance (x”d). The market occasionally requests alternators with subtransient reactance less than 12%, as a method of securing harmonic performance. Though this is a good starting point, if the non-linear load is a typical 6-pulse, unfiltered device, the equivalent reactance needs to be closer to 6%. This source impedance is usually given in percentage values, typically ranging from 10% to 18% for three-phase generators, and 14% to 24% for single-phase equipment. Alternator impedances are much higher on small machines and single phase equipment, so there is a greater need for proper harmonic analysis with these devices. It is common for small single phase alternators to be sized five times the non-linear load level. One common tactic for reducing generator-set impedance is to upsize the alternator, because larger alternators have lower source impedance. However, because alternator reactances are in a percentage system, it is important to recognize that the reactance number is relative to the base kVA of the machine. This means that specifying an alternator with a larger base kVA effectively lowers the absolute value of the machine’s impedance. As this discussion illustrates, sizing generator applications based on harmonics can be challenging. Various tools exist to aid in this analysis. When using generator sizing tools, scrutinize the effectiveness of the non-linear load sizing based on the tool’s inputs and outputs. If the sizing tool uses inputs of harmonic currents and alternator reactance and the output includes a calculated voltage distortion, the tool is performing a harmonic analysis. >> Moving from the harmonic current of the device to a resulting system voltage distortion is as simple as applying Ohm’s law. Matching a generator to an application’s load is, generally, a very straightforward process—the total load is measured or calculated, in kW, and the generator is selected based on the result. However, when the sizing involves loads that create significant amounts of harmonic currents (non-linear loads), the process can become very imprecise, often relying on rules of thumb and simplistic multipliers. Rules of thumb are valuable references, but non-linear generator sizing should incorporate some of the fundamentals of harmonic analysis. Non-linear loads consume input power through their internal-switching power electronics. These switching devices consume 60/50 Hz current and produce other current frequencies—harmonics—as a byproduct. The harmonic currents vary, based on the type of switching devices, the number of switching devices (referenced as pulses) and the amount of harmonic filtration. The harmonic current’s amplitude usually is expressed as a percentage of the input 60/50 Hz current, while its frequency usually is expressed based on its multiple of the rated input frequency. Since non-linear loads produce many odd numbered multiples of the input frequency, harmonics are often referred to as a vectorial summation of the various frequencies. The common summations are THID (total harmonic current distortion) and THVD (total harmonic voltage distortion). Not all non-linear loads are created equal. Typical three-phase equipment varies dramatically from 35% to less than 10% THID. So the first step in sizing a generator to a non-linear load is to recognize the load’s harmonic characteristics. A device with 35% distortion may need an alternator rated three times the size of the device, but a 10% distortion may require no alternator upsizing. As a general guide, loads with THID less than 12%, don’t require alternator upsizing. So on what criteria should harmonic sizing be based? Traditionally, the market attempted to address this issue from a harmonic heating perspective, requiring alternators to be rated at class B and/ or class F temperature rises. This approach may have worked with moderate applications, but when the load is predominately 6-pulse, unfiltered (35% THID) devices, the resulting voltage distortion will be extreme. IEEE 519, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, recommends that the maximum THVD level should be limited to 10% for a dedicated system. Keeping voltage distortion to a range of 10% to 12% is a good target—application experience tells us 6 november2008

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Consulting-Specifying Engineer - November 2008

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