Quick Guide
Locating Cable PD using IPEC ASM & iPD
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Contents
Locating PD in Cables with ASM.............................................................................................................................................. 3
1: HFCT Sensors ................................................................................................................................................................... 3
HFCT installed at Cable Terminations ............................................................................................................................. 3
Where HFCT Sensors can be installed? ........................................................................................................................... 4
HFCT installed at Cable Joints ......................................................................................................................................... 6
2: iPD ................................................................................................................................................................................... 7
Time of Flight Method .................................................................................................................................................... 7
Mapping Graphs .............................................................................................................................................................. 8
Multiple Defects .............................................................................................................................................................. 9
How to Set up the iPD ................................................................................................................................................... 10
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Locating PD in Cables with ASM
When an ASM finds PD in a cable, an iPD unit can be deployed to locate the PD source to within 1% of the overall cable
length. In almost all cases the PD will be in a joint or termination which is easily identified by the location process for
further inspection. This document covers an overview of how to use HFCT sensors and the iPD unit in conjunction with
an ASM to locate cable PD.
1: HFCT Sensors
IPEC’s PD sensors can be used to detect PD, on-line, in cable circuits of all voltage levels from MV upwards. Typically,
3.3kV1 to 750kV or higher (theoretically no limit)
HFCT sensors are designed in different sizes and materials, depending on the application, but generally they operated
similarly.
HFCT installed at Cable Terminations
Typical Voltage Ranges 3.3kV to 750kV+
Sensor Types HFCT (High Frequency Current Transformer)
HFCT
Detection Type Internal PD: Inside insulation, 97% will be in cable joints or terminations, some PD will be inside pure
insulation
Coupling Method Inductive sensor, HFCT is installed around earth or core
Detection Range 5km +, depending on cable condition, additional substations/impedance changes
PD in Underground Cables HFCT (60KHz to 80MHz)
Substation 1 Substation 2
Switchgear PD Test Device Switchgear
CT
Underground cable High frequency CT
1
3.3kV insulation is predominantly mechanical rather than electrical, and PD is unlikely to occur, though often clients still choose to
monitor these circuits. IPEC recommend 5kV or higher for optimal deployment
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Where HFCT Sensors can be installed?
HFCT Sensors rely on detection of the PD signal, which is travelling on the earth sheath AND the core. The signals on
these electrodes will be equal and opposite, so we must install HFCT where they can be separated, or where the PD may
be detected in special cases.
It must be considered in each case where a HFCT can be applied, generally at EHV level, the HFCT installation is not a
problem, but at MV there can be some challenges due to earthing arrangements. Sensor location options:
HFCT connected
around earth
HFCT connected
around core (above
earth)
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Earth and Core signal HFCT position above HFCT is installed around HFCT is installed around
cancel each other out insulation, this is very Earthing core
(+ and -) dangerous PD will be detected PD will be detected
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HFCT installed at Cable Joints
Typical Voltage Ranges 110kV to 750kV+
Sensor Types HFCT (High Frequency Current Transformer)
HFCT
Detection Type Internal PD: Inside insulation, 97% will be in cable joints or terminations, some PD will be inside pure
insulation
Coupling Method Inductive sensor, HFCT is installed around earth (core possible, but HFCT likely too small)
Detection Range 5km +, depending on cable condition, additional substations/impedance changes
PD in Underground Cables HFCT (60KHz to 80MHz)
High frequency CT
Notes on PD Detection in Cable joints
HFCT
• At EHV levels, local cross bonding and local grounding at joints offer another potential location for HFCT sensors to
be installed, as the cable earth and core are separated
• The application can vary depending on the circuit layout and earthing types
• Cross bonded joints do not specifically cause issues to the application of HFCTs on earthing systems, though these
must be considered when the next step, of PD location, is considered
• HFCT’s may also be installed inside link boxes at joints
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2: iPD
The iPD is a transponder unit optimised for use with IPEC systems. The unit incorporates ground breaking hardware PD
recognition intelligence to distinguish cable PD from noise in real time. The ASM’s mapping feature uses received PD
pulses and pulses sent by the iPD to locate the PD using the time of flight method
Time of Flight Method
The ASM has a very high acquisition rate of data and therefore can accurately measure the timing of each PD pulse.
When a PD occurs in the monitored cable, the PD signal will propagate along the cable in both directions. The pulse will
be detected by both the iPD and ASM. When the iPD receives a PD pulse it instantly sends a much larger pulse down the
cable. This pulse is also detected by the ASM. The ASM can process the timing of the initial PD signal with the iPD pulse
and using the cable length will calculate the location of the PD.
Cable PD location iPD & ASM
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Mapping Graphs
This process is repeated over time to give a map of pulse locations like the one seen below. The graph clearly indicates a
spike at the location of the PD which is used to identify the defective joint.
PD Mapping Graphs iPD & ASM
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Multiple Defects
The system is capable of locating more than 1 defect whilst mapping. In the case of having multiple defects in the cable
the system will show 2 output spikes such as the case below
Multiple Defect Points iPD & ASM
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How to Set up the iPD
To perform cable mapping the iPD unit must be installed at the other end of the cable to the ASM and the cable length
must be known. The iPD HFCT is installed in the same way as ASM HFCT sensors.
iPD Set up iPD
Main Substation Far End of Cable
ASMTM Auto-mapping iPDTM
Once the hardware is set up, the mapping feature can be accessed in iSM. The iPD and cable mapping is then configured
from within iSM and left to run for the desired period. The longer the map runs the better the data will be. Suggested
running time is 1 week with 24 hours as a minimum guide.
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