Transformers are devices which transfer energy from one electrical circuit to another by means of a common magnetic field. In all the cases except auto-transformers, there is no direct electrical connection from one circuit to the other. Transformers are important equipment in power distribution system as well as in power electronic system. They can step-down high voltages in transmission at sub-stations or step-up currents to the needed level at the end-users. Additionally, several functions, for example, isolation, noise decoupling, or phase-shifting can be achieved through transformers. The transformers are suitable for operation at altitudes of up to 1,000 metres (m) above sea level. Site altitudes which are above 1, 000 m need the use of special designs.

Industrial transformers lower the voltage from the net-work to the application level so that currents can be maximized. The range of industrial transformers includes regulating and rectifier transformers, alternating current (AC) arc furnace / induction furnace transformers, series reactors for alternating current arc furnaces, direct current (DC) arc furnace transformers, and large drive / converter transformers. Industrial transformers are key elements in the processes into which they are integrated. Reliability is crucial for ensuring uninterrupted power supply to motors, furnaces, and smelters used in a wide variety of applications including primary aluminum, steel, and power plants, and rail networks. A wide range of specialty transformers serves unique end markets such as technology and data centres, electric vehicle charging, light rail networks, and solar and wind turbine generating facilities.

Transformers are highly engineered products, needing sophisticated technical and industry knowledge as well as tailored procurement, design, and testing capabilities. The main components of transformers include a laminated magnetic core, as well as primary and secondary windings. The primary winding is connected to the power source and is where the magnetic flux (a magnetic field-inducing electrical current) is produced in the core by the current and the windings and is constant in all windings.

If the number of secondary turns is higher than the number of primary turns, then the secondary voltage is higher than the primary voltage (step-up). On the other hand, if the number of secondary turns is less than the number of primary turns, then the secondary voltage is less than the primary voltage (step-down).

Transformers form the foundation of the electric grid and are critical components from power generation to distribution and end use. Primary uses of an electrical transformer include (i) raising or lowering the voltage level in the circuit of an alternating current system, (ii) preventing the passage of direct current from one circuit to another, (iii) isolating two electric circuits, (iv) stepping up the voltage level at power generation and stepping down the voltage level at the distribution phase,  and (v) in rare cases, mainly for power factor correction, increasing or decreasing the value of an inductor or capacitor in an alternating current circuit.

When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based.

A transformer is a static (or stationary) piece of equipment by means of which electric power in circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in the current. The physical basis of a transformer is mutual induction between two circuits linked by a common magnetic flux. In its simplest form, it consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance as shown in Fig 1.

Fig 1 Magnetic linkage of inductive coils and principle of transformer

The two coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, majority of which is linked with the another coil in which it produces mutually-induced EMF (electro-motive force) as per Faraday’s laws of electro-magnetic induction which states that whenever a conductor is placed in a varying magnetic field, an electro-motive force is induced. If the second coil circuit is closed, a current flow in it and so electric energy is transferred (entirely magnetically) from the first coil to the second coil. The first coil, in which electric energy is fed from the alternating current supply mains, is called primary winding and the other coil from which energy is drawn out, is called secondary winding (Fig 1).

In brief, a transformer is a device which has such characteristics as (i) it transfers electric power from one circuit to another, (ii) it does so without a change of frequency, (iii) it accomplishes this by electro-magnetic induction, and (iv) where the two electric circuits are in mutual inductive influence of each other.

ANSI (American National Standards Institute) / IEEE (Institute of Electrical and Electronics Engineers) defines a transformer as a static electrical device, involving no continuously moving parts, used in electric power systems to transfer power between circuits through the use of electro-magnetic induction. The term ‘power transformer’ is used to refer to those transformers which are used between the generator and the distribution circuits, and these are normally rated at 500 kVA (kilovolt ampere) and above. Power systems typically consist of a large number of generation locations, distribution points, and inter-connections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers are to be used at each of these points where there is a transition between voltage levels.

Some small transformers for low-power applications are constructed with air between the two coils. Such transformers are inefficient since the percentage of the flux from the first coil which links the second coil is small. Power transformers are selected based on the application, with the emphasis toward custom design being more apparent when the larger is the unit. Power transformers are available for step-up operation, which are primarily used at the generator and which are referred to as generator step-up (GSU) transformers, and for step-down operation, which mainly used to feed distribution circuits (Fig 1). Power transformers are available as single-phase equipment or three-phase equipment.

There are two basic types of transformers which are categorized by their winding / core configuration. These are (i) core type, and (ii) shell type. The difference in their construction is shown in Fig 2. In core type transformer, windings are cylindrical former wound, mounted on the core limbs as shown in the Fig 2a. The cylindrical coils have different layers and each layer is insulated from each other. Materials like paper, cloth, or mica can be used for insulation. Low voltage windings are placed nearer to the core, as they are easier to insulate. In the shell type transformer, the coils are former wound and mounted in layers stacked with insulation between them. A shell type transformer can have simple rectangular form (as shown in Fig 2b), or it can have a distributed form.

Fig 2 Types of transformers

Because of the intrinsically better magnetic shielding provided by the shell type arrangement, this is particularly suitable for supplying power at low voltage and heavy current, as for example, in the case of arc furnace transformers. In a shell-type transformer the flux-return paths of the core are external to and enclose the windings. Core-type designs predominate throughout the world. Core-type transformers have their limbs surrounded concentrically by the main windings as shown in Fig 2a. Fig 2a shows a two-limb arrangement. In a three-phase, three-limb configuration, having top and bottom yokes equal in cross-section to the wound limbs, no separate flux-return path is necessary, since for a balanced three-phase system of fluxes, these summate to zero at all times. In the case of a very large transformer which can be subject to height limitations, normally because of transport restrictions, it can be necessary to reduce the depth of the top and bottom yokes. These can be reduced until their cross-sectional area is only 50 % of that of the wound limb so that the return flux is split at the top of the limb with half returning in each direction. Clearly in this case return yokes are to be provided.

