Corona and conductor
Corona discharge isn't a matter of voltage but of field strength. The electrical field strength depends on voltage, but also on the area the charge is distributed on:

$$E = \frac{1}{A} \cdot \frac{Q}{\varepsilon_0}$$

A conductor cable or hollow conductor has a surface area of

$$A = 2\pi r \cdot l$$

as the inner surface is free from charge. For a bundled conductor, you have to run tests to get estimates on the equivalent surface area.