Phasor groups for 3-phase transformers


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PHASOR GROUPS:

Phasor groups are used to denote different connection methods for poly phase transformers. This knowledge is required for connecting the poly phase transformers in parallel.

To understand the designation of these groups one has to know the assumptions.

These are:

  1. In the phasor diagrams, the phasors rotate in the counter clockwise direction.
  2. We use Clock Method of Angle Designation
    According to this method, the HV line phasor is considered as the minute hand always set at 12 O’clock (zero hour) position and the corresponding LV line phasor is represented by hour hand .

As we know that each digit in clock differs by adjacent one with 30°.

For ex: Take Yd11

Here capital letter(Y) denotes HV winding, small letter (d) denotes LV winding and NUMBER (11) denotes phase displacement between HV and LV emfs expressed as clock hour number.
It represents

A poly phase transformer with HV winding in star, LV winding in delta and the LV line phasor 11 O’clock i.e 30° ahead of the zero hour position of the HV line phasors.

Different Phasor Groups:

Group-1 (0° phase displacement)

• In this connection both HV and LV phasors are in phase with each other.
• In this group Yy0 and Dd0 connections are available.
• Here zero denotes both minute and hour hands are in same position i.e. at 12 O’ clock.
• LV phasors are inphase with the HV phasors .

Group-2 (180° phase displacement)

• In this connection both HV and LV phasors are at 180° phase displacement with each other.
• In this group Yy6 and Dd6 connections are available.
• Here 6 denotes minute hand is on 12 O’ clock and hour hand is on 6 O’ clock.
• LV phasors lead the HV phasors by 180°.

Group-3 (-30° phase displacement)

• In this connection both HV and LV phasors are at 30° phase displacement with each other.
• In this group Yy1 and Dd1 connections are available.
• Here 1 denotes minute hand is at 12 O’ clock and hour hand is at 1 O’ clock.
• LV phasors lag the HV phasors by 30°.

Group-4 (+30° phase displacement)

• In this connection both HV and LV phasors are at 30° phase displacement with each other.
• In this group Yy11 and Dd11 connections are available.
• Here 11 denotes minute hand is at 12 O’ clock and hour hand is at 11 O’ clock.
• LV phasors lead the HV phasors by 30°.