<P> Ontario Hydro (Hydro One) introduced trapezoidal - shaped wire ACSR conductor designs in the 1980s to replace existing round - wire ACSR designs (they called them compact conductors; these conductor types are now called ACSR / TW). Ontario Hydro's trapezoidal - shaped wire (TW) designs utilized the same steel core but increased the aluminium content of the conductor to match the overall diameter of the former round - wire designs (they could then use the same hardware fittings for both the round and the TW conductors). Hydro One's designs for their trapezoidal ACSR / TW conductors only use even numbers of aluminium layers (either two layers or four layers). They do not use designs which have odd number of layers (three layers) due to that design incurring higher hysteresis losses in the steel core . Also in the 1980s, Bonneville Power Administration (BPA) introduced TW designs where the size of the steel core was increased to maintain the same Aluminium / Steel ratio . </P> <P> Self - damping (ACSR / SD) is a nearly obsolete conductor technology and is rarely used for new installations . It is a concentric - lay stranded, self - damping conductor designed to control Aeolian - type vibration in overhead transmission lines by internal damping . Self - damping conductors consists of a central core of one or more round steel wires surrounded by two layers of trapezoidal shaped aluminium wires . One or more layers of round aluminium wires may be added as required . </P> <P> SD conductor differs from conventional ACSR in that the aluminium wires in the first two layers are trapezoidal shaped and sized so that each aluminium layer forms a stranded tube which does not collapse onto the layer beneath when under tension, but maintains a small annular gap between layers . The trapezoidal wire layers are separated from each other and from the steel core by the two smaller annular gaps that permit movement between the layers . The round aluminium wire layers are in tight contact with each other and the underlying trapezoidal wire layer . </P> <P> Under vibration, the steel core and the aluminium layers vibrate with different frequencies and impact damping results . This impact damping is sufficient to keep any Aeolian vibration to a low level . The use of trapezoidal strands also results in reduced conductor diameter for a given AC resistance per mile . </P>

Explain why lots of small strands of aluminium are used rather than a single core aluminium cable