Operation of the generator with full converter and long tower cables leads to common mode and bearing current in the wind turbine. Common mode and bearing current are almost always present in such systems, due to the harmonics reflections within large tower cables. An electrical system should be carefully designed to perform in such environments. Bearing insulation thickness should be carefully chosen to minimize the impact of bearing current. Ideally these currents should flow through the dedicated path within the nacelle and pass through the tower to the ground. Due to complex structure of the nacelle in the onshore geared turbine and dealing with a very high frequency current, this is usually not the case. A complex cable routing and formation of different conducting path leads the common mode current to flows into an unexpected direction. It is the part of a good system design to monitor the path and evaluate the potential impact on the sensitive components to avoid potential failure in the field. A higher downtime of the turbine corresponds to a less reliable turbine, this is also closely associated with the levelized cost of electricity. This paper presents an overview of the system for the preferred common mode current routing and recommend various method by which formation of unwanted grounding loops can be avoided.

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