Electrical cables are the major arteries for electrical power. The power utilities are facing new challenges due to liberalization of electricity. It means reliability demand on distribution network is increasing day by day. The distribution networks are mainly cable networks of different voltage grade and dielectrics. Therefore, the proof design of cables is required after installations. During installation the wear and tear of the cables are must due to mechanical and environmental stresses. Cables are buried in the ground, laid in conduits, hung down on the mines haft/trays/ducts them to survive the elements and man .Today ,it is possible to, on site, do cable diagnostics of all types of high voltage cable. This paper will mainly concentrate on PILC & XLPE cable of different voltage grade. There are three major categories of cable tests as noted below: (a) High Potential Withstand Test (HI-POT)
(b) General Condition Assessment(GCA]
(c) Partial discharge (PD)
In this paper, the emphasis has been given to the diagnostic method for VLF & TD, PD for PILC & XLPE cables.
Key words-Water tree, Damped Alternating Current (DAC), Very Low Frequency (VLF) & Tan Delta (TD), ) Partial discharge (PD), Oscillating Wave Test Method (OWTS).
Liberalization of electricity markets brings about important changes in the working environment of power utilities. The power utilities are facing new challenges everyday pertaining to the reliability of power system. It means reliability demands on distribution networks are increasing. To meet this demand in proper way, it is necessary to avoid shutdowns/breakdowns of the networks. To maintain the reliability demand of utilities, it is important to do right things at .right place at right time. As a result the network operators have to concentrate more on asset management to reduce the costs, to postpone the investment, to optimize technical management keeping at the same time the reliability and power quality at high level. And to achieve the above cited goal i.e. avoid outage of supply, it is important to diagnose the network system regularly i.e. the diagnostic test to asses the condition of dielectrics of cables which are the major vital components of power system. Before switching over to field diagnostics, it is important to be acquainted with insulation condition assessment rules .
2. General Aspect
The actual ageing processes generated by the interaction of partial discharges are still not known well and understood at present time. In particular, taking into account in distribution power cables important aspect etc. the detecting the defects can be used in combination with PD knowledge rules to support the condition best management. The important symptom of degradation process can be related to discharging local insulation imperfection or defects, which may occur in the particular cable part or cable accessories. Before performing diagnostic on the cable, it is important to know insulation defect for different type of power cable, shown in table -1
Power Cable Insulation Defects
|Cable PILC |Accessories |Insulation | | |Low oil level |Damage outer sheath | | |Sharp edge on conductors |Tracking internal | | |Moisture penetration |damages | | |Air/ gas bubble |(as a result of | | | |bending) | |XLPE |Sharp edge on conductors |Damage outer sheath | | |Moisture penetration | | | |Air gas bubble | | | |Field grading movement | | |...