ACSR aluminum conductors
High Voltage Conductors are the most important element of an overhead power line because they have to provide economical and reliable transport and contribute significantly to the total cost of the line.
For many years, aluminum and its alloys have been the main conductive materials of power lines due to the favorable price, low weight, and the need for certain minimum sections.
However, aluminum is a very corrosive metal. But a dense oxide layer is formed which stops further corrosive attacks. Therefore, up to a certain level, aluminum Voltage conductors are ideal for areas where corrosion is a problem, for example, the maritime climate.
For aluminum Voltage conductors, different models are used. All aluminum conductors (AAC) have the highest conductivity for a given section; however, they have only low mechanical strength, which limits their application to short spans and low tensile forces.
To increase the mechanical strength, wires made of aluminum-magnesium-silicon alloys are adopted. Their strength is about twice that of pure aluminum.
But single-material conductors such as all-aluminum and aluminum alloy conductors have shown susceptibility to wind vibration. Steel core composite conductors, also known as Steel Reinforced Aluminum Conductor (ACSR), avoid this inconvenience.
The ratio of aluminum to steel ranges from 4.3: 1 to 11: 1 . An aluminum to steel ratio of 6.0 or 7.7 is an economical solution. Conductors with a ratio of 4.3 should be used for lines installed in areas with heavy wind and ice. Conductors with a ratio greater than 7.7 provide superior conductivity. But due to the low force of the conductor, the dips are larger, which requires higher towers.
Experience has shown that ACSR conductors, just like conductors made of aluminum and aluminum alloys, provide the most economical solution and offer a lifespan of over 40 years. The conductors are selected according to the electrical, thermal, mechanical, and economic aspects.
The resulting electrical resistance of the conductive material and its cross-section is the most important characteristics affecting the voltage drop and energy losses along the line and, therefore, the transportation costs. The cross-section must be chosen so that the admissible temperatures are not exceeded both in normal operation and in short-circuit.
With an increasing section, the line costs increase, while the costs for losses decrease.
Depending on the length of the line and the power to be transmitted, a section can be determined t which results in the lowest transmission costs. The thermal balance of ohmic losses and solar radiation against convection and radiation determines the temperature of the conductor. A current density of 0.5 to 1.0 A / mm 2 based on the aluminum section has proven to be an economical solution in most cases.
High voltage results as a result of high voltage gradients on the conductor surface and corona related effects such as visible discharges, radio interference, audible noise, and energy loss
When choosing conductors, the alternating voltage gradient must be limited to values between 15 and 17 kV / cm. Since the sound of audible noise from DC lines is mainly caused at the positive pole, and this sound differs from that of AC lines, the subjective feeling also differs.
Therefore, the maximum surface voltage gradient of DC lines is higher than that of AC lines. A maximum value of 25 kV / cm is recommended. The line voltage and the conductor diameter are one of the main factors that influence the surface tension gradient. In order to keep this slope below the limit value, the conductor can be divided into sub-conductors.
This results in an equivalent conductor diameter that is larger than the diameter of a single conductor with the same cross-section. This aspect is important for lines with voltages of 245 kV and above.
Therefore, so-called conductor bundles are mainly adopted for extra-high voltage (EHV) lines. Table 1 below shows typical conductor configurations for AC lines
Electrical characteristics of AC power lines
(data refer to one circuit of a double circuit line)
Electrical characteristics of overhead AC power lines (data refer to one circuit of a double-circuit line)
From a mechanical point of view, conductors must be designed for everyday conditions and to withstand the maximum loads exerted on the conductor by wind and ice. Approximately, daily stress of about 20% of the nominal tensile stress of the conductor can be adopted, which limits the risk of damage to the conductor.
The maximum tensile stress at work should be limited to about 40% of the nominal tensile stress.
Earth wires (Shieldwires or Earthwires)
Ground wires, also called wire or ground wire, can protect a line against direct lightning strikes and improve the behavior of the system in the event of a short circuit; therefore, lines with single-phase voltages of 110 kV and above are usually equipped with earth wires. Earth wires made of ACSR conductors with a sufficiently large aluminum cross-section meet both requirements.
Since the early 1990s, more and more ground wires for overhead power lines with extra high voltage optical ground wires (OPGW). This type of ground wire combines the functions just described for the typical ground wire with the additional installation for high data transfer capacity via optical fibers built into the OPGW.
This data transfer is essential for communication between two converter stations within an HVDC interconnection or for the remote control of central units. In this case, the OPGW becomes the main communication link within the interconnection. OPGWs are primarily designed in one or more layers of aluminum alloy and/or aluminum-coated steel wires.
Single-layer designs are used in areas with low keraunic levels (low lightning possible per year) and low short circuit levels.