Introduction:

Three phase transformer consists of three sets of primary windings, one for each phase, and three sets of secondary windings wound on the same iron core. Separate single-phase transformers can be used and externally interconnected to yield the same results as a 3-phase unit.

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).

It’s important to remember that the secondary voltage waveforms are in phase with the primary waveforms when the primary and secondary windings are connected the same way. This condition is called “no phase shift.” But when the primary and secondary windings are connected differently, the secondary voltage waveforms will differ from the corresponding primary voltage waveforms by 30 electrical degrees. This is called a 30 degree phase shift. When two transformers are connected in parallel, their phase shifts must be identical; if not, a short circuit will occur when the transformers are energized.”

Basic Idea of Winding:

• An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path. The phase relationship of the two voltages depends upon which ways round the coils are connected. The voltages will either be in-phase or displaced by 180 deg
• When 3 coils are used in a 3 phase transformer winding a number of options exist. The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.

Six Ways to wire Star Winding:

Six Ways to wire Delta Winding:

Polarity:

• An ac voltage applied to a coil will induce a voltage in a second coil where the two are linked by a magnetic path.  The phase relationship of the two voltages depends upon which way round the coils are connected.  The voltages will either be in-phase or displaced by 180 deg.
• When 3 coils are used in a 3 phase transformer winding a number of options exist.  The coil voltages can be in phase or displaced as above with the coils connected in star or delta and, in the case of a star winding, have the star point (neutral) brought out to an external terminal or not.

• When Pair of Coil of Transformer have same direction than voltage induced in both coil are in same direction from one end to other end.
• When two coil have opposite winding direction than Voltage induced in both coil are in opposite direction.

Winding connection designations:

• First Symbol: for High Voltage: Always capital letters.
•  D=Delta, Y=Star, Z=Interconnected star, N=Neutral
• Second Symbol: for Low voltage: Always Small letters.
•  d=Delta, y=Star, z=Interconnected star, n=Neutral.
• Third Symbol: Phase displacement expressed as the clock hour number (1,6,11)
• Example – Dyn11
Transformer has a delta connected primary winding (D) a star connected secondary (y) with the star point brought out (n) and a phase shift of 30 deg leading (11).
• The point of confusion is occurring in 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.

Vector Group of Transformer:

• The three phase transformer windings can be connected several ways. Based on the windings’ connection, the vector group of the transformer is determined.
• The transformer vector group is indicated on the Name Plate of transformer by the manufacturer.
The vector group indicates the phase difference between the primary and secondary sides, introduced due to that particular configuration of transformer windings connection.
• The Determination of vector group of transformers is very important before connecting two or more transformers in parallel. If two transformers of different vector groups are connected in parallel then phase difference exist between the secondary of the transformers and large circulating current flows between the two transformers which is very detrimental.

Phase Displacement between HV and LV Windings:

•  The vector for the high voltage winding is taken as the reference vector. Displacement of the vectors of other windings from the reference vector, with anticlockwise rotation, is represented by the use of clock hour figure.
• IS: 2026 (Part 1V)-1977 gives 26 sets of connections star-star, star-delta, and star zigzag, delta-delta, delta star, delta-zigzag, zigzag star, zigzag-delta. Displacement of the low voltage winding vector varies from zero to -330° in steps of -30°, depending on the method of connections.
• Hardly any power system adopts such a large variety of connections. Some of the commonly used connections with phase displacement of 0, -300, -180″ and -330° (clock-hour setting 0, 1, 6 and 11).
• Symbol for the high voltage winding comes first, followed by the symbols of windings in diminishing sequence of voltage. For example a 220/66/11 kV Transformer connected star, star and delta and vectors of 66 and 11 kV windings having phase displacement of 0° and -330° with the reference (220 kV) vector will be represented As Yy0 – Yd11.
• The digits (0, 1, 11 etc) relate to the phase displacement between the HV and LV windings using a clock face notation. The phasor representing the HV winding is taken as reference and set at 12 o’clock. Phase rotation is always anti-clockwise. (International adopted).
• 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.
• Example:
• Digit 0 =0° that the LV phasor is in phase with the HV phasor
Digit 1 =30° lagging (LV lags HV with 30°) because rotation is anti-clockwise.
• Digit 11 = 330° lagging or 30° leading (LV leads HV with 30°)
• Digit 5 = 150° lagging (LV lags HV with 150°)
• Digit 6 = 180° lagging (LV lags HV with 180°)
• When transformers are operated in parallel it is important that any phase shift is the same through each. Paralleling typically occurs when transformers are located at one site and connected to a common bus bar (banked) or located at different sites with the secondary terminals connected via distribution or transmission circuits consisting of cables and overhead lines.
 Phase Shift (Deg) Connection 0 Yy0 Dd0 Dz0 30 lag Yd1 Dy1 Yz1 60 lag Dd2 Dz2 120 lag Dd4 Dz4 150 lag Yd5 Dy5 Yz5 180 lag Yy6 Dd6 Dz6 150 lead Yd7 Dy7 Yz7 120 lead Dd8 Dz8 60 lead Dd10 Dz10 30 lead Yd11 Dy11 Yz11
•  The phase-bushings on a three phase transformer are marked either  ABC, UVW or 123 (HV-side capital, LV-side small letters). Two winding, three phase transformers can be divided into four main categories
 Group O’clock TC Group I 0 o’clock, 0° delta/delta, star/star Group II 6 o’clock, 180° delta/delta, star/star Group III 1 o’clock, -30° star/delta, delta/star Group IV 11 o’clock, +30° star/delta, delta/star Minus indicates LV lagging HV, plus indicates LV leading HV

