Method for detecting a ground fault, a control unit and an energy storage system

A method for detecting a ground fault in an energy storage system comprising at least one string having a plurality of energy storage modules, wherein said at least one string is connected between a first direct current (DC) pole and a second DC pole providing a DC side, and wherein the first DC pole is connected to an AC side through a first converter and the second DC pole is connected to the AC side through a second converter, the method comprising:

• receiving at least one phase-to-ground voltage measured at the AC side and at least one pole-to-ground voltage measured at the DC side,
• determining a first co-phasal harmonic component based on the at least one phase-to-ground voltage measured at the AC side,
• determining a second co-phasal harmonic component based on the at least one pole-to-ground voltage measured at the DC side,
• determining a presence of a ground fault if the first co-phasal harmonic component increases and/or the second co-phasal harmonic component decreases, and
• tripping the energy storage system when the ground fault is detected.