Changing from 3 Pole to 4 Pole Circuit Breaker

Since source of supply is same , and there is no parallel operation, there is no need to change the 3 pole ACB to 4 pole ACB.

This is my opinion.

Hey Ramvinod

Thanks for your opinion

Let me just be more specific. REMEMBER WE HAVE TWO 22KV/400 V Transformers WHICH SUPPLY THE MAIN SUBSTATION. So at Sub 1&2, we have 2 incomers, from Transformer 1 and from Transformer 2 , with the bus coupler. And also we have 2 neutral bus bars which are supplied from Transformer 1&2, and the 2 neutral bus-bars are coupled together with a piece of a bus-bar link, which is removable.

If you supply the Loads at Sub 1&2 with both Transformers/Incomers at the same time, with the Bars Coupler open, the neutral link must be removed, otherwise the Core balance/CT will trip the Main CB at the Main Substation because of the circulation of the neutral currents.

The only time when you have to connect the neutral link is when you supply the Loads from one incomer and close the Bars Coupler.

So we don't want that manual removing and connection of the Neutral Link that is why we considering the change.

You are correct.

You need to change only the Buscoupler to 4 pole. So it will break or make the neutral provided that you must have interlock between two incomers and buscoupler to make sure any 2 breakers will ON at a time.

Yes , the neutral at the bus coupler to be opened to avoid nuisance tripping due to core balance CT and earth leakage relay.

You have two options:

1) Replace the existing 3 pole breaker of bus coupler with 4 pole

OR

2) Provide one Contactor in the neutral bus removing the link between two neutral buses.

This contactor will be open when buscoupler is open and this contactor will be in closed when bus coupler is in closed.

Rating of the contactor can be 1/6 of the rating of bus coupler breaker.

The contactor poles are to be connected in parallel.

suppose bus coupler breaker is rated 2400Amps, provide 400 Amps , 4 pole contactor AC 3 duty.

How about a telephone conversation with the original switchgear manufacturer?

I wouldn’t alter the main board neutral. As you’re using E/L to feed the sub boards the sub board bus coupler should be 4 pole.

Please clarify the system, maybe I misunderstand. Diagram below shows system with two transfo with neutrals earthed by rods at each transfo. Each input to board has core balance CT with a relay which trips the input breaker. The resistances of neutral cable from transfo to switch board are r1, r2 respectively, probably r1 = r2 =r. If your transfos are 1 MVA, full load is about 1400 amps.

Assuming resistance r drops 1% phase (230V)volts at rated current, the voltage across r will be about 2 volts. A resistance of 2 ohms between earths would circulate 1 amp, which could be negligible.

According to your first paragraph, "the neutrals of the the two transformers are linked". In that case there is a definite neutral loop, my resistance Re ~= 0, and the circulating current could be 750 amp and a trip results.

My questions is...

If you supply all loads from one transfo with the switchgear neutral link IN, then there will be a circulating neutral current and a trip - unless there is something you did not tell us, like taking a neutral link out at de-energised transfo??

I feel that with neutrals joined only at transfos, the bus coupler could be closed to parallel the transfos briefly, as part of a "no-break" load transfer sequence to one transfo. With a neutral tie breaker and four pole main breakers (or neutral breaker on each transfo incomer) there could be an auto transfer, provided transfo core balance CT relays have time delay (surely must have delay else earth fault way down system takes out main breaker).

67model

Looking at your drawing: The neutrals are linked before the CT or Core-balance not after the Core balance

If two or more LV(400V) sources feed the same bus and TN system of earthing is adopted in network, then the neutral of such incomers should be switched meaning 4P incomers to avoid incorrect operation of earth fault / leakage relay or other residual current device due to existence of parallel paths of neutral to earth currents. Neutral pole should be rated for the switchboard neutral bus rating.