The magnetic circuits of the three-phase five-limb core-type transformers behave differently in relation to zero-sequence and third-harmonic fluxes than do the more commonly used three-phase three-limb cores. Of course, it is always necessary to provide a return-flux path in the case of single-phase core-type transformers and different configurations are possible according to whether these have one or two wound limbs. A three-phase transformer has considerable economic advantages over three single-phase units used to provide the same function so that the large majority of power transformers are of three-phase construction. The exceptions occur at each end of the size range.

Single-phase transformers are used at the remote ends of rural distribution systems in the provision of supplies to consumers whose load is not high enough to justify a three-phase supply. These transformers almost invariably have both limbs wound. Single-phase units are also used for the largest generator set-up transformers. Frequently the reason for this is to reduce the transport weight and dimensions but there are other factors which influence the argument such as limiting the extent of damage in the event of faults and the economics of providing spare units as well as the ease of moving these around in the event of failures in service.

In the case of these very large single-phase units, the high initial cost justifies a very careful study of all the economic factors affecting each individual design. Such factors include the merits of adopting a one-limb wound or a two-limb wound arrangement. Since the cost of windings norm constitutes a considerable proportion of the total cost of these units, it is normally more economic to adopt a single-limb wound arrangement.

The other factor descriptive of the type of transformers which constitute the large majority of power transformers is that they are double wound, i.e., they have two discrete windings, i.e. a low-voltage and a high-voltage winding. This fact is of high importance to the designers of electrical power systems in that it provides a degree of isolation between systems of different voltage level and limits the extent that faults on one system can affect another.

Transformer action – Transformer action depends upon magnetic lines of force (flux). At the instant, a transformer primary is energized with alternating current, a flow of electrons (current) begins. During the instant of switch closing, build-up of current and magnetic field occurs. As current begins the positive portion of the sine wave, lines of magnetic force (flux) develop outward from the coil and continue to expand until the current is at its positive peak. The magnetic field is also at its positive peak. The current sine wave then begins to decrease, crosses zero, and goes negative until it reaches its negative peak. The magnetic flux switches direction and also reaches its peak in the opposite direction. With an alternating current power circuit, the current changes (alternates) continually 50 times per second (frequency). Some countries use frequency of 60 cycles per second.

Several transformers have separate coils, and contain several turns of wire and a magnetic core, which forms a path for and concentrates the magnetic flux. The winding receiving electrical energy from the source is called the primary winding. The winding which receives energy from the primary winding, through the magnetic field, is called the secondary winding. Either the high-voltage or low-voltage winding can be the primary or the secondary. With generator set-up at power plants, the primary winding is the low-voltage side (generator voltage), and the high voltage side is the secondary winding (transmission voltage). Where power is used, the primary winding is the high-voltage side, and the secondary winding is the low-voltage side.

The quantity of voltage induced in each turn of the secondary winding is the same as the voltage across each turn of the primary winding. The total quantity of voltage induced is equal to the sum of the voltages induced in each turn. Hence, if the secondary winding has more turns than the primary, a higher voltage is induced in the secondary winding, and then the transformer is known as a step-up transformer. If the secondary winding has fewer turns than the primary winding, a lower voltage is induced in the secondary winding, and then the transformer is a step-down transformer. It is to be noted that the primary winding is always connected to the source of power, and the secondary winding is always connected to the load. In actual practice, the quantity of power available from the secondary winding is slightly less than the quantity supplied to the primary winding because of losses in the transformer itself.

Strength of a magnetic field depends on the quantity of current and number of turns in the winding. When current is reduced, the magnetic field shrinks. When the current is switched off, the magnetic field collapses. When a coil is placed in an alternating current circuit, as shown in Fig 3, current in the primary coil is accompanied by a constantly rising and collapsing magnetic field. When another coil is placed within the alternating magnetic field of the first coil, the rising and collapsing flux induces voltage in the second coil. When an external circuit is connected to the second coil, the induced voltage in the coil causes a current in the second coil. The coils are said to be magnetically coupled. They are, however, electrically isolated from each other.

Fig 3 Transformer action in magnetically coupled coils

When an alternating current generator is connected to the primary coil of a transformer (Fig 3), electrons flow through the coil because of the generator voltage. Alternating current varies, and accompanying magnetic flux varies, cutting both transformer coils and inducing voltage in each coil circuit. The voltage induced in the primary circuit opposes the applied voltage and is known as back voltage or back electro-motive force (back EMF). When the secondary circuit is open, back electro-motive force, along with the primary circuit resistance, acts to limit the primary current. Primary current is to be sufficient to maintain enough magnetic field to produce the required back electro-motive force.

When the secondary circuit is closed and a load is applied, current appears in the secondary coil because of the induced voltage, resulting from flux created by the primary current. This secondary current sets up a second magnetic field in the transformer in the opposite direction of the primary magnetic field. Hence, the two fields oppose each other and result in a combined magnetic field of less strength than the single field produced by the primary coil with the secondary coil open. This reduces the back voltage (back EMF) of the primary magnetic field and causes the primary current to increase. The primary current increases until it reestablishes the total magnetic field at its original strength. In transformers, a balanced condition always exists between the primary and secondary magnetic fields. Volts times amperes is also to be balanced (be the same) on both primary and secondary coils. The needed primary voltage and current is required to be supplied to maintain the transformer losses and secondary load.

