SMALL DISTRIBUTION TRANSFORMERS, SDT
Power range Up to 315 kVA
Primary voltage Up to 36kV
Available fluids Mineral oil, dimethyl silicone,
esters and synthetic
Transformers of this type are used to step down three-phase high
voltage to low voltage for energy distribution, mainly in the
countryside or low-density populated areas.
The transformers are three phase oil immersed hermetically sealed,
adaptable for pole mounting or assembly in substations.
On customer request, the transformer can be equipped with oil
Hot dip galvanizing is often the preferred surface treatment for
A typical product has power rating 100 kVA, primary voltage 22 kV, secondary voltage 420 V.
DRY-TYPE DISTRIBUTION TRANSFORMERS
Dry-type transformers are used to minimize fire hazard and other environmental contamination on
surroundings and people, like in large office buildings, hospitals, shopping centers and
warehouses, sea going vessels, oil and gas production facilities and other sites where a fire has
potential for catastrophic consequences.
SGB ENERGY offers a full range of dry-type transformers with primary voltages up to 52 kV, full filling the
requirements in IEC, CENELEC and ANSI standards.
Application areas for both types are quite similar; however ResiN has an advantage in extreme
Vacuum cast resin dry-type transformers
Power range 50 kVA up to 30 MVA
Primary voltage Up to 52 kV
Climate class C2
Vacuum cast means that the high voltage windings are cast-in in
epoxy and cured in vacuum. The high voltage windings are typically
A typical product has power rating 1000 kVA, primary voltage 22 kV,
secondary voltage 420 V.
Single phase transformers
Power range 15 – 100 kVA
Voltage Up to 36 kV
Applicable fluid Mineral oil
Transformers of this type are generally oil immersed and suitable for pole
mounting. They represent an economical option for certain networks,
particularly those with low population densities. Depending on customer
requirements, transformers may be connected between two phases of a
three phase system (two HV bushings) or from one phase to ground (single HV bushing). They are
suitable for residential overhead distribution loads, as well as light commercial or industrial loads
and diversified power applications.
Variable Speed Drive (VSD) transformers provide the voltage
transformation as well as electric isolation that is necessary for
motor drives applications. Converter drives are normally fed by
medium voltage networks from 5 kV up to 36 kV and the converter
supply voltage usually ranges from 400 V up to 4 kV. The VSD
transformer transforms the medium network voltage to the
converter supply voltage. A typical application is submersible oil
pump drives and similar equipment where only HV motor
applications are available. VSD transformers are produced in oil
insulated and dry-type configurations up to 6 MVA ratings for
various types of converters and output voltages. Transformers are
individually designed and manufactured according to system requirements.
• Earthing transformers,
• Starter transformers,
• Booster transformers,
• Auto transformers,
• Convertor transformers.
The following information shall be given in all cases:
• Particulars of the specifications to which the transformer shall comply,
• Kind of transformer, for example: separate winding transformer, auto-transformer or booster
• Single or three-phase unit,
• Number of phases in system,
• Dry-type or oil-immersed type. If oil-immersed type, whether mineral oil or synthetic
insulating liquid. If dry-type, degree of protection (see IEC 60529),
• Indoor or outdoor type,
• Type of cooling,
• Rated power for each winding and, for tapping range exceeding ± 5 %, the specified
maximum current tapping, if applicableIf the transformer is specified with alternative methods
of cooling, the respective lower power values are to be stated together with the rated power
(which refers to the most efficient cooling),
• Rated voltage for each winding
• For a transformer with tappings:
Which winding is tapped,
the number of tappings, and
the tapping range or tapping step,
Whether ‘off-circuit’ or ‘on-load’ tap-changing is required,
If the tapping range is more than ±5 %, the type of voltage variation, and the location of
the maximum current tapping, if applicable, see 5.4,
• Highest voltage for equipment (Um) for each winding (with respect to insulation, see IEC
• Method of system earthing (for each winding),
• Insulation level (see IEC 60076-3), for each winding,
• Connection symbol and neutral terminals, if required for any winding,
• Any peculiarities of installation, assembly, transport and handling. Restrictions on
dimensions and mass,
• Details of auxiliary supply voltage (for fans and pumps, tap-changer, alarms etc),
• Fittings required and an indication of the side from which meters, rating plates, oil-level
indicators, etc., shall be legible,
• Type of oil preservation system,
• For multi-winding transformers, required power-loading combinations, stating, when
necessary, the active and reactive outputs separately, especially in the case of multi-winding
In addition for example limitations on oil and winding temperature rises (e.g. 65/60 or 55/50 °C)
may be given.
