Purchase for your line crews today! Standard Transformer kVA's and Voltages. Explanation of Voltage Ratings. Transformer Polarity. Terminal Markings for Single- and Three-phase Transformers. Single-Phase Paralleling. Three-phase Connections Using Single-phase Transformers.
Three-phase Paralleling. Secondary Banking. Notes to the Connection Diagrams. Single-Phase Installations. Transformer Banks Wye - Wye. Wye - Delta. Open Wye - Open Delta. Delta - Wye. Delta - Delta. Three-Phase Transformers. Safety Tips. Installation Procedure for Overhead Transformers. Single-Phase Transformer Loads. Load Checks on Single-Phase Transformers. Three-Phase Bank Loads. Load Checks on Delta, Wye Banks.
Phasing and Paralleling Three-Phase Installations. Minimum Pole Class Guidelines. Strength of Wood Poles. Grounding Transformers. Fusing Transformers. Padmounted Transformer Installation. Safety Clearances Around Padmount Transformers. Guard Posts. Distribution transformers are installed overhead on poles, at grade level on pads, and totally underground in vaults. For years, the most widely used transformer has been the single-phase, overhead version, installed either to deliver single-phase service or in a bank of transformers to deliver three-phase service.
Padmount transformers are becoming more popular because the higher cost of underground distribution is being offset by increased interest in aesthetics, safety, and system reliability. Primary high side distribution voltage is from volts to 34, volts.
Connections to the transformer primary windings are at the top of overhead and underground transformers, and the left panel of padmount transformers.
Secondary low side service voltages are typically volts, volts, volts, volts, volts, volts, and volts. Connections to the transformer secondary windings are at the side of overhead and underground transformers, and the right panel of padmount transformers. When an AC voltage is applied to one coil, current flowing in that coil magnetizes the core — first in one direction, then in the opposite direction. This oscillating magnetic field intersects the second coil, inducing a voltage in it.
The voltage across the secondary terminals causes current to flow through its coil and through any load connected across the secondary terminals. The secondary voltage is determined by the primary voltage and the effective ratio of the number of turns in the primary coil to the number of turns in the secondary coil. The secondary current is the secondary voltage, divided by the load impedance. Core Magnetic flux Primary Secondary Core — The part of the transformer in which the magnetic field oscillates.
It is built from thin laminated sheets, each coated with a thin layer of insulation, and cut to form the shape around which the coils are wound. Laminations are used instead of solid cores to reduce core losses. The ease with which a material can be magnetized is known as its permeability.
Iron or a special type of steel is used for transformer cores because these materials have high permeability. Transformer Concepts Coil — A coil consists of insulated conductors, wound around the core.
The type of insulation depends on the voltage across the coil. The higher voltage input coil is the primary, the lower voltage output coil is the secondary. The primary coil has many turns of small wire, The secondary coil has fewer turns and its conductors are large wire or strips with rectangular cross-sections. Turns ratio — The number of turns on the primary coil, divided by the number of turns on the secondary coil.
Effective turns ratio — The relationship between the input and output voltage. Also called: voltage ratio. Bushing — Porcelain bushings bring the high and low voltage leads from the coils out through the tank, to external connections.
Tank — The enclosure for the core, coils, and transformer oil. The outer surface of the tank dissipates heat generated in the core and coils. Note: A transformer does not work on DC. DC produces a magnetic flux that flows constantly in one direction, only.
Transformation requires a changing magnetic flux. In practice, small losses in the transformer make kVA out slightly less than kVA in. Transformers are rated by their ability to deliver apparent power. Watthour meters measure active power, which is what most customers are billed for. Reactive power circulates in the wires. It is consumed in alternatively building and collapsing AC magnetic fields in transformers and motor windings — and electrostatic fields in capacitors.
The ratio of active power to apparent power is the power factor of the circuit. Adding capacitors to distribution lines makes the angle between these vectors smaller, bringing the power factor closer to 1. This reduces the total power kVA the utility must generate.
