Minute Lectures
Four analogies to explain reactive power
Why an analogy?
Reactive power is an essential aspect of the electricity
system, but one that is difficult to comprehend by
non-experts
By presenting four different analogies, we hope the
reader will
• For non-experts: develop insight in the phenomenon
• For experts: acquire ideas to explain the phenomenon
Of course, none of these analogies are 100% correct
Minute Lectures
Four analogies to explain reactive power
What does the analogy need to
explain?
The difference between active power (W), reactive
power (VAR), and apparent power (VA)
The idea of compensating reactive power
Why reactive power increases energy losses in the grid
Why reactive power limits the capacity of cables and
lines in the grid
Minute Lectures
Four analogies to explain reactive power
I. The bicycle analogy (1/3)
Power stations, producing electrical energy, are
represented by bikers
At the backseat of the bike there are passengers, the
consumers of electrical energy (the loads)
Minute Lectures
Four analogies to explain reactive power
I. The bicycle analogy (2/3)
A reactive load can be represented by
a passenger leaning to one side
The fact that the passenger is leaning
to one side, does not influence directly
the work that the biker has to deliver,
but without compensation, the bike
might fall over
Minute Lectures
Four analogies to explain reactive power
I. The bicycle analogy (3/3)
The biker compensates the movement
of his passenger by leaning in
opposite direction (= by generating
inductive power)
Consequences:
• A pedalling figure leaning to one
side cannot work as comfortably as
before (Æ limiting capacity)
• The bike catches more head wind
(Æ extra losses)
Minute Lectures
Four analogies to explain reactive power
II. The horse-and-boat analogy
(1/4)
Take a boat on a canal, pulled by a
horse at the bank
Minute Lectures
Four analogies to explain reactive power
II. The horse-and-boat analogy
(2/4)
The fact that the horse is not walking straight in front
of the boat, does not influence the work it has to do
to pull the boat. But without compensation by the
rudder, the boat will be pulled towards the bank of
the canal.
Consequences:
• The turned rudder leads to extra losses
• The fact that the rope is pulling at the flank of the
horse and not straight behind it, limit’s the horse’s
capacity to deliver work
Minute Lectures
Four analogies to explain reactive power
II. The horse-and-boat analogy
(3/4)
Minute Lectures
Four analogies to explain reactive power
II. The horse-and-boat analogy
(4/4)
The vector representation of the force to pull the boat,
is similar to the vector representation of power in
an electric system:
Minute Lectures
Four analogies to explain reactive power
III. The inclined-plane analogy (1/4)
Suppose men have to push a large ball from one side
of an inclined plane to another (A to B)
Minute Lectures
Four analogies to explain reactive power
III. The inclined-plane analogy (2/4)
The active power needed is the same as if the plane
were flat, but a man needs to keep the ball up on
his path.
Consequences:
• A loss of capacity (this man cannot be used for
pushing)
• Extra friction losses (since this man will have to
touch the ball)
Minute Lectures
Four analogies to explain reactive power
III. The inclined-plane analogy (3/4)
Minute Lectures
Four analogies to explain reactive power
III. The inclined-plane analogy (4/4)
Vector representation:
Minute Lectures
Four analogies to explain reactive power
IV. The trampoline analogy (1/4)
Suppose someone has to run from point A to point B
The harder the surface, the less the runner will jump
up during his sprint, the faster he will be able to run
Minute Lectures
Four analogies to explain reactive power
IV. The trampoline analogy (2/4)
But now suppose he has to move to a platform B from
A using a series of trampolines
He will start at the same height A, compensating for
the height (reactive load) of B
Minute Lectures
Four analogies to explain reactive power
IV. The trampoline analogy (3/4)
His work to go from A to B will be the same
But the trajectory has some consequences:
• Since the surface is a trampoline, he can’t use all
his force to go full speed forward
• He will encounter increased resistance of ground
and air
Minute Lectures
Four analogies to explain reactive power
IV. The trampoline analogy (4/4)
Vector representation:
Minute Lectures
Four analogies to explain reactive power
Round-up
Four analogies represent the idea of active and reactive
power in an electric system:
•
•
•
•
The tandem analogy
The horse-boat analogy
The inclined-plane analogy
The trampoline analogy
All analogies convey the same idea, but depending on the
person, one analogy might work better than another
We hope they will increase the reader’s insight in the
subject, or help experts to develop ideas to explain it to
others
Minute Lectures
Four analogies to explain reactive power
Links and references
• The Electricity System as a Tandem Bicycle
>>
• What are VARs?
>>
• Capacitors in Harmonic-Rich Environments
(technical application note)
>>