Post by Big Bad Bob
Each week, the Diesels would be fired up for 30 minutes or so for
testing. There was a big meter mounted between the Diesels. Apparently,
the needle had to be centred, when the Diesels had reached an in-phase
operation. Then they could be switched in. I asked what would happen if
they were switched when not in phase. Apparently all the trainees asked
that question. No one knew for sure and they didn't want to find out.
Probably a lot of structurual damage would occur.
Well it's kinda like this...
When I was in the Navy I was qualified to run the nuclear reactor on a
sub. This included being qualified to run the electric plant as well
(turbine generators, motor/generator sets, and a diesel generator mostly
for emergencies). The phase meter you refer to is used to parallel two
3-phase generators. The Navy used a manual control system that was
relatively foolproof, somewhat universally throughout the fleet.
Submarines were pretty much the same that way.
If you plot the sine wave patterns for the generator output vs power
already on the bus (let's say with an o-scope), in order to minimize the
inevitable current spike you get when shutting the generator's breaker,
the output and the line have to be in phase and at the same AC voltage.
Ideally it's a zero volt difference, but in practice the best way to do
a) speed regulator on the engine has a "load curve" so that increasing
its setting may keep the same frequency, but takes up more load from the
other generators and utility power.
b) The incoming generator is moving slightly faster. As the phase
needle spins clockwise, you time shutting the breaker [which has a delay
- you get used to how long that is] so that it physically shuts the
moment the needle is at the top [in phase].
c) once the breaker shuts you immediately put load on the generator by
increasing its speed regulator setting. You don't want a turbine
"windmilling" nor an engine being driven by incoming power for any
length of time. [some generators might have reverse-power trips on
their breakers to prevent this]
d) when you 'secure' the generator, you decrease the speed regulator
setting until all the load is off, then pop open the breaker just as it
Hopefully that explains how those phase meters work and why they are there.
Now, for the question "what could happen if you do it wrong"... well, in
the worst case scenario (180 degrees out of phase)
a) melt the contacts on the breaker, hopefuly not causing a phase-phase
short in the process
b) start a major electrical fire (buswork or generator itself)
c) physically torque the generator and damage the shaft and/or windings
d) snap the crankshaft on the diesel generator
e) torque the engine/generator so hard they either separate from one
another, or jump off of their mounts and damage things while rolling
around the room
Yes. You do NOT want to parallel them out of phase. EVAR. It is worse
than suddenly putting a shorting bar across all 3 phases simultaneously.
(aka 'Bombastic Bob' in case you wondered)
'Feeling with my fingers, and thinking with my brain' - me
'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"
SO - you was a squid, huh? Me too - avionics technician. My class
the tubes. That was in 1968. Those old training rigs were really
each end. The instructors had a big box of BAD components
skills in order to find the fault. Sometimes, you found it. ;-) I wonder
what happened to all those old training rigs...