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
1. Removing harmonics: Dy connection – y winding nullifies 3rd harmonics, preventing it to be reflected on delta side.
2. Parallel operations: All the transformers should have same vector group & polarity of the winding.
3. 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..
4. 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.
5. 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.
6. 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.

Point to be remembered:

• For Group-III & Group-IV: By doing some unconventional connections externally on both sides of the Transformer, an internal connected Group-III or Group-IV transformer can be changed to any of these two groups.
• Thus by doing external changes on both sides of the Transformer an internal connected Dyn1 transformer can be changed to either a: Dyn3, Dyn5, Dyn7, Dyn9 or Dyn11 transformer, This is just true for star/delta or delta/star connections.
• For Group-I & Group-II: Changes for delta/delta or star/star transformers between Group-I and Group-III can just be done internally.

Why 30°phase shift occur in star-delta transformer between primary and secondary?

• 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 120 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 120 electrical degrees apart, the sum is inevitably shifted by 30 degrees.
•  The Main reason for this phenomenon   is that the phase voltage lags line current by 30degrees.consider a delta/star transformer. The phase voltages in three phases of both primary and secondary. you will find that in primary the phase voltage and line voltages are same, let it be VRY(take one phase).but, the corresponding secondary will have the phase voltage only in its phase winding as it is star connected. the line voltage of star connected secondary and delta connected primary won’t have any phase differences between them. so this can be summarized that “the phase shift is associated with the wave forms of the three phase windings.

Why  when Generating Transformer is Yd1 than Distribution Transformer is Dy11:

• This is the HV Side or the Switchyard side of the Generator Transformer is connected in Delta and the LV Side or the generator side of the GT is connected in Star, with the Star side neutral brought out.
• The LV side voltage will “lag” the HV side voltage by 30 degrees.
• Thus, in a generating station we create a 30 degrees lagging voltage for transmission, with respect to the generator voltage.
• As we have created a 30 degrees lagging connection in the generating station, it is advisable to create a 30 degrees leading connection in distribution so that the user voltage is “in phase” with the generated 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 Dyn11.
• 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-11 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 in Generating-Transmission-Distribution System:

• Generating TC is Yd1 transmitted power at 400KV, for 400KV to 220KV Yy is used and by using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV so that their phase shifts can be cancelled out. And for LV (400/230V) supplies at 50 Hz are usually 3 phase, earthed neutral, so a “Dyn” LV winding is needed. Here GT side -30lag (Yd1) can be nullify +30 by using distribution Transformer of Dy11.
• A reason for using Yd between e.g. 220 and 66 kV, then Dy from 66 to 11 kV is that their phase shifts can cancel out and It is then also possible to parallel a 220/11 kV YY transformer, at 11 kV, with the 66/11 kV (a YY transformer often has a third, delta, winding to reduce harmonics). If one went Dy11 – Dy11 from 220 to 11 kV, there would be a 60 degree shift, which is not possible in one transformer. The “standard” transformer groups in distribution avoid that kind of limitation, as a result of thought and experience leading to lowest cost over many years.

Generator TC is Yd1, Can we use Distribution TC Dy5 instead of Dy11.

• With regards to theory, there are no special advantages of Dyn11 over Dyn5.
• In Isolation Application: In isolated applications there is no advantage or disadvantage by using Dy5 or Dy11. If however we wish to interconnect the secondary sides of different Dny transformers, we must have compatible transformers, and that can be achieved if you have a Dyn11 among a group of Dyn5’s and vice versa.
• In Parallel Connection: Practically, the relative places of the phases remain same in Dyn11 compared to Dyn5.
• If we use Yd1 Transformer on Generating Side and Distribution side Dy11 transformer than -30 lag of generating side (Yd1) is nullify by +30 Lead at Receiving side Dy11) so no phase difference respect to generating Side and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from left to Right.
• This will make the Transmission lines have same color (for identification) whether it is input to or output from the Transformer.
• If we use Yd1 Transformer on Generating Side and Distribution side Dy5 transformer than -30 lag of generating side (Yd1) is more lag by  -150 Lag at Receiving side (Dy5) so Total phase difference respect to generating Side is 180 deg (-30+-150=-180) and if we are on the HV side of the Transformer, and if we denote the phases as R- Y-B from left to right, the same phases on the LV side will be R- Y -B, but from Right to Left.
• This will make the Transmission lines have No same color (for identification) whether it is input to or output from the Transformer.
• The difference in output between the Dyn11 and Dny5 and is therefore 180 degrees.