Transformer voltage and current – If the small quantity of transformer loss is ignored, the back-voltage (back EMF) of the primary coil is to equal the applied voltage. The magnetic field, which induces the back-voltage in the primary coil, also cuts the secondary coil. If the secondary coil has the same number of turns as the primary coil, the voltage induced in the secondary coil is equal to the back-voltage induced in the primary coil (or the applied voltage). If the secondary coil has twice as many turns as the primary coil, it is to be cut twice as many times by the flux, and twice the applied primary coil voltage is to be induced in the secondary coil. The total induced voltage in each winding is proportional to the number of turns in that winding. If ‘E1’ is the primary voltage and ‘I1’ is the primary current, ‘E2’ is the secondary voltage and ‘I2’ is the secondary current, ‘N1’ is the primary turns and ‘N2’ is the secondary turns, then ‘E1/E2 = N1/N2 = L2/I1’ (equation 1). It is to be noted that the current is inversely proportional to both voltage and number of turns. This means that if voltage is stepped up, the current is to be stepped down and vice versa. The number of turns remains constant unless there is a tap changer.

The power output or input of a transformer equals volts times amperes (E x I). If the small quantity of transformer loss is disregarded, input power equals output power or ‘E1 x I1 = E2 x I2’ (equation 2). If the primary voltage of a transformer is 110 volts (V), the primary winding has 100 turns, and the secondary winding has 400 turns, then the secondary voltage is calculated as ‘E1/E2 = N1/N2 or 110/E2 = 100/400 or E2 = 440 volts. If the primary current is 20 amperes, the secondary current is calculated as ‘E2 x I2 = E1 x I1 or 440 x I2 = 110 x 20 or ‘I2’ = 5 amperes. Since there is a ratio of 1 to 4 between the turns in the primary circuit and secondary circuit, there is to be a ratio of 1 to 4 between the primary voltage and secondary voltage and a ratio of 4 to 1 between the primary current and secondary current. As voltage is stepped up, the current is stepped down, keeping volts multiplied by amperes constant. This is referred to as ‘volt-amperes’. Fig 4 shows relationship between voltage and current in transformers.

Fig 4 Relationship between voltage and current in transformers

As shown in Fig 4, when the number of turns or voltage on the secondary coil of a transformer is higher than that of the primary coil, it is known as a step-up transformer. When the number of turns or voltage on the secondary coil is less than that of the primary coil, it is known as a step-down transformer. A power transformer used to tie two systems together can feed current either way between systems, or act as a step-up transformer or step-down transformer, depending on where power is being generated and where it is consumed. As stated above, either winding can be the primary or secondary. For eliminating this confusion, in power generation, windings of transformers are frequently referred to as high-side winding and low-side winding, depending on the relative values of the voltages. It is to be noted that kilovolt-ampere (amperes times volts) remains constant through-out the above circuit on both sides of each transformer, which is why, they are called constant wattage devices.

Efficiencies of well-designed power transformers are very high, averaging over 98 %. The only losses are because of the core losses, maintaining the alternating magnetic field, resistance losses in the coils, and power used for cooling. The main reason for high efficiencies of power transformers, compared to other equipment, is the absence of moving parts. Transformers are called static alternating current machines.

Construction of transformer – A transformer has no internal moving parts, and it transfers energy from one circuit to another by electro-magnetic induction. External cooling can include heat exchangers, radiators, fans, and oil pumps. The large horizontal tank at the top is a conservator as shown in Fig 5 which shows the schematic of a large, liquid immersed power transformer. Transformers are typically used since a change in voltage is needed.

Fig 5 Schematic of a large, liquid immersed power transformer

The construction of a transformer depends upon the application. Transformers intended for indoor use are primarily of the dry type but can also be liquid immersed. For outdoor use, transformers are normally liquid immersed.

Larger transformers are oil-filled for insulation and cooling. A typical generator step-up transformer can contain several thousand litres of oil. One is to be always aware of the possibility of spills, leaks, fires, and environmental risks which are posed by this oil.

Transformers smaller than 500 kVA are normally called distribution transformers. Pole-top and small, pad-mounted transformers which serve residences and small commercial establishments are typically distribution transformers. Generator step-up transformers, used in power plants, receive electrical energy at generator voltage and increase it to a higher voltage for transmission lines. On the other hand, a step-down transformer receives energy at a higher voltage and delivers it at a lower voltage for distribution to different loads.

All electrical devices using coils (in this case, transformers) are constant wattage devices. This means voltage multiplied by current is to remain constant. Hence, when voltage is stepped-up, the current is stepped-down (and vice versa). Transformers transfer electrical energy between circuits are completely insulated from each other. This makes it possible to use very high (stepped-up) voltages for transmission lines, resulting in a lower (stepped-down) current. Higher voltage and lower current reduce the needed size and cost of transmission lines and reduce transmission losses as well.

Transformers have made possible economic delivery of electric power over long distances. Transformers do not need as much attention as majority of the other equipment. However, the care and maintenance they do need is absolutely critical. Because of their reliability, maintenance is sometimes ignored, causing reduced service life and, at times, outright failure.

Principle of operation – Transformer function is based on the principle that electrical energy is transferred efficiently by magnetic induction from one circuit to another. When one winding of a transformer is energized from an alternating current source, an alternating magnetic field is established in the transformer core. Alternating magnetic lines of force, called ‘flux’, circulate through the core. With a second winding around the same core, a voltage is induced by the alternating flux lines. A circuit, connected to the terminals of the second winding, results in current flow.

Each phase of a transformer is composed of two separate coil windings wound on a common core. The low-voltage winding is placed nearest the core, while the high-voltage winding is then placed around both the low voltage winding and core. Fig 6 shows internal construction of one phase. The core is typically made from very thin steel laminations, each coated with insulation. By insulating between individual laminations, losses are reduced. The steel core provides a low resistance path for magnetic flux. Both high-voltage and low-voltage windings are insulated from the core and from each other, and leads are brought out through insulating bushings. A three-phase transformer typically has a core with three legs and has both high-voltage and low-voltage windings around each leg. Special paper and wood are used for insulation and internal structural support.

