Learn about the principal types of power transformer construction and how the typical magnetic circuits and winding arrangements affect the flow of triple-harmonics and zero-sequence currents.

Two principal types of transformer construction embody the requirements of economics, ease of manufacture, insulation, mechanical strength, and ventilation: core-type and shell-type. The key distinction between the two types lies in the core and winding placement. For core-type transformers, the windings encircle the core, while in shell-type transformers, the core encircles the windings.

Figure 1. Core-type and shell-type transformers work well in high-power applications.

There are four ways to connect the primary and secondary windings of the transformers symmetrically. These combinations, along with the typical magnetic circuits configurations, have a profound impact on the transformer and power system performance.

Arrays of Iron and Coils in Single-phase Transformers

Again, there are two general types of transformers depending on the arrangement of the iron and copper: core-type and shell-type.

In core-type transformers, the iron core is in the form of a hollow square made up of sheet-steel laminations, and the windings surround it. Figure 2 represents, diagrammatically, a core-type transformer.

Figure 2. Single-phase core type transformer

Manufacturing the core-type transformer with the primary and secondary coils on separate legs creates large leakage fluxes giving poor voltage regulation and unsatisfactory general performance. However, placing both coils on each leg, as shown in Figure 3, reduces the leakage flux to a small value.

Figure 3. Core-type arrangement of coils and core

In the shell-type transformers, the iron nearly surrounds the coils, as shown in Figure 4. 

Figure 4. Shell-type arrangement of coils and core

The core is shaped like a figure eight. The entire flux passes through the central part of the core, but outside of this central core it divides, half going in each direction. The coils have the shape of pancakes, and primary and secondary coils are usually stacked so that each primary is adjacent to a secondary. This design keeps the leakage fluxes low.

Shell-type transformers are not typically used as distribution transformers. Distribution transformers are usually single-phase and serve mainly single-phase loads in residential areas.

Assemblies of Iron and Coils in Three-phase Transformers

The bulk of all transformers, except for large, extra-high voltage and distribution units, are three-phase units. In the early days of the industry, it was almost a universal practice in the USA to use three single-phase units connected in a three-phase bank. Three-phase transformers have considerably less weight and occupy much less floor space than three single-phase transformers of equal rating.

Figure 5 shows the basis of the three-phase core-type transformer.

Figure 5. Basis of three-phase core-type transformer

Three single-phase transformers each have a primary winding on one leg connected to one wire of a three-phase system. The three cores are separated by 120° and the empty legs of the three are in contact forming one leg that carries the sum of the fluxes produced by the phase currents I1, I2, and I3.

The sum of the three currents at any instant is zero, and the three fluxes cancel in the joint leg. Eliminating this leg does not disturb the system because two legs act as the return for the third.

Figure 6 shows a more accurate construction arrangement.

Figure 6. Functional winding arrangement of three-phase core-type transformer

Figure 7 shows a three-phase shell-type transformer array similar to three single-phase units laid side by side. 

Figure 7. Array of coils and core in a three-phase shell-type transformer

This type of transformer uses less iron than three equivalent single-phase units because of the joint use of the magnetic paths between the coils. Also, the three phases are more independent of one another than they are in core-type transformers because each phase has an individual magnetic circuit.

Triple-harmonics and Transformer Winding Connections

There are several ways to connect the windings of the transformers to manage three-phase power, employing two or three single-phase units or one three-phase piece. The most frequent connections are wye-wye, wye-delta, delta-wye, and delta-delta. The wye connection is conventional in distribution systems because a neutral is available to supply single-phase, line-to-neutral loads.

The wye-wye connection has neutrals on both sides. Connecting the primary neutral to the ground ensures a balanced line-to-neutral voltage and provides a path for the exciting currents’ third-harmonics, and multiples (triple-harmonics).

The neutral connection carries no fundamental component of current under balanced conditions, because they are displaced 120° and add to zero. The third harmonics are 3 · 120° = 360° apart (they are in phase), add to three times the magnitude of one phase, and flow through the neutral. Likewise, all multiples of third harmonics will also return to the source through the neutral connection.

In an ungrounded wye-wye connection, the triple-harmonics are absent from the exciting current, so they will appear in the flux waveform, distorting the line-to-neutral voltages. These line-to-neutral, cophasal voltages will cause the secondary or load side neutral to oscillate.

The line-to-line voltages will not exhibit distortion because they are the phasor difference between the line-to-neutral voltages, such that which cancels the triple-harmonics cancel out.