The following additional information may need to be given:
• If a lightning impulse voltage test is required, whether or not the test is to include chopped
waves (see IEC 60076-3),
• Whether a stabilizing winding is required and, if so, the method of earthing,
• Short-circuit impedance, or impedance range (see IEC 60076-1 Annex C). For multi-winding
transformers, any impedance’s that are specified for particular pairs of windings (together
with relevant reference ratings if percentage values are given),
• Tolerances on voltage ratios and short-circuit impedance’s as left to agreement in IEC
60076-1 Table 1, or deviating from values given in the table,
• Whether a generator transformer is to be connected to the generator directly or switchgear,
and whether it will be subjected to load rejection conditions,
• Whether a transformer is to be connected directly or by a short length of overhead line to
gas-insulated switchgear (GIS),
• Altitude above sea level, if in excess of 1 000 m (3 300 ft),
• Special ambient temperature conditions, (see IEC 60076-1 section 1.2 b)), or restrictions to
circulation of cooling air,
• Expected seismic activity at the installation site which requires special consideration,
• Special installation space restrictions which may influence the insulation clearances and
terminal locations on the transformer,
• Whether load current wave shape will be heavily distorted. Whether unbalanced phase
loading is anticipated. In both cases, details to be given,
• Whether transformers will be subjected to frequent over-currents, for example, furnace
transformers and traction feeding transformers,
• Details of intended regular cyclic overloading other than covered by 4.2 (to enable the rating
of the transformer auxiliary equipment to be established),
• Any other exceptional service conditions,
• If a transformer has alternative winding connections, how they should be changed, and
which connection is required ex works
• Short-circuit characteristics of the connected systems (expressed as short-circuit power or
current, or system impedance data) and possible limitations affecting the transformer design
(see IEC 60076-5),
• Whether sound-level measurement is to be carried out (see IEC 60551),
• Vacuum withstand of the transformer tank and, possibly, the conservator, if a specific value
Any tests not referred to above which may be required should be mentioned separately and may
lead to additional cost and longer delivery time.
Any information regarding the required corrosion resistance of the surface treatment in reference to
the geographic zone and pollution zone.
If parallel operation with existing transformers is required, this shall be stated and the following
information on the existing transformers given:
• Rated power,
• Rated voltage ratio,
• Voltage ratios corresponding to tappings other than the principal tapping,
• Load loss at rated current on the principal tapping, corrected to the appropriate reference
• Short-circuit impedance on the principal tapping and at least on the extreme tappings, if the
tapping range of the tapped winding exceeds ±5 %,
• Diagram of connections, or connection symbol, or both.
NOTE: On multi-winding transformers, supplementary information will generally be required.
This is a list of available accessories to be selected in cooperation with the user.
Accessory type Type of transformer
SDT/MDT LDT Dry
• Voltage regulation relay
• Electrical position indicator
• Parallel operation equipment
Conservator O S NA
Silicagel dehydrating breather O1) S NA
Pocket, top liquid thermometer O2) S NA
Dial thermometer O S S
Pressure relief valve S4) O NA
Pressure relief device with signal O O NA
Gas-actuated relay O S NA
Integrated protective device mounted
on the cover
O O NA
Temperature monitoring system NA NA O
Earthing terminals S S S
Wheels, (bi-directional) S2) S S
Drain valve at tank bottom S S NA
Liquid level gauge O3) S NA
Liquid sampling valve O3) S NA
Cable boxes on LV – and HV side
mounted on the cover
O5) O5) NA
Plug in HV & LV bushings equipped
with the robust protections
O O NA
(of different protection class)
NA NA O
Overvoltage protection devices
• Surge arrestors
• Arching horns
Overcurrent protection, fuses O
Built-in current transformers O O O
Multi function devices; pressure,
temperature, oil level and gas
O O NA
The abbreviations S (standard accessory), O (optional) and NA (not applicable) are an indication of
where the different accessories are most common in use.
1) Only used when conservator
2) Thermometer pocket is an option on SDT and standard on MDT.
3) Standard on SDT/MDT with conservator
4) Standard on hermetically sealed
5) Plug-in bushings is the preferred ABB solution instead of cable boxes due to easy assembly
and lower cost and reduced distances in the substation.
