GE K-45404F Manuel d'utilisateur télécharger pdf (Page 4)

GEK-45404

APPLICATION

These are single-phase, sensitive, time-delay overvoltage relays. They are principally applicable for

sensitive overvoltage ground detection for high-impedance grounded generators or for ungrounded

power systems. These relays all have a capacitor in series with the operating coil. For the IAV51K

and IAV52K relays, this capacitor is external to the relay because of its large size. The effect of the

capacitor is to tune the relay coil circuit so that the relay pickup on third harmonic (180Hz) voltage is

approximately eight times the relay pickup at 60 Hz. This is particularly important in overvoltage

ground detection for generators, since they produce a large third- harmonic component of voltage

during normal operation.

The external connection diagram for the IAV51K or the IAV52K relay for sensitive generator ground

detection is shown in Fig. 8. This is a typical protection scheme for a unit generator transformer

using a distribution transformer connected in the neutral with a resistor across the transformer

secondary. The IAV51K/52K relays are usually used for this application because they provide the

very sensitive 5.4 volt pickup rating. This enables the relay to detect a ground fault within a few

percent of the winding from the neutral. Since the power transformer and the station service

transformer in a unit generator transformer scheme are both connected in delta on the generator side,

coordination with other protection is virtually unnecessary and the ground relay used can be very

sensitive. Some time delay in relay operation is usually used to coordinate with fuses protecting

against faults on the secondary side of potential transformers that are normally connected to the

generator terminals. The usual practice is to use these relays to trip the generator breaker and shut

down the machine. If the relay is used only to sound an alarm when a ground fault is detected, the

application should be reviewed. It may be necessary to add another relay to provide ground

protection with good sensitivity simultaneously with a high continuous-voltage rating.

The resistor, shown in Fig. 8 connected across the broken delta secondary of the PT's, or alternatively

connected in series in the primary neutral connection, is usually required to prevent the occurrence of

ferro-resonance. This phenomenon could occur due to the interaction of the PT inductance with the

distributed capacitance-to-ground of the primary power system.

The external connection diagram for the IAV51D, 52D, or the IAV52C relay for ground-fault

detection on an ungrounded power system is shown in Fig. 9. The relay operates to detect the first

ground fault that occurs on the system, connections so that it can be removed before a second

ground occurs, causing a double phase-to-ground fault, which requires a service interruption. Since

the relay may be applied merely to sound an alarm in this application, it is necessary that it be

continuously rated for the full broken-delta voltage expected for a single phase-to-ground fault

located right at the potential transformers, or some automatic means must be employed to disconnect

the relay from the voltage supply.

When this fault occurs it is equivalent to shorting out one phase of the potential transformer primary.

The other two unfaulted phases of the PT primaries now have full phase-to-phase voltage applied

and their corresponding dJelta secondaries will be supplying √3 times their normal rated voltage.

The broken delta equivalent voltage will be these two voltages added in series at a 60

angle and the

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GE K-45404F Manuel d'utilisateur Page 4