In case of secondary output of electronic current transformer

the linear preservation of characteristic information and the restriction of non-characteristic information are the physical essential requirements of primary and secondary fusion.The factor affecting the primary and secondary fusion is mainly compound error

time constant

structural symmetry and electromagnetic interference of adjacent elements.The parameter correlation equation is used in this paper to design the optimal design scheme.The nano-crystalline alloy with high permeability is used as the core for measurement

protection and zero sequence

and the same core is used for measurement and protection

while the whole core is used for zero sequence current core.When rating the accurate limit current of short circuit

1.1 T of protective magnetic hermetic and0.96 T of zero sequence magnetic hermetic are measured so to avoid nonlinear phenomena.The current/voltage converting resistor is placed in the input interface of the control unit and the regulation is based on current.It is shown by the experiment that in the full current range

the measurement protection reaches 0.5 S level

the zero sequence ratio difference and composite error are less than 1%

which can meet the requirements of primary and secondary fusion.