| The voltages and currents on four distribution sites (FPL{dollar}sb-{dollar}1 to FPL{dollar}sb-{dollar}4) have been monitored. The load behavior with and without the EV load has been recorded. By performing laboratory tests on the GMC EV1 charger charging a fully discharged battery, the characteristics of the charger have been determined. The harmonic spectrum of the transformer current charging a fully discharged battery has been determined. The snap shots of the current and voltage waveforms have been recorded and studied. Based on the laboratory data a computer model of the charger and the distribution system has been developed. Comparing the simulation results to the laboratory results and the field data the simulation results have been validated. The simulated currents are roughly 4 to 10% away from the field recorded currents, in their magnitude and distortion. Based on the field data, laboratory results and the simulation results the derating of the distribution transformer has been calculated. The two methods suggested for the calculation of derating have been presented and compared. The maximum and minimum limits of derating have been suggested. Using the first method the maximum derating is 20% and the minimum value is 99%. Using the modified method the derating remains close to 99% throughout the charging cycle. (11 must be noted here, that a derating to 20% means that the transformer can safely handle only 20% of its full rated load. Likewise a derating to 99% means that the transformer can handle 99% of its full rated capacity. Thus, a transformer derated to 20% is highly derated and a transformer derated to 99% is slightly derated.) After establishing the reliability of the simulation, by comparing the simulation results with the field data and laboratory results, a charging scenario with shorter charging times (45 minutes and 1.2 hours) has been simulated. The distribution transformer derating corresponding to these short-charging times has been calculated and presented. Furthermore, recommended scope for further research has been suggested which among other things, involves validating the derating formula for distribution transformers. |
