shore power supply to vessel 2

[tweeky]

I work for a ferry company with 30 ships. All our ships have a three phase 440v insulated neutral system. So basically the neutral terminal at the generators are not used, with only the three phase wires going to the main breakers in the control room. The ships hull is being used as a 'floating earth'. I know this system is used as it has better reliability for essential gear on vessels. This is because if an earth fault appears on one phase it will not trip the breaker as there would not be a path back to neutral. If an earth fault appears on two of the phases then the breaker would trip.Having earth meters on board gives the engineer time to solve problems before the breaker trips. I know also some ships have a neutral earthed system as well, which would trip the breaker if a fault appeared on any one phase.
My concern is that with all the new earth friendly regulations some of our vessels are planning on using shore power to stop generators running all night. If the vessel uses shore power what would be the best way to connect the earth cable from shore to stop electrolysis happening.
The shore power box on certain vessels have the earth tag from the box connected straight onto the ships hull. I dont think this is correct! As the vessel is an insulated neutral system it probably is not as bad as it would be if it was a neutral earthed system. Fitting an isolating transformer would islolate phases but what about earthing.
As the PD of the shore power earth will be different to the ships hull this would cause electrolysis. Also, if another vessel (or even piles from ashore) have earth faults it can cause stray currents to appear and cause severe deteriation of the hull.
What i need to know is if there is anybody with any experience in this matter and suggestions regards the best way to earth the vessel safely.

 

[waross]

You want to avoid having ANY connection between your Hull and your power system. I have worked on vessels that had a knife switch to open the ground connection so as to be able to go safely to shore power.
The cause of hull corrosion when on shore power is most often neutral currents returning through the hull. The neutral currents may not originate in the vessel that has the corrosion issues.
Consider a long feeder to a shore power panel. The good ship "Floating Power" ties up and goes on shore power. "Floating Power" has a large unbalanced load and as a result a heavy neutral current. This results in a voltage drop in the feeder neutral. "Floating power" has an isolated ground, so the hull is not part of the power circuit. No problem. No hull currents. No corrosion.
Now the good ship "Grounded Power" ties up and goes on shore power. The hull gives a low impedance connection back to the ground grid of the main power system. The neutral current from "Floating Power" arrives at the shore power panel and now has a choice of two paths back to the supply transformer.
The first path is through the neutral conductor in the feeder.
The second choice is to follow the neutral conductor to "Grounded Power" and return through the ground connection and hull of "Grounded Power".
This is not good for the hull and is probably not good for the system ground grid either.
This second ground path will share the neutral current with the feeder neutral in the inverse proportion of the impedances of the two paths. It is entirely possible that the grounding path through the hull of "Grounded Power" has a lower impedance than has the neutral conductor in the feeder cable and the greater part of the neutral current may pass through the hull.
Even with no other ships alongside, shore loads fed from the feeder that services the shore power panel may cause problematic neutral currents. If there is a large amount of discharge lighting on the same feeder as the shore power panel, there will be neutral currents and a neutral voltage drop.
The first solution is to ensure that the electrical system onboard is completely isolated from ground. Ground faults on one phase "waiting for the engineer to find and clear them" may cause either hull currents or breaker tripping depending on the impedance of the fault.
The second solution may be to install an isolation transformer onboard for shore power isolation. Put the transformer onboard each vessel that may have hull current issues. That way the transformer may be matched to the expected load on the vessel and the ship's engineer will have control and responsibility for it.
I would suggest a delta/wye configuration with the wye feeding the ship and the delta connected to the shore power. Neutral currents onboard will be translated to unbalanced phase currents on the delta connected shore power connection and there should be no hull currents.
A transformer mounted ashore will only service one vessel, and there may be capacity issues if new, larger ships are added to the fleet.
If more than one vessel is connected to a shore mounted isolating transformer, one vessel may still cause problems for the other.
The shore mounted isolating transformer would have to have the secondary neutral floating to be effective and that may cause code compliance issues.
A ships power system may be grounded to the hull if there is no connection to shore power.


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

 

[tweeky]

Thanks for that Bill.
If a isolating transformer was installed on the vessel how would this be wired for three phase and earth with no neutral?
Say for example the load on the vessel was a constant 160A ( which it is) overnight.

Any suggestions.

Many thanks

Garry

 

[rovineye]

tweety,
I am a little confused by your description. You say you have a 440 volt 3 wire with an insulated neutral and there is no neutral connection at the generator. So you really don't have a neutral at all, right? Just 3 phase isolated power.

