While there are numerous physical analogies for this quantity
called reactive power, one that is reasonably accurate is the process of
filling a water tower tank with water - one bucket at a time. Suppose you want
to fill a water tower tank with water, and the only way that you can do that is
by climbing up a ladder carrying a bucket of water and then dumping the water
into the tank. You then have to go back down the ladder to get more water.
Strictly speaking, if you simply go up a ladder (not carrying anything) and
come back down (not carrying anything), you have not done any work in the
process. But, since it did take work to go up the ladder, you must have gotten
all that energy back when you came down. While you may not feel that coming
down the ladder completely restores you to the condition you were in before you
went up, ideally, from an energy conversion viewpoint, you should! If you don’t
agree, get out your physics book and check out the official definition of doing
work.
OK, if you still
don’t agree that walking up a ladder and coming back down does not require any
net work, then think of it this way. Would you pay anyone to walk up a ladder
and back down without doing anything at the top? Probably not. But, if they
dumped a bucket of water in the tank while they were at the top, then that
would be something worth paying for.
When you carry a
bucket of water up the ladder you do a certain amount of work. If you dump the
water at the top and carry an empty bucket down, then you have not gotten all
your energy back (because your total weight coming down is less than going up),
and you have done work during that process. The energy that it takes to go up
and down a ladder carrying nothing either way requires reactive power, but no
real power. The energy that it takes to go up a ladder carrying something and
come down without carrying anything requires both real power and reactive
power.
A reminder here is
that power is the time rate of energy consumption, so consuming 500 Watts of
real power for 30 minutes uses 250 Watt-hours of energy (or 0.25 kilowatt-
hours which costs
about 2.5 cents to generate in the U.S.). The analogy is that voltage in an AC
electrical system is like the person going up and down the ladder. The movement
of the water up the ladder and then down into the tank is like the current in
an AC electrical system.
Now, this
pulsating power is not good in an electrical system because it causes
pulsations on the shafts of motors and generators which can fatigue them. So,
the answer to this pulsation problem is to have three ladders going up to the
water tower and have three people climb up in sequence (the first person on the
first ladder, then the second person on the second ladder, then the third
person on the third ladder) such that there is always a steady stream of water
going into the tank. While the power required from each person is pulsating,
the total result of all three working together in perfect balanced, symmetrical
sequence results in a constant flow of water into the tank – this is why we use
“3-phase” electrical systems where voltages go up and down in “sequence” –
(first A phase, then B phase, and finally C phase).
In AC electrical
systems, this sequential up/down pulsation of power in each line is the heart
of the transmission of electrical energy. As in the water tower analogy, having
plenty of water at ground level will not help you if you cannot get it up into
the tower. While you may certainly be strong enough to carry the bucket, you
cannot get it there without the ladder. In contrast, there may be a ladder, but
you may not be strong enough to carry the water. However, the people do take up
room around the water tower and limit how much water can go up and down over a
period of time - just as reactive power flow in an electrical system requires a
larger current which limits how much real power can be transmitted.1
To make the system
more reliable, we might put two sets of three ladders leading up to the tank on
the tower. Then, if one set fails (maybe the water plus the person get too
heavy and the ladder breaks), the other set picks up the slack (that is, has to
carry more water). But, this could eventually overload the second set so that
it too fails. This is a cascading outage due to the overloading of ladders.
No comments:
Post a Comment