(7) Auto Transformer Connection:

• An Ordinary Transformer consists of two windings called primary winding and secondary winding. These two windings are magnetically coupled and electrically isolated. But the transformer in which a part of windings is common to both primary and secondary is called Auto Transformer.
• In Auto Transformer two windings are not only magnetically coupled but also electrically coupled. The input to the transformer is constant but the output can be varied by varying the tapings.
• The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer. Autotransformers are also used on radial distribution feeder circuits as voltage regulators. The connection is shown in Figure
• The primary and secondary windings of a two winding transformer have induced emf in them due to a common mutual flux and hence are in phase. The currents drawn by these two windings are out of phase by 180◦. This prompted the use of a part of the primary as secondary. This is equivalent to common the secondary turns into primary turns.
• The common section need to have a cross sectional area of the conductor to carry (I2−I1) ampere.
• Total number of turns between A and C are T1. At point B a connection is taken. Section AB has T2 turns. As the volts per turn, which is proportional to the flux in the machine, is the same for the whole winding, V1 : V2 = T1 : T2
• When the secondary winding delivers a load current of I2 Ampere the demagnetizing ampere turns is I2T2. This will be countered by a current I1 flowing from the source through the T1 turns such that, I1T1 = I2T2
• A current of I1 ampere flows through the winding between B and C. The current in the winding between A and B is (I2 − I1) ampere. The cross section of the wire to be selected for AB is proportional to this current assuming a constant current density for the whole winding. Thus some amount of material saving can be achieved compared to a two winding transformer. The magnetic circuit is assumed to be identical and hence there is no saving in the same. To quantify the saving the total quantity of copper used in an auto transformer is expressed as a fraction of that used in a two winding transformer As
•  copper in auto transformer / copper in two winding transformer =((T1 − T2)I1 + T2(I2 − I1))/T1I1 + T2I2
• copper in auto transformer / copper in two winding transformer = 1 –(2T2I1 / (T1I1 + T2I2))
• But T1I1 = T2I2 so
• The Ratio = 1 –(2T2I1 / 2T1I1) = 1 –(T2/T1)
• This means that an auto transformer requires the use of lesser quantity of copper given by the ratio of turns. This ratio therefore the savings in copper.
• As the space for the second winding need not be there, the window space can be less for an auto transformer, giving some saving in the lamination weight also. The larger the ratio of the voltages, smaller is the savings. As T2 approaches T1 the savings become significant. Thus auto transformers become ideal choice for close ratio transformations.
•  The auto transformer shown in Figure is connected as a boosting auto transformer because the series winding boosts the output voltage. Care must be exercised when discussing ‘‘primary’’ and ‘‘secondary’’ voltages in relationship to windings in an auto transformer.
• In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage. In the case of a boosting autotransformer, however, the primary (or high) voltage is associated with the series winding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding.

Limitation of the autotransformer

• One of the limitations of the autotransformer connection is that not all types of three-phase connections are possible. For example, the ∆-Y and Y- ∆ connections are not possible using the autotransformer. The Y-Y connection must share a common neutral between the high-voltage and low-voltage windings, so the neutrals of the circuits connected to these windings cannot be isolated.
• A ∆- ∆ autotransformer connection is theoretically possible; however, this will create a peculiar phase shift. The phase shift is a function of the ratio of the primary to secondary voltages and it can be calculated from the vector diagram. This phase shift cannot be changed or eliminated and for this reason, autotransformers are very seldom connected as ∆ – ∆ transformers.

• There are considerable savings in size and weight.
• There are decreased losses for a given KVA capacity.
• Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation. Its efficiency is more when compared with the conventional one.
• Its size is relatively very smaller.
• Voltage regulation of autotransformer is much better.
• Lower cost
• Low requirements of excitation current.
• Less copper is used in its design and construction.
• In conventional transformer the voltage step up or step down value is fixed while in autotransformer, we can vary the output voltage as per out requirements and can smoothly increase or decrease its value as per our requirement.

