Power Design Pro™ Tips - Harmonic Analysis Overview
Most sizing programs in the market use simple rule of thumb multipliers to size the generator for nonlinear loads. However, given the vast variation between different types of loads and the variations in alternator performance based upon size and configuration, simple multiplier sizing is just not adequate for today’s complex loads. To solve this problem, Power Design Pro™ utilizes harmonic analysis to limit the harmonic voltage distortion to acceptable levels. The program automatically selects an appropriate harmonic current signature for each load type but also gives the user the ability to modify it to accurately model any load. Once the loads are entered, Power Design Pro calculates the resulting harmonic voltage distortion as that load is applied to various generators.
Harmonic analysis is the process in which each load is assigned a harmonic current spectrum based on its typical harmonic characteristics. This harmonic current signature is largely associated to the type of rectification the device is utilizing. In the photo above, a simple six pulse, unfiltered VFD has a very different harmonic signature compared to an IGBT device.
The harmonic current signature is actually the description of the various harmonic frequencies and their amplitudes. For example, the 5th harmonic is actually 300 hertz (5 x 60) and in this illustration below, has an amplitude of 26.1% based on the fundamental, 60 hertz current. Each device’s information is then harmonically summed together to get the total harmonic current spectrum. This harmonic current is then mathematically passed through the alternators’ source impedance, defined by its subtransient reactance (x”d). Using a harmonic equivalent of ohms law, the actual voltage distortion spectrum can be calculated and summarized as total harmonic voltage distortion (THVD). Power Design Pro uses these calculations to ensure that THVD and the sine wave distortion are within acceptable limits (typically 10-12% maximum). This helps ensure proper sizing for complex loads.
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