Earth Fault Tripping

i feel that the unbalanced current flowing through the neutral is NOT an earth fault current, and is not unhealthy. The relay should not trip for any magnitude of this current up to its max rating. The output of the CT sensing the current in the neutral should not go to the EF relay. In fact, the CT should not be there. On the contrary, there should be a SUMMATION CT or CORE-BALANCE CT (CBCT) through which ALL three phases AND the neutral must pass. This way, under healthy conditions, the vector sum of all the currents is always zero, and the EF Relay will not act. In the event of an earth fault, the vector sum of the 4 currents is no longer zero, and that should cause the EF to trip.

For more information on this, please check out this superb paper from Schneider Electric:

schneider-electric ect114.pdf

GA.I was searching for the right words, I could not find them because you had already placed them here in the right order!!If his diagram is correct, it would appear to be missing an earth wire connecting the neutral point at the Generator, to any frames/housings of the equipment being supplied, via the earth in the supply cable.

I personally would NEVER work on a system with an earth return via "mother earth" as under the wrong circumstances this could allow the frame of a faulty equipment to become live and kill/injure anyone touching it.

Something like this:-

It would therefore appear that someone was saving money on cables....I get the impression (here at least) that some people take the term "Earth" a little too simply......maybe we should call it something else to identify it as being required....?

No Andy,

Protective earth wire could not be wired to the neutral point of generator, you are short-circuiting the earth-neutral resistor.

I agree an earth wire is missed but it should be wired to the neutral point of the star at load side, any current, including ground fault current, have to flow back to the reference point of the generator which is the neutral of the generator star connection.

in case of a ground fault the current will flow back through the earth wire to the neutral thru the resistor.

A CT should be installed there to detect it and trip the main breaker.

the neutral current resulti of an unbalance will not flow thru the resistor so there is no trip . Cheers

Hi Andy and Le Noble.

i have seen this in that paper from Schneider. It seems to be ok as per IEC to use one wire for Neutral and Earth (PEN)...please check this out, and if possible the ECT 114 also.

Rather than putting a CT on the Earth conductor, would it not be simpler (and more economical too) to use a CBCT ? i would welcome your views, thanks.

Yes it is ok for PEN arrangement, but it is not the case elaborated by the OP, he does have an IT arrangement, so the PE should be connected the the earth side of the resistor ( as shown in schneider document) not to star neutral point as Andy suggested.

A CBCT can detect a non zero vectorial sum of the 3 currents, but it may trip in case of unbalance occure or single phase loading, a resisor in the neutral-earth link will reduce also the fault current, the CT installed in this link is not sensible to unbalance

B/R

The CBCT must encompass all four wires...then, unbalance will not trigger the RCBO/RCCB.

but it still not limit the fault current,

as shown in the diagram, there is resistor connecting the generator neutral to the earth, the purpose of this resistor is to limit the fault current

Oh, earth fault vs earth leakage....ok. Thanks.

Anyway, coming to the op's issuem he needs to put that neutral CT somewhere else, to prevent nuisance tripping.

I agree. the CT is misplaced, it should be installed somewhere between earth pit and generator star node..

In Europe, I do believe that we are limited to having a max of 1 ohm resistance from earth to neutral. I have not heard that this has been changed.....

Someone who knows the European codes better than I should chime in here please......

I am very surprised to hear that some countries put resistors in the earth/neutral path.......to reduce fault current!

Surely one needs ANY earth fault to blow fuses for safety reasons....

Of course, if an ELCB is installed, then one can detect a minor short far better and quicker and safer, but nobody mentioned having an ELCB as far as I remember....here I have ELCBs and a neutral/earth link connection only in my house.

From the substation I only get 3 phases and neutral......no earth. It looks something like this as far as I remember:-

Not here in Europe at least, no resistor and a link is to be made at the substation between earth and neutral AND ONLY THERE. NOWHERE ELSE.

GA from me...

