Page 11 - ECLECTIC MARCH-2022 INSIDE PAGES_p2.indd
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The net loss occurred due to harmonics in a year is ➤ In a system with a substantial amount of nonlinear
calculated for each DTRs and one sample calculation single-phase loads, the neutral current may rise to
is given below: a dangerously high level. There is a possibility of
excessive heating of the neutral conductor since
Table 7: Load (A) THD (%) Power there are no circuit breakers in the neutral conductors
Factor like in the phase conductors. It is important to take
Phase 135 9.2 care of the size of the neutral conductor if harmonics
165 11.2 0.89 are prevalent in the system.
R 147 10.7 0.89
Y 16.2 33.6 0.80 ➤ Since the presence of triple-N harmonics in the
B 0.86 neutral conductor results in higher heat generation,
N cable size selection must make allowance for this.
When the current rating of a cable is defined, it is
Chaulpatty Rd (C) P/T assumed that in the situation where unbalance
(Load Nature:Industrial) current flows in the neutral due to unbalanced
load, the joule loss in the neutral is offset by the fact
From acquired test results, that at least one phase conductor is carrying less
than rated load. When the load is non-linear there
➤ Total Active Power = 113 kW is a neutral current contributing to thermal loss in
➤ Active Power due to fundamental component = addition to the phase currents, leading to derating
of the ampacity of the cable.
86 kW
➤ Active power loss due to harmonics = 27kW ➤ Reference to sizing the neutral conductor in case of
non-sinusoidal currents can be found in IEC 60364-
Estimating similar distortion and load during commercial 5-524. Clause 524.2 which indicates that the neutral
hours (10 hrs.), daily loss = 270 units conductor shall have at least the same section as the
phase conductors:
➤ Yearly unit loss due to harmonics (assuming 300
active days for the industrial belt) = 81000 units ➣ in two-conductor single-phase circuits and for
all conductor cross-sections
➤ Yearly monetary loss (assuming per unit cost
` 7.30) = ` 5,91,300 ➣ in multi-phase circuits and in three conductor
single-phase1 circuits when the cross-section
It is to be noted that all these calculations have been of the phase conductors is equal to or less than
done based on instantaneous measurement of THD 16 mm2 for copper or 25 mm2 for aluminium.
values which may not be the true indicator the actual loss
figures. So we must go for long term THD measurement ➤ Clause 524.3 states that, for other multi-phase
before further analysis. circuits, the neutral conductor may have a reduced
cross section if all the following conditions are met:
➣ the maximum expected current, including
➤ Under balanced load conditions without harmonics, harmonics, if any, in the neutral conductor
the phase currents cancel each other in neutral and during normal service is not greater than the
the resultant neutral current is zero. However, in a current carrying capacity of the reduced cross-
4 wire system with single-phase nonlinear loads, sectional area of the neutral
odd-numbered multiples of the third harmonics (3,
6, 9, etc.) do not cancel, rather add together in the ➣ the neutral conductor is protected against
neutral conductor. overcurrent

➣ the size of the neutral is at least 16 mm2 in
copper or 25 mm2 in aluminium

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