Abbreviations k kilo, a prefix indicating one thousand. Thousands of volt-amperes. VA volt-ampere. A unit of apparent power. VAR volt-ampere reactive.
A unit of reactive power. W watt. A unit of active power. The example shown here is for a 10 kVA transformer operating at full load. Core Magnetic flux Primary Secondary Volts 1. For example, a 10 kVA transformer could accept any of these primary inputs, and deliver any of these secondary outputs. Typically, five settings are available. A common application for tap-changing occurs near the end of a long distribution line where the primary voltage is low, and service voltages delivered to customers are below acceptable limits.
Changing the taps on the transformer raises the secondary voltages. Taps are installed on the high-side primary winding. The tapchanging handle is usually located inside the transformer above the oil, and accessed by removing the lid. In some cases, the operating handle is on the outside of the tank.
The actual tap-changing contacts are below the oil level. Caution: Operate tap changers only when the transformer is de-energized. Iron losses are caused by magnetic hysteresis — the opposition by atoms in the core to being aligned first in one direction and then in the other, by the AC field.
Iron losses are also caused by small circles of current that flow, like eddy currents in a pool of water, within the core laminations. Iron losses are called no-load losses because they occur regardless of the loading on the transformer.
Copper losses, also called I2R losses, are produced by the resistance in the transformer windings and the currents flowing through them. Total losses within a transformer are typically a small percentage of its kVA rating. The H1 bushing is always at the upper left.
The lowest and highest numbered terminals are across the full winding. Secondary terminals on additive transformers are numbered right-to-left. This designation makes the phases of H1 and X1 coincide. Note: For details on subtractive and additive transformers, see Polarity, below. Examples: H0, X0. POLARITY Transformer polarity refers to the instantaneous relationship between the oscillating voltage at the primary, and the oscillating voltage at the secondary.
Polarity is unimportant when a transformer is installed alone, but is extremely important when transformers are installed in parallel, or as a bank. If one transformer in a bank has a different polarity, the connections to either the primary or the secondary bushings of that transformer must be reversed. The upper graph shows the primary voltage and secondary voltage, with both measurements taken left-to-right.
The voltages are in-phase. The lower graph shows the voltage difference between the two graphs. The voltage between them is less than the primary voltage, as indicated by the shaded areas. The waveforms subtract.
The transformer has subtractive polarity. The secondary bushings are numbered left-to-right. Apparent power is the power generated by the utility. Transformers are rated by their ability to deliver apparent power. Watthour meters measure active power, which is what most customers are billed for. Reactive power circulates in the wires. It is consumed in alternatively building and collapsing AC magnetic fields in transformers and motor windings and electrostatic fields in capacitors.
The ratio of active power to apparent power is the power factor of the circuit. Adding capacitors to distribution lines makes the angle between these vectors smaller, bringing the power factor closer to 1. This reduces the total power kVA the utility must generate.
Abbreviations k kilo, a prefix indicating one thousand. Thousands of volt-amperes. VA volt-ampere. A unit of apparent power. VAR volt-ampere reactive. A unit of reactive power. W watt. A unit of active power. The example shown here is for a 10 kVA transformer operating at full load. Rated kVA is the full-load capacity for either the primary or the secondary they are the same.
For example, a 10 kVA transformer could accept any of these primary inputs, and deliver any of these secondary outputs. Secondary Outputs V Transformers are manufactured in the ratings listed here. Single-Phase kVA Overhead 1. Typically, five settings are available. A common application for tap-changing occurs near the end of a long distribution line where the primary voltage is low, and service voltages delivered to customers are below acceptable limits.
Changing the taps on the transformer raises the secondary voltages. Taps are installed on the high-side primary winding. The tapchanging handle is usually located inside the transformer above the oil, and accessed by removing the lid.
In some cases, the operating handle is on the outside of the tank. The actual tap-changing contacts are below the oil level. Caution: Operate tap changers only when the transformer is de-energized. Iron losses are caused by magnetic hysteresis the opposition by atoms in the core to being aligned first in one direction and then in the other, by the AC field. Iron losses are also caused by small circles of current that flow, like eddy currents in a pool of water, within the core laminations.
