This paper describes Very Low Frequency (VLF) Tan δ experiments performed on field-aged and non-aged distributionMedium Voltage (MV) cable samples. The fieldaged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of Crosslinked Polyethylene (XLPE) and Water Tree Retardant Cross-linked Polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan δ diagnostic measurements at VLF of 0.1 Hz.
Results help in clarifying issues that arise when characterizing MV cable insulation byTan δ diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan δ diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.
Buried cables are often routed through short segments of conduit, and when this situation occurs, the ampacity must be reduced or the cable will overheat as a result of the high thermal resistance created at the location of the conduit. This problem is examined for extruded cables by using a finite element heat transfer software program to determine the derating in ampacity that cables in conduits must experience in order to remain below a maximum conductor temperature. The derating factors are provided as a function of conduit length, soil resistivity, burial depth and number of cables in the conduit. The results show that once the length of conduit exceeds about 20 times its outer diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in the conduit. Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors.
Index Terms-Ampacity, cable in conduit, thermal ratings, underground cables. NOMENCLATURE outer diameter of cable [m] nominal diameter of conduit [m] DF ampacity derating factor burial depth below the surface [m] current [A] length of conduit [m] heat generation per unit length of circuit [W/m] radial distance from center of cable [m] conduction shape factor temperature [ C] Greek Symbols thermal resistivity [cm C/W] Subscripts ambient air value that exists in the air layer inside the conduit conductor cond value that exists when a conduit is present db value that exists when the cable is direct buried equiv value for single cable that is thermally equivalent to a triplexed cable
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