Some Considerations of Power Quality in HVAC Applications
One of the most misunderstood design elements in an HVAC building is the role harmonics play in the electrical system. Harmonics are harmful, energy-wasting currents which flow in the electrical system as a result of electronic switching devices such as variable-speed drives, computer power supplies, and energy-efficient lighting. These harmonics are at multiples of the 60 Hz current, typically 180 Hz (3rd harmonic) and the 320 Hz current (5th harmonic) but are also prevalent above these frequencies.
Many devices in the electrical power system lose efficiency when harmonics are introduced. For example, distribution transformers can lose a full 1 percent or more of efficiency when carrying 100 percent harmonic current, and motors lose 0.1 percent efficiency with each 1 percent of harmonic voltage distortion. These losses can cause fan and pump motors to reduce their lifespan dramatically due to extra heating in the motor stator. For a large HVAC system with 500 hp of fan and pump load, running at 5 percent voltage distortion instead of 1 percent can cost $15,000 per year at 10¢ per kWh in addition to the reduced lifespan of the motors caused by additional heating effects of the harmonic currents.
In addition to the general applications, some systems have even more stringent requirements due to special requirements inherent to the systems. Hospitals and data centers have millions of dollars of sensitive equipment that can be damaged or rendered inoperable by high system harmonics.
These same systems can very often, by nature of their building design, have high harmonics due to the high HVAC and lighting loads as a percentage of their total system loads. HVAC loads typically have high drive content, and standard six-pulse drives, even with harmonics fighting “chokes,” can have 35 percent or more harmonics at full load. Without the choke, this number can be up to 80 percent harmonic content.
In a hospital there are special considerations due to equipment such as MRIs or CTs, which require a very clean sine wave in the power system to work properly. In fact, a manager for a Wisconsin-based MRI manufacturer stated that electrical harmonics are “the No. 1 problem” faced in equipment performance. Clearly harmonics can’t be ignored as the electrical engineer’s problem.
There are many solutions that can be employed in applying variable-speed drive systems, some of them newer and some that have been employed for years. From a new equipment standpoint, one is to apply clean power drives for all drives 75 hp and above (and possibly smaller for hospital applications).
Eaton Corp., as well as many other manufacturers, provide 18-pulse clean power drives which, when applied will help, meet the IEEE 519 standards for harmonics in every case, by having current distortion at or below 5 percent. Other technologies, such as passive filters can effectively reduce individual drive harmonics to 8 percent current distortion for that drive, allowing the other linear loads in the system to bring the total system harmonics to allowable levels.
Passive filters can be challenging to employ in some older, smaller systems where existing harmonics are problematic due to the capacitors in these filters that can act as magnets for harmonics or cause other applications issues in unloaded conditions when inappropriately specified.
Finally, 12-pulse drives have been used for decades and have now been largely superseded by the more effective 18-pulse technology, which offers 50 percent better harmonics (10 percent for 12 pulse versus 5 percent for 18 pulse) at a very slight price premium.
Harmonic system analysis should be completed in the design stage of system design. Harmonic problems can be prevented easily and cost effectively with standard products available today, leading to lower operating costs and long equipment life.
Publication date: 04/09/2007