Iron losses are called no-load losses because they occur regardless of the loading on the transformer. Copper losses, also called I2R losses, are produced by the resistance in the transformer windings and the currents flowing through them. Total losses within a transformer are typically a small percentage of its kVA rating. Viewed from the front of the transformer: The primary terminals are numbered left to right. The H1 bushing is always at the upper left. X designates a secondary, or low-side terminal.
The subscript sequence Example: X1, X2, X3 indicates progress along the coil windings, connected in series. The lowest and highest numbered terminals are across the full winding. For single-phase transformers: Viewed from the front of the transformer: Secondary terminals on subtractive transformers are numbered left-to-right.
Secondary terminals on additive transformers are numbered right-to-left. This designation makes the phases of H1 and X1 coincide. Note: For details on subtractive and additive transformers, see Polarity, below. For three-phase transformers: Neutral terminals are designated by the subscript 0. Examples: H0, X0. POLARITY Transformer polarity refers to the instantaneous relationship between the oscillating voltage at the primary, and the oscillating voltage at the secondary.
There are two possibilities: the voltages are either in-phase, or out-of-phase it depends on whether the primary and secondary windings are wound in the same direction, or in opposite directions. Polarity is unimportant when a transformer is installed alone, but is extremely important when transformers are installed in parallel, or as a bank. If one transformer in a bank has a different polarity, the connections to either the primary or the secondary bushings of that transformer must be reversed.
In the subtractive transformer shown above, the windings are in the same direction. The upper graph shows the primary voltage and secondary voltage, with both measurements taken left-to-right. The voltages are in-phase. The lower graph shows the voltage difference between the two graphs.
The voltage between them is less than the primary voltage, as indicated by the shaded areas. The waveforms subtract. The transformer has subtractive polarity. The secondary bushings are numbered left-to-right. In the additive transformer shown above, the windings are in opposite directions.
The voltages are out-of-phase. The voltage between them is more than the primary voltage, as indicated by the shaded areas. The waveforms add. The transformer has additive polarity. The secondary bushings are numbered right-to-left. Single-phase transformers below kVA with primary voltage below volts, usually have additive polarity. All other single-phase transformers usually have subtractive polarity. These rules apply to all transformers, regardless of polarity: H1 is the left primary bushing.
What goes into H1 goes out of X1 the voltage at the X1 bushing is in-phase with the voltage at the H1 bushing. If in doubt, this test will determine the polarity of a transformer: 1. Connect two adjacent terminals of the high and low voltage windings. Apply a moderate volts voltage across the high voltage terminals. Do not apply the volts to the secondary terminals. This will induce a lethal voltage across the primary terminals. Measure the voltage across the other high and low voltage terminals.
The polarity is additive if the measured voltage is higher than the applied voltage. The polarity is subtractive if the measured voltage is lower than the applied voltage. One end of a coil is plus and the other end is minus. To make a delta connection, connect unlike markings of each coil together. Line-to-line voltage is the same as the voltage across a transformer winding. Line current is 1. Delta configurations:. To make a wye connection, connect like markings of each coil together.
The remaining terminals are the output terminals. Line-to-line voltage is 1. Line current is the same as the current in a transformer winding.
Wye configurations:. Heat Protection Transformers can deliver considerably more current than their nameplate indicates, for a short while. For cooling, distribution transformers are oil-filled. The oil carries heat away from the core and coils, to the tank wall which dissipates the heat to the surrounding air. Oil around the core and coils heats and rises to the top of the tank, then flows away from the center to the walls of the tank.
At the tank walls, the oil cools and sinks to the bottom, and the cycle repeats. To circulate easily, transformer oil has a low viscosity resistance to flow. Oil in older transformers may contain PCBs, a chemical whose use is now banned. Use caution when handling this substance.
