What? Legacy Ship DOESN'T Need an Isolating Transformer After All ?!? 3

[crshears]

Hello again all,

See

http://www.eng-tips.com/viewthread.cfm?qid=356842

for the background to this post.

Some of you will recall my gratitude at having a retired electrical engineer with relevant marine experience join the electrical team on the ship where I volunteer.

Based on take-away from the above thread, I've been militating to have an isolating transformer installed in the shore power supply. But to my consternation, my engineer colleague sent me the following [slightly redacted to protect identities]:

<begins>

On another matter...

When we met last week there was a brief discussion about re-installation of an isolation transformer in the hydro power service supply to the boat, similar to that removed when the vessel returned from the US. This is to reduce or eliminate galvanic / electrolysis of the hull due to ground currents into the water...

I feel this is entirely unnecessary, for the following reasons:

First, the vessel is no longer a working ship. She will never be travelling to other ports where there could be a risk of connecting to "shore power" that is mis-wired with the opposite (wrong) polarity.

Second, she is hard wired to hydro power, not connected through a shore power cord as is commonly used for pleasure craft and active ships. Therefore there is no risk of faulty connections or mis-wiring.

Third, she is permanently moored; the hull should be solidly connected with several heavy (welding) cables to a substantial grounding system on-shore along side the slip. This would prevent any currents flowing to ground from the hull
through the water and provide a good path for lightning hits should one occur.

Just some ideas...maybe someone could enquire of some of the other "museum ships" around the world (Haida, Norisle, Capt' John's, etc.; there are many) to ask what they have done...

<ends>

I plan on referring him to the original thread noted above, but are there any points I'm missing or that my colleague has mis-apprehended? Part of my thinking is that I'd still rather err on the side of caution and have an isolating transformer installed anyway...after all, even though we're a shoestring volunteer organization, it's only money; and if the ship sinks to the bottom, or needs to be scrapped because we can't afford to repair a corroded hull, the organization won't have any remaining reason to exist...

I value your thoughts.

Carl

p.s.: I got my fuse question sorted out.

CR

 

[waross]

Currents may flow into the hull as well as out of the hull. The issue is not so much ground currents as it is neutral currents flowing in the ground path as a result of an unbalanced load and the resulting voltage drop/rise on the neutral relative to ground.
Neutral currents in the ground path may be and often are caused by adjacent loads, on shore or nearby vessels.

First, the vessel is no longer a working ship. She will never be travelling to other ports where there could be a risk of connecting to "shore power" that is mis-wired with the opposite (wrong) polarity.

Second, she is hard wired to hydro power, not connected through a shore power cord as is commonly used for pleasure craft and active ships. Therefore there is no risk of faulty connections or mis-wiring.

This type of miswiring generally results in a tripped breaker or blown fuses. It does not often last long enough to cause serious corrosion.

Third, she is permanently moored; the hull should be solidly connected with several heavy (welding) cables to a substantial grounding system on-shore along side the slip. This would prevent any currents flowing to ground from the hull

In practice this may be hard to locate and may have to be installed at a greater cost than the transformer.
Nevertheless your colleague makes some good points. If this is a direct connection from a dedicated transformer the risk of unbalanced neutral currents from adjacent loads may be greatly reduced.
If the ship has passive or active cathodic protection I would recommend regular inspection of the protection grounding electrodes. I suspect that hull damage will be preceded by a rapid sacrifice of the sacrificial electrodes. This is a worth considering even with an isolating transformer.
Location may be important. A ship moored away from industrial activity adjacent to a park or public area will be in much less danger than a ship moored near or at a working port with heavy electrical loads nearby.
I respect your colleagues knowledge and position, however I support your desire to err on the side of safety.
I hope we get some response from those with firsthand experience with other display vessels.

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

 

[crshears]

Hi Bill,

Further to your concerns I learned recently that the local electrical utility has plans on the books to install an underground 4.8/8.32 kV distribution network in the immediate vicinity of the ship's berth, with the intent of supplying a number of loads in the area including a marina, 'clubhouse,' restaurant and a commercial small-vessel operator, just to name a few. Additionally, a new 10 MVA municipal station located < 1 km away will eventually be commissioned to supply these loads...

Given the foregoing I'm wondering if it would be simpler and less expensive to use an isolating transformer in this application than to bond the vessel to a shore ground gradient control network via multiple cables; maybe it's just me, but my instinct is to essentialy eliminate galvanic connectivity rather than depend on numerous cables to try and minimize the flow of stray currents based on a hopefully low value of V/R.

Carl

 

[MikeHalloran]

Ever since multiple misadventures with Marketing Eddie, I get a twitch when I hear 'hopefullly' in any context related to engineering.

Positive Mental Attitude is a wonderful thing, but it can only take you so far.




Mike Halloran
Pembroke Pines, FL, USA

 

[crshears]

Fer sure; I get far more warm & fuzzy from the thought of going with an isolating transformer than the other route...

CR

 

[stevenal]

Okay, my thoughts. Hard wired versus cord connected, permanent versus mobile makes little difference. The issue is the effectiveness of the cathodic protection system and how much metal is involved. Think of pipelines, also not very mobile. Galvanic isolation is provided so the DC currents generated are contained to an area that can be dealt with more easily. The power types in this forum may not be the best source, since the real question here is about salt water corrosion.

