reliable transformers - basic transformer theory...

Reliable Transformers - Basic Transformer Theory Course

Reliable Transformers, South Africa, [email protected]

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©Reliable Transformers

Index

Direct Current (DC)........................................................................................................................................................ 2

Alternating Current (AC) ............................................................................................................................................... 2

Transformer Theory ...................................................................................................................................................... 3

What are Transformers used for? ................................................................................................................................. 4

1. Transformer Enquiries

Electrical Specification .................................................................................................................................................. 5

a. VA ...................................................................................................................................................................... 6

b. Input Voltage ..................................................................................................................................................... 6

c. Output Voltage .................................................................................................................................................. 6

d. Frequency .......................................................................................................................................................... 6

e. Winding Connections (Vector Groups) ............................................................................................................. 7

f. Cooling .............................................................................................................................................................. 8

g. Auto / Double wound transformers .................................................................................................................. 9

h. Impedance......................................................................................................................................................... 9

i. HV Terminations ............................................................................................................................................... 9

j. LV Terminations ................................................................................................................................................ 9

k. Standards ........................................................................................................................................................ 10

l. Size & Weight Considerations ......................................................................................................................... 10

m. Applications ................................................................................................................................................. 10

n. Tap Requirements ........................................................................................................................................... 11

o. Fittings ............................................................................................................................................................. 12

2. Other Considerations

a. Copper versus Aluminium ................................................................................................................................... 13

b. Installation of Transformers ................................................................................................................................ 13

c. Free Breathing or Sealed Transformer and Conservators. ................................................................................. 14

d. Transformers in parallel ...................................................................................................................................... 14



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The Basics

Electricity is available mainly in two forms, namely DC or direct current and AC or alternating current.

Direct Current (DC) The most common source of DC is a battery or DC Generator. DC current flows in one direction only in a circuit.

Fig 1. DC Circuit

Alternating Current (AC) Alternating current is generated by an alternator such as the ones used in power stations or mobile generators.

Alternating current continuously changes direction in a circuit and therefore has no polarity.

Fig 2. Alternating Current waveform.



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Transformer Theory

The simplest form of a transformer is a metal core with two coils wound around it.

If Alternating current is injected into the first or primary coil, the electricity will be converted into a magnetic field in

the core. The magnetic energy in turn will be converted back into electricity in the second coil.

The magnitude or voltage that appears at the second coil is proportionate to the turn’s ratio between the first and

second coil. If the second coil had twice as many turns as the first one, the voltage at the second coil would be twice

that of the first coil.

Fig 3. A simplified single phase transformer circuit.



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What are Transformers used for?

When electricity has to be transported over long distances, heat losses occur in the cables due to electrical

resistance in the cables. These losses are proportionate to the square of the current flow (Amps) in the cable.

If one doubled the voltage in a cable but kept the power constant, the current would halve. As a result, the losses in

the cable would reduce by 75%. This is a significant saving and it is this saving that has led to the use of transformers

to step voltages up and down in modern electrical reticulation networks.

Typically a power-station or generator will generate electricity at a low, manageable voltage. A step-up transformer

will step the voltage up to as high as is economically possible, considering the infrastructure available. At the

receiving end, the voltage will be stepped down again to whatever voltage the customer requires. The stepping or

changing of the voltages is accomplished by means of transformers.

Fig 4. A Typical power generation network



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1. Transformer Enquiries

Electrical Specification

When ordering or enquiring about a transformer, it is important to provide Reliable Transformers with as much

information about the transformer, the environment where it will be used and the application.

The following info is essential to ensure the correct response to your enquiry:

a. VA

b. Input Voltage/s

c. Output Voltage/s

d. Frequency

e. Winding Connections

f. Cooling Required

g. Auto or Double Wound

h. Impedance

i. HV Terminations

j. LV Terminations

k. Applicable Standards

l. Size & weight constraints

m. Application

n. Taps required

o. Fittings

**** Please see on Page 16 is a Table for all the required information.



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a. VA

The VA of a transformer is the power rating of a transformer. It is derived from the Volts multiplied by the

Amps rating of a Single Phase Transformer.

A Single Phase Transformer calculation will be the Volts multiplied by the Amps rating.

A Three Phase Transformer calculation will be the Volts multiplied by the Amps rating then multiplied by the

Square Root of 3.The VA can also roughly be equated to the Watts rating of a transformer. Prefixes such as

Kilo or Mega are added to the VA to indicate thousands or millions, e.g. - a 100 000VA transformer is called a

100KVA and a 1000 000VA transformer a 1MVA transformer.

b. Input Voltage

The input voltage is the supply voltage to a transformer. If this voltage is supplied by the power utility, It

would typically be a high voltage such as 11KV, 22KV, 33KV or higher.

