TRANSFORMER
Knowledge Sharing
Mahersa ALI
Content
• Basic Theory
• Definition Of Transformer
• Electrical Parameters
•
•
•
•
•
•
Types of Transformer
Cooling and Insulation
Surface Treatment
Monitoring & Protection Device
Testing
Installation, Operation &
Maintenance
Basic Theory
Definition, Electrical Parameters,
Basic Theory
Transformer is…
an electrical
equipment which convert
electrical energy from one
level voltage to another level
voltage at a same frequency
…
Confidential Property of Schneider Electric | Page 4
Transformation Ratio
I1
I2
P = U x I x √3
n1 U1 I2
=
=
n2 U2 I1
U1, n1
U2, n2
kVA = kV x A x √3
P = rated power
U = rated voltage (at no-load)
I1
I2
U2 / √3
U1
I = rated current
Transformer ratio :
U1
I2
n1
=
=
U2
I1
n2 × 3
n= number of turns
Rated voltage: step-up or step-down
• Step-down:
• Primary = the higher voltage
• Secondary = the lower voltage
i.e. 20000/400 V
• Step-up:
• Primary = the lower voltage (supply)
• Secondary = the higher voltage (operating)
i.e. 400/20000 V
Tapping
To adjust the rated voltage of the transformer to the system.
Example: 20 000 V ± 2.5 %
⇐ plus tapping
Magnetic
core
⇐ nominal tapping
⇐ minus tapping
U1
n1
= pos 1
20 500
1904
= pos 2
20 000
1858
= pos 3
19 500
1812
tapping range 2.5 %
 46 turns by step
Iron Core
Sometimes referred as
Magnetic Core, it is made
of laminations of grain
oriented silicone steel
insulated with mineral
oxide. Its performance
depends on the grade of
the steel, the cutting pattern
and the method of
assembly
Confidential Property of Schneider Electric | Page 8
No Load Loss (P0) and Parameters Related to Magnetic Core
No Load Loss (P0)
•Composed of:
– Hysteresis losses (reversing of magnetic flux)
– Eddy - current losses (the magnetic core is an electrical conductor)
No load current
Noise/Sound/Acoustic Level
• is the current required for the core magnetisation
• acoustic power level Lw(A)
• is often expressed as a percentage of the rated current
of the same winding.
• acoustic pressure level Lp(A)
Windings
Iron
core
LV winding
HV winding
● Conductor material:
● Copper (Cu)
● Aluminum (Al)
● Conductor shape:
● Round wire
● Rectangular wire
● Foil
● Strip
Load Loss (PCU)
Load loss
● Is composed of ohmic losses (R x I 2), eddy-current losses and
stray-load losses
● Resistance increases as conductor temperature increase
● Reference temperature :
●75°C for the oil immersed transformers
●120°C for the cast resin transformers
● Thus, load loss should be corrected according to reference
temperature
Confidential Property of Schneider Electric | Page 11
Vector Group
The vector group of a transformer defines the connection of the HV and LV windings
• 1st letters group :
CAPITAL letters
⇒the higher voltage
if the neutral is brought out : YN or ZN
• 2nd letters group :
small letters
⇒the lower voltage
if the neutral is brought out : yn or zn
• 3rd letters group : number
⇒clock-hour figure, i ndicating phase
displacement between HV and LV windings
Vector Group - example
Dyn11
HV : Delta
LV : Star
Phase Angle =11x30°= 330°
Confidential Property of Schneider Electric | Page 13
Types of Transformer
Oil Immersed Transformer, Dry Type Transformer
Types of Transformers Based on Cooling and Insulation
Based on insulation and cooling medium:
●Oil immersed transformers
● Sealed type transformer
● Hermetically Sealed Totally Oil Filled
● Sealed type with gas (N2) cushion
● Breathing type with conservator
●Dry type transformers
● Cast Resin
● Encapsulated Resin
● Vacuum Pressure Impregnated (VPI)
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Types of Transformers Based on Cooling and Insulation
Oil Immersed Transformer. Use oil as cooling and insulation
medium
Confidential Property of Schneider Electric | Page 16
Dry Type Transformer. Use air as cooling medium
Oil Immersed Transformer
Based on method to absorb oil expansion
Hermetically sealed totally oil filled
Sealed type with gas cushion
Breathing Type with Conservator
• Transformers is fully filled with oil.