INSTALLATION, OPERATION AND MAINTENANCE
This section describes SGB ENERGY’s general experience/recommendations and minimum requirements,
however it may be overruled or supplemented by local regulations and the supplier’s or purchaser’s
Please observe the following and any other safety advices before installation, commissioning and
• Never work on transformers or any installed electrical equipment alone,
• Do not move or lift a transformer without applying adequate equipment and safety
• Do not make any connections which do not comply with the rating plate,
• Do not apply abnormal mechanical strain on the terminals,
• Do not reconnect when the transformer is energized,
• Do not attempt to change tap setting while the transformer is energized,
• Do not energize or perform maintenance on the transformer without proper earth connection,
• Do not operate the transformer without alarm and monitoring systems connected,
• Do not remove any enclosure panels while the transformer is energized,
• Do not tamper with interlocks, alarms and control circuits,
• Be aware of possible need for magnetic field protection,
• Perform a final inspection prior to energizing:
o All external connections have been made properly,
o All connections are tight and secure,
o All accessory circuits are operational,
o All tap connections are properly positioned,
o The neutral and earth connections have been properly made,
o Fans – if supplied – are operational,
o Proper clearance is maintained from high voltage bus to terminal equipment,
o The correct transformer ratio exists for units with internal terminal boards,
o All windings are free from un-intended earths. A relevant megger is recommended,
o There is continuity in all windings,
o There is no dust, dirt or foreign material on core and coils (dry-type transformers),
o There is no visible moisture on or inside the core and coils (dry-type transformers),
o All plastic wrappings are removed from the core and coils (dry-type transformers),
o All shipping members have been removed,
o There are no obstructions in or near the openings for ventilation,
o No tools or other articles are left inside or on top of the core, coils, tank or
o All protective covers are closed and tightened,
• Comply with any instructions supplied by the transformer manufacturer,
• Comply with relevant Internal Control Regulations.
See also relevant safety sections in Extracts from IEC 61936-1 (2002-10) Power installations
The transformer is supplied filled with liquid and normally all accessories fitted, except for the
largest units. The radiators may be dismantled during transport.
During transport the following should be considered:
• Angle of tilting exceeding 10º must be specified in the contract,
• Prevention of damage to bushings, corrugated panels or radiators and accessories,
• Larger transformers should preferably be positioned with the longitudinal axis in the direction
• Secure against movement by means of e.g. wooden blocks and lashes,
• Adapt vehicle speed to the road conditions,
• Vehicle capacity shall be adequate for the transport weight of the transformer,
• Any impact recorders to be specified in the contract,
• Any use of crates or containers.
Transfer of responsibility (Incoterms)
Incoterms defined by International Chamber of Commerce (ICC) make international trade easier
and help traders in different countries to understand one another.
Incoterms are standard definitions of trade terms and are internationally recognized as
indispensable evidence of the buyer’s and seller’s responsibilities under a sales contract. Incoterms
will not apply unless specifically incorporated into the contract
Those standard trade definitions that are most commonly used in international contracts are
protected by ICC copyright, however a wall chart may be viewed on
http://www.iccwbo.org/index_incoterms.asp where a wall chart in preferred language may also be
Only approved and suitable lifting equipment shall be used.
Use a forklift only on transport pallets or transformer bottom.
Do not apply load to corrugated fins or radiators and their supports.
Use the provided lifting lugs only.
When lifting a transformer with cable boxes on the cover, special care must be taken.
When hydraulic jacks are used, only provided jacking points shall be used, and in such a way that
twisting forces on the transformer tank are avoided.
Receiving the transformer at site
Transformers manufactured by SGB ENERGY‘S are thoroughly tested and inspected prior to shipping, but
upon receiving the transformer at site, it should be inspected carefully.
To be inspected:
• The way in which the transformer has been secured on the trailer,
• That the delivery is complete according to order confirmation,
• Compare the packing list with the goods received,
• The transformer nameplate,
• Liquid level, when applicable. Any leakages?
• External damage, e.g. cracks in bushings,
• Impact recorders indications when applicable.
The receipt of the unit shall be signed for, and the result of the inspection shall be noted.
Transformer shipments are normally insured.
