Effects On The System - AEMC 721 User Manual

Measure each feeder for har-
monic content. A high degree at
this location is often heard as a
buzzing sound. A voltage THD
reading is also useful at this loca-
tion.
IEEE standard 519-1992 is a
guidance document for utilities
and electric power users which
specifies both the maximum dis-
tortion levels and recommended
correction levels. The harmonic
distortion limit of 5% is proven
to be the point where harmon-
ics begin to have a detrimental
effect on an electrical distribution ��������������������������
system.
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Harmonic current measurements define the harmonic generating charac-
teristics of the load, so measurements should be taken at the load when
possible. Voltage measurements define the system response and are usu-
ally taken at the individual busses.
4.2

Effects on the System

To compound the problems that harmonic currents present to the system,
the nonlinear harmonic load currents also have an Ohm's Law relationship
with the source impedance of the system to produce voltage harmonics.
Consider a heavily loaded transformer that is affected by one branch cir-
cuit feeding a nonlinear load; the resulting voltage harmonics can then be
passed down to all the remaining circuits fed by that transformer.
Voltage harmonics may cause havoc within the electrical system. Motors
are typically considered to be linear loads; however, when the source volt-
age supply is rich in harmonics, the motor will draw harmonic current. The
result is typically a higher than normal operating temperature and short-
ened service life.
Different frequency harmonic currents can cause additional rotating fields
in the motor. Depending on the frequency, the motor will rotate in the oppo-
site direction (counter-torque). The fifth harmonic, which is very prevalent,
is a negative sequence harmonic causing the motor to have a backward
rotation, shortening the service life.
21
Harmonic Meter Model 721