Schneider Electric HRB1684301-01 user manual Chapter Power quality, Harmonics overview

Chapter 13 Power quality

This section describes the meter’s power quality features and how to access power quality data.

The meter measures voltage and current harmonics up to the 63rd harmonic, and calculates Total Harmonic Distortion (THD) and Total Demand Distortion (TDD and tdd).

Harmonics overview

Harmonics are integer multiples of the fundamental frequency of the power system. Harmonics information is valuable for power quality analysis, determining properly rated transformers, maintenance and troubleshooting.

Harmonics measurements include per-phase magnitudes and angles for the fundamental and higher harmonics relative to the fundamental frequency. The meter’s power system setting defines which phases are present and determines how line-to-line or line-to-neutral voltage harmonics and current harmonics are calculated.

Harmonics data provide information to determine how non-linear loads affect the power system. For example, power system harmonics can cause current flow on the neutral conductor, increase heating in electric motors, and eventually damage connected equipment. Power conditioners or harmonic filters can be used to minimize unwanted harmonics.

Crest factor and K-factor

Crest factor is the ratio of peak to RMS voltage values. For a pure sinusoidal waveform, crest factor is equal to 1.414. The meter uses the following equation to calculate crest factor:

Vpeak

Crest factor = --------------

VRMS

K-factor relates the heating effect of a distorted current in a transformer to a sinusoidal current with the same RMS magnitude — it describes a transformer’s ability to serve non-linear loads without exceeding rated temperature rise limits. The K-factor is equal to the sum of the squares of the harmonic currents multiplied by the squares of the harmonic order. The meter uses the following equation to calculate K-factor:

∞

K-factor = ∑ (Ih2 × h2)

h = 1

Where h is the harmonic order and Ih is the true RMS current of harmonic order h.

Total Harmonic Distortion and Total Demand Distortion

Total Harmonic Distortion (THD) is a measure of the total per-phase voltage or current harmonic distortion present in the power system. It provides a general indication of the quality of a waveform. THD is calculated for each phase of both voltage and current.

Total Demand Distortion (TDD) is the per-phase harmonic current distortion against the full load demand of the electrical system. TDD indicates the impact of harmonic distortion in the system. For example, if your system is showing high THD values but a

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