• The autotransformer connection is not available with certain three-phase connections.
• Higher (and possibly more damaging) short-circuit currents can result from a lower series impedance.
• Short circuits can impress voltages significantly higher than operating voltages across the windings of an autotransformer.
• For the same voltage surge at the line terminals, the impressed and induced voltages are greater for an autotransformer than for a two-winding transformer.
• Autotransformer consists of a single winding around an iron core, which creates a change in voltage from one end to the other. In other words, the self-inductance of the winding around the core changes the voltage potential, but there is no isolation of the high and low voltage ends of the winding. So any noise or other voltage anomaly coming in on one side is passed through to the other. For that reason, Autotransformers are typically only used where there is already some sort of filtering or conditioning ahead of it, as in electronic applications, or the downstream device is unaffected by those anomalies, such as an AC motor during starting

Applications:

• Used in both Synchronous motors and induction motors.
• Used in electrical apparatus testing labs since the voltage can be smoothly and continuously varied.
• They find application as boosters in AC feeders to increase the voltage levels.
• Used in HV Substation due to following reasons.
1. If we use normal transformer the size of transformer will be very high which leads to heavy weight, more copper and high cost.
2. The tertiary winding used in Auto transformer balances single phase unbalanced loads connected to secondary and it does not pass on these unbalanced currents to Primary side. Hence Harmonics and voltage unbalance are eliminated.
3. Tertiary winding in the Auto Transformer balances amp turns so that Auto transformer achieves magnetic separation like two winding transformers.

Transforming 3 Phase to 2 Phase:

• There are two main reasons for the need to transform from three phases to two phases,
1. To give a supply to an existing two phase system from a three phase supply.
2. To supply two phase furnace transformers from a three phase source.
• Two-phase systems can have 3-wire, 4-wire, or 5-wire circuits. It is needed to be considering that a two-phase system is not 2/3 of a three-phase system. Balanced three-wire, two-phase circuits have two phase wires, both carrying approximately the same amount of current, with a neutral wire carrying 1.414 times the currents in the phase wires. The phase-to-neutral voltages are 90° out of phase with each other.
• Two phase 4-wire circuits are essentially just two ungrounded single-phase circuits that are electrically 90° out of phase with each other. Two phase 5-wire circuits have four phase wires plus a neutral; the four phase wires are 90° out of phase with each other.

• The easiest way to transform three-phase voltages into two-phase voltages is with two conventional single-phase transformers. The first transformer is connected phase-to-neutral on the primary (three-phase) side and the second transformer is connected between the other two phases on the primary side.
• The secondary windings of the two transformers are then connected to the two-phase circuit. The phase-to-neutral primary voltage is 90° out of phase with the phase-to-phase primary voltage, producing a two-phase voltage across the secondary windings. This simple connection, called the T connection, is shown in Figure
• The main advantage of the T connection is that it uses transformers with standard primary and secondary voltages. The disadvantage of the T connection is that a balanced two-phase load still produces unbalanced three-phase currents; i.e., the phase currents in the three-phase system do not have equal magnitudes, their phase angles are not 120° apart, and there is a considerable amount of neutral current that must be returned to the source.

The Scott Connection of Transformer:

• A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.
• Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and Teaser. The MAIN and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on the primary side.
• MAIN and Teaser outputs are on separate cores. An external jumper is also required to connect the primary side of the MAIN and Teaser sections.
• The schematic of a typical Scott T Transformer is shown below:
• Scott T Transformer is built with two single phase transformers of equal power rating. The MAIN and Teaser sections can be enclosed in a floor mount enclosure with MAIN on the bottom and Teaser on top with a connecting jumper cable. They can also be placed side by side in separate enclosures.
• Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection consists of a center-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5√3) ratio teaser transformer, T2. The center-tapped side of T1 is connected between two of the phases on the three-phase side. Its center tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of the transformers then connects directly to the two pairs of a two-phase four-wire system.

• The Scott-T transformer connection may be also used in a back to back T to T arrangement for a three-phase to 3 phase connection. This is a cost saving in the smaller kVA transformers due to the 2 coil T connected to a secondary 2 coil T in-lieu of the traditional three-coil primary to three-coil secondary transformer. In this arrangement the Neutral tap is part way up on the secondary teaser transformer . The voltage stability of this T to T arrangement as compared to the traditional 3 coil primary to three-coil secondary transformer is questioned

Key Point:

• If the main transformer has a turn’s ratio of 1: 1, then the teaser transformer requires a turn’s ratio of 0.866: 1 for balanced operation. The principle of operation of the Scott connection can be most easily seen by first applying a current to the teaser secondary windings, and then applying a current to the main secondary winding, calculating the primary currents separately and superimposing the results.

