TERTIARY-WINDING OVERCURRENT PROTECTION BASIC INFORMATION

The tertiary winding of an autotransformer, or three-winding transformer, is usually of much smaller kVA rating than the main windings. Therefore, fuses or overcurrent relays set to protect the main windings offer almost no protection to tertiaries.

During external system ground faults, tertiary windings may carry very heavy currents. Hence, to guard against failure of the primary protection for external ground faults, separate tertiary overcurrent protection may be desirable.

The method selected for protecting the tertiary generally depends on whether or not the tertiary is used to carry load. If the tertiary does not carry load, protection can be provided by a single overcurrent relay connected to a CT in series with one winding of the Δ.

This relay will sense system grounds as well as phase faults in the tertiary or in its leads. When tertiary windings are connected by cables, the overcurrent protection provided to the tertiary winding should account for the thermal withstand of the cables.

Alarming and tripping as a result of a prolonged unbalance condition or load tap changer malfunction should prevent damage to cables. If the tertiary is used to carry load, partial protection can be provided by a single overcurrent relay supplied by three CTs, one in each winding of the Δ and connected in parallel to the relay.

This connection provides only zero sequence overload protection and does not protect for positive and negative sequence overload current. In this case, the relay will operate for system ground but will not operate for phase faults in the tertiary or its leads.

Where deemed necessary, separate relaying such as differential type should be provided for protection for phase faults in the tertiary or its leads. The setting of the tertiary overcurrent relay can normally be based on considerations similar to those in line time overcurrent.

However, if the tertiary does not carry load, or if load is to be carried and the three CT, zero sequence connection is used, the associated overcurrent relay can be set below the rating of the tertiary winding. This relay should still be set to coordinate with other system relays.

DIFFERENTIAL PROTECTION OF AUTOTRANSFORMERS USING HIGH-IMPEDANCE RELAYS BASIC INFORMATION AND TYPICAL SCHEMATIC DIAGRAM

Some utilities provide protection for large high-voltage and extra-high-voltage autotransformers by using voltage-operated bus-type high-impedance differential relays. Typical connections of this protective system for autotransformers, with the neutral point of the wye winding solidly grounded, are shown below.

Typical schematic connections for high-impedance differential protection of a Y autotransformer with unloaded tertiary

This arrangement provides protection for all types of phase faults and ground faults, but not turn-to turn

faults. In this application, three sets of three-phase CTs are required, one set on the high-voltage side, another set on the low-voltage side, and the third set in the neutral ends of the winding.

All CTs should have the same turns ratio and should be reasonably matched in accuracy class. A single high-impedance relay connected in a ground differential scheme is also applicable for autotransformer protection.

This protection is immune to the effects of magnetizing inrush current because inrush current is cancelled by the neutral CTs. Also, there is no imbalance current in the relay circuit due to the load tap changing equipment.

Thus a high-impedance differential relay can be applied without any harmonic restraint, load bias, or time delay. Autotransformers are often provided with a Δ tertiary winding. It should be noted that with this type of scheme no protection is afforded for faults occurring in the Δ tertiary winding.

Where the terminals for this winding are not brought out to supply load, one corner of the Δ can be connected between the end of one phase of the main winding and its neutral CT. This connection is shown above.

In such an arrangement, the tertiary winding is included in the differential protection zone, and the relay would sense ground faults in the tertiary winding. This scheme does not provide protection for phase faults or turn-to-turn faults in the tertiary winding.

Where the tertiary winding is used to supply load, the Δ winding corner connection cannot be used. Hence, separate protection is required.

Information can be found here.