DOOBS,
I am agree with you.

Particularly in this connection, the neutral contactor at alternator side is meaning less except one situation. When only alternator is running, in that case neutral unbalanced current have to flow through ground if that contactor is not in line. And that ground path may not be sufficiently able to withstand that load.

It is my guess that, to avoid that situation, this contactor is provided.

 

Barindra... I'm curious!

Why do you accept the two neutrals remaining connected via the Ground/earth link, as normal?

Phil

 

>Why do you accept the two neutrals remaining connected via
>the Ground/earth link, as normal?

Case-1: say the contactor part is eliminated from the circuit.

In this case, the unbalanced neutral current has to flow thru earth link line via earth pit (or common point). This earth connection may be weak or contain high resistance (normally in India GI strip is used). In case the unbalanced current is high, in the range of 200A (which is feasible in case of single phase lighting circuit) the per phase voltage will suffer. Say the earth line has a resistance of 0.1 ogm and un balanced current is 150A then the phase voltage will be 15V less.

Case-II: say the earth link part is eliminated

In this case when the alternator is in island mode then for any unbalanced current neutral shift will occur and phase voltages will be unequal, also this will be the case of unearthed neutral which is not preferable as this is low voltage and home appliances may be connected in line like computers, ac, fridge, tv etc.

NB Both the connections are necessary i.e. a connection of direct neutral to neutral and a connection between earth and neutral.
One can connect the grid neutral directly to earth pit also.
Here only the contactor is not necessary i.e. direct grid neutral may be connected to earth.

 

Barindra... I didn't ask for a "what if" situation.

The PRESENT situation is that the two neutrals are now CONTINUOUSLY connected together via the earth-block or earth-bus. Thus, both sources will contribute to an Earth-fault. Isn't the purpose of "Neutral-Switching" to insure that only one earth-source exist when both the utility and the generator are operated in parallel?

Now ignore the earth-fault situation and consider the paralleling situation. Since both the Utility and Generator are switched with 4-pole ACBs (not the usual ATS) there is the possibility of isolating the Load from its neutral-bus. Furthermore, even if the duration of "Open-Neutral" phenomenon is of the order of milliseconds, great damage can occur. The purpose of the contactor, is to prevent isolation of the neutral until paralleling is underway. And then, the contactor is opened leaving the Utility only connected to the load-neutral bus.

However, while the idea is sound, implementation is another story because the two neutrals are always connected together via the earthing-block.

Phil

 

Hi Doobs,

I am a learner and was reading this post. I would be thankful to you if you can please send me a hand-drawn sketch of your system to lakmalp at gmail.com. I am eager to read and understand the replies to your post.

Lakmal

 

Mr. Phil Corso,

Please elaborate the situation of open neutral situation in this case? When and how it will occur? And please explain to the point like what if analysis.

Your languages are really very high type...not understandable to me.

As the system is small and low voltage, I think earth fault level is not a matter of concern here.

 

Gentlepeople... if anyone would like a schematic drawing illustrating my concerns, contact me at:

[email protected]

<b>Moderator's Note:</b> you might want to give them the original as well so they can compare them.

 

Barindra...

You concluded that the closure of the neutral-contactor is meaningless, except when only the Genset is carrying load.

Please explain, then, the false tripping of the Genset's ACB. It is my conclusion that it will will occur if the utility's Xfmr is 'Energized', but its ACB is open.

The cause is the fact that both Utility-Xfmr, and the Genset's neutral points are still connected together via what is called a "sneak" path. the conductive ground/earth path. This 'path' leads to GFP current-imbalance.

Regards,
Phil Corso

 

ModSquad... I did send the OP's schematic to those that requested it!

Regards,
Phil

 

Dear Phil,

Actually,

The REF protection may operate (restricted earth fault), which is a kind of differential protection. To avoid that, it is recommended to engage the neutral line CT at GEN terminal box or before the junction point of earth & neutral line at GEN side.

