Earth Leakage Relay question..

[FBW]

I have a 400VAC generator here that will be used on an offshore platform, 4 wires out (1,2,3,N). There is a toroid type earth leakage relay on it inside the generator main breaker enclosure with all 3 phases and neutral running through it. My understanding is that the toroid will sense any unbalance between the currents travelling on all 4 wires. In any normal situation, any current travelling out of the gen will also be travelling back in, whether it be on another phase or on the neutral. So the net sum of current travelling on all lines should always be 0 at the breaker panel.

This all makes sense to me, but here is the confusing part: The neutral and frame of the generator are not bonded. Which means that even in the event of a line to frame fault, there will not be any fault detected, as there is no path for the fault current to travel back to the star point of the gen. Which makes me wonder, what’s the point of the earth leakage relay? Where can any stray current possibly “leak” to?

Even if they are relying on neutral to ground bonding to take place in a main distribution panel in the field, that still means that a line to frame fault on the generator would not cause an imbalance on the ELR, as fault current would still be returning on the neutral wire to the start point.

Is there a gap in my knowledge somewhere? I cant see how the ELR can do anything, unless current from the generator actually “leaks” out of the electrical system completely…..to where??

Sorry if it's a noob question, this is my first time dealing with the thing...

 

[ScottyUK]

How is the neutral of the system earthed? Generally LV power systems are solidly earthed, but emergency power systems occasionally use a resistance-earthed 3-wire system. It sounds like you have an ungrounded installation, either by design or by oversight. The intended earthing arrangement is key to understanding what can or can't be done in terms of protection.

 

[sibeen]

Somewhat stepping outside my bailiwick of knowledge here, but many years ago I had to work on a shore power system, which provided power to naval ships when they were at dock. One of the requirements was that the system had to be floating - the phases and the neutral had no reference to the earthing system. It may be that an offshore platform has the same requirement.

If that is the case, it is difficult to see how this earth leakage scheme would operate. Perhaps a specification error, if my floating system has any legs.

 

[Skogsgurra]

Ships and platforms all have, to my knowledge, floating ground. Lots of different resistance supervising devices, from Megohm meters with or without signaling contacts to sophisticated relays like Bender and similar, are used.
Never seen an earth fault current relay in any such installation.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[FBW]

I'll have to check but im fairly sure that neutral is solidly grounded in their main distribution panel as required by NORSOK for this voltage level.

The geneset came from the manufacturer in this orientation. I guess it's just a matter of satisfying my own curiosity here...if the star point of the gen is not connected to the frame of the genset, and grounding takes place in the distribution panel, any and all fault currents would have to return to the star point via the genset's neutral wire, which travels through the toroid....which means there will never be an imbalance to cause the ELR to trip. It seems as though i'm missing something. My understanding of the relay might be wrong..

 

[ScottyUK]

If the scheme is configured as you describe then the relay would trip for a stator earth fault within the machine. Current would flow to the stator core without passing through the CBCT but would return to the neutral point via the CBCT.

 

[Skogsgurra]

To avoid confusion: I didn't mean floating GND, I meant floating Neutral (very often no Neutral at all).

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.