Heat rise in the tank is accompanied by a rise in pressure in the air space above the oil. Pressure relief valves automatically discharge this pressure to the atmosphere, and pop out to provide a visual indication that they were activated.
Current Protection Fused cutouts protect transformers from excessive currents and short circuits. Cutouts are installed between the primary line and the transformer. The fuse in the cutout must be carefully sized to blow only when abnormal conditions occur. Voltage Protection Arresters protect transformers from high voltage spikes, such as lightning.
If lightning strikes a power pole or line, it seeks the easiest path to ground, which could be through a transformer. Arresters create a safe, low-resistance path for lightning to get to ground, that bypasses the transformer. Lightning strikes can exceed one million volts, so the connections at the arrester must be tight, and the ground wire properly sized for surge currents that accompany these high voltages.
Wildlife protector Discourages wildlife from resting on a transformer primary wire and shorting a high-side line to ground. The Completely SelfProtected CSP transformer has this protection built-in: A high-voltage fuse in series with the primary bushing, for protection in the event of an internal failure in the transformer An arrester mounted externally on the tank A circuit breaker on the secondary side to protect it from overloads and short circuits Conflicts can arise between protective devices when a CSP is installed on the same circuit as other protective devices.
CSP transformers should not be used in three-phase four-wire delta banks serving combined three-phase power and single-phase lighting loads. This voltage is often more than five times the normal voltage, and sometimes as much as 15 times normal voltage. This high voltage can damage the transformer, the primary cable insulation, and other equipment. When ferroresonance is present, the transformer usually makes a rattling, rumbling, or whining noise which is considerably different from the normal transformer hum.
In any AC circuit, when the inductive reactance is equal to the capacitive reactance, a resonant circuit, or ringing occurs. Ferroresonance can occur in a distribution system when the inductive reactance of one winding of a three-phase transformer is approximately equal to the phase-to-ground capacitive reactance distributed along the primary cable to that winding. A high voltage appears between the transformer winding and ground, not the usual phaseto-ground voltage.
If a transient voltage also occurs at the same time, the voltage between the transformer winding and ground will go even higher. To decrease the possibility of ferroresonance, design engineers: Keep cable lengths from the switching point to the transformer well within design limits Convert three-phase closed delta banks to wye-wye connections Use a triplex core cable to the transformer. When a transformer core is magnetized and demagnetized, its core laminations expand and contract.
These physical changes to the laminations happen twice during each 60 hertz cycle, on the positive and negative sides of the flux cycle, causing the laminations to vibrate at hertz. The vibrations are conveyed by the cooling oil to the tank wall, where they escape into the air as sound waves.
Transformer hum also occurs at higher harmonics of hertz, but these tones are less audible. Vectors are usually used instead of sine waves. The length of the vector arrow illustrates the value of the electrical quantity for example, how many volts.
The angle of each vector shows its relationship, relative to other vectors in the circuit. In this diagram, vector A, which rotates around the origin, is shown at 0. At this starting position, the portion of the vector projected on the vertical axis, is zero. As the vector rotates up to 90, the portion of the vector projected on the vertical axis increases, to a maximum when the vector is at The projected value then falls to zero at , become maximum negative at , and returns to zero at or 0.
The process then repeats. Each revolution of the vector describes one cycle of a sine wave. For 60 hertz systems, vectors make 60 revolutions per second. Technically, vector is a mechanical engineering term that defines the magnitude of a force and its direction. Phasor is an electrical engineering term that defines the magnitude of an electrical quantity and its phase relationship to other electrical quantities.
While phasor would seem to be the correct term to use for transformer applications, vector is more widely used. Voltages in a three-phase system are illustrated here: 90 C A Volts A 1 cycle 4. Vector A is shown at 0, and the A sine wave on the graph starts by crossing 0, just like the graph on the preceding page.
Vector B starts at and initially projects a negative value on the vertical axis. The B sine wave on the graph also starts at a negative value. Vector C starts at Its sine wave has passed maximum value, and is heading for zero.