Your engineer speaks of how well grounded the ship is as if it will mitigate the problem. The low impedance connections are the problem, not the solution, since the low voltage DC has an easy path to take.

Try this circuit for example: Your steel ship with is moored at one side of the bay, and a slightly more noble metal hulled boat is docked at the other. We have two dissimilar metals immersed in an electrolyte, so we have an electro/chemical cell. Since neither craft or any marina involved is providing galvanic isolation, the two hulls are metallicaly joined together through the common system neutral/ground using those nice heavy welding cables your engineer likes as part of the path. The circuit is complete, with the less noble metal corroding away to discharge this cell. The tick is to open this external circuit while still providing a good AC safety ground. [URL unfurl="true"]http://www.deimarine.com/galvanic-corrosion[/url]

 

[waross]

lps Thanks stevenal.

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

 

[crshears]

Er...ahem...I may have shot myself in the foot here...although I'm not completely convinced I have...

I failed to mention in this thread that the vessel's permanent mooring is in fresh water, not salt water. [I've heard that in such situations magnesium should be used for cathodic protection instead of zinc...]

The above notwithstanding, should there not still be an isolating transformer in the main power supply, rubber wheels on the gangways, etc., etc?

Side note: I have been subjected to very mild ridicule for worrying about electrolytic / galvanic hull corrosion / pitting for a vessel floating in fresh water; but my thinking is, "Oh? Really? Oxidation isn't a problem in fresh water? The process may be slower in fresh water than salt, but rust never sleeps." Plus, she isn't just any old planned-obsolescence ship; she's a heritage vessel, and I'd rather err on the side of caution.

CR

 

[waross]

I am under the impression that this ship is in the Toronto area.
While not sea water, I wouldn't call lake Ontario fresh water.
Every year many tons of road salt ends up in the lake. I remember the very expensive repairs to the Gardiner Expressway due to salt corrosion damage some years back.

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

 

[crshears]

Hello again Bill,

Chafe at it though I might, I remain hobbled by being enjoined from disclosure, and can neither confirm nor deny your speculation as to locale...but I like the way you think.

CR

 

[waross]

I apologize. I didn't realize that you were under restrictions. Sitting in a lake the corrosion will be much less than sitting in the ocean, but some stray current through the hull may greatly accelerate the corrosion in fresh water. I am sure that virtually any lake in North America has enough impurities to be somewhat conductive. Good luck in your endeavours.

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

 

[dpc]

In fresh water, shock hazard to someone in the water from energized ground conductors or miswiring is a hazard - no question. With no isolation, your green wire will be bonded to all of the other ships around you. GFCI protection can help reduce this hazard quite a bit.

In salt water, the green wire completes the circuit for a battery consisting of all the wetted parts of your vessel and any other nearby vessels. Any steel and aluminum in the water will become sacrificial to bronze, etc on other boats. On smaller boats, the most common solution to this issue is to install a galvanic isolator on the ground connection. This is basically diodes (usually 2 in series) connected back-to-back. The forward voltage drop of the two diodes in series is great enough to eliminate the galvanic current, but will still conduct for short circuits. This is done in lieu of the isolation transformer approach.

 

[crshears]

Thanks, dpc!

Questions:

1] We have a 30 kW VFD wired 'single-phase open corner delta' supplying the main engine turning gear, plus a number of smaller electronic devices for our on-board Wi-Fi. Need we be concerned about harmonic issues causing undesired leakage through such a galvanic isolator?

Also, since our power supply is the rough equivalent of a 200 ampere residential or small commercial service, would it not be in our best interests to go with something higher than the standard 1.2 volt breakdown value?

The admittedly few recommendations I've found so far suggest we'd need an isolator with a limited time withstand rating in the order of 300 amperes, and I haven't yet found a mfr of such a beast...


Carl

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

 

[dpc]

The size of the service is not really relevant to the galvanic isolator voltage threshold. This is not a breakdown voltage. These are standard diodes connected back-to-back. The 1.2 volts is the forward voltage drop when the diodes are conducting. They will conduct in either direction if the voltage exceeds 1.2 V. The 1.2 V is high enough to block any dc current associated with galvanic corrosion since the voltage difference between the dissimilar metals will never be that high. What you do have to be concerned about is the current rating of the diodes. They have to be capable of surviving the current magnitude and duration of a ground fault.

However, switched mode power supplies can be an issue is there is enough ac voltage impressed on the ground wire to keep the diodes conducting.

You might look here:

http://www.smartgauge.co.uk/galv_tran_02.html

So you only have single-phase power available? One of the VFD experts will have to educate us on how this might impact the ac voltage seen by the green ground wire. I really don't know. If there is enough ac voltage to get the diodes conducting, they will not be as effective in reducing dc current.

 

[crshears]

Hi again dpc,

Doh!

Thanks for the link! I now get what you mean by diodes 'back-to-back,' namely that it takes a forward bias in excess of 1.2 volts before either diode will begin to conduct, and below that no ground current flows.

There is a ham radio group [of which my retired electrical engineer colleague is a member] with their 'clubhouse' on board; perhaps they'd be willing to assist with taking some voltage, current and waveform measurements to assist in determining whether we do or don't have issues witrh harmonics, and if 'tis better to harness an isolating tranformer or a galvanic isolator in this circumstance.

Thanks again, all.

Follower of ten eng-tips forums,

Carl

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