Shielded vs Unshielded Medium Voltage Cable

[FaradayFF]

Greetings, All -
Recently, I've witnessed an installation where the existing MV(11kV) circuit consisted of unshielded cable. The cable was installed and commissioned years ago. What's more, it's been in continuous operation for 50+ years.
In my recent training, I was taught that the medium voltage cable needs to be shielded so that the electric field within the cable is confined, the cable is protected from induced voltage, and other reasons. However, here I come across the installation that seems to be operating fine without the shield. It appears the MV cable construction utilizing the shield has come on scene only recently?
Does anyone can recommend a good source of literature that talks about this topic? I'd like to learn more in depth about this.
I've also just recently seen an installation where a star point of a generator is grounded to Neutral Grounding Transformer utilizing a cable with insulation rated for 1kV. The generator is rated for 11kV. If the machine has experienced a ground fault, how can the cable sustain the voltage level that is greater than what its insulation is rated for??
Thanks,
EE

 

[electrofelon]

Greetings, All -
Recently, I've witnessed an installation where the existing MV(11kV) circuit consisted of unshielded cable. The cable was installed and commissioned years ago. What's more, it's been in continuous operation for 50+ years.
In my recent training, I was taught that the medium voltage cable needs to be shielded so that the electric field within the cable is confined, the cable is protected from induced voltage, and other reasons. However, here I come across the installation that seems to be operating fine without the shield. It appears the MV cable construction utilizing the shield has come on scene only recently?
Does anyone can recommend a good source of literature that talks about this topic? I'd like to learn more in depth about this.
I've also just recently seen an installation where a star point of a generator is grounded to Neutral Grounding Transformer utilizing a cable with insulation rated for 1kV. The generator is rated for 11kV. If the machine has experienced a ground fault, how can the cable sustain the voltage level that is greater than what its insulation is rated for??
Thanks,
EE

I would be curious too as to the history and track record of unshielded cables. About all I know is the NEC has recently become more unfriendly toward non shielded conductors/cables, IIRC recently lowering the allowed voltage for unshielded. Also 300.50 requires underground non shielded cables to be installed in metal conduit or non metallic with concrete encasement. I havent really heard much of a history or failures in non shielded cables. It is commonly stated that shielding is generally not needed at or below 2400V.

 

[synchro]

In my recent training, I was taught that the medium voltage cable needs to be shielded so that the electric field within the cable is confined, the cable is protected from induced voltage, and other reasons. However, here I come across the installation that seems to be operating fine without the shield.

As well as confining the electric field, a shield establishes and limits the voltage gradient through the insulation all along the cable. If it was unshielded and say a grounded metal object with a sharp edge or small radius was in close proximity to the cable, then that would concentrate the field (i.e., increase its gradient) in that area and cause more stress on the insulation.