If this voltage is supplied by a generator, it would typically be 400V

c. Output Voltage

The output voltage of a transformer is the voltage that is required by the customer to supply their load. This

voltage is normally quoted as an open circuit voltage. When a transformer is fully loaded, a small reduction

of voltage will occur in the output. This reduction, typically 5% is caused by the internal losses of the

transformer. This is normally compensated for by increasing the open circuit voltage rating of a transformer

or by providing taps for fine tuning of the voltage.

d. Frequency

ESKOM generates electricity at 50Hz and all networks connected to it, operates at 50Hz. Countries such as

the USA generate electricity at 60Hz. It is important to note that all transformers used in a 50Hz grid are

rated for 50Hz. Transformers designed for 60Hz are 20% smaller than their 50Hz counterparts and are likely

to overheat when used in a 50Hz environment.

It is important to verify the frequency when exporting as various countries throughout the world use 60Hz

networks.



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e. Winding Connections (Vector Groups)

AC electricity is almost always generated as three phase. The reason for this is that it is economical and it

makes electrical machines more efficient than if they were only supplied from a single phase source.

There are two main 3phase connections, namely STAR and DELTA.

Star Connection (Y or WYE connection)

A star connection is made by connecting one end of each of three loads to a common or neutral point and

the three phase supply to the remaining ends.

Delta Connection

A delta connection is made by connecting the start of each load to the end of the adjacent load until a

triangle or delta is formed.

One can now rotate and swop the three wires in a myriad of different configurations, each with their own

characteristics. These connections are called vector groups and the customer must specify this when

ordering a transformer.

A typical vector group for a step-down distribution transformer is Dyn11. The “D” is in uppercase, indicating

that the high voltage side of the transformer is connected in Delta. The lowercase “y” indicates that the low

voltage side is connected in star. The “n” indicates that the common star point or neutral of the star

connection is available to the customer. The “11” indicated the specific wire rotation according to an

internationally recognized table.

A standard vector group for step-up or generator transformers is YNd1. The “Y” & “N” are uppercase,

indicating that the high voltage is star with a neutral and the low voltage is delta connected.

Specify a star connection wherever a neutral is required.

When in doubt, make the input to a transformer delta as delta connected windings suppress harmonics.

Fig 5. Delta and Star connections.



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f. Cooling

Transformers generate heat when in operation. This heat must be dissipated in order to regulate the

temperature of transformers otherwise they may overheat and fail.

There are various ways to deal with the heat generated by transformers.

The simplest way is to ensure that the surface area of the transformer is large enough to dissipate any heat

generated without overheating. This type of cooling is called AN or Air Natural cooling and is widely used for

small panel mount and transformers used in electronic devices.

If a transformer is used in a confined space or it is not able to dissipate all the heat generated, one can

improve the dissipation by blowing air across the transformer by means of a fan. This type of cooling is called

AF or Air Forced cooling.

Transformers that are designed for outdoor use generally have to be hermetically sealed to prevent them

from being damaged by the elements and rain.

The best way to achieve this is to mount transformers in a steel tank and to fill the tank with an insulating

substance such as transformer oil. The oil vastly improves the transformers insulating properties whilst also

improving its ability to dissipate heat. Oil conducts heat far better than air.

The heat generated by the transformer is now transferred to the oil, which in-turn transfers the heat to the

tank surface. As the tank surface is much larger than the transformer surface, more heat is dissipated,

making for better cooling. This type of transformer is called an ONAN or Oil Natural, Air Natural transformer.

One could also add oil radiators to allow more oil to come in contact with tank surface. If the radiators are

remote from the transformer, one could pump the oil from the main tank to the radiator and back. This type

of cooling is called OFAN or Oil Forced Air Natural.

To further improve cooling, one can install fans to blow across the radiators. This type is called OFAF or Oil

Forced, Air Forced.

The most efficient form of cooling is by means of a heat exchanger. Oil is pumped into a device (heat

exchanger) that extracts the heat from the oil by exposing it to chilled water in an insulated jacket. The

water is then continuously cooled and circulated through the jacket. This type of cooling is called OFWF or

Oil Forced, Water Forced.

There are many more combinations of the above, depending on the application.



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g. Auto / Double wound transformers

Double wound transformers consist of a metal core with two windings, an input and an output winding.