Fig 6 Internal construction of one phase of a transformer

The magnetic circuit – A magnetic circuit or core of a transformer is designed to provide a path for the magnetic field, which is necessary for the induction of voltages between windings. A path of low reluctance (i.e., resistance to magnetic lines of force), consisting of thin silicon steel sheet laminations, is used for this purpose. In addition to providing a low reluctance path for the magnetic field, the core is designed to prevent circulating electric currents within the steel itself. Circulating currents, called eddy currents, cause heating and energy loss. They are because of the voltages induced in the steel of the core, which is constantly subject to alternating magnetic fields. Steel itself is a conductor, and changing lines of magnetic flux also induce a voltage and current in this conductor. By using very thin sheets of steel with insulating material between sheets, eddy currents (losses) are largely reduced.

The two common arrangements of the magnetic path and the windings are shown in Fig 7. In the core-type (core form) transformer, the windings surround the core. A section of both primary winding and secondary winding is wound on each leg of the core, the low voltage winding is wound next to the core, and the high voltage winding is wound over the low voltage. In a shell-type (shell form) transformer, the steel magnetic circuit (core) forms a shell surrounding the windings. In a core form, the windings are on the outside, while in a shell form, the windings are on the inside. In power transformers, the electrical windings are arranged so that practically all of the magnetic lines of force go through both the primary and secondary windings. A small percentage of the magnetic lines of force goes outside the core, and this is called leakage flux. Larger transformers, such as generator set-up, are almost always shell type.

It is to be noted that, in the shell form transformers, (Fig 7) the magnetic flux, external to the coils on both left and right extremes, has complete magnetic paths for stray and zero sequence flux to return to the coils. In the core form, it can easily be seen from Fig 7 that, on both left and right extremes, there are no return paths. This means that the flux is to use external tank walls and the insulating medium for return paths. This increases core losses and decreases overall efficiency and shows why the majority of the large transformers are built as shell form units.

Fig 7 Magnetic circuits in transformer units

Core losses – Since magnetic lines of force in a transformer are constantly changing in value and direction, heat is developed because of the hysteresis of the magnetic material (friction of the molecules). This heat is to be removed. Hence, it represents an energy loss of the transformer. High temperatures in a transformer drastically shortens the life of insulating materials used in the windings and structures. For every 8 deg C temperature rise, life of the transformer is cut by one-half, hence, maintenance of cooling systems is critical. Losses of energy, which appears as heat because of both the hysteresis and the eddy currents in the magnetic path, is known as core losses. Since these losses are because of the alternating magnetic fields, they occur in a transformer whenever the primary is energized, even though no load is on the secondary winding.

Copper losses – There is some loss of energy in a transformer because of the resistance of the primary winding to the magnetizing current, even when no load is connected to the transformer. This loss appears as heat generated in the winding and is also to be removed by the cooling system. When a load is connected to a transformer and electrical currents exist in both primary winding and secondary winding, further losses of electrical energy occur. These losses which are because of the resistance of the windings, are called copper losses or the ‘I2R’ losses.

Transformer rating – Capacity (or rating) of a transformer is limited by the temperature which the insulation can tolerate. Ratings can be increased by reducing core losses and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it can withstand higher temperatures. A physically larger transformer can dissipate more heat, because of the increased area and increased volume of oil. A transformer is only as strong as its weakest link, and the weakest link is the paper insulation, which begins to degrade around 100 deg C. This means that a transformer is to be operated with the ‘hottest spot’ cooler than this degradation temperature, or service life is greatly reduced. Reclamation typically orders transformers larger than needed, which aids in heat removal and increases transformer life.

Ratings of transformers are obtained by simply multiplying the current times the voltage. Small transformers are rated in volts amperes (VA). As the size increases, 1 kilo volt-ampere (kVA) means 1,000 volt-amperes, 1 mega volt-ampere (MVA) means 1 million volt-amperes. Large generator set-up transformers can be rated in hundreds of MVAs. A generator set-up transformer has a very high costs and a long delivery time. Each transformer is designed for a specific application. If one fails, this can mean a unit or whole power plant can be down for a long time, resulting huge losses in lost generation, in addition to the replacement cost of the transformer itself. This is one reason why proper maintenance of the transformer is critical.

Transformer impedance – Transformer impedance is defined as the ratio of the voltage drop across the transformer under full-load conditions to the rated current. It is an important parameter which reflects the resistance of the transformer to the flow of electrical current. The impedance of a transformer can be calculated for each winding. However, a rather simple test provides a practical method of measuring the equivalent impedance of a transformer without separating the impedance of the windings.

When referring to the impedance of a transformer, it is the equivalent impedance which is meant. In order to determine equivalent impedance, one winding of the transformer is short circuited, and just enough voltage is applied to the other winding to create full load current to flow in the short-circuited winding. This voltage is known as the impedance voltage. Either winding can be short-circuited for this test, but it is normally more convenient to short circuit the low-voltage winding. The transformer impedance value is given on the name plate in percent. This means that the voltage drop because of the impedance is expressed as a percent of rated voltage. For example, if a 2,400 / 240-volt transformer has a measured impedance voltage of 72 volts on the high voltage windings, its impedance (Z), expressed as a percent, is ‘% Z = (72/2,400) x 100 = 3 %.

This means there is a 72-volt drop in the high-voltage winding at full load because of the losses in the windings and core. Only 1 % or 2 % of the losses are because of the core, and around 98 % are because of the winding impedance. If the transformer is not operating at full load, the voltage drop is less. If an actual impedance value in ohms is needed for the high-voltage side then it is found using equation 3, which is Z = V/I, where ‘V’ is the voltage drop or, in this case 72 volts, and ‘I’ is the full load current in the primary winding. If the full load current is 10 amperes, then Z = 72 volts/10 amperes = 7.2 ohms. Of course, it is to be remembered that impedance is made up of both resistive and reactive components.

