A modular energy storage device for a high voltage electrical power system

A modular energy storage device (1, 40) for a high voltage electrical power system is provided comprising at least two modules (3a-c, 4a-c) connected in series, each module (3a-c, 4a-c) including at least one dc power-source unit (5) enclosed in a container (7) and a positive (9a, 10a) and a negative (9b, 10b) terminal, and the device further comprising a positive (12) and a negative (11) pole, a first (14, 30) and a second (17) conductor arranged to form a current path between the positive (9a, 10a) and negative (9b, 10b) poles, the first conductor (14, 30) including a plurality of conductor parts (23) connected to the terminals (9a-b, 10a-b) of the modules (3a-c, 4a-c) to provide a series connection of the modules (3a-c, 4a-c), a first module (3a, 3c, 4a, 4c) of the series connection of the modules (3a-c, 4a-c) being connected to one of the poles (11-12), and the second conductor (17) is connected between a last module (3c, 3a, 4c, 4a) in the series connection of modules (3a-c, 4a-c) and the other pole (11-12), wherein the first (14, 30) and second (17) conductors are arranged to pass through the containers (7) in parallel such that a current flows through the first conductor (14, 30) in a first direction and through the second conductor (17) in a second direction opposite to the first direction.