• Oil expansion is absorbed by
conservator tank on top of main tank.
• Expansion of oil is compensated with
deformable fins radiator
• Not fully filled with oil, this
transformer has N2 gas cushion on
the top of tank to absorb oil
expansion.
Standard solution. Up to 36 kV and 4000 kVA.
Customized solution. Up to 36 kV and 10 MVA.
Suitable for high rating up to 170 kV and 100 MVA.
Confidential Property of Schneider Electric | Page 17
Cooling and Insulation
Confidential Property of Schneider Electric | Page 18
Temperature Rise
Part
Temperature rise °K
Windings :
65, for natural or forced oil
circulation,
Temperature class A
(measured by resistance) 70, for forced and directed oil
circulation
Top oil
60, when the transformer is
(measured by
sealed or equipped with a
thermometer)
conservator,
55, when the transformer is not
sealed
For Oil Immersed Transformer
Part
Cooling method
Class of
temp.
Temp.
rise °K
Windings
(measurement
by resistance)
Air natural or forced
A
E
B
F
H
60
75
80
100
125
150
For Dry Type Transformer
Cooling
Symbols
Kind of cooling medium
Symbol
Mineral oil or synthetic insulating fluid with flame
point ≤ 300°C
Insulating fluid with flame point > 300°C
Askarel
Gas
Water
Air
Solid insulant
Kind of circulation
Natural
Forced
Directed
O
K
L
G
W
A
S
ONAN
N
F
D
Order of symbols
1st letter
2nd letter
Indicating the cooling medium that is in contact with windings
Kind of medium
3rd letter
4th letter
AN
Indicating the cooling medium that is in contact with external cooling
system
Kind of circulation
Kind of medium
Kind of circulation
A
N
Example :
O
N
Examples
Surface Treatment
Confidential Property of Schneider Electric | Page 21
Corrosivity Categories
ISO 12944-2 Standard
EXTERIOR
Atmospheric corrosivity category
ISO12944-2
C1
● Heated buildings with clean atmospheres
● Examples : Offices, shops, schools, hotels
-
Very Low Corrosivity
C2
Low Corrosivity
C3
Medium Corrosivity
C4
High Corrosivity
C5M arine
● Atmospheres with very low level of pollution
● Mostly rural areas
● Unheated buildings where condensation may occur
● Examples : Depots, sports halls
● Urban and industrial atmospheres, moderate
sulphur dioxide pollution
● Coastal areas with low salinity
● Productions rooms with high humidity and some air
pollution
● Examples : Food-processing plants, laundries,
breweries, dairies
● Industrial areas and coastal areas with moderate
salinity
● Chemical plants, swimming pools, coastal ship-and
boatyards
● Coastal and offshore areas with high salinity
● Buildings or areas with almost permanent
condensation and with high pollution
● Industrial areas with high humidity and
aggressive atmosphere
● Buildings or areas with almost permanent
condensation and with high pollution
Very High Corrosivity
C5Industrial
Very High Corrosivity
INTERIOR
Durability Categories
ISO 12944-2 Standard
Atmospheric corrosivity category
ISO12944-2
LOW
MEDIUM Durability
HIGH Durability
2 - 5 years
Maintenance : 5 - 15 years
Maintenance : > 15 years
Ri 3 – 5 years
Ri 2 – 5 years
Guarantee
C3
Medium Corrosivity
C4
High Corrosivity
C5 Marine
Very High Corrosivity
C5 Industrial
Very High Corrosivity
> 240 Hours
> 480 Hours
> 480 Hours
> 720 Hours
> 720 Hours
> 1440 Hours
> 720 Hours
> 168 Hours (Chemical Resistance –
> 1440 Hours
> 168 Hours
(Neutral Salt Spray
Resistance - ISO 7253 standard)
ISO 2812-1)
Monitoring & Protection Device
Confidential Property of Schneider Electric | Page 24
Off circuit tap changer
• To adjust the voltage of the
transformer at off circuit (not only at
no-load) co ndition, the transformer
has to be disconnected from the
network.
• Tap changer is operated by means
of a handle on the co ver or a hand
wheel on top or on sidewall of the
transformer.
