Specification of rotor side voltage source inverter of a doubly-fed induction generator for achieving ride-through capability

The ride-through capability of a doubly-fed induction generator under three-phase balanced voltage sags is examined, under the condition that the machine should have the capability of real and reactive power control during the external fault. Mathematical formulae for the peak rotor fault current and the required rotor voltage output under vector control are derived. Moreover, the DC link dynamics are incorporated into the analysis and it is shown that they can have a decisive impact on fault behaviour of the machine during voltage sags. Combined, a design methodology for the rotor side voltage source inverter aiming to achieve a ride-through capability at the lowest cost is described. Simulation results in PSCAD/EMTDC show very good agreement with the theoretical analysis.     

Stability of doubly-fed induction generator under stator voltage orientated vector control

The stability of a doubly-fed induction generator (DFIG) under vector control in stator voltage orientation (SVO) is investigated. Prior art has tended to assume that the inner current loop dynamics can be neglected when an SVO is employed. As a result, the poorly damped poles of the DFIG system were considered unaffected by the inner current loop tuning. The state-space model of the machine including the inner current closed loop dynamics is developed for schemes where different feed-forward compensation terms are used. The interaction between inner current loop dynamics and damping of the critical poles of the system is illustrated through analysis and simulation. The main outcome of the analysis is that the stability of the machine system in an SVO depends solely on the parameters of the proportional-integral controllers. Erroneous tuning can lead to instability, irrespective of the particular feed-forward compensation scheme, which could cause the disconnection of the machine as a result of rotor current oscillations of unacceptable magnitude in an actual case. The main contribution is to provide the necessary methodology in order to ensure the stable operation of a DFIG under SVO vector control.