1. CTs
Randhir Singh

2. Approximate Population of Substation Equipments
Transformers Nos.
Reactors Nos.
Circuit Breakers Nos.
Current Transformers Nos.
Capacitive Voltage Transformers 3200 Nos.
Lightning Arresters Nos.
Isolators Nos.

3. Current Transformers

4. Population of CTs in POWERGRID
Make
Population
(Approx.)
ABB
1100
BHEL
1200
CGL
1000
AREVA/ WSI
TELK
200
Others/ Imported
300
Total
5200

5. Types of CTs Dead Tank Design Live tank Design Hair Pin Design
Eye Bolt Design
Live tank Design

6. Hair Pin Design Hair-Pin design 1. Dome 2. Nitrogen filling valve
3. Primary terminal
4. Collar
5. Porcelain insulator
6. Primary conductor with insulation
7. Adaptor cylinder
8. Secondary cores
9. Base
10. Oil drain plug
IT 400 Cross section
Hair-Pin design

7. Internal details Eye bolt design 1. Oil filling plug 2. Dome
3. Nitrogen filling valve
4. Collar
5. Primary terminal
6. Porcelain insulator
7. Insulated primary
8. Cover plate for tank
9. Tank
10. Secondary cores
Eye bolt design

8. Active Part Manufacturing
IT range

9. Live Tank CTs

10. Dead tank CTs

11. CT Standards
IEC – 1
IS 2705

12. CT Design
Core Material – The main aim is to give high accuracy with low saturation factor.
Core Material is made of CRGO Silicon steel

13. CT accuracies As per IEC-60044(1)
Metering Core – ±0.2 or 0.5% at rated Currents
Protection Cores – ± 1% at rated current

14. Accuracies as per IEC-60044-1
Class
5% of rated I
20% of rated I
100% of rated I
120% of rated I
0.2
0.75
0.35
0.5
1.5

15. Protection Cores Class Current Error at rated Primary Current
Composite Error at rated accuracy limit Primary Current 5P
±1%
±5%
10P
±3%
±10%

16. Factors for Protection
1. Accuracy Limiting Factor/composite error
For e.g if the class designation is 5P20
20 is the Accuracy limiting factor which signifies that when
20 times the rated primary current is applied the composite
error of 5P( +/- 5%) is maintained.
Typical Class designations are
5P10, 5P20, 10P10, 10P20 etc.,

17. Ratio Error Ratio Error = (KnIs- Ip)*100/ Ip
Kn = Rated transformation ratio
Ip = Actual primary current
Is = Actual secondary current

18. Instrument Transformer Error
Secondary
Primary Ip Is K
Current transformer :
:Ratio error Ip
: Phase error
K.Is
TC : K= Ip Is

19. Phase Angle Error
The difference in Phase between the Primary and Secondary current vectors

20. Knee Point Voltage
10% increase in Voltage will lead to 30% or more increase in Current.

21. Insulation Levels
For Windings having Um greater than 300kV, the rated insulation level is determined by rated switching and lightning impulse withstand voltage
For voltages < 300kV, insulation levels are decided by lightning impulse and power-frequency withstand voltages

22. Insulation Levels System Voltage 1 min Power Freq. Voltage
Switching Impulse withstand Voltage
Lightning Impulse withstand Voltage
220kV
395kV
460kV -
950kV
1050kV
400kV
630kV
1425kV

23. Creepage Distances Pollution Levels Creepage distance Light 16mm/kV
Medium
20mm/kV
Heavy
25mm/kV
Very Heavy
31mm/kV

24. Pre Commissioning Tests
Polarity Test
Magnetization Curve Test
Ratio Test
Primary Current Injection Test
Secondary Current Injection Test

25. Reasons of CT Failures
About 30 nos. CTs have failed due to poor impregnation and paper wrapping at works.
About 90 nos. CTs have failed due to pre-mature ageing of almost all makes
2 no. CT failed after repair at site.
Moisture entry due to N2 gas leakage

26. Primary Insulation Failure due to moisture entry

27. Raipur CT failure (AREVA)

28. Insulation puncture

29. Failure of Primary Insulation

30. Condition Monitoring Checking of Bellow expansion - M
Visual Inspection for leakages - M
Tan Delta Measurement – 2Y
Thermovision Scanning - Y
Nitrogen Pressure Checking – 2Y
DGA testing of Oil - SOS

31. CT TESTING –TAN DELTA

32. CT Insulation

33. Capacitance and Tan Delta Measurement
CTs with Test Tap- Ungrounded Specimen Test mode (UST)
CTs without Test Taps – Grounded Specimen Test (GST) mode with jumpers disconnected
Values to be monitored w.r.t. factory/ pre- commissioning values
Sudden change in measured values indicate faster deterioration of insulation.
Precautions: P1/P2 to be shorted. Porcelain surface to be thoroughly cleaned. Test Tap to be reconnected to Earth after the Test

34. Capacitance and Tan Delta Measurement – Contd.
Connection of Test Tap to be ensured otherwise it may lead to slow arcing in the soldering area and insulation may fail in due course of time.
Measurement of Tan Delta of C2 (insulation between last foil on which test tap wire is soldered to the ground) to be carried out.
Measurement in GSTg mode with P1/P2 terminal guarded.

35. CVTs

36. Population of CVTs in POWERGRID
Make
CVTs
ABB
800
BHEL
315
CGL
AREVA/WSI
1200
Others 85
Total
3200

37. CVT Construction Details

38. Capacitor stack
