On-Line Partial Discharge HV Cable Mapping Technology

The OSM-Longshot™ On-Line Cable PD Mapping System in the Substation	PDMap© Software Screenshot (Data Input Page)	PDMap© Software Screenshot (Data Processing Page)

PTT 2000-CT Portable Transponder
(2 units) (top – Pulse Generator Unit,
bottom –Discharge Trigger Unit)

The OSM-Longshot™ On-Line Cable PD Mapping Technology from IPEC High Voltage represents the world's first commercially-available on-line pd mapping technology for locating incipient faults on high voltage cables.

The technology applies the same Time-Of-Flight (TOF) analysis of PD signals as used in the off-line cable pd location methods (ie: VLF,OWTS and resonant test systems) with the key advantage of being able to be done on-line (no outage required).

The core technology applies the portable and flexible OSM-Longshot™ PD Spot Tester, which is combined with the PD Map© Software, 'PTT-2000-CT' Portable Transponder and 3x Shielded HFCT Sensors (2x Measurement CT's and 1x Pulse Injection CT).

The On-Line Cable Mapping system provides for the accurate location of PD sites (to an accuracy of less than 1% of cable length). The system is suitable for On-Line PD Mapping of high voltage cables of lengths of up to 5km.

• PD mapping measurements are made on-line, with measurements in picoCoulombs (pC's).
• The Battery-Powered Portable Transponder can be used at any position on a cable feeder with access to the screen of the cable.
• The OSM-Longshot™ Unit can also be used in Monitoring Mode to assess load-related PD.
• Can also be used for Off-Line Testing (OWTS etc)

• The cable is tested in its 'normal' conditions (working voltage, load and temperature).
• Lower cost when compared with off-line testing.
• The switching out of circuits (as per off-line testing) is not necessary = more flexibility.
• Teed circuits can be tested by adjusting the location of the Transponder.

PD Pulse Propagation in HV cables - when a PD event occurs, a PD pulse is generated on both the conductor and earth screen of the cable (these being of opposing polarity). These pulses travel outwards in both directions from the originating site (as shown below).

The first pulse to arrive at the measurement end is the pulse which has travelled directly to that end (the 'Direct Pulse'). The pulse which allows the PD site to be located is the pulse which set off in the opposite direction, and has been reflected from the far end ('Reflected Pulse'). In an ideal situation the pulses measured would look like:

If both of these pulses are identifiable, the location of the site of the PD event is relatively easy to do by measuring the time difference between the direct and reflected pulses, ΔT. It can be noted that the two pulses will continue to travel up and down the cable, until they become too small to be seen above the noise level. During this time, the pulses are reflected at exactly a 'Cable Return Time = L' away from the previous pulse. It has been found in practice that it is often difficult to carry out the PD mapping locations using the above single-ended method due to the following reasons:

• Signal Attenuation is too large in long cables
• Waveforms are too tricky to interpret
• Teed or jointed cables with multi-reflections
• Cables with many Ring Main Units which cause signal attenuation and (part) reflections of pulses
• Cables with little change in impedance at far end.

It is for the above practical reasons that IPEC HV have developed the Portable Transponder (Type PTT-2000- CT). The Transponder effectively converts the single-ended location into a Double-Ended Location Method (the test set-up for this is shown below).

The Portable Transponder detects the PD Pulse arriving at the Remote End of the cable via a Detection HFCT connected to the Discharge Trigger Unit (DTU). If the signal level exceeds an adjustable 'Trigger Level', then the DTU emits a signal to the Pulse Generator Unit (PGU) which outputs a 1.0 to 2.0V pulse back onto the earth screen of the cable via a Pulse-Injection HFCT. The major advantage of this location system is that the PD waveform interpretation is automatically done by the Transponder's DTU.

Waveform Processing Pages from PDMap© (from on-line mapping test on 33kV ‘mixed’ paper/polymer feeder)	PD Map drawn from 30x Processed Waveforms: (from test on 33kV ‘mixed’ paper/polymer feeder shown above) Results showed two distinct sites of PD activity on the feeder: • Site 1, PD activity of up to 6,200pC at 58.4% cable length (1564m) • Site 2, PD activity of up to 9,100pC at 62.8% cable length (1681m)