1. M. Nageswar Rao, Sr.Mgr.(EMD) Date: 24.03.2015
Venue: EDC, Simhadri

2. Contents
 Introduction
 Types of Transformers
 Transformers in SMPP
 Specifications
 Components
 Loading & Overloading capacity
 Condition Monitoring
 Testing
2

3. Types of transformers
 Power transformation
 Power transformer ,
 Distribution transformer,
 Auto transformers
 Voltage transformation (Step up, Step down)
 Voltage class (LT, MV, EHV)
 Cooling
 Oil filled (ONAN, ONAF, OFAF, ODAF)
 Dry type
 Core material (CRGO, AMT)
 Construction (Core type, Shell type)
 Phase (1Φ, 3Φ, Multi-phase)
3

4. Transformers in SMPP
 St#1 Transformers
 St#2 Transformers
4

5. Rating/ specifications
 Voltage HV/LV
 Power rating, MVA
 Cooling method
 % impedance
 Vector group & clock
 Taps, tap changer
 Losses & Loss capitalization charges
 Terminal arrangement
5

6. Rating/ specifications
 % Impedance
6

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

13. Name plate details
 St#2 GT
 St#2 ST
 St#2 UT
 St#2 UAT
13

14. Components
of a Transformer
 Windings
 Core
 Insulation (paper)
 Cooling medium (oil)
 Bushings
 Protective devices
 PRD, Buch.Relay
 Monitoring devices
 MOG, Oil level indicators,
 RTD & Temperature gauges (WTI, OTI)
 Accessories
 Tap changer,
 Radiators, Cooling fans & pumps,
 Oil flow indicators.
 Conservator, Breather, Aircell
 Aux. power supply (Marshalling box)
 Lightning protection
 Earthing arrangement
14

15. Oil filled Transformer
15

16. Dry type transformer
16

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

20. Bushing
 Types
 Plain type bushings (<25kV)
 Oil-impregnated, Paper-insulated Condenser Bushing
 Air-Oil
 SF6 Gas- Oil
20

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.

23. OTI
 measures the Top oil
Temperature
23

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

26. PRD
 Qualitrol catalogue
26

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

49. 49

50. TRANSFORMER EXPLOSION PREVENTION AND
FIRE EXTINGUISHING SYSTEM
50

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