On Load Tap Changer (OLTC)
• To adjust the voltage automatically,
without de-energizing the transformer
Monitoring & Protection
Devices
1. Integrated Safety Detector
2. Oil Level Indicator
3. Pressure Relief Device
4. Oil Temperature Indicator
5. Winding Temperature Indicator
6. Buchholz Relay
7. Pressure Vacuum Gage
8. Sudden Pressure Relay
Integrated Safety Detector (R.I.S®, DMCR®, DGPT2®)
Hermetically Sealed Totally Oil Filled type
Function:
• Oil level indication
• Oil temperature indication
• Over pressure detection
All functions are provided with
contact for alarm and/or trip
Confidential Property of Schneider Electric
Page 28
Confidential
Property of Schneider Electric |
Standard monitoring device
Oil Level Indicator
For Sealed Type with N2 Gas Cushion Type and Breathing Type
Function :
to measure the level reached
by the oil in a transformer
conservator
Pressure Relief Device
Standard device for all type
Function :
A device to relieve large volumes of gas or
insulating fluid rapidly when the pressure inside
a transformer reaches a pre-determined limit.
Oil Temperature Indicator
For Sealed Type with N2 Gas Cushion Type and Breathing Type with Conservator
Function :
to measure the oil temperature inside transformer tank
Winding Temperature Indicator
Standard device for high rated power
Function:
• to measure the winding temperature by means of a special
bulb surrounded by heating resistance, through which
passes a current proportional to the current passing through
the transformer winding.
Buchholz Relay
for breathing type with conservator
Function :
• to trip out the transformer if there is a release of gas arising
from the decomposition of the solid and liquid insulating
material due to the action of heat or electric arcing.
Pressure Vacuum Gauge
for sealed type with gas cushion
Function :
To check the vacuum pressure inside the tank,
applicable for sealed type transformer with N2
gas cushion
Test ing
Confidential Property of Schneider Electric | Page 35
Transformer Testing
For distribution transformer, according to IEC 60076
ROUTINE TEST
TYPE TEST
1. Measurement of Winding Resistance
2. Measurement of Turn Ratio and Vector Group Check
1. Temperature Rise Test
3. Measurement of No Load and No Load Current
2. Lightning Impulse Test
4. Measurement of Load Losses and Impedance Voltage
5. Induce Test
6. Applied Test
7.
Oil Break Down Voltage Test
Performed on all units, free of charge.
Performed on one unit to validate design. Subject to additional cost.
Winding Resistance Test
Test Objective:
To Measure the resistance in
ohms of each of the windings
of the transformer
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Turn Ratio Test
Test Objective:
To check conformity to
specification of the transformer
turn ratio and vector group
connection
Acceptance Criteria:
Tolerance : ± 0,5% of theoretical
ratio
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No Load Losses & Current Measurement
Test Objective:
1. To measure no load losses
and no load current of the
transformer
2. To check whether the
characteristics conform the
applicable standards
Acceptance Criteria:
+15% tolerance from declared
value according to IEC 60076
Confidential Property of Schneider Electric
Load Losses & Impedance Measurement
Test Objective:
1. Measurement of the
transformer’s load losses
2. Measurement of impedance
voltage
Acceptance Criteria:
•Load losses tolerance: +15%
from declared value
•Impedance tolerance:± 10%
from declared value
• All according to IEC 60076
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Induce Overvoltage Test
Test Objective:
The test the insulation strength
inter turns and inter layer of the
windings.
Acceptance Criteria:
The transformer is considered
to have passed when there is
no voltage collapse or variation
in current
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Applied Voltage Test
Test Objective:
To test the insulation strength
between live parts to earth
Acceptance Criteria:
The transformer is considered
to have passed when there is
no voltage collapse in 60
second
HV
HV side
kV
LV Side
TEST FOR HV SIDE
HV
HV side
kV
LV Side
TEST FOR LV SIDE
Confidential Property of Schneider Electric
LV
LV
Temperature Rise Test
Test Objective:
1. To determine the temperature rise of the oil (ΔT oil)
2. To determine the temperature rise of the HV and LV windings
3. To confirm the rated power of the transformer
Acceptance Criteria:
•According to approved documents and specifications; ex ΔT oil : 60°C, ΔT windings : 65°C
Lightning Impulse Test
Test Objective:
•To check the impulse withstand of each winding relative to earth, relative and to other
windings and along the tested winding
Acceptance Criteria:
•Reduced and full wave should be similar
Function Test of Measuring & Protection Devices
Test Objective:
•
Check of control and protection devices : relay function, accessories function and
continuity of wiring from devices into wiring box (Regulator Control Panel, Lamp, Power
socket, Heater, Oil level gauge, Pressure Relief Device, Oil Thermometer, Winding
Thermometer, Buchholz Relay, Junction Relay (RS. 2001, OLTC), Control Panel, Current
Transformer (CT), etc.)
