# Re: Impedance of Corona or Arcs vs Voltage (fwd)

```

---------- Forwarded message ----------
Date: Thu, 16 Jul 1998 08:39:22 -0700
From: Jim Lux <James.P.Lux-at-jpl.nasa.gov>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Impedance of Corona or Arcs vs Voltage (fwd)

Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Wed, 15 Jul 1998 21:33:27 -0700
> From: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Impedance of Corona or Arcs vs Voltage (fwd)
>
> Jim Lux wrote:
>
> > The corona onset voltage (for AC) is roughly where the E field strength
> > gets over 30 kV/cm.  This is mostly determined by the radius of
> > curvature. If the radius of your "thing" is 1 cm, then when it reaches
> > 30 kV, corona will start. The presence of nearby conductors will
> > definitely affect the E field distribution, and hence, the corona onset
> > voltage.
>
> The rule about the 1 cm of radius for each 30 kV is correct for a polished sphere,
> or a surface curved in all directions, for peak voltage values, including DC.
>
> Do you know what is the rule for a long cylindric conductor, as a high-voltage
> cable? It is not this, as 500 kV power line cables don't appear to have 47 cm
> of diameter (500*1.4142/30*2).
>
> (too lazy to make the derivation)

That's why 500 kV power lines are usually "bundled" conductors. The
bundle increases the effective radius of the conductor, and also reduces
the inductance, both of which are desirable, particularly the latter.

The max field at a subconductor of the bundle is:

Emax = V/ ln(4*h^2/(r*d))*(1/r + 1/d)

where h is the height above ground, r is the conductor radius, d is the
spacing between the conductors.

(from Khalifa, page 19,Eq 2.15)

The other thing is that you can use Peterson's formula and use the