How to Do Vector Group Test of Transformer: Conditions, Diagrams & Procedures

The vector group test is one of the most critical pre-commissioning tests performed on three-phase transformers to verify the phase relationships and angular displacement between the high voltage (HV) and low voltage (LV) windings. This test ensures that transformers are properly configured and suitable for parallel operation, preventing costly failures and hazardous conditions in power systems.

Proper understanding and correctly performing the vector group test is essential for electrical engineers involved in transformer testing, commissioning, and maintenance. In this blog post we will discuss the the theory, procedure, and practical applications of vector group testing of Transformer in electrical engineering.

What is a Vector Group?

A vector group is the International Electrotechnical Commission (IEC) standardized method of categorizing the winding configurations of three-phase transformers and the phase angle difference between them. The vector group designation provides critical information about:

• High voltage (HV) winding connection (Delta, Star, or Zigzag)
• Low voltage (LV) winding connection (delta, star, or zigzag)
• Phase displacement between HV and LV windings in multiples of 30°

The notation follows the IEC 60076-1 standard, with the HV winding designated by uppercase letters and LV winding by lowercase letters, followed by a numeric digit representing the phase shift.

Vector Group Notation System

The vector group uses a clock notation system where:

• The HV winding is fixed at the 12 o’clock position (reference)
• The LV winding position indicates its phase displacement
• Each hour represents 30 degrees of phase shift
• Phasor rotation is counterclockwise

Common examples include:

• Dyn11: Delta HV, star LV with neutral, LV leads HV by 30° (11 o’clock position)
• YNd11: Star HV with neutral, delta LV, LV leads HV by 30°
• Yyn0: Star HV and LV, 0° phase shift (both at 12 o’clock)
• Dd0: Delta HV and LV, 0° phase shift

Importance of Vector Group Testing

Vector group testing is important for several reasons in power system operations:

1. Parallel Operation of Transformer

Transformers can only be connected in parallel if they have the same vector group. Mismatched vector groups result in large circulating currents that can cause transformer overheating and failure, create short circuit conditions, damage insulation systems that may lead to system instability.

2. System Compatibility

The vector group test ensures that the transformer matches the power system’s phase and grounding requirements. This is essential for proper load balancing, harmonic mitigation, fault protection and isolation, etc.

3. Manufacturing Verification

The test verifies that the transformer has been manufactured according to customer specifications and nameplate ratings. Any manufacturing defects or incorrect winding connections are identified before commissioning.

4. Safety Assurance

Incorrect phasing or vector group mismatches can result in equipment damage, personnel safety hazards, and power system disturbances. The vector group test is a critical safety verification step.

Principle of Vector Group Test

The vector group test is based on the principle of phase-relation verification. By applying a three-phase voltage to one winding and measuring voltages at specific terminals, engineers can determine the angular displacement and relative phase sequence between the HV and LV windings.

The test procedure involves:

1. Connecting specific HV and LV terminals together to create a reference point
2. Applying a three-phase supply (typically 415V) to the HV winding
3. Measuring voltages between various terminal combinations as per the vector group of the transformer (detailed below)
4. Comparing measured voltages with theoretical values derived from vector diagrams

The measured voltage values, when analyzed according to vector diagram principles, reveal the phase displacement and confirm whether the transformer matches its specified vector group.

Vector Group Test Equipment

Required Equipment

1. Three-phase power supply: 415V AC source for test voltage application
2. Digital voltmeters: High-accuracy voltmeters for voltage measurements
3. Connection cables: Properly rated test leads with adequate insulation
4. Phase sequence meter: To confirm positive phase sequence of applied voltage
5. Safety equipment: PPE, insulating mats, and safety barriers

Modern Testing Instruments

Transformer Turn Ratio (TTR) Testers: Advanced TTR equipment with automatic vector group detection capabilities can simplify testing. These instruments:

• Perform ratio tests phase-by-phase
• Measure phase shift automatically
• Display vector group results directly
• Provide excitation current measurements
• Store test data for documentation

Vector Group Test Procedure – General Steps

Preparation and Safety

• Ensure the transformer is de-energized and isolated from all power sources
• Verify all terminals are open and accessible
• Check that the transformer is at normal tap position
• Confirm proper grounding arrangements
• Set up safety barriers and wear appropriate PPE

General Testing Steps

1. Verify phase sequence of the supply before applying voltage
2. Connect specified terminals between HV and LV according to vector group as detailed below
3. Apply 415V three-phase voltage to the HV winding
4. Measure voltages at specified points systematically
5. Record all readings and verify test conditions

Connection Diagram

Test Conditions for Each Vector Group

1. YNyn0 Vector Group Test

The YNyn0 vector group represents a Star-Star connection with 0° phase shift. Both HV and LV windings are star-connected with their neutral points grounded.