All three vectors rotate counterclockwise. The next vector to pass through 0 will be B, followed by C. The phase sequence is ABC counterclockwise. Why 3, or 1. Why is the line-to-line voltage in wye circuits, 3 or 1. For example, if the phase-to-neutral voltage measures volts, why is the phase-to-phase voltage , not twice volts? As illustrated on the previous page, each of the three phases passes through maximums and minimums at different times.
To make a wye connection, connect like markings of each coil together. The remaining terminals are the output terminals. Line-to-line voltage is 1. Line current is the same as the current in a transformer winding. Heat Protection Transformers can deliver considerably more current than their nameplate indicates, for a short while.
For cooling, distribution transformers are oil-filled. The oil carries heat away from the core and coils, to the tank wall which dissipates the heat to the surrounding air. Oil around the core and coils heats and rises to the top of the tank, then flows away from the center to the walls of the tank.
At the tank walls, the oil cools and sinks to the bottom, and the cycle repeats. To circulate easily, transformer oil has a low viscosity resistance to flow. Oil in older transformers may contain PCBs, a chemical whose use is now banned. Use caution when handling this substance. Heat rise in the tank is accompanied by a rise in pressure in the air space above the oil. Pressure relief valves automatically discharge this pressure to the atmosphere, and pop out to provide a visual indication that they were activated.
Current Protection Fused cutouts protect transformers from excessive currents and short circuits. Cutouts are installed between the primary line and the transformer. The fuse in the cutout must be carefully sized to blow only when abnormal conditions occur. Voltage Protection Arresters protect transformers from high voltage spikes, such as lightning. If lightning strikes a power pole or line, it seeks the easiest path to ground, which could be through a transformer. Lightning strikes can exceed one million volts, so the connections at the arrester must be tight, and the ground wire properly sized for surge currents that accompany these high voltages.
CSP transformers should not be used in three-phase four-wire delta banks serving combined three-phase power and single-phase lighting loads. Arrester Weak-link fuse Secondary breaker Secondary breaker x3 x2 Com pletely self-protected transform er. This voltage is often more than five times the normal voltage, and sometimes as much as 15 times normal voltage.
This high voltage can damage the transformer, the primary cable insulation, and other equipment. When ferroresonance is present, the transformer usually makes a rattling, rumbling, or whining noise which is considerably different from the normal transformer hum. Ferroresonance can occur in a distribution system when the inductive reactance of one winding of a three-phase transformer is approxi- mately equal to the phase-to-ground capacitive reactance distributed along the primary cable to that winding.
A high voltage appears between the transformer winding and ground, not the usual phase- to-ground voltage. If a transient voltage also occurs at the same time, the voltage between the transformer winding and ground will go even higher. When a transformer core is magnetized and demagnetized, its core laminations expand and contract. These physical changes to the laminations happen twice during each 60 hertz cycle, on the positive and negative sides of the flux cycle, causing the laminations to vibrate at hertz.
The vibrations are conveyed by the cooling oil to the tank wall, where they escape into the air as sound waves. Transformer hum also occurs at higher harmonics of hertz, but these tones are less audible. Vectors are usually used instead of sine waves. The length of the vector arrow illustrates the value of the electrical quantity — for example, how many volts.
The angle of each vector shows its relation- ship, relative to other vectors in the circuit. At this starting position, the portion of the vector projected on the vertical axis, is zero.
The process then repeats. Each revolution of the vector describes one cycle of a sine wave. For 60 hertz systems, vectors make 60 revolutions per second.
Technically, vector is a mechanical engineering term that defines the magnitude of a force and its direction. Phasor is an electrical engineering term that defines the magnitude of an electrical quantity and its phase relationship to other electrical quantities.
While phasor would seem to be the correct term to use for transformer applications, vector is more widely used. The B sine wave on the graph also starts at a negative value. Its sine wave has passed maximum value, and is heading for zero. All three vectors rotate counterclockwise. The phase sequence is ABC counterclockwise. Why is the line-to-line voltage in wye circuits, 3 or 1. For example, if the phase-to-neutral voltage measures volts, why is the phase-to-phase voltage , not twice volts?
