I have a tough time wrapping my head around these three powers and why they would be so important in a power plant setting.
In: Engineering
Real power is the power burnt off in the resistive parts. Whenever you do a quick power calculation this is what you’re considering. This is not Vrms x Irms, it is the sum of V x I at each point
Reactive power is the energy that is stored and released by capacitors and inductors each line cycle. It briefly looks like energy is being consumed but you get it back
Apparent power is the combination of the two. You need to be able to source the apparent power and in certain applications it matters a lot. This is Vrms x Irms
Real power is actual work output by the equipment plus waste heat.
Reactive power is energy stored in the equipment which is then sent back down the supply line, but not utilized by the machine. The result of this energy storage is that the apparent current consumed by the machine is greater than would be expected by the rated machine output.
You can think of this as a certain “springiness” in the electrical characteristics of a machine.
A good analogy might be if your bicycle used an elastic rubber band to drive the wheel instead of a rigid chain.
Therefore, attempting to push on the pedals requires you to wind up tension on the band before effort is supplied to the rear wheel. As the pedals near top dead center, you need to apply force to the opposite pedal, but the elastic resists this too, causing a “kickback force” on the pedals. Obviously this is not ideal. In a bicycle this kind if characteristic is easy to avoid by using a rigid mechanical connection. It generally is hard to avoid in electrical machines like motors.
If RMS is the “root mean square” function, then
RMS(Real Power) + RMS( Reactive Power) = Apparent Power.
I use the idea of RMS here because at some points in the AC cycle the reactive power may be negative, meaning the reactive effects may be momentarily supplying power to the circuit and at it her times consuming it. Again, think of the rubber band bicycle analogy.
Reactive power comes in two forms, charge stored in capacitors in the machine, and magnetic field stored in inductive components such as transformers and electric motor cores.
In a purely resistive machine, current consumption is always directly proportional to AC voltage at all times Thus the current waveform is always exactly phase with the supply voltage waveform.
Since inductors resist changes in current, in machines that have some degree of inductive load (LR circuits) the current waveform lags behind the voltage. This lag is measured by a phase angle Φ or *Phase shift* in the circuit.
The reason for this is that the inductors store energy by setting up a magnetic field. As the supply voltage begins to drop, this stored energy manifests itself as a positive voltage *gain* across the inductor relative to the supply voltage, so the net voltage across the resistance inside the machine may be more or less than the supply voltage depending if the current is rising or falling. As supply voltage drops, the stored magnetic energy is released across the resistor. As voltage begins to rise again as the AC waveform turns over, energy is absorbed again by setting up the field, which prevents current from traveling through the resistor.
Capacitors resist changes in voltage. The net effect of this is that the current waveform rises ahead of the voltage waveform instead of lagging behind, in machines that have an appreciable degree of capacitance. (RC circuits).
So, the total phase shift is
Tan Φ = { V(across inductors) – V(across capacitors) } / V(across load).
Using Ohm’s law: V(across load) = I * R.
Ideally, in a machine the inductance is balanced with the internal capacitance. Thus the capacitive current phase *lead* is balanced with the inductive phase *lag* and the total phase shift is more or less zero. That is to say that any current produced in inductors in a machine is capable to be absorbed by capacitors in the machine.
In practice the line current has it’s own LC characteristics that depend on the current draw, that will interact with any devices connected to it. Specifically, a building or group of homes will have a distribution transformer that converts several thousand or tens of thousands of volts to several hundred. It does this by the primary winding storing energy in the magnetic field inside the transformer core, and that energy being siphoned off by the secondary coil.
So one needs to consider both the inductance of the machine or machines connected to the supply transformer and that of the supply itself.
You can to some extent mitigate the effects of inductive load by adding and additional capacitor bank at the power meter connection that is commensurate to the total inductance of the building. This is called Power Factor Correction.
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