Diagnostics of the transmission properties of the slip ring system in doubly fed induction generators

The slip ring apparatus in doubly fed induction generators is used to transfer the rotor current from the rotating to the static part of the generator. During the current transfer micro brush fires occur in the contact area when the minimum contact force F_C is not maintained. This depends on the spring force applied to the brush top side, the sliding speed V_s of the slip ring, the electrical variables (e.g. amplitude of brush current, contact voltage) and environmental influences (e.g. air humidity, ambient temperature, dust). All these variables are measured in our slip ring apparatus. Consequently, this rises the contact temperature that affects the wear of the contact materials (graphite). Using the envelope analysis method, the electrical variables were examined in the frequency domain, where the first and the even harmonic were considered. Since the system is asymmetrical, the mean of the side bands amplitudes for each position is calculated. To evaluate the influence of v and F_C, the quotient with the fundamental amplitude A_C is formed and is defined as the sparking intensity. With this method, both parameters the sliding speed and the contact force must be approximately constant and the measurement time must be at least 30 seconds.