In case of damage revealed during the receiving inspection, do any of the following:
• Make necessary arrangement to avoid further damage,
• Contact the insurance company concerned and ABB,
• Make a report of the damage immediately,
• No repairs should be started
Storage prior to energizing
When storage of the transformer is required, the following recommendations should be noted:
• Preferably in dry and clean locations, without any possibilities of mechanical damage and on
a solid foundation,
• If the transformer does not have a structural steel base, it should be placed upon supports to
allow ventilation under the bottom of the transformer base,
• The liquid conservator and dehydrating breather must be checked to ensure that dry air is
breathed. (Conservator type only) Liquid samples to be analysed regarding moisture content
prior to energizing,
• Humidity/condensation in control cubicles, driving mechanism for on-load tap changer, cable
boxes, dry-type transformers etc. should be inspected/removed,
• Minimum storage temperature for dry-type transformer is in general -25°C, however for
• Prior to energizing, perform a megger test between the different windings, and from the
windings to earth. This applies to dry-type in particular.
Erection at site
In determining the location of a transformer, give careful consideration to accessibility, safety,
ventilation and ease of inspection. Make sure the foundation for mounting the transformer is
Ventilate the erection site properly. As a guide each kilowatt of losses requires 4 cubic meters of air
circulation per minute. Fresh air intake at floor level, and a ventilation duct leading to outer air must
be built on the ceiling or upper part of the wall. The intake and outlet openings should be located
diagonally across the room. The duct cross section of the outlet should be 10 % more than the
cross section of the inlet opening due to increase of volume of the hot air.
Observe local authorities regulations for civil engineering of transformer cells, safety regulations,
fire protection regulations. A liquid containment tank may be required.
A transformer equipped with wheels must be prevented from moving by chocking the wheels.
Connecting transformer terminals to the networks
Observe local authorities safety- and regulations for electrical installations.
Conductors, bus bars and cables shall be installed such that minimal mechanical stress is
transferred to the bushings.
Conical washers shall be used in order to obtain the required contact pressure. Nuts should be
Flexible connectors shall be used between the terminals and the bars, when connecting to low
voltage busbars. Suitable cable lugs shall be used when connecting low voltage copper cables.
High voltage connection is normally performed by copper cables and copper cable lugs. In
some cases heat shrinkable connectors or elbow connectors are used.
For aluminium-copper joints the copper is coated with tin, or bi-metal sheets (one side of copper
and the other of aluminium) can be used between the joint.
The aluminium surface must be larger than the copper surface.
Aluminium parts shall always be placed above copper parts so that water cannot drain from the
copper parts onto the aluminium (corrosion).
It must be remembered that good contact between joined aluminium surfaces can be achieved
only if the no conducting oxide film is removed with a wire brush, file or similar immediately
before joining, and renewed oxidation is prevented by applying a thin protective film of grease
Jointing compound, which prevents the access of air and humidity into joints, must be used in
the joint. The zinc crystals of the compound break down the layer of oxide on the aluminium.
Minimum electrical clearances shall be obtained.
Suitably strong steel bolts and nuts have to be used for tightening the joint. The tightening
torque’s given below are recommended to be used in external transformer joints.
Cable and busbar load capacities are outside the scope of this handbook, however dealt with in
detail in SGB ENERGY‘S Switchgear Manual.
Earth the transformer at the earthing terminals provided.
Earthing resistance according to electricity utilities- or national standards
Connect the equipment, and check the functions:
• Thermometer with contacts for alarm and tripping signals,
• When applicable, oil level indicator with contacts for alarm signal,
• Gas relay with contacts for alarm and tripping signals,
• Pressure relief device with contacts for alarm signal. When provided,Thermometer settings for dry-type transformers according to the relevant temperature class.
Insulation resistance, off-circuit/on-load tap changer
Check the insulation resistance between HV and LV as well as windings to earth. Minimum value is
1000 ohm per volt service voltage (Maximum 1mAmp.),
Off-circuit/on-load tap changer
• Check the galvanic contact between the winding and the tap-changer in all positions,
• Compare the voltage ratio of the transformer and the network voltages, and select the
suitable tap changer position.
• Check the liquid level,
• Tighten all leaking gaskets carefully,
• After completed installation work, surface damage caused by transportation and installation
work shall be repaired,
• Condition of dehydrating breather. (For conservator type only),
• Finally the transformer must be cleaned.
After the transformer has been found to be in good condition and the protective equipment is found
in order, the transformer can be connected to the network.
When connecting the transformer to the network, fuses may blow immediately caused by high
inrush current. This does not necessarily mean that there is a fault in the transformer. Replace
blown fuses and try energizing again because the magnitude of the inrush current is a statistical
variable large spread. Modern over-current and differential relays contain a control circuit which
makes the relays in sensitive to inrush currents. Older relays may trip the circuit breaker
immediately. See also section 6, paragraph “Inrush current”.
