MV design guide - Schneider Electric

Design rules
Dielectric withstand
A few orders of magnitude
Dielectric strength
(20°C, 1 bar absolute): 2.9 to 3 kV/mm
Ionization limit
(20°C, 1 bar absolute): 2.6 kV/mm
c The dielectric withstand depends on the following 3 main parameters:
v the dielectric strength of the medium
v the shape of the parts
v the distance:
- ambient air between the live parts
- insulating air interface between the live parts.
The dielectric strength of the medium
This is a characteristic of the fluid (gas or liquid) making up the medium.
For ambient air this characteristic depends on atmospheric conditions
and pollution.
The dielectric strength of air depends
on the following ambient conditions
c Pollution
Conductive dust can be present in a gas, in a liquid, or be deposited on
the surface of an insulator.
Its effect is always the same: reducing the insulation performances by a
factor of anything up to 10!
c Condensation
Phenomena involving the depositing of droplets of water on the surface of
insulators which has the effect of locally reducing the insulating
performance by a factor of 3.
c Pressure
The performance level of gas insulation, is related to pressure.
For a device insulated in ambient air, altitude can cause a drop in
insulating performance due to the drop in pressure.
We are often obliged to derate the device.
c Humidity
In gases and liquids, the presence of humidity can cause a change
in insulating performances.
In the case of liquids, it always leads to a drop in performance.
In the case of gases, it generally leads to a drop (SF6, N2 etc.) apart from
air where a low concentration (humidity < 70%) gives a slight
improvement in the overall performance level, or so called "full gas
performance"*.
c Temperature
The performance levels of gaseous, liquid or solid insulation decrease as
the temperature increases. For solid insulators, thermal shocks can be the
cause of micro-fissuration which can lead very quickly to insulator
breakdown. Great care must therefore be paid to expansion phenomena:
a solid insulator expands by between 5 and 15 times more than a
conductor.
* We talk about "full gas" insulation.
Pollution level
Pollution may originate: from the external gaseous medium (dust), initial
lack of cleanliness, possibly the breaking down of an internal surface,
pollution combined with humidity causes electrochemical conduction
which will worsen discharge phenomena.
Its scope can be a constraint of the external medium (exposure to
external elements).
38 Merlin Gerin MV design guide Schneider Electric