Internal forces – During the normal operation of the transformer, internal structures and windings are subjected to mechanical forces because of the magnetic forces. By designing the internal structure very strong to withstand these forces over a long period of time, service life of the transformer can be extended. However, in a large transformer during a ‘through fault’ (fault current passing through a transformer), forces can reach millions of kilograms, pulling the coils up and down and pulling them apart 50 or 60 times per second. The internal low-voltage coil is pulled downward, while the high voltage winding is pulled up, in the opposite direction. At the same time, high-voltage and low-voltage coils are being forced apart. It is to be kept in mind that these forces are reversing 50 or 60 times each second. It is obvious why internal structures of transformers are to be built incredibly strong.

Several times, if fault currents are high, these forces can rip a transformer apart and cause electrical faults inside the transformer itself. This normally results in arcing inside the transformer which can result in explosive failure of the tank, throwing flaming oil over a wide area. There are protective relay systems to protect against this possibility, although explosive failures do occur occasionally. It is possible to get transformer action by means of a single coil, provided that there is a ‘tap connection’ somewhere along the winding. Transformers having only one winding are called auto-transformers, as shown schematically in Fig 8a.

Fig 8 Auto transformer and connections of instrument transformer

An auto-transformer has the normal magnetic core but only one winding, which is common to both the primary and secondary circuits. The primary is always the portion of the winding connected to the alternating current power source. This transformer can be used to step-up the voltage or step-down the voltage. If the primary winding is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding (as shown in Fig 8a), the secondary voltage is ‘stepped-down’. If only a portion of the winding is the primary and is connected to the supply voltage and the secondary includes all the winding, then the voltage is ‘stepped-up’ in proportion to the ratio of the total turns to the number of connected turns in the primary winding.

When primary current ‘I1’ is in the direction of the arrow, secondary current, ‘I2’ is in the opposite direction, as in Fig 8b (ii). Hence, in the portion of the winding between points b and c, current is the difference of ‘I1’ and ‘I2’. If the requirement is to step the voltage up (or down) only a small quantity, then the transformer ratio is small, i.e., ‘E1’ and ‘E2’ are nearly equal. Currents ‘I1’ and ‘I2’ are also nearly equal. The portion of the winding between ‘b’ and ‘c’, which carries the difference of the currents, can be made of a much smaller conductor, since the current is much lower. Under these circumstances, the auto-transformer is much cheaper than the two-coil transformer of the same rating.

However, the disadvantage of the auto-transformer is that the primary circuit and secondary circuit are electrically connected and, hence, cannot be safely used for stepping down from high voltage to a voltage suitable for plant loads. The auto-transformer, however, is extensively used for reducing line voltage for step increases in starting larger induction motors. There are normally four taps or five taps which are changed by timers so that more of the winding is added in each step until the full voltage is applied across the motor. This avoids the large in-rush current needed when starting motors at full line voltage. This transformer is also extensively used for ‘buck-boost’ when the voltage needs to be stepped up or down is only a small percentage. One very common example is boosting 208 V up from one phase of a 120 V / 208 V three-phase system to 220 V for single-phase loads.

Instrument transformers – Instrument transformers (Fig 8b) are used for measuring and control purposes. They provide currents and voltages proportional to the primary, but there is less danger to instruments and personnel. Those transformers used to step voltage down are known as potential transformers (PTs) and those used to step current down are known as current transformers (CTs). The function of a potential transformer is to accurately measure voltage on the primary side, while a current transformer is used to measure current on the primary side.

Potential transformers – Potential transformers are used with volt-meters, watt-meters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and undervoltage and overvoltage trip coils of circuit breakers. One potential transformer can be used for a number of instruments, if the total current needed by the instruments connected to the secondary winding does not exceed the transformer rating. Potential transformers are usually rated 50 volt-amperes to 200 volt-amperes at 120 secondary volts. The secondary terminals are never to be short circuited since a heavy-current results, which can damage the windings.

Current transformers – The primary conductor of a current transformer typically has only one turn. This is not really a turn or wrap around the core but just a conductor or bus going through the ‘window’. The primary conductor never has more than a very few turns, while the secondary coil can have a large number of turns, depending upon how much the current is to be stepped down. In majority of the cases, the primary conductor of a current transformer is a single wire or bus bar, and the secondary coil is wound on a laminated magnetic core, placed around the conductor in which the current needs to be measured, as shown in Fig 9a.

Fig 9 Current transformer and test connections for determination of polarity

If primary current exists and the secondary circuit of a current transformer is closed, the winding builds and maintains a counter or back electro-motive force to the primary magnetizing force. In case the secondary winding is opened with current in the primary winding, the counter electro-motive force is removed, and the primary magnetizing force builds up an extremely high voltage in the secondary winding, which is dangerous to personnel and can destroy the current transformer. Hence, for this reason, the secondary winding of a current transformer is always to be shorted before removing a relay from its case or removing any other device which the current transformer operates. This protects the current transformer from overvoltage.

Current transformers are used with ammeters, watt-meters, power-factor meters, watt-hour meters, compensators, protective and regulating relays, and trip coils of circuit breakers. One current transformer can be used to operate several instruments, provided the combined loads of the instruments do not exceed that for which the current transformer is rated. Secondary windings are normally rated at 5 amperes. Several times, current transformers have several taps on the secondary winding to adjust the range of current possible to measure on the primary.

Transformer taps – Majority of the power transformers have taps on either primary windings or secondary windings to vary the number of turns and, hence, the output voltage. The percentage of voltage change, above or below normal, between different tap positions varies in different transformers. In oil-cooled transformers, tap leads are brought to a tap changer, located beneath the oil inside the tank, or brought to an oil-filled tap changer, externally located. Taps on dry-type transformers are brought to the insulated terminal boards located inside the metal housing, accessible by removing a panel.

