Effect of converter packaging techniques on device electrical conduction

High power converters handling several thousands of amperes and employing solid state switches (silicon controlled rectifiers, or SCRs, for example), often have to use multiple devices in parallel. Additionally, if the converter operates across a high voltage bus, it may also require a series/parallel combination of SCRs. It is clear, therefore, that multiple conducting paths are created in any one leg of the converter. Because of mechanical requirements and practical considerations regarding the mounting of the SCRs and the routing of power to them, differential reactances are generated among the different paths, leading unavoidably to unequal current sharing among the various SCRs. This serious consequence may limit the effective current that can be switched or force the use of larger devices, if this is possible. It is very important, therefore, to be able to estimate the actual current distribution among the various power switches for a given realization of the converter. This paper discusses the problem as it was experienced in the actual implementation of a three-phase SCR bridge converter. The system geometry is described and the results of the electromagnetic finite-element analysis of one leg of the converter are given, highlighting the expected current distribution unbalance.