مدل سازی و تحلیل کاهش حد مجاز ترانسفورماتورها تحت بارهای غیر سینوسی
Abstract
I- Introduction
II- Modeling of De-Rating
III- Simulation and Experimental Results
IV- Conclusion
References
Abstract
When supplying transformers under non-linear loads, electromagnetic losses including core loss and copper losses are increased. This results in boasting temperature and consequently reducing transformer lifetime due to the insulation deterioration. To solve the problem, the rated capacity of the transformer connected to a harmonic load is usually reduced which is called the de-rating of transformer. In the present paper, the conventional method for calculating derating of transformer is introduced first and its essential equations are given. Based on finite element method (FEM), a new method is then developed for de-rating determination. To evaluate the proposed methods, the simulation and experimental results are presented for a 2 KVA single-phase transformer.
Introduction
One of the main components of the power system network is the distribution transformer and design of the distribution transformers is normally performed for pure sinusoidal voltage and current waveforms. However, the majority of the loads from modern electric installations are nonlinear and therefore they generate non-sinusoidal currents. In other word, the power system network is subjected to high levels of harmonic currents due to extensive usage of solid state electronics in electrical devices such as power supplies and electric drives. The current harmonics flowing through the transformer windings lead to additional losses and temperature rise and consequently the transformer life-time expectancy could be reduced significantly. In order to avoid this matter, the reduction of the apparent power rating of transformer is deliberately done which is known as de-rating. To calculate the reduced kVA for a distribution transformer connected to harmonics load, some mathematical equations are presented in [1]. Adjusting the total losses to the rated losses, measurement of de-rating is considered in [2] for the single-phase transformers operating with non-sinusoidal currents and voltages. Using the harmonic loss factor, eddy-current loss coefficient and iron-core losses, de-rating is also calculated and it is compared to measured data. In addition to mathematical modeling, the finite element (FE) analysis is carried out in [3] to calculate the de-rating of three-phase transformers.