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Grasping power factor and solid state lighting

Posted: 19 Apr 2013 ?? ?Print Version ?Bookmark and Share

Keywords:solid state lighting? power factor? PF driver?

Lighting amounts to approximately 17.5% of global electricity consumption. As the world transitions from incandescent to solid state lighting (SSL) technology, utilities and government regulatory agencies worldwide are concerned that, as this large segment of the consumption base switches to SSL, it will increase infrastructure costs. This is due to the reactive nature of LED-based solid state lighting, which results in higher distribution currents that adversely affect power factor (PF) and, in turn create a larger demand on the power grid.

Regulators have been working with utilities companies to enact rigid standards to control the impact of SSL technology on the power grid (figure 1). The move to LED-based solid state lighting promises a significant reduction in the carbon footprint of the electrical power grid simply due to the dramatic reduction in real power consumption. However, if power factor is not managed, the grid will still need to be able to provide a much higher power level than is actually needed at the load, eliminating a significant portion of the benefits of moving to solid state lighting.

Figure 1: Regional standards for power factor and total harmonic distortion.

Historically, incandescent bulbs have had near-perfect power factor. Therefore, solid state lighting is being held to a much higher PF standard compared to legacy AC/DC power supplies. In most cases, power supplies are free from any form of power factor regulation for supplies rated up to 75W. However, for solid state lighting, PF regulations typically kick in as low as 5W or below.

In order to effectively design an LED-based luminaire, designers need to understand power factor, the impact LED drivers have on it, and different techniques for integrating power factor correction cost-effectively in the LED driver design.

Understanding power factor
Power factor is a simple, unit-less ratio of real power to apparent power. Real power is the power used at the load measured in kilowatts (kW). Apparent power is a measurement of power in volt-amps (VA) that the grid supplies to a system load. In a highly reactive system, the current and voltage, both angular quantities, can be highly out of phase with each other. This results in the power grid needing to supply a much larger reactive power to be able to supply the actual real power at any given time.

The ratio of the reactive power to the real power is called power factor (PF). This basically means that for an equivalent real power consumed by a highly reactive load, for example 5W, the actual current that the grid needs to supply to the load in order to provide the real power has to be higher than the real power by the power factor ratio. For the previous 5W example, for a load with a PF of 0.5, the grid needs to provide 2x the current actually required by the load at any given time. This adverse impact on the power grid does not apply to incandescent lighting, which is purely resistive and has a unity power factor.

Figure 2: Power factor C ratio of real power (kW) to reactive power (kVA).

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