Letting the triple harmonics flow by connecting at least one of the neutrals to an external circuit causes a slight distortion in the flux waveshape — just the amount required to generate the harmonics of the exciting current.

The three connections involving deltas provide a path for the circulation of the triple-harmonics inside the deltas, so no voltage distortion will occur.

Triple Harmonics and Three-phase Magnetic Circuits

The flow of fluxes that are in phase depends on the configuration of the transformer magnetic circuit.

In the three-legged core-type arrangement shown in Figure 8, each of the three legs has the primary and secondary windings overlapped. 

Figure 8. Magnetic circuit for three-legged core-type transformer

With balanced voltages, the fluxes are 120° out of phase, so they add up to zero at all times. The flux of phase a completes its circuit through the legs of phases b and c. This arrangement reduces the effective length of the magnetic circuit per phase with the consequent reduction of the magnetizing current and the core losses compared to a bank of three single-phase transformers of the same power with independent magnetic circuits.

When the triple-harmonics appear in the flux waveform, the flux components created by them are in phase and close the magnetic circuit through the air outside the magnetic core, the tank, and various structural elements. This region has a high reluctance, and the net effect is to inhibit triple-harmonics flux and reduce the distortion in the line-to-neutral voltage.

The flux forced outside of the magnetic core may cause heating to the tank walls and the end frames of the core. Providing a low reluctance return path for this flux often solves this heating problem. Conventional practice is to use a five-legged magnetic core structure when using core-type transformers.

Figure 9 shows a five-legged core-type transformer. The three intermediates are winding legs and the two outer legs provide a return path for the triple-harmonics flowing in the winding legs.

Figure 9. Magnetic circuit for five-legged core-type transformer

This low reluctance path for the flux components created by the triple-harmonics will allow line-to-neutral voltage distortion as previously described.

In three-phase shell-type transformers, the outside legs provide a path for the flux components created by the triple-harmonics (shown in Figure 10).

Figure 10. Magnetic circuit for three-phase shell-type transformer

The Method of Symmetrical Components

The method of symmetrical components helps us to analyze the power system when there is no symmetry in the three phases as the result of unbalanced loads, unbalanced faults, or short-circuits. This method transforms unbalanced currents and voltages into three balanced systems analyzed as single-phase. The components resulting from the transformation are positive-, negative-, or zero-sequence.

With the phasors labeled as a, b, and c, the positive-sequence phasors are equal in magnitude, displaced 120°, and, with phase sequence a, b, c, the negative-sequence phasors have a phase sequence a, c, b and the zero-sequence phasors are in phase. The three sets rotate at the same angular velocity, usually counterclockwise.

The current of any one sequence flows in a sequence circuit comprising the source, if any, and the sequence impedance. The sequence impedances Z1 (positive), Z2 (negative), and Zₒ (zero) may have different magnitudes.

The Exciting Current Harmonics and Symmetrical Components

If we consider the odd harmonics of the exciting current flowing in the symmetrical component circuits, the distribution will be:

Positive sequence: seventh, thirteenth, nineteenth, …

Negative sequence: fifth, eleventh, seventeenth, …

Zero-sequence (triple harmonics): third, ninth, fifteenth, …

The triple-harmonics currents are in phase in a three-phase system, constituting a zero-sequence set of currents. The third harmonic is the most significant harmonic component of the exciting current. It represents a zero-sequence set of currents of triple frequency, and the third-harmonic voltages are a zero-sequence set of voltages of triple frequency.

Zero-sequence Currents and Transformer Winding Connections

When the power systems supply unbalanced loads or have faults involving ground, zero-sequence currents and voltages appear.

The transformers, being static devices, respond identically to positive and negative sequence currents and voltages. Their reaction to zero-sequence components is distinct and largely depends on the connection configurations of the primary and secondary windings and the characteristics of the magnetic circuits.

The behavior of the zero-sequence components, according to the different connection modes of the transformer windings, is similar to that previously identified for triple harmonics.

The basic rules of functioning are:

A zero-sequence current can flow in a wye-connected winding only with a neutral linked to the power source or the ground. The line and phase currents are identical.

A zero-sequence current can flow in a delta-connected winding only with a magnetic coupling with a wye-connected winding through which a zero-sequence current flows.

According to the basic rules, the system reacts to different transformer connections as follows:

Wye-wye with isolated neutrals. Zero-sequence current cannot flow in any winding. The primary and secondary windings have infinite impedance.

