Three Phase Transformers

Three-phase Transformers are the backbone of electrical energy distribution, whether delta or star-linked windings.

So far, we have looked at the structure and operation of a single-phase, two-winded voltage transformer that increases or decreases secondary voltage relative to primary supply voltage.However, voltage transformers can be made not only for a single phase, but also for connection for detailed combinations of up to 24 phases for two-phase, three-phase, six-phase and even some DC straightening transformers.

If we take three single-phase transformers and connect their primary windings together in a fixed configuration, we can use the transformers in a three-phase feed.

The resources, written in three phases, as well as 3 phases or 3φ, are used for all industrial uses as well as for the production, transmission and distribution of electrical energy.Three-phase sources have many electrical advantages over single-phase power, and when considering three-phase transformers, we have to deal with three alternating voltages and currents that differ by 120 degrees in phase-time, as shown below.

three-phase transformer
Three Phase Voltages and Currents

Where: V L is line-line voltage and V P is phase-neutral voltage.

A transformer cannot act as a phase changer and convert a single phase into three phases or three phases into a single phase.To harmonize transformer connections with three-phase sources, we need to connect them in a certain way to create a Three Phase Transformer Configuration.

The advantages of building a single three-phase transformer are that for the same kVA value, copper and iron cores are used more efficiently, making them smaller, cheaper and lighter than three separate interconnected single-phase transformers.

Three Phase Transformer Connections

three-phase transformer

Combinations of the three windings can be with primary triangular connection and secondary star connection or star-triangle, star-star or triangle-triangle, depending on the use of the transformer.Transformers are often referred to as Multi-Phase Transformers when used to provide three or more phases.

Three Phase Transformer Star and Triangular Configurations

But what do we mean by "star" (also known as Wye) and "delta" (also known as mesh) when dealing with three-phase transformer connections?A three-phase transformer has three sets of primary and secondary windings.Determines whether the connection is an asterisk or triangular configuration, depending on how these winding sets are connected.

The three existing voltages, separated by 120 degrees of electricity, not only decide the type of electrical connections used on both primary and secondary sides, but also determine the flow of transformer currents.

With three interconnected single-phase transformers, the magnetic flux in the three transformers differs in phases to 120 time degrees.In a single three-phase transformer, the nucleus has three magnetic fluxes that differ 120 degrees in the time phase.

The standard method for marking three-phase transformer windings is to label the three main windings with capital letters A, B and C, which are used to represent three separate phases of RED, YELLOW and BLUE.Secondary windings are labeled lowercase (lowercase) letters a, b, and c.Each winding has two ends, normally labeled 1 and 2, so that the second dressing of the primary winding, for example, has ends to label as B1 and B2, the third winding of the secondary will be labeled c1 and c2 as shown.

Transformer Star and Delta Configurations

three-phase transformer

Symbols are typically used in a three-phase transformer to indicate the type or types of connections used with capital letter Y for star-linked, D for triangular connection, and Z for interconnected star primary windings, with lowercase letters y, d, and z for related secondarys.It is then labeled Star-Star Yy, labeled Delta-Delta Dd, and Zz for transformers connected to the same type of star connected from the interconnected star.

Transformer Winding Identification

ConnectionPrimary windingSecondary Winding
DeltaDD
StarYy
interconnectedZz

We now know that there are four different ways in which three single-phase transformers can be connected between their primary and secondary three-phase circuits.These four standard configurations are: Delta-Delta (Dd), Star-Star (Yy), Star-Delta (Yd), and Delta-Star (Dy).

Star-linked high-voltage operating transformers have the advantage of reducing voltage in a single transformer, reducing the number of rotations required and increasing the size of conductors, making issulating coil windings easier and cheaper than delta transformers.

Nevertheless, the delta-delta connection has a great advantage over the star-delta configuration, which is that in the event that a transformer from the triple group fails or fails, the remaining two will continue to provide three phases of power equal to capacity.

Transformer Delta and Delta Connections

three-phase transformer

In a group transformer with delta connection (Dd), the line voltage is equal to the V L supply voltage, V L = V S .However, the current in each phase winding is given as follows: 1/√ 3 × I L line current I Lis line current.

One drawback of delta-connected three-phase transformers is that each transformer must be wrapped for full line voltage (over 100V in our example) and 57.7 percent line current.A greater number of windings in the winding, together with insulation between the windings, require a larger and more expensive coil than the star connection.Another drawback of delta-connected three-phase transformers is that they are not "neutral" or common connection.

In star-star editing (Yy ), (y-y), each transformer has a terminal connected to a common connection or a neutral point with the remaining three ends of the primary windings connected to the three-phase main feed.The number of windings in a transformer winding for the star connection is 57.7 percent of what is required for the triangular connection.

