Lighting ballasts and power quality

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Canadian Consulting Engineer, Jan/Feb., p. 24, 1993-01

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Construction construction

Lighting Ballasts and Power

Quality

by D. Finn and M.J. Ouellette

Revised text of article published in Canadian Consulting Engineering Jan./Feb., 1993, p.24

(IRC Paper No. 3109)

Lighting can account for 40% of the energy consumed in commercial and institutional buildings, making it a

favourite target of energy

management initiatives. Energy-

efficient T8 and T I 0 fluorescent and compact fluorescent lamps, including those coupled with energy-efficient magnetic ballasts or high-frequency electronic ballasts, are popular choices for low-energy lighting systems in retrofits and new installations.

But some ballasts can have an adverse effect on the quality of the power supply in buildings. Power quality problems are not necessarily inherent in the power delivered to

the building: they are usually

caused by devices (like magnetic and electronic ballasts) that end users connect to the system.

Overheated transformers and

capacitors, computer glitches,

frequent tripping of circuit breakers, scrambled clocks, and radio and telephone interference are some problems blamed on poor power quality. One can minimize these problems by paying attention to the

performance characteristics of

ballasts.

Magnetic and Electronic Ballasts

The decades-old magnetic "core- coil" ballast has begun to give way to

the electronic ballast as

semiconductor technology has

leaped forward in recent years. Magnetic ballasts

-

-

supply. Power losses appear as

heat dissipated in the core and windings. Recently, energy-efficient magnetic ballasts have appeared that use larger windings and improved core materials to reduce power losses.

An electronic ballast uses diodes and transistors to convert the 60 Hz input to direct current, and then convert it back to a high frequency current in the 25 to 40 kHz range. Electronic ballasts have a higher initial cost, but are lighter, smaller and more efficient than magnetic ballasts, and they drive fluorescent lamps to operate more efficiently, because the phosphors in the lamp produce more light when bombarded with higher frequency.

Power Factor: The higher, the better

Ballasts can induce a phase shift between the voltage and the current supplied by the utility, requiring more current to do the same amount of work. Harmonic distortions in the power supply can also produce the same effect. The amount of current a ballast draws to do its job can be determined from the Power Factor (PF):

Power Current =

PF x Voltage

A PF of 1 (or 100%) is ideal because it doesn't increase the current requirement. A ballast with a PF of 0.5 will double the amount of current required by a similar ballast with a

PF of 1. Interestingly, but

unfortunately, ballasts with PF less than 0.5 are called Normal Power Factor (NPF) ballasts, even though PF less than 0.5 is poor. Ballasts with PF of 0.9 or higher are called High Power Factor (HPF) ballasts.

As an example, a new hotel with 347 V distribution may use a 300 VA stepdown transformer to bring a

local circuit voltage down to 120 V. Energy-efficient lighting in one section of the hotel may use a dozen 13 W compact fluorescent lamps, with ballasts that may consume an additional 2 W each, for a total load of 180 W. If NPF ballasts are used, this installation could draw up to 3 A of current (1 80 Wl(0.5 x 120 V)), well

above the 2.5 A limit of the

transformer. The results may be an overheated transformer and a reluctance on the part of hotel management to ever again use

energy-efficient lighting. One

solution is to specify HPF ballasts. Another solution, especially for new buildings, is to design the circuit to

compensate for the increased

current.

Minimize Total Harmonic

Distortion (THD)

Besides inducing a current phase shift, ballasts can distort current waveforms. These distortions show

up as higher order current

harmonics that can produce many of the problems associated with current overload, plus others such as telephone and radio interference and

glitches in electronic devices like computers. It is extremely difficult, if not impossible, to use corrective

devices to prevent harmonic

distortion transmission once the distortion has been created. The best way to avoid problems is to buy ballasts which produce as little Total

Harmonic Distortion (THD) as

possible.

Europe has introduced a power quality standard (IEC 555) of THD less than 33.8%, and the American National Standards Institute (ANSI)

and the Canadian Standards

Association (CSA) are considering similar limits. Some electric utilities will only offer rebates on ballasts with THD less than or equal to 20%. For critical areas such as computer rooms, ballasts with even lower THD are available.

The good news...

The Canadian Code for Energy Efficiency in New Buildings, set to appear in 1995, will effectively spell the end for inefficient ballasts.

Lighting manufacturers have

responded to the power quality problems associated with earlier versions of energy-efficient magnetic and electronic ballasts, and there are many good models on the market now.

To minimize problems, use ballasts with as high a PF (preferably > 0.9) and as low THD (preferably < 20%) as the budget will allow, especially in critical areas where good power quality is essential. Awareness of the performance characteristics of ballasts and their possible effects is the first step to avoiding power quality problems with lighting.

David Finn is an engineer with IRC's Industry Liaison Branch. He coordinated the recent seminar series "Effective and Efficient Lighting".

Michael Ouellette is a researcher at IRC's Lighting Laboratory. His current work focuses on fluorescent lighting and power quality.