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Dielectric spectroscopy
Water tree diagnostics on polymeric cables.
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Severe water treeing will significantly reduce the remaining life of polymeric cables. Water tree diagnostics based on the dielectric spectroscopy (DS) method assists asset managers in identifying polymeric cable circuits having water tree deterioration. A maintenance program can then be developed placing a high priority on those cables shown to have the severest deterioration, and applying a “wait and see” approach for the cables having less deterioration. The DS method consists of energizing your cable with a variable voltage Very Low Frequency (VLF) power source, then measuring the electric losses and capacitance at different voltage/frequency levels. By analyzing the change in electric losses and in capacitance at different voltages and frequencies, the water tree deterioration level of the cable can then be categorized as: very severe, severe, moderate, or no deterioration. An additional benefit of the DS method is that the influence of other losses, such as discharges or contaminated terminations, can also be identified and excluded from the water tree diagnosis.
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Water tree transformed into an electrical tree
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[download] Watertree Diagnostics - Dielectric Spectroscopy (DS) method (.pdf 481 kb)
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Dielectric Spectroscopy (DS) method
The DS method is based on measuring the losses (tan d) and the capacitance values of the cable at different voltage levels (0.25Uo, 0.5Uo, 0.75Uo, Uo, and 0.25Uo) as a function of the frequency of the ac voltage applied. The frequency is varied from 1 Hz to 0.01 Hz.
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Loss factor and change in capacitance as indicators
Water trees are indicated by a non-linear change of the losses
and capacitance values with respect to voltage and frequency. Very
large water trees will initiate leakage currents, which contribute
to the losses, and the measured values change significantly at
higher voltages. Using the measured values of the losses and
capacitance, a very good physical interpretation of the insulation
condition can be made. It is also possible to identify the
influences of deterioration in cable accessories and contamination
on the terminations.
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Change in capacitance as indicator
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Loss factor as indicator
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Examples
Figures 1a and 1b give examples of the behavior of a new and an aged 24 kV XLPE cable. The values of the losses of the new cable are small and linear with voltage. The loss factor is the same at 6 kV as it is at 12 kV. The same is true for the capacitance value. The aged cable, however, shows increased losses at the higher voltage level, with strong non-linearity of the loss factor and capacitance. This is a typical response from a water tree deteriorated cable.
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Results of a new and of a water tree deteriorated cable
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Influence of accessories
The influence of dirt on terminations or of bad insulation
inside accessories can be readily identified by increased losses
and no corresponding change in the capacitance value at lower
frequencies.
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Experiences and benefits
The University of Stockholm in Sweden developed the DS method, in close cooperation with some electric utilities. Extensive laboratory and field investigations have proven the effectiveness of the method compared with other methods. KEMA has applied this method in the USA and in the Netherlands.
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KEMA services
KEMA offers cable-consulting services for CBM on power cables including the following:- Diagnostic measurements such as partial discharge diagnostics on all types of MV cable circuits
- Water tree diagnostics on MV polymeric circuits
- Partial discharge diagnostics on MV and HV cable a ccessories
- Outer sheath testing and voltage withstand testing of the cable insulation
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