600 kVA Generator installation - 3 phase, 4 wire

[StublueKPL]

Hi Guys, I am presently on a ship which is about to be stacked. This means we will no longer be using our 1500kVA Kato Generators. The company plan to install a deck generator just for running lights/motors/galley etc. ON the ship we have no neutral connection, its 3 phase, 480V, 60HZ. This is then stepped down to 208V 2 phase for domestic appliances (no neutral). All the motors are connected 3 phase 480V and grounded. The generator they have suggested for installation is 3 phase, 4 wire, 440V. (i.e Neutral). Is there any reason why I cannot just connect our 3 phase and earth as per usual on this or need I do something with the Neutral? I was planning on not connecting anything at the neutral. Any help would be very much appreciated. (ALSO, my apologies but I cannot seem to get the return key to work on this forum so no paragraphs!!!). Thanks again. Stu

 

[Spoon]

Can the 3 phase motors run off 440V affectively? Wont you still need a step-down transformer for the 208V domestic appliances?

 

[Knobhead]

The early systems I worked on were all 550V. Star connected with the star point grounded but not used as neutral. Without grounding you can’t detect earth faults so it is a must. It may be worth looking at fitting a neutral earth resistor to limit fault current.

All our ancillary supplies at 230V were fed from 550/230V ∆Y transformers. Again with the star point grounded. This gives 132V to ground.

What have the generator manufacturers had to say on the matter? You may be able to adjust the AVR to give 480V.

 

[StublueKPL]

HI spoon, thanks for the help. it will be feeding a switchboard which already has 480-208v transformers. our emergency generator is only 440v and I have already successfully backfed onto the equipment rated for 480v. I know its not ideal and we have quite a bit of volt drop but it definitely does work. they wont let me use the emergency generator however

 

[Engineer54]

Rather than coming onto a forum to seek guidance, as to what type of generator is required to suit your ships needs, i would have thought you should be in consultation with generator manufacturer that will be supplying the said generator.

They will after all, KNOW the answers and will be in a position to give/provide guidance to you on any connection and protective measures that need to be incorporated with the generator they suggest will suite your needs!!

 

[StublueKPL]

if I was to just generally ground the whole generator set, im guessing that would not be good enough? is there any issues not using the neutral and going straight onto 3 wire? (and ground). I have no issues once it goes onto the main switchboard I am feeding as it will then be using a local transformer for lighting etc. I am trying to get in touch with the manufacturer but its not been purchased yet. I just have to give an answer to the boss in the morning (8 hours ahead of uk) as to whether we can use this unit or not. really appreciate all your help

 

[StublueKPL]

its a second hand unit and its just up for consideration at present. im just trying to give my boss an answer for the morning. the manufacturer did not reply to my email earlier today. sorry if I troubled you!

 

[Engineer54]

if I was to just generally ground the whole generator set, im guessing that would not be good enough? is there any issues not using the neutral and going straight onto 3 wire? (and ground). I have no issues once it goes onto the main switchboard I am feeding as it will then be using a local transformer for lighting etc. I am trying to get in touch with the manufacturer but its not been purchased yet. I just have to give an answer to the boss in the morning (8 hours ahead of uk) as to whether we can use this unit or not. really appreciate all your help

Even more reason to get in touch with the generator company!! It's down to your company to give complete details of your current and/or proposed installation to the generator company. How can you tell the Boss something you don't know about. You need details, and those details can only be provided by the generator company providing the new unit. They in turn can only advise and suggest a suitable unit etc, if they have been given the full details of your ships install, and what you want this generator to provide.


I doubt very much if any of the guy's on this forum have the ocean going ship experience, to the level you are requesting here. I myself have a good deal of generator experience in both MV and LV, but i'm not in a position to advise on a ships installation, that i have no experience on.

 

[tissparks]

You will have an insulated system (no earth) on board as its LV. Tankers and other HV ships may well be different. When you say the generator company are giving you a four wire supply i would think they mean 3 phase and an earth so you can bond the generator to the ships hull. Contact them to confirm. If its a certain well known company then i would almost guarantee that is what they will supply you.

 

[Knobhead]

I’ve worked on earth free systems, they are a pain in the arse as well a danger if earth monitoring isn’t in place. That’s mind blowing to get you’re head around. The last system I designed, I used DC injection on to the AC wave form.
A phase goes down to earth, the potential on the other two phases rises to Ph→Ph voltage to earth, not a good idea when a second fault occurs.

Due to this being ship board I suggested the NER. The entire body is steel. L→E fault levels can be horrendous.
Statutory testing for M&Q plants often gave 2,000% better results for earth return than the live conductor. 25,000 tons of steel bolted/riveted together tends to give a good reading.

 

[tissparks]

I always found the monitoring not the problem, it was the lads monitoring the earth monitor. As soon as an earth fault comes up you're meant to isolate sections of the switchboard to identify the equipment with the fault. More than often this was ignored. As you said Tony, two earth faults can then cause all sorts of problems. I never saw it but heard of a short from the engine controls 24V system to the ships 440V, caused a right mess of the engine control panel apparently. The only real issue i experienced was an earth fault bouncing from one switchboard to another. Took us days to find it. Turned out to be one of the phases from the genny had worked its nut loose so was touching the terminal housing as the ship rolled. Could have turned out a lot worse than just having to tighten the nut up!!!

 

[Knobhead]

The fact you say it took days to find the fault sums it up with “earth free” systems.

It took me months to find an earth fault on a 660V 2.6MW AC system that had water cooled cables feeding a furnace. When I found it I wished I hadn’t, I’m stood under a furnace leaking molten iron! (The works was shut down for a fortnight).

I’ve mainly worked on DC “earth free” which isn’t too bad. Its earth free in so far as you have a slight bleed through matched series/ parallel resistances to earth. All you do is monitor the voltage difference across the two centres of the series bleed resistors to trigger an alarm.

 

[Engineer54]

You don't Have To use the double fault scenario normally associated with a IT NER earthing system. Monitoring control can be used to open a circuit or sub-main on a first fault. I can imagine having impedance monitoring devices all over the show, becoming quite an expensive set-up, but it will make fault finding on such systems that much quicker and easier to locate.


The OP's ship sounds as if it could be quite an old one, which makes things much more interesting when you try to integrate or modify an existing electrical distribution system. Having no experience with such ship installations, i'm not going to even try giving any advice as to what generator is going to be suitable for integration into his present electrical distribution system. ...lol!!

 

[Knobhead]

When has double fault come in to NER IT systems? It’s a standard system with fault current limiting to prevent damage to the earth return path. Line to line fault currents aren’t affected unless line reactors are used. I’ve never come across line reactors on modern systems and only once on a very old system.

An earth free system is where double fault comes in to play and where earth monitoring is required. Unmonitored a line to earth fault can go unnoticed, get a second fault then you have line to line fault current flowing through the structure. There is no CPC for the current to go through. Earth fault monitoring will show you have a fault on one leg, not where it is.

 

[Engineer54]

NER or no NER, it's still double fault based IT system. The earth as such, is the whole shell of the ship you don't need any CPCs. Impedance earth monitoring will, when strategically located, help in the location of the fault, just depends how big or how much of the system is being monitored by that particular earth fault monitor. I did say that an extensive monitoring system would be expensive!! lol!!


As i stated the monitor(s) can be used to trip out that section, that's being monitored instead of just raising an alarm.
Line to line faults are taken care of by standard OCPD's...