7. Vector group
Counter-clockwise phase rotation
Clock angle
1: LV lags HV by 30 deg
11: LV leads HV by 30 deg
Ex.:
Dd0: No phase shift
Dyn11: LV leads HV by 30 deg
YNd5 : LV lags HV by 150 deg
7
8. Vector group
The standard connection for
a Δ-Y transformer is to have
the HV side voltages lead the
LV side voltages by 30°.
8
9. Vector group
A-B-C, 1–2-3 phase rotation. A-B-C, 3–2-1 phase rotation 9
Interchanging the phase connections in a Δ-Y
transformer not only reverses the phase rotation but
changes the phase angle displacement from a standard -
-30° to +30° displacement.
10. Cooling Arrangement
ONAN
For smaller T/f ratings
Oil is kept in circulation
by the gravitational
buoyancy in the closed-
loop cooling system
ONAF
Fans are mounted just
below the radiators, to
blow air from bottom.
more fans should be
mounted at the top of
radiator height. 10
11. Cooling Arrangement
OFAF ODAF
11
Heat dissipation of winding is
less
Oil ducting system is used to
direct the oil over the windings,
hence more heat dissipation.
12. Cooling Arrangement
Cooling pump
Used for circulating oil against low and high frictional head
losses respectively.
Two types of pump designs:
axial flow in-line type and
radial flow type
The axial flow type offers less resistance when switched-off,
and is used with
mixed cooling (ONAN/ ONAF/OFAF) since it
The radial flow type pumps, which offer very high resistance
to oil flow under the switched-off condition, are used with
oil-to-air heat exchangers (unit cooler arrangement) or
oil-to-water heat exchangers in which no natural cooling is provided 12
17. Core
Thickness Grade
Core
losses(w/Kg)
1.7T/ 50 Hz
0.23 MM M3 0.90
0.27 MM M4 1.12
0.30 MM M5 1.30
0.35 MM M6 1.45
0.23 MM 23ZDKH85 0.85
0.27 MM 27ZDKH90 0.90
0.23 MM 23M-0H 1.00
0.23 MM TCH-0 0.90
0.27 MM TCH-1 1.00
To provide a low-
reluctance path for the
magnetic flux linking
primary and secondary
windings.
Types of core
CRGO
AMT
17
18. Oil
Oil parameters
BDV
MOISTURE CONTENT
TAN DELTA
RESISTIVITY
ACIDITY/SLUDGE CONTENT
INTERFACIAL TENSION
If BDV and Moisture content deteriorate oil filtration is to
be done.
If Tan Delta & Acidity content deteriorate beyond
permissible values, oil needs to be replaced. 18
19. Insulation
Minor insulation Like inter turn insulation, is achieved
using cellulosic paper.
Major insulation like between primary and secondary,
phase to phase and inner coil to core is achieved by
Bakelite, wooden blocks, cellulosic paper cylinders.
19
21. Bushing
21
Plain type bushings
Consisting of a single
porcelain tube through
which passes a central
conductor.
Advantageous when
used as an opening of
equipment to be placed
in a busduct
22. Bushing
22
Oil-impregnated, Paper-insulated
Condenser Bushing
consisting of a condenser cone of oil-
impregnated insulating paper.
High reliability and easy maintenance.
Partial discharge free at test voltage.
Provided with test tapping for measuring
electrostatic capacity and tan δ.
Provided with voltage tapping for
connecting an instrument transformer if
required.
24. WTI
Winding is the highest
temperature component
of transformer
Thermowell method
24
25. Buchholz relay
This is mounted in the connecting pipe line between
conservator and main tank.
This has two Floats, one of them with surge catching baffle
and gas collecting space at top.
Alarm float operates when
Broken down core bolt insulation
Shorted Laminations
Bad Contacts
Overheating of winding parts
Trip float operates when
Short Circuit between Winding Phases or within Windings
Puncture of Bushing
25
27. Overloading capacity
IS:6600 – Guide for loading of oil immersed
transformers.
k1 = initial load, S1 as a fraction of rated kVA, Sr.
