On-Line Partial Discharge Mapping (PD Location) on HV Cables

The OSM-Longshot PD Spot Tester, when combined with the PD Map© Software, PTT 2000-CT Portable Transponder and HFCT Sensors from IPEC Engineering and HVSL can be successfully applied to the location (mapping) of PD sites along the length of high voltage cables. The technique utilises the principle of Time-Of-Flight (TOF) measurement of the PD pulses, which travel along the cable sheath and conductor in power-line carrier mode.

Figure 1: Single-Ended PD Site Location Method

When a PD event occurs, the PD pulses travel outwards in both directions along the cable earth sheath (and conductor) from the originating PD site as illustrated in Figure 1 above. The first pulse ('Direct Pulse') to arrive at the measurement end of the cable is the pulse which has travelled directly to that end. The pulse which then allows the PD site to be located is the 'Reflected Pulse' which originally sets off in the opposite direction, and is then reflected back from the remote end back to the measurement end. This technique is called 'Single-Ended PD Location' and is, when possible in ideal conditions, the simplest and quickest way to provide PD mapping of cables.

If both the 'Direct Pulse' and the 'Reflected Pulse' are identifiable (as in the ideal case) then location of the site of the PD event is relatively easy with the Single-Ended Location Method. Results would look like:

Figure 2: PD Pulse Trains as seen from the Measurement End

With reference to Figure 2, the time difference between the first two pulses (the direct pulse and the reflected pulse) ΔT, locates the site of the PD event. It can be noted from Figure 2 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 arrival at the measurement end. This gives rise to sets of pulses of diminishing size, each spaced at the cable return time, L. If L is the return time of the cable length (this can be easily measured with the OSM-Longshot with Cable Map Software) then the location of the PD event is:

Location from Measurement End (in % Cable Length) = 100*(1- D T/L)

Whilst the single-ended location method illustrated above is possible in an ideal environment it has been shown through testing in the field (both on-line and off-line) that the Single-Ended mapping methods can be made more difficult if the cables are long or other circuit constraints apply. Difficulties can be encountered in the following cases:

• PD pulse signal attenuation is large - long cables with high attenuation can reduce the magnitude of the reflected pulse so that it is lost in the 'background noise'.
• PD Waveforms are difficult to interpret due to interference such as switching noise from motors attached to the feeder.
• Teed or jointed cables producing attenuation and reflections.
• Cables with many Ring Main Units (RMU's) producing attenuation and (part) reflections of pulses.
• Cables with no change in impedance at the far end.

Transponder Unit Type PTT 2000-CT
Above: Pulse Generator Unit
Below: Discharge Trigger Unit

The solution to these practical problems which are encountered in the field is to apply the Portable Transponder Type: PTT 2000-CT shown right which has been developed by IPEC and HVSL for use in PD location in the above cases.

The concept of operation is that if a PD pulse is received by the Signal HFCT connected to the Transponder Trigger Unit which exceeds the Transponder's adjustable 'trigger level', then the Trigger Unit will emit a signal to the Pulse Generator Unit which outputs a large, 100V pulse (into 50 Ohms) to a Pulse-Injection HFCT which sends a large pulse back down the cable.

This process essentially converts the Single-Ended location system into a Double-Ended location system.

The major advantage which the Double-Ended location system has over the Single-Ended system, is that no waveform interpretation is required by the user for the Double-Ended case as this is done by the Transponder's Discharge Trigger Unit.

A typical waveform, as seen at the measurement end of the cable, would then look like:-

Example of PD Waveform with Transponded Pulse

PTT 2000-CT Transponder
- PD Discharge Trigger Unit

This instrument is designed to act as a Trigger unit for partial discharge pulses passing along the screen/core of HV cables. The unit can also be used as a rough level guide for PD activity, and is in the Transponder System to trigger the Pulse Generator unit which can be used to locate partial discharges on cable networks.

The Trigger unit is powered from a rechargeable battery, which has the capacity to last 6-8 hours of continual usage between charges. The Trigger Unit is shown right.

The Portable Pulse Generator is used in conjunction with the PD Trigger Unit as a source of fixed amplitude, variable-width pulses for inductive coupling onto a power cable via a High Frequency Current Transformer. The Pulse Generator has a variable pulse width output which can be varied by the user between 100ns and 10us.

PTT 2000-CT Transponder
- Portable Pulse Generator

The output pulse height is 100V nominal peak when matched into 50 Ohms which produces a pulse of around 2V on the cable sheath when injected onto it using an HFCT. A low voltage pulse is also provided for scope triggering purposes with an amplitude of 2V nominal. As per the Trigger Unit the Pulse Generator unit is powered from a rechargeable battery, which has the capacity to last 6-8 hours of continual usage between charges.

The battery-powered Transponder System has significant advantages for PD Mapping at the the remote ends of cables, secondary substations or at RMU's where it is unusual to have an AC 'mains' power supply. The Portable Pulse Generator Unit is shown right.

The standard test set-up for locating the site(s) of PD activity in an HV cable feeder using the OSM Longshot PD tester with Portable Transponder is shown below. In this example measurements are made at Substation A with the OSM Longshot Unit with the Trigger Unit and Pulse Generator positioned at Substation B to generate the amplified, transponded reflected pulses which are passed back along the cable for detection at the measurement end.

Standard Test Set-Up for Double-Ended On-Line PD Cable Mapping

PDMap© Software - Data Input Window

The PDMap© Software is used with the OSM-Longshot PD Spot Tester and Portable Transponder to provide the user with the ability to record, process and display cable PD data maps and waveforms to provide for the on-line location of Partial Discharge activity in HV cable feeders. The PDMap© Software is used to draw PD maps from on-line testing, as well as with the traditional off-line, directly-coupled methods (VLF testing etc).

The Software is easy-to-use and allows the user to prepare PD Maps quickly and easily with the equipment 'live'. PDMap© consists of three main User pages: The Data Input Page, The Data Processing Page and the PD Mapping Page as shown.

PDMap© Software - Data Processing Window	PDMap© Software - PD Mapping Window

Example 1: On-Line PD Mapping of a 33kV XLPE Cable Feeder

PD Data Processing Window for 33kV XLPE Cable measured using an HFCT

The first two cursors (green and yellow) have been placed at the start (green cursor line) and end (yellow cursor line) of the initial pulse to calculate pulse magnitude (in pC). The third (blue) cursor has been placed at the start of the reflected pulse, to enable the PD location to be calculated. The fourth (white) cursor has been placed at exactly Tl following the first (green) cursor to show the 'cable return time' (this is the relection of the first pulse). After processing of 30 of the above waveforms the following PD Location Map was produced by the PD Map© Software.

PD Map for 33kV XLPE Cable (drawn from 30x processed waveforms)

The above PD Map shows there is a PD site with PD activity of up to 3,000pC at 45% along the cable length from the measurement end. The Map also shows some smaller level switchgear/termination PD activity at the measurement end (0% along cable).

Example 2: On-Line PD Mapping of a 33kV XLPE/PILC 'Mixed' Cable Feeder

Examples of Data Processing Waveforms for a 33kV XLPE/PILC 'Mixed' Cable

In this example two PD sites were detected on the cable using the PDMap© Software which produced the following PD Map.

PD Map for 33kV XLPE Cable (drawn from 30x processed waveforms)

The PD mapping results showed 2 distinct sites of PD activity, the first had a magnitude of 6,200pC at a distance of 58.4% (1564m) from the measurement end and the second with a magnitude of 9,100pC at 62.8% (1681m) from the measurement end.