There seems to be NO earth current. To quote the OP...

For e.g. if the load b/w the three phases L1: L2: L3 is 220 A: 210 A: 230 A, the earth current is around 10A-40A (which is considered safe or OK). However, if the load on the three phases is 200 A: 250 A: 270A, the earth current rises to 70A and keeps increasing as the difference b/w the phases (load) increases. If it rises to 150A, the genset trips on 'Earth Fault' and plant shuts down. To prevent this we usually cut some load on the phase which is high and add some load on the phase which is low in order to decrease the difference b/w the phases, plus we water the earthing pit where the generators' neutral is connected

Seems to be a case of wrong wiring...the CT on the neutral (need not have been there actually) is wired to trip the EF relay...naturally it is sensing unbalance and tripping ... all he has to do is to relocate the neutral CT to sense the EF current.

KVSRHIDAR,

I agree with you, there is NO FAULT

This assembly is done by caterpillar as understood from OP, and there is little chances to be done wrongly, I dont think Caterpillar misplaced the CT-dispite what a said before- and not clear from the OP that there is a CT, may be y are using another arrangement, if it is an Holgrem connection to detect the earth current, sure it will sensitive to big unbalanced current

If the original poster is still following the discussion, please to send more information if there is neutral CT and how it is connected

B/R

Yes, you are quite right that the neutral CT would be correctly wired if done by Cat. i think therefore that the wiring of the single phase loads (which the user would have to do) may have ignored some conventions. For example the housing of an appliance may have been connected to the neutral rather than being earthed ? As you say, the OP needs to come back and tell us more.

Yes , it could be,

people some time connect their loads to PE in place of Neutral wire, especially if done by unqualified persons, and this will be seen as a earth current.

But the OP talks about a Oil and Gas plant, they should follow the highest standards...

OP should come back, or let the Admin close this discussion

Good explanation, you Got a GA vote!! Thanks to Andy!!

But the OP is clear, they do have trip when they have current unbalance more than 25%.... there is NO EARTH FAULT and the megger will not find anything....

The problem is that the normal neutral current is seen by earth fault detection circuit where it should not...

we were guessing about neutral CT, but unless the OP elaborate more how the fault detection circuit is wired, we can do nothing but guessing

Has the earth fault gone yet?

What were the Megger insulation test readings?

Which item was causing the earth fault and has it been replaced yet?

This is an interesting thread. Finally, I took my time to read it – lot of good comments here. Let me add few more.

Some concerns:

(1) In the drawing, the OP showed two CT's – one in neutral and another one in grounding (earthing) but he didn't mention which one feed to the ground (earthing) fault protection.

(2) The OP showed the generator grounding but it is not clear that it is solidly grounding or Low Resistance Grounding (LRG) or High Resistance Grounding (HRG)

(3) OP didn't mention the type of earth fault device

It is already clearly pointed out by other participants that the unbalance current flowing through the neutral and ground fault current flowing through the ground wire is different. The ground fault device should trip only when the ground fault current flows through the ground system, not because of the unbalance current flowing through the neutral.

Now come to the answers to the OP's points:

(1) Yes but the magnitude of the earth fault current depends on the grounding system (solidly or LRG or HRG)

(2) This concept is wrong – as it is already pointed (in above bold text) out the load unbalance current and the earth fault current are not same.

(3) Earth fault current setting and its tripping doesn't relate to the generator loading. If there is an earth fault, it should trip even if the generator runs at no load.

(4) This is also a wrong concept. There should not be any effect on earth fault how the generators are synchronized.

(5) There shouldn't be any effect on earthing system if the earth pit is watered. If it does, the earthing is not done properly and the system is in danger.

Suggestions:

It seems to me, people around you are having the wrong concepts and I would suggest hiring good electrical consulting engineer for the thorough checking of the system and find the proper reason of the trip.

- MS

That is a good post with a lot of common sense.....thanks.