In my power engineering course, we were introduced to reactive power. I believe I understand what reactive power is in essence: there is energy transfer between the source and inductor/capacitor. This makes sense to me, and I can do many calculations just fine. However, I am failing to understand what the value of reactive power represents. From my notes:
$$\begin{align*} P=VI\cos\psi \\
Q=VI\sin\psi
\end{align*}$$
where P and Q are active and reactive power respectively. I understand the formula for P, and its derivation is presented in my notes. Physically, P represents the amount of energy dissipated through the resistor. What does Q represent?
Considering the graphs of power vs time in capacitor/inductor only circuits, the average power is 0. Shouldn't this mean that on average reactive power is just 0?
Apologies if the question is a bit unclear, I am having a difficult time just explaining my confusion.
Best Answer
Let's ignore the power aspect for a second, and think about what reactance really is.
You know a math and theory, you can talk about things in abstract theoretical constructs using complex numbers, phasors, and all that. But abstract models are exactly that: abstract models. The math can model a thing, but it won't really help you understand the physical system it is modeling except in other abstract ways.
So let's ignore the math for a second, and talk about what reactance really is.
Actually, let's talk about resistance. The real component to impedance. Ultimately, resistance represents energy loss. Resistance consumes some of the kinetic energy of electrons moving through the circuit, and that manifests as the familiar ohmic voltage drop we see across any resistive load. Electrons smack into stuff, set them vibrating, and the resistive load heats up as joules are lost by the electrons and transferred into the load. The faster the rate of energy that moves through this load, the faster the rate of power lost, and the harder you have to push to make that happen.
But that's just one side of the coin. Besides simply dissipating energy to the environment, there is another option that can happen: energy can be stored. Capacitance and inductance are often talked about as being 'duals' of each other because they're both measures of energy storage. Capacitance is a measure of energy stored in an electric field, while inductance is a measure of energy stored in a magnetic field.
Energy being stored looks just like energy being dissipated, at least at first. In both cases, energy that was in the circuit is no longer present. The only difference between resistance and reactance here is that with resistance, that energy is gone for good, but reactance will eventually return that energy back to the circuit at a later time. Well, and of course as a measure of storage, they eventually reach a maximum amount of storage given a static circuit. A capacitor will need a higher voltage to store more energy, an inductor likewise will need higher current to store more energy. This is the 'reactance' aspect. As energy is stored, less power is seemingly dissipated by this reactance until it vanishes entirely. If the power starts going down, the stored energy is released back into the circuit.
So what is apparent power? It's simply the rate at which a circuit or part of a circuit (depending on what you're calculating for/looking at) is storing energy, or, if the magnitude is opposite, the rate at which it is releasing energy. That's all. It's not weird, or strange, and it is a real, physical, quantifiable thing. If you charge a massive capacitor bank, from a battery, it will consume joules from that battery, and it will do so at a certain rate, one that is highest at first, but will eventually fall to zero. This is, technically, reactive power. But it's still measured in watts, and watts are, well, always watts. You just are measuring the rate at which something stored joules, rather than the rate at which it simply dissipated them.
Your confusion I think is that you've actually sort of arrived at the answer already without realizing it. If you have a circuit with only capacitors and inductors, then there is no 'P' as there is no energy being dissipated at some number of joules per second. There is only energy being stored, and it will eventually get released, and so yes, it averages to 0. Reactive power always does. It's ultimately just storage, not consumption, so yes, the it will always always always average to 0. Those joules got loaned out, but inductors and capacitors have terrific credit ratings and always pay you back eventually, so you didn't actually lose any money/joules in the long run.
So, you don't need to talk about this in terms of math at all. In fact, if you understand that reactive power is simply the rate energy is being stored and then released, measured in joules per second or watts like anything else involving power, then the behavior and math should all just make logical sense, because that's ultimately what you are modeling with said math.
Now, one might wonder why reactive power even matters if it averages to zero.
Let's talk power factor real quick. Power factor is, of course, the ratio of real to apparent power. This might seem like a rather strange or pointless thing to have a ratio of. I mean, who cares? The apparent power isn't actually being lost, why even measure it?
The issue is that this energy storage is never (excepting in the case of perhaps superconductors) totally efficient. Electrons have to move onto the negative plate of a capacitor, while an equal number of electrons are pushed off the positive plate. Moving charge is current. Conductors (again, except in the case of superconduction) always have some resistance, so you have losses. In the context of alternating current, where storing energy will have a profound effect in this regard, you wind up having electrons flowing in and out over and over, uselessly storing energy for no reason. So even though the energy is getting returned to the circuit, you're still suffering losses in the form of current flowing but without it doing any work. Really, the idea of current and voltage phase is just a way of looking at how reactance is effectively dropping voltage but because it is storing energy, or maintaining voltage (or increasing it to maintain current instead), by releasing energy.
Never forget that important concept, that all of this is ultimately just different sort of abstract ways of looking at or modeling one true, physical process going on, which is actually very simple at its core. The storage of energy, and that there are two different fields with which it can be stored. From that concept, everything else can be derived.