3phase generator quiery 1

[Jamie9241]

first of all thanx to anyone who has taken the time to read this. I have a theoretical question i dont have any data it is just a general quiery. I have a friend who is doing a job the job is near a farm they have created something similer to a football pitch, what they requier us to do is install 7meter lighting collumbs with 2 x 400watt son light fitting to this thier will be 12 lights in total. The problem is because it is a remote location it is going to be supplied via a 3phase generator like i said earlier i do not know the size but they say it is large. Now some one i was talking to at work advised us to put all the load on 1 phase the reason being they said is that if you lose the neutral conductor you would have 400v acrross the load. I dissagread with this and said realy it should be ballanced across all phases. my question is ,is thier any problem or potentiol to cause damage to the generator by just loading one phase i think the load would be round 30 amp,

angain thanx for any feed back

 

[davidbeach]

Wow! Do you think that maybe you could try again with complete sentences, proper grammar, proper punctuation, proper capitalization, and correct spelling? That was way too painful to attempt to read.

 

[ScottyUK]

Jamie must be from the UK to have such poor written English...

Balance the load across the phases, connecting the luminaires line-neutral. Putting all the load on one phase is a bad idea for at least two reasons:

1) You will need a larger generator larger because the genset rating assumes a balanced 3-phase load. Unless you have money to burn that doesn't make economic sense.

2) Unbalanced loads in three-phase machines cause negative phase sequence heating of the rotor, which eventually kills the genny. That is generally considered to be a bad thing.

Sodium lamp ballasts are usually horrible loads with high inrush, distorted load currents and a lousy power factor. Don't skimp on the size of the generator - I would suggest at least 30% larger than the connected load, and try to avoid switching all the lamps on together. Check the luminaire for the actual current draw - if you just add 12 x 400W you will end up with an undersized set.


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

 

[waross]

What Scotty said.
In addition:
Is it safe to assume that this is a star connected generator?
Is it safe to assume that the ballast voltage equals the line to neutral voltage of the generator?
If these assumptions are inoperative print a response and we will go from there.

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

 

[Jamie9241]

Thanks for your feed back. Scotty UK you are correct in assuming that I am from the UK. Reading back over what I wrote I can see how bad the grammar is and apologise for that, usually I only write like that when it is my notes. I was in a bit of a rush and had promised to my friend who is doing the job a second opinion a,s,p.
Warross referring to what you said you are correct in your assumptions.
I am an installation electrician so my knowledge on generators is very minimal.
Now referring back to my original question each lighting coulomb will have some means of switching. When the generator has been started they will then switch each coulomb on one by one so we are avoiding dumping the entire load on the generator at once. What my concern is that if all the load is just on one phase will this reduce the life of the generator. Something that I didn’t mention is that they already have the generator it is built into some sort of agricultural tractor. As I mentioned in my first thread I have no data on the generator but if it is assumed that it is capable of supplying the load via one phase for this question.

Once again I thank you for your time in reading this and any feedback.

 

[waross]

It sounds as if your friend is a single phase electrician on a three phase job.
I have great respect for Scotty. We do have different backgrounds and experience. Scotty works with very large generators and most of the generators that I encounter would be too small to supply excitation to Scotty's machines.
When I disagree with Scotty it is usually a matter of scale.
The smaller machines withstand rotor heating better than the large machines. The heating is there but the small machines are better able to reject the heat without damage. I mention this so that you may avoid confrontation with the person who has seen a small three phase generator successfully loaded on only one phase.
But to add to his other points:
Dual voltage ballasts usually have a 2:1 voltage ratio and the phase to neutral voltage ratio is 1.73:1

The best installation will be to run 3 phases and a neutral to the distribution point and distribute the load equally on the three phases. Ballasts produce harmonics so the neutral should be the same size as the phase conductors.
FYI and to reinforce this point, the Canadian code prohibits reducing the size of the neutral when feeding ballasted lighting loads.
This method will require less copper than a single phase installation.
If you have only single phase distribution equipment available, you have three options:
1> Use a double delta connection to convert the machine to single phase output.
2> Use a zig-zag connection to convert the machine to single phase output.
3> Put the load on two phases connected line to neutral. The neutral current will equal the line current.
In all three cases the neutral should be full size.
A final note. All of these options will develop the rotor heating that Scotty mentioned. None will generate as much heat as loading across only one phase. Two of these connections have been used by major manufacturers to convert small three phase machines from three phase to single phase and will reject the extra rotor heat without damage. (Zig-zag is less common than it once was due to the symmetry of the double delta connection.)


