Why to use 4 Pole Breaker for Generator 1

[vp1959]

Can anyone explain why certain authorities insist to use 4Pole circuit breaker for generators and transformers?

1) What type of action/ protection is expected from the 4P breaker fitted on generator when its neutral is solidly grounded (TN-S system for 3P4W distribution)?
2) Is it recommended to link the neutral and body earthing at the source (generator or transformer)?

Appreciate your input

Regards

Prakash

 

[waross]

This is often for standby sets. When a system has ground fault protection it is often necessary to switch the neutral when switching between sources to avoid issues with the ground fault detection scheme.
My first choice is a three pole breaker but this is not always possible.

Bill
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"Why not the best?"
Jimmy Carter

 

[sibeen]

One thing to be very careful of when using a 4 pole automatic transfer switch (ATS)between mains and generator is when the site has down stream UPS systems. With sites with a UPS system...don't do it!

A UPS will normally have multiple single phase loads and the output load balance can often be quite uneven. During the transfer of the upstream ATS the Earth and Neutral bond will be broken and the Neutral point at the output of the UPS will float to the centre of the three phase output delta voltage. The phase with the lowest amount of load will see its voltage increase and the highest loaded phases will see their voltage decrease.

In systems where the phase imbalance is quite high there then is the chance of the voltage going outside equipment limits and smoke begins to be let out. This then cascades as when a piece of equipment gives up the ghost, the load decreases and the voltage goes up and then another bit of kit decides to let go...and so on.

 

[davidbeach]

An excellent example as to why a large UPS should only supply load via PDUs (Power Distribution Units) with transformers. The UPS sees only the delta windings and the single phase load never sees a interruption of ground or neutral.

 

[sibeen]

David, I know PDUs are the normal way of distributing UPS power in North America. I suspect a lot of this has to do with the requirements to step down the voltage for final distribution, with the subsequent added extras of a solidly bonded E-N and triplen harmonic isolation, but in other regions of the world they ain't so common.

I'm in Australia and as a gross estimate less than 5% of sites with large UPS systems would be using an isolating transformer in their distribution switchboards. The UPS output will be 400/230 and all the distribution done at this voltage. It's one of the reasons that I'm so aware of the four pole switching issues that can arise.

I mentioned in the earlier post the problems that can occur with systems where the output loads are not distributed evenly but that is not the only issue that can arise out of the four pole switching. As david alluded to, one of the strengths of using a PDU with an isolating/step down transformer is that the N-E is solidly bonded. Many years ago I was called to investigate a site where every time they experienced a power failure half of their computer infrastructure would shut down. In this case they had a balanced load system so the computers didn't see any major variation in the active-neutral voltage during the changeover but the E-N voltage went from around 2 volts up to 120 volts during the period the four pole switching was being undertaken. This was enough to send the power supplies of approximately half their computers into a glitch.

 

[vp1959]

Dear all,

Please see the attached SLD to know my concern, as no one answered my qusestions.

What is the advantage of using 4pole MCCB instead of 3Pole MCCB for standby generator with neutral solidly grounded?

It is mounted on the generator, always closed to start and feed the power through the ATS upon main failure?

The 3P MCCB, manually operated is having protection for SC, OC, GF.

The loads fed from the utility transformer or standby generator are combination of 3phase and single phase (unbalanced)

Regards

Prakash

 

[vp1959]

See the attached SLD

 

[waross]

The diagram as shown does not require any four pole switching, either for breakers or for the ATS. However when protection and grounding details are added to the diagram it may become apparent that a four pole transfer switch is needed. Maybe not.
I have seen too much damage done by late neutral switching and failed neutral switching to use any four pole breakers or switches unless it is absolutely required by grounding and protection issues.
There may be exceptions for unique site conditions.
A possible exception may be if there is a long distance between the main service and the generator location AND poor grounding conditions. If a ground fault analysis shows the possibility of dangerous touch and step potential voltages at the generator as a result of a fault elsewhere on the system when on grid power but not when on generator power, a four pole generator breaker may be indicated. With the relatively high source impedance of a generator compared to the grid impedance, ground return fault currents may be expected to be much higher when on grid power. Some consultants may address such an unusual issue by improving the grounding system. A four pole generator breaker MAY be a solution.
It depends.
A definitive answer requires much more site and protection information than is given.


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

 

[vp1959]

Thanks,

What other information is required to conclude, as I have shown everything?

The neutrals for the transformer and generator are solidly grounded and shown in the SLD.

Yes, problems due to late neutral switching or failed neutral are possible, but it is common like phases (sometimes phase also missing due to bad contact/ loose connections)

The only benefit by having a 4P MCCB is that the generator can be isolated totally from the system during maintenance, but it is not applicable here as the neutral is switching thru the ATS (4ople).

I tried to ask the consultant/ their followers about the advantage, but could not explain except saying it is required to protect the generator.

 

[ScottyUK]

If you have multiple neutral-earth bonds then your earth fault protection may mis-operate correctly if it was not designed for a multiple earthed system. Unit protection schemes on the generator or transformer can also mis-operate if earth fault current returns through an unintended path such as a second neutral-earth bond. Complex schemes aren't common on small machines of a few hundred kW but the modern relays do so much for so little cost that it is economically viable to protect a large LV set.

We don't know what protection you have because you haven't detailed anything about the scheme, so we can't comment.


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If we learn from our mistakes I'm getting a great education!

 

[sibeen]

What sort of ATS is it? You mention that it is a 4 pole ATS but how does it operate? Do the neutrals overlap during a transfer or is there a neutral break?

Are the generator and transformer close together and connected to the same earthing point?

 

[vp1959]

There are two sites:

One ATS is contactor type, while the other one is motorized MCCB type:

4Pole type contactors and MCCB are provided with electrical and mechanical interlock and they breaks the neutral when changeover

Please see the attached SLD for better understanding the application/ arrangements