I believe you're discussing the spacers in such as twin feeder kind transmission lines.
The impedance of a transmission line is dependent on a range of variables, but one is that the spacing between the ACSR Conductors. So to maintain the impedance exactly the same over the line spacers are added to maintain the correct spacing.
Obviously this can be of more concern in RF where transmission line impedance is of all significance.
They are called package conductor spacers. Bundle conductors include many conductor wires connected by non-conducting spacers (although those conductors are at precisely the exact same potential).
Impedance, energy factor, corona discharge and weight reduction effects are enhanced by spacing quite a few conductors apart to make a bigger effective single bulb.
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