Vector groups | Application of Transformer According to Vector Group ๏ฟผ

Vector groups

Vector groups identify the winding connections and polarities of the primary and secondary. From a vector group, one can determine the phase shift between primary and secondary.

The transformer vector group depends upon:

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

Application of Transformer according to Vector Group

1.)  Dyn11, Dyn1, YNd1, YNd11

  • Common for distribution transformers.
  • Normally Dyn11 vector group uses as a distribution system. Because Generating Transformer is YNd1 for neutralizing the load angle between 11 and 1.
  • We can use Dyn1 in the distribution system, when we are using Generator Transformer are YNd11.
  • In some industries 6 pulse electric drives are used due to this 5thharmonics will generate if we use Dyn1 which will suppress the 5th harmonics.
  • Star point facilitates mixed loading of three-phase and single-phase consumer connections.
  • The delta winding carries 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 the cost of insulation.
  • But delta-connected HV winding is common in the distribution networks, for feeding motors and lighting loads from the LV side.

2.) Star-Star (Yy0 or Yy6)

  • Mainly used for large system tie-up transformers.
  • The most economical connection in the HV power system is 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, the load can be connected between line and neutral, only if
    (a) the source side transformers are 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 โ€“ Delta (Dd 0 or Dd 6)

  • This is an economical connection for large low-voltage transformers.
  • A large unbalance of load can be met without difficulty.
  • Delta permits a circulating path for triple harmonics and thus attenuates the same.
  • It is possible to operate with one transformer removed in an open delta orโ€ Vโ€ 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 affects 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 is 15% more than a corresponding star or delta winding. This is extensively used for earthing transformers.
  • Due to the zig-zag 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 a normal phase connection. In cases where delta connections are weak due to a large number of turns and small cross sections, then a zigzag star connection is preferred. It is also used in rectifiers.

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

  • A zigzag connection is obtained by the interconnection of phases. A 4-wire system is possible on both sides. Unbalanced loading is also possible. The 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 costs 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 sequences fundamental mmf content also does not affect its performance. Even with the Yy type of polyphase connection without a 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 Transformers in large Power Stations and Transmission Substations.
  • The Neutral point can be loaded with rated Current.

7.)  Yz-5

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

Application of Transformer according to Uses

Step-up Transformer: โ€“ It should be Yd1 or Yd11.
Step-down Transformer: โ€“ It should be Dy1 or Dy11.
Grounding purpose Transformer: โ€“ It should be Yz1 or Dz11.
Distribution Transformer: โ€“ We can consider a vector group of Dzn0 which reduce the 75% of harmonics on the secondary side.
Power Transformer: โ€“ Vector group is deepened on the application for Example Generating Transformer: Dyn1, Furnace Transformer: Ynyn0.

Transformer Vector Group Phasor diagram: V

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