The stray-loss factor for copper conductors varies as the square of the load current and the square of the frequency, and will therefore vary with the harmonic mix in the power supply. Although the percentage contribution to distortion by higher harmonics decreases as the harmonic frequency rises, its heating effect, even if the harmonic percentages are low, could rise substantially.
🔺 The harmonics generated by nonlinear loads such as variable-frequency drives (VFD) will impose non-sinusoidal current on the power transformers that supply such loads, resulting in a substantial increase in losses and temperature rise.
With the addition of harmonic currents, standard design transformers must be derated to limit the temperature rise to be within the insulation temperature-rise rating or the transformer needs to be replaced with a special “K”-rated transformer or the best and easy solution is Emerich Active Harmonic Mitigator (AHM) is the cost effective solutions for existing transformers
K-Factor Calculation
The K-factor is a number derived from a numerical calculation based on the summation of harmonic currents generated by the non-linear load. The higher the K-factor, the more significant the harmonic current content
The algorithm used to compute K-factor is:
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Σ ih^2
Details of the calculation method can be found in IEEE Standard 1100-1992.
The “K” factor has been established by Underwriter Laboratories (UL) to define the ability of a transformer to serve varying degrees of nonlinear load current without exceeding the rated temperature rise.
The “K” factor is based on the predicted losses as specified in ANSI/IEEE C57.110 [S14].