After the transformer has been connected to the network, gas may be present which gives an
alarm in the gas relay. It could be a false alarm caused by an air bubble, trapped under the cover,
and then moved into the gas relay.
Air is colour less and odor less. If not air, a gas and an oil sample should be taken for analysis.
The lifetime of a transformer can be divided into two categories; economical and technical.
Economical lifetime ends when the capitalized cost of continued operation of the existing
transformer exceeds the capitalized cost of a new investment.
In practical terms; typically when the cost of the total losses of the old transformer is too high.
Consequential risks and costs associated with electricity downtime are of increasing importance.
Solid insulation materials consist mainly of organic materials. These materials change over time,
they become brittle and the mechanical strength is reduced, while there is very little reduction in the
These detrimental processes, which are affected by temperature, humidity and oxygen, are called
ageing. Ageing consists of several oxidation processes, where the chemical reaction rate increases
strongly with temperature.
As a rule of thumb, lifetime is halved at a temperature rise of 7-8 ºC and vice versa, this is called
Thus, the expected technical lifetime for a transformer depends mainly on its accumulated load
cycle and the ambient temperature. This varies with customer, application, location etc.
IEC 60354 provides guidelines on how to overload oil-immersed transformers.
There is no clear definition of “end of life” in any international standard. This is a matter where high
degree of judgment is involved.
To illustrate this, an old transformer can work perfectly well in normal conditions for several years
more, but as soon as it gets a surge, be it a voltage peak, heavy overload or short- circuit currents,
it may collapse.
Also a number of transformers are exchanged due to change in system voltage or insufficient
A typical technical lifetime of a distribution transformer under normal conditions is at least 30 years.
Temperature rise and load capacity
Thermal class 105 is defined to withstand a continuous temperature of 105° C for 7 years, without
loosing more than 50 % of its original mechanical strength. Class 130 is defined to withstand a
continuous temperature of 130° C for 7 years, and so forth for all other insulation classes.
The sum of ambient + average temperature rise of any of the windings + 10° C (to allow for that
maximum temperature rise in a winding is 10° C above average) should not exceed 105° C.
Ageing sets certain limitations to the load capacity of the transformer. These limitations are defined
in IEC publication 60354 “Loading guide for oil immersed transformers”.
Continuous load capacity at different constant ambient air temperatures has been calculated
according to IEC 60354.
Permissible continuous loading capacity at different ambient air temperatures for
ONAN distribution transformers.
In practice the transformer is not continuously fully loaded. The load and the temperature fluctuate
by seasons and time of the day.
The permissible short-term overload depends on the starting conditions, plus duration and the
repetitive cycle of the overload. This is defined in the IEC 60354, and must be analysed on a caseto-
Based on the above there is a certain emergency overload capacity for the transformer.
Special attention shall be paid to the fact that the transformer’s short term load capacity cannot be
judged on the basis of the oil temperature alone, because the oil temperature changes much
slower with the load than the temperature rise of the winding.
Overloading a transformer implies that the construction and the accessories, such as the tap
changer and the bushings are correspondingly rated. Normally the accessories are selected with
rated current not less than 120% of rated current of the transformer. (For bushings see IEC 60137)
Corresponding considerations are made for dry-type transformers, and are described in
IEC 60905 Loading guide for dry-type power transformers.
Transformer liquid and insulation
The task of liquid in a transformer is to act as an electrical insulation and to transfer heat from the
transformer’s active parts into coolers. Liquid acts as a good electrical insulation only as long as it
is satisfactorily dry and clean.
Humidity balance between the oil and the insulation implies that most of the humidity will gather in
the paper insulation.
Testing of liquid in transformers should normally be performed 12 months after filling or refilling,
subsequently every six years. ABB offers different tests and analyses of liquid samples depending
of transformer type, size, service record and strategic importance for safe electricity supply.
Testing of oil in on load tap changers must be performed according to the tap changer supplier’s
To take liquid samples from hermetically sealed transformers is normally not necessary, and
should only be performed after consultation with ABB. The liquid in this type of transformers is not
in contact with the atmosphere, and less exposed to moisture.
Especially for large LDT transformers, liquid regeneration may be economically motivated. Liquid
regeneration implies drying, filtering, de-gassing and possibly addition of inhibitor.
Bushings and joints
The porcelain insulators of transformer bushings ought to be cleaned during service interruptions
as often as necessary. This is particularly important for places exposed to contamination and
Methylated spirit or easily evaporating cleaning agents can be used for cleaning.
