Voltage Unbalance Reason

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mrmuneeb

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dear All,

Hi, I am having a problem of unbalanced strange values of system voltages in of our power generation plant,
the disturbance in voltage is being reflected on Low voltage side only,

system diagram is attached.

View attachment 20244

what the system must show at low voltage side is;
1. P-P 480V
2.P-N 277V

but the current scenario on delta side of transformer is;

L1-L2= 316 V
L2-L3= 147 V
L3-L1= 221 V

L1-N= 594 V
L2-N= 599 V
L3-N= 530 V

neutral is kept floating on both sides, i.e. both LV & MV neutral is floating.
is there any fault in on of the system phases?
 
There seem to be two separate problems, unless you can somehow identify a coupling between the two.

1. The relationship between the P-N and P-P voltages is not possible for a three phase system with equal 120 degree rotation from phase to phase. If you draw the phasors out on paper with scaled lengths I think that you will find they match if you have roughly 550V for each P-N but one of the three windings of the star is reversed in polarity. That will leave the P-P voltages smaller than the P-N voltages and will have one P-P voltage approximately twice the other two, since the end points of the three phasors are closer to a center tapped single phase system than to a proper star.
Possibly the leads were incorrectly identified when the generator was manufactured or rewound or else somebody simply miswired them.
The problem would be obvious if you were able to look at the voltages on an oscilloscope (always triggered off one phase) instead of just measuring the amplitude with a meter.

2. The P-N voltages are far too high. This might be explained by the error in 1 wreaking havoc on the operation of the voltage regulation system.
 
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Ungrounded Neutral Symptions

Ungrounded Neutral Symptions

Is this a new installation? If not has there been any electrical work (PM/ routine NETA testing, equipment replacement/ rewiring, etc) been done in the problem area lately? Has it ever operated properly?
I agree that the P-P voltages on the Delta winding are a problem. What is confusing is you state neutral is kept floating on both sides, i.e. both LV & MV neutral is floating. But since the gen winding is Star connected then by code the Xo must be grounded at the gen for proper voltage output. If this connection is loose or non-existent then the P-P and P-N voltages would be unstable and all over the place just as you show.
As Ron stated earlier I would check this connection on both gens.
Were these measurements taken with the gens in parallel or separately? I would find it hard to believe they would even parallel with these voltage readings.
The only other area I would suspect are the gens voltage regulators. Load bank testing would, for sure, weed these out.
 
was it running normally before and this problem just occured?
or was this identified during commissioning?

L-N ~ 550-600
Ph-Ph 150-300
???
not sure how that can be

has each gen been isolated/measured?
have the gen winding connections been verified?
 
Ingenieur said:
was it running normally before and this problem just occured?
or was this identified during commissioning?

L-N ~ 550-600
Ph-Ph 150-300
???
not sure how that can be

has each gen been isolated/measured?
have the gen winding connections been verified?
Click to expand...
See post #6 for how it can be. :happyyes:
 
he has the same set up on the other sets
n is floating, but the others read ok

has it ever worked correctly?
is it a new install being commissioned?

the only way I can see those magnitudes
l-n ~600
l-n ~ 150 with a 300
is the neut having 600/0 +/- 15-20 deg phasing vs +/- 120 deg
no idea how you would get that
 
mrmuneeb said:
dear All,

Hi, I am having a problem of unbalanced strange values of system voltages in of our power generation plant,
the disturbance in voltage is being reflected on Low voltage side only,

system diagram is attached.

View attachment 20244

what the system must show at low voltage side is;
1. P-P 480V
2.P-N 277V

but the current scenario on delta side of transformer is;

L1-L2= 316 V
L2-L3= 147 V
L3-L1= 221 V

L1-N= 594 V
L2-N= 599 V
L3-N= 530 V

neutral is kept floating on both sides, i.e. both LV & MV neutral is floating.
is there any fault in on of the system phases?
Click to expand...
The vector sum of line or phase voltages, whether balanced or not, of the generator should equal zero i.e they form a closed triangle. If it is not so with your readings i.e a closed triangle is not possible even approximately, it means the phase angle between phase windings are not correct i e the winding issue.
 
ATSman said:
Gentlemen
pls re-read post #8 that addresses these issues
and comment back
Click to expand...

a floating neut will not make v l-n 3 x v l-l
either he made a posting error and v l-l is 600 and v l-n 150-300 or there are major connection issues

the ONLY way for this to occur
v l-n for a, b, c ~ 600/0, +15-20, -15-20 deg

this will give a l-l using a ph ~ +/- 17 deg
v a + (- v b) = vab
va = 600 + 0j
vb = -(575 + 175j)
vab = 25 + 175j = 175/82
similarly vac ~ 175/-82

vbc
0 + 350j ~ 350/90
 
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Sahib said:
The vector sum of line or phase voltages, whether balanced or not, of the generator should equal zero i.e they form a closed triangle. If it is not so with your readings i.e a closed triangle is not possible even approximately, it means the phase angle between phase windings are not correct i e the winding issue.
Click to expand...
If vector sum of line or phase voltage readings of the generator by OP does not equal zero ie they do not form a closed triangle even approximately, it implies erroneous readings or a break in generator windings.:)
 
+(vab = 25 + 175j) or 175/82
-(vac = 25 - 175j) or 175/-82
-(vbc = 0 + 350j) or 350/90
= 0 + 0j

something does not compute
one machine in reverse rotation
messed up connections
etc or some combination
 
I don't understand, if the neutral is kept floating, then aren't any line to neutral voltage readings meaningless?


If the voltage readings are true, I can't really wrap my head around it either except to speculate that a neutral and a phase are reversed or one generator has its windings in series rather than parallel. I would compare connections to the other six generators that are working fine, try to rule out any obvious errors
 
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