Load connected between phaseY1 and phase Y2 of the secondary:

• Secondary current from the teaser winding into phase X1 =1.0 <90°
• Secondary current from the teaser winding into phase X2 =-1.0< 90°
• Primary current from H3 phase into the teaser winding= 1.1547< 90°
• Primary current from H2  phase into the main winding= 0.5774 <90°
• Primary current from H1 phase into the main winding= -0.5774< 90°
• The reason that the primary current from H3 phase into the teaser winding is 1.1547 due to 0.866: 1 turn’s ratio of the teaser, transforming 1/0.866= 1.1547 times the secondary current. This current must split in half at the center tap of the main primary winding because both halves of the main primary winding are wound on the same core and the total ampere-turns of the main winding must equal zero.

Load connected between phase X2 and phase X1 of the secondary:

• Secondary current from the main winding into phase X2 =1.0< 0°
• Secondary current from the main winding into phase X4= -1.0 <0°
• Primary current from H2 phase into the main winding =1.0 <0°
• Primary current from H1 phase into the main winding=- 1.0 <0°
• Primary current from H3 phase into the teaser winding= 0
• Superimpose the two sets of primary currents:
• I H3= 1.1547 <90° +0= 1.1547 <90°
• I H2 =0.5774 <90° +1.0< 0°= 1.1547 < 30°
• I H1 =0.5774 <90°+ 1.0 <0°=1.1547 <210°
• Notice that the primary three-phase currents are balanced; i.e., the phase currents have the same magnitude and their phase angles are 120° apart. The apparent power supplied by the main transformer is greater than the apparent power supplied by the teaser transformer. This is easily verified by observing that the primary currents in both transformers have the same magnitude; however, the primary voltage of the teaser transformer is only 86.6% as great as the primary voltage of the main transformer. Therefore, the teaser transforms only 86.6% of the apparent power transformed by the main.
• We also observe that while the total real power delivered to the two phase load is equal to the total real power supplied from the three-phase system, the total apparent power transformed by both transformers is greater than the total apparent power delivered to the two-phase load.
• The apparent power transformed by the teaser is 0.866 X IH1= 1.0 and the apparent power transformed by the main is 1.0X IH2 =1.1547 for a total of 2.1547 of apparent power transformed.
• The additional 0.1547 per unit of apparent power is due to parasitic reactive power owing between the two halves of the primary winding in the main transformer.
• Single-phase transformers used in the Scott connection are specialty items that are virtually impossible to buy ‘‘off the shelf ’’ nowadays. In an emergency, standard distribution transformers can be used

Advantages of the Scott T Connection:

• If desired, a three phase, two phase, or single phase load may be supplied simultaneously
• The neutral points can be available for grounding or loading purposes

• This type of asymmetrical connection (3 phases, 2 coils), reconstructs three phases from 2 windings. This can cause unequal voltage drops in the windings, resulting in potentially unbalanced voltages to be applied to the load.
• The transformation ratio of the coils and the voltage obtained may be slightly unbalanced due to manufacturing variances of the interconnected coils.
• This design’s neutral has to be solidly grounded. If it is not grounded solidly, the secondary voltages could become unstable.
• Since this design will have a low impedance, special care will have to be taken on the primary protection fault current capacity. This could be an issue if the system was designed for a Delta-Star connection.
• The inherent single phase construction and characteristics of this connection produces a comparatively bulky and heavier transformer when compared with a normal three phase transformer of the same rating.

Application:

• For Industrial Furnace Transformer.
• For Traction Purpose: The power is obtained from the 220 kV or 132 kV or 110 kV or 66 kV, three-phase, effectively earthed transmission network of the State Electricity Board, through single-phase transformers or Scott connected transformer installed at the Traction Substation. The primary winding of the single-phase transformer is connected to two phases of the transmission network or Where Scott-connected transformer is used, the primary windings are connected to the three phases of the transmission network.
• The single-phase transformers at a Traction Substation are connected to the same two phases of the transmission network (referred as single-phase connection), or alternatively to different pairs of phases- the three single phase transformers forming a delta-connection on the primary side. Out of three single-phase transformers, one transformer feeds the overhead equipment (OHE) on one side of the Traction Substation, another feeds the OHE on the other side of the Traction Substation, and the third remains as standby. Thus the two single-phase transformers which feed the OHE constitute an open-delta connection (alternatively, referred as V-connection) on the three-phase transformers network. The Scott-connected transformer and V-connected single-phase transformers are effective in reducing voltage imbalance on the transmission network. The spacing between adjacent substations is normally between 70 and 100 km.

