Application of open-air capacitive sensors for voltage monitoring near terminations in HV and EHV insulated connections

Contactless capacitive sensors are applied to monitor
(over)voltages near the overhead line terminations of a
substation or at cable to line transitions. The sensor response is
the signal time derivative when loaded with a resistive
impedance and the waveform is restored by integration. As part
of this differentiating/integrating (D/I) measuring concept, the
use of open-air sensors results in excellent EMC characteristics
but the inherent cross-coupling to other phases has to be dealt
with. Three applications are presented: 1) For a 150 kV cable
termination the partial discharge activity needs to be related to
momentary phase voltages; 2) Measured slow front overvoltage
at a 380 kV cable termination from line energization
are compared with predictions from numerical simulation; 3)
The perspectives of employing the D/I method for (very) fast
front overvoltages near a 380 kV circuit breaker are examined.