Operation of LV bus

[LittleInch]

In the engineering and Disasters forum there is a debate going on about how a 440V board with a bus tie and two incoming transformers should or is normally operated.

https://www.eng-tips.com/viewthread.cfm?qid=517698#newpost

The big ship which crashed into the bridge in Marland, was running a 6.6 kV board with 4 generators, only two running, bus tie closed, then feeding a 440V board through one of two Txs connected, which they switched over every now and then. So when one TX tripped for some reason, all the lights went out and the main engine stopped because all the auxiliaries stopped as well

Is that normal?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dpc]

There are many ways to configure and operate with multiple buses, multiple sources and tie breakers. Not really enough information to draw any conclusions. The operational logic will also depend on the equipment's tolerance for momentary outages, short circuit ratings and other factors.

It's common to NORMALLY operate double-ended buses using both sources and the tie breaker open, at least in my experience. Shipboard systems may be different.

 

[che12345]

"..was running a 6.6 kV board with 4 generators, only two running, bus tie closed, then feeding a 440V board through one of two Txs connected, which they switched over every now and then. So when one TX tripped for some reason, all the lights went out and the main engine stopped because all the auxiliaries stopped as well..."
I have the following opinion.
1. I noticed that most of the learned contributors in this forum are NOT involved/familiar with marine switchboard design. In most cases, we refer to IEC or IEEE standards that are for land installations. Marine boards design are different from land boards. They are designed, approved and tested to ship classification society rules/regulations by e.g IEC(international), LRS (Gr. Britain), ABS (America), BV, GL DNV, RINa, MRS...
2. For example, for LV, 3ph 3W without Neutral is the norm. The metallic body (Hull) is the Earth. All equipments are Earthed. There is NO solid conductor connecting the Hull to the Earth (soil) that we are standing on.
Che Kuan Yau (Singapore)

 

[FacEngrPE]

IEEE Std 45 IEEE Electric Installation on Shipboard is cited in the US Coastguard regulations as the go to document for shipboard wiring design. The National Electric code does not apply shipboard, except as cited in 46CFR

90.2 Scope ...

(B) Not Covered
This Code does not cover the following:

Installations in ships, watercraft other than floating buildings, railway rolling stock, aircraft, or automotive vehicles other than mobile homes and recreational vehicles
Informational Note: Although the scope of this Code indicates that the Code does not cover installations in ships, portions of this Code are incorporated by reference into Title 46, Code of Federal Regulations, Parts 110—113. ...

However I do not think it good operating practice to let a large dry transformer sit cold. It is probably better (for the transformers service life) to ensure it stays dry by leaving both transformers on line and the low voltage buss tie open, HV Buss tie closed, except when maintenance requires a different configuration.

However I am not a marine engineer, so training and experience may direct a different approach.

 

[LittleInch]

Fac eng, and everyone, thanks for responding.

There may be something in the sitting cold as apparently the unit they were using initially had been in service several months before they switched over to the second TX once they had two trips on their system whilst in port.

Second question is whether it would be normal for all the 440V breakers to drop out on loss of power to the 440V board. I have no idea if they had a UPS on the control circuitry or not.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[che12345]

Dear all, I have the following experience for your consideration.
1. Electrical installations in Ships are very much different from the land installations, which are usually from the Utility; without having any Gen of your own. In some cases may have a single Emergency Gen, usually NOT syned with the Utility.
Note: In Ship installations, there are min TWO main Gen and an Emergency Gen.
2. The Gens are always designed to be syned. Knowledge on how to syn, Voltage + Frequency (V, A, Hz, kW, kVA, Pf.. ) adjustments; Load-sharing, Load shedding, Load transfer from Gen A to Gen B , is "routine".
Note: kW meter with 20-30% Reverse power scale, Earth test lamp (Insulation monitoring meter) , Synscope ( Syn lamps) are items NOT on land boards taking supply from the utility.
3. IEC 60092-xx , IEEE 45 etc. ; each publishes their own documents.
Note: They are NOT classification organization. They neither approve the design nor conduct/witness any test or police their document are complied with.
4. The Classification organization Approves the design, Witness/verify the operation/protection during the Dry test at the fabrication workshop and witness/verify the operation/protection during the Sea trial.
5. Navigation light circuitry, Number of Gen starts and over-crank limit, Steering gear starters from different sources + routing, Automatic starting of the Emergency Gen within the time limit are some items that are not in the land boards, supplied by the utility.
6. In summary: Marine board design differs from that of the Land board.
Che Kuan Yau (Singapore)

 

[cranky108]

As far as I know, many 480V contactors are magnetically held. But not being an expert on low voltage, I may be wrong.
So on the loss of control voltage, the contactors would open.

 

[crshears]

So on the loss of control voltage, the contactors would open.

True; therefore good transmission system practice commonly dictates the use of DC relaying and control, with batteries having eight hours of emergency capacity at normal loading being "floated" on the control voltage supply, so that a momentary loss of AC supply while equipment transfers to another source may cause a momentary panic that is quickly resolved.

I have not seen everything, but in my experience most UPS equipment will not normally come with that extended of a capability.

As to marine practice, I certainly cannot quote chapter and verse of the various pertinent regulations the way che12345 can, but I'm with him when it comes to the more stringent requirements applicable to marine operations. In my view, an occurrence like "all the lights went out and the main engine stopped because all the auxiliaries stopped as well" should never, ever become a reality; stuff like that topples bridges and kills people, and any behaviour that tolerates that sort of situation is criminal and should be prosecuted as such.

Indeed, properly designed and operated marine propulsion systems will be configured in such a way that independent fuel, lube oil, and cooling pumps ensure continued operation of main propulsion, steering gear, etc. so as to ride through AC system issues without jeopardizing the vessel in question.

CR

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

 

[dpc]

Low voltage power circuit breakers will generally NOT open on loss of power unless configured with an undervoltage release. (This is similar to HV breakers). An UV release is uncommon in the USA, but seems to be more common in the rest of the world. I don't know about shipboard standard practices. Molded case circuit breakers will also not open on power loss unless specially configured with an UV release.

I think we'll have to wait for a more detailed report from the NTSB to really know what happened. One takeaway for me was that the fuel quality was apparently not an issue - unless I missed something.

 

[RRaghunath]

If a transformer is off, it saves power as the transformer has no losses when it is switched off. But, this approach compromises on the availability of standby power supply to the loads downstream.
I have always seen both transformers ON and the bus coupler open to be the arrangement.
The power supply switchover is done rapidly after confirming that the upstream supplies are in synchronism and that the trip of the incomer is not due to any protection operation.
Automatic dropping of all the outgoing feeder breakers is adopted through no-volt release, if there is restriction on block loading of the generator. Automatic dropping of all loads facilitates gradual loading of the generator by switching the loads one by one, by the operator.

R Raghunath