Fig. 5 shows data from a period during the test when the
voltage was increased from 1.1·U0 to 1.5·U0 (at approximately
4200 seconds from start) and then turned off (after 5000
seconds). U0 is the rated voltage for the tested bushing. In Fig.
5, the measured electric field strength has been normalized
with the capacitive field.
The increase at t=4200 s is smaller than expected from a
capacitance controlled step. The decline is faster at 1.5·U0 than
at the 1.1U0 turn on. This can be explained by some non-linear
effect such as corona from some grounded object that increases
the air conduction. The step down at t=5000 s is also smaller
than the expected "capacitive step". The final decline of the
field, after grounding, is relatively fast.
In Fig. 6 the angle of the field is shown. At the switch off at
5000 s the angle flips around 180º, as expected. The large
scatter in the end of the measurement is a consequence of that
the field strength approaches zero.