Protection of Generators

 Protection of generators present for illustration several schemes for protecting generators by relays which are able to detect improper circuit behavior. Some protection schemes are followings,

1)  Generator Differential Protection:

Current differential protection is a scheme by which the current measured at two points are compared. These currents under normal conditions are equal but may become unequal on the occurrence of a fault. This scheme applied to one winding of a generator is Fig. 1. A current transformer at the neutral end of the terminal end of the winding. A current relay is connected. In normal operation the currents at both ends of the winding are equal and in phase. The secondary current around the loop then in the same at all points and no current flows through the relay coil.
If a fault develops at winding which causes current to flow from the winding to the grounded frame or to another winding, the current at the neutral end differs from the current at the terminal end. The current in the secondary circuit to the left of the relay must correspond to the current in the neutral end of the winding, while the current to the right of the relay must correspond to the current at the terminal end. The difference of these two currents must pass through the relay current coil. This relay then closes its contacts to clear the generator from the system. This usually the throttle on the prime mover driving the generator.
Note that this relay system responds to a ground fault at any point between CT 1 on the generator neutral and CT 2 on the machine terminal. The system does not respond to a fault at Q, for now the currents in CT 1 and CT 2 are identical and the difference current to the relay is zero. The region in which a fault causes operation of a relay is known as the protected zone.
The above arrangement assumes that the two current transformers CT 1 and CT 2 are identical, a situation that really never exists. If ratio errors in one are slightly different from those in the other, relay current may be present during normal operation and particularly during faults external to the generator. To avoid this difficulty, the relay is modified to respond to the differential current in terms of its fractional relation to the current flowing in the generator winding. Thus under heavy load, a greater differential current through the relay operating coil is required for its operation than under light load conditions. Such a relay is termed a percent differential relay.

Figure 1 : Current differential protection of a generator

2)  Split-Winding Protection:

A balanced winding differential protection scheme may be used on large generators which have two parallel paths through the machine for each winding. As shown in Fig. 2, a current transformer connected in each branch of the winding permits a comparison of the path currents. If these two path currents differ by a predetermined amount, the relay operates to clear the generator from the system. A percent differential relay may be used in this application.

The protection zone for the system of Fig. 2 extends from the machine neutral to the current transformers on the machine terminals. A fault at P, outside the protection zone, will not cause operation of the relay shown. An additional relay system would be necessary to initiate the clearing of a fault at this point.

Figure 2: Split-Winding Protection

3)  Ground Fault: 

A scheme for generator ground protection is in Figure 3 shows a scheme for protection a generator against a ground fault on one of its terminals or within the winding itself. Such a condition will cause current to flow in the neutral ground terminal, which will be detected by an over current relay. The protected zone extends from the machine neutral to an indefinite distance out on the lines connecting directly to the machine terminals.

Figure 3: Ground fault protection ( neutral current measurement)

Another scheme for generator ground-fault protection is shown in Figure 4. A potential transformer is connected between the generator neutral and ground. Across the secondary of this transformer is connected an impedance Z and an over voltage relay. Under normal conditions, a small current flows in the impedance as unbalances in the power system cause the machine neutral to be slightly different from zero potential. The voltage across the relay is low. On the occurrence of a ground fault, the machine neutral rises to value equal to normal line-to-ground voltage, causing operation of the over voltage relay. The relay operates for a ground fault at scheme of protection has the advantage that the current in the fault is low, being controlled in magnitude by the value of the impedance Z in the potential transformer secondary.

Figure 4: Ground fault protection (neutral voltage measurement)

4)  Ground-Fault Differential:

Another scheme for generator fault protection is shown in Figure 5. The secondaries of each of the three current transformers connected at the terminals of the machine are connected in parallel. Under ordinary operation, the currents flowing in the generator leads and hence the current flowing in the secondary winding add to zero and no current flows to the relay. Similarly, under normal conditions the neutral current is zero and no current is supplied by the neutral lead current transformer to the relay. If a ground fault develops at A external to the generator protective scheme, the sum of the currents at the terminals of the generator is exactly equal to the current in the neutral connection and the current supplied to the relay is zero. However, for a ground fault at B flows through the relay. Operation of the relay will clear the machine from the system. The protected zone is limited to the region between the neutral and the current transformers on the machine terminals.

Figure 5: Differential ground fault protection 

5)  More Relay Schemes:

• Over current protection,
• Reversed power,
• Reversed vars,
• Excess temperature,
• Loss of bearing oil pressure.