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Selection of HV Conductors And Earth Wires by Siemens

Aluminium ACSR Conductors

Conductors represent the most important component of an overhead power line because they have to ensure economical and reliable transmission and contribute considerably to the total line costs.


Selection of HV Conductors And Earth Wires (on photo: Conventional ACSR and modern ACCC conductors; via Wikipedia).

!!! For many years, aluminum and its alloys have been the prevailing conducting materials for power lines due to the favorable price, the low weight and the necessity of certain minimum cross-sections.

However, aluminium is a very corrosive metal. But a dense oxide layer is formed that stops further corrosive attacks. Therefore, up to a certain level, aluminium conductors are well-suited for areas in which corrosion is a problem, for example, a maritime climate.


For aluminium conductors, there are a number of different designs in use. All-aluminium conductors (AAC) have the highest conductivity for a given cross-section; however, they possess only a 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 approximately twice that of pure aluminu."

ACSR – Aluminium conductor steel-reinforced (photo credit: myanmartc.com)

But single-material conductors like all-aluminium and aluminium alloy conductors have shown susceptibility to eolian vibrations. Compound conductors with a steel core, so-called aluminium conductor, steel-reinforced (ACSR), avoid this disadvantage.


The ratio between aluminium and steel ranges from 4.3:1 to 11:1. An aluminium-to-steel ratio of 6.0 or 7.7 provides an economical solution. Conductors with a ratio of 4.3 should be used for lines installed in regions with heavy wind and ice loads. Conductors with a ratio higher than 7.7 provide higher conductivity. But because of lower conductor strength, the sags are bigger, which requires higher towers.

!!! Experience has shown that ACSR conductors, just like aluminum and aluminum alloy conductors, provide the most economical solution and offer a life span greater than 40 years. Conductors are selected according to electrical, thermal, mechanical and economic aspects.

The electric resistance as a result of the conducting material and its cross-section is the most important feature affecting the voltage drop and the energy losses along the line and, therefore, the transmission costs. The cross-section has to be selected so that the permissible temperatures will not be exceeded during normal operation as well as under short-circuit conditions.


"With increasing cross-section, the line costs increase, while the costs for losses decrease."

Aluminum Conductor Steel reinforced (ACSR) cross-section

Depending on the length of the line and the power to be transmitted, a cross-section can be determined that results in the lowest transmission costs. The heat balance of ohmic losses and solar radiation against convection and radiation determines the conductor temperature. A current density of 0.5 to 1.0 A/mm2 based on the aluminium cross-section has proven to be an economical solution in most cases.

!!! High-voltage results in correspondingly high-voltage gradients at the conductor’s surface, and in corona-related effects such as visible discharges, radio interference, audible noise and energy losses.


"When selecting the conductors, the AC voltage gradient has to be limited to values between 15 and 17 kV/cm. Since the sound of the audible noise of DC lines is mainly caused at the positive pole and this sound differs from those of AC lines, the subjective feeling differs as well.

Therefore, the maximum surface voltage gradient of DC lines is higher than the gradient for 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 voltage gradient. In order to keep this gradient below the limit value, the conductor can be divided into sub-conductors.

This results in an equivalent conductor diameter that is bigger 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.

Aluminium conductor steel-reinforced (ASCR) bundle cable (photo credit: jcmiras.net)

Therefore, so-called bundle conductors have mainly been adopted for extra-high-voltage (EHV) lines. Table 1 below shows typical conductor configurations for AC lines.



Electric characteristics of AC overhead power lines (data refer to one circuit of a double-circuit line)

Electric characteristics of AC overhead power lines (data refer to one circuit of a double-circuit line)

From a mechanical point of view, the conductors have to be designed for everyday conditions and for maximum loads exerted on the conductor by wind and ice. As a rough figure, everyday stress of approximately 20 % of the conductor rated tensile stress can be adopted, resulting in a limited risk of conductor damage.

The maximum working tensile stress should be limited to approximately 40 % of the rated tensile stress.


Earth wires (Shieldwires or Earthwires)

HexaCore Optical Ground Wire (OPGW)
HexaCore Optical Ground Wire (OPGW)

Earth wires, also called shield wire or earth wire, can protect a line against direct lightning strikes and improve system behaviour in the event of short-circuits; 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 high aluminium cross-section satisfy both requirements.

!!! Since the beginning of the 1990s, more and more earth wires for extra-high-voltage overhead power lines have been executed as optical earth wires (OPGW). This type of earth wire combines the functions just described for the typical earth wire with the additional facility for large data transfer capacity via optical fibers that are integrated into the OPGW.

Such data transfer is essential for the communication between two converter stations within an HVDC interconnection or for remote controlling of power stations. The OPGW in such a case becomes the major communication link within the interconnection. OPGW is mainly designed in one or more layers of aluminium alloy and/or aluminium-clad steel wires.



One-layer designs are used in areas with low keraunic levels (small amount of possible lightning strikes per year) and small short-circuit levels.

 

Reference: Power Engineering Guide // Power Transmission and Distribution Solutions by SIEMENS



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