Thermal Inspection of Medium Voltage Cables

Inspection conducted by Ermanno Peruta, a Level III thermographic operator, from the company Nuova C.M.M.E., specializing in the maintenance of electrical installations and multi-brand low-voltage (LV) and medium-voltage (MV) switches.

The inspection of medium-voltage conductors is crucial for safety and preventing plant downtimes. Having a thermal camera always in our pocket or bag is definitely a plus, providing an immediate thermal response in case of uncertainties. This is why Ermanno Peruta, with his 25 years of experience in thermography, has opted for the Hikmicro Mini3, a model he always keeps handy during site visits and client inspections. Its compact size and high performance were key factors in his choice to complement his array of classic professional thermal cameras.

During a preliminary visit to a facility, by connecting the device to his smartphone, Ermanno confirmed his thermal suspicions regarding two defect indicators that had emerged during an initial visual inspection.

Both defects involved the heads of medium-voltage conductors, one on a transformer and the other on a switch in a cell.

                                     Defect on the switch cable head in the cell                                                                                The 3D graph highlights the magnitude of the defect

                                      Medium-voltage cable head on transformer                                                                            Inspection through perimeter protection network

Both defects, identified by a hot spot, originate from the same cause and require the same intervention, which Ermanno summarizes as follows:

TECHNICAL EXPLANATION:
Improper assembly of the specified components results in the concentration of equipotential lines, thereby creating a high electric field at the point where the outer semiconductor is interrupted (tip effect), leading to a rapid deterioration of the insulation.

  Behavior of the electric field following the disruption of the external semiconductor

Conversely, adding a material with a high dielectric constant to the outer part of the cable's insulation reduces the electric field.

              Behavior of the electric field with the addition of high dielectric constant material

The phenomenon is clearly evident both thermally and visually.

As a result, it was necessary to replace the MV terminals on all cables exhibiting the detected thermal anomaly.

Note: When conducting inspections through openings or mesh, as in the case of the terminal on the transformer, it is almost essential to use a thermal camera with adjustable focus to accurately view the subject of the investigation. Both the Hikmicro Mini2Plus and Mini3 models, which connect to mobile devices, tablets, or laptops, feature a manual focus ring. This attribute provides maximum usage flexibility, and the sensors, with resolutions of 256×192 (49,152 pixels) and 384×288 (110,592 pixels) respectively, ensure good image quality.