Transformer Derating - AEMC 721 User Manual

Noise can be picked up in communication equipment and telephone
systems when harmonics at audio or radio frequencies are inductively or
capacitively coupled into communication or data lines.
When induction-disc watt-hour meters are monitoring nonlinear loads,
depending on the content of the harmonics, the disk may run slower or
faster, resulting in erroneous readings.
4.3

Transformer Derating

Most generators and transformers base their operating characteristics on
non disturbed 60 Hz waveforms. When the waveforms are rich in harmonics,
shortened service or complete failure is sure to follow.
The derating K factor can be applied specifically to transformers to ensure
that dangerous heating will not result due to the transformer supplying load
currents which are rich in harmonic content.
The K factor is determined by measuring the True RMS current of each har-
monic, multiplied by the harmonic order and squared. The total sum of this is
then multiplied by the eddy current losses. Transformer parasitic heating due
to harmonic currents is frequency-dependent; higher frequency harmonic cur-
rents cause a higher degree of transformer heating and failure.
The K factor can be thought of as an index of the transformer's ability to
handle nonlinear load current without abnormal heating. Some distribution
transformers today are being designed with magnetic cores and windings to
accommodate harmonic contents.
A K-rated transformer is specifically designed to handle nonlinear loads. The
higher the K factor value, the better the ability of the transformer to handle
nonlinear loads.
IEEE C57.110-1986 is a prescribed procedure used to derate the transformer
loading based on the specific harmonic content. Each specific electrical appli-
cation is unique in type and amount of harmonic interaction.
IEEE C57.12.00-1987 has proposed a limit of 5% for transformer harmonic
current factor.
An alternate method for derating transformers is available for buildings which
supply single-phase, 120 V receptacles. This method is established by The
Computer & Business Equipment Manufacturers Association (CBEMA).
1.414
CBEMA Derating Factor =
Crest Factor
22
Harmonic Meter Model 721