Operating Electrical
Generators in Parallel
One of the most delicate and intimidating aspects of
operating a power plant can be synchronizing to the grid. This is especially the case for
someone new to cogeneration or power generation. I’ve seen seasoned and knowledgeable
operators turn into frightened puppies at the mere mention of this evolution. Lack of
training on the subject, rather than lack of confidence usually cause this situation. Many
a gray hair can be avoided if the situation is explained properly. That is just what we
are going to try and do today.
You see, operating a generator is much like the operation
of a car.
NOTE: For the purposes of this lesson, we will assume that your turbine, when carrying
load, is always connected to some larger source, like a utility power grid. There is more
to talk about if your plant can run isolated from the grid, but that is for another time.
A modern turbine generator has a speed control. So does a car. To accelerate a car from 0
to 60 mph, you press on the gas. In simple operation, to increase the speed of a generator
from 0 to 60 Hz, you do the same thing, usually by depressing the speed increase button.
This allows more fuel (or steam) into the turbine causing it to accelerate.
"Gee Thanks Prof. Could not have figure that one out on my own...duh?" Okay. I
realize that this might seem obvious, but things can get confusing when paralleling to the
power grid. Cause ya see, once on the grid, you can push that button all day and nothing
will happen to your speed or frequency. Why?
We’ll get back to that. First, lets parallel. You increase your speed to match the
grid. 60 Hz usually. Now you turn on your synchroscope. This is a dial and/or lights which
will tell you when you are "In Phase" with the grid. You want the scope rotating
slow in the fast direction....this is the phrase that gives you a headache.
"Slow" in the "Fast" direction? Okay. All this means is that when the
dial on the scope is rotating in the fast direction, your turbine speed (frequency) is
higher than the grid. This is good. This means you will pick up load when you synchronize.
It is like merging onto the highway. You have to "Synchronize" your speed to
that of the other cars, but you usually want to be going a little faster, especially if
traffic is heavy. If you come into 60 mph traffic at 50 mph...well...its gonna hurt.
Likewise, you don’t want to merge at 70 in heavy traffic doing 60. That would hurt
too. You wanna be just above 60. Synchronizing to the grid is the same. You are matching
phases (speed) with the power flowing on the grid (traffic). This is where
"Slow" in the "Fast" direction comes in. The fast direction shows that
your turbine is faster than the grid, so you can "merge" properly. The
"Slow" means that you are not going too fast. This will ensure a smooth
transition to the grid (highway). Rotating in the "Slow" direction would mean
that your turbine is going slower than the grid. This could cause you to reverse power
when you shut the breaker.
Okay...so how come once you are on the grid, you can’t
change speed. I said we would get back to that, so here we go.
The reason is that most cogeneration plants have generators which produce anywhere from 1
to 50 MW, but most power grids are producing thousands of MW’s all at 60 hz. Since
speed and frequency in a generator are functionally identical, you are never going to
increase your wimpy 5 Mw turbine’s frequency beyond what the big bad grid’s is
at. You will however pick up load. When you synch to the grid, as you press the increase
speed button, your frequency will not change, but the electrical load your unit is
carrying will increase.
Imagine being on the highway in your brand new Miata. Nice little car. Now imagine a huge
tractor trailer is in front of you on a single lane road. He’s doing 60 mph. This
upsets you. You wanna do 70. Lets assume you are a little nuts (Maybe its not your car.
Maybe it is borrowed and you really don’t like the guy anyway). Your gonna try and
push the trailer.
You ease on up behind him, touch his bumper, and slam the pedal to the metal. Nothing
happens right? Why? Because your little 4 banger cannot push that truck any faster than he
wants to go. You will however "Pick Up Load" from the truck. The amount of
energy your Miata is expending will have a minute effect on the amount of load being
placed on the truck engines. It will be using less energy to carry its several tons due to
the fact that you are pushing him from behind.
In reality, you will ALSO increase his speed, but it will be so slight, neither of you
will probably notice. What you will never do is get him to 70 mph. This goes the same for
your generator. As you increase speed on your turbine, you will pick up load from the
grid. This will have some effect on the frequency (speed) of the grid but it will be so
minute as to be imperceptible.
So....your turbines speed control will cause changes in your turbine generators speed when
you are not on the grid, but will change turbine LOAD when synchronized.
