Differential Protection of Auto-Transformers

Autotransformers are designed either as three-phase units or are made up of three single-phase units. They are used for interconnecting solidly earthed EHV and HV networks if the rated voltages of both networks do not differ by more than a factor of two to three. Use of auto- transformers saves material and consequenly weight as well as losses compared with the use of transformers with separate windings for each voltage level.

Autotransformers with wye connected primary and secondary windings (serial and common winding) are usually equipped with a delta-connected tertiary winding that is rated about one third of the throughput rating. The serial, common and tertiary winding arrangements of an auto- transformer are shown in Figure 38.

Figure 38  An auto transformer with a delta connected tertiary winding

Shunt reactors or capacitors can be connected to the tertiary winding for power factor correction. A booster transformer, consisting of energizing and regulating windings, can be accommodated in the same tank for voltage control by in-phase or phase-angle regulation as is shown in Figure 39.

It is possible to use different features with differential protection of autotransformers depending on the application. Features of four scenarios are listed in Table 8; these senarios are then discussed in this section.

Figure 39  Autotransformer with energizing and regulating windings Table 8  Features of four scenarios concerning autotransformers

Delta tertiary without feeders

Neutral earthing with phase-segregated CTs

CTs in series with delta tertiary

CTs outside delta tertiary

Differential protection Two-end Three-end Three-end Three-end

Amplitude matching Vnom,a Vnom,b1 Vnom,a = Vnom,b = Vnom,c

1 _

3∙Vnom,c1 Vnom,c 1

Vector group matching Ga-b = 0 Ga-b = 0; VGa-c = 0 Ga-b = 0; Ga-c = 0 Ga-b = 0; Ga-c = odd

Zero sequence current filter With Without Without With

Inrush stabilization With Without With With

Phase-segregation No Yes Yes No

Voltage adjustment effect Yes No No Yes

Sensitivity to earth faults Low High High Low

Turn-to-turn fault protection Yes No Yes Yes

Delta tertiary protection No No Yes Yes

1

a,b,c: ends of the differential protection

A two-ended differential protection may be applied if the tertiary winding is connected in delta, is used for stabilizing only and no load or voltage regulating capacitors or reactors are connected to this winding. This arrangement is shown in Figure 40. The setting of the differential protection corresponds to the setting of a two-winding transformer with neutral grounded at both ends.

Figure 40  Differential protection of an autotransformer if the tertiary is not connected to any load

3.1.2.1 Neutral grounding with phase-segregated CTs

The use of phase segregated CTs on an autotransformer is shown in Figure 41. In case of neutral grounding with phase-segregated CTs the ideal solution is to apply a three-end differential protection. The protected zone corresponds to a galvanic connected electrical node.

Figure 41  Differential protection of an autotransformer with three segregated CTs in the neutral

3.1.2.2 Amplitude matching:

Because of the galvanic connected electrical node, amplitude matching has to be based on a common primary nominal voltage (nominal voltage of the serial or of the common winding) for all three ends.

3.1.2.3 Vector group matching:

Because of the galvanic connected electrical node vector group matching of end b (vector group a-b) and vector group matching of end c (vector group a-c) have to be set corresponding to vector group number „0‟.

3.1.2.4 Zero-sequence current filtering:

Because the neutral-to-earth current is included, zero-sequence current filtering may be disabled for all three ends.

3.1.2.5 Inrush stabilization:

The inrush restraint is not required because current summation for differential protection is done at an electrically connected node with no transformer coupling.

The differential protection described above works strictly on phase-segregated CT arrangements only because inrush stabilization is not included. While sensitivity to earth fault detection is high, turn-to-turn faults and faults on the tertiary winding cannot be detected.

3.1.2.6 CTs in series with phase windings of delta-connected tertiary

Information on the coupling between the autotransformer winding and the tertiary winding is available if the current through the tertiary winding is measured instead of the neutral-to-earth current per phase as shown in Figure 42.

Figure 42  Differential protection of an autotransformer using three buried CTs in the tertiary winding

3.1.2.7 Amplitude matching:

Amplitude matching is based on the nominal primary voltages of the individual terminals because of coupling of the transformer winding. Considering that the CTs of the third end are located in

series with the delta-connected tertiary winding, 3 times the primary nominal voltage has to be

3.1.2.8 Vector group matching:

Vector group matching of end b (vector group a-b) and vector group matching of end c (vector group a-c) have to be set corresponding to vector group number „0‟ because of the limb related measuring systems.

3.1.2.9 Zero-sequence current filtering:

Zero-sequence current filtering may be disabled for all three ends because the neutral-to-earth current is available from measurement of the current in the tertiary winding.

3.1.2.10 Inrush stabilization:

Stabilization of the relay during magnetizing inrush should be enabled to take care of the coupling between the transformer windings. The turn-to-turn faults can be detected in principle because of the coupling of the transformer windings and the tertiary winding is included in the protection zone. Earth faults on the regulating winding will be detected too whereas the differential measuring systems are not affected by voltage adjustment. Only the requirement of stabilizing during magnetizing inrush is unfavourable.

3.1.2.11 CTs outside delta tertiary winding

In many cases, CTs of the tertiary winding are not located in series with the phase windings but are located outside the delta winding as shown in Figure 43. A three-end differential protection may be applied in this case. This differential protection offers the largest protection zone in comparison to all the applications described above. However, the requirement of zero-sequence current filtering leads to reduced in earth fault sensitivity. The setting of the differential protection corresponds to the setting of a separate-winding transformer. The differential measuring systems are affected by in-phase or phase-angle regulation.

3.1.2.12 Tripping characteristic:

This overall differential protection is affected by voltage adjustment. This has to be taken into consideration for the setting of the tripping characteristic.

Figure 43  Differential protection of an autotransformer with CT in the circuit connected to the tertiary winding