Page 785 - Industrial Power Engineering and Applications Handbook

23/742 Industrial Power Engineering and Applications Handbook

lhl
A - R
WY vh
/, = 0

Harmonic circuit is not completed,
lt,
:_ 0 v, = 0
=
(a) Isolated neutral system
Ihl Harmonics can circulate freely to other systems and from other
systems to this system
lh and Vh, = High
(c) Grounded neutral system
1

L
h = Ihl + /h2 + /h3
/h = 0
Harmonic circuit is made only internally. There is no flow of harmonics
to outside or from outside to the system, /h = 0
:. I, = low v, = 0 v,, = low (d) Delta connected system
(lh Vh are harmonic quantities)
and
(b) Ungrounded neutral system
Figure 23.12 Circulation of triple harmonic quantities

fifth harmonic waveforms. Higher harmonics are not Inferericr
shown being of lower magnitudes. Harmonics tend to The study based on which the table is produced has
diminish in amplitude at higher orders, Le. amplitude at revealed the following important aspects:
h3 > h5 > h7 etc., where h3, h5 and h7 etc. are the
harmonic quantities at the third, fifth and seventh harmonic
orders respectively. The effective value under the influence 1 Hexaphase electronic circuits generate maximum
of such harmonics will be the summation of all such harmonic disorders.
harmonics present in the system. Each harmonic wave 2 Inductive equipment generates moderate amounts of
may have a different phase relationship with the harmonic disorders, except the third harmonics on an
fundamental waveform, as illustrated in the figure, and LT system.
add or subtract from it to give the waveform a shape 3 The odd harmonics (non-multiples of 3), generated
somewhat of a pulsating nature as shown in Figure 23.7. by hexaphase rectifiers, are more severe than any other
Accurate measurement of harmonic quantities and their harmonic disorders present in the system, except LT
cumulative effect is a complex subject, and is not possible systems, where third harmonics too are pronounced.
to determine theoretically with the help of algebraic An LT system and the devices connected on it must
equations. But it can be easily measured with the help of take cognisance of these effects and suitable measures
a harmonic analyser. Cigre (1989) has made a com- be taken, either to diminish such effects or to select
prehensive study of the likely harmonics and their higher ratings of the devices to operate on such systems
amplitudes that may be present in a power system. These without overloading. HT systems must take particular
are briefly described in Table 23.1. cognizance of odd harmonics.