Earth Continuity Conductor Location in Single-Circuit Underground Transmission Line Using the Generalized Model

This paper investigates the impact of earth continuity conductor (ECC) placement in duct banks for triangular and flat configurations of three-phase single-core underground cables using the single-point bonding method. Several works in the literature suggest that the ECC should be placed on the right side of the duct/trench for half of the underground transmission line (UGTL) and on the left side for the remaining length, implying the necessity of ECC transposition. However, this technique poses significant civil engineering challenges, requiring extensive work that increases the cost and construction time of the UGTL. Therefore, it is essential to propose a better ECC position that mitigates these problems while satisfying electrical, civil, and safety requirements. To address these issues, this study proposes a better ECC position that meets electrical, civil, and safety requirements while minimizing induced voltage on the sheath and induced current in the ECC. We focus on an underground transmission line operating at 220 kV, evaluating both triangular and flat configurations. The paper employs accurate formulations for the series impedance calculation of the cable and computes the induced factors under steady-state conditions, considering practical aspects such as civil works and ease of installation. Our findings indicate that in triangular formations, the optimal ECC position is precisely in the middle of the cables. This placement minimizes induced current and voltage on the sheath, eliminating the need for ECC transposition, contrasting with IEEE 575 guidelines. Conversely, ECC transposition is crucial for flat formations to minimize circulating current. Our study recommends placing the ECC at the top of the duct at a specific distance, differing from IEEE 575 and CIGRE 531 guidelines. When ECC transposition is performed, the induced voltage on the sheath can be calculated as if the ECC were not present.