Neutral distribution

[LMSANZ]

Hi everyone,

I've been recently involved in a brown field project. It is a 25 years old plant which was powered by generator at first "stage 0" and later on a electrical overhead line was built so they included a distribution transformer.

As you can See under Stage 0, we have two generators connected to a 3Phase 3Wire MCC. So for having neutral (3P+N) for auxiliaries we have a isolation transformers, one of them 750KVA.
Under Stage 1, one distribution transformer 1.6MVA, Dyn has been added, so the neutral is available but not distributed. Transformer is protected through 87T upstream. We still maintain the Aux transformer 400/400V for having neutral downstream.

Apart of the advantages of the isolation transformer, I was thinking in remove it (it is 20 year old now) and make a Neutral distrubition, joining all the neutral point of the sources (generators and transformer).

The protection in the MCC is a Digitrip RMS510 Cutler - Hammer, with LTI and Ground protection in the incoming of the MCC.

What do you think about neutral distribution for avoiding Isolation transformers?
We should protect the Neutral with additional CT and/or relay? or just with the existing protections is enough.

Regards
Luis

 

[Mbrooke]

You will either need a 4 pole ATS, or remove the grounds (neutral to Earth bonds) on the generator and ground at a single point only.

Make sure the increased fault current passed were the isolation transformers were previously is taken into account.

 

[waross]

As Mbrooke mentioned:
Circuits fed from a .75 KVA transformer will now be fed from a 1500KVA transformer.
With a ratio of 2000:1 your fault currents at the equipment fed by the 0.75 KVA transformer may be somewhat higher.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LMSANZ]

Well,

the transformer is 750 KVA so is almost half size of the distribution transformer 1600 kVA. In any case, all the auxiliary circuit connected to this transformer are properly protected through a Main CB and Outgoing CB with Ground protection.

The goal of distribute the neutral is to remove the 750kVA which have low insulation resistance values during last inspection.

Not sure, if any protection needed and the implication of interconnecting generators (2) neutrals and Distribution transformer neutral at one point and grounded at one point only (transformer).

Luis

 

[waross]

Sorry. I misread the size of the smaller transformer. I thought that I saw a decimal point in the diagram. When I went back and zoomed in the point went away.

Not sure, if any protection needed and the implication of interconnecting generators (2) neutrals and Distribution transformer neutral at one point and grounded at one point only (transformer).

That is my preference. That avoids changing the transfer switches to four pole switches.
I don't like four pole ATS switching. I have more than once seen appliances damaged by the malfunction of the neutral pole of an ATS with a switched neutral.
One other consideration:
If a transformer has been added to provide a neutral, then I assume that there are or may be line to neutral loads. If that is the case then in North America the neutral must be solidly grounded.
A single, solid ground point may compromise your ability to use resistance grounding for the MCC.
Choices:
1. Remove the transformer and use single point solid grounding.
2. Replace the transformer.
3. Use multi point grounding and change the ATSs to four pole switches.
Your codes may be different. You may be allowed to use a floating neutral, or perhaps you do not have any line to neutral loads.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

X2 with what waross said, in addition to that Id recommend a fault study to determine if the LV MCB can handle it, just to be on the safe side safe.

 

[waross]

There are exceptions to my preferences for instance when long distances are involved between the source transformer, the MCC, the generators and/or the transfer switches.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

Howdy LMSANZ,
My first thought was "Where are the hi-resistance grounding resistors on the two generator neutrals?". After a second read and some careful consideration my second thought was "Where are the hi-resistance grounding resistors on the two generator neutrals?".

I like HRG resistors connected to all neutrals as this makes a really safe system. Don't forget that 95% of all faults are single-phase-to-ground type. Of course with any HRG system, you cannot supply line-to-neutral loads without the use of an isolation transformer (ie delta-wye with the secondary wye solidly grounded).

The other option would be to establish an artificial neutral with a zig-zag grounding transformer, together with a HRG resistor. This zig-zag could be connected to MCC-501.

What is the nature of the load on PT-504 (ie the 750kVA xfmr)? ie how much of this load is actually connected line-to neutral, and how much is 3-phase (ie w/o the neutral)? My thinking here is to ditch the 750kVA transformer and connect all of the 3-pahse loads direct to MCC-501. A smaller isolation transformer could then supplied to feed only the line-neutral loads.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[LMSANZ]

Hi GG,

well the loads connected to PT-504 (750kVA Xfmr) are basically all single phase. The transformer give service to a Restaurant and living quarters in the plant. The loads are quite well balance and only few of them are three phase.

On the other hand, all loadas connected to MCC are pure 3 phase motors and feeders.

My understanding is that having a neutral for generators and transformer connected to ground we can distribute the neutral without isolation transformer, and in case of fault the high value of current will activate the circuit breaker on the affected phase.

Luis

 

[waross]

I would prefer a scheme where the 1.6 MVA transformer and the generators were all protected by a high resistance grounding scheme and the 750 KVA transformer was replaced with a new, similar transformer.
I would be checking the Available Short Circuit Current with the ratings of the downstream devices.
You may end up considering splitting the load across two or more smaller transformers to stay within the ASCC ratings of downstream equipment.
There are other acceptable solutions.

Bill
--------------------
"Why not the best?"
Jimmy Carter