(5) The Zigzag Connection:

• The zigzag connection is also called the interconnected star connection.  This connection has some of the features of the Y and the ∆ connections, combining the advantages of both.
• The zigzag transformer contains six coils on three cores. The first coil on each core is connected contrariwise to the second coil on the next core. The second coils are then all tied together to form the neutral and the phases are connected to the primary coils. Each phase, therefore, couples with each other phase and the voltages cancel out. As such, there would be negligible current through the neutral pole and it can be connected to ground
• One coil is the outer coil and the other is the inner coil. Each coil has the same number of windings turns (Turns ratio=1:1) but they are wound in opposite directions. The coils are connected as follows:
• The outer coil of phase a1-a is connected to the inner coil of phase c2-N.
• The outer coil of phase b1-b is connected to the inner coil of phase a2-N.
• The outer coil of phase c1-c is connected to the inner coil of phase b2-N.
• The inner coils are connected together to form the neutral and our tied to ground
• The outer coils are connected to phases a1,b1,c1 of the existing delta system.
• If three currents, equal in magnitude and phase, are applied to the three terminals, the ampere-turns of the a2-N winding cancel the ampere-turns of the b1-b winding, the ampere-turns of the b2-N winding cancel the ampere turns of the c1-c winding, and the ampere-turns of the c2-N winding cancel the ampere turns of the a1-a winding. Therefore, the transformer allows the three in-phase currents to easily flow to neutral.
• If three currents, equal in magnitude but 120° out of phase with each other, are applied to the three terminals, the ampere-turns in the windings cannot cancel and the transformer restricts the current flow to the negligible level of magnetizing current. Therefore, the zigzag winding provides an easy path for in-phase currents but does not allow the flow of currents that are 120°out of phase with each other.
• Under normal system operation the outer and inner coil winding’s magnetic flux will cancel each other and only negligible current will flow in the in the neutral of the zig –zag transformer.
• During a phase to ground fault the zig-zag transformer’s coils magnetic flux are no longer equal in the faulted line. This allows zero sequence.
• If one phase, or more, faults to earth, the voltage applied to each phase of the transformer is no longer in balance; fluxes in the windings no longer oppose. (Using symmetrical components, this is Ia0 = Ib0 = Ic0.) Zero sequence (earth fault) current exists between the transformers’ neutral to the faulting phase. Hence, the purpose of a zigzag transformer is to provide a return path for earth faults on delta connected systems. With negligible current in the neutral under normal conditions, engineers typically elect to under size the transformer; a short time rating is applied. Ensure the impedance is not too low for the desired fault limiting. Impedance can be added after the secondary’s are summed (the 3Io path)
• The neutral formed by the zigzag connection is very stable. Therefore, this type of transformer, or in some cases an auto transformer, lends itself very well for establishing a neutral for an ungrounded 3 phase system.
• Many times this type of transformer or auto transformer will carry a fairly large rating, yet physically be relatively small. This particularly applies in connection with grounding applications. The reason for this small size in relation to the nameplate KVA rating is due to the fact that many types of grounding auto transformers are rated for 2 seconds. This is based on the time to operate an over current protection device such as a breaker. Zigzag transformers used to be employed to enable size reductions in drive motor systems due to the stable wave form they present. Other means are now more common, such as 6 phase star.

• The ∆ -zigzag connection provides the same advantages as the ∆-Y connection.
• Less Costly for grounding Purpose: It is typically the least costly than Y-D and Scott Transformer.
• Third harmonic suppression: The zigzag connection in power systems to trap triple harmonic (3rd, 9th, 15th, etc.) currents. Here, We install zigzag units near loads that produce large triple harmonic currents. The windings trap the harmonic currents and prevent them from traveling upstream, where they can produce undesirable effects.
• Ground current isolation: If we need a neutral for grounding or for supplying single-phase line to neutral loads when working with a 3-wire, ungrounded power system, a zigzag connection may be the better solution. Due to its composition, a zigzag transformer is more effective for grounding purposes because it has less internal winding impedance going to the ground than when using a Star type transformer.
• No Phase Displacement: There is no phase angle displacement between the primary and the secondary circuits with this connection; therefore, the ∆-zigzag connection can be used in the same manner as Y-Y and ∆- ∆ transformers without introducing any phase shifts in the circuits.

Application:

• An Earthing Reference: Occasionally engineers use a combination of YD and zigzag windings to achieve a vector phase shift. For example, an electrical network may have a transmission network of 220 kV/66 kV star/star transformers, with 66 kV/11 kV delta/star for the high voltage distribution network. If a transformation is required directly between the 220 kV/11 kV network the most obvious option is to use 220 kV/11 kV star/delta. The problem is that the 11 kV delta no longer has an earth reference point. Installing a zigzag transformer near the secondary side of the 220 kV/11 kV transformer provides the required earth reference point.
• As a Grounding Transformer:The ability to provide a path for in-phase currents enables us to use the zigzag connection as a grounding bank, which is one of the main applications for this connection.
• We rarely use zigzag configurations for typical industrial or commercial use, because they are more expensive to construct than conventional Star connected transformers. But zigzag connections are useful in special applications where conventional transformer connections aren’t effective.
• D or Y / Zig-zag are used in unbalanced low voltage system – mostly with single phase appliances

(4)  Star-Delta Connection:

• In this type of connection, then primary is connected in star fashion while the secondary is connected in delta fashion as shown in the Fig.

