On-Line Cable PD Mapping
Site Location on MV & HV Cables

On-Line Cable PD Mapping Expansion System

The HVPD On-line Cable PD Mapping Expansion System is used in conjunction with the HVPD Longshot™ PD Spot Tester to locate the site(s) of PD activity along the lengths of in-service MV and HV cables. The Cable Mapping Expansion System consists of the HVPD Portable Transponder PTT 2000-CT hardware and the PDMap© Cable Mapping Software.

The On-line Cable PD Mapping technique utilises the principle of Time-Of-Flight (TOF) measurement of the high-frequency PD pulses which travel along the cable sheath and conductor in power-line carrier mode. The system enables the user to carry out 'double-ended' cable PD mapping on MV and HV cables of lengths of up to 5km through the placement of the Portable Transponder unit at the remote end of cable. The battery-powered portable transponder unit detects the PD event and then forces a reflection of the end of the cable. By measuring the time difference between the detected PD pulse and injected pulse (which has been reflected from the far end of the cable) it is possible to locate the position of the PD site on the cable to less than 1% of the cable length.

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 below.

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:

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 HVPD 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)

Requirements for Portable Transponder (Trigger and Pulse Booster)

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 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.

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 HVPD 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:-

PTT 2000-CT Transponder - 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 above in Figure 5.

PTT 2000-CT Transponder - Portable Pulse Generator Unit

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.

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 above in figure 6.

Transponder & HFCT Connections for On-Line Cable Mapping

The standard test set-up for locating the site(s) of PD activity in an HV cable feeder using the HVPD Longshot PD tester with Portable Transponder is shown below. In this example measurements are made at Substation A with the HVPD 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.

Hardware Specification Portable Transponder (PD Trigger Unit and Pulse Generator) Specification

Output pulse magnitude	100V into 50Ohms
Output pulse width	1 m Sec
Pulse delay from trigger	200nSec
Settable output pulse delays	20 m Sec, 50 m Sec, 100 m Sec
Trigger level range (high gain)	5mV -> 150mV
Trigger level range (low gain)	100mV -> 1000mV
Trigger polarity	Either +ve or -ve pulses will trigger device
Input bandwidth	10MHz
Input inhibit	No inputs will give triggers for 100 m Sec following a trigger. This eliminates reflected pulses re-triggering the device
Free run mode	Switched mode with device generating pulses continuously at fixed interval
Free run mode intervals	1mSec, 1 Sec, 1 Min
Input/output protection	All protected by gas discharge tubes
Input/output connections	BNC type
Power	Battery Powered - 6-8 hours continual use
Weight	4 Kg

PDMap© Software for PD Mapping

The PDMap© Software is used with the HVPD-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

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 reflection of the first pulse). After processing of 30 of the above waveforms the following PD Location Map was produced by the PD Map© Software.

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

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

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.