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Explaining reactive power - 4 further analogies
 

Explaining reactive power - 4 further analogies

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Following the Minute Lecture Four analogies to explain reactive power, we received many reactions. Several writers suggested further analogies. ...

Following the Minute Lecture Four analogies to explain reactive power, we received many reactions. Several writers suggested further analogies.

This inspired us for making a sequel to this minute lecture, with four more analogies worked out.

When explaining reactive power to lay persons, you can pick any analogy of your choice, according to your target audience.

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    Explaining reactive power - 4 further analogies Explaining reactive power - 4 further analogies Presentation Transcript

    • Minute LecturesAnalogies to explain reactive power (part 2)Following the Minute Lecture ‘Four analogies to explain reactive power’, we received many reactions. Several writers suggested further analogiesThis inspired us for making a sequel to this minute lecture, with four more analogies worked outWhen explaining reactive power to lay persons, you can pick any analogy of your choice, according to your target audience
    • Minute LecturesAnalogies to explain reactive power (part 2)What does the analogy needs to explain? Remember: these analogies are didactic tools; none of them are 100% correct They elucidate the difference between active power (W), reactive power (VAR), and apparent power (VA) Some of them also explain why reactive power limits the capacity of the lines in the grid, why reactive power increases energy losses in the grid, or other related phenomena
    • Minute LecturesAnalogies to explain reactive power (part 2)I. The beer analogy (1/2)Suppose you want to quench your thirst by a cool beer. This beer is the active power (W)Along with your ale comes a bit of foam, which doesn’t quench your thirst. This is the reactive power (VAR)
    • Minute LecturesAnalogies to explain reactive power (part 2)I. The beer analogy (2/2)The total content of the glas, is theapparent power (VA)The more foam, the bigger glas youneed (= you need a line with ahigher capacity)
    • Minute LecturesAnalogies to explain reactive power (part 2)II. The marble-in-a-tube analogy (1/3) Suppose your goal is to push marbles out of one end of a horizontal pipe You accomplish this by pushing marbles into the pipe
    • Minute LecturesAnalogies to explain reactive power (part 2)II. The marble-in-a-tube analogy (2/3) The first ten marbles will give you no result, they will just fill the tube Only the 11th marble will result in a marble coming out of the other end of the pipe
    • Minute LecturesAnalogies to explain reactive power (part 2)II. The marble-in-a-tube analogy (3/3) The ‘marbles in’ can be compared to apparent power (VA), the ‘marbles out’ to active power (W) and the length of the pipe to reactive power (VAR) The longer the length of the pipe (more reactive power), the more marbles are needed (apparent power) for the same result (active power)
    • Minute LecturesAnalogies to explain reactive power (part 2)III. The cash flow analogy (1/2) Suppose you are running a business. You have to spend an amount of money C (cost) on one day, for earning a larger amount of money E (earnings) one month later. Your profit will be P = E - C C is not lost money. However, without spending C, you will not be able to make the profit P The profit P can be compared to the active power. The earnings E are the equivalent of the apparent power. The cost C is the reactive power
    • Minute LecturesAnalogies to explain reactive power (part 2)III. The cash flow analogy (2/2) To bridge the time between the spending of C and the earning of E, you will need some money. You could borrow this money (take reactive power from the power grid). That will however cost interest (reactive power penalty) A better solution would be to have some cash money available at your business (= a capacitor bank at your site).
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (1/7) Suppose a water tower and men that have to bring water to the tank by climbing ladders. After emptying their bucket, they return by another ladder.
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (2/7) The climbing ladder = the electricity line The number of men on the ladder = the voltage The amount of water each man carries = the current The amount of water added to the tank per time interval = the frequency
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (3/7) Energy = voltage x current The active power = the height of the tower. If the tower is high (high power), there are two solutions to keep the frequency: using more men (higher voltage) or increasing the amount of water each man carries (higher current)
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (4/7) There are three ladders = three phases
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (5/7) Width of the tower = reactive power Consequence: the length of the ladder (apparent power/voltage) depends on the height of the tower (active power) and the width of the tower (reactive power). The higher the reactive power, the longer the ladder. So if the same voltage is kept (same amount of men on the ladder), each man has to carry more water to keep up the same frequency.
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (6/7)
    • Minute LecturesAnalogies to explain reactive power (part 2)IV. The water tower (7/7) If a man is carrying more water (higher current), he will lose some along the way. So more reactive power means more losses. If the reactive power is so high that the men have to carry really a lot of water, one of the men could collapse  the other men have to carry even more water  if no support from new men is given, the system risks to experience a complete breakdown
    • Minute LecturesAnalogies to explain reactive power (part 2)Round-up Another four analogies represent the idea of active and reactive power in an electric system: • The beer analogy • The marble-in-a-tube analogy • The cash flow analogy • The water tower analogy Some people prefer one or another analogy We hope they will increase the reader’s insight in the phenomenon, or help explaining the phenomenon to others
    • Minute LecturesAnalogies to explain reactive power (part 2)Links and references • P Sauer, What is Reactive Power?, PSERC, Sep 2003 • Minute Lecture: 4 analogies to explain reactive power