J.M. PANG & SEAH PTE LTD 1

On-Line Partial Discharge Measurement of a 230kV XLPE Cable Termination Using High Frequency CT Sensors

n a cable system consisting of the cables, cable terminations and cable joints, one of the common

point of premature failure will be at the cable termination. This article will show the use of a on-

line partial discharge measurement system using high frequency CT for a 230kV XLPE cable

termination. Partial discharge measurement is a useful assessment tool because it provides an

instantaneous measure of the integrity of the cable termination and also serves as a sensitive monitor of

the deterioration of insulation as a function of time.

High Frequency Current Transformer The high frequency CT or HFCT sensor has a split core construction and can be connected to the

earth connection for the cable screen at either one end or both ends of the cable termination. Figure

12.1 and Figure 12.2 illustrates the two methods of earthing for the cable screen. When a cable has

earthing at both ends of the cable screen, there will be a power frequency 50 hertz circulating current in

the earth cable connected to the cable screen. This 50 hertz current is proportional to the length of the

cable and the current in the phase conductor of the cable. However, the HFCT will not detect the 50

hertz current in the earth cable connected to the cable screen because it has soft ferrite core whose

frequency response is constant from 1MHz to 10MHz.

I

Chapter

12

FIGURE 12.1 : Earthing at Both Ends

J.M. PANG & SEAH PTE LTD 2

Figure 12.3 shows the frequency characteristic of the HFCT. This frequency range is consistent

with other capacitive and inductive sensors of about 10MHz which will achieve the best signal to noise

ratio. The HFCT sensor is easy to use because it can easily be connected to the earth cable connected to

the cable screen and does not require to de-energize the cable system for the partial discharge

measurement. In addition, the split core construction does not require the disconnection of the earth

cable in order to insert the HFCT sensor. Figure 12.4 is a picture of the HFCT used by the author.

FIGURE 12.2 : Earthing at Single End

Frequency (kHz) 1 5 10 50 100 500 1000 5000 10 000 13 000

Input (V) 5 5 5 5 5 5 5 5 5 5

Output (mV) 0 1.3 6.7 104 180 222 222 224 218 200

FIGURE 12.3 : Frequency Response of HFCT

J.M. PANG & SEAH PTE LTD 3

The Measurement System The output of the HFCT sensor is

connected to a typical digital oscilloscope with

100MHz bandwidth and a sampling rate of 1

Giga sample per sample. The oscilloscope has

4 input channels with 3 of the input channels

connected to L1, L2 and L3 phase of a 3 phase

cable system. The last input channel is

connected to an antenna to detect external

noise. The oscilloscope is interfaced to a

partial discharge software using a standard

GPIB (general purpose interface board).

The partial discharge software will display and detected partial discharge activity in the phase resolved

mode, which has the horizontal axis in 0 to 360 degree of the line voltage. This will provide the real time

information of the partial discharge across the phase of the 50 hertz AC line voltage. When the AC line

voltage is zero, there will be no partial discharge activity. Partial discharge can only happen when there is

voltage whose magnitude exceeds the partial discharge inception voltage. This basic behavior will allow

pattern recognition of real partial discharge activity. The partial discharge software will set up the

oscilloscope for the following:

• The trigger source of the oscilloscope will be on AC line voltage.

• The measurement is set to peak detection

• The input channel will detect only the AC component of the measured signal. Any DC

component will be ignored.

Figure 12.5 shows the basic block diagram of the measurement system. The measurement duration can be

as short as 5 minutes or as long as 60 minutes. The typical duration is 10 minutes.

Case History

The A short length of new 230kV XLPE

cable connect an existing 230kV GIS to a new

230/66kV, 150MVA transformer. The

transformer was successfully energized to the

utility 230kV line voltage. Partial discharge

activity was detected at the cable termination

FIGURE 12.4 : High Frequency Current Transformer (HFCT)

FIGURE 12.5 : Measurement System

J.M. PANG & SEAH PTE LTD 4

at the transformer by the utility

company and I was requested to

measure the partial discharge activity

using the HFCT sensors. Figure 12.6

shows the measurement for phase L1.

The phase L1 shows partial discharge

activity near the positive peak and

negative peak of the line voltage. The

cable termination at the transformer

was for a 230kV XLPE cable into an

oil filled box at the transformer. The

cable was unplugged from the oil filled

box, cleaned-up, plugged back to the

oil filled box, and re-energized to the

utility line voltage. Figure 12.7 shows

the re-measurement for phase L1. The

previous “double hump” shape did not

appear and the re-measurement was

relatively constant across the phase

angle of the utility line voltage. This

indicates no partial discharge activity.

Conclusion The partial discharge activity was

measured using the actual utility line voltage, and hence will represent the insulation condition at the

actual operating voltage. The split core construction of the HFCT sensor allows easy connection to the

cable termination. These two factors make the partial discharge measurement of the cable termination

useful and easy to use.

- END --

Phase (Degrees)350300250200150100500

PD M

agni

tude

(mV)

65605550454035302520151050

FIGURE 12.6 : Measurement Before Repair for Phase L1.

Phase (Degrees)350300250200150100500

PD M

agni

tude

(mV)

3

2

1

0

FIGURE 12.7 : Measurement After Repair for Phase L1.