K Factor - AEMC 725 User Manual

K Factor

K-Factor information can be accessed while measuring AC current with
the rotary switch set to A~ in the HARM mode by pushing the yellow
(Shift) key.
The K factor can be thought of as an index of the transformer's ability to
handle nonlinear load current without abnormal heating. The K factor is
determined by measuring the True RMS current of each harmonic, mul-
tiplied by the harmonic order and squared.
K-Factor information can be used to select the proper "K-rated" trans-
former that properly matches your nonlinear electric load. Typical rat-
ings would be:
K=1: No derating necessary
K=4: Typical value for most commercial buildings
K=9: Upper limit for commercial buildings
A "K-4" rating indicates the ability of a transformer to accommodate four
times the eddy current losses of a K-1 transformer.
Most generators and transformers base their operating characteristics on
undisturbed 60 Hz waveforms. When the waveforms are rich in harmon-
ics, shortened service or complete failure of non-K-rated transformers is
sure to follow. The derating K factor can be applied specifically to dry-type
transformers to ensure that dangerous heating will not result due to the
transformer supplying load currents which are rich in harmonic content.
Existing transformer derating is not recognized by the National Electrical
Code ® . Therefore, it is recommended that ANSI/IEEE C57.110-1986 be
used as a guidance document before applying derating to an existing
transformer. This document is the "Recommended practice for establish-
ing transformer capability when supplying non-sinusoidal load currents."
Transformer parasitic heating due to harmonic currents is frequency-
dependent; higher frequency harmonic currents cause a higher degree
of transformer heating and failure. K-rated distribution transformers
today are being specifically designed with magnetic cores and windings to
accommodate harmonic contents.
Clamp-On Harmonic Probe Model
39