Case Study
On-line PDM of 161kV Cable Termination
2013-12-09 16:20:58
Measurement Period :2007/09/19~2008/02/12
Customer:Picture tube manufactory
Measurement Item:On-Line PDM of 161kV cable termination
Operator:Boden Huang, Hung-Lin Chang, Chang-Hsing Lee, Javis Chiu

  Test Methodology  

The on-line PDM (Partial Discharge Measurement) of 161 kV cable terminations is done by TM (Transient Magnetic field) sensor attaching to grounding wire. Because of the high background noise from ground wire, the measurement frequency is raised to reduce the influence of background noise. The waveforms, phase-resolved-patterns, and spectrums of partial discharges are all monitored and compared among three phases.
 
(a) Sensor  (b) Instrument
Fig. 1 Test arrangement

  Measurement Result  

On Sep. 19th 2007, there are strong partial discharge activities measured in the S-phase cable termination, and the phase-resolved-pattern is shown in fig. 2. The measured partial discharge areas is not similar to the typical areas, 0~90 and 180~270, due to take the outlet voltage as reference voltage. And the partial discharge source is located by comparing the apparent partial discharge magnitudes of all tested cable terminations. As shown in fig. 2, the max. apparent partial discharge is about 200 pC, and the phase-resolved-pattern shows the inside partial discharge instead of noise. Moreover, oscilloscope and frequency spectrum are used to figure out the partial discharge source, as shown in fig. 3. Measurement results show that the partial discharge signal is inside the S-phase cable termination rather than from outside. Therefore, the S-phase cable termination is considered that there is a defect inside it, and a suggestion of replacement is proposed...

 
Fig. 2 Phase-resolved-pattern of S-phase 
cable termination
(a) waveform of partial discharge (b) frequency spectrum of partial discharge
Fig. 3 Additional diagnosis

  Review Results  

The first review of on-line PDM is done on Nov. 12th , 2007. The measured apparent partial discharge is about 380 pC as indicated in fig. 4. Comparing with first measurement, apparent partial discharge is slightly increasing, and the repetitive rate are almost the same. Fig. 4 shows the partial discharge area is almost shifted by 180 degree, and this is resulting from taking different outlet as reference voltage. At this time, acoustic emission sensor is additionally applied to do on-line PDM, and no partial discharge activity is measured by AE sensor as shown in fig. 5. It implies that the partial discharge source is inside the cable termination instead of between cable termination and oil-to-gas bushing.

 
Fig. 4 Phase-resolved-pattern of 1st review Fig. 5 Phase-resolved-pattern measured by AE sensor (non PD signal) Fig. 6 Phase-resolved-pattern of 2nd review

The second review of on-line PDM is done on Dec. 12th 2007. The measured apparent partial discharge is slightly decreasing to 250 pC (shown in fig. 6), and the partial discharge still remains. This implies that abnormal signals could not be random noise or resulting from unexpected switching transients. Therefore, the internal partial discharge of cable termination is validated.

  Replacement of Cable Termination  

Cable termination is replaced on Feb. 26th 2008, and the details are as following:

 
(a) Inside GIS  (b) Surface of cable termination
No abnormal condition observed
Blacked insulated oil
Serious trace of partial charge activities

  Discussion  

The structure of 161 kV cable termination is shown in fig. 7. Because of improper workmanship, the stress cone is the common place causing partial discharge. The PD source of the mentioned cable termination locates at the front part of the termination, as indicated in fig. 7. Comparing with the standard procedure illustrated in fig. 8, there is no tape wrapped in the junction of terminal and XLPE. Therefore, the insulated oil inside termination infiltrated into the interspace between conductor and inner-semiconductor. The partial discharge activities finally happen.


  Fig. 8 Standard procedure of installation of cable termination
Fig. 7 Structure of cable termination


  Conclusion  

1. Because of the connection to GIS, on-line PDM is the only way to assess the insulation condition of 161 kV cable terminations.
2. On-line PDM can assess the insulation condition of 161 kV cable terminations, but there is technique issues remaining about the noise suppression and partial discharge source location. Therefore, an expert is necessary.
3. The root cause of partial discharge inside the mentioned cable termination is improper workmanship. For a new installation cable termination, the workmanship should be demanded. For an in-serviced cable termination, periodical on-line PDM should be used to assess the insulation condition.
4. On-line PDM can detect the defect inside cable termination, but the remaining life is still unavailable.  Therefore, the partial discharge trend can be an effective way to estimate the aging of cable termination.