Bezinal Voltage 32luik - Bekaert

Operating the conductor at higher
temperatures increases power grid
capacity
Whereas in the past electricity production
has been strongly localised in the
area where it is consumed, the current
trend to buy electricity where it is cheapest
strongly increases transport over
large distances, using the existing networks.
The result is that the installed capacity
of high voltage power lines is getting
insufficient. Economic and ecological
objections prevent the building of new
lines in many places.
The only solution is
significantly increasing
the capacity of existing
lines, i.e. increasing
line capacity.
Capacity (MVA)
P 250°C
2x
P 50°C
As a consequence, more heat is generated
(heat generation increases quadraticly
with current), causing the nominal
operating temperature to rise from 50°C
up to even 300°C.
Existing power grids are typically equipped
with traditional ACSR or AAAC conductors,
which are not suitable for operation
at these high temperatures.
Therefore, ACSS and T-ACSR conductors
have been developed.
50 100 150 200 250
Temperature of the conductor (°C)
Different conductor types
ACSR:
Aluminium
Conductor Steel
Reinforced
The conductor consists
of steel wires around
which hard-drawn 1350
aluminium wires are
wound. Both the steel
core and the aluminium
wires mechanically
support the conductor.
ACSS:
Aluminium
Conductor Steel
Supported
ACSS and ACSR are
similar from a geometric
point of view. The
ACSS is annealed in the
factory. The main aim is
to permanently soften
the hard-drawn 1350
aluminium wires. In all
operating conditions the
ACSS is only supported
by the steel.
AAAC:
All Aluminium Alloy
Conductor.
A drawn and age-hardened
6201 aluminium
alloy yields a considerably
higher mechanical
strength than the pure
1350 aluminium. No
steel is needed for supporting
the conductor.
T-ACSR:
Temperature
resistant Aluminium
Conductor Steel
Reinforced
The conductor is similar
to an ACSR, except that
the aluminium is alloyed
with zirconium (Zr),
which prevents the
mechanical properties
of the drawn aluminium
to change irreversibly at
elevated temperatures.
Determining design parameters:
sag and safety limit (force)
Two design parameters determine the applicability
of a conductor (technical constraints related to
the towers are not considered): the sag and the
force required to keep the conductor in the air.
The maximum allowable force for a conductor
corresponds typically with one third of the yield
strength. Relating the applied force
to the yield strength is straightforward
(force = strength
60
x cross section of the supporting
area of the conductor).
50
The maximum allowable sag is often
related with local (legal) restrictions.
40
Force
Wind load
Definition of sag and force
applied force (kN)
Sag
30
20
10
Own weight
and ice load
The sag is often expressed in terms of the radius,
typically 1600 m. This value (excl. external
load) is the starting point for further calculations.
Other constants are: a span of 400 meter, external
load of about 500 kg, (conductor geometry =
Hawk (ASTM) or equivalent).
Force
15 20 25
sag/m
For a constant temperature, the
sag is function of the total load
(conductor weight, ice and
wind load) and the modulus of
elasticity of the supporting
metal (typically 200 GPa for
steel and 70 GPa for aluminium).
When the sag decreases, the
force increases substantially.
Influence of temperature
on the mechanical
properties
When determining the sag at elevated temperatures
(Operating temperatures up to 300°C are
taken into consideration), we need to consider:
- the thermal expansion coefficient of the different
metals (typically 12 µm/m°C and 25
µm/m°C for the used steel and aluminium grades,
resp.)
- the temperature dependence of the different
metals’ elasticity modulus. The temperature
related changes of the modulus are reversible.
The yield strength –and hence the “safe operation
limit” corresponding with 1/3 of the yield
strength- decreases with temperature. In contrast
with the modulus of elasticity, temperature induced
changes of the yield strength are almost completely
irreversible for the hard-drawn 1350 aluminium
in ACSR and the 6201 aluminium alloy in
AAAC, thanks to recrystallisation and over-aging
effects. After heating to 300°C, both alloys will
have mechanical properties similar to those of
annealed 1350 aluminium. The yield strength of
zirconium-alloyed aluminium used for T-ACSR
conductors is reversibly changing up to a certain
temperature. This temperature depends on the
amount of zirconium in the alloy. It is typically
180°C for standard grades and 250°C for high
grades with more zirconium. The temperature
induced change in yield strength of the steel will
be reversible.
BEKAERT
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