These windings are magnetically coupled through the core.

Auto transformers consist of a single winding with a tap-off where the lower of the two voltages are

obtained.

Advantages of Auto-wound Transformers

Size – often much smaller than double wound transformers, depends on the spec.

Price – often much cheaper than double wound transformers, depends on the spec.

Disadvantages of Auto-wound Transformers

Isolation – Auto wound transformers do NOT provide electrical isolation between the supply and the

load.

h. Impedance

Normally represented as a percentage, impedance is the percentage of the output voltage that the output

will drop by under full load conditions. For example, if the impedance of a transformer with an output of

400V is 5%, its output voltage will drop by 5% to 380V when loaded to 100% of it’s capacity.

Impedance is closely related to the efficiency of a transformer. Transformers are very efficient devices,

typically 95%. This means that 95% of what you put in, comes out and only 5% goes to waste as heat and

noise.

i. HV Terminations

High voltage terminations are often made from overhead lines. In these cases it is often most practical to

have the HV bushings protrude from the top of the transformer or pointing up at an angle from the sidewall

of the tank. These bushings are of an outdoor type and not enclosed.

When HV supply is by means of underground cable, enclosed cable boxes are recommended. Cable boxes

provide effective anchoring of the cable to prevent undue strain on the bushings.

j. LV Terminations

Low voltage terminals are frequently enclosed in cable boxes, especially when the cables are wired in from

below. Cable boxes are convenient for providing protection from electrocution and for providing a means to

support heavy cables in order to reduce the mechanical loading on the bushings.



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It is important to establish in advance how many and what size cables will be wired into a cable box before

ordering transformers. Multiple cables will often require over-sized cable boxes with pre-manufactured

connection plates, greatly simplifying on-site installation.

k. Standards

In South Africa we generally rely on the SANS 780 specification for distribution transformers to guide us

when manufacturing transformers. The spec describes a wide variety of aspects in the manufacture of

transformers, from testing to shape and dimensions. The SANS 780 spec was however designed to cover a

specific range of step-down transformers of specific sizes. Often when quoting the SANS 780 spec, only parts

of it are referred to.

It is important to compare apples with apples when comparing transformers. Different standards dictate

different losses and cooling and as a result, the same KVA transformer can vary dramatically in size and cost

depending on which standard is being adhered to.

l. Size & Weight Considerations

When shipping transformers to sites, one must always consider transport and installation constraints. It is

often practical to containerize transformers before shipment for ease of movement and to reduce damage

and vandalism. In these cases, the manufacturer must be notified in advance as this would often require

partial dismantling of the transformers before they can be loaded into containers.

Another consideration is overall dimensions. One must ensure that the overall dimensions are compatible

with the mode of transport used. A transformer that will comfortably fit on a low-bed truck, could be an

abnormal load when transported on a normal truck due to the increase in overall height.

If it is known that a transformer will be transported over very rough terrain, the Reliable Transformers will

often adapt the design accordingly, so be sure to notify him of this fact.

m. Applications

If special conditions are not specified, we, the manufacturer, make a couple of assumptions namely:

1. The transformer will be stationary. (Mounted on an immovable surface).

2. There will be ample natural ventilation to draw heat away from the transformer whilst in operation.

3. The transformer will not be installed at an altitude beyond 1800m above sea level.

4. The ambient temperature will not exceed 45 deg C.

It is therefore important to stipulate any unusual environmental or application requirements when enquiring

about a transformer. Transformers for use in tropical environments are often galvanized for extra

protection.

Transformers for use in mobile installations are subjected to continuous vibration and shocks. We tend to

design low profile transformers for these applications with lots of extra bracing and re-enforcement. These

extras do affect the price of the transformers but the reliability is substantially increased.



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Ventilation ties in with cooling methods. We can only recommend the correct cooling method if we

understand the application.

At very high altitudes, transformer radiators and fans become very inefficient. We can compensate for the

reduced efficiency by adding more cooling surfaces but we must know about the conditions.

n. Tap Requirements

Supply voltages are often slightly higher or lower than specified. Tap-changers allow one to closely match a

transformers input circuit with the actual available supply voltage.

This is important as it ensures that the transformer operates within its design parameters and it ensures that

one obtains the correct output voltage from the transformer.

Tap-changers can be automatic of off-load type. Automatic or on-load tap-changers are very expensive and

are only used in specialized applications.

Off-load tap-changers are common and cheap, but they require that the transformer be turned off before

adjustment can be made.



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o. Fittings

SKIDS

Skids are fitted to the base of a transformer to make it possible to drag it across a flat surface.

