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Generator out of sync.

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torhav

New Member
Could you please help me solving these two questions?

1: A generator is connected to a ships main supply. The generator is out of sync 10 degrees. The generators voltage (6.6kV) matches the voltage in the main supply. What is the voltage on the breaker?
2: What happens if the generator is connected with opposite fases?

I have googled the subject for hours, but can not find how this could be solved. For generators in series I can find the solution, but I guess this is a case where the generator is connected in paralell?
 

user_88

Member
If I may suggest, delete the post in this section, and repost it in the Electonic Chat section of this forum.
 

Sceadwian

Banned
It's not really an electronics problem user 88 it's a math problem, this is the right forum.
 

john1

Active Member
Hi Torhav,

I will certainly try to answer your questions.

1: A generator is connected to a ships main supply. The generator is out of sync 10 degrees. The generators voltage (6.6kV) matches the voltage in the main supply. What is the voltage on the breaker?


This seems an odd question.
It could be that there is a mistake here somewhere.
You see, if the generators breaker is closed then the voltage would be the same either side of the breaker.
So the voltage on the breaker is 6.6kV

The current from the generator can be 'out of phase' and this shows on the power factor meter. If its in reverse then it may have to be corrected within a given period, as some arrangements are set to reject the generator taking power from the main supply, and may trip it out.

The current from generators is always out of phase, the power factor on most generators is around 0.7 roughly. (0.6 to 0.8)
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2: What happens if the generator is connected with opposite fases?


Again, an odd question.
I am going to guess that you mean what would happen if the breaker is closed when the incoming voltage(s) is/are 180 deg out of synchronisation ?

In that case, there would be a rather loud bang. Various fuses would blow.
The breaker would have to be taken out of service for overhaul. The service electricians would be brought in to check lots of things. The operator who was in charge of the switching would be questioned, if he/she was at fault they might be reprimanded or sent for re-training.
There would be a jolt of some sort through the ships electrical system, and many sections would have to be subsequently checked out to make sure that they are Ok.
This sort of thing has happened before, and no doubt will again.
There will be a procedure and probably a list of checks for the service electricians to go through.

Most latter day generators are made to synch up and go on line from their own control units. It is unusual these days to have manually operated switching on these powerful 6.6 kV units.

*********************************************

What has prompted these questions ?

John :)
 

Sceadwian

Banned
It's not an odd question, it's a couple questions on someone's test. Almost all 'aptitude tests' involve totally made up but real world relevant questions. The questions seem to be in relation to some established system 'on a ship' which he probably has as a reference to.
 

be80be

Well-Known Member
I wouldn't want to be there when you slap it out phase I have seen some small generators ripe the engine off
1: A generator is connected to a ships main supply. The generator is out of sync 10 degrees. The generators voltage (6.6kV) matches the voltage in the main supply. What is the voltage on the breaker?
Now I'm not that good at the math but I no for fact a generator 10 out will be pulled in phase then you bring it on line. And I would speed the generator up to bring it in to phase so if its out by 10 it not going to put the full 6.6 kV on line Your going to have a lost due to phase shift
 
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torhav

New Member
Sceadwian: You are almost right. I am doing some self studies (full job and two kids) to get a Bachelor degree. This is what you can call home work...

The generator is not correctly synchronized with the main system, but both the frequence and voltage is the same.
I think I might found the answer to problem 1:
delta U=(6600/squareroot 3)*sin (10/2)
delta U= 332,1V

Voltage on breaker in the moment the generator is connected to the main system: 6600V-332V=6268V

Does this sound right?
 

be80be

Well-Known Member
Like I said I don't remember all the math I was looking for my book that i have but that looks right. I do no for sure it less by how much it's out. I was going to post how you get
the figure to day when I go to my shop. And to bring it in phase you speed up the generator which would rises the volts to 6.8 or 6.9kv
 

Sceadwian

Banned
Looks right, but I'm the wrong person to ask about math problems =)
 

be80be

Well-Known Member
I came up with 366.66volt drop by being out of sync 10 degrees have about 36.66 volt drop per 1 degree below sync.
 

Hero999

Banned
This looks like a homework to me.

Hint: use phasers or complex numbers
 

Sydney

New Member
As an electrical officer in the navy.............a petty officer tried to allign with the shore power supply................nearly blew our gen out of it's bed
 

Warpspeed

Member
As an electrical officer in the navy.............a petty officer tried to allign with the shore power supply................nearly blew our gen out of it's bed
Hehehe, I spent some time down in the Antarctic a very long time ago.
When the diesel guy that ran HIS power station was not available, I as the radio guy used to look after the power generation for him.

Anyhow, starting up and connecting large power generators on line can be pretty dramatic if you close that breaker, and the voltages are even very slightly different or slightly out of synch. Very scary.
 
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