As illustrated on the previous page, each of the three phases passes through maximums and minimums at different times. The length of each vector represents its voltage. If we measure from B from C, it is 1. So, the voltage between any two phases is 1.
Angular displacement can refer to the relationship between the primary and secondary voltages of a three- phase transformer, or the relationship between two circuits such as two secondaries. The diagrams below illustrate angular displacements for combinations of wye and delta transformers. C A A c a b Wye-delta a Delta-w ye C B N B b n c Compare any corresponding pair of primary and secondary voltage vectors, for example, AB and ab: In the left diagram, a new vector drawn from the tip of A to the tip of B points approximately west-southwest.
Vector ab points directly west. Transformer Concepts 25 In the right diagram, AB points approximately north-northeast. A new vector drawn from the tip of a to the tip of b points approximately east-northeast.
C A A b c a n c Wye-delta b Delta-wye C B N B a Compare any corresponding pair of primary and secondary voltage vectors, for example, AB and ab: In the left diagram, a new vector drawn from the tip of A to the tip of B points approximately west-southwest. Vector ab points directly east. In the right diagram, AB points approximately north-northeast. A new vector drawn from the tip of a to the tip of b points approximately west-southwest.
Single-Phase Paralleling To increase the capacity of a single-phase service, a second singlephase transformer may be connected in parallel. Transformers of either additive or subtractive polarity may be paralleled, provided the primary phase sources are the same and the H and X terminals are correspondingly connected.
One way to determine if the angular displacements match, is to take voltage readings between corresponding pairs of bushings. For details on paralleling three-phase transformers, see pages A wye-wye bank can be paralleled with another wye-wye bank or with a delta-delta bank.
A wye-delta bank can be paralleled with another wye-delta bank or with a delta-wye bank. A wye-wye bank and a delta-delta bank cannot be paralleled directly with a wye-delta bank or a delta-wye bank. Primary phases are indicated by capital letters: A, B, C, N. Secondary phases are indicated by lower case letters: a, b, c, n.
Your utility might use singlebushing transformers, with H1 connected to a primary phase and the other end of the primary winding connected to ground through the transformer case.
If a subtractive transformer is used instead, make connections to the same terminal numbers marked in the diagrams. The secondary terminals will be physically located on the transformer tank in the opposite sequence from that shown in the diagrams. Other outputs are possible, depending the primary voltage and transformers used.
However, if there is a clearance problem trees overhead, restricted climbing space, etc. This practice is not followed by all utilities. The diagrams in this handbook illustrate the most popular configurations.
Many others are possible. If another configuration is used by your utility, you might sketch it for reference on a blank page at the back of the book. Note: Send a copy of your sketch to us and we will return to you a computer-precise illustration, ready to paste in your handbook. Some utilities ground one corner of the delta secondary.
Transformer Connections 37 Open Wye - Open Delta Three-phase, f our-wire open wy e primary Three-phase, three-wire open delta secondary Secondary services volts, phase-to-phase This conf iguration is relativ ely inef f icient.
Trace from the neutral up to X2 into T1 2. Go one-half w inding to X1 3. Go across the common connection into T2 4. Go across one full w inding 5. If the vertical clearance above the transformer is limited, use a spreader bar in place of a sling, and install cover-up on any energized conductors nearby.
Sling angle Do not lift a transformer from beneath a bushing, pressure relief valve, drain plug, or any other attachment not specifically designed for lifting. Do not move or shift a transformer by grasping the bushings, fins, or plugs. Porcelain bushings can be damaged during handling in ways not visually obvious, then fail when the unit is put into service. When handling a transformer, take care to not damage the tank finish. Paint scratches can lead to rust.
The nameplate shown here is for an overhead single-phase transformer. The low number is the phase-to-neutral coil voltage. The high number is the phase-to-phase voltage.
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