Vector group

In electrical engineering, a vector group is the International Electrotechnical Commission (IEC) method of categorizing the primary and secondary winding configurations of three-phase transformers. It indicates the windings configurations and the difference in phase angle between them. For example, star(H.V)-delta(L.V) 30 degree lead is denoted as Yd11.

The phase windings of a polyphase transformer can be connected internally in different configurations, depending on what characteristics are needed from the transformer. For example, in a three-phase power system, it may be necessary to connect a three-wire system to a four-wire system, or vice versa. Because of this, transformers are manufactured with a variety of winding configurations to meet these requirements.

Different combinations of winding connections will result in different phase angles between the voltages on the windings. This limits the types of transformers that can be connected between two systems, because mismatching phase angles can result in circulating current and other system disturbances.

Symbol designation

The vector group provides a simple way of indicating how the internal connections of a particular transformer are arranged. In the system adopted by the IEC, the vector group is indicated by a code consisting of two or three letters, followed by one or two digits. The letters indicate the winding configuration as follows:

• D: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings form a triangular configuration with the terminals on the points of the triangle.
• Y: Wye winding, also called a star winding. Each phase terminal connects to one end of a winding, and the other end of each winding connects to the other two at a central point, so that the configuration resembles a capital letter Y. The central point may be connected outside of the transformer.
• Z: Zigzag winding, or interconnected star winding. Basically similar to a star winding, but the windings are arranged so that the three legs are "bent" when the phase diagram is drawn. Zigzag-wound transformers have special characteristics and are not commonly used where these characteristics are not needed.
• III: Independent windings. The three windings are not interconnected inside the transformer at all, and must be connected externally.

In the IEC vector group code, each letter stands for one set of windings. The HV winding is designated with a capital letter, followed by medium or low voltage windings designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, with HV winding is taken as a reference. The number is in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected HV winding and a wye-connected LV winding. The phase angle of the LV winding lags the HV by 30 degrees.

Phase displacement

Phase rotation is always anti-clockwise. (international adopted convention) Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°. Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°. The minute hand is set on 12 o'clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point. Because rotation is anti-clockwise, 1 = 30° lagging (LV lags HV with 30°)and 11 = 330° lagging or 30° leading (LV leads HV with 30°)

The point of confusion is in how to use this notation in a step-up transformer. As the IEC60076-1 standard has stated, the notation is HV-LV in sequence. For example, a step-up transformer with a delta-connected primary, and star-connected secondary, is not written as 'dY11', but 'Yd11'. The 11 indicates the LV winding leads the HV by 30 degrees.

Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.

See also

• Single phase electric power
• Three-phase electric power

   Transformer vector group depends upon
       Removing harmonics: Dy connection – y winding nullifies 3rd harmonics, preventing it to be reflected on delta side.
       Parallel operations: All the transformers should have same vector group & polarity of the winding.
       Earth fault Relay: A Dd transformer does not have neutral. to restrict the earth faults in such systems, we may use zig zag wound transformer to create a neutral along with the earth fault relay..
       Type of Non Liner Load: systems having different types of harmonics & non linear Types of loads e.g. furnace heaters ,VFDS etc for that we may use Dyn11, Dyn21, Dyn31 configuration, wherein, 30 deg. shifts of voltages nullifies the 3rd harmonics to zero in the supply system.
       Type of Transformer Application: Generally for Power export transformer i.e. generator side is connected in delta and load side is connected in star. For Power export import transformers i.e. in Transmission Purpose Transformer star star connection may be preferred by some since this avoids a grounding transformer on generator side and perhaps save on neutral insulation. Most of systems are running in this configuration. May be less harmful than operating delta system incorrectly. Yd or Dy connection is standard for all unit connected generators.
       There are a number of factors associated with transformer connections and may be useful in designing a system, and the application of the factors therefore determines the best selection of transformers. For example:

For selecting Star Connection:

   A star connection presents a neutral. If the transformer also includes a delta winding, that neutral will be stable and can be grounded to become a reference for the system. A transformer with a star winding that does NOT include a delta does not present a stable neutral.
   Star-star transformers are used if there is a requirement to avoid a 30deg phase shift, if there is a desire to construct the three-phase transformer bank from single-phase transformers, or if the transformer is going to be switched on a single-pole basis (ie, one phase at a time), perhaps using manual switches.
   Star-star transformers are typically found in distribution applications, or in large sizes interconnecting high-voltage transmission systems. Some star-star transformers are equipped with a third winding connected in delta to stabilize the neutral.