WHEELS

Wheels make it possible to roll a transformer into place on a firm surface such as concrete.

JACKING PADS

Protrusions on a transformer tank that allows for it to be jacked into place where lifting facilities are not

available.

CABLE BOXES

Enclosures around terminals that provide protection and anchoring facilities for cables.

WINDING THERMOMETERS

Devices that indicate the current temperature of the transformer windings.

OIL THERMOMETERS

Devices that indicate the temperature of the top of the oil in a transformer tank.

THERMOMETER POCKETS

Fittings where oil thermometers can be installed on transformer tanks.

BUCHOLTZ RELAYS

Devices that detects the presence of gas inside a transformer tank.

NER’s and NERM’s

Devices that provide a neutral path to earth and monitors and trips on earth faults.

PRESSURE RELIEF VALVES

Devises that will open in case of excessive pressure inside transformer tanks.

SILICA GEL BREATHERS

Devises that absorbs moisture in the air passing through them.

FILLER & DRAIN VALVES

Valves that allow for oil to be added or drained from transformer tanks.

SURGE ARRESTORS

Devices that provide suppression of surges and spikes that may damage transformers.



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2. Other Considerations

a. Copper versus Aluminium

There are many misconceptions about the differences and advantages and disadvantages of using copper

and/or aluminium in the manufacture of transformers.

The table above addresses some of these:

b. Installation of Transformers Transformers are often the first equipment to arrive at new sites. This could mean that lifting equipment is

not available or rudimentary. Reliable Transformers fit jacking pads as standard fittings on all transformers

from 800KVA up but we can fit them on virtually any size.

Our transformers have large lifting lugs, generally mounted in opposite corners of the main tank. These are

for lifting and rigging.

Smaller lifting lugs can also be found on all removable fittings such as cable boxes, conservators etc. DO NOT

ATTEMPT TO LIFT THE TRANSFORMER BY THESE SMALLER LIFTING LUGS. They were only designed to lift the

fittings they are attached to and serious structural damage may result from incorrect use.

TRUE FALSE

Aluminium-wound transformer terminations are incompatible with copper line & load cables

*

Properly terminating onto aluminium connections is more difficult than with copper connections

*

Connections onto copper connections are more reliable than those made onto aluminium conductors.

*

Aluminium wound transformers weigh less than copper wound transformers *

Copper wound transformers perform better under high impact loads than aluminium wound transformers.

*

Aluminium wound transformers have higher losses because copper is a better conductor.

*

Aluminium wound transformers have more hot-spots because copper conducts better thermally.

*



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c. Free Breathing or Sealed Transformer and Conservators. The two main reasons why one would install a breather on a transformer is firstly to allow for the expansion

and contraction of the transformer oil due to heating and cooling, and secondly, to remove moisture from

the transformer.

The oil inside a transformer tank expands when the transformer heats up and contracts again when it cools

down.

If a transformer is filled to the top and a conservator is installed, a breather must be fitted to allow for the

contraction and expansion of the oil in the conservator.

If a transformer is installed in an environment where temperature extremes are experienced, a breather

might also be a good idea.

Most small and medium sized distribution transformers can be designed to operate without breathers.

These transformers are called “sealed” transformers. Space is normally provided in the tops of these

transformer tanks to allow for the expansion of the oil. The advantage of these transformers is that their oil

cannot easily be contaminated. The disadvantages are that one cannot have cover mounted bushings as

their tanks are not filled to the top. One can also not install Gas actuated relays (Bucholtz relays) in sealed

transformers.

Lastly, the degradation process of oil and insulating materials generate small amounts of water as a by-

product. Because oil floats on water, the water will sink to the bottom of the tank and will generally not

cause any problems inside the transformer, except when the transformer voltages are very high. Silica gel

breathers capture any moisture that is generated inside a transformer tank, and prevents moisture from

entering a transformer tank, provided that the silica gel inside the breather is not already saturated with

moisture.

d. Transformers in parallel Unless transformers are identical in every way, connecting them in parallel must be done only after careful

consideration.

Unmatched transformers will not share the load equally and this may cause one transformer to be over-

loaded whilst another is operating at only a fraction of its capacity.

Be sure to notify Reliable Transformers before ordering if you are planning to use transformers in parallel.



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Please contact me if you have any comments on this document or you need more information.

Reliable Transformers

South Africa

Gerhard Duvenhage

+27 11 421 2333

[email protected]

www.reltrans.co.za

mailto:[email protected]

http://www.reltrans.co.za/

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