Analysis of derating necessity of oil-immersed 10/0.4 kV/kV power transformers–the case studies of four PV power plants

In the last few years many PV power plants have been connected to the electric distribution network. They are nonlinear sources of electric power and consequently generate current harmonics. These harmonics cause heating of power transformers due to the additional eddy current and core losses, skin effect, etc. This paper analyzes the necessity of derating power transformers installed at the points of common coupling (PCC) of PV power plants. Calculations are done in accordance with EN 50464-3:2007. Four two-winding three-phase oil-immersed 10/0.4 kV/kV power transformers with rated power equal to 150 kVA (two of them), 250 kVA, and 1250 kVA are analyzed. In all cases, it is presented that there is no need to derating power transformers because analyzed PV inverters generate low current harmonics at medium and high power.