Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit. Understanding Vector Group of Transformer Part 1 The primary windings are connected in one of several ways. The two most common configurations are the delta, in which the polarity end of one winding is connected to the non-polarity end of the next, and the star, in which all three non-polarities or polarity ends are connected together. The secondary windings are connected similarly. This means that a 3-phase transformer can have its primary and secondary windings connected the same delta-delta or star-star , or differently delta-star or star-delta. This is called a 30 degree phase shift.
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Clock Notation: 11 Points to be consider while Selecting of Vector Group: Vector Groups are the IEC method of categorizing the primary and secondary winding configurations of 3-phase transformers.
Windings can be connected as delta, star, or interconnected-star zigzag. Winding polarity is also important, since reversing the connections across a set of windings affects the phase-shift between primary and secondary. 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. Transformer vector group depends upon Removing harmonics: Dy connection — y winding nullifies 3rd harmonics, preventing it to be reflected on delta side.
Earth fault Relay: A Dd transformer does not have neutral. Type of Transformer Application: Generally for Power export transformer i. For Power export import transformers i.
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. 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. 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.
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. 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. Yd1 or another vector group i. 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. Normally Dyn11 vector group using at distribution system. Because Generating Transformer are YNd1 for neutralizing the load angle between 11 and 1.
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. 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. Large unbalance of load can be met without difficulty. Delta permits a circulating path for triple harmonics thus attenuates the same. Three phase units cannot have this facility.
Mixed single phase loading is not possible due to the absence of neutral. 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.
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.
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. Unbalanced loading is also possible. Oscillating neutral problem is absent in this connection. 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. The Neutral point can be loaded with rated Current. Step down Transformer: It should be Dy1 or Dy Grounding purpose Transformer: It should be Yz1 or Dz The same is true for internal connected Dd4 or Dd8 transformers.
The phase shift is a natural consequence of the delta connection. The currents entering or leaving the star winding of the transformer are in phase with the currents in the star windings. Therefore, the currents in the delta windings are also in phase with the currents in the star windings and obviously, the three currents are electrical degrees apart.
But the currents entering or leaving the transformer on the delta side are formed at the point where two of the windings comprising the delta come together — each of those currents is the phasor sum of the currents in the adjacent windings. When you add together two currents that are electrical degrees apart, the sum is inevitably shifted by 30 degrees.
The phase voltages in three phases of both primary and secondary. Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.
And, as the transmission side is Delta and the user might need three phase, four-wire in the LV side for his single phase loads, the distribution transformer is chosen as Dyn There is magnetic coupling between HT and LT. When the load side LT suffers some dip the LT current try to go out of phase with HT current, so 30 degree phase shift in Dyn keeps the two currents in phase when there is dip.
So the vector group at the generating station is important while selecting distribution Transformer.
Vector Group of Transformer
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