The condition of external conductor and bus bar joints of transformer bushings shall be checked at
regular intervals because reduced contact pressure in the joints leads to overheated bushings etc.
and may cause the adjacent gasket to be destroyed by the heat.
Off-circuit tap changer
The transformation ratio can be adjusted with an off-load tap changer when the transformer is not
energized. Usually the turns ratio of the HV winding is regulated ±2×2.5 %.
The control shaft of the off-load tap changer is brought through the cover or the tank wall. The
shaft end is provided with a handle, position indicator and locking device. When the tap changer is
turned the locking device must be secured, because that assures that the off-load tap changer has
been set to operating position.
It is recommended that the off-circuit tap changer is moved from one extreme position to the other
a few times during service interruption. This is necessary especially when the tap changer is
For dry-type transformers the off-circuit tap changing is generally done by means of bolted links.
On-load tap changer
Maintenance of on load tap changer to be performed according to the instructions given by the
supplier of the tap changer.
Liquid conservator with rubber sack
The system consisting of oil conservator with rubber sack does normally not require any other
maintenance than inspection of the silicagel breather. The silicagel shall be changed when approx.
2/3 of the silicagel has changed from blue to red colour.
Zinc coated surfaces
Zinc coated surfaces have a self-repairing, passivating, characteristic. Small damage as scratches
do normally not need repairing. Larger areas, above 50 mm2, may need repair. After thoroughly
cleaning apply zinc-rich (between 65-69% zinc by weight, or >92% by weight metallic zinc in dry
film) paint to at least the same thickness as the original zinc coating. Do not remove any original
zinc during cleaning. The paint may be one-component (preferred) or two-component.
Transformers contain valuable materials, which may be reused either as is or after reprocessing.
Insulation material, pressboard and paper, represent energy.
A transformer is a static device with two or more windings that are linked to each other by means of
a strong magnetic field. Transformers are designed for specific purposes, such as the
measurement of voltage and current, or the transfer of signals or electric power. The design
requirements of transformers depend on the application. In measurement transformers, the
quantity measured must be transferred from the primary circuit to the secondary as exactly as
possible, while in signal transformers the signal must be transferred with a minimum of distortion.
This text concentrates on power transformers, where the main requirement is that it shall transfer a
certain amount of electric power at a constant frequency while the voltage is being changed from
one level to another with a minimum of power losses.
Large power stations where the electric energy is generated are often situated far away from the
numerous places where the electric energy is consumed. The need for high voltage levels in
electric power transmission is illustrated in the following.
The power loss p in a 3-phase transmission line with a resistance R per phase and a current I
flowing in each phase is:
p = 3 ⋅R ⋅ I2 (W) (1)
At a system voltage U the transmitted active power is:
P = 3 ⋅U⋅ I ⋅ cos ϕ (W) (2)
Equation (2) can be rewritten as:
⋅ ⋅ ϕ
3 U cos
I (A) (3)
Inserted in the equation (1) gives:
= 2 2
p P (W) (4)
Equation (4) indicates that the power loss in the line is proportional to the square of the transmitted
active power and inversely proportional to the square of the system voltage,
BASIC PHYSICS OF THE TRANSFORMER
The function of a transformer is mainly based on two physical phenomena.
One of them is the electromagnetic induction, which was discovered by Faraday in the 1830’s. The
induction law was formulated by Neumann in 1845.
Consider a loop of a conducting material enclosing a magnetic field Φ. If a change ΔΦ takes place
during a short time-interval Δt, a voltage will be induced which will drive a current around in the
loop. The induced voltage ui is proportional to the quotient ΔΦ/ Δt, or more correctly written on
In other words, the power loss will be lower when the system voltage is increased.
The choice of system voltage is a matter of economic balance. A high system voltage reduces the
transmission losses, but on the other side it requires more expensive lines, cables and
Several hundreds of kilovolts are used for lines for lines transporting large quantities of electric
power over long distances. Closer to the consumers the power is distributed at lower voltage
levels. The voltage is taken down in several steps. While generator transformers are step-up
transformers, the distribution transformers are normally step-down transformers. The power rating
of distribution transformers is usually lower the closer to the consumer the transformers are
In no load condition (open secondary terminals) the total magnetising current and the active power
consumption are measured at rated voltage. The current has one dominant inductive component
and one smaller active component. These can be calculated from the measurements, and the real
and the imaginary components of transformer’s no load impedance Z0 can be found. This
impedance is not constant but will vary non-linearly with the applied voltage due to the non-linearity
of the magnetisation curve. The real part of Z0 represents the no load losses.
