Simplified equation for the shielding factor of multiple shielding conductors

Abstract

Electromagnetic interference problems have become increasingly important due to the development of high-voltage bulk power transmission lines and the expansion of ac railway systems. The shielding factor of the shielding conductor is very important in calculating the electromagnetically induced interference voltage. In conventional methods of calculation, the resultant shielding factor of multiple shielding conductors is represented by the product of shielding factors of individual conductors and the effect of mutual interaction between shielding conductors are neglected. As a result, the calculated induced voltage does not always agree with the actual one and the calculated results are often too optimistic. On the other hand, the exact expressions of induced voltage which are derived taking into account the mutual interaction between shielding conductors are too complicated and impractical. This paper proposes a simplified expression for the shielding factor of multiple shielding conductors. A feature of the proposed method is that it always gives a result on the safe side. The proposed method can be applied to a railway system as well as to a transmission system.