Draft Guide for Harmonic Limits for Single-Phase Equipment



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8.5 Suggested Protocol


  • With the nonlinear load disconnected, measure the voltage source distortion to ascertain that THDV  0.5 %.

  • Adjust the Thevenin impedance to the prescribed value.

  • Check the linear frequency response of the Thevenin impedance. This procedure requires injection of harmonic currents into the voltage source. In many situations, the injection can be provided by the nonlinear load under test, otherwise a 2 kVA or larger nonlinear load, with THDI > 1.0, can produce the right harmonic source for this task. The analyzer is used to measure the harmonic voltages (Vh) and harmonic currents (Ih). The Thevenin harmonic impedance is computed from


T
he normalized values

a
nd must be found within the lines

pu Zs = (1  0.05) h; h > 1



If a background harmonic voltage, VhO, (i.e., a source harmonic voltage) has the same order of magnitude as the measured voltage, Vh, then the measurement of the Thevenin impedance at this particular frequency may be compromised since, in this case, the expression of ZS is

T
he requirement that the supply voltage is sinusoidal addresses this need; nevertheless, it is necessary to verify that the condition

Vh = ZS (h) Ih  20 VhO

is always satisfied, especially when the measured harmonic voltage Vh < 0.5 % V1.


  • Connect and energize the nonlinear load to be evaluated. Allow for the unit to stabilize thermally. Measurements are taken for different input rms voltages (e.g., 0.95, 1.00 and 1.05 rated voltage) and, if applicable, a set of output powers (e.g., no-load, 0.25, 0.50, 0.75, 1.00 and 1.10 rated load). It is recommended to start with the conditions that cause the largest temperature rise, i.e., maximum power.




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