Circuit for the Dynamic Control of Magnetic Flux in Current Transformers

Incorrect relay operation has often been related to saturation of current transformers. In many cases, the obvious solution to this saturation problem is an increased core cross-section of the current transformers, in order to allow a larger flux amplitude. In this respect, the transient asymmetrical component of fault currents is one of the most determining factors governing the dimensioning of the core cross-section of current transformers. A new technique using the characteristics of asymmetrical fault currents, has been developed in order to achieve a dynamic control of the core flux, thus allowing a significant reduction of the required cross-section. It is shown theoretically and experimentally that, taking into account all accuracy requirements and overvoltage limitations, the flux control circuit achieves a net reduction of the core crosssection by a factor of at least 3, even in the worst conditions imposed by C.T. or system parameters.