ship to shore power system 1

[jimmy2times]

I’m hoping to get some assistance with understanding some issues we are experience with our shore to ship power system and whether they could be having impact on corrosion of a ship. Basically our wharf structure sees noise potential when our shore power system is energised and this noise disappears when ship reverts to ship on board power generators. I know there are few guys on here who seem to have good knowledge on shore power systems and keen to hear their thoughts especially.

See attached sketch for basic overview of system.

We have two independent shore transformer substations, one feeding aft end of the ship and other feeding the fwd section.

System works on floating, isolated, neutral principal.

Given a healthy system I am trying to understand if there will be current flow through the ship to shore bond cable (see green cables on sketch).

From thinking it through, for the condition of healthy balance power system with no faults, could we expect some zero sequence current through the system capacitance to the ship’s metallic hull, via bonding cables, through wharf pier structure and then back to the source via transformer capacitance to wharf? Therefore would we expect even in a healthy system (with no faults) some low level current in the ship bonding cables.

Under a fault condition (e.g. bolted phase fault) on the ship side, would one expect this current in the bond cable to reduce if not, disappear? i.e The fault current flows through the faulted phase and then returning via the healthy capacitance of the other non-faulted phases on the ship, therefore bypassing the bond cable?

How do I go about estimating the network system capacitance, it is quite a large distributed system on board. Does anyone have any similar examples or references papers where the distributed network capacitance of a large network is reduced by a few rules of thumb, or is it considered I need to model this in detail in modelling package.

 

[Compositepro]

No sketch is attached.

 

[jimmy2times]

here it is

 

[waross]

The capacitances to ground and the resistances to ground will form a wye.
With conductors in a conduit, and cables with a ground conductor one phase is often farther than the other two phases from the ground conductor.
This causes different currents to ground from the phase conductors.
This difference also causes a shift in the wye point away from the expected neutral point.
If the neutral shift in the onshore system is not the same nor in the same direction as the neutral shift in the onboard system, then there will be a difference in potential between the ship ground and the shore ground. This may be the cause of your problem.
Check for any difference in AC potential between the hull and the wharf. Even differences of less than a volt may cause corrosion.
Your zinc cathodes may cause a slight DC voltage.
The area of the ships hull will result in a low impedance to ground. Any potential difference between the hull and the wharf will cause a current that will split between the grounding cables and the hull-water path.
There nay be ore current through the hull-water path than through the grounding conductors.
We see this problem more often and more severely when a grounded neutral system has a voltage drop on a long, onshore neutral conductor.

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

 

[Power0020]

AC currents don't cause corrosion.

The corrosion may appear to rectification of very small amount of AC currents (IEEE 80,cl.14.6) while passing through concrete-rebars interface on whare but whether this will reach the ship? maybe.

 

[jimmy2times]

Thanks waross, so have you found this as a cause on insulated neutral source before, how can it be addressed if offset differs, it sounds painstaking exercise to address cabling layouts to counter the offset, or is there a more practical solution?

How have you measured the neutral displacement offset at wharf and ship, e.g with a VT. Is this just done at main switchboard level or also required at lower levels of the distribution system on the ship.

there are some measurements of wharf and hull potentials taken at the back end of last year, i will see if i can upload in next few days, i believe there is difference (less than 1V) as you say and this raised question from our corrosion guys whether power system could be at fault as it appears to be linked to shore power.

 

[waross]

Measure the voltage difference with a voltmeter between the ship and the wharf.
The solution may be an isolating transformer.
Another solution that comes to mind is active cathodic protection.
A DC voltage of the proper polarity and of a slightly higher voltage than the peak AC voltage will probably avoid corrosion.



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

 

[crshears]

A galvanic isolator may also be needed to optimize the ship-to-shore connection; hopefully it won't come to needing ICCP [impressed current cathodic protection], as this can be and very often is both difficult and expensive to install in an existing vessel, at least based on the research I once did...

A ways back controlsdude [another participant here] sent me the following link; the info there may be of use to you...

http://www.uscg.mil/sflc/mlclant/vdiv/docs/SFLCStdSpec0450.pdf

 

[jimmy2times]

Thanks for link crshears, interesting that they advocate to not use a bond cable from wharf to ship. I can see that logic with a solidly earthed neutral on the shore side but not so much with an isolated neutral. Our standing procedures dictate there must be bond cable connected.

We have an iccp system on the ship. We also have two wharves where the ship berths, one wharf is not cathodically protected, the other is. Ship switches its own iccp system off when at the iccp protected wharf, but leaves its iccp system running at the non protected wharf.

The noise we are seeing is at the non protected wharf.[pre][/pre]

 

[waross]

Nice information CR and controlsdude.

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

 

[crshears]

You liked it the first time too, Bill!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]