High level of harmonics is very influential on the transformer, and the transformer performance parameters are determined through loss - power losses that occur on the transformer and a decrease in working capacity (derating) which may occur as a result of the harmonic distortion. In this research, analysis of THD (Total Harmonic Distortion) measurement and simulation results with the IEEE 519-1992, analysis of losses in the transformer after affected harmonics, derating analysis that occurs on the transformer, as well as the analysis of additional losses due to current THD. The analysis showed that the value of the voltage across each phase THD Sub Distribution Panel, THD voltage has a value that meets the standard, current THD measurement results mostly not meet the standards. Results of analysis of losses due to harmonics of 2.25 kW/day. Derating that occurs in the transformer is equal to 403.24 kW.