GPU Harmonic Distortion And Power Factor Values

    May 18, 2015
    Severe harmonic distortion leads to premature failure of transformers, capacitors, wiring insulation, noise in telephone lines and computer signals.

    Well, harmonic distortion is not what we hear when an inconsiderate teenager walks by with his headphones acting like speakers spewing some sort of sound into the surrounding area. Nor is it the dissonance we hear in some modern concert pieces.

    Rather, it is the distortion of the sinusoidal waveform of the electrical current that is powering the equipment at the airport. When there is severe harmonic distortion, there is premature failure of transformers, capacitors, wiring insulation, noise in telephone lines and computer signals. Therefore, the wrong decision at the time of procurement may lead to considerable costs for the airport once the equipment has been installed.

    During the early 20th century, we weren’t worried about power quality and issues like harmonic distortion, because everything was pretty much linear loads. However, after the development of solid state components and their use in Variable Frequency Drives (VFD) and Solid State Frequency Converters (SSFC) in the 1980s and beyond, which are non-linear loads and thus sources of harmonic distortion, power quality has become much more important to facility planners and maintenance staffs.

    We can see an ideal waveform in Figure 1. But as can be seen from the waveform in Figure 2, the ideal waveform can be severely distorted by non-linear loads such as older 6-pulse VFD’s or SSFC’s. This distortion is caused by multiple waveforms, which are harmonics of the fundamental waveform. For a 60 Hertz waveform the harmonics are at 120 Hz, 180 Hz, 240 Hz, etc.

    The total harmonic distortion (THD) is thus the sum of all of the harmonic waveforms imposed onto the fundamental voltage or current waveform (VTHD and ITHD) and it is expressed as a percentage of the fundamental waveform. The higher the percentage, the more distortion there is in the fundamental waveform.

    Current harmonic distortion causes the most problems in electrical equipment. The distortions in the current waveform cause excess heat to develop in motors, transformers and capacitors. This extra heat degrades the insulation and can cause premature failure of the devices and thus unexpected down time. The higher frequency harmonics can also interfere with communication transmission lines when they are run in the same conduits.

    While there is no national or international standard dictating the THD limits on electrical systems, there are recommended values for acceptable harmonic distortion. IEEE Std. 519 provides suggested harmonic values for power systems of no more than 5 percent harmonic distortion factor, with the largest single harmonic being no more than 3 percent of the fundamental voltage.

    GREATEST CONTRIBUTORS

    At an airport, the greatest contributors of ITHD in the electrical system are the VFDs in the baggage handling system and HVAC systems, the charging systems for electric vehicles and the SSFC units at the boarding gates and in central 400 Hz power plants. While the VFDs are typically small to mid-size motors (5 to 20 kVA), there are a lot of them and thus their contribution to the ITHD of the system can be large. The SSFC units are typically 60 kVA to 180 kVA, while there are fewer of them they can have a large effect on the electrical systems at the airport.

    Over the last two decades, SSFC manufacturers have developed different versions of their units which have lower values of full load ITHD. The lowest priced units typically have 6-pulse rectifiers which have full load ITHD values of over 30 percent. By doubling the number of SCR elements in the rectifier to 12 or even 24, the full load ITHD values have been lowered to 12 percent and 10 percent respectively. Active rectifiers using IGBT’s offer values between 7 percent and 10 percent, while the new “Magnetic Wave Shaping” feature in the Hobart PoWerMaster® 2400 unit has a full load ITHD of 5 percent.

    Of course, most SSFC units are not operated at full load while an aircraft is at the boarding gates, most of the time the SSFC units are operating between 30 percent and 70 percent of their full load capability. However, the ITHD values at these lower loads are actually much higher than at full load. A 6-pulse unit can have ITHD values ranging from 60 percent to 100 percent. 12-pulse units might range from 20 percent to 35 percent, and the new Hobart unit will range from 9 percent to 15 percent. While there is less power behind these high distortion values, there is still a lot of distortion in the older, cheaper 6-pulse units.

    While high ITHD values are a cause for concern, if there is less power behind them then the potential harm that can be done is lower as well. Conversely, if there is a lot of power in the system, it doesn’t take a lot of distortion to cause problems.

    To get a better understanding of this, we can compare an electrical power system to a river of water. The number and size of the rocks in the river are equivalent to the ITHD in an electrical system. The amount of water and its speed is equivalent to the power (kVA) of the electrical system/equipment. If the river is flowing slowly (say 1 percent to 25 percent of maximum), any rocks in the river aren’t causing much of a disturbance. (No rapids to maybe a Class 1 rapid.) But, as the river flows faster the rocks cause more distortion in the flow. And so a Class 1 rapid can easily become a class 4 or 5 rapid.

    FUTURE PURCHASES

    So, when considering future purchases of VFDs, electric vehicle chargers, and SSFC’s, it might be prudent to request the ITHD values at various load points. This will provide a better understanding of the potential disturbances in the electrical system during normal operations.

    The other long term energy consideration that should be taken during the purchasing decision, is the power factor at the input of the unit. The power factor of a piece equipment (or aircraft) is the amount of energy required to do actual work versus the total amount of energy consumed by the equipment. An easier way to look at this is to say that if a beer mug contains the total power to be consumed, then the ratio of beer to foam is the power factor. A mug that is 80 percent filled with beer and 20 percent with foam is equivalent to 0.8 PF (power factor) which is what all legacy aircraft are rated as requiring from a power source.

    Utilities have to provide the full amount of power to the airport, whether the power is useful (beer) or not (foam). Because of this, they often charge a penalty if the overall airport power factor is too low. There are devices that can be added where the power enters the facility, to help correct the overall power factor. But, using equipment that has an input power factor that is 1.0 or close to it, minimizes the need for or the size of the power factor correcting equipment.

    So when considering the purchase of new equipment for the airport, remember that equipment that costs less initially may cost much more (system wide) over a longer period of time. For the lowest cost over a long time period, choose equipment with a high input power factor and low input distortion values across its load profile, possibly also with Magnetic Wave Shaping as this will considerably lower the operating and maintenance costs in the long run. 

    About the Author: Mark Frink is sales engineer for Hobart Ground Power - ITW GSE  Group. He  is responsible for engineering support to the company's sales force and customers of utility and diesel-powered PCA units, 400 Hz frequency converters and 28V DC power supplies.