Wye-wye with grounded neutrals. Zero-sequence current flows in both the primary and secondary windings limited by the low leakage impedances of both windings and the zero-sequence impedances of the system. With this connection, the effective leakage impedance per phase is virtually identical for positive-, negative- and zero-sequence currents.

Wye-wye with grounded secondary neutral. Zero-sequence current flow is only possible in the secondary circuit. Therefore, the transformation of this current is not allowed. The primary winding has infinite impedance to zero sequence current and behaves like an open circuit.

Wye-delta with ungrounded neutral. Flow is not possible for zero-sequence current in any line conductor, whether primary or secondary. Both windings have infinite zero sequence impedance.

Wye-delta with neutral connected to the source or ground. Zero-sequence current can flow between the line and the neutral in the wye-connected windings, inducing a current that circulates within the delta-connected secondary. The delta winding poses an infinite impedance to the flow of zero-sequence current in the lines. At the same time, the wye-connected winding displays a low leakage impedance and zero-sequence currents flow in line and neutral circuits.

Three-winding transformers, with the tertiary connected in delta, are typical. The tertiary allows for the flow of the triple harmonics, improving the flux waveform, reducing the triple-harmonic voltage, and providing a low zero-sequence impedance to ground both to the primary and secondary circuits.

Sequence Impedances of Three-phase Banks and Transformers

Transformers react differently to balanced positive- or negative-sequence voltages and zero-sequence voltages. As with triple-harmonics, the flow of zero-sequence fluxes depends on the configuration of the transformer magnetic circuit.

The test to estimate the sequence impedances of transformers consist of short-circuiting one set of windings, and measuring the impedance on the other windings. Testing for positive- and negative-sequence impedances (they are equivalent) requires balanced positive-sequence voltages. The zero-sequence impedance requires a single-phase voltage applied from the three line terminals (tied-up) to ground.

The impedance measured for either positive- or zero-sequence is approximately the sum of primary and secondary leakage impedances in parallel with the exciting impedance.

When a positive-sequence voltage is applied, the exciting impedance is much higher than the leakage impedance. The leakage impedance is the positive- and negative-sequence equivalent, ignoring the exciting impedance branch in parallel.

Applying a single-phase (zero-sequence) voltage to the three terminals of a wye-connected winding, we might expect a negligible small magnetizing impedance compared to the zero-sequence leakage impedance. But this is not necessarily so.

When zero-sequence currents flow in a three-phase, three-legged core-type transformer, the fluxes behave as in the case of triple-harmonics. The fluxes produced in the three legs are equal in magnitude and in phase, so their sum must return through the air, the tank, and structural elements (shown in Figure 11). 

Figure 11. Zero-sequence flux in a three-phase core-type transformer

The high reluctance path in this kind of transformer with some winding connections causes a much lower exciting impedance than in the case for positive- and negative-sequence voltage and is not ignored.

In a bank of three single-phase units (for three-phase, five-legged core-type and shell-type transformers), the legs offer a path for the zero-sequence flux. Then, the magnitude of the zero-sequence exciting impedance in these arrangements is similar to that obtained for positive and negative sequences (see Figure 12).

Figure 12. Zero-sequence flux in a three-phase shell-type transformer

Review of Transformer Construction and Phases

There are two general types of transformer construction: core-type and shell-type.

Single-phase core-type construction consists of a core that provides a single-path magnetic circuit. Each leg of the core has overlapped high and low-voltage windings. In shell-type, all windings form a single ring with the core encircling each side of the winding ring.

Doing a three-phase power transformation requires either a three-phase transformer unit or the connection of three single-phase units to form a three-phase bank. Three-phase transformers require less material meaning higher efficiency, smaller size, and less cost.

If the triple-harmonics of the exciting current cannot flow because of transformer connections, the flux will contain a triple-harmonic component, which, in turn, induces a triple-harmonic voltage in the transformer windings.

With single-phase banks or three-phase shell-type transformers, the triple-harmonic voltages may be high when compared to the fundamental-frequency voltage. In three-phase core-type transformers, the path of the triple-harmonic flux is outside the core and has a high reluctance. Therefore, the triple-harmonic flux in the magnetic circuit is small, even though the winding connections inhibit the triple harmonics of the exciting current, and the induced triple-harmonic voltage is small.

The triple-harmonic currents and voltages are zero-sequence in character.

The paths permitting the flow of triple-harmonic and zero-sequence currents depend on the system and transformer zero-sequence equivalents.

For confirming the specifications and performances of an electrical power transformer it has to go through a number of testing procedures. Some tests are done at a transformer manufacturer premises before delivering the transformer.

