EuroWire September 2023
A technical view of the importance of medium voltage (MV) cable testing By BASEC (British Approvals Service for Cables) A technical insight on critical testing and the difference between MV and LV cables, and additional testing needed for MV and the role of an independent third party and accredited body
For medium voltage cables, testing is even more critical than with their low voltage counterparts due to the designs’ complexity, and the need to ensure the highest level of safety and proper operations with the higher voltages. Testing is always a necessary action to verify the quality.
transition from the conducting medium to the insulation. For example, a conductor screen is used to guarantee a uniform electric field in the interface between the insulation and the conductor, and to prevent any electric field concentration, as this concentration will create high-stress points at the conductor/ insulation interface. Conductor and insulation screens are semiconductive, so they are neither a conductor nor an insulator. Furthermore, the processing of the three layers for MV insulation should be done simultaneously by a unique head/machine, and the extruder itself is entirely different from the regular horizontal extrusion lines used for low voltage cables. The medium voltage machine is called a CCV line (catenary continuous vulcanising), and includes triple heads to simultaneously extrude the three layers (conductor screen, insulation and insulation screen).
The insulation layer withstands higher voltage levels experienced by medium voltage cables – the insulation layer’s purpose is to provide sufficient resistance to ensure that the current maintains its path alongside the conductor at the higher system voltage ratings. Therefore, an appropriate insulation layer should be applied. The insulation layer in the cable differs in the material used, the processing and even the testing done. The insulation layer for medium voltage cables consists of three layers: conductor screen, insulation and insulation screen. The higher voltage levels applied to these cables need a progressive
This complexity in the construction and process of medium voltage cable is reflected in the testing, making testing of medium voltage cables more complex than regular low voltage cables. Not only are higher voltages applied, but additional tests should be done to medium voltage cables such as a partial discharge (PD) test, tan delta measurements, heating cycle test, impulse voltage test, and adherence of screens at short circuit temperature test; these tests are not required to be performed on LV cable. Partial discharge is a measurable indicator of developing insulation defects in power cables due to internal weak points in the insulation of medium voltage cables, such as voids, cracks or contaminant particles. In medium voltage cable standards, the maximum detected level of partial discharge within the insulation of MV cable during the type testing is 5 pC (picocoulombs) or greater, to be considered successful and pass the test.
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