K-Factor rated transformers are preferred over oversized (derated) conventional transformers because they are designed to supply nonlinear loads, are equipped with 200% rated neutral bus, and are likely to be smaller and less expensive. Disadvantages of an over-sized standard transformer may include the requirement for a higher short-circuit rating on circuit breakers and the drawing of a higher inrush current. Derating a standard transformer is only a temporary fix that often translates into lower efficiency operation.
The “K-Factor” conveys a transformer’s ability to serve varying degrees of nonlinear loads without exceeding the rated temperature rise limits. For any given nonlinear load, if the harmonic current components are known, the K-Factor can be calculated and compared to the transformer’s nameplate K-Factor. As long as the load K-Factor is equal to or less than the transformer’s rated K-Factor, the transformer does not need to be derated. The higher the K-Factor, the more non-linear loads the transformer can handle. The actual formula to determine K-Factor takes into account the frequency and current intensity of each individual harmonic. The chart below portrays typical load K-Factors.
In an electrical power system, harmonics are current and voltage with frequencies that are integer multiples of the fundamental power frequency. That is, in a power system with a fundamental frequency of 60Hz, the second harmonic is 120Hz, the third harmonic is 180Hz, et cetera. Harmonics have no useful purpose, yet contribute to losses and lower system efficiency. Harmonics return over the neutral and are dissipated as heat in cables and load devices.
Transformers serving nonlinear loads have increased winding temperatures due to harmonic currents generated by those loads. This overheating likely will result in a shortened service life for the transformer. For example, operating a transformer at 10 degrees C above its insulation-rated class will cause approximately a 50% reduction in the life of the transformer. If the over temperature gets high enough or lasts long enough, the transformer will prematurely fail. Other warning signs of harmonics include: overheated neutral conductors and panels, unexplained tripping of circuit breakers, line voltage distortion, equipment failure, electrical fires, control equipment mis-operation, and interference on communication lines.