The other impedance in the diagram, Z, is found by short-circuiting the secondary terminals and
applying a voltage at the primary side. The current in the windings during this measurement shall
be equal to the rated current. To achieve this current an applied voltage of just a fraction of the
rated voltage will be sufficient. This voltage is called the short-circuit voltage and is usually
expressed as a percentage of the rated voltage. The impedance of the circuit is given by the
quotient of the short-circuit voltage divided by the rated current. The circuit is a parallel connection
of Z0 and Z. Because Z0>>Z the impedance of the parallel connection is equal to Z with a negligible
difference. The real part of Z represents the load losses of the transformer, and the imaginary part
is attributed to the magnetic leakage field. That is the part of the magnetic field, which is situated
outside the core.
VOLTAGE RATIO AND VOLTAGE DROP (OR RISE)
The voltage ratio of a transformer is normally specified in no load condition and is directly
proportional to the ratio of the number of turns in the windings.
When the transformer is loaded, the voltage on the secondary terminals changes from that in no
load condition, depending on
• the angle φ between the voltage on the secondary terminals of the transformer U2 and the
secondary current I2
• the value of the secondary current I2
• the short-circuit impedance of the transformer Z and its active and reactive components, r
and ±jx respectively
At no load the secondary voltage is U20. With the load ZL connected, the voltage at the secondary
terminals changes to U2. The corresponding vector diagram is shown in Figure 4. In the following
considerations symmetrical loading is assumed. The influence of
The voltage drop is due to the consumption of active and reactive power in the transformer.
According to the IEC definitions (see clause 4.1 of IEC 60076-1) it is implied that the rated power of
a two-winding transformer is the input power. The output power differs from the rated power.
This is different from the definition in ANSI/IEEE, which states that the output power shall be equal
to the rated power, and that the voltage applied on the primary side shall be adjusted to
compensate for the voltage drop (or rise) in the transformer.
Users and installation planners are recommended to take the variation of the secondary voltage
during loading into account when specifying the transformer data. This may be especially important
for example in a case where a large motor represents the main load of the transformer. The highly
inductive starting current of the motor may then be considerably higher than the rated current of the
transformer. Consequently there may be a considerable voltage drop through the transformer. If
the feeding power source is weak, this will contribute to an even lower voltage on the secondary
side of the transformer.
The no load current and no load losses are attributed to the core and the special magnetic
properties of the core steel.
Ferromagnetic materials are characterised by their particular high relative permeability, up to 280
- This means that a relatively small number of magnetising ampereturn per meter length of the
magnetic flux lines is required to obtain a strong magnetic field, which gives a strong coupling
between the windings of a transformer.
Unlike the permeability of other materials, which have constant permeability, a diagram is
necessary to describe the permeability of ferromagnetic materials, see Figure 6-7.
When a sinusoidal ac voltage is applied to the terminals of a transformer winding, a magnetising
current will flow through the winding and a magnetic flux will float in the core. The magnetic flux will
also be of sinusoidal shape, lagging 90 degrees after the applied voltage. The magnetising current
will not be sinusoidal but considerably distorted.
Figure 6-7 shows corresponding values of magnetic flux and magnetising current during one cycle
of the applied voltage. Starting at point a in the diagram, where the flux density and the
magnetising current have their maximum negative value. When the magnetising current and the
magnetic flux proceed towards smaller negative values, they follow the curve from a to b. At point b
the magnetising current is zero, but there is still remaining a magnetic flux in the core. This flux is
called the remanent magnetic flux.
When proceeding further along the curve to the right in the diagram the magnetising force changes
to positive direction. At point c the flux density in the core becomes zero. This value of the
magnetising force is called the coercive force.
Increasing the magnetising force further from point c, a flux in the positive direction starts to flow in
the core. At point d the current starts to decrease. The flux decreases also, but corresponding
values of corresponding values of current and flux do not follow the curve to the right in the
diagram. They follow the curve to the left.
At point e the current has decreased to zero. And again there is a remanent flux floating in the
core, this time with the opposite direction compared to the remanent flux at point b. Increasing the
current now in the negative direction, the flux decreases further and becomes zero at point f. From
point f on the flux changes direction and increases in the negative direction until it reaches point a.
Now one cycle of the applied voltage is completed.
