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Water tree is one of the degradation aspects of XLPE cables used for under-ground distribution or transmission lines. We have developed the loss current method using 3rd harmonic in AC loss current for cable diagnosis. Harmonic components in loss current arise as a result of the non-linear voltage-current characteristics of water trees. We confirmed that the 3rd harmonic in AC loss current had good correlation with water tree growth and break down strength. After that, we have applied this method to the actual 66 kV XLPE cable lines. Up to now, the number of the application results is more than 120 lines.In this method, it is sometimes said that the degradation signal (3rd harmonic in loss current) is affected by the 3rd harmonic in the test voltage. To indicate and solve this problem, we investigated the extent of influence by 3rd harmonic in the test voltage, and found the rule of the influence. As a result, we developed a new technique of harmonic-noise reduction in loss current method that enabled a more highly accurate diagnosis and confirmed the effectiveness of this new technique by simulations and experiments with actual cables. CV

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No abstract

Water tree is one of the degradation aspects of XLPE cables used for underground distribution or transmission lines. We have developed the loss current method using 3rd harmonic in AC loss current for cable diagnosis since 2002. Harmonic components in loss current arise as a result of the non-linear voltage-current characteristics of water trees. We confirmed that the 3rd harmonic in AC loss current had good correlation with water tree growth and break down strength. After that, we have applied this method to the actual 66 kV XLPE cable lines more than 130 lines. However, in case of cable lines terminated at gas-insulated switchgear (GIS), we have to remove the lightning arrester (LA) and the potential transformer (PT) out of the test circuit. The reason is that we are afraid that each of LA and PT disturbs the degradation signal from cable line. Figure 1 shows typical measurement circuit. In this case, it takes extra time (1 or 2 days) and costs more to remove LA and PT in GIS out of a circuit. Consequentry, in order to achieve easy and reasonable diagnosis, we have developed a new method for cable lines terminated at GIS by utilizing a technique, which enables to reduce signal of LA and PT from disturbed signal of cable lines. We proposed the measurement circuit showed in Fig.2,which reduced influence from LA and PT.