Inter turn stator winding fault can easily be detected by stator differential protection or stator earth fault protection. Hence, it is not very essential to provide special protection scheme for inter turn faults occurred in stator winding. This type of faults is generated if the insulation between conductors (with different potential) in the same slot is punctured. This type of fault rapidly changes to earth fault.
The high voltage generator contains a large number of conductors per slot in the stator winding hence, in these cases the additional inter turn fault protection of the stator winding may be essential. Moreover in modern practice, inter turn protection is becoming essential for all large generating units.
Several methods can be adopted for providing inter turn protection to the stator winding of generator. Cross differential methods is most common among them. In this scheme the winding for each phase is divided into two parallel paths.
Each of the paths is fitted with identical current transformer. The secondary of these current transformers are connected in cross. The current transformer secondary’s are cross connected because currents at the primary of both CTs are entering unlike the case of differential protection of transformer where current entering from one side and leaving to other side of the transformer.
The differential relay along with series stabilizing resistor are connected across the CT secondary loop as shown in the figure. If any inter turn fault occurs in any path of the stator winding, there will be an unbalanced in the CT secondary circuits thereby actuates 87 differential relay. Cross differential protection scheme should be applied in each of the phases individually as shown.
An alternative scheme of inter turn fault protection of stator winding of generator is also used. This scheme provides complete protection against internal faults of all synchronous machines irrespective of the type of the winding employed or the kind of methods for connection. An internal fault in the stator winding generates second harmonic current, included in the field winding and exciter circuits of the generator. This current can be applied to a sensitive polarized relay via a CT and filter circuit.
The scheme operation is controlled by a direction of negative phase sequence relay, in order to prevent operation during external unbalanced faults or asymmetrical load conditions. Should there be any asymmetry external to the generator unit zone, the negative phase sequence relay prevents a complete shutdown, only allowing the main circuit breaker to be tripped, to prevent the rotor damage due to the over rating effects of second harmonic currents.