Calculation of metal sheath circulating currents in multi-circuit power cables laid in tunnels

Abstract

Abnormal circulation currents in metal sheaths are common in multi-circuit power cables. This study proposes an improved method for calculating the induced voltage in cable sheaths to characterize these currents. The new method improves the magnetic flux cross-section for calculating induced voltage in cable sheaths from a cross-section parallel to the ground to a cross-section formed by the equivalent circuit between the sheath and the ground, thus eliminating multiple limitations of the traditional method for calculating induced voltage in cable sheaths. Using the proposed approach, the metal sheath circulating currents (MSCC) in eight circuit cables were calculated, and the effects of cable laying configuration, grounding resistance, and metal sheath segment length on MSCC were analyzed. The results indicate that the MSCC in cables laid in the equilateral triangular configuration was slightly lower than in the parallel configuration, while no clear trend was observed in the mixed configuration. Increasing the grounding resistance reduced the MSCC but increased the induced sheath voltage. Under specific conditions, an appropriate increase in grounding resistance can effectively suppress MSCC. Additionally, uneven segmentation in cable cross-interconnection units can considerably increase MSCC, affecting surrounding cables to varying degrees.