some basics to design a transformer.

1. The different topics to be covered are –
Working principle , Transformer types & their application, Transformer Design ,
Components of Transformer ,Transformer Materials ,Testing of Transformer

2. Basically a transformer consists of two inductive coils; primary winding and
secondary winding. The coils are electrically separated but magnetically
linked to each other. When, primary winding is connected to a source of
alternating voltage, alternating magnetic flux is produced around the
winding. The core provides magnetic path for the flux, to get linked with the
secondary winding. Most of the flux gets linked with the secondary winding
which is called as 'useful flux' or main 'flux', and the flux which does not get
linked with secondary winding is called as 'leakage flux'. As the flux
produced is alternating (the direction of it is continuously changing), EMF
gets induced in the secondary winding according to Faraday's law of
electromagnetic induction. This emf is called 'mutually induced emf', and
the frequency of mutually induced emf is same as that of supplied emf. If
the secondary winding is closed circuit, then mutually induced current flows
through it, and hence the electrical energy is transferred from one circuit
(primary) to another circuit (secondary)

3. Transformers can be classified on different basis, like types of construction, types of cooling etc.
(A) On the basis of construction, transformers can be classified into two types as;
1. Core type transformer and
2. Shell type transformer,
(B) On the basis of their purpose
1. Step up transformer: Voltage increases (with subsequent decrease in current) at secondary.
2. Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.
(C) On the basis of type of supply
1. Single phase transformer
2. Three phase transformer
(D) On the basis of their use
1. Power transformer: Used in transmission network, high rating
2. Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.
3. Special application transformers – VSD , FSD , phase shift , isolation , pad mounted , earthing transformer.
The type of cooling in the transformer can be
Air Natural (AN) ,Air Blast or forced (AF) , Oil Natural Air Natural (ONAN) ,Oil Natural Air Forced(ONAF),Oil Natural Water Forced(ONWF),Oil Forced Water
Forced(OFWF)
For voltage regulation Tap changers are provided in transformers which can be either off load or on load tap changer depending on customer requirement.

4. Key inputs required while designing a transformer are -
Rating, Voltage ratio , Vector group , Impedance , NLL , LL , type of cooling, type of conductor , Insulation level , temperature rise , frequency.
We will take example of a 1000KVA 33/0.433KV Transformer.
(A) Rating - 1000KVA
The power rating of a transformer is usually expressed in kVA (kilovolt-amperes), because kVA is a measure of apparent power,
which is the product of the voltage and current in an AC system.
(B) Voltage ratio – 33/0.433KV
The ratio of voltage transformation of a transformer is equal to the number of turns on the secondary coil divided by the number of turns on the primary
coil.
(C) Vector Group – Dyn11
A vector group is the International Electrotechnical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding
configurations of three-phase transformers. The vector group designation indicates the windings configurations and the difference in phase angle between them.
(D) Impedance – 5%
The percent impedance (%Z) is the percent of the rated load impedance possessed by a transformer. The percent impedance is important in that it allows
us to: Calculate available fault currents (both individual and bank). Determine whether two transformers are suitable for paralleling.
(E) NLL & LL – 1.80kw & 10.0kw
No-load losses are caused by the magnetizing current needed to energize the core of the transformer, and do not vary according to the loading on the
transformer. They are constant and occur 24 hours a day, 365 days a year, regardless of the load, hence the term no-load losses.
Load losses vary according to the loading on the transformer. They include heat losses and eddy currents in the primary and secondary conductors of the
transformer. Heat losses, or I 2R losses, in the winding materials contribute the largest part of the load losses.

5. (F) Type of cooling - ONAN
ONAN is a form of transformer cooling in which mineral oil is used inside the transformer tank to cool and cool the transformer. KNAN is another cooling
method for transformers, which means that vegetable oil is used to cool the transformer inside the oil tank.
(G) Type of conductor - Electrolytic copper
Copper and aluminum are the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less
expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper
(H) Insulation Level – power frequency KVrms 70/3 , Basic Impulse level KVp 170 / NA
Power frequency test - The design efficacy of the insulation system is ascertained through the one minute Power Frequency Withstand test.
Impulse testing of transformers is done to determine the ability of ​insulation to withstand transient voltages
(I) Temperature rise over ambient – Oil 45 deg , Winding 50 deg
Temperature rise tests are used to make sure that a product does not get overheated during operation.
This test's results are really important for the operational economy to show the performance of the transformer in the fields. The mean purpose of this test
is to check whether the oil and winding temperatures of the transformer meet the values specified in the standard and technical projects.

