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.

DIFFERENT TYPES OF TRANSFORMER PROTECTION BASIC INFORMATION

The protection of the transformer is as important a part of the application as the rating values on the transformer. Entire texts are devoted to the subject of transformer protection.

When investigating a failure, one should collect all the protection-scheme application and confirm that the operation of any tripping function was correct.

Surge Arresters

Surge arrester protective level must be coordinated with the BIL of the transformer. Their purpose, to state what may seem obvious, is to protect the transformer from impulse voltages and high-frequency transients.

Surge arresters do not eliminate voltage transients. They clip the voltages to a level that the transformer insulation system is designed to tolerate. However, repeated impulse voltages can have a harmful effect on the transformer insulation.

Overcurrent Protection

Overcurrent devices must adequately protect the transformer from short circuits. Properly applied, the time–current characteristic of the device should coordinate with that of the transformer.

These characteristics are described in IEEE C57.109-1993, Guide for Liquid-Immersed Transformer Through-Fault Duration. Overcurrent devices may be as simple as power fuses or more complex overcurrent relays.

Modern overcurrent relays contain recording capability that may contain valuable information on the fault being investigated.

Differential Protection

Differential relays, if applied, should be coordinated with the short-circuit current available, the transformer turns ratio and connection, and the current transformers employed in the differential scheme.

If differential relays have operated correctly, a fault occurred within the protected zone. One must determine if the protected zone includes only the transformer, or if other devices, such as buswork or circuit breakers, might have faulted.