Inter Turn Fault protection of Stator
Turn to turn or inter turn fault on the same phase winding of the stator, can be never sensed by the percentage differential protection relay, which is otherwise called as modified Merz-price differential relay. This is because, the current that these types of fault produces, flows in a local circuit between the turns involved and does not create a difference between the currents entering and leaving the winding at its two ends where the CTs are placed.
If the stator has only one coil per slot, inter turn fault cannot occur. For multi coils per slot, inter turn faults are likely to occur.
Hence instead of longitudinal differential relay, transverse differential relay is used, to detect the stator inter turn fault.
In case of multi coil wound stator, as used in hydro generators, each phase winding is divided into two halves, owing to very high currents they have to carry.
Parallel Wound Generator:
The schematic diagram of a biased transverse differential protection for stator inter-turn faults is shown in the figure above.
This type of protection requires special arrangements for the winding. To identify the fault occurring between turns, each winding (phase) is split into two as shown in the fig. Under normal conditions, the current through the two windings are equal. Hence, the currents through the secondary of the two CTs are equal and no current flows through the operating coil of the relay and so the relay will not operate. If there is any fault between the turns the current through the two CTs will not be equal and hence the difference of the currents flows through the relay operating coil. The relay will pickup and close the trip circuit to isolate the fault sections.
Generator with single winding per phasor having inaccessible parallel winding
This type of protection uses zero sequence components of voltage caused by the reduction of emf in the faulted phase, When the fault occurs, the zero sequence voltage appears across tertiary winding of the voltage transformer which is connected to the operating coil of the three element directional relay and hence the relay operates.
Restricted Ground Fault Protection by Differential System
If the generator neutral is solidly earthed or grounded, it is possible to protect complete windings, against phase to ground fault.
As neutral is earthed through a resistance to limit the earth fault current, with this type of earthing , it is not possible to protect complete winding from earth-fault and percentage of winding protected, depends upon the value of the neutral earthing resistor and the relay setting. The setting should be such that the protection does not operate for earth faults on EHV side. Earth faults are not likely to occur near the neutral point due to less voltage with reference to earth. It is the usual practice to protect about 80 to 85% of generator winding against earth-faults. The remaining 20 to 15% winding from neutral side is left unprotected by the differential relay. Hence a separate earth fault protection is provided to take care of the complete winding against earth faults.
During earth fault in the generator winding (G.W), the fault current IF, flows through a part of the winding and neutral to ground circuit. The corresponding secondary current IS flows through the operating coil (OC) and the restricted earth fault (R.E.F) relay coil of the differential protection shown in the figure above.
If the earth fault IF occurs at point F of the generator winding, VaF is available to drive earth fault current IF through the neutral to ground connection. If point ‘F’ in nearer to ‘a’ i.e., nearer to neutral point, the forcing voltage VaF is relatively less. Hence earth fault current IF will be small. It is not practicable to keep the relay setting too sensitive to sense the earth fault currents of small magnitudes. Hence, the practice is to protect about 85% of the generator winding against phase to earth fault and to leave the 15% portion unprotected by the differential protection against earth faults.
The resistance R limits the earth-fault current show in the figure above. If R is too small (solid earthing) is not used. Solid earthing is limited to 3.3KV. For low resistance earthing, the resistance R is such that full current passes through neutral for a full line to neutral voltage. For a 60 MW generator, the resistance is of such a value that the earth is about 200A. In high resistance earthing, maximum earth fault current is of the order of 10A, which is normally used in distribution transformers and generator transformer units. With higher neutral resistance, the earth fault current is reduced. Hence lesser percentage of winding is protected by the restricted earth fault protection.