Propagation of intersheath modes on underground cables

• Analysis of circuits proposed by CIGRE for excitation of intersheath modes in field testing of normal-bonded cables shows that if the source is grounded, which is usually the case in field tests, different propagation modes arise and it becomes difficult to observe intersheath modes.
• The homogeneous model is used to calculate the propagation characteristics of a cross-bonded cable system and to show that inter-core modes change substantially when the cable sheaths are cross-bonded.
• Comparison through transient simulation of a discrete and a homogeneous model of a cross-bonded cable makes clear that even though intersheath modes cannot be excited in a cross-bonded cable, they can still be induced by the core conductors and impact the transient response of the cable.
• It is shown that the accuracy of a homogeneous model is improved if the number of sheath grounding points is high.

Wave propagation and transients associated to intersheath modes on normal-bonded and cross-bonded cables are the main topic of this paper. Intersheath mode characteristics are not easily identified in practice using the standard circuits for field testing of normal-bonded cables because the source grounding introduces several overlapping modal components. The differences between cross-bonded and normal-bonded cables are highlighted based on the propagation characteristics of the two systems. In particular, inter-core mode propagation is substantially affected by the cross-bonding of sheaths. Discrete and homogeneous models of a cross-bonded cable are compared in a time-domain simulation. The accuracy of the homogeneous model improves when the number of sheath grounding points is high. It is clearly shown that even though intersheath modes cannot be excited in a cross-bonded cable they can still be induced by the currents in the core conductors and have an impact on the cable's transient response.