TRANSFORMER - VECTOR GROUP

HOW TO DETERMINE THE VECTOR GROUP OF A TRANSFORMER WITHOUT NAMEPLATE DETAILS

A transformer vector group is a classification system that describes the phase shift between the primary and secondary voltages of a transformer. The vector group is determined by the way the transformer windings are connected.

Determining the vector group of a transformer is crucial to ensure its compliance with customer specifications. This information holds significant importance, particularly when,

• Transformers operate in parallel. Transformer vector groups are important because they must be matched when connecting transformers in parallel. Transformers with different vector groups cannot be connected in parallel because this would cause circulating currents to flow.
• and when implementing differential protection schemes. Accurate knowledge of the phase shift is essential for proper settings in the protection relay to avoid false trips during normal operation.

The vector group test is performed before commissioning the transformer. To find the vector group, follow these steps.

1. Isolate the Transformer: Ensure the transformer is disconnected from the power source.
2. Short Circuit HV R-Phase and LV R-Phase: You can select any combination if you have a good understanding of the vector diagram.
3. Apply 415V AC to the HV Side: Connect a 415V AC source to the HV side of the transformer, minimizing high voltage risks.
4. Measure Voltages in All Combinations: Measure voltages for all possible combinations, as outlined in the table below.

1. Compare the readings with the following conditions; if they match, you have determined the vector group of the transformer:

HOW TO CREATE A VECTOR GRAPH

Ever wondered where these graphs come from, what these conditions mean, or how to draw them yourself? Let's explore how to draw a vector graph independently.

Consider a transformer with the vector group DYn11, where "D" represents delta connection on the HV side, "Yn" signifies star with neutral connection on the LV side, and "11" implies a +30 degree phase shift.

where,

• HV: High Voltage
• LV: Low Voltage

For reference:

Decoding DYn11:

• "HV" is a delta connection, so you can represent it like this:

"LV" is a star connection, so you can draw it smaller due to its lower voltage:

"DYn11" indicates an 11 o'clock position, equivalent to a +30 degree phase shift, which you can represent like this:

Ensuring the Vector Group: To confirm the vector group of the transformer, short-circuit the HV and LV R-phase, effectively connecting R and r. The diagram will look like this:

• R-r Short circuited

From the above drawing, these conditions apply:

1. Voltage between R and Y = Voltage between R and n + Voltage between Y and n.
2. Voltage between B and y is greater than Voltage between B and b.
3. Voltage between Y and b is approximately equal to Voltage between Y and y

If these conditions match the measured voltages, the transformer is identified as DYn11.

Testing Connection Diagram for DYn11 Transformer:

Thank you for reading.

For additional information: The outcome of short-circuiting the HV Y-phase and LV Y-phase is depicted in the following resultant vector diagram (in the case of Y-y short circuit).

Article updated with assistance from ChatGPT.