1.
HFCT Installation and Location:
o The HFCT can be applied either around the core of a cable or on the earth sheet.
o It’s crucial to note that HFCT should not be installed in a location where it captures both the core
and the earth simultaneously.
o
2. Reason for Separate Installation:
o When partial discharge occurs in an underground cable, equal and opposite signals are induced on
both the core and the earth.
o Placing the HFCT in a location that captures both signals simultaneously results in the
cancellation of the signals, making detection impossible.
o
3. Installation on Medium Voltage Networks:
o In medium voltage networks, HDFC tends to be installed in cable terminations, back in the
substation, and often on the earth for ease of access, especially during partial discharge testing.
o
4. Core vs. Earth Installation:
o For permanent installations, installing HDFC on the core can provide a more accurate signal and
allow detection of PD at lower levels.
o However, installation on the core might not always be feasible due to the need for shutdowns
during installation.
o
5. PD Signal Propagation:
o During partial discharge, signals are induced on the inside of the earth shield and an equal and
opposite signal on the outside of the core.
o These signals are equal and opposite, making them cancel each other out if a sensor is placed
around the cable capturing both signals simultaneously.
6. Faraday Cage Effect:
o The cable acts as a Faraday cage, trapping the high-frequency signals inside and preventing their
escape.
7. Connection to Earth:
- By connecting the HFCT to the earth, the equal and opposite signals induced on the core and the earth can be
effectively measured separately.
- This separation allows for the detection of partial discharge signals that would otherwise cancel each other out
if captured simultaneously.
In summary, connecting the HFCT to the earth helps in isolating and detecting partial discharge signals more
effectively by avoiding the cancellation effect caused by the equal and opposite signals induced on both the core
and the earth during partial discharge events.