Transformer manufacturers perform two main types of transformer testing – type test of transformer and routine test of transformer.

Some transformer tests are also carried out at the consumer site before commissioning and also periodically in regular and emergency basis throughout its service life.

Type Test of Transformer

To prove that the transformer meets customer’s specifications and design expectations, the transformer has to go through different testing procedures in manufacturer premises. Some transformer tests are carried out for confirming the basic design expectation of that transformer. These tests are done mainly in a prototype unit not in all manufactured units in a lot. Type test of transformer confirms main and basic design criteria of a production lot.

Routine Tests of Transformer

Routine tests of transformer is mainly for confirming the operational performance of the individual unit in a production lot. Routine tests are carried out on every unit manufactured

Special Tests of Transformer

Special tests of transformer is done as per customer requirement to obtain information useful to the user during operation or maintenance of the transformer.

Pre-Commissioning Test of Transformer

In addition to these, the transformer also goes through some other tests, performed on it, before actual commissioning of the transformer at the site. The transformer testing performed before commissioning the transformer at the site is called the pre-commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the test results of all the low voltage tests with the factory test reports.

Type tests of transformer include:

1. Winding resistance test of transformer
2. Transformer ratio test
3. Transformer vector group test
4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test)
5. Measurement of no-load loss and current (Open circuit test)
6. Measurement of insulation resistance
7. Dielectric tests of transformer
8. Temperature rise test of transformer

Routine tests of transformer include

1. Winding resistance test of transformer
2. Transformer ratio test
3. Transformer vector group test
4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test)
5. Measurement of no-load loss and current (Open circuit test)
6. Measurement of insulation resistance
7. Dielectric tests of transformer.
8. Tests on on-load tap-changer.
9. Oil pressure test on transformer to check against leakages past joints and gaskets

That means Routine tests of transformer include all the type tests except temperature rise and vacuum tests. The oil pressure test on transformer to check against leakages past joints and gaskets is included.

Special Tests of transformer include

1. Dielectric tests.
2. Measurement of zero-sequence impedance of three-phase transformers
3. Short-circuit test
4. Measurement of acoustic noise level
5. Measurement of the harmonics of the no-load current.

Various Types of Transformer Testing Methodology

Transformer Winding Resistance Measurement

Transformer winding resistance measurement is carried out to calculate the I2R losses and to calculate winding temperature at the end of a temperature rise test. It is carried out as a type test as well as routine test. It is also done at site to ensure healthiness of a transformer that is to check loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings etc.

There are different methods for measuring of the transformer winding, likewise:

Current-voltage method of measurement of winding resistance.

Bridge method of measurement of winding resistance.

Kelvin bridge method of Measuring Winding Resistance.

Measuring winding resistance by Automatic Winding Resistance Measurement Kit.

Note: Transformer winding resistance measurement shall be carried out at each tap.

Transformer Ratio Test

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ratio test is an essential type test of transformer. This test also performed as a routine test of transformer. So for ensuring proper performance of electrical power transformer, voltage and turn ratio test of transformer one of the important tests.

The procedure of the transformer ratio test is simple. We just apply three phase 415 V supply to HV winding, with keeping LV winding open. We measure the induced voltages at HV and LV terminals of the transformer to find out actual voltage ratio of the transformer. We repeat the test for all tap position separately.

Magnetic Balance Test of Transformer

Magnetic balance test of transformer is conducted only on three-phase transformers to check the imbalance in the magnetic circuit.

Procedure of Magnetic Balance Test of Transformer

Keep the tap changer of transformer in normal position.

Now disconnect the transformer neutral from ground.

Then apply single phase 230 V AC supply across one of the HV winding terminals and neutral terminal.

Measure the voltage in two other HV terminals in respect of neutral terminal.

Repeat the test for each of the three phases.

transformer testing table

In case of an autotransformer, a magnetic balance test of transformer should be repeated for LV winding also.

There are three limbs placed side by side in a core of the transformer. One phase winding is wound in one limb. The voltage induced in different phases depends upon the respective position of the limb in the core. The voltage induced in different phases of a transformer in respect to neutral terminals given in the table below.

Magnetizing Current Test of Transformer

Magnetizing current test of transformer is performed to locate defects in the magnetic core structure, shifting of windings, failure in between turn insulation or problem in tap changers. These conditions change the effective reluctance of the magnetic circuit, thus affecting the current required to establish flux in the core.

