Shore Power isolation transformer help 1

[Spreedriver]

I have a 100 foot aluminum workboat that has the capability of using shore power. The shorepower comes in from shore as 240 Delta. Inside the shore power distribution panel the neutral and ground are connected, as per local code. 4 wires come to the boat in the shore power cable, 3 phases and neutral/ground. Ship's power is 208 Wye produced by either of 2 Northern Lights 65 kW generators, with 3 phases and the center tap connected to the neutral bus bar in the ship's distribution panel. In the electrical room is a 45 kVa sola/Hevi-duty isolation transformer that has 3 "H" taps, and 4 "X" taps. Currently, the transformer is wired so that the 3 incoming lines go to the 3 "H" taps, and the incoming neutral/ground go to the "X0" tap. The 3 load side lines go to the 3 "X" taps, and the load side neutral is tied to the "X0" tap along with the shore side neutral/ground. This arrangement worked successfully for years, but about 2 months ago something changed, not on the ship.

I'm currently running about 60-90 amps per phase shore power, and 24-30 amps on the ship's neutral line. I can't tell you what the neutral line was running "before", because I never had cause to troubleshoot before. Now, I'm eating anodes off the boat so fast you can almost see them fizz, and I've burned up one $4,000 transformer and the new one installed yesterday is so hot you can't touch the enclosure. Everything I've read says that there should be only one path to ground, and that path has a CT on it going to a meter. It usually reads 1 1/2 to 2 amps to ground. The generator neutrals have a knife switch to disconnect them, they've never been disconnected in the 15 years I've been on this boat.

I think that the problem is that something in shore power changed a couple of months ago and I should disconnect the shore power neutral/ground to the ship's transformer. All of my Navy electrician buddies think so too. Anyone got any thoughts? Burning up another transformer is not on my list of happy thoughts. Oh, we keep tripping the ships side shore power breaker at 100 amps. The shore side shore power breaker (100 amps) does not trip. When operating on ship's power, everything is ducky.

 

[itsmoked]

Sounds like you have a phase shorted to your hull and the shore neutral or ground is missing.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

I would definitely disconnect shore power until this is resolved. If you think it's just the anodes melting off you're mistaken. A diesel fuel bill is less than than the price of your hull. I've seen your scenario melt the screw off the prop shaft. You can buy a lot of fuel for the price of another transformer.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Spreedriver]

No phase shorted to hull, it would show on the ground detect circuit. Lifting the neutral/ground from the high leg of the transformer helped tremendously with the heat. Only time will tell if the electrolysis has abated.... And the transformer stopped buzzing.

 

[waross]

Disconnect all neutral and ground cables between the shore and the ship. The "something changed" may be another ship connected that is drawing a lot of neutral current. Part of that current is entering your ship on the neutral cable and finding a low impedance path home through your anodes.
You only need hree phase wires to feed the transformer primary.
Also, a cable or transformer on shore may be failing to ground and part of the fault current is being "shared" on your neutral connection to shore through your anodes. This may also be welding activity somewhere near with inadequate grounding. Again, disconnecting the cable to the shore neutral/ground will solve your problem.

Bill
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"Why not the best?"
Jimmy Carter