For selecting Delta Connection:

   A delta connection introduces a 30 electrical degree phase shift.
   A delta connection ‘traps’ the flow of zero sequence currents.

For selecting Delta-Star Connection:

   Delta-star transformers are the most common and most generally useful transformers.
   Delta-delta transformers may be chosen if there is no need for a stable neutral, or if there is a requirement to avoid a 30 electrical degree phase shift. The most common application of a delta-delta transformer is as tan isolation transformer for a power converter.

For selecting Zig zag Connection:'

   The Zig Zag winding reduces voltage unbalance in systems where the load is not equally distributed between phases, and permits neutral current loading with inherently low zero-sequence impedance. It is therefore often used for earthing transformers.
   Provision of a neutral earth point or points, where the neutral is referred to earth either directly or through impedance. Transformers are used to give the neutral point in the majority of systems. The star or interconnected star (Z) winding configurations give a neutral location. If for various reasons, only delta windings are used at a particular voltage level on a particular system, a neutral point can still be provided by a purpose-made transformer called a ‘neutral earthing.

For selecting Distribution Transformer:'

    The first criterion to consider in choosing a vector group for a distribution transformer for a facility is to know whether we want a delta-star or star-star. Utilities often prefer star-star transformers, but these require 4-wire input feeders and 4-wire output feeders (i.e. incoming and outgoing neutral conductors).
   For distribution transformers within a facility, often delta-star are chosen because these transformers do not require 4-wire input; a 3-wire primary feeder circuit suffices to supply a 4-wire secondary circuit. That is because any zero sequence current required by the secondary to supply earth faults or unbalanced loads is supplied by the delta primary winding, and is not required from the upstream power source. The method of earthing on the secondary is independent of the primary for delta-star transformers.
   The second criterion to consider is what phase-shift you want between primary and secondary. For example, Dy11 and Dy5 transformers are both delta-star. If we don’t care about the phase-shift, then either transformer will do the job. Phase-shift is important when we are paralleling sources. We want the phase-shifts of the sources to be identical.
   If we are paralleling transformers, then you want them to have the same the same vector group. If you are replacing a transformer, use the same vector group for the new transformer, otherwise the existing VTs and CTs used for protection and metering will not work properly.
   There is no technical difference between the one vector groups (i.e. Yd1) or another vector group (i.e. Yd11) in terms of performance. The only factor affecting the choice between one or the other is system phasing, ie whether parts of the network fed from the transformer need to operate in parallel with another source. It also matters if you have an auxiliary transformer connected to generator terminals. Vector matching at the auxiliary bus bar

Application of Transformer according to Vector Group:

(1) (Dyn11, Dyn1, YNd1, YNd11)

   Common for distribution transformers.
   Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1.
   We can use Dyn1 at distribution system, when we are using Generator Transformer are YNd11.
   In some industries 6 pulse electric drives are using due to this 5thharmonics will generate if we use Dyn1 it will be suppress the 5th harmonics.
   Star point facilitates mixed loading of three phase and single phase consumer connections.
   The delta winding carry third harmonics and stabilizes star point potential.
   A delta-Star connection is used for step-up generating stations. If HV winding is star connected there will be saving in cost of insulation.
   But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

(2) Star-Star (Yy0 or Yy6)

   Mainly used for large system tie-up Transformer.
   Most economical connection in HV power system to interconnect between two delta systems and to provide neutral for grounding both of them.
   Tertiary winding stabilizes the neutral conditions. In star connected transformers, load can be connected between line and neutral, only if
   (a) the source side transformers is delta connected or
   (b) the source side is star connected with neutral connected back to the source neutral.
   In This Transformers. Insulation cost is highly reduced. Neutral wire can permit mixed loading.
   Triple harmonics are absent in the lines. These triple harmonic currents cannot flow, unless there is a neutral wire. This connection produces oscillating neutral.
   Three phase shell type units have large triple harmonic phase voltage. However three phase core type transformers work satisfactorily.
   A tertiary mesh connected winding may be required to stabilize the oscillating neutral due to third harmonics in three phase banks.