• The voltages on primary and secondary sides can be represented on the phasor diagram as shown in the Fig.

Key point:

• As Primary in Star connected
• Line voltage on Primary side = √3 X Phase voltage on Primary side. So
• Phase voltage on Primary side = Line voltage on Primary side / √3
• Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage
• Secondary Phase Voltage = K X Primary Phase Voltage.
• As Secondary  in delta connected:
• Line voltage on Secondary side = Phase voltage on Secondary side.
• Secondary Phase Voltage = K X Primary Phase Voltage. =K X (Line voltage on Primary side / √3)
• Secondary Phase Voltage = (K/√3 ) X Line voltage on Primary side.
• There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary Phase Voltage

• The primary side is star connected. Hence fewer numbers of turns are required. This makes the connection economical for large high voltage step down power transformers.
• The neutral available on the primary can be earthed to avoid distortion.
• The neutral point allows both types of loads (single phase or three phases) to be met.
• Large unbalanced loads can be handled satisfactory.
• The Y-D connection has no problem with third harmonic components due to circulating currents inD. It is also more stable to unbalanced loads since the D partially redistributes any imbalance that occurs.
• The delta connected winding carries third harmonic current due to which potential of neutral point is stabilized. Some saving in cost of insulation is achieved if HV side is star connected. But in practice the HV side is normally connected in delta so that the three phase loads like motors and single phase loads like lighting loads can be supplied by LV side using three phase four wire system.
• As Grounding Transformer: In Power System Mostly grounded Y- ∆ transformer is used for no other purpose than to provide a good ground source in ungrounded Delta system. Take, for example, a distribution system supplied by connected (i.e., un-grounded) power source. If it is required to connect phase-to-ground loads to this system a grounding bank is connected to the system, as shown in Figure

• This system a grounding bank is connected to the system, as shown in Figure. Note that the connected winding is not connected to any external circuit in Figure.
• With a load current equal to 3 times i, each phase of the grounded Y winding provides the same current i, with the  -connected secondary winding of the grounding bank providing the ampere-turns required to cancel the ampere-turns of the primary winding. Note that the grounding bank does not supply any real power to the load; it is there merely to provide a ground path. All the power required by the load is supplied by two phases of the ungrounded supply

• In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.
• One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts.
• If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem

Application:

• It is commonly employed for power supply transformers.
• This type of connection is commonly employed at the substation end of the transmission line. The main use with this connection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that there is phase difference of 30° between primary and secondary line voltages.
• Commonly used in a step-down transformer, Y connection on the HV side reduces insulation costs the neutral point on the HV side can be grounded, stable with respect to unbalanced loads. As for example, at the end of a transmission line. The neutral of the primary winding is earthed. In this system, line voltage ratio is 1/√3 Times of transformer turn-ratio and secondary voltage lags behind primary voltage by 30°. Also third harmonic currents flow in the to give a sinusoidal flux.

(3) Delta-Star Connection of Transformer

• In this type of connection, the primary connected in delta fashion while the secondary current is connected in star.

• The main use of this connection is to step up the voltage i.e. at the begining of high tension transmission system. It can be noted that there is a phase shift of 30° between primary line voltage and secondary line voltage as leading.

Key point:

• As primary in delta connected:
• Line voltage on primary side = Phase voltage on Primary side.
• Now Transformation Ration (K) = Secondary Phase Voltage / Primary Phase Voltage
• Secondary Phase Voltage = K X Primary Phase Voltage.
• As Secondary in Star connected
• Line voltage on Secondary side = √3 X Phase voltage on Secondary side. So,
• Line voltage on Secondary side = √3 X K X Primary Phase Voltage.
• Line voltage on Secondary side = √3 X K X Primary Line Voltage.
• There is s +30 Degree or -30 Degree Phase Shift between Secondary Phase Voltage to Primary Phase Voltage

