To accurately verify the physical principle and to explore appropriate technical routes for the anti-interference testing of heavy current transformers（HCT）

we discuss the optimal design of testing coils for "non-uniformly distributed equal ampere-turns testing method" in detail. Specifically

an improved physical model of the interfering magnetic field is illustrated

and a core-pickup factor is introduced for calibration

which is valued by an empirical equation summarized from simulation studies. Based on the physical model

two different approaches for the optimization of the testing coil are proposed

namely

L2 norm minimum method and partial summation equivalent method. The superiority of the proposed testing coil optimization methods

compared to the previous designs

are verified by both simulations and experiments

which are also applied for validating the theoretical analysis.