Bezinal Voltage 32luik - Bekaert

wire coating technology
BEKAERT
High strength, Bezinal ® coated steel wires,
the solution when increasing power grid capacity
Bezinal ®
Allows
power grid
capacity
increase
Suitable for
existing concepts
(ACSS, T-ACSR)
Superior corrosion
resistance with
respect to
galvanised
More versatile
than
clad steel

Increasing the capacity of high voltage power lines goes together
with a substantial increase in operating temperature. Safe and
reliable operation conditions require high strength steel wires
coated with
Bezinal ®
Zn + Al
95% 5%
Fe
Advantages:
▲
Allow substantial power grid capacity increase
• Safe operation at low and high temeratures
• Mechanically stable in a broad temperature domain
• No flaking of the coating at elevated temperatures
• Retained corrosion resistance at elevated temperatures
▲
Ease of application - suitable for existing conductor
concepts (ACSS, T-ACSR)
BEKAERT
▲
▲
• Bezinal ® coated and galvanised steel cores are
geometrically similar
• Withstands pre-annealing of ACSS conductors
Superior corrosion resistance with respect to galvanised
More versatile than clad steel
2

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
3

Why are ACSS and T-ACSR suitable for operation
at high temperatures
Taking into account the composition of each conductor type and the described temperature effects of the composing metals, the temperature related
performance of each conductor type can be simulated.
AAAC: Thermal elongation and a decreasing modulus of elasticity
lengthen the conductor (increasing sag) and hence decrease
the exerted force. The yield strength, which determines the maximum
tolerable load, is also decreasing. The result is a more or
less stable load/yield ratio in a temperature domain up to 150°C.
Due to the steep decrease of the yield strength when the conductor
temperature is higher than 150°C, the conductor will plastically
deform or even break (steep increase of the graph).
Safe operation of an AAAC type is only possible up to
100°C. When, by accident, the conductor is heated up
to higher temperatures, irreversible metallurgical
changes prevent recovery of the original mechanical
properties, resulting in lower yield strength and an
unsafe operation after several temperature cycles.
Load/Yield limit
100%
75%
50%
25%
0%
0
AAAC
AAAC limit
AAAC
50 100 150
Sag-m
25
20 AAAC limit
AAAC
15
10
0 50 100 150 200 250 300
Temperature - °C
200 250 300
ACSR: In “cold” conditions, both the steel and the aluminium
mechanically support the conductor. The aluminium wires will be
loaded to 75% of their yield strength (which corresponds to limiting
the elongation to less than 1% in order to avoid fracture of
the brittle aluminium). The steel is only loaded to about 10% of
its yield strength.
Due to the difference in thermal elongation, the forces in the pretensioned
aluminium change from pull to push, with as a consequence
that the aluminium wires will not contribute in carrying
the load anymore. The steel core totally takes over the supporting
action when the operating temperature exceeds 80°C. Therefore
only the elongation of the steel determines the sag. This results
in a change (decrease) of the slope in the sag vs. temperature
curve.
Al further temperature rise will irreversibly soften the drawn aluminium.
When cooling down, the increasing force will cause a
permanent (plastic) deformation of the aluminium wires, and only
the steel core will mechanically support the conductor.
The temperature dependence of the load distribution
and the sag under permanently changing temperature
conditions is unwanted as it creates uncertainty. Safe
operation is only possible as long as no significant
changes of the metallurgical and mechanical properties
of the aluminium occur. Operating temperatures therefore
have to be limited to maximum 100°C.
T-ACSR: T-ACSR essentially behaves similar to ACSR. In
contrast with ACSR, however, the mechanical properties of the
zirconium alloyed aluminium wires will hardly change irreversibly
when heated up to 180°C (or 250°C depending on the
alloy). As long as this temperature limit is respected, the aluminium
wires will keep on playing their supporting role in
cold conditions.
A stable operation is ensured up to operating temperatures
of 180° C (or 250°C).
ACSS: The result of starting with soft-annealed aluminium
wires before installing the conductor is that all temperature
related changes during operation will only be linked to changes
of the mechanical properties of the steel, i.e. they are
reversible. This means that the steel wires can be used at full
strength. A stable operation is ensured up to even
300°C. Moreover, ACSS features considerably lower high
temperature sag than ACSR at equal diameter and conductivity.
Also here, the main advantage is that the sag is reversible
in a very broad temperature domain.
Load/Yield limit
Load/Yield limit
Load/Yield limit
100%
75%
50%
25%
0%
0
100%
75%
50%
25%
0%
0
100%
75%
50%
25%
0%
0
ACSR
ACSR
steel core
T-ASCR
ACSS
ACSS
Al 1350
ACSR limit
50 100 150 200 250 300
Al Zr alloy
steel core
Temperature - °C
Temperature - °C
Temperature - °C
Sag-m
T-ACSR limit
Sag-m
25
20
15
10
0
10
0
50 100 150 200 250 300
50 100 150 200 250 300
Temperature - °C
Sag-m
25
20
15
25
20
15
10
0
ACSS limit
T-ACSR limit
ACSR limit
50 100 150 200 250 300
Temperature - °C
T-ACSR limit
50 100 150 200 250 300
Temperature - °C
ACSS
ACSS limit
50 100 150 200 250 300
Temperature - °C
BEKAERT
4

