III. DE-RATING OF DISTRIBUTION TRANSFORMER Posr-r 2te the structural part stray losses under rated conditions. The factor 0.8 is accepted by IEEE after manufacturer’s verification. For oil filled transformers, these stray losses increase the oil temperature and thus the hot spot temperature. Total load losses in both oil cooled and dry type transformer under non-sinusoidal load condition with current harmonics are calculated by the following expression, According to the IEEE dictionary, de-rating is defined as "the intentional reduction of the stress/strength ratio (e.g., real or apparent power) in the application of an item (e.g., transformer), usually for the purpose of reducing the occurrence of stress-related failure (e.g., reduction of lifetime of transformer due to increased temperature beyond the rated temperature)."Harmonic currents and voltages result in harmonic losses increasing the temperature rise. This rise beyond its rated value results in a reduction of lifetime. ALL PARAMETERS AND LOSSES OF 200K VA TRANSFORMER WORKING AT FULL LOAD A. Base Case of 200kVA Distribution Transformer with Linear Nature of Load at 0.8 P.F. The single-line diagram of the simulated power system is shown in Fig.1 (a).Both primary and secondary full load currents, current THD, and total losses are calculated. It is matching with standard full load test data of the 200kVA distribution transformers with 15% tolerance given by distribution Company. Efficiency of the transformer under this case is 98.12% at 0.8 lagging power factor and current harmonics are below the IEEE standard. In this case transformer, performance is checked without power factor improvement for non-linear load only in which load is adjusted at THDi=28.09% up to 35" harmonics level. From the simulation, it is observed that the losses are increased drastically, which results in efficiency at 96.60%. The transformer maximum current delivery capacity is reduced by 15% as compared to secondary full load current. The voltage profile is also reduced due to increased voltage drop in distribution lines. This arrangement is simulated and is shown in Fig.1(c). The current spectrum and its harmonic current level of a single phase are shown in Fig.2 (a) and (b) respectively. The results are shown in Table 2. Fig.2. (a) Harmonic current spectrum of phase A ; (b)Current harmonic bar chart of phase A at non-linear full load without power factor improvement. In this case transformer, performance is checked with power factor improvement capacitor and non-linear load nature at full load. From the simulation results it is observed that the THDi is increased to 35.42%.It is also observed that the current harmonics are increased in each level compared to previous case. The losses are increased drastically results in efficiency reduction at 92.80%. For the same load distribution, line is overloaded by 4.5% and the load carrying capability of a transformer is reduced by 30% compared to previous case. The transformer maximum current delivery capacity is reduced by 44% as compared to secondary full load current. The voltage profile is also disturbed due to increased voltage drop in distribution lines results in reduction in apparent power capacity. Here under non-sinusoidal load condition, power factor improvement is impossible with simple capacitor banks only. An ordinary capacitor bank also acts as a source of harmonics as current THD is increased. This arrangement is simulated and is shown in Fig.1(d). The current spectrum and harmonics level is shown in Fig.3(a)&(b) respectivelv.The results are shown in Table? Hence, correct form of true power factor under linear and non-linear load environments is given by the following expression, Under the harmonic load conditions, total harmonic distortion or distortion factor is used for its level measurement. It is the ratio of the rms value of the harmonics (voltage or current) above fundamental to the rms value of the fundamental. It can be given by the following expression, SIMULATION RESULTS OF 200KVA TRANSFORMER TESTEDWHEN PASSIVE FILTERS ARE TESTED Fig.4. (a) Harmonic current spectrum of phase A; (b) Current harmonic level bar chart of phase A at non-linear full load with passive filters. International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) The calculations shown in Table I and Table II, it can be observed that the K-factor rating increases with total current harmonic distortion. The relation between K- factor and THDi is shown in Fig.5, as given below. Power factor capacitor contributes for increase in total current harmonic distortion level with non-linear load. It alone doesn’t helpful for improving total power factor but can improve displacement power factor. This relation is plotted in Fig 6. Fig.6. Relation between total power factor and THDi. International Journal of Emerging Technology and Advanced Engineering Website: www.iietae.com (ISSN 2250-2459. Volume 1. Issue 2. December 2011) The maximum current delivered by the transformer is inversely proportional to the total current harmonic distortion. Transformer KVA capacity also reduced with current harmonic level. When total current harmonic distortion level is 28.09%, K-13 rating and_ for THDi=35.52%, K-20 rating transformer is recommended. When passive filters are used, the K-factor is reduced to K-1.Other mitigation measures suggested are given below:
