Vector Group and Clock Number: How to Read Them

Short answer

The connection symbol is a short code that says how the windings are connected and how far they are out of phase with each other. Clause 6 of IEC 60076-1 sets its rules. With them you can read any symbol, from the simplest to one with three windings. For how zero-sequence impedance changes with the connection, see zero-sequence impedance by transformer connection.

Writing rules

Clause 6 applies to three-phase transformers and to banks of three single-phase transformers.

The clock number

The phase displacement of a three-phase winding (clause 3.10.6) is the angular difference between the voltage phasors of two windings. A positive-sequence voltage system is applied to the high-voltage terminals. The phasors rotate counterclockwise.

The high-voltage phasor is the reference. The displacement of any other winding is expressed as a clock hour: the hour shown by its phasor when the high-voltage one is at 12. Higher numbers mean greater lag.

Each hour is 30°. An 11 equals 330° of lag. A 5 equals 150° of lag. A 0 means they are in phase.

Clause 6 fixes these drawing conventions:

• The high-voltage winding goes on top and the low-voltage winding below.
• The phasor of phase I of the high voltage points to 12.
• The phasor of phase I of the low-voltage winding is oriented according to the induced voltage that results from the connection.
• The sequence is I - II - III, counterclockwise.

Phase numbering is arbitrary. Terminal marking follows national practice.

Three examples from the standard

Figure 2 of clause 6 illustrates three cases.

For a three-phase autotransformer with the same internal connection, the symbol is the same.

Reading YNyn0d5, winding by winding:

1. YN: 123 kV, star with accessible neutral.
2. yn0: 36 kV, star with accessible neutral, displacement 0 (in phase with the 123 kV).
3. d5: 7.2 kV, delta, lagging 5 hours (150°).

Stabilizing winding

A stabilizing winding is a delta-connected winding that is not brought out to terminals for an external three-phase load. It is indicated with the symbol “+d” after the symbols of the loadable windings. It carries no clock number. The example in the standard is YNauto+d. Further detail on the advantages of wye, delta and zigzag windings: wye, delta and zigzag windings.

Reconnectable windings

If the transformer is specified with a reconnectable winding, in series-parallel or between Y and D, both connections are written together with the corresponding rated voltages. The standard gives two examples:

• 220(110)/10.5 kV YN(YN)d11
• 110/11(6.35) kV YNy0(d11)

The full information must be on the nameplate, under clause 7.2. If connections are changed inside the transformer, it must be shown which were made at the factory. See the power transformer nameplate. To dig deeper into the data a transformer nameplate must carry, read how to read a power transformer nameplate.

Commonly used connections

Annex D of the standard gives diagrams of the connections in general use, with the same drawing conventions as figure 2. It also shows the designation of three-phase autotransformers and of a bank of three single-phase transformers forming a three-phase group with symbol Yd5. More on the conditions for operating transformers in parallel in parallel operation of transformers.

The standard allows the nameplate to show diagrams with terminal markings and with built-in current transformers, together with the text data.

Why it matters for protection and testing

The phase displacement between high and low voltage determines how currents are compensated in differential protection. A misread symbol produces the wrong compensation. The guide on transformer differential protection 87T develops this. To check the angle calculation, use the vector group compensation 87T tool.

In acceptance tests, the measured displacement is checked against the nameplate symbol. It is a routine verification that appears in transformer factory tests.

What not to do

• Do not read the number as an angle in degrees. It is an hour, and each hour is 30°.
• Do not ignore the N. It means the neutral point is brought out to a terminal.
• Do not assume that 11 and 1 are equivalent. One lags by 330° and the other by 30°.
• Do not set the compensation of a protection with a symbol copied from another unit.

When to call us

At Tiza Group we verify the vector group when the equipment is received and calculate the differential protection compensation. See our service of power transformer testing and commissioning in Colombia, or elsewhere on request. Write to us and we will send you a quote.

Related guides

• Transformer differential protection 87T
• Power autotransformer
• Grounding transformer
• Transformer factory tests
• Power transformer
• Zero-Sequence Impedance by Transformer Connection
• Power Transformer Nameplate: How to Read It
• Wye, Delta and Zigzag Transformer Windings Explained
• Parallel Operation of Transformers: Conditions

References

• IEC 60076-1, Power transformers - Part 1: General, clauses 3.10.6, 3.10.7, 6 and 7.2, figure 2 and annex D. Clause numbering of edition 2.1 (1993, with amendment 1 of 1999).

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