Re: power-grid physics

[James Frysinger <frysingerj@COFC.EDU>]

Hugh, it is damned important to match phases when paralleling generators! I 
have seen steam-driven turbine generators paralleled as little as 10 degrees 
out of phase and those puppies jumped on their mounts like a cat dropped into 
a bathtub of ice-cold water. What was really scary was that those large, hgh 
pressure steam lines (pipes) feeding the turbines had to flex and jump with 
them!

Let's say that a generator is configured to provide 3-phase power in a 
wye-configuration. That is perhaps the most common configuration in industry 
and it means that one end of each of the three generator windings is 
connected to one point (the "neutral" point which is usually grounded). Now 
let's say that each phase is set up to provide a potential of 1000 V. The 
"phase-to-ground" voltage is then 1000 V and the "phase-to-phase" voltage is 
equal to that times the square root of three, or 1732 V.\

If the generator is paralleled in phase to the system, then its phase A 
instantaneous potential equals the system's phase A instantaneous potential 
and so at that instant of breaker closure there is a zero volt difference 
across the breaker points.

If, however, that generator is paralleled 120° out of phase, there will be an 
instantaneous 1732 volt difference across the breaker points. Worse (yep, 
worse!), since AC generators have synchronous style windings the oncoming 
generatore tries to *immediately* jump ahead or back by 120° of phase 
rotation to get into synch. It's a sight to behold --- from as great a 
distance as is possible. And of course, if the paralleling is done 180° out 
of phase, then the voltage difference across those zero-ohm contacts is 2000 
V and the generator has to make up its mind whether to jump ahead or to jump 
back by 180° of phase rotation. Imagine the torque!!!!

One brings a generator online by running it at a slightly higher speed 
(frequency) than the system and the operator "tells his hand" to flip the 
control switch to shut the breaker just before the generator is in phase. 
With practice, the generator is actually in phase then following operator 
reaction time and breaker armature time of travel. In commercial plants I 
would imagine that they have control circuits do this rather than human 
operators. However it's done, the breaker is commanded shut when the oncoming 
generator is "[say,] five before 12 o'clock and moving in the fast direction" 
on the synchronization meter. That meter is a synchro-driven meter with two 
sets of windings, one for the system and one for the generator. Thus, the 
meter output (needle position) represents the phase difference and "12 
o'clock" is "in phase". (In the olden days, there were three lights and the 
operator watched the flicker of the lights to determine phase differences and 
match-up --- yowza!)

I can guarrantee that your flight engineer did it that way. Either that or he 
did some brig time. The same principles apply and the same procedure is used 
for 400 Hz systems as for 60 Hz systems. Been there, done that at both those 
system frequencies.

Jim

On Wednesday 2003 August 20 21:54, you wrote:
> This is an area about which I know almost nothing, but I do have one
> piece of information that may or may not be relevant, a memory
> triggered by the comments that the generators are not phase-locked.
> When I first finished navy flight training, my assignment was to a
> squadron flying Constellations. These aircraft had a 400 Hz ac
> electrical system. It was powered by a an alternator on each of the
> four engines. To insure frequency matching, each alternator was
> driven through a constant speed transmission. On engine startup the
> alternators were not brought on line until they were up to speed. All
> this was done by the flight engineer and I never had to actually do
> it. But my recollection (which could easily be faulty since this was
> 45 years ago) was that the process of bringing the alternators on
> line was tricky since they had to be phase-locked, or the electronics
> on board would go haywire.
>
> If my memory is correct, and phase-locking in this admittedly small
> system was important, why isn't it important in the large scale power
> grid?
>
> Hugh
> --
>
> Hugh Haskell
> <mailto:haskell@ncssm.edu>
> <mailto:hhaskell@mindspring.com>
>
> (919) 467-7610
>
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-- 
James R. Frysinger
Lifetime Certified Advanced Metrication Specialist
Senior Member, IEEE

http://www.cofc.edu/~frysingj
frysingerj@cofc.edu
j.frysinger@ieee.org

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