Abstract:There are abnormal temperature rise and difficult measurement of loss in the lap joint area of transformer laminated core joint. To solve these problems, this study proposes the joint building factor (JBF) method to obtain the accurate calculation ofloss mutation at the lap joint. Firstly, a threedimensional equivalent model of theoverlapped zoneof core joint isformulatedandtheequivalent rangeof theoverlapped zone isdeterminedbythegradient curve.Meanwhile,thecalculation formula of JBF is determinedandtheinfluencing factors of JBF are studied. Then, the whole temperature rise model of transformer considering JBF is established. The temperature rise and influence factors of different hot spots on iron core and structural parts under open circuit test are calculated and analyzed. Finally, the temperature rise test platform of transformer core is built to measure the temperature of joint lap area and hot spots of clamps. Experimental results show that the relative error between the measured value and the calculated value is less than 250%, which proves the effectiveness of the calculation method. Therefore, this method of equivalent loss in the lap area provides a theoretical basis for transformer core loss assessment and engineering design. It is of great significance for transformer safety operation and online monitoring.