Among many factors that control the frequency spectrum (harmonics) of the line current supplying a nonlinear load, five items are essential:

The design of the nonlinear load (circuit topology, components, geometry and physical properties of the active materials)

The rms value and the frequency spectrum of the applied voltage.

The value and frequency dependence of the Thevenin impedance measured at the bus supplying the observed nonlinear load.

The amount of power converted and delivered by the nonlinear load and the characteristics of the output power. For example, a mechanical load will require a certain torque and speed, and an arc welder will be characterized by the arc current and gap.

Nonelectrical environmental parameters such as temperature and pressure. Some electrical discharge lamps are very sensitive to such parameters.
We learn from here that, when measuring the current harmonic emission produced by a nonlinear load, it is imperative to energize the observed nonlinear load from a voltage supply with a standardized internal impedance and a nearly perfect sinusoidal voltage. This approach will help provide a common reference level for comparing different nonlinear loads.
The basic measurement circuits are presented in Fig. 81. The voltage source must have a frequency of 60 0.01 Hz and very low distortion (THD_{V} < 0.5%). It is suggested that the Thevenin impedance, measured at 60 Hz and including the current transducer CT and the analyzer voltage input impedance, is:
Z_{s} = 10 + j 50 m for a 240 V load
and
Z_{s} = 3 + j 15 m for a 120 V load.
A tolerance of 5% to the above values is permitted. The frequency dependence of Z_{s} must be linear (see Fig. 82) with a margin of error of 5%. This requirement is intended to help avoid resonance’s that will lead to erroneous measurements.
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