Heating in cable
The
temperature rise of cable depends on the following factors:
1.
The production of heat within the external periphery of the cable.
2.
The conveyance of the heat as far as the periphery - that is, up to the
boundary of the surrounding medium.
3.
The conveyance of the heat through this medium, and therefore away from the
cable.
4.
The current rating of the cables.
5.
The nature of the load, i.e. whether continuous or intermittent; not
infrequently the rating under short-circuit conditions has to be considered.
Heat generation in
cable
Following
are the sources of heat generation in the cable
a) I2R
losses in the conductors
b)
Dielectric losses in the cable
insulation
c) Sheath and armour loss
a).I2R
losses in the conductors
Copper
loss is the term often given to heat produced by electrical currents in the conductors,
or other electrical devices. Copper losses are an undesirable transfer of
energy, as are core losses, which result from induced currents in adjacent
components. The term is applied regardless of whether the windings are made of
copper or another conductor, such as aluminium.
Resistance
of conductor at an temperature of 70 deg. C (assumed) is determined from the
resistance given in standard table (usually at 20 deg,C) from the following
relation-
Rh
= Ra(1+α (70-20))
Where
Rh, Ra are the hot resistance, resistance at 20deg.C.
b).Dielectric losses in
the cable insulation
The
energy losses occurring in the dielectric of cables are due to leakage and so
called dielectric hysteresis.
·
One being true capacitance current which
is equal to ωC V and leads the applied voltage by 90deg.
·
The other being the energy component
which in phase with the applied voltage and represents the dielectric loss
components of current.
If
V is the applied voltage, C is the capacitance, of cable, Ф is the phase angle
between voltage and current called the power factor of the cable and δ is the
loss angle of the dielectric,
Charging
current, Ic = V/Xc= ωC V
The
dielectric loss, due to leakage and hysteresis effects in the dielectric, is
usually expressed in terms of the loss angle,δ:
δ=
90-φ
Where,
φ is the dielectric power factor angle.
Dielectric
loss =ωC
V2tanδ,
Where,
C=
capacitance to neutral
V=
phase voltage
A
typical value of tanδ lies in the range 0.002 to 0.003. In low voltage cables
the dielectric loss is negligible, but is appreciable in EHV cables.
c).Sheath loss
In
3 core cable the effect is negligible but for single core cable the effect is
of great importance. The electromagnetic fields produced by the current flowing
through the conductors induce emfs in sheath and under certain condition heavy
currents are set up therein. The actual current flowing along the sheath depends
magnitude and frequency of the current in the conductor, the arrangement and
spacing between the cables. Two different cables having sheath electrically
connected are bounded or unbounded. The induced sheath currents are of two
types-
i)
The currents, which have both outward
and inward directions, called the sheath eddies.
ii)
The currents, which have outward and
inward current path in separate sheath called the sheath circuit eddies.
The
approximate formulae for eddy loss for unbounded cables given by Arnold is as
under-
Where,
I
= current per conductor,
r
= mean radius of sheath,
d=inter
axial spacing of conductors
Rs
= sheath resistance in ohm
NB: The core loss, sheath loss, dielectric loss constitute the heating of cable.
reference:
a practical guide to cable installation and toolbox talk
Price:
Rs. 375/- excluding delivery charges
reference:
a practical guide to cable installation and toolbox talk
Available with book
shop and -
Heating cables are versatile cables with numerous applications including frost protection, ice and snow melting and many more.
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