Installation, Operation & Maintenance
INSTALLATION
Installation should be carried out in compliance with following rules / conditions:
1. All external cables connections are tight and secure
2. All accessories circuits operate properly
3. The tap changer is securely in correct position
4. The neutral and earth connections have been properly installed
5. Fans (if any) has been installed and operates properly
6. Proper clearance is maintained from high voltage bus to terminal equipment
7. All windings are free from un-intended earths
8. There is no dust, dirt or foreign material on core and coils (dry-type transformers)
9. There is no visible moisture on or inside the core and coils (dry-type transformers)
10. All plastic wrappings are removed from the core and coils (dry-type transformers)
11. There are no obstructions in or near the openings for ventilation
12. Commissioning should be carried out prior to energizing
OPERATION
Checklist before operation (part 1)
No.
CHECK ITEMS
DESCRIPTION
SPECIFIED
1
Out line drawing
Specification
Compliance with drawing and specification
2
Oil level
Main tank
Level gauge to read correctly for existing
level and oil temperature
1. Valves
3
Valve position
2. Radiator valves
Open and close position
3. Other valves
4
Off load tap changer
Manual operation through all tap position
As specified
REMARK
OPERATION
Checklist before operation (part 2)
No.
CHECK ITEMS
DESCRIPTION
1. Insulation resistance of windings
2. Ratio and polarity
5
Electrical tests
3. Dielectric strength of oil
4. Alarm, tripped, control circuits, and auxiliary circuit
6
7
8
Earthing
Bushing
Other
Tank and Marshalling kiosk
1.
Porcelain
2.
Spark Gap
3.
Connected
Cleans rust
SPECIFIED
No insulation breakdown
REMARK
As reference
Acc. to specification
>30 kV
1.
Wired properly and comply with wiring
2.
Contacts open or close as specified.
It should be connected to earth
Clean
Proper continuity and tightness
Cleans
Clean and
touch point
ROUTINE MAINTENANCE
No
Task
Acceptance Criteria
Frequency
Remarks
1
Check the pressure relief disc
No crack
Yearly
Contact the transformer supplier
2
Check ventilation and ambient
temperature inside the transformer
room
< 40o C
Yearly
Ensure adequate ventilation and the compliance
with Section 7 - Installation.
3
Check insulating liquid temperature
< 100o C
Yearly
Conduct investigation if the insulating liquid
temperature is abnormally high with due respect to
the load current and ambient temperature
4
Check for leakage of insulating liquid
level
No insulating liquid leak
Yearly
If leakage is found, refer to Appendix 1.1
5
Check insulating liquid level
Middle between Max & Min
Yearly
If level on the RIS insulating liquid level indicator is
too low, refer to Appendix 1.2
6
Check the RIS settings
Refer to the RIS Catalogue &
Instructions
2-yearly
Reset the settings if different from that in the RIS
Catalogue & Instruction
7
Check dielectric strength of the
insulating liquid
Breakdown voltage test >
30kV/2.5mm
2-yearly
For sampling details, please refer to Appendix 1.3
8
Check paintwork
No paint peeling or rust
2-yearly
9
Arrange outage to check and clean
bushings
No dust
5-yearly
The frequency should be increased for hostile
environment
The frequency should be increased to 4 years or
less for hostile environment
Diagnostic Method for Transformer
Electrical Test:
•Winding Resistance
•No Load Current
•Capacitance
•Transmission ratio
Periodic/Routine Maintenance
Advance (Special Test):
•Partial Discharge
•Dielectric Response
•SFRA
Oil Analysis:
•Color
•BDV
•Water Content
•DGA
On Line Monitoring:
•Voltage, Current, Temperature
•Oil, tap changer, cooling
•Bushings
Post Mortem :
• Degree of polymerization
•Moisture in pressboard/paper
Questions?
Confidential Property of Schneider Electric | Page 52