Copper conductors are  surrounded with many layers of paper insulation.

铜导体周围有多层纸绝缘体。

As you might know, many materials expand or contract with changes  in temperature, and that's true for the copper conductors used in underground transmission lines.

您可能知道， 许多材料会随着温度的变化而膨胀或收缩，对于地下传输线路中使用的铜导体来说也是如此。