Friday, July 27, 2012

K-Rated Transformers

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]. The K factor can be calculated from the equation:

  • The core has a larger cross section to compensate for increased flux density.
  • Delta primary winding utilizes a heavier conductor due to increased heating from circulating triplen harmonics.
  • Secondary winding uses small parallel conductors to minimize the skin effect.
  • An electrostatic shield is installed between the core and LV winding, and between the HV and LV windings.
  • The transformer can operate at more than 10% system voltage without core
  • A double-size neutral bar and lug pad are installed.
For standard transformers, K = 1 and 1.5 for single- and three-phase transformers, respectively. For nonlinear load applications, the standard K factors are 4, 9, 13, 20, 40, and 50. An example for calculating a nonlinear load K factor is given in
table 5.15, based on table 4.5 of IEEE 1100 [S30]. The J&P Transformer Book  has also provided the following guidelines for estimating the K factor if the data on harmonic current is not available:

  • K1.5 when the nonlinear load is about 15% of the transformer bank rating
  • K4 when the nonlinear load is about 35% of transformer bank rating
  • K13 when the nonlinear load is about 75% of transformer bank rating
  • K20 when the nonlinear load is about 100% of transformer bank rating


Source: IEEE 1100-1992, IEEE Recommended Practice for Powering and Grounding Electronic Equipment, 1992.&Industrial Power System Shoaib Khan

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