Steel grades with higher yield strengths are beneficial
As only ACSS and T-ACSR conductors are
suitable for operation at high temperatures,
describing the effect of increasing the
strength of the steel core will only be limited
to these two conductor types.
The exerted load is highest in “cold” conditions,
due to tensioning of the conductor (full
black line).
The safety limit (Load/Yield limit = 1/3) is
plotted for four steel grades with different
yield strengths: 1300, 1500, 1700 and 1900
(N/mm 2 at 25°C). The yield strength –and
hence the “safety limit” will decrease with
temperature. Safe operation requires that the
load in the considered temperature range
remains lower than the maximum allowable
load. This is possible by choosing the right
steel grade.
For ACSS, the most critical condition is at the
lowest temperatures, whereas for T-ACSR the
highest failure risk occurs at slightly elevated
temperatures, i.e. when -due to the difference
in thermal expansion coefficient between the
aluminium and the steel- the load distribution
switches over from steel plus aluminium
to only steel. Increasing the strength of
the steel wires is the best way to ensure
safe operation conditions. Another
advantage is that a high strength steel core
allows a tauter pre-tensioning in circumstances
where sag has to be limited to a minimum.
Load - kN
Load - N
80
60
40
20
0
80
60
40
20
0
ACSS
1900 MPa
1700 MPa
1500 MPa
1300 MPa
50 100 150 200 250 300
Temperature - °C
T-ACSR
1900 MPa
1700 MPa
1500 MPa
1300 MPa
50 100 150 200 250 300
Temperature - °C
Why Bezinal ®
Increasing the operation temperature is only possible with an adapted steel wire coating, retaining the corrosion performance at elevated temperatures.
Traditionally for ACSR, a zinc coating has been used to protect the steel from corrosion. A zinc coating on steel does not resist high temperatures;
it will not withstand the pre-annealing process of ACSS and it is not suitable for being used in conductors that will be operated at temperatures
above 200°C.
The high temperature performance of a Bezinal ® coating compared to a zinc coating is required here. Bezinal ® (BEkaert ZINc ALuminium) contains
about 95% zinc and 5% aluminium, is temperature resistant and has better overall corrosion resistance than zinc.
1
No flaking when exposed at high temperatures
Bezinal ®
When exposing zinc coated steel, typically used for ACSR, to high temperatures (higher than 200°C) the entire
coating transforms into a brittle FeZn alloy, with severe flaking as the consequence. The result is a steep decrease
in corrosion resistance (the Zn coating is partly removed). Moreover, hard particles between the conductor
wires may induce fretting, with a sudden fracture as the possible consequence.
As Bezinal ® is an alloy, some metallurgical transformations occur at elevated temperatures. These changes
have, however, no negative impact on the coating stability and on the corrosion performance of
the coating. Tests up to 350°C prove this.
Zinc
2
Superior
corrosion resistance
BEKAERT
Bezinal ®
Zinc
Three main reasons explain the superior corrosion resistance of the Bezinal ® coating when compared to a
zinc coating:
• For Bezinal ® , the first red rust signs become visible when the coating is almost completely consumed,
while on galvanised wire first red rust appears when there is still a significant amount of unconsumed zinc
left.
• A dense aluminium oxide film develops spontaneously over time and forms a natural barrier against corrosive
agents. Outcome: the overall corrosion slows down.
• The specific electrochemical potential interface between the steel substrate and the coating improves the
overall corrosion performance of Bezinal ® .
5

BEKAERT
Bekaert is a specialist in metal transformation and advanced coatings. With
about 17.500 employees and 96 production sites in 29 countries, the company
achieves a turnover of 2,8 billion euro. An important part of this turnover is
achieved with steel wire.
Bekaert service
You want :
• further technical information
• technical support for a
special project
Please contact:
new.products@bekaert.com
Bekaert has seen a growing tendency to replace existing conductors with new
ones with a higher current carrying capacity at equal weight, diameter and sag.
A popular method to increase a conductor’s current carrying capacity is to
operate it at higher temperatures, but this may cause some problems on ACSR
(Aluminium Conductor Steel Reinforced). The hard drawn aluminium wires
become annealed and loose their strength, thus jeopardising the conductor’s
mechanical strength and increasing sag. The zinc coating of the core wires
does not withstand high temperatures and will come off the wire. Operating an
AAAC (All Aluminium Alloy Conductor) at higher temperatures is not possible
due to the poor mechanical properties of aluminium at elevated temperatures.
To cope with this, alternative conductor types were developed. The most
known are ACSS (Aluminium Conductor Steel Supported) and T-ACSR
(Temperature resistant Aluminium Conductor Steel Reinforced). Bekaert’s
contribution is in the development of Bezinal ® coated, high
strength wire strands, because: - Bezinal ® contains about 95% zinc and
5% aluminium and is temperature resistant; - Bezinal ® also has a better corrosion
resistance than zinc, which even improves after heating; - the high
strength ensures a safe and reliable operation of the conductor.
Modifications reserved.
All details describe our products in general
form only. For ordering and design only use
official specifications and documents.
© N.V. Bekaert S.A. 2003
The Bekaert Group is a world leader in
advanced metal transformation and coating technologies.
All Bekaert company names are trademarks owned by Bekaert.
N.V. Bekaert S.A.
Global Market Management
Flip Verhoeven
Bekaertstraat 2
B-8550 Zwevegem
Tel. +32/56/76 61 93
Telefax +32/56/76 79 53
flip.verhoeven@bekaert.com
http://www.bekaert.com
BEKAERT
Resp. Edit.: D. Gonnissen, Roeselare/Joof// 12/2003