| 摘要 |
For a synchronous electric generator an earth fault detection or protection circuit is provided using extracted (35A, 35B, 35C) third order harmonic currents (Ia3, Ib3, Ic3) in the phase windings (1A, 1B, 1C) at locations at the neutral point (N, 5). From the third order harmonic currents, directional criteria (Da, Db, Dc) for each phase winding are calculated (35) indicating the relative magnitude of the third order harmonic current reduced by the other third order harmonic currents. The directional criteria are compared (37) to a normal value valid and it is signalled, when at least one of the directional criteria deviate too much from the normal value, to a control device (39) for disconnecting or disabling the phase windings. Two other criteria, a fault detection criterion (FC) and a range control criterion (RC) are also calculated (41; 49) that can prevent the control device from issuing any signal. The earth fault protection circuit can detect earth faults occurring close the neutral point and can be used in all synchronous machines, which have a neutral point that is isolated or earthed through a large impedance, and which are connected directly to either a non-effectively earthed power network or an effectively earthed power network, for instance high voltage generators of the type Powerformer<TM>. |
