Case Study
On-line Partial Discharge (PD) Testing
and Location of a 33 kV cable for a Metro
Light Rail System
Cables
Middle East
Project
A Middle Eastern, metropolitan light rail company experienced several cable failures leading to regular disruptions of their entire 33 kV rail
network. The customer requested On-line PD testing of a 5 km 33 kV XLPE cable, in order to identify and locate any further faults. Five years
before these failures when the network had been in-service for just over one year, On-line PD testing had identified three cable faults, all at
33 kV cable joints/splices.
Solution
HVPD Kronos®
Spot Tester
TEV Sensor
HFCT Sensor
HVPD Portable
Transponder
On-line PD Testing was conducted using the HVPD Kronos® Spot Tester
connected to High Frequency Current Transformer (HFCT) and Transient Earth
Voltage (TEV) sensors. The HFCT and TEV Sensors were installed at the cable
termination while in-service.
•
The HFCT sensors are split-core enabling them to be easily installed around
the cable core and earth for detection of PD activity within the cable.
•
The TEV is a capacitive probe designed to detect local, high frequency PD
pulses originating within the switchgear and terminations.
HVPD Kronos® Spot Tester
HFCT Sensors Attached to Cable Terminations
HVPD LTD
128 Metroplex Bus. Park
+44 (0)161 877 6142
Broadway, MediaCityUK,
info@hvpd.co.uk
Salford, M50 2UW
www.hvpd.co.uk
United Kingdom
Certificate Number 9329
ISO 9001, ISO 14001,
ISO 45001
MMCS-022-2
Case Study
Background Theory
Cable mapping is conducted by detecting the original PD pulse from the fault location as well as a reflected PD pulse (caused when the
original PD pulse reaches the far end of the cable). Using the time delta between these pulses, and the signal propagation speed the
approximate location of the PD source can be identified.
In cables of 5 km and longer, it is often necessary to use an HVPD Portable Transponder at the opposite end of the cable to amplify PD
signals, allowing the HVPD Kronos® Spot Tester to receive and interpret the relative arrival times of pulses at each end of the cable to give an
accurate location. This is important because when testing from just a single end, PD pulses reflected from the far end of long cable runs can
often become difficult to interpret due to large signal attenuation and increased levels of noise.
Results
High levels of PD were detected and the location was pinpointed to a
cable joint, 475 m along the 33 kV cable. The PD trace clearly showed
the Direct PD pulse injected, the Reflected PD pulse, and the amplified
Transponder pulse.
Analysis of the Phase Resolve PD Pattern (PRPD) and PD signals
indicated that the PD was likely to be the result of a non-conducting
cavity/void within the cable insulation.
Direct
PD pulse
Reflected
PD pulse
PD Trace
Transponder
pulse
PRPD Pattern
Conclusions and Recommendations
•
Due to high levels of PD detected and located during On-line PD testing, a cable fault was identified to a cable joint, 475 m along the
33 kV cable. If left undetected, this fault would have likely caused another unplanned outage.
•
The customer purchased an HVPD Kronos® Spot Tester and continues to perform routine On-line PD testing as part of their
Condition-Based Maintenance.
HVPD LTD
128 Metroplex Bus. Park
+44 (0)161 877 6142
Broadway, MediaCityUK,
info@hvpd.co.uk
Salford, M50 2UW
www.hvpd.co.uk
United Kingdom
Certificate Number 9329
ISO 9001, ISO 14001,
ISO 45001
MMCS-022-2