Accurate switching loss evaluation is playing an increasingly important role in the reliability design of large power converters and devices. For instance， the unbalanced power loss of two switches in one arm has occurred in the operational test of 50 MVA STATCOM. In view of such a problem， this paper analyzes the electrical relations between the commutating switches， develops a new electrical-magnetic model and power loss calculating method for IGCTs. With the new method， the unbalanced loss is attributed to the redundant PWM state and an improved PWM is presented. Laboratory experiment and simulation results have proved that the method can be used in the design of thermal systems and for improving the balance of power loss. The waveform given by the electrical-magnetic model is close to the actual waveform.