The physical explanation for the described course of events is, expressed in a simplified way, that
a ferromagnetic material has numerous small magnets attached to its crystalline molecular
structure. Within certain domains these magnets have the same orientation. In the original state of
the material these domains are randomly orientated, and the magnetic field from each of them
practically cancels each other so there is no resulting magnetic field.
If the material is placed in an external magnetic field, this will have an impact on the orientation of
the domains. As the external field is increasing, more and more of the domains will change their
direction so the direction of their magnetic field coincides with the direction of the external field.
If the external field gradually decreases, more and more domains will slide out of the orientation
they got due to the external field. But when the external field has disappeared, there will still be a
considerable number of domains, which remain in the same direction as they got under influence of
the foregoing external field.
Exposed to an increasing external field of opposite direction, more and more domains will change
orientation. At a certain value of the external field, the orientation of the domains will be so mixed
that there is no resulting magnetic field from them. Further increase of the external field will cause
more and more domains to change their direction gradually so the direction of their magnetic field
coincides with the new direction of the external field.
Due to the many magnetic domains that become unidirectional, the total magnetic field will be
thousands of times higher than the original external field that directed the domains.
Reorientation of the domains is a gradual process that requires some time. This is the reason why
the magnetic flux lags behind the magnetizing force, which the diagram in Figure 6-7 illustrates.
This diagram is called the hysteresis loop. The word hysteresis comes from the Greek hystereo =
The hysteresis loop illustrates also that the slope of the curve decreases with increasing
magnetizing force. At a certain flux density the slope of the curve will become equal to μ0, the
permeability of air.
This means that no further increase in the magnetic flux can be obtained from the ferromagnetic
material. All the domains have now aligned their field with the external field. This is called magnetic
saturation. For the best commercially available core material today the saturation flux density is
slightly above 2,0 Tesla (Vs/m2).
The permeability is equal to the slope of the hysteresis loop. Because this slope varies during a
cycle of the applied voltage, the permeability is not a constant but varies during the cycle and
varies also with the peak value of the flux density.
To turn the direction of the magnetic domains requires supply of active energy. The required
energy is represented by the area within the hysteresis loop, which has the unit of Ws/m3 of core
material. The supplied energy changes to heat, which increases the temperature of the core. One
could imagine that there is friction present in the material when the domains turn around. The
supplied energy appears as losses in the transformer. They are called hysteresis losses, and they
are proportional to the frequency. At 50 Hz the hysteresis loop is run through 50 times per second.
The hysteresis losses per second will then be 50 times the area of the loop. The loop can be
displayed on the screen of an oscilloscope.
Materials with a narrow hysteresis loop, that is low coercive force, have low hysteresis losses.
As mentioned in the foregoing, the no load current is not sinusoidal.
Eddy current losses
Another component of the no load losses is the eddy current losses. The time-variable magnetic
flux induces currents running in paths perpendicular to the direction of the flux. These currents
produce losses in the core plates. These losses can be calculated by means of the following
24 d B V
P 1 2 2 2
0eddy = ⋅ σ ⋅ ω ⋅ ⋅ ⋅ (W) (17)
σ is the conductivity of the core material
ω is the angular frequency
d is the thickness of the core plates
B is the peak value of the flux density
V is the volume of the core
The formula deserves some comments. It appears that the eddy current losses are proportional to
the conductivity of the core material. In modern core plate a few percents of silicon is alloyed to
reduce the conductivity and the eddy current losses. But if the silicon content exceeds a certain
limit, the material will be difficult to roll, slit and cut.
The eddy current losses are proportional to the square of the thickness of the plates. The choice of
plate thickness is a matter of economic balance between the capitalised value of the losses and
manufacturing costs. In practice the thickness is normally within the range 0,23 – 0,30 mm.
Inrush currents have sometimes been a problem from a protection point of view. Fuses have blown
or relays have disconnected the transformer immediately after energizing. This may create the
suspicion that there is something wrong with the transformer. An explanation of the phenomenon
will be given in the following.
The magnitude of the inrush current is a statistical variable depending on where on the sinusoidal
voltage curve the circuit breaker connects the transformer to the voltage source. The highest inrush
current occurs when the circuit breaker connects the transformer when the voltage passes through
When the circuit breaker disconnects the transformer from the voltage source, there will always
remain a remanent flux in the core, unless the disconnection takes place exactly when the flux
passes zero. The remanent flux will be a statistical variable. It will be at its maximum if the
disconnection happens when the flux has its maximum. The polarity of the remanent flux may be
positive or negative.
The induction law states that the induced voltage is proportional to the time derivative of the