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

 

[ScottyUK]

Hi Bill,

We don't disagree that often do we? I find your posts very helpful for the smaller sets we use as standby power and for temporary supplies. You're absolutely right that a small set will withstand negative sequence heating better than a big one: percentagewise a small set will tolerate levels of load imbalance which would seriously damage a big utility class machine. Jamie should probably listen to you since you play with diesel sets more than I do - I might just chip in on some of the UK code requirements! I think we have very similar ideas for how to implement this.


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

 

[ccjersey]

It's hard to say about the ballast taps, they could be almost anything. In the US, 120/240 and 120/208 are both common as well as "universal" models with taps for 120-277 volt supplies.

The available taps will determine the connection, but you should try to balance the load as much as possible across all three phases by connecting either line-line or line-neutral as needed. With a certain generator, using line-line (high voltage)connections would cut the size of the wiring required to cover the distances involved in lighting an athletic field. It would probably be best to buy the lights with taps for the line-line voltage the generator produces and save on copper.

6 poles with 2 lights each would divide across 3 phases quite well!

 

[TurbineGen]

I second ccjersey's post. Why is your friend afraid of losing the neutral?

In an ideal world with 4 lamps per phase you load would be evenly balanced and theoretically you would have no neutral current. Now in the real world, there are harmonics and other distortions associated with ballasts which will likely return harmlessly via the neutral conductor. ccjersey may have a point with going to a line-line voltage. 1600 Watts of lighting per phase may have some significant voltage drop issues depending on how far away the load is from the generator.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[Jamie9241]

Thanks for all your feedback. In response to ccjersey the ballast taps for high pressure sodium lighting is generally 230v or 240v L-N in the UK. I totally agree with the idea of balancing the load across the 3 phases.

The reason that this other person gave for only distributing the load across one phase is as I mentioned in my earlier thread. If for some reason their was a failure in the generator and the neutral conductor was lost, then their would be 400v applied across the load if the lighting was distributed across the 3phases.

This could be said for any 3phase installation thou, unless he has had previous problems of a similar nature using light duty 3phase generator sets I do not understand why he has advised my friend to do it this way.
Anyhow I thank you for your time and will forward this thread to my friend.

Regards Jamie s

 

[waross]

In any system, the loss of a neutral results in a voltage division that is related to the impedance of the loads on either side of the neutral. In a single phase system the voltage can approach 2 times the rated voltage.
In a three phase system the voltage can approach 1.73 times the rated voltage.
The loss of a neutral in a system constructed with any kind of workmanship at all is very rare.
When someone is over their head, they often give invalid reasons for their choices.


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

 

[Jamie9241]

Yes well we had an 80 bed room nursing home were we did the electrical installation work and we had major problems their.
When the utility company installed their supply they didn’t make sure that the neutral connection was correctly tightened. It was fine for a bit but over a month or two it burnt out due to the bad connection or be it high resistant connection.
Due to loss of the neutral and it being a 3 phase system you can guess what happened.
All the single phase equipment i.e., computers, servers, telecoms, security equipment all was destroyed. When they accessed how much it would cost to replace it totalled over £50,000 worth of damage. Was a very expensive lesson.

 

[TurbineGen]

The nursing home is a far more complex system than what you have in your lighting. I can easily see a cascaing chain reaction destroying the equipment with a loss of neutral. The loads in the nursing home were likely not balanced all of the time, thus causing voltage changes in the system and destroying sensitive equipment. However, you are installing a (essentially) balanced lighting system. The only way I can see a real proble is if you were to lose a neutral and have a few burned out bulbs... maybe. And even then, with this chance being quite remote, how costly would it be to replace a few lights versus buying a larger generator to power it all on one phase?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.