Shedding light on power quality for energy efficient bulbs

One aspect of energy efficient lighting that is rarely discussed is the way in which it will affect the utility power system. With this in mind, we decided to measure the new modern light bulbs. We tested incandescent light bulbs (ICBs), compact fluorescent light bulbs (CFLs), and light emitting diode bulbs (LEDs). This is not an exhaustive test in that it covers only a few representative samples available from large retail chains. Nevertheless, we feel the results are a good snapshot of how present-day lighting products behave.

The efficiency of the light bulb can be measured by how much light the bulb emits as a function of the power it consumes. This may seem simple and straightforward, but several caveats apply to how the bulb emits light and to its power consumption. The emitted light should be in a range of wavelengths that the human eye can see. Light emitted in the infrared range would be of no use for conventional illumination.

The ac voltage and current with a 60-W GE soft light incandescent lamp is in phase with minimal distortion and almost no reactive power.
Select figure to enlarge.

First a few basics. The common measure of light strength or intensity is lumens. The power a light bulb consumes should be divided into two parts, power and harmonics. The power is a function of the real power in watts, the total power in volt-amperes and the reactive (imaginary) power in volt-amperes reactive. Power factor is a ratio of the power used to do work, typically in kilowatts, divided by the total power in kilovolt-amperes. The power utility must generate, transmit, and transform the total power. The customer usually only gets billed for kilowatts used over a period of time measured in kilowatt-hours (kWh).

Power line harmonics are the other piece of the power puzzle. The harmonics are the whole number multiples (one, two, three, and so on) of the fundamental frequency that the utility delivers, 60 Hz in the U.S. Harmonics are caused by the rapid switching of modern power supplies that convert the 120-V ac supply into some dc voltage. The measure of harmonic distortion is the percent Total Harmonic Distortion (%THD) for voltage and percent Total Demand Distortion (%TDD) for current.

The Institute of Electrical and Electronic Engineers’ (IEEE) 519-1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems” standard for Current Distortion Limits, Table 10.3, lists 15.0% for the 2nd through 10th harmonic and 20.0% TDD for small loads where the short circuit current divided by the load current is greater than 1,000. (The IEEE term TDD will be used to differentiate between voltage total harmonic distortion [THD] and current total demand distortion [TDD]). The IEEE 519-1992 voltage distortion limits in Table 11.1 for Bus voltages less than 69 kV are 3.0% for individual voltage harmonics and 5.0% THD.

The voltage waveform for a 13-W CFL has minimal distortion but the current that the CFL uses has a 114.5% TDD with 77.7% on the 3rd and 47.5% on the 5th harmonic. Readings also show the utility must supply 3.950 VAR and contend with the harmonic current circulating on the power system.
Select figure to enlarge.

The accompanying figure shows the voltage and current waveform for an old GE 60-W ICB. The voltage supplied by our local utility (Avon, Co.) is the larger of the sine wave traces; the current is the smaller trace. Note that the voltage and current are in phase with reasonable sine waves. The waveforms have a voltage distortion of only 2.4%.

The ICB current causes only 2.3% TDD. The power used by the ICB is 61.14 W, 61.15 VA, with only 0.367 VAR. Of the 61.15 VA that the utility must generate, transmit and transform, the customer with a 60-W ICB is billed for 61.14 W. The power factor (PF) is 61.14 W/ 61.15 VA = 0.9998364 or basically 1. The displacement power factor (DPF) is the real power in watts at the fundamental (60 Hz) divided by the total power in volt-amperes at the fundamental.

Now consider the voltage and current waveform for the Wal-Mart Great Value 13-W CFL, and how its current waveform differs from that of the ICB. The voltage supplied by the local utility is still 2.4%THD but the current that the CFL uses has a 114.5% TDD with 77.7% on the 3rd (60 Hz × 3 = 180 Hz) and 47.5% on the 5th (60 Hz × 5 = 300 Hz) harmonic. The Wal-Mart Great Value 13-W CFL uses 11.45 W, 12.12 VA, and 3.950 VAR with a power factor taking the harmonics into account of 0.61.

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