This paper proposes a radial-grouping-based planning method for the electrical collector systems (ECSs) to design the topology and select the cross-sections of ECS submarine cables in tidal current generation farms (TCGFs) while minimizing investment and operation costs. First, an angle-based radial-grouping method is proposed to group the tidal current turbines (TCTs) in a TCGF, considering their capacity limitations while avoiding the trans-region crossing of submarine cables. Second, an optimal planning model is presented to determine the connection scheme for the TCTs within the same group in a TCGF. The investment and trenching costs of submarine cables and ECS power losses are modeled to minimize the comprehensive annual equivalent cost of ECS, while satisfying the submarine cables' voltage drop and capacity limitation constraints. Third, an efficient solution method is developed by modifying a genetic algorithm with the Prufer number, which retains the ECS radial topology and connectivity with high computational efficiency. Finally, two TCGFs with distinct characteristics in the North Orkney Islands, Scotland, and their measured tidal current data are used to demonstrate the effectiveness, correctness and adaptability of the proposed method. (C) 2020 Elsevier Ltd. All rights reserved.