Keep the tap changer in the lowest position and open all IV and LV terminals

Then apply three phase 415 V supply on the line terminals for three-phase transformers and single phase 230 V supply on single phase transformers

Measure the supply voltage and current in each phase

Now repeat the magnetizing current test of transformer test with keeping tap changer in normal position

Repeat the test while keeping the tap at highest position

Normally, there are two similar higher readings on two outer limb phases on transformer core and one lower reading on the center limb phase, in the case of three phase transformers.

An agreement to within 30% of the measured exciting current with the previous test is usually considered satisfactory. If the measured exciting current value is 50 times higher than the value measured during factory test, there is a likelihood of a fault in the winding which needs further analysis.

Caution: This magnetizing current test of a transformer is to be carried out before DC resistance measurement.

Vector Group Test of Transformer

In a 3 phase transformer, it is essential to carry out a vector group test of transformer. Proper vector grouping in a transformer is an essential criteria for parallel operation of transformers.

There are several internal connections of three-phase transformer are available on the market. These several connections give various magnitudes and phase of the secondary voltage; the magnitude can be adjusted for parallel operation by suitable choice of turn ratio, but the phase divergence cannot be compensated.

So we have to choose a transformer suitable for parallel operation whose phase sequence and phase divergence are same. All the transformers with the same vector ground have same phase sequence and phase divergence between primary and secondary.

Before procuring an electrical power transformer, you should ensure the vector group of the transformer, whether it will be matched with his or her existing system or not. The vector group test of transformer confirms his or her requirements.

Insulation Resistance Test or Megger Test of Transformer

Insulation resistance test of transformer is essential type test. This test is carried out to ensure the healthiness of the overall insulation system of an electrical power transformer.

Procedure of Insulation Resistance Test of Transformer

Disconnect all the line and neutral terminals of the transformer

Megger leads to be connected to LV and HV bushing studs to measure insulation resistance IR value in between the LV and HV windings

Megger leads to be connected to HV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the HV windings and earth

Megger leads to be connected to LV bushing studs and transformer tank earth point to measure insulation resistance IR value in between the LV windings and earth

NB: It is unnecessary to perform insulation resistance test of transformer per phase wise in three-phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.

Measurements are to be taken as follows:

For autotransformer: HV-IV to LV, HV-IV to E, LV to E.

For two winding transformer: HV to LV, HV to E, LV to E.

Three winding transformers: HV to IV, HV to LV, IV to LV, HV to E, IV to E, LV to E.

Oil temperature should be noted at the time of insulation resistance test of the transformer, since the IR value of transformer insulating oil may vary with temperature.

IR values to be recorded at intervals of 15 seconds, 1 minute and 10 minutes.

With the duration of application of voltage, IR value increases. The increase in IR is an indication of dryness of insulation.

Absorption coefficient = 1 minute value/15 secs. value.

Polarization index = 10 minutes value/1 minute value.

Dielectric Tests of Transformer

Dielectric test of a transformer is one kind of insulation test. This test is performed to ensure the expected overall insulation strength of the transformer. There are several tests performed to ensure the required quality of transformer insulation; the dielectric test is one of them. Dielectric test of the transformer is performed in two different steps.

First one is called Separate Source Voltage Withstand Test of transformer, where a single phase power frequency voltage of prescribed level, is applied on transformer winding under test for 60 seconds while the other windings and tank are connected to the earth, and it is observed that whether any failure of insulation occurs or not during the test.

The second one is the induced voltage test of Transformer where, three-phase voltage, twice of rated secondary voltage is applied to the secondary winding for 60 seconds by keeping the primary of the transformer open circuited.

The frequency of the applied voltage should be double of power frequency too. Here also if no failure of insulation, the test is successful.

In addition to dielectric tests of transformers, there are other types of test for checking insulation of transformer, such as lightning impulse test, switching impulse test and partial discharge test.

Induced Voltage Test of Transformer

The induced voltage test of the transformer is intended to check the inter-turn and line end insulation as well as main insulation to earth and between windings-

Keep the primary winding of transformer open circuited.

Apply three-phase voltage to the secondary winding. The applied voltage should be twice of the rated voltage of secondary winding in magnitude and frequency.

The duration of the test shall be 60 seconds.

The test shall start with a voltage lower than 1/3 the full test voltage, and it shall be quickly increased up to the desired value.

The test is successful if no breakdown occurs at full test voltage during the test.

