How to ensure total harmonic distortion (THD) doesn’t dramatically increase following a lighting retrofit
Lighting can be a large part of any building's electrical load — 30% or more if it is heated by non-electrical means. As an energy cost-cutting measure, consider upgrading your lamps from incandescent to compact fluorescents (CFLs) or switching from T12 to T8 fluorescents. You may also be motivated to look into a lighting upgrade based on complaints from your staff about poor color rendering, flicker, and glare. Announcements about legislative bans that will make incandescents, T12 fluorescents, and mercury lamps obsolete may also push you in that direction. Although all of these factors are valid reasons to pursue a lighting retrofit, if you're not careful, your project can lead to overheated transformers and nuisance tripping of overcurrent devices, overloaded motors and neutral conductors, and premature failure of power factor correction capacitors.
Total harmonic distortion (THD) can drop or dramatically increase following a lighting retrofit. That's why it's critical to reduce THD at the source (clean load) to prevent interactions with your building system (Promoting Power Quality Prowess on page 15), instead of creating power quality problems that have to be solved by installing very expensive filters.
How to decrease THD when replacing incandescents
Although incandescent lamps have been historically inexpensive to install, even budget-conscious contractors are now turning to more energy-efficient alternatives, such as CFLs, for lighting retrofits in an effort to achieve increased energy savings in the long run.
Install commercial-grade, clean CFLs — Commercial-grade CFLs typically generate THD under 80% — sometimes even under 33%. Unfortunately, these lamps are more expensive (more than $10 each), prompting many building operators to purchase residential-grade lamps (around $2 each) that can result in THD levels of more than 150%. Residential-grade lamps are usually of the screw-in variety, with ballasts built into the base of the lamp. However, very small and inexpensive components generate high THD.
Note that if one CFL is mixed with another load, it may not cause a power quality problem. However, adding several CFLs may be enough to cause a problem.
Install CFL luminaires where ballasts are separate from lamps — In CFL luminaires where the lamp and ballast are separate, THD tends to be low, typically less than 30% — even 10%. Some CFL luminaires are designed as “hard-wired” units from scratch, while others are set up as a quick “upgrade” unit for older incandescent units. According to several ballast manufacturers, upgraded luminaire units often overheat and cause premature ballast failure (one to two years). Mockup and review of existing installations usually address any concerns about construction and reliability.
Aside from power quality issues, the location of the ballast in the fixture is also important from a maintenance standpoint and for heat dissipation, which can have a dramatic effect on the life of the ballast.
Install new linear fluorescent T8 or T5 systems — If larger size T8 luminaires can be installed in the space, this may be the most economical solution. Lamps have long life (20,000-plus hr), and ballasts are economical and heat-resistant, often designed for 90°C operation.
How to decrease THD when replacing T12 fluorescents
Magnetic fluorescent ballasts are now being replaced by more efficient electronic ballasts, which rectify the supplied power and invert it to high frequency, resulting in a lamp that runs more efficiently. Fluorescent lamps with electronic ballasts typically run at frequencies above 30kHz.
Install low THD (premium) ballasts — T8 electronic ballasts are usually more linear than T12 ballasts. They have THD under the 10%, 20%, or 30% mark; however, distribution is spread over a larger frequency spectrum — from the 3rd to the 15th harmonic and beyond. When choosing a new ballast, select the lowest THD you can afford (preferably one with THD less than 10%). Note that if you do not see THD published in the specification, it can be much higher than 30% — up to more than 150%.
If you need budget support for installing premium ballasts, check with your local utility or government agency, as some offer financial incentives on premium ballasts.
How to confirm manufacturers' specs
Here are several ways to verify manufacturers' claims about their lighting products:
Look for independent lab tests — In the past, many manufacturers hired independent laboratories to perform verification testing. Unfortunately, this option is used less often today because of cost-cutting and frequent product redesign. Review product reports carefully; sometimes old reports may be submitted with new, redesigned products.
Beware of factory internal testing — Design engineers can use advanced computer tools supported by internal factory laboratories to create specifications from sophisticated sets of data. Be aware that while such specifications may be optimistic, they are only as good as the brand name of the manufacturer.
Request a mockup — Seeing really is believing in this case. Install a few fixtures, and use a light meter/power meter to gain a true picture of how well the lighting system measures up in real-life settings.
CSA/UL or similar tests — During their certification work, CSA/UL performs tests that you may be able to use to support claims in the manufacturers' specifications. Unfortunately, CSA/UL usually covers electrical safety-related testing, so its files may be incomplete for what you require.
Testing by independent utilities — Various utilities offer incentive programs for installing energy-efficient lighting components and systems. To confirm that performance, energy savings, and other aspects of newly installed lighting systems warrant the payment of financial incentives, they conduct detailed investigations of the product specifications and often perform in-house testing. Access to this data is typically free, and there are no brand affiliations. Qualifying technical specifications are also typically posted on utilities' Web sites.
Note that these groups test products for performance only, not reliability, so ensure your system is protected by a sufficient warranty.
How to protect your investment
Check with your potential supplier for the warranty on all major components instead of the entire system. For example, there may be a 12-month warranty on each complete luminaire but an additional 4-yr warranty on ballasts and an additional 6-month warranty on lamps. When you use a combination of ballasts and recommended lamps, manufacturer warranty periods can be even longer. A warranty not only protects your investment, but it also is a good indicator of the expected life of the product.
When selecting new lighting equipment for an upgrade project, assess its construction not only for convenient cleaning optics, but also for easy lamp and ballast replacement. You may not plan to replace ballasts, but trust me…you will!
Witkowski is a senior electrical systems engineer, customer engineering services, with Manitoba Hydro, Winnipeg, Manitoba, Canada. He can be reached at email@example.com.
Sidebar: Promoting Power Quality Prowess
If you're thinking of upgrading your existing lighting system — or designing an entirely new one — follow these rules for a green, easy-to-maintain system that does not create power quality problems:
Minimize harmonics at the source by using ballasts with a total harmonic distortion of less than 10%.
Verify manufacturers' claims by having specs checked at a test lab operated by a utility, government department, or independent lab.
Protect your investment by checking the warranties on major components of your lighting system rather than on the overall system.
Review the construction of fixtures for maintainability.