If that is the case, you don't have any neutral currents to worry about, and no ground connection needed. Typically the shore power station for you will be a transformer with 3 phase receptacles. The only issues you will have if sharing amongst hulls is voltage drop when your neighbor loads up, and possibly some ground detector issues. Some are just not compatible, including some of those built in to the pier shore power station.

You will still want some cathodic protection, either from the pier, or your system left on. If you can't leave yours on, there are temporary anodes you can just drop over the side.

 

[waross]

Don't connect a neutral from shore power. If some of the new ships require a neutral, use an onboard isolating transformer to develop the neutral.
Be wary of connecting a shore based ground to your hull. You need a system analysis of the shore system to check for and predict possible elevated ground potentials on the shore power ground terminal.
If codes require a grounded system on board the ship, use an isolating transformer to generate the on board neutral.
You may run three phases but no ground or neutral to the ship. However, the system capacity will establish a neutral point on the shipboard system. Possibly not a problem on a small vessel, possibly an issue on a larger vessel.
Be aware that a medium resistance ground fault on one phase that the engineer has not got around to fixing may eat your zincs up in a few days. Many vessels go on shore power without an isolating transformer but an isolating transformer is the safest.

I know also some ships have a neutral earthed system as well, which would trip the breaker if a fault appeared on any one phase.

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

 

[tweeky]

Just to clear things up. The vessel that will be starting this project has a three phase insulated neutral from generators to switchboard. (three cables). The ships hull is being used as a 'floating earth' with earthing cables connected to generator casings to hull. The vessel has the facility to electronically match the phases from shore power to generators to stop any blackouts happening. The shore power as far as im aware has a RCD device feeding the shore power to the vessel. ( which is another concern with the idea of phasing with generators and the RCD device tripping during phasing together). The vessel itself takes approx 160A constant power at night.
The usuall practice which other older vessels do at the moment is they plug into shorepower and use three phases and an earth cable from shore and connect the earth cable from shore direct to vessel hull (these vessels dont have the ability to phase the shore and generators together)
The vessels all have cathodic protection installed (cathelco).

 

[rovineye]

I have never seen an RCD device at a shore power station. Doesn't mean there couldn't be one however. Mine have ground detector lights.

Bill is correct; do not connect any ground or neutral from the pier station to your hull. This assumes there is no earthed neutral at the pier shorepower station. If there is a ground on your hull will come back to neutral via hull and gangway etc. In that case an isolation transformer should be used.

I never have put shore power isolation transformers into a ship design, only due to expense, weight, room, and inrush difficulties. But I usually have 16 OR MORE 400 amp 460 volt shore power cables for each ship.

Your demand is small, and unless you are certain the piers are compatible, then add the isolation transformer. If all your ships go to the same piers time after time, then make the pier compatible or make sure they already are.

 

[waross]

Good advice, Rovineye.

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

 

[Rodmcm]

From my experience in the naval and white boad industry the shore connections are all 3 phase with no neutral. The ships earth solidly to the pier earth to stop any potential difference between the ships hull and the pier earth

 

[rovineye]

Rodmcm,
It is OK or even good to bond the hull to pier, IF there is no earthed neutral on the pier transformer NOR hull. If there are there are other considerations that COULD include bonding hull to pier.
There is a mix and match of pier and ship configurations. Shore power is more and more being mandated. Not so much for economics of fuel and watch standers: the driving force is emission control. Like buses, they won't allow our ships to idle pier-side.

There are only a very few configurations possible. But the repercussions of mismatching are great; hull corrosion, paint deterioration, shock, and unreliable service. But the solutions are simple (altho for some of us that should not be confused with cheap). For tweeky, we are talking cheap, if we ignore the number of hulls and the great unknown budget.

And tweeky, I would stop saying you have an insulated neutral. You have no neutral.

 

[cer339]

rovineye,
I am a little confused. If tweeky has a three phase generator that is wound as a wye machine there is a neutral whether it is in use or not. If the neutral is not connected to the hull then it would seem appropriate to call it insulated.
Additionally, having no ground wire to the ship would only be appropriate if the isolation transformer is shore side. If a grounded power supply is supplied to the ship (e.g. the isolation transformer is on the ship) then a ground wire would be required to carry fault current and open the breaker in the event of a phase to ground fault on the incoming shore power cable. With a 480/277 volt three phase four wire system the hull could be at 277 volts above ground in this scenario.

cer339
cer339