29-11-2012, 05:30 PM
What is Reactive Power?
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Engineering talk
Reactive power is a quantity that is normally only defined for alternating current (AC) electrical systems. Our U.S. interconnected grid is almost entirely an AC system where the voltages and currents alternate up and down 60 times per second (not necessarily at the same time). In that sense, these are pulsating quantities. Because of this, the power being transmitted down a single line also “pulsates” - although it goes up and down 120 times per second rather than 60. This power goes up and down around some “average” value - this average value is called the “real” power and over time you pay for this in kilowatt-hours of energy. If this average value is zero, then all of the power being transmitted is called “reactive” power. You would not normally be charged for using reactive power because you are consuming some energy half the time, and giving it all back the other half of the time - for a net use of zero. To distinguish reactive power from real power, we use the reactive power unit called “VAR” - which stands for Volt-Ampere-Reactive. Voltage in an electrical system is analogous to pressure in a water system. Current in an electrical system is analogous to the flow of water in a water system.
Let’s go back to this notion that voltage and current may not go up and down at the same time. When the voltage and current do go up and down at the same time, only real power is transmitted. When the voltage and current go up and down at different times, reactive power is being transmitted. How much reactive power and which direction it is flowing on a transmission line depends on how different these two times are.
Two extreme examples of the time relationship between voltage and current are found in inductors and capacitors. An inductor is a coil of wire that is used to make motors. A capacitor is made of parallel conductive plates separated by an insulating material. The electrical properties of these two devices are such that if they are both connected to the
same AC voltage source, the inductor absorbs energy during the same “half cycle” that the capacitor is giving energy. And similarly, the inductor produces energy during the same “half cycle” that the capacitor absorbs energy. Neither of them absorbs any real power over one complete cycle. Thus, when a motor needs reactive power, it is not necessary to go all the way back to electric power generators on the transmission grid to get it. You can simply put a capacitor at the location of the motor and it will provide the VARs needed by the motor. This relieves the generator and all the lines between the generator and the motor of having to transmit those VARs. They are provided “locally” by the capacitor. This means that with the capacitors installed, the current in the lines will be smaller than when the capacitors are not installed. This is a good thing because current in the lines causes heat and every line can only handle a limited amount of current. Since the line current is smaller when the capacitors are installed, the voltage drop along all the lines is also less, making it more likely that the motor will have a voltage closer to the desired value. When there are not enough VARs flowing locally to the loads, the generators must supply them remotely, causing unnecessarily large currents and a resulting drop in voltage everywhere along the path.
A physical analogy for reactive power
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.
How is reactive power related to the problem of voltage collapse?
In terms of this water-carrying analogy, the frequency of going up and down the ladder should be nearly constant (that, is like our 60 cycles per second electrical frequency). So, when more water is needed, the amount that each person carries up the ladder must get bigger (since they are not allowed to go faster or slower). Well, if this water gets too heavy, either the ladder might break, or the person might get too tired to carry it. We could argue that if the ladder breaks, that is like the outage of a transmission line that either sags or breaks under the stress of too much current. There are devices called relays in an electrical system that are supposed to sense when the load is too much and send a signal to a “circuit breaker” to remove the line from service (like removing the set of three ladders).