In the previous episode, we examined the different types of electric breakdown.
These have largely assumed a DC current. Since the advent of using alternating current in
power transmission it was quickly realised that breakdown in dielectric insulators
occurs much more easily in AC compared to DC. The reasons for this are not well understood.
Numerous works have examined the breakdown phenomenon under DC conditions. These showed
that the space charge contributes greatly to the field distortion within insulating materials,
and breakdown happens instantly after local electrical fields reach a certain value.
The understanding of space charge behaviours under ac stress is very limited and their
influence on the breakdown process is actively debated. One of the problems is
that the detection of space charges under ac stress is much harder compared to dc stress,
due to the extremely fast periodic motion of the applied voltage amplitudes. There is also the
issue of the resolution of equipment to measure the space charges is not high enough currently.
An assumption can be made that for each insulating material, an intrinsic breakdown strength exists,
and whenever the local electric field within the material reaches that value, breakdown of
the overall material will be triggered. Based on this George Chen created a numerical model.
This model has strong experimental correlation in high voltage DC breakdown.
This model was then extended to AC and the results paint an intriguing picture.
The apparent breakdown strength of insulating materials are strongly related to the region
where breakdown happens. It occurs more rapidly and closer to the surface of the material
when the frequency of the applied field is greater. This would seem to suggest
that in an AC breakdown it is the severely unbalanced and distorted electric fields
which lead to a lower apparent breakdown strength under high voltage AC compared to high voltage DC.
In the example here we can see that ac breakdown strength decreases with increasing applied voltage
frequency. At 50Hz it occurs at 135 kV/mm. By comparison, the DC breakdown strength is 233
kV /mm. As the frequency increases, this drops significantly to around 60 kV / mm.
If we examine what happens to the space charge within the insulator
we can see that both positive and negative charges are accumulating in the vicinity of the electrodes
compared to under dc stress. It is also much shallower due to the much lower mobility which
controls the migration of the charges. Although the total amount of space charges in ac stress
is relatively small, the charge density in the vicinity of the electrodes is extremely large.
The density of both positive and negative charges increases with increasing applied frequency.
If we examine the distribution of charges then it can be seen that although in an
ac stress case of 50Hz the overall field strength is 135 kV/mm
the maximum electric field reaches 233 kV / mm near the electrode.
AC breakdown strength is therefore determined by
space charges in the vicinity of the electrode interface.
The lower breakdown strength compared to dc and the decreasing trend with increasing applied
frequency are due to space charge induced electric field distortion in the vicinity of the electrode.
Breakdown happens more rapidly and closer to the surface of the material
when the frequency of the applied fields is higher.
The strange relationship between breakdown strength and sample thickness.
Based on numerous experimental results,
the apparent breakdown strength of insulating materials seems to decrease
with increasing thickness. This downward trend is commonly acknowledged to be a geometrical effect
and it is widely observed. Many attempts have been made to explain this but so far
have failed to describe the dynamics and back this up with experimental evidence.
Here we once more see a large difference between AC and DC.
AC seems to undergo a bigger change as thickness increases.
When frequency is added in we see that at lower frequency there is a much bigger difference
between various thicknesses and this tends towards the same value as frequency increases.
As the material gets thicker it allows a much greater charge dynamic to build
which may ultimately be the cause for the decrease in the breakdown with increased thickness.