K-FactorTransformer (K系数变压器及非线性负载).pdf

K-Factor Transformers and Nonlinear Loads SUMMARY Harmonic currents generated by nonlinear loods can cause problems in the power system. Transformers are particularly vulnerable to overheating and premature failure. To protect against transformer overheating caused by harmonics, designers can specify derated equipment, i.e., oversized transformers that will run at a fracton of their rated capacity, or K-factor transformers spedfically designed to accommodate harmonic currents. K-factor transformers are preferred because they have additional thermal capacity of known limits, design features that minimize harmonic current losses, and neutral and terminal connections sized at 200% of normal. K-factor transformers allow operation up to nameplate capacity without derating. In recent years, there has been an increased concern about the effects of nonlinear loads on the electric power system. Nonlinear loads are any loads which draw current which is not sinusoidal and include such equipment as arc furnaces, gas discharge lighting, solid state motor drives, battery chargers, UPS systems, and the increasingly common electronic power supply. While nonlinear loads are not new, their increased use means a larger percentage of any power system tends to be nonlinear. Additionally, nonlinear loads were once thought to be a concern only to industrial power systems where large static power converters are used. Such is not the case today. With the widespread application of electronics to virtually every electrical load, nonlinear loads are also prevalent in commercial and even residential power systems. Nonlinear loads generate harmonic currents which flow from the load toward the power source, following the paths of least impedance. Harmonic currents are currents which have frequencies that are whole number multiples of the f