K2 = permissible load, S2 as a fraction of rated kVA, Sr.
h = duration of k2 in hours
θa = temp. of oil (weighted average)
27
28. Overloading capacity
Sr = 1000kVA
S1 = 500 kVA for h = 2
hrs.
k1 = 500/1000 = 0.5
From table, k2 = 1.43
S2 = k2* Sr = 1430kVA.
28
29. Condition Monitoring
Dissolved gas-in-oil analysis (DGA)
Moisture level in oil
Condition assessment of cellulose insulation
Degree of Polymerization (DP)
Furanic compounds
Metals in oil
Insulation p.f. test-
insulation degradation,
excessive water in cellulose structures
Infrared thermograph (IRT)- detect localized hotspots.
29
30. DGA- Key gases
When the insulation system (Oil & cellulose) is electrically or thermally stressed,
gases are produced and they will dissolve in the oil.
Gas are produced due to
Corona (Partial discharge) :
Low level energy fault
Caused due to gas filled voids/ bubbles surrounding impregnated material.
Key gases: Hydrogen & Methane
Thermal heating (Pyrolysis)
Key gases:
Hydrogen,
methane, ethane (<300 degC)
ethylene (>300 degC)
Acetelene (>1000 degC)
Arcing
High level energy fault
Key gases:
Hydrogen and acetylene
CO, CO2 (If the cellulose material (paper, insulating board etc.) is involved)
30
31. DGA
The distribution of released gases can be related to the type
of electrical fault and the rate of gas generation can
indicate the severity of the fault.
Methods
Rogers Ratio Method,
IEC Basic Ratio Method,
Duval Triangle method and
Key Gas Method
31
32. DGA (Rogers Ratio Method)
Three ratios
Ratio 1 (R1)=CH4/H2
Ratio 2
(R2)=C2H2/C2H4
Ratio 5
(R5)=C2H4/C2H6
No minimum levels
suggested when normal
levels exceeded
32
33. DGA (IEC 60599)
Identifies 6 different fault types
PD: Partial Discharge
D1: Discharge of low energy
D2: Discharge of high energy
T1: Thermal fault, t <300°C
T2: Thermal fault, 300°C < t < 700
°C
T3: Thermal fault, t > 700 °C
Uses a combination of ratios
(based on Roger’s Ratios), gas
concentrations and rates of gas
increase.
33
34. Moisture content in cellulose insulation
Water heat run test
Relative saturation (RS) of water in oil
35. Water heat run test
Test should be carried out
For 3 days
With top oil temp at 60 deg.
Moisture will migrate from
solid insulation to oil
35
36. Relative saturation (RS) of water in oil
Accurate test
Results should be corrected
to top oil temperature
36
37. Transformer testing
Voltage ratio test
1Φ Excitation Current at 415V
Polarity check
Winding Resistance
IR of winding & core
Capacitance & Tan delta of winding & bushing
37
38. Voltage ratio test
To measure the voltage ratio
of one winding to another
associated with a lower or
equal voltage.
Accepted criteria
% Ratio Error : ± 0.5 % of declared
ratio on all taps
Phase Angle error: 0.5% radian
% Ratio error (Deviation) =
[(measured ratio – calculated ratio) /
calculated ratio] x 100
Calculated voltage ratio = HV
winding voltage / LV winding voltage
38
39. 1Φ Excitation Current at 415V
To measure the excitation current at 415v in order to
cross check the results at site before commissioning.
Procedure:
Single phase 415V, 50Hz supply is given to low voltage
winding. Other windings are kept open.
Excitation current is measured of supplied winding.
39
40. Polarity check
Procedure
1-ph supply given to 1.1 &
2.2
1.2 & 2.1 are shorted.
Voltage of 1.1 -1.2 & 2.1 -
2.2 are measured.
Acceptance criteria
Sum of voltages 1.1-1.2 &
2.1-2.2 = supply voltage
40
41. Winding Resistance
Purpose
To measure the winding
resistance & Calculation of
I2R component of conductor
losses.
To check faulty joints or
breaks.
To check loose connection.
41
42. IR of winding & core
To determine the
insulation resistance
from individual windings
to ground or between
individual windings.
Insulation Resistance
will be measured
between
(HV+N) / LV
(HV+N) / (LV+E)
LV / (HV+N+E)
42
43. IR of winding & core
Calculate the Polarization index (P.I) which is equal to
ratio of IR after 600 sec.to IR after 60 sec.
