Criteria and Methodologies for Selecting Sheath Voltage Limiters (SVLs)

To mitigate excessive dielectric stress and prevent insulation faults resulting from slow- and fast-front transient over voltages, sheath voltage limiters (SVLs) should be installed at sheath terminations and cross-bonding positions. These SVLs are housed within link boxes at joint bays and cable terminations; the location of cable joints and termination structures determines whether their installation is done underground or above ground.

The maximum power-frequency voltage difference between the metal sheath and the ground is one of the main factors in the selection of SVLs, requiring analysis of the induced power frequency sheath voltages during normal, emergency, and through-fault conditions. However, when sizing SVLs it is also important to ensure that the peak residual voltage Ures of the selected SVL does not exceed the impulse withstand level of the cable and accessories. Various publications - including standards, guidelines, and technical brochures provide similar recommendations for the selection of SVLs for cable circuits. However, discrepancies exist: while some references incorporate explicit safety margins between the maximum calculated induced metal sheath voltage and the rated withstand capability of the SVL

(typically expressed as continuous operating voltage (Uc) and rated voltage (Ur)), others don’t.

This paper presents a comparison of criteria for selecting SVL in accordance with the following documents:

• IEC 60099-5: Surge arresters - Part 5: Selection and application recommendations [1]

• Electra 128-2: Guide to the protection of specially bonded cable systems against sheath overvoltages [2]

• CIGRE TB 283: Special bonding of high voltage power cables [3]

• CIGRE TB 797: Sheath bonding systems of AC transmission cables - Design, testing, and maintenance [4]

• ENA Engineering Recommendation C55 Insulated sheath power cable systems [5]

The above documents provide easy to handle formulas for calculating induced sheath voltage at power frequency. Estimating sheath overvoltages during system faults can be challenging and often requires specialized software. To address this, simplified formulas are used at the design stage of projects for calculating the maximum expected overvoltage. When greater precision and modelling of switching and lightning impulses is required, the calculation can be done with specialized software.

The paper presents an illustrative case study highlighting variations in the continuous operating voltage (Uc) and rated voltage (Ur) of sheath voltage limiters (SVLs), based on differing criteria outlined in relevant technical documents. For the selected example cable circuit, a comparative analysis of various SVL specifications is conducted. The aim is to compare standardized selection criteria with simulated induced voltages under various fault scenarios, including single-phase-to-ground fault, three phase fault and switching.