Some transformers taps can be changed under load, while other transformers are to be de-energized. When it is necessary to change taps frequently to meet changing conditions, taps which can be changed under load are used. This is done by means of a motor which can controlled either manually or automatically. Automatic operation is achieved by changing taps to maintain constant voltage as system conditions change. A common range of adjustment is plus or minus 10 %. At power plants, de-energized tap changers (DETC) are used and can only be changed with the transformer off-line. A very few load tap changers (LTC) which are normally used at switch-yards are between the 500 kilovolt (kV) and 220 kV switchyards. A bypass device is sometimes used across tap changers for ensuring power flow in case of contact failure. This prevents failure of the transformer in case excessive voltage appears across faulty contacts.

Transformer bushings – The two most common types of bushings used on transformers as main lead entrances are solid porcelain bushings on smaller transformers and oil-filled condenser bushings on larger transformers. Solid porcelain bushings consist of high-grade porcelain cylinders through which the conductors pass through. Outside surfaces have a series of skirts to increase the leakage path distance to the grounded metal case. High voltage bushings are normally oil-filled condenser type. Condenser types have a central conductor wound with alternating layers of paper insulation and tin foil and filled with insulating oil. This results in a path from the conductor to the grounded tank, consisting of a series of condensers. The layers are designed for providing around equal voltage drops between each condenser layer.

Acceptance and routine maintenance tests very frequently used for checking the condition of bushings are Doble power factor tests. The power factor of a bushing in good condition remains relatively stable throughout the service life. A good indication of insulation deterioration is a slowly rising power factor. The most common cause of failure is moisture entrance through the top bushing seal. This condition gets revealed before failure by routine Doble testing. If Doble testing is not performed regularly, explosive failure is the eventual result of a leaking bushing. This, several times, results in a catastrophic and expensive failure of the transformer as well.

Transformer polarity – With power or distribution transformers, polarity is important only if the need arises to parallel transformers to gain additional capacity or to hook up three single-phase transformers to make a three-phase bank. The way the connections are made affects angular displacement, phase rotation, and direction of rotation of connected motors. Polarity is also important when hooking up current transformers for relay protection and metering. Transformer polarity depends on which direction coils are wound around the core (clock-wise or counter-clock-wise) and how the leads are brought out.

Transformers are sometimes marked at their terminals with polarity marks. Frequently, polarity marks are shown as white paint dots (for plus) or plus-minus marks on the transformer and symbols on the name-plate. These marks show the connections where the input and output voltages (and currents) have the same instantaneous polarity.

More frequently, transformer polarity is shown simply by the American National Standards Institute (ANSI) designations of the winding leads as H1, H2 and X1, X2. By ANSI standards, if a person faces the low-voltage side of a single-phase transformer (the side marked X1, X2), the H1 connection is always be on the person’s far left as seen in the single-phase diagrams in Fig 9b. If the terminal marked X1 is also on the person’s left, it is subtractive polarity. If the X1 terminal is on the person’s right, it is additive polarity. Additive polarity is normal for small distribution transformers. Large transformers, such as generator set-up at power plants normally have subtractive polarity.

It is also helpful to think of polarity marks in terms of current direction. At any instant when the current direction is into a polarity marked terminal of the primary winding, the current direction is out of the terminal with the same polarity mark in the secondary winding. It is the same as if there is a continuous circuit across the two windings.

Polarity is a convenient way of stating how leads are brought out. If a person wants to test for polarity, then the transformer is to be connected as shown in Fig 9b. A transformer is said to have additive polarity if, when adjacent high-voltage and low-voltage terminals are connected and a voltmeter is placed across the other high-voltage and low-voltage terminals, the voltmeter reads the sum (additive) of the high-voltage and low-voltage windings. It is subtractive polarity if the voltmeter reads the difference (subtractive) between the voltages of the two windings. If this test is conducted, then the lowest alternating current voltage available is to be used to reduce the potential hazards. An adjustable alternating current voltage source, such as a ‘variac’, is desired to keep the test voltage low.

Single-phase transformer connections for typical service to buildings – Fig 10 shows a typical arrangement of bringing leads out of a single-phase distribution transformer. For providing flexibility for connection, the secondary winding is arranged in two sections. Each section has the same number of turns and, hence, the same voltage. Two primary leads (H1, and H2) are brought out from the top through porcelain bushings. Three secondary leads (X1, X2, and X3) are brought out through insulating bushings on the side of the tank, one lead from the centre tap (neutral) (X2) and one from each end of the secondary coil (X1 and X3). Connections, as shown, are typical of services to residences and small commercial places. This connection provides a three-wire service which permits adequate capacity at minimum cost. The neutral wire (X2) (centre tap) is grounded. A 120-volt circuit is between the neutral and each of the other leads, and a 240-volt circuit is between the two ungrounded leads.

Fig 10 Single phase transformer

Parallel operation of single-phase transformers for additional capacity – In perfect parallel operation of two or more transformers, current in each transformer is directly proportional to the transformer capacity, and the arithmetic sum is equal to one-half of the total current. In practice, this is seldom achieved because of small variations in transformers. However, there are conditions for operating transformers in parallel. They are given below.

In the first condition, any combination of positive and negative polarity transformers can be used. However, in all cases, numerical notations are to be followed on both primary connection and secondary connection, i.e., H1 is connected to H1, H2 is connected to H2, X1 is connected to X1, X2 is connected to X2, and X3 is connected to X3. It is to be noted that each subscript number on a transformer is to be connected to the same subscript number on the other transformer as shown in Fig 11a. With positive and negative polarity transformers, the location of X1 and X2 connections on the tanks are reversed. Hence, care is to be exercised for ensuring that terminals are connected, as stated above (Fig 11a).

Fig 11 Single phase paralleling and three phase connections

The second condition is that the tap settings is to be identical. The third condition is that the voltage ratings are to be identical, which, of course, makes the turns ratios also identical. The fourth condition is that the percent impedance of one transformer is to be between 92.5 % and 107.5 % of the other. Otherwise, circulating currents between the two transformers are excessive. The fifth condition is that the frequencies are to be identical. Standard frequency of 50 hertz or 60 hertz which are normally used in several countries do not present a problem.

