1 Design of 25 KA Current Injection Transformer Core with Finite Element Method HOSSEIN HEYDARI, MOHSEN ARIANNEJAD, FARAMARZ FAGHIHI Iran University of Science and Technology, Tehran, Iran Abstract—.Since Current Injection Transformers (CIT) are within the major group of the standard type test equipments in electrical industry, their performances are very crucial. CIT is widely used for standard tests in electrical industry for a variety of purposes, such as relay protection systems consisting of insolated HV conductor, fuses, SF6 switches, low voltage switches (ACB, and MCCB), sectionalizing switches, and so on With the help of Finite Element Method (FEM), some of the prominent problems can be solved. This paper attempts to fill this void by making direct comparison of the localized flux in 25KA CIT with three-limb single phase transformer cores. Flux distribution is solved by Ansys5.4 software .Then we compare results of the three-limb single phase transformer cores together and we obtain effect of overlap size of the core laminations on its hot-spot temperature. Key-Words: Current Injection Transformer (CIT), Finite Element Method (FEM), Local Flux density. 1-INTRODUCTION As an electrical utility you have the right to demand that every piece of power equipment installed on your system meets your exact specifications. The reliability and safety of your system depends on all of the components performing as intended. As a manufacturer, the performance and conformance to industry standards of your product are of prime importance. Testing can assure that your product meets or exceeds performance standards. Current Injection Transformers (CIT) is within the major group of the standard type test equipments in electrical industry and its performance is very critical. . Secondary voltage of this transformer are very low, is less than 10 volts, and its output current varied from 1 to 100KA. The final objective of this paper is to establish a FEM method which leads to localized fluxes of the CIT [1, 2]. Case study in this paper is dry- type transformer that its output current, is 25KA, its secondary voltage is 5 volts and short-circuit time duration is about 3 seconds for 25KA test. 2-CURRENT INJECTION TRANSFORMER EQUATIONS 2.1- The Voltage Equation Faraday’s Law relates the impressed voltage on a winding to the rate of change of flux density (29 1 dt dB NA v m - = Where N is the number of turns and A m is the effective cross sectional area of the magnetic core. In the case of laminated and tape-wound cores, this is less than the physical area c A due to interlamination space and insulation. The layout of the transformer is shown in Fig (1) and the physical parameters are illustrated. Fig.1, Typical layout of Current Injection Transformer The two areas are related by the core stacking factor K f ( c f m A k A = . Typically, K f is 0.9 for laminated cores.