With the growing adoption of energy-efficient technologies, electronics have become integral to modern power systems. Switched Mode Power Supplies (SMPS) in office equipment and LED lighting, Variable Frequency/Speed Drives (VF/SDs) for induction motors, and inverters that convert DC power from photovoltaic cells to Mains Frequency AC are prime examples of how these systems enhance power usage efficiency. However, a significant drawback of these electronic devices is their non-linear operation, which leads to the generation of harmonics—distorted waveforms that increase the total harmonic content of both voltage and current. These harmonics, if not properly managed, can propagate upstream through supply lines, impacting transformers by causing excessive heating and accelerated aging. In extreme cases, this can result in severe equipment damage or failure.
This article introduces three key metrics to evaluate the impact of current harmonic heating on transformers: Harmonic Loss Factor, K Factor, and Factor K. These metrics help assess how harmonics contribute to transformer heating, allowing for more informed decisions in managing transformer health and longevity.
Consider a transformer rated at 200A connected to a Variable Speed Drive (VSD), whose input current spectrum mimics that of a six-pulse rectifier and is normalized to 104.1A RMS. In this scenario, the rated eddy current loss (PEC-R), as well as other coefficients like e and q, are set to 10%, 0.1, and 1.7 respectively. The transformer harmonic derating metrics—Harmonic Loss Factor (FHL), K Factor, Factor K, and others—are computed using predefined equations to evaluate how harmonics impact transformer performance.
Per IEEE Standard C57.110-1998, the maximum harmonic number is capped at 25 due to the increasing prominence of the skin effect at higher frequencies, which leads to conservative values for eddy-current loss predictions, particularly above the 19th harmonic. As illustrated in Figure 1, the harmonic derating metrics increase as the harmonic number rises. For instance, at the 25th harmonic, the values for FHL, K Factor, FHL-STR, and Factor K are 8.35, 2.26, 1.34, and 1.15, respectively.
The cumulative impact of harmonics on transformers can be seen in the derating factors. Figure 2 highlights that dry-type transformers face a derating factor of 77.4%, while oil-filled transformers fare slightly better with a derating factor of 87.2%. These derating factors translate to equivalent operating currents of 155A for dry-type transformers and 174A for oil-filled transformers. This shows that oil-filled transformers are better equipped to handle harmonics, though both types still experience a decrease in capacity due to the presence of harmonics.
To mitigate the harmful effects of harmonics on transformers and extend their operational life, YT Electric offers a cutting-edge solution in the form of Active Harmonic Filters (AHF). These devices actively monitor and filter out harmful harmonics before they can propagate into transformers and other sensitive equipment. By installing YT Electric’s AHF, facilities can significantly reduce harmonic distortion in their power systems, improve energy efficiency, and prevent transformer overheating, thereby minimizing the need for costly derating and reducing the risk of equipment failure. This solution is ideal for industries relying heavily on VSDs, SMPS, and other non-linear loads, ensuring smooth and reliable power distribution even in complex systems.
Incorporating YT Electric's Active Harmonic Filters not only preserves the health of transformers but also enhances the overall power quality of your electrical infrastructure.
Subscribe to us to enjoy event prices and get some of the best prices.