AMORPHOUS VS. CRGO CORE TRANSFORMER LOSSES DUE TO HARMONICS

(From npsc2010.uceou.edu)

The performance of transformer, the most ancient power system static element, is affected due to presence of distortion in the input supply voltage and non-linearity in the load current. Due to extensive use of modern power electronics controlled devices, the degree of non-linearity in the load current has increased in recent years. 

This has resulted in substantial increase in two of the major loss components namely core and coil losses. It is well known that transformer efficiency is as high as 99.5%, where as efficiency decreases manifold in above situation. While the criteria for evaluation of the transformer load losses while feeding non-linear load current are well defined in IEEE Standard C57 110, 1998 , the same is not true for the core losses.  Journals 7 and Handbooks gives formulae which requires the design data like thickness of the lamination, t, mass of the core, m, resistivity,ρ, density of the core material,  δ, coefficient of hysteresis loss,  σh  and value of Steinmetz constant, n; the resultant delivers specific core loss only. 

The actual value of core loss can only be computed if the value of Building Factor (which ranges between 1.07 to 1.29) is known, which depends on geometry of construction, stacking factor, mass of the core not in the rolling direction and its angle with respect to rolling direction. The journals, handbooks and standards use proportion of Hysteresis Loss to Eddy Current Loss as 50:50 for cold rolled material.  During the sets of experimentation conducted at Author’s laboratory, it is found that this ratio differs depending upon the material and geometrical configuration of the core. 

So there is a need to develop a novel method for evaluation of core losses of Distorted Input Voltage Fed Transformer. The methodology evolved finds the coefficients of Hysteresis and Eddy Current Loss with distorted waveform. This methodology is independent of design data.  The methodology finds certain constants which remain valid for all the transformers with same design facing any combination of distortions in voltage and current.

The benefits are to utilities in selection of site specific optimized performance of transformer. ERDA has developed a simple method to find the empirical constants required to evaluate core losses of a Distorted Voltage Fed Transformer through experimental results taking design value of flux density as input. The expression model is as below: 

W = k1 . B_s^p. f + k₂ . B_s^q . f^m

 For a given design, the empirical co-efficient so derived in above mathematical model through sets of experiments are valid for any given situation having different combinations of voltage and current distortions. The above mathematical model is independent of design variables such m, ρ, δ, t, coefficient of hysteresis loss, σh used in theoretical formula given.

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