- Vector Group Test Definition: The vector group test of a transformer checks the phase sequence and angular difference to ensure transformers can operate in parallel.
- Phase Sequence Importance: Matching the phase sequence is essential for parallel operation of transformers to avoid short circuits.
- Secondary Connections: Different primary three-phase connections can lead to various secondary voltages and phases.
- Vector Group Classification: Transformers are grouped based on their phase sequence and angular differences, affecting their compatibility for parallel operation.
- Test Procedure: The vector group test involves connecting specific terminals, applying voltage, and measuring outputs to determine the transformer’s vector group.
Vector Group Test of Transformer
The vector group of a transformer is the HV-to-LV phase displacement. The vector group test checks that displacement so units can share parallel operation of transformers. Every electrical power transformer is given this factory check so the built group matches the specified group. IEC 60076-1 lists voltage ratio and vector-group check as a routine test.
Phase sequence, the order in which phases reach peak voltage, must be the same on transformers that run in parallel. If it is not, each pair of phases is shorted once per cycle.
A three phase transformer can use several secondary connections for a given primary three-phase connection. For the same primary three-phase voltage, secondary line voltages can differ in magnitude and phase according to the internal connection of the transformer.
A worked example shows the 30° clock-hour shift.
The primary and secondary coils on any one limb have induced emfs that are in time phase. Take two transformers with the same number of primary turns and with star-connected primaries. Each also has the same secondary turns per phase. The first has a star secondary and the second a delta secondary. With the same primary voltages, each secondary phase emf stays in time phase with its own primary phase, because those coils sit on the same limb of the core of transformer. On the star secondary the line voltage is √3 times the induced voltage of one secondary phase coil. On the delta secondary the line voltage equals the induced voltage of one secondary phase coil. The secondary line-voltage vectors then sit 30o apart. That is the usual star-delta clock-hour displacement in IEC 60076 (groups 1 and 11).
If those two units are paralleled, a circulating current flows because of the phase angle between their secondary line voltages. Ratio cannot cancel that displacement. Units with different secondary phase displacements are not used for parallel operation of transformers.
The table below groups connections that can run in parallel when phase sequence and angular displacement match. Three-phase transformers are classed by that vector relation. Units in the same vector group can be paralleled if the other parallel-operation conditions are also met.
| GROUP | Connection | Connection | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0 (0o) |
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| 1 ( 30o) |
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| 6 ( 180o) |
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| 11 ( – 30o) |
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Procedure of Vector Group Test of Transformer
Take a YNd11 transformer as the example.
- Connect neutral point of star connected winding with earth.
- Join 1U of HV and 2W of LV together.
- Apply 415 V, three phase supply to HV terminals.
- Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral.
- Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W.
For a YNd11 transformer the voltages should satisfy
2U-1N > 2V-1N > 2W-1N
2V-1W > 2V-1V or 2W-1W .
The vector group test of transformer for other groups follows the same jumper-and-voltmeter method, with different inequality checks. A 415 V three-phase HV supply is a common workshop method, not the only factory method.