It can be noticed from the above requirements, that paralleled transformers do not have to be the same size. However, to meet the percent impedance requirement, they are to be nearly of the same size. Majority of the utilities do not parallel transformers if they are more than one standard kVA size rating different from each other, since otherwise circulating currents are excessive.

Three-phase transformer connections – Three-phase power is attainable with one three-phase transformer, which is constructed with three single-phase units enclosed in the same tank or three separate single-phase transformers. The methods of connecting windings are the same, whether using the one three-phase transformer or three separate single-phase transformers.

Wye and delta connections – The two common methods of connecting three-phase generators, motors, and transformers are shown in Fig 11b. The method shown in at Fig 11b(i) is known as a delta connection, since the diagram bears a close resemblance to the Greek letter ‘delta’. The other method at 11b(ii) is known as the star or wye connection. The wye differs from the delta connection in that it has two phases in series. The common point ‘O’ of the three windings is called the neutral since equal voltages exist between this point and any of the three phases. When windings are connected wye, the voltage between any two lines is 1.732 times the phase voltage, and the line current is the same as the phase current. When transformers are connected delta, the line current is 1.732 times the phase current, and the voltage between any two is the same as that of the phase voltage.

Three-phase connections using single-phase transformers – As stated above, single-phase transformers can be connected to get three-phase power. These are found at several facilities, at shops, offices, and warehouses. The same requirements are to be observed as given under heading ‘parallel operation of single-phase transformers for additional capacity’ in the manner connections are made between individual single-phase units. ANSI standard shows the connections. There are other angular displacements which also work but are seldom used. Also, attempt is not to be made to connect single-phase units together in any combination which does keep the exact angular displacement on both primary circuit and secondary circuit, since a dangerous short circuit can be the result. Additive and subtractive polarities can be mixed. The banks also can be paralleled for additional capacity if the rules are followed for three-phase paralleling. When paralleling individual three-phase units or single-phase banks to operate three phase, angular displacements are to be the same.

Delta-delta connections for single-phase transformers for three-phase operation are frequently used. Wye-wye connections are seldom used at several facilities because of inherent third harmonic problems. Methods of dealing with the third harmonic problem by grounding are described below. However, it is easier just to use another connection scheme (i.e., delta-delta, wye-delta, or delta-wye), to avoid this problem altogether. In addition, these schemes are much more familiar to the plant personnel.

Paralleling three-phase transformers – Two or more three-phase transformers, or two or more banks made up of three single-phase units, can be connected in parallel for additional capacity. In addition to requirements listed above for single-phase transformers, phase angular displacements (phase rotation) between high voltages and low voltages are to be the same for both. The requirement for identical angular displacement is required to be met for paralleling any combination of three-phase units and / or any combination of banks made up of three single-phase units.

This means that some possible connections do not work and produce dangerous short circuits (Tab 1). For delta-delta and wye-wye connections, corresponding voltages on the high-voltage and low-voltage sides are in phase. This is known as zero phase (angular) displacement. Since the displacement is the same, these can be paralleled. For delta-wye and wye-delta connections, each low-voltage phase lags its corresponding high voltage phase by 30-degree. Since the lag is the same with both transformers, these can be paralleled. A delta-delta, wye-wye transformer, or bank (both with zero-degree displacement) cannot be paralleled with a delta-wye or a wye-delta which has 30 degrees of displacement. This results in a dangerous short circuit. Table 1 shows the combinations which operate in parallel and the combinations which do not operate in parallel.

Tab 1 Operative and inoperative parallel connections of three phase transformers voltage sideHigh voltage side
Transformer ATransformer BTransformer ATransformer B
Operative parallel connections
Inoperative parallel connections

Wye-wye connected transformers are seldom, if ever, used to supply plant loads or as generator set-up units, because of the inherent third harmonic problems with this connection. Delta-delta, delta-wye, and wye-delta are used extensively at several facilities. Some rural electric facilities use wye-wye connections which are supplying to structures in remote areas. There are three methods to negate the third harmonic problems found with wye-wye connections. These are (i) primary and secondary neutrals can be connected together and grounded by one common grounding conductor., (ii) primary and secondary neutrals can be grounded individually using two grounding conductors, and (iii) the neutral of the primary can be connected back to the neutral of the sending transformer by using the transmission line neutral.

In making parallel connections of transformers, polarity markings are to be followed. Regardless of whether transformers are additive or subtractive, connections of the terminals are to be made as per the markings and as per the method of the connection (i.e., delta or wye). Also as mentioned above regarding paralleling single-phase units, when connecting additive polarity transformers to subtractive transformers connections are to be in different locations from one transformer to the next.

Methods of cooling – Increasing the cooling rate of a transformer increases its capacity. Cooling methods are not only to maintain a sufficiently low average temperature but also to prevent an excessive temperature rise in any portion of the transformer (i.e., it is to prevent hot spots). For this reason, working parts of large transformers are normally submerged in high-grade insulating oil. This oil is to be kept as free as possible from moisture and oxygen, dissolved combustible gases, and particulates. Ducts are arranged to provide free circulation of oil through the core and coils. Warmer and lighter oil rises to the top of the tank, cooler and heavier oil settles to the bottom. Several methods have been developed for removing heat which is transmitted to the transformer oil from the core and windings (Fig 12a).

Fig 12 Cooling of transformers

Oil-filled – self-cooled transformers – In small-sized and medium-sized transformers, cooling takes place by direct radiation from the tank to surrounding air. In oil-filled, self-cooled types, tank surfaces can be corrugated to provide a larger radiating surface. Oil in contact with the core and windings rises as it absorbs heat and flows outward and downward along tank walls, where it is cooled by radiating heat to the surrounding air. These transformers can also have external radiators attached to the tank to provide larger surface area for cooling.