The star connection requires the use of three transformers, and if any transformer fails or is disabled, the entire group may be disabled.However, the star-linked three-phase transformer is particularly suitable and economical in electrical power distribution systems, since a fourth wire can be connected as a neutral point (n) of three star-linked secondarys, as shown.

Transformer Star and Star Connections

three-phase transformer

The voltage between any line of the three-phase transformer is called "line voltage", V L, while the voltage between any line and neutral point of a star-connected transformer is called "phase voltage", V P.This phase voltage between the neutral point and any of the line connections is 1/√ 3 × V L of the line voltage.Then the primary side phase voltage above is given as V P.

three-phase transformer

The secondary current in each phase of a star-linked group of transformers is the same as the line current of the feed, then I L = I S .

Then, the relationship between line and phase voltages and currents in a three-phase system can be summarized as follows:

Three Phase Voltage and Current

ConnectionPhase VoltageLine voltagePhase CurrentLine Current
StarV P = V L ÷ √ 3V L = √ 3 × V PI P = ben LI L = I P
DeltaV P = V LV L = V PI P = I L ÷ √ 3I L = √ 3 × I P

Here again, V L is a phase-neutral voltage over line-line voltage and V P primary or secondary edge.

Other possible connections for three-phase transformers are star-triangular Yd , where the primary winding is star-bound and secondary triangular connected, or a triangular-linked primary and a star-bound secondary and delta-star Dy.

Delta-star-connected transformers are widely used in low power distribution, while primary windings provide a balanced load of three wires to the grid company, while secondary windings provide the required 4th string neutral or soil connection.

When the primary and secondary have different types of winding connections, stars or deltas, the total rotation rate of the transformer becomes more complex.If a three-phase transformer is connected as delta-delta (Dd) or star-star (Yy), the transformer could potentially have a rotational rate of 1:1.In other words, the inlet and output voltages of the bandages are the same.

However, if the 3-phase transformer is connected as a star-triangle, each star-linked primary winding (Yd) will receive the phase voltage, V P,which is equal to 1/√ 3 × V L of the feed.

Then the same voltage is induced inside each corresponding secondary winding, and since these windings are delta-connected, the 1/√ 3 × V L voltage becomes secondary line voltage.Then a star-triangular connected transformer with a rotation rate of 1:1 will provide a √ 3:1 downline voltage ratio.

Then, for a star-triangle (Yd) connected transformer, the rotation rate is as follows:

Star-Delta Rotation Rate

three-phase transformer

Similarly, the connected transformer for a delta-star, the ratio of windings 1: 1, has an incremental line voltage ratio of 1 3: √ that the transformer will provide.Then the rotation rate for a delta-star-connected transformer is as follows:

Delta-Star Rotation Rate

three-phase transformer

Next, for the four basic configurations of a three-phase transformer, we can list the secondary voltages and currents of transformers according to the primary line voltage, V L and primary line current I L, as shown in the table below .

Three Phase Transformer Line Voltage and Current

three-phase transformer

Where: n , the "rotation rate" (TR) of transformers obtained by dividing the number of secondary windings by the primary winding number of N S is N P. (N S /N P) and V L are even voltages from the line, where V P is phase-neutral voltage.

Three Phase Transformer Question Sample 1

The primary winding of a delta-star (Dy) connected 50VA transformer is provided by a three-phase feed of 100 volts, 50 Hz.If the transformer has 500 rotations in the primary and 100 in the secondary winding, calculate secondary side voltages and currents.

Data Given: transformer value, 50VA, supply line voltage, 100v, primary turns 500, secondary turns, 100 .

three-phase transformer

Then the secondary side of the transformer provides a line voltage, VLINE gives a secondary phase voltage of about 35v, V FAZI is 20v at 0.834 amps.

Three Phase Transformer Structure

We have previously said that the three-phase transformer is three single-phase transformers connected to each other on a single-layer core, and that combining the three windings on a single magnetic circuit can result in significant savings in cost, size and weight.

A three-phase transformerhas three handheld magnetic circuits of a uniform distribution of dielectric flux between the highest and lowest voltage coils in general.The exception to this rule is a three-phase shell type transformer.In shell-type construction, they do not pass, although the three cores are together.

three-phase transformer
Three Phase Transformer Structure

The three-limb core type three-phase transformer is the most common method of making a three-phase transformer that allows the phases to be magnetically connected.The flux of each limb uses the other two limbs for the way back with three magnetic fluxes in the nucleus produced by line voltages that differ 120 degrees in the time phase.Thus, the flux in the nucleus remains almost sinusoidal and produces a sinusoidal secondary feeding voltage.

The shell type five extension type three-phase transformer structure is heavier than the core type and more expensive to build.Five-limbed cores are often used because they can be made with reduced height for very large power transformers.Shell transformers core materials, electrical windings, steel housing and cooling are almost identical to larger single-phase types.