Temperature Rise Test of Transformer

Temperature rise test of transformer is included in type test of transformer. In this test, we check whether the temperature-rising limit of the transformer winding and oil as per specification or not. In this type test of the transformer, we have to check oil temperature rise as well as winding temperature rise limits of an electrical transformer.

Short Circuit Test on Transformer

The connection diagram for the short circuit test on the transformer is shown in the figure below. A voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. A low voltage of around 5-10% is applied to that HV side with the help of a variac (i.e. a variable ratio auto transformer). We short-circuit the LV side of the transformer. Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the HV side.

After reaching the rated current of the HV side, we record all the three instrument readings (Voltmeter, Ammeter and Watt-meter readings). The ammeter reading gives the primary equivalent of full load current IL. As the voltage applied for full load current in a short circuit test on the transformer is quite small compared to the rated primary voltage of the transformer, the core losses in the transformer can be taken as negligible here.

The electrical power transformer features primary, secondary, and tertiary windings. The transformer is driven by the flux between the windings. Magnetic cores are used in transformers, which serve as the pathway of the flux. The core can be made from various materials, this post discusses some of them.

The magnetic core is basically a material with magnetic permeability, which helps confine magnetic fields in transformers. The following are the types of materials used for producing magnetic cores for the transformers:

Amorphous Steel: This is one of the popular options for creating magnetic cores in transformers. These cores are made from several paper-thin metallic tapes, which help reduce the flow of eddy currents. Amorphous steel cores have fewer losses than other magnetic cores and can easily operate at high temperatures than standard lamination stacks. The amorphous steel cores are most used in high-efficiency transformers that operate at medium frequencies.

Solid Iron Core: These cores provide magnetic flux and helps retain high magnetic fields without iron saturation. The cores are not recommended for transformers operating at AC applications because large eddy currents are produced by the magnetic field. These eddy currents produce heat at high frequencies.

Amorphous Metals: Also known as vitreous metals, these metals are glassy or non-crystalline. These metals are used for creating high-performance transformers. The materials have low conductivity, which helps reduce eddy currents.

Ferrite Ceramics: Ferrite ceramics are a class of ceramic compounds made from iron oxide, and one or multiple metallic elements. The magnetic cores made from ferrite ceramics are used in high-frequency applications. The ceramic materials are produced in different specifications to meet diverse electrical requirements. These ceramic materials serve as efficient insulators and help decrease eddy currents.

Laminated Magnetic Cores: These cores feature thin iron sheets covered with an insulating layer. These insulators prevent eddy currents and confine them to narrow loops within each single lamination layer. The thinner lamination minimizes eddy current effects.

Carbonyl Iron Cores: These magnetic cores are made from powdered carbonyl iron, and provide stable service across a wide range of magnetic flux and temperature levels. The carbonyl iron powder cores feature small iron spheres coated with a thin insulating layer. These cores help reduce the effects of eddy currents at high temperatures.

Silicon Steel: Silicon steel has high electrical resistivity. The silicon steel core provides stable performance over the years. Silicon steel offers high saturation flux density. A few years ago, characteristics of silicon steel was altered with chemical changes, and today, the new product is known as AISI type M6. The M6 steel has high permeability and low losses, and it is used in high-performance applications.

Introduction

The 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.

Introduction

The 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.

Vector Group of Transformer

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 degrees.

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

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 degrees.

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, S=Star, Z=Interconnected star, N=Neutral

Second Symbol: for Low voltage: Always Small letters.

 d=Delta, s=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 especially 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.

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

Clock Notation 0 (Phase Shift 0)

Clock Notation 1 (Phase Shift -30)

Clock Notation 2 (Phase Shift -60)

Clock Notation 4 (Phase Displacement -120)

Clock Notation 5 (Phase Displacement -150)

Clock Notation 6 (Phase Shift +180)

Clock Notation 7 (Phase Shift +150)

Clock Notation 11 (Phase Shift +30)

Or, would you prefer a straightforward response that covers key points and makes sense of abstract concepts? Though clarity is key, Google also places a high value on being thorough. This means marketers need to be careful…

Or, would you prefer a straightforward response that covers key points and makes sense of abstract concepts? Though clarity is key, Google also places a high value on being thorough. This means marketers need to be careful…

Or, would you prefer a straightforward response that covers key points and makes sense of abstract concepts? Though clarity is key, Google also places a high value on being thorough. This means marketers need to be careful…

Or, would you prefer a straightforward response that covers key points and makes sense of abstract concepts? Though clarity is key, Google also places a high value on being thorough. This means marketers need to be careful…