6. CORE
ASSEMBLY
LV
WINDING
HV
WINDING
CORE COIL
ASSEMBLY
HV & LV
CONNECTION
TANKING &
FITTING
TESTING ACCECORY &
FINAL
PAINTING
SKID/PSS
ASSEMBLY
DISPATCH
Core assembly MPC copper strip MPC copper strip Press board,
blocks , crepe
paper
Brazing rods , solder
iron
Transformer oil WTI , NCT PRD, PRV Open/close skid
cage
Silica gel breather
Insulation paper Copper Foil SE conductor Job locking,
support hardware
Crepe paper Job locking/tank
fitting hardware
Marshalling box Vacuum gauge PSS enclosure Lugs & glands
Cotton tape Insulation kraft
paper
Insulation kraft
paper
off load tap
changer
Connection lugs ,
ferrules
Radiator as per
drawing
Special fuse Marshalling box & its
material
SF6 RMU
with/without OLU
Termination kit
Cotton tape Cotton tape Earthing &
connection CU
jumper
Copper flats, braided
wire, MPC wire
Shut off valve Ammeter CT WTI CT, NCT , special
fuse, Termination kit,
lugs & glands
LV panel
Press board Press board FRP nuts & rods Paper tube, Bakelite
tube
HV bushing Cable box and DC
fitting
Fitting &
Connection
accessories for PSS
Adhesive tape Adhesive tape PVC sleeve , glass tube LV bushing Cable support & CB
support assembly
Termination kit for
RMU in PSS
Crepe paper Crepe paper MOG/oil gauge WTI,OTI fitting Epoxy insulator
Epoxy dotted
paper
Drain valve/ filter
valve
Buchholz relay Connection flat
Transformer tank Tinned CU flat,
braided wire , sleeves
Panel Mounting
Screw/bolt
SS threaded rod Epoxy insulator , SS
threaded rod
RD & tag plate ,
accessories labels ,
Phase & other labels
Paint
COMPONENTS & MATERIALS IN A FINISHED TRANSFORMER

7. Tests done at Factory / 3rd party labs
1. Type tests
To prove that the transformer meets customer’s specifications and design expectations, the transformer has to go through different
testing procedures in manufacturer premises. Some transformer tests are carried out for confirming the basic design expectation of
that transformer. These tests are done mainly in a prototype unit not in all manufactured units in a lot. Type test of transformer
confirms main and basic design criteria of a production lot.
2. Routine tests
Routine tests of transformer is mainly for confirming the operational performance of the individual unit in a production lot. Routine
tests are carried out on every unit manufactured.
3. Special tests
Special tests of transformer is done as per customer requirement to obtain information useful to the user during operation or
maintenance of the transformer.
Tests done at Site
1. Pre-commissioning tests
In addition to these, the transformer also goes through some other tests, performed on it, before actual commissioning of the
transformer at the site. The transformer testing performed before commissioning the transformer at the site is called the pre-
commissioning test of transformer. These tests are done to assess the condition of transformer after installation and compare the
test results of all the low voltage tests with the factory test reports.

8. Type tests of transformer include:
Winding resistance test of transformer
Transformer ratio test
Transformer vector group test
Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test)
Measurement of no-load loss and current (Open circuit test)
Measurement of insulation resistance
Dielectric tests of transformer
Temperature rise test of transformer
Tests on on-load tap-changer
Vacuum tests on tank and radiators
Routine tests of transformer include
Winding resistance test of transformer
Transformer ratio test
Transformer vector group test
Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test)
Measurement of no load loss and current (Open circuit test)
Measurement of insulation resistance
Dielectric tests of transformer.
Tests on on-load tap-changer.
Oil pressure test on transformer to check against leakages past joints and gaskets
That means Routine tests of transformer include all the type tests except temperature rise and vacuum tests. The oil pressure test on transformer to check against leakages past joints and gaskets is
included.
Special Tests of transformer include
Dielectric tests.
Measurement of zero-sequence impedance of three-phase transformers
Short-circuit test
Measurement of acoustic noise level
Measurement of the harmonics of the no-load current.
Measurement of the power taken by the fans and oil pumps.
Tests on bought out components / accessories such as Buchholz relay, temperature indicators, pressure relief devices, oil preservation system etc.