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POWER SYSTEM HARMONIC EFFECTS ON DISTRIBUTION TRANSFORMERS AND NEW DESIGN CONSIDERATIONS FOR K FACTOR TRANSFORMERS TECHNICAL PAPER

POWER SYSTEM HARMONIC EFFECTS ON DISTRIBUTION TRANSFORMERS AND NEW DESIGN CONSIDERATIONS FOR K FACTOR TRANSFORMERS TECHNICAL PAPERA technical paper completed by N.R Jayasinghe, J.R Lucas, K.B.I.M. Perera

ABSTRACT
This paper presents the effects of harmonic distortion of load current & voltages on distribution transformers, the standard ways of calculating the harmonic effects & design & development of K Factor transformer, which can operate under a specific harmonic environment. The usage of nonlinear loads on power systems has increased the awareness of the potential reduction of a transformer’s life due to increased heat losses.

The performance analysis of transformers in a harmonic environment requires knowledge of the load mix, details of the load current harmonic content & total THD. The additional heating experienced by a transformer depends on the harmonic content of the load current & the design principals of the transformer. Both No load & Load losses are affected by the presence of harmonics in load currents. But the variation in load losses contributes more to excessive heat generation in distribution transformer. Increment in no load losses in a distribution transformer due to harmonics is less compared to the load loss but it has a significant contribution to the capitalization cost when operating in longer term.

 The load loss components get affected by the harmonic current loading are the I2R loss, winding eddy current loss & the other stray losses. The K-FACTOR method is an approximation of the total stray loss heating effect, including the fundamental and harmonic contributions & finally new design techniques for K-FACTOR transformers are discussed. In designing of K-FACTOR transformers different design techniques like parallel conductor arrangement for windings, lower flux density & introduction of static shields are discussed & the estimated results are compared with actual implemented results.

CONCLUSION
The new design considerations proposed, helped to reduce the effects of harmonics, and the experiments done with the 5kVA transformer certify the facts. It is observed that the current harmonics are playing a dominant role in additional heating effects, but always there should be a proper attention by the utility engineers, on the voltage harmonics also.

The case study done with the 12-pulse transformer shows that there can be severe cases than expected. The proposed modification done to the winding construction and the clamping structure has contributed significantly to the stray loss reduction. But for low capacity transformers (below 50kVA) experiences less effects due to harmonics as their construction it self can withstand to some extent.

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