ELECTRICAL ENGINEERS

Generator Inter-turn Fault Protection Inter-turn fault have commonly been disregarded on the basis that if they occur they will quickly develop into earth faults. This is probably true if the fault is in the slot portion but will take a little longer in the region of the end connection. An approach of this kind is never attractive and may be entirely unjustified. There is a possibility of the machine being very seriously damaged before the fault evolves to a condition that can be detected by the longitudinal system. Modern medium size and large size turbo generators have the stator winding designed with only one turn per phase per slot. For these machines Inter-turn faults can only occur in case of double ground faults or as a result of severe mechanical damage of the stator end winding. The latter is considered rather unlikely to occur. It is generally considered difficult to obtain a reliable protection against short-circuiting of one turn if the stator winding has a la

Rotor Earth Fault Protection (64R1/64R2) The field circuit of generator (i.e. rotor winding) is an isolated D.C. circuit and not earthed anywhere. The field can be exposed to abnormal mechanical or thermal stresses due to vibration, excessive currents or choked cooling medium flow. This may result in a breakdown of the insulation between the field winding and the rotor iron at one point where the stress has been too high. A single earth fault in the field winding or its associated circuits, therefore, gives rise to a negligible fault current and does not represent any immediate danger. If however a second ground fault should occur, heavy fault current and severe mechanical unbalance may quickly arise and lead to serious damage. It is essential therefore that any occurrence of insulation failure is discover and that the machine is taken out of service as soon as possible. Normally the machine is tripped instantly on occurrence of second rotor earth fault. Three method are avai

Stator Earth Fault Protection In most countries, it is a common  practice  to ground the generator neutral through a Grounding Transformer having a loading resistor across its secondary. This method of earthing is called high impedance earthing where the earth fault current is limited to 5-10 Amps. Tuned reactors which limit the ground fault current to less than 1.0 A are also used. The generator grounding resistor normally limits the neutral voltage transmitted from the high voltage side of the unit transformer in case of a ground fault on the H.V. side to maximum 2-3% of rated generator phase voltage. Short circuits between the stator winding in the slots and the stator core are the most common electrical fault in generators. Interturn faults, which normally are difficult to detect, will quickly develop into a ground fault and will be tripped by the stator ground fault protection. 95% stator ground fault relay for generator For generators with unit transformer

Class-C Protection : When a fault in equipment outside the “UNIT” is not isolated for some reason by the relays/switchgear of that equipment, has to be cleared by “back up” relays provided in the generator protection system. In such cases it is sufficient to isolate the unit from the system side and “run it back to house load” which will be supplied by the Generator to the 6.6 KV systems through UAT. Such trips when only 400 KV breakers (generator main CB) are opened are called “Class-C Trips”. After restoration of normalcy in the system, the generator can be resynchronized. Such trip also initiates HP and LP by pass arrangement to save boiler from probable tripping.

Class-B Protection : It initiates the closing of turbine stop and interceptor valve. The load reduces to 1% or less within a few seconds; this situation is detected by a: Low Forward Power” relay which, after a short delay, initiates a main CB trip. The process takes 4 to 5 sec from the original initiation and is intended to prevent a possible speed run away if over speed governor does not function correctly (Tripping of unit after a time delay). A.                  Class B trip through unit lock out relay 86G-3 under following conditions 1.                    Distillate flow to generator stator winding decreases to 13 tons/hour. 2.                   Distillate specific resistance is very low (50K-ohms-cm). 3.                    Generator transformer winding temperature increases to 1250 C. 4.                    UAT A&B winding temperature increases to 1250 C. 5.                    Generator transformer back up over current and eart

Class A Protection : It initiates a main CB trip as fast as can be arranged. Electrical failure damage propagates so quickly that class A tripping is essential. Taking the trip relay and CB operating time into account, this means about 120 ms (6 cycles) after initiation. With most modern CBs this time may be reduced to about 80 ms. The following situations are commonly protected by class A trip through unit lock out relays 86G-1 and 86G-2(Instantaneous Tripping of unit). A. Tripping conditions for 86G-1 will energize on the operation of the following protections : 1.                   Differential protection of Generator Transformer and Unit Auxiliary transformers-A/B. 2.                    Generator and Transformer overall differential protection. 3.                    Generator transformer and UAT fire protection. 4.                   Generator inter winding protection. 5.                    Generator Transformer over flux protection.