Underwater LV Connection - Possibility of neutral corrosion/open neutral 1

[Oblsss]

Dear sirs,

I am currently working on a case of connecting an island (300 m from the coast) with the LV distribution system.
Two (for redudancy reasons) 3-core PILC/SWA lead sheathed cables will depart from the secondary of a distribution transformer (wye connection with the common point grounded)to travel the underwater section of the connection before transitioning to a three-core waveform (copper concentric conductor) cable in the island side.
The loading of the circuit is expected to be assymetrical, therefore the neutral of the system will be lightly loaded.

11kV-PILC-Cable
Waveform-Cables-3-Core

My main concern is the following:

1. At the transition joint, corrosion is expected due to the dissimilar metals of the metal shields, ie copper on one side with lead + steel on the other. The presence of moisture, or in the worst case contact with sea water in combination with the continuous loading of the shield due to the asymmetry of the phases can only speed up the corrosion process.
In case of heavy corrosion there is a risk of an open neutral condition on the island side.

My thought is to lay an extra 1-core cable to act as a neutral, ie connected from the neutral point of the distribution transformer's secondary to the copper concentric conductor of the three-core waveform cable in the island side.
The lead sheath + steel armour of the three-core PILC/SWA lead sheathed cables will be grounded on one point only with the other end insulated from ground and will only serve as a fault current path in case of single phase fault to ground.

Am i missing something here?

Thank you in advance,

George

 

[waross]

In a similar situation we lost one phase.
A single conductor was laid about one meter away from the existing 3 core cable.
The reactance of the asymmetrical phase was excessive.
As a result we had chronic low voltage issues on one phase.
Expect to find a higher than expected voltage drop on the neutral conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Oblsss]

Waross,

[highlight #FCE94F]A single conductor was laid about one meter away from the existing 3 core cable.[/highlight]--> if I lower the distance, the reactance will get smaller.
[highlight #FCE94F]Expect to find a higher than expected voltage drop on the neutral conductor.[/highlight]--> this can create issues with the utility operation principles ...

Any comments on the corrosion of the neutral? Have you had any experience with this or with jointing a PILC/SWA cable to a copper waveform?

Thanks again!

 

[waross]

As I remember, (it was almost 15 years ago) we had a single conductor copper neutral cable running close to the three core cable.
The neutral unbalance current is typically much less than the phase currents.
Our neutral had been laid together with the three core cable and the reactance was less.
I don't remember any issues with the neutral voltage drop.
We did have issues with the greater voltage drop on one phase after we lost one core.
We had been metering four wire three phase with delta connected CTs and two element KWHr meters.
We had to change our revenue meters to three element meters.
("We had been metering four wire three phase with delta connected CTs and two element KWHr meters" Not a Blondel solution. The meter will register the phase currents accurately but makes the assumption that the phase voltages are equal. With the voltage on one phase chronically low, the metering was no longer accurate. Hence the change to three element meters and a full Blondel solution.)
As for connecting the waveform conductors:
I would try to contact the cable manufacturer for their suggestions.
This link may be of use to you. Link

Bill
--------------------
"Why not the best?"
Jimmy Carter