James Lyall

Probabilistic cable rating based on cable thermal environment studying

This paper describes a new approach to establish the probabilistic cable rating based on cable thermal environment studies. Knowledge of cable parameters has been well established. However the environment in which the cables are buried is not so well understood. Research in Queensland University of Technology has been aimed at obtaining and analysing actual daily field values of thermal resistivity and diffusivity of the soil around power cables. On-line monitoring systems have been developed and installed with a data logger system and buried spheres that use an improved technique to measure thermal resistivity and diffusivity over a short period. Based on the long-term continuous field data for more than 4 years, a probabilistic approach is developed to establish the correlation between the measured field thermal resistivity values and rainfall data from weather bureau records. Hence, a probabilistic cable rating can be established based on monthly probabilistic distribution of thermal resistivity.

New Techniques for Determining Condition of XLPE Cable Insulation from Polarization and Depolarization Current Measurements

This paper describes how a commercially available test set has been used to make preliminary determinations of condition of XLPE cable insulation at remote sites in NE Queensland. Polarization and depolarization currents are measured with an injection voltage of 5 kV with observed depolarisation period of 60 seconds. It is shown that misleading results may be obtained by this technique due to rapid drying out of cable insulation after removal of the cable 50 Hz normal supply and laboratory experiments to systematically examine this effect are reported. Procedures needed to obtain consistent field test results are recommended as are diagnostic criteria to provided an indication of cable insulation condition.. It is concluded that whilst the technique can provide evidence of poor insulation quality, it is not possible to distinguish between water trees in insulation and poor joint insulation; additional research work to solve this problem is suggested.

Improved GPR image quality by truncating the bandwidth of the Fourier transform in 2D diffraction tomographic inversion

Diffraction tomography (DT) has been applied in ground penetrating radar (GPR) to obtain images of buried objects. In order to obtain a unique solution for the object function, a maximum frequency range should be used in a DT inversion routine. In this paper, a method is proposed to appropriately truncate the frequency range of the Fourier-transformed (FT) scattered field for a 2D DT inversion routine. This is achieved by determining the variation of the phase in the FT scattered field. The number of zero-crossings of the FT phase along the spatial axis is counted and a moving-average filter is applied to enable proper bandwidth selection. Numerical simulations are demonstrated using synthetic data with additive white Gaussian noise.

Water migration in degraded XLPE cables

During dielectric response measurements on samples of XLPE 22 kV cable that were known to have been severely-degraded in service by water trees, it was observed that the insulation of the cable appeared to degrade considerably after water-immersed cable sample were connected to the rated voltage AC supply. This paper describes results of systematic experiments made to investigate this factor further using polarisation and depolarisation current (PDC) equipment to measure the condition of cable samples. It is found that the cable condition appears to improve from poor to good condition within a few days after short circuiting of the previously-energised cable commenced. Conversely it is found that the condition of the cable appears to degenerate over time following re-application of the rated voltage to the cable samples. It is hypothesised that electrostatic forces maintain moisture in the trees during energisation and that the moisture retreats from the trees when the supply voltage is removed. The implication of this phenomenon on dielectric response measurements is discussed in the paper.

Asset Management of Medium Voltage Cable Networks

This paper describes the background to research currently underway associated with asset management (AM) of degraded cable networks in North Queensland. In the paper the main failure mode of cables installed twenty years ago is identified as the growth of water trees through the cable insulation. The costs of managing water tree degradation in a large distribution network are considered and the economics of cable replacement in a regulated monopoly are examined. Condition monitoring techniques for assessing the condition of cables are reviewed and the possibility of silicone liquid injection as an alternative to cable replacement is discussed: a research methodology to support improved cable AM is also presented.