Draft Guide for Harmonic Limits for Single-Phase Equipment

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8.4.1 Current Transformers

The major source of errors caused by current transformers is traced to its magnetizing current. The magnetizing current is nonsinusoidal and contains harmonics that are included in the measured current. When the measured harmonic current is of the same order of magnitude as the magnetizing current harmonic, then large phase angle errors occur and even the measurement of harmonic magnitudes may be compromised. It is imperative to use metering class current transformers, with magnetizing current harmonics (Imh ≤ 5 % Ih, i.e., the magnetizing current harmonic is less than 5% of the measured harmonic current).

The burden of the current transformer, consisting of the input impedance at the current terminals of the analyzer plus the impedance of the conductors that connect the analyzer with the secondary terminals of the current transducer, must not exceed the rated burden of the current transformer. The lower the actual burden, he lower are the phase and amplitude errors caused by the current transformer.

Clamp-on type current transformers should not be used for such measurements. Their magnetizing current is large. Oxidation, impurities and nicks on the butt-joint surfaces of the core cause the increase of the magnetizing current. Moreover, many of the commercially available clamp-on units are overly sensitive to stray magnetic fields produced by adjacent current-carrying conductors.

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