(3) Delta – Delta (Dd 0 or Dd 6)

   This is an economical connection for large low voltage transformers.
   Large unbalance of load can be met without difficulty.
   Delta permits a circulating path for triple harmonics thus attenuates the same.
   It is possible to operate with one transformer removed in open delta or” V” connection meeting 58 percent of the balanced load.
   Three phase units cannot have this facility. Mixed single phase loading is not possible due to the absence of neutral.

(4)  Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

   These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading.
   This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes.
   The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer.
   Due to zigzag connection (interconnection between phases), third harmonic voltages are reduced. It also allows unbalanced loading. The zigzag connection is employed for LV winding. For a given total voltage per phase, the zigzag side requires 15% more turns as compared to normal phase connection. In cases where delta connections are weak due to large number of turns and small cross sections, then zigzag star connection is preferred. It is also used in rectifiers.

(5) Zig- zag/ star (ZY1 or Zy11)

   Zigzag connection is obtained by inter connection of phases.4-wire system is possible on both sides. Unbalanced loading is also possible. Oscillating neutral problem is absent in this connection.
   This connection requires 15% more turns for the same voltage on the zigzag side and hence costs more. Hence a bank of three single phase transformers cost about 15% more than their 3-phase counterpart. Also, they occupy more space. But the spare capacity cost will be less and single phase units are easier to transport.
   Unbalanced operation of the transformer with large zero sequence fundamental mmf content also does not affect its performance. Even with Yy type of poly phase connection without neutral connection the oscillating neutral does not occur with these cores. Finally, three phase cores themselves cost less than three single phase units due to compactness.

(6)  Yd5:

   Mainly used for machine and main Transformer in large Power Station and Transmission Substation.
   The Neutral point can be loaded with rated Current.

(7)  Yz-5

   For Distribution Transformer up to 250MVA for local distribution system.
   The Neutral point can be loaded with rated Current.

== Application of Transformer according  according to Uses: ==

    Step up Transformer: It should be Yd1 or Yd11.
   Step down Transformer: It should be Dy1 or Dy11.

   Grounding purpose Transformer: It should be Yz1 or Dz11.
   Distribution Transformer: We can consider vector group of Dzn0 which reduce the 75% of harmonics in secondary side.
   Power Transformer: Vector group is deepen on application for Example : Generating Transformer : Dyn1 , Furnace Transformer: Ynyn0.

Convert One Group of Transformer to Other Group by Channing External Connection:

(1) Group I: Example: Dd0 (no phase displacement between HV and LV).

   The conventional method is to connect the red phase on A/a, Yellow phase on B/b, and the Blue phase on C/c.
   Other phase displacements are possible with unconventional connections (for instance red on b, yellow on c and blue on a) By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd0 transformer can be changed either to a Dd4(-120°) or Dd8(+120°) connection. The same is true for internal connected Dd4 or Dd8 transformers.

(2) Group II: Example: Dd6 (180° displacement between HV and LV).

   By doing some unconventional connections externally on one side of the Transformer, an internal connected Dd6 transformer can be changed either to a Dd2(-60°) or Dd10(+60°) connection.

(3) Group III: Example: Dyn1 (-30° displacement between HV and LV).

   By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn1 transformer can be changed either to a Dyn5(-150°) or Dyn9(+90°) connection.

(4) Group IV: Example: Dyn11 (+30° displacement between HV and LV).

   By doing some unconventional connections externally on one side of the Transformer, an internal connected Dyn11 transformer can be changed either to a Dyn7(+150°) or Dyn3(-90°) connection.