M.V. cable insulation test
Before the commissioning of an MV system the regulations recommend carrying out the check to verify that the assembly is compliant and that the cables have not deteriorated during the laying operations.
Regulatory ref.
All M.V. cables in the system must be subjected to the acceptance tests following installation in the manner indicated in paragraph 8.3 in the CEI 11-17 Standards
The DF PROJECT Studio is able to carry out diagnostics of the M.V. cables and the relative sheath with test voltages up to 62 kV in alternating current (VLF).
Interventions performed
Cable insulation test
with frequency f = 0.1 Hz (refer to Guide for connections to the Enel distribuzioni electricity network)Testing of equipment and of M.V. cables
of new and old installation through rigidity tests according to CEI 11-17 standardsDiagnosis of the dissipation factor or delta tangent
at 0.1 Hz to obtain a differentiated evaluation of the state of ageing of the cables insulated in impregnated paper and of the PE/XLPE cables.Cable testing up to 62 kV
The measurement of the dissipation factor (tan delta)
The measurement of the dissipation factor (tan delta measurement) is non-destructive and provides a global parameter, which is used to assess the state of ageing of the entire section. The ratio between the effective power and the reactive power of the cable is measured with the dielectric dissipation factor tan delta. The measurement provides clear information on the state of insulation of the cable and the respective state of ageing.
With the measurement of the dissipation factor we identify:
- points damaged by water (water trees) in the insulation of cross-linked polyethylene (XLPE) cables, which can subsequently cause electrical trees and that represent the natural cause of a failure in a cable,
- damaged points of cables with paper insulation caused by drying,
- insufficient insulation of paper-insulated cables due to moisture,
- humidity in the accessories (joints/terminals) and
- possible partial discharges.
Tan delta diagnostic procedure
The tan delta measurement is performed on various voltage steps preset in our devices. In aged cables there is a characteristic rise in the dissipation factor as the measurement voltage increases. Thus, it is possible to carry out a classification of the cables, which contributes to the planning of the repair measures.
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