Forced-air and forced-oil cooled transformers – Forced-air cooled transformers have fan-cooled radiators through which the transformer oil circulates by gravity, as shown in Fig 12b(i). Fans force air through radiators, hence cooling the oil. Forced-air / oil / water-cooled transformers have a self-cooled (kVA or MVA) rating and one or more forced cooling ratings (higher kVA or MVA). Higher ratings are because of forced cooling in increasing quantities. As temperature increases, more fans or more oil pumps are turned on automatically. The forced-cooling principle is based on a trade-off between extra cooling and manufacturing costs. Transformers with forced-cooling have less weight and bulk than self-cooled transformers with the same ratings. In larger-sized transformers, it is more economical to add forced cooling, even though the electricity needed to operate fans and pumps increases the operating cost.

Transformer oil – In addition to dissipating heat because of the losses in a transformer, insulating oil provides a medium with high dielectric strength in which the coils and core are submerged. This allows the transformers to be more compact, which reduces costs. Insulating oil in good condition withstands far more voltage across connections inside the transformer tank than air. An arc jumps across the same spacing of internal energized components at a much lower voltage if the tank has only air. In addition, oil conducts heat away from energized components much better than air.

Over time, oil degrades from normal operations, because of the heat and contaminants. Oil cannot retain high dielectric strength when exposed to air or moisture. Dielectric strength declines with absorption of moisture and oxygen. These contaminants also deteriorate the paper insulation. For this reason, efforts are made to prevent insulating oil from contacting air, especially on larger power transformers. Using a tightly sealed transformer tank is impractical, because of the pressure variations resulting from thermal expansion and contraction of insulating oil. Common systems of sealing oil-filled transformers are the conservator with a flexible diaphragm or bladder or a positive pressure inert-gas (nitrogen) system. Generator set-up transformers are normally purchased with conservators, while smaller station service transformers have a pressurized nitrogen blanket on top of oil. Some station service transformers are dry-type, self-cooled or forced air cooled.

Conservator system – A conservator is connected by piping to the main transformer tank which is completely filled with oil. The conservator also is filled with oil and contains an expandable bladder or diaphragm between the oil and air to prevent air from contacting the oil. Fig 13a is a schematic representation of a conservator system. Air enters and exits the space above the bladder / diaphragm as the oil level in the main tank goes up and down with temperature. Air typically enters and exits through a desiccant-type air dryer which is to have the desiccant replaced periodically. The main parts of the system are the expansion tank, bladder or diaphragm, breather, vent valves, liquid-level gauge and alarm switch. Vent valves are used to vent air from the system when filling the unit with the oil. A liquid-level gauge indicates the need for adding or removing transformer oil to maintain the proper oil level and permit flexing of the diaphragm.

Fig 13 Conservator and typical nitrogen system

Oil-filled, inert-gas system – A positive seal of the transformer oil may be provided by an inert-gas system. Here, the tank is slightly pressurized by an inert gas such as nitrogen. The main tank gas space above the oil is provided with a pressure gauge (Fig 13b). Since the entire system is designed to exclude air, it is required to operate with a positive pressure in the gas space above the oil, otherwise, air is admitted in the event of a leak. Smaller station service units do not have nitrogen tanks attached to automatically add gas, and it is common practice to add nitrogen yearly early winter as the tank starts to draw partial vacuum, because of the cooler weather. The excess gas is expelled each summer as loads and temperatures increase.

Some systems are designed to add nitrogen automatically (Fig 13b) from the pressurized tanks when the pressure drops below a set level. A positive pressure of around 3.5 kilopascal (kPa) to 35 kPa is maintained in the gas space above the oil for preventing the ingress of air. This system includes a nitrogen gas cylinder, three-stage pressure-reducing valve, high-pressure and low-pressure gauges. high-pressure and low-pressure alarm switch, an oil / condensate sump drain-valve, an automatic pressure-relief valve, and necessary piping.

The function of the three-stage, automatic pressure-reducing valves is to reduce the pressure of the nitrogen cylinder to supply the space above the oil at a maintained pressure of 3.5 kPa to 35 kPa. The high-pressure gauge normally has a range of 0 to 27.5 MPa psi and indicates nitrogen cylinder pressure. The low-pressure gauge normally has a range of about -35 kPa to +70 kPa and indicates nitrogen pressure above the transformer oil.

In some systems, the gauge is equipped with high-pressure and low-pressure alarm switches to alarm when gas pressure reaches an abnormal value. The high-pressure gauge can be equipped with a pressure switch to sound an alarm when the supply cylinder pressure is running low. A sump and drain valve provide a means for collecting and removing condensate and oil from the gas. A pressure-relief valve opens and closes to release the gas from the transformer and, hence limit the pressure in the transformer to a safe maximum value. As temperature of the transformer rises, oil expands, and internal pressure increases, which is have to be relieved. When temperature drops, pressure drops, and nitrogen has to be added, depending on the extent of the temperature change and pressure limits of the system.

Indoor transformers – When oil-insulated transformers are located indoors, because of fire hazard, it is frequently necessary to isolate these transformers in a fire-proof vault. Today, dry-type transformers are used extensively for indoor installations. These transformers are cooled and insulated by air and are not encased in sealed tanks like liquid-filled units. Enclosures in which they are mounted need to have sufficient space for entrance, for circulation of air, and for discharge of hot air. Dry-type transformers are enclosed in sheet metal cases with a cool air entrance at the bottom and a hot air discharge near the top. They can or do not have fans for increased air flow.

In addition to personnel hazards, indoor transformer fires are extremely expensive and detrimental to the plants, needing extensive clean-up, long outages, and lost generation. Larger indoor transformers, used for station service and generator excitation, need to have differential relaying so that a fault can be interrupted quickly before a fire can ensue. Experience has shown that transformer protection by fuses alone is not adequate to prevent fires in the event of a short circuit.

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