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Keep neutral floating
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

2. YNyn6 Vector Group Test

The YNyn6 vector group represents a Star-Star connection with 180° phase shift. Both HV and LV windings are star-connected but with opposite polarity.

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Keep neutral floating
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

3. Dd0 Vector Group Test

The Dd0 vector group represents a Delta-Delta connection with 0° phase shift. Both HV and LV windings are delta-connected with no phase displacement.

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Both windings are delta-connected (no neutral available)
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

4. Dd6 Vector Group Test

The Dd6 vector group represents a Delta-Delta connection with 180° phase shift. Both HV and LV windings are delta-connected but with opposite polarity.

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Both windings are delta-connected
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

5. YNd1 Vector Group Test

The YNd1 vector group represents Star HV with Delta LV connection, with HV leading LV by 30° (or equivalently, LV lagging HV by 30°, at 1 o’clock position).

Connection Method:

• Connect HV R-phase neutral point (1N) to LV delta connected neutral point reference (virtual)
• Connect HV R-phase (1U) to LV R-phase (2u)
• Keep neutral point for star winding available for reference
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

6. YNd11 Vector Group Test

The YNd11 vector group represents Star HV with Delta LV connection, with LV leading HV by 30° (at 11 o’clock position). This is one of the most common distribution transformer configurations.

Connection Method:

• Connect neutral point of star winding (1N) to earth
• Connect HV R-phase (1U/1R1) to LV R-phase (2u/2R1)
• Keep all other terminals open
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

7. Dyn1 Vector Group Test

The Dyn1 vector group represents Delta HV with Star LV connection, with HV leading LV by 30° (or LV lagging HV by 30°, at 1 o’clock position).

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Connect LV neutral point (2N) to earth
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

8. Dyn11 Vector Group Test

The Dyn11 vector group represents Delta HV with Star LV connection, with LV leading HV by 30° (at 11 o’clock position). This is the most widely used vector group in distribution transformers.

Connection Method:

• Connect HV R-phase (1U) to LV R-phase (2u)
• Ground the neutral point of LV star winding (2N)
• Apply 415V three-phase voltage to HV winding

Test Conditions to be satisfied:

Measurement and Recording Format

Standard Test Data Sheet Format

For each vector group test, record the following:

Acceptable Criteria

The vector group test is considered successful when:

1. All test conditions are satisfied within acceptable tolerance
2. Measured voltages match theoretical values derived from vector diagrams
3. Percentage deviation is minimal (typically less than 2-3%)
4. Phase sequence is correctly identified
5. No indication of winding faults or connection errors

Testing Tips

1. Use higher test voltages when possible (within safe limits) for better accuracy
2. Verify phase sequence of the supply before applying voltage
3. Document all connections with photographs for reference
4. Take multiple readings to ensure consistency
5. Compare with factory test reports when available
6. Use calibrated instruments for accurate measurements

Vector Groups and Parallel Operation

Parallel Operation Requirements

For successful parallel operation of transformers:

1. Same vector group is mandatory
2. Same voltage ratio and tap position
3. Same percentage impedance and X/R ratio
4. Same phase sequence
5. Same polarity
6. Similar KVA ratings (within acceptable range)

Vector Group Compatibility

Transformers can be paralleled only within specific groups:

• Group 1 (0° displacement): Yy0, Dd0, Dz0 – can parallel within group only
• Group 2 (180° displacement): Yy6, Dd6, Dz6 – can parallel within group only
• Group 3 (-30° displacement): Yd1, Dy1, Yz1 – can parallel within group only
• Group 4 (+30° displacement): Yd11, Dy11, Yz11 – can parallel within group only

Note: Groups 3 and 4 can sometimes be paralleled with each other under specific conditions, but this is generally not recommended.

Consequences of Incorrect Parallel Operation

Connecting transformers with different vector groups in parallel results in:

• Large circulating currents even at no-load condition
• Severe overheating and accelerated insulation degradation
• Potential short circuit conditions
• Protection system operation and nuisance tripping
• Reduced transformer life and reliability
• Safety hazards for personnel and equipment

Vector Group Test Conditions Summary

Conclusion

The vector group test is an essential pre-commissioning procedure that verifies the phase relationships and winding configurations of three-phase transformers. Proper execution of this test ensures transformer compatibility for parallel operation, prevents costly failures, and guarantees safe and reliable power system operation.

Mastering vector group testing with detailed knowledge of test conditions for each vector group is fundamental for transformer testing and commissioning professionals. With proper understanding of theory, careful execution of test procedures, and thorough analysis of results, engineers can ensure that transformers are correctly configured and ready for safe, reliable operation in electrical power systems.

Frequently Asked Questions (FAQs)