For the power transmission between unsynchronised AC distribution systems and
over undersea cables HVDC systems will be used more and more.
The general principle for a HVDC system is the following:
In a first converter station the AC voltage will be rectified to a DC voltage. After the
transmission over a short or long distance in a secound converter station a pulse
wide modulated voltage will be produced. The transformers in this converter station
will be loaded by these voltage pulses with short rise times and high repetition rate.
Because of the nonlinear voltage distribution in the coils and the possibility of
resonances, the winding insulation could be stressed much higher than at ac voltage.
The influence of impulse voltages on the breakdown behaviour of oil-solid-insulations
with small insulation distances is partly unknown. Therefore the breakdown behaviour
of insulations between solid insulated wires with and without small oil gaps was
investigated. The breakdown voltage at voltage rising tests and the breakdown time
at lifetime experiments were measured at AC and different impulse voltages.
The measured breakdown voltages at ac voltage and at different kinds of impulse
voltages show for all specimen, that the breakdown voltage at impulse voltage is a
little bit higher than at ac. There is no significant influence of the time parameters and
the polarity change of the impulses.
For a better understanding of the discharge processes at AC voltage also PD
measurements were carried out. Outgoing from field calculation the inception voltage
was estimated by help of the streamer criterion and compared with the experimental
results
Lifetime experiments with ac and impulse voltage show a very high life time
exponent. Otherwise at ac voltage partial discharges (PD) has been observed in the
range of 50 % of the breakdown voltage. The intensity of the partial discharges
increases with increasing voltage but at constant voltage the intensity decreases after
some minutes to very small values.
The PD inception voltage of the insulation was estimated with the streamer criterion.
There is a good agreement between the calculated values and the measured partial
discharge inception voltages at ac.