Real Power vs Apparent Power vs Reactive Power: What is the difference?
On the grid, electricity power is supplied with an AC signal. In a perfect condition, the load would be purely resistive, but due to motors in factories and houses, the load is in fact inductive.
A difference of phase appears between the power in the grid and the power in the load. It can be seen as a simple RL circuit and as shown figure 1, the various powers are called, real power, reactive power and apparent power:
- Real power
- Reactive power
- Apparent power
Figure 1: Power triangle illustration
Types of Electrical Power
Reactive power represents electrical energy stored in the coil that then flows back to the grid. Ideal coils do not consume any electrical energy, but create a significant electric current. Real power is the power actually consumed due to the resistive load and apparent power is the power the grid must be able to withstand. The unit of real power is watt while apparent power unit is VA (Volt Ampere)
Real, reactive and apparent power comparison
A famous analogy is made with the glass of beer and the froth of the beer. Real power if what you end up drinking. The glass is the apparent power and must be large enough to contain liquid and froth.
The issue of reactive power is not only technical but has potentially large economic consequences. Indeed, a utility company must build a grid able to transport the apparent energy, but only bills the real power. If the difference were too large, it would be unsustainable. The ratio between real power and apparent power is known as power factor. Power factor must be as close as possible to one. Electronics components, called power factor correctors (PFC) help in this task. Governments regularly pass new regulations for electronic devices that must comply with stricter norms in order to obtain a good energy label.
Conventional AC to DC converters usually employ a full wave rectifier bridge with a simple capacitor filter to draw power from the AC line. Consequently, the line current waveform is a narrow pulse, and the power factor is poor (0.5-0.6) due to the high harmonic distortion of the current (see Figure 3).
Figure 2: AC to DC converter equation
Various methods exist to improve the power factor corrector. For low power, a passive solution with discrete components is often enough. As said previously, a load is most of the time inductive and putting a capacitor in parallel will improve the power factor. When applications need a few tens of watts, an active PFC is necessary. The most common topology is the boost topology that can be differentiated in 2 sub categories:
- Transition Mode (TM) or Critical conduction Mode (CrM) for a few tens of watts to hundreds of watts
- Continuous Conduction Mode (CCM) for a few hundreds of watts to several thousand of watts
Figure 3 shows the PFC stage is implemented in front of the bulk capacitor as a boost converter circuit.
Figure 3: PFC - power factor corrector stage
The goal is to shape the input current in a sinusoidal fashion, in-phase with the input sinusoidal voltage. An internal sinusoidal reference is generated. This reference is compared to the external signal, and when the error is too large, the MOSFET is turned off. Then, when the current reaches zero, the MOSFET is turned on again. The transition mode has a fixed ON time period and has a curve like in Figure 4.
Figure 4 : MOSFET timing and inductor current waveform - transition mode
The system operates (not exactly but very close to) the boundary between continuous and discontinuous current mode and that is why this system is called a Transition Mode PFC. The current has large amplitudes and the peak current is twice the average current. Hence, for high power, it is necessary to get a current closer to a sine wave curve. The Continuous Conduction Mode is the solution, applying a fixed frequency that limits the variations of the current as shown figure 5. This is the most complex design but a power factor of 0.99 is achievable.
Figure 5 : MOSFET timing and inductor current waveform timing - continuous conduction mode
More methods exist like the Fixed Off Time (FOT) timing where the modulation happens on the On Time. In some conditions, it can provide results similar to the Current Continuous Mode but with an implementation similar to Transition Mode. When power must be increased and a single Transition Mode is not adequate anymore, an interleaved PFC can be the solution. This kind of solution uses more components but can be much easier to design.
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