• Cross section area of winding is less at Primary side: On primary side due to delta connection winding cross-section required is less.
• Used at Three phase four wire System: On secondary side, neutral is available, due to which it can be used for 3-phase, 4 wire supply system.
• No distortion of Secondary Voltage: No distortion due to third harmonic components.
• Handled large unbalanced Load: Large unbalanced loads can be handled without any difficulty.
• Grounding Isolation between Primary and Secondary: Assuming that the neutral of the Y-connected secondary circuit is grounded, a load connected phase-to-neutral or a phase-to-ground fault produces two equal and opposite currents in two phases in the primary circuit without any neutral ground current in the primary circuit. Therefore, in contrast with the Y-Y connection, phase-to-ground faults or current unbalance in the secondary circuit will not affect ground protective relaying applied to the primary circuit. This feature enables proper coordination of protective devices and is a very important design consideration.
• The neutral of the Y grounded is sometimes referred to as a grounding bank, because it provides a local source of ground current at the secondary that is isolated from the primary circuit.
• Harmonic Suppression: The magnetizing current must contain odd harmonics for the induced voltages to be sinusoidal and the third harmonic is the dominant harmonic component. In a three-phase system the third harmonic currents of all three phases are in phase with each other because they are zero-sequence currents. In the Y-Y connection, the only path for third harmonic current is through the neutral. In the ∆ -Y connection, however, the third harmonic currents, being equal in amplitude and in phase with each other, are able to circulate around the path formed by the ∆ connected winding. The same thing is true for the other zero-sequence harmonics.
• Grounding Bank: It provides a local source of ground current at the secondary that is isolated from the primary circuit. For suppose an ungrounded generator supplies a simple radial system through ∆-Y transformer with grounded Neutral at secondary as shown Figure. The generator can supply a single-phase-to-neutral load through the -grounded Y transformer.
• Let us refer to the low-voltage generator side of the transformer as the secondary and the high-voltage load side of the transformer as the primary. Note that each primary winding is magnetically coupled to a secondary winding The magnetically coupled windings are drawn in parallel to each other.

• Through the second transformer law, the phase-to-ground load current in the primary circuit is reflected as a current in the A-C secondary winding. No other currents are required to flow in the A-C or B-C windings on the generator side of the transformer in order to balance ampere-turns.
• Easy Relaying of Ground Protection: Protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier.  If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection.  On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult.  Actually, ground fault protection is one of the primary advantages of delta-wye units.

• In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.
• One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts.
• If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem.

Applications:

• Commonly used in a step-up transformer:As for example, at the beginning of a HT transmission line. In this case neutral point is stable and will not float in case of unbalanced loading. There is no distortion of flux because existence of a Δ -connection allows a path for the third-harmonic components. The line voltage ratio is √3 times of transformer turn-ratio and the secondary voltage leads the primary one by 30°. In recent years, this arrangement has become very popular for distribution system as it provides 3- Ø, 4-wire system.
• Commonly used in commercial, industrial, and high-density residential locations: To supply three-phase distribution systems. An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.
• Used as Generator Transformer:The ∆-Y transformer connection is used universally for connecting generators to transmission systems because of two very important reasons. First of all, generators are usually equipped with sensitive ground fault relay protection. The ∆-Y transformer is a source of ground currents for loads and faults on the transmission system, yet the generator ground fault protection is completely isolated from ground currents on the primary side of the transformer. Second, rotating machines can literally be

(2)  Delta-Delta Connection:

• In this type of connection, both the three phase primary and secondary windings are connected in delta as shown in the Fig.

•  The voltages on primary and secondary sides can be shown on the phasor diagram.

• This connection proves to be economical for large low voltage transformers as it increases number of turns per phase.

Key point:

• Primary Side Line Voltage = Secondary Side Line Voltage.
• Primary Side Phase Voltage= Secondary Side Phase Voltage.
• No phase shift between primary and secondary voltages

• Sinusoidal Voltage at Secondary: In order to get secondary voltage as sinusoidal, the magnetizing current of transformer must contain a third harmonic component. The delta connection provides a closed path for circulation of third harmonic component of current. The flux remains sinusoidal which results in sinusoidal voltages.
• Suitable for Unbalanced Load: Even if the load is unbalanced the three phase voltages remains constant. Thus it suitable for unbalanced loading also.
• Carry 58% Load if One Transfer is Faulty in Transformer Bank: If there is bank of single phase transformers connected in delta-delta fashion and if one of the transformers is disabled then the supply can be continued with remaining tow transformers of course with reduced efficiency.
• No Distortion in Secondary Voltage: there is no any phase displacement between primary and secondary voltages. There is no distortion of flux as the third harmonic component of magnetizing current can flow in the delta connected primary windings without flowing in the line wires .there is no distortion in the secondary voltages.
• Economical for Low Voltage: Due to delta connection, phase voltage is same as line voltage hence winding have more number of turns. But phase current is (1/√3) times the line current. Hence the cross-section of the windings is very less. This makes the connection economical for low voltages transformers.
• Reduce Cross section of Conductor: The conductor is required of smaller Cross section as the phase current is 1/√3 times of the line current. It increases number of turns per phase and reduces the necessary cross sectional area of conductors thus insulation problem is not present.
• Absent of Third Harmonic Voltage:  Due to closed delta, third harmonic voltages are absent.
• The absence of star or neutral point proves to be advantageous in some cases.