This work was carried out for the Martinlaakso power plant of Vantaa Energy ltd. The goal was to define through both theoretical analysis and measurements what causes the large currents in the rotor earth fault detection system. Rotor earth currents have caused detection system failures and breakdowns.

This study first looked at the principles of synchronous generators and static excitation as well as the rotor earth fault detection system. Next the study focused on carrying out measurements, conducting a simulation at the laboratory and making calculations. Finally, the results thus obtained were examined and compared. Conclusions were made from the results.

This study generated a great deal of information regarding the compatibility of thyristor bridge and old AC injection. The measurements showed that the three phase thyristor bridge along with circuit impedances were causing harmonic currents to the rotor earth fault detection system and AC injection breakdowns.

To avoid this problem this study proposes three different options to protect the earth fault detection system from harmonic currents.