Patent ID: 7518256

Claim:
A control system for a double-fed induction generator (DFIG) comprising a rotor ( 1 ) having rotor windings and a stator ( 2 ) having stator windings connectable to a grid for electric power distribution; said control system comprising a converter ( 170 , 171 ), said converter comprising the following components: a rotor-inverter ( 71 - 73 ) connectable to the rotor windings of the generator, a grid-inverter ( 74 - 76 ) connectable to the grid and/or to the stator windings, and a DC-link ( 77 ) for feeding the rotor-inverter; the converter ( 170 , 171 ) further comprising a clamping unit ( 190 ) for protecting the converter from damage due to over-currents in the rotor windings, said clamping unit ( 190 ) being connectable over the rotor windings and arranged to be triggered from a non-operating state to an operating state following detection of an over-current in the rotor-windings, said clamping unit comprising a clamping element ( 290 ) arranged so that when the clamping unit is in its non-operating state, currents in the rotor windings cannot pass through said clamping element, and when the clamping unit is in its operating state, currents in the rotor windings can pass through said clamping element, the clamping unit further comprising for each phase of the rotor, a connector ( 300 ) for connection to the respective rotor phase, each connector being connected to a trigger branch comprising, in series: a point of connection ( 297 ) of the clamping unit, to the connector ( 300 ) for connection to the respective rotor phase; a thyristor ( 295 ) for triggering the clamping unit; the clamping element ( 290 ); a diode ( 296 ); and the point of connection ( 297 ) to the connector ( 300 ) for connection to the respective rotor phase, said clamping element comprising at least one passive voltage-dependent resistor element ( 291 , 292 , 293 , 294 ) for providing a clamping voltage over the rotor windings.