Also, During parallel operation,
Because the current will divide into two path hence earth fault protection will operate at higher than its set point.
R's

 

Barindra, your comment "because the current will divide into two path" is exactly what I said in my 22-Dec post, point b). So, you do understand me after all. However, you still haven't explained false tripping of the Genset's ACB.

Phil

 

Gentlepeople... in conclusion, the GFP operates falsely because of current imbalance:

Consider the GFPs. Whether it's provided using the residual-connection method (3-Phase CTs) or the window-type method (encompassing all 4 phase and neutral conductors), an error always exists.

The error... the neutral-path and earth-path are in parallel, causing a circulating-current flow between neutrals of both the Utility and Genset.

Phil Corso

 

But he said when the Genset and Grid is synced, neutral contact is Open. Then how there can be circulating-current flow?

> The error... the neutral-path and earth-path are in
> parallel, causing a circulating-current flow between
> neutrals of both the Utility and Genset.

 

Lakmalp... you have missed an important fact.

When the connection between the Genset's neutral-terminal and the Load's Neutral-bus is open-circuited (via the single-pole contactor), the Utility-neutral and Genset-neutral are still connected.

Note the line labeled "E" at the bottom of Doob's schematic! It is connected to the Ground/Earth bus, and it is a conductive metallic path.

Note, also, the current-path between the Genset's neutral-terminal, the Load's neutral-bus, the Utility's neutral-terminal, and the Load's neutral-bus.

That the Genset is still connected to the load, thus unbalancing the GFP.

Phil

 

To all requesting additional info... Doob mentioned that the Genset GFP measures the sum of phase-currents L1, L2, & L3, but Neutral-current is not measured.

Instead it's taken as the 'residual' or unbalanced sum of phase-currents. Of course with the Neutral-contactor open, its share of Neutral-current via the neutral-bus is zero.

But, Genset Neutral-current still flows... via the ground/earth link. Although it's not 'seen' by Genset GFP, it adds to the Utility's contribution of the Load's Neutral-current.

Thus, false-tripping of the Genset ACB is related to unbalanced phase-currents! And, that should be easy to corroborate:

1) Has the ACB tripped with the Neutral-Contactor closed?
2) Is there current-flow in the Genset Neutral-terminal?

Phil

 

Hello!!

So I think it's safe to say you guys have the idea of this setup now.

To answer the last two questions. No it doesn't trip when the contractor is closed and yes there is current measured in the ne connection.

However I have another question.

Say I "remove" this connection from the alternator star point to ground. And run the generator in parallel with the mains supply all (mains 3 phases and neutral). generator only l1 l2 and l3 connected neutral open. How do you think the system would react with regard our unbalanced site load.

Is it that the mains tx will provide us with the balancing action we require?

Thank you guys!

 

Doobs

I sent the SLD illustrating why the Genset GFP trips to you. With the 'Neutral' contactor closed the system becomes a 3-loop symmetrical network and the Genset neutral-current is the sum of its contribution to load neutral-current and its ground-path current.

With the contactor open a 2-loop un-symmetrical network results. There is no Genset contribution to load neutral-current. And, the ground-return path is virtually in series with the Utility's neutral-connection. This unbalances Genset line-currents tripping the GFP because it only measures phase-currents (excluding neutral current).

There is a relatively simple fix, but more info is needed. Please provide Line, Neutral, and Ground-currents for both the Utility and Genset. Include data for several operating conditions: a) Utility only; b) Genset only; and c) parallel operation.

Regards,
Phil Corso

 

Doobs...

Four more questions:

1) What type of Neutral Contactor is being used?

2)What is Utility Xfmr kVBA rating?

4)What is Genset KW or kVA rating?

5) Interrupting-Duty of Swgr?

Phil

 

Hi Phil thanks for your reply.

I will get the info ASAP.

However just for the time being. They system has issues with the nuisance gfp tripping. Let's just take that out of the equation for a second. I'm curious about this scenario.

Gen set running in parallel with no neutral or star point connected to the system at all. The mains has its star point and neutral connected as normal. Will the single phase loads only be supplied via the mains and the gen set will export only a balanced load. IE the generator cannot supply the single phase / unbalanced portion of the load.

Kind regards.