2. Contents
Applicable Standards
Working Principle
Construction
Type of Transformer
Sizing of Transformer
Testing of Transformer
3. Applicable Standards
IEC 60076 (Part 1)- Power Transformer, General
IEC 60076 (Part 2)- Power Transformer, Temperature Rise
IEC 60076 (Part 3)- Power Transformer, Insulation Levels,
dielectric tests and external clearance in air
IEC 60076 (Part 5)- Power Transformer, Ability to withstand short
circuit
IEC 60076 (Part 7)- Power Transformer, Loading guide for oil-
immersed power transformer
IEC 60076 (Part 8)- Power Transformer, Application guide
IEC 60076 (Part 11)- Dry Type Transformer
4. Why do we need Transformers?
If power losses in electricity distribution networks are to be
kept within reasonable limits, then large amount of electric
power have to be transmitted across long distances at the
highest feasible voltage.
In absolute terms, higher the transmission voltage, smaller
the current and hence smaller the (resistive) power losses
in transmission cables.
5. Working Principle
Transformer is a static piece of apparatus by means of
which electric power in one circuit is transformed into
electric power of same frequency in another circuit. It
can raise or lower the voltage in a circuit but with a
corresponding increase or decrease in current.
In simplest form, it consists of two inductive coils which
are electrically separated but magnetically linked
through a path of low reluctance.
6. Type of Transformer
The two types are know as:
1. Core-Type Transformer – the windings surround a
considerable part of the core.
2. Shell-Type Transformer – the core surrounds a
considerable portion of the windings.
7. Types of Transformer
• Distribution Transformer
• Power Transformer
• Phase Shift Transformer
8. Parts of Transformer
MAIN TANK
RADIATORS
CONSERVATOR (With or Without)
TAP CHANGER (ON / OFF Load)
EXPLOSION VENT
LIFTING LUGS
AIR RELEASE PLUG
OIL LEVEL INDICATOR
BUCHHOLZ RELAY
BREATHER
WHEELS
HV/LV BUSHINGS
FILTER VALVES
OIL FILLING PLUG
DRAIN PLUG
CABLE BOX
9. Sizing of Transformer
With maximum demand (Peak Load) + spare capacity
transformer sizing done. Also need to check highest
motor starting.
11. Transformer Capacity on basis of concepts
• 1 x 100% - Only a single power-transformer of rated
capacity 15 to 20% higher than the calculated
maximum demand.
• 2 x 100% - Two power transformers each capable of
meeting the full maximum demand.
• 3 x 50% - Three power transformers each rating higher
than 50% of maximum demand.
13. Ceiling on MVA ratings
The ceiling on individual transformer capacity is due to the necessity to limit HV system
fault level and is also non availability of single HV-CB for larger current ratings.
Secondary Voltage of
Power Transformer
Max MVA rating of power
transformer
3.3kV secondary ……12.5MVA
6.6kV secondary …… 25MVA
11kV secondary …… 40MVA
14. Letter denotes form of Cooling
First letter: Internal cooling medium in contact with the windings
O – mineral oil or synthetic insulating liquid with fire point < 300°C
K – insulating liquid with fire point > 300°C
L – insulating liquid with no measurable fire point
Second letter: Circulation Mechanism for internal cooling medium
N – natural convection flow through cooling equipment and windings
F – Forced circulation through cooling equipment (cooling pumps), natural
convection flow in windings (non-directed flow)
D – forced circulation through cooling equipment, directed from the cooling
equipment into at least the main windings
15. Letter denotes form of Cooling
Third letter: External cooling medium
A – Air
W – Water
Fourth letter: Circulation Mechanism for external cooling medium
N – natural convection
F – Forced circulation (fans, pumps)
16. Type of Tanks
Conservator Type:
This is fitted with an overhead tank which is approximately half full of the oil. The overhead tank is
allowed to breath to atmosphere as the oil level varies with the average temperature inside the
transformer.
Sealed Type:
An alternative design which has the advantage of reduced periodic maintenance of oil in the sealed
type. The main tank is designed not to breath and is provided with gas or vapour space between the
top of surface of the liquid and the underside of the tank lid.
17. Vector Group of Transformers
• Delta – delta : Generally used where it needs to carry large currents on
low voltage
• Star – star : this type of transformer is rarely used due to problem with
unbalanced loads.
• Delta – star : Commonly used as Step Up transformer at the beginning of
a HT transmission line, Commercial, industrial and residential locations
• Star – delta : commonly used as Step Down transformer at the end of
transmission line
• Delta – zigzag : to reduce harmonics in distribution system. Normally in
transmission line
• Star – zigzag : to reduce harmonics in distribution system. Normally in
transmission line
18. Vector Group of Transformers
High voltage (not primary) line emf is placed at 12 o’clock
position and corresponding low voltage line emf position is
noted. Low voltage line emf position denotes Hour hand of
the clock and accordingly nomenclature is given (vector
group is denoted).
HV connection is denoted by capital letter, LV connection is
denoted by small letter and corresponding LV line emf
position is termed as HOUR hand of the clock and is
denoted by suffix.
For HV delta connected and LV star connected, and LV line
emf 30° leading is written as Dy11
21. Min. Information required for Enquiry
• Particulars of specification
• Kind of transformer (separate winding, auto or booster transformer)
• Single or three phase unit
• Number of phases in system
• Frequency
• Dry type or liquid filled
• Indoor or outdoor type
• Type of cooling
• Rated power
• Rated voltage
• Method of system earthing
• Details of auxiliary supply voltage
• Insulation Level for each winding
23. Testing of Transformer
TYPE TESTS (TO BE CARRIED OUT ONLY ON ONE TRANSFORMER OF THE LOT)
• Lightning Impulse Test
• Temperature Rise Test
24. Testing of Transformer
ROUTINE TESTS ( TO BE CARRIED OUT ON EACH JOB)
• Measurement of winding resistance
• Measurement of insulation resistance
• Separate source voltage withstand test (High Voltage
tests on HV & LV)
• Induced Over voltage Withstand test (DVDF test)
• Measurement of voltage ratio
• Measurement of NO LOAD LOSS & current.
• Measurement of LOAD LOSS & IMPEDENCE.
• Vector Group Verification
• Oil BDV test.
• Tests on OLTC (if Attached)
25. Testing of Transformer
SPECIAL TESTS
• Additional Impulse Test
• Short Circuit Test
• Measurement of Zero Phase sequence Impedance Test
• Measurement of acoustic noise level
• Measurement of harmonics of the no load current
• Magnetic balance test
