S6 Alternator Protection.pdf

Of protection also known as merz price system is most

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of protection (also known as Merz-Price system) is most commonly employed due to its greater sensitivity and reliability. This system of protection is discussed in the following section. 6.1.3 Differential Protection of Alternators: The most common system used for the protection of stator winding faults employs circulating-current principle. In this scheme of protection, currents at the two ends of the protected section are compared. Under normal operating conditions, these currents are equal but may become unequal on the occurrence of a fault in the protected section. The difference of the currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate protected section from the system. This form of protection is also known as Merz-Price circulating current scheme . Schematic arrangement: Fig. 22.2 shows the schematic arrangement of current differential protection for a 3-phase alternator. Identical current transformer pairs CT 1 and CT 2 are placed on either side of each phase of the stator windings. The secondaries of each set of current transformers are connected in star; the two neutral points and the corresponding terminals of the two star groups being connected together by means of a four-core pilot cable. Thus there is an independent path for the currents circulating in each pair of current transformers and the corresponding pilot P . The relay coils are connected in star, the neutral point being connected to the current- transformer common neutral and the outer ends one to each of the other three pilots.
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Operation: Referring to Fig. 22.2, it is clear that the relays are connected in shunt across each circulating path. Therefore, the circuit of Fig. 22.2 can be shown in a simpler form in Fig. 22.3. Under normal operating conditions, the current at both ends of each winding will be equal and hence the currents in the secondaries of two CTs connected in any phase will also be equal. Therefore, there is balanced circulating current in the pilot wires and no current flows through the operating coils (R1, R2 and R 3) of the relays. When an earth- fault or phase-to-phase fault occurs, this condition no longer holds good and the differential current flowing through the relay circuit operates the relay to trip the circuit breaker. (i) Suppose an earth fault occurs on phase R due to breakdown of its insulation to earth as shown in Fig. 22.2. The current in the affected phase winding will flow through the core and frame of the machine to earth, the circuit being completed through the neutral earthing resistance. The currents in the secondaries of the two CTs in phase R will become unequal and the difference of the two currents will flow through the corresponding relay coil (i.e. R1), returning via the neutral pilot.
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