Acceptance:
P.I value should be greater than or equal to 1.3
43
44. Capacitance & Tan delta of
winding & bushing
Purpose
To check the tan delta and
capacitance of the transformer
windings.
For comparison with field
measurements in order to assess
the probable condition of the
insulation
Method
Test voltage of 0.5 to 12 kV @
50Hz.
Test done for windings to ground ,
between windings.
Acceptance
Tan Delta of winding @ 20°C =
0.5% Max. 44
45. Diagnostic testing
Frequency response analysis (SFRA)
Degree of polymerization index
Furan analysis (furfural content)
45
46. SFRA
To detect movement of windings under short
circuit conditions
Procedure
Low voltage sinusoidal output to one end of winding
from Network Analyser (NA).
Voltage transfer function measured from 5hz to 10mhz
NA programmed to measure amplitude and phase shift
at discreet intervals of frequencies.
46
47. SFRA
Condition assessment based on
comparison of responses from different phases for each
winding and tap position
comparison of responses from different transformer of
the same design
Frequency bandwidth
5hz to 2khz indicate mag circuit condition
2khz to 200 khz changes in leakage impedance
1mhz and above indicate capacitance changes
47
48. dP index
Length of cellulose molecule is measured in terms of
dp index.
Depends on dp index of paper
New paper 1200 to 2000
Below 250 a matter of concern
Below 150 little life left and close to failure
48
51. Air cell is made from Nylon fabric coated with Nitrile
rubber
51
52. Transportation
With oil
10% of the tank volume is left clear for expansion to limit
the excess internal air pressure to 0.35 kg/cm2.
Fittings dismantled before transport are packed in
packing cases in line with shipping list.
With N2
Transformer tank is filled with dry Nitrogen at a positive
pressure of 0.175 kg/cm2 (2.5 psi)
N2regulator reduces cylinder high pressure of 120 to 140
kscto required low pressure of 0.175 ksc.
52
Hinweis der Redaktion
The angle is called “phase angle displacement”.
Phase angle displacement must not be confused with phase rotation. Phase rotation is the order in which the phase voltages reach their peak values. In three-phase systems, the phase rotation can be either A-B-C or C-B-A.
The phase rotation can be reversed by interchanging any two phases of the primary circuit or any two phases of the secondary circuit. Interchanging two phases on both the primary circuit and the secondary circuit will revert back to the original phase rotation.
The angle is called “phase angle displacement”.
Phase angle displacement must not be confused with phase rotation. Phase rotation is the order in which the phase voltages reach their peak values. In three-phase systems, the phase rotation can be either A-B-C or C-B-A.
The phase rotation can be reversed by interchanging any two phases of the primary circuit or any two phases of the secondary circuit. Interchanging two phases on both the primary circuit and the secondary circuit will revert back to the original phase rotation.
SLD St#2
the 1-3 winding is still magnetically linked to the 1′-N winding, the 2-1 winding is still magnetically linked to the 2′-N winding, and the 3-2 winding is still magnetically linked to the 3′-N winding in exactly the same way.
When the oil is forced into the transformer, its flow is governed by the least resistance path as well as the buoyancy. Hence, part of the oil may not enter either windings or core, and may form a parallel path outside these two.
The Amorphous Metal, an alloy of Fe78-B13-Si9, has non crystalline structure which is formed by cooling molten metal rapidly at cooling rates of 106 °C/sec.
The atoms do not get arranged as proper grain structure, but are arranged randomly. After annealing under a magnetic field, this alloy exhibits low losses as it gains excellent Magnetic and Chemical properties as compared to the conventional material.
Amorphous Metal Distribution Transformers are the proven energy savers and this is because the Amorphous Metal core reduces the No load losses by about 75% when compared to the conventional CRGO grade.
Thermal faults:As the fault temperature rises, the formation of the degradation gases change from methane (CH4) to ethane (C2H6) to ethylene (C2H4).
When the oil is forced into the transformer, its flow is governed by the least resistance path as well as the buoyancy. Hence, part of the oil may not enter either windings or core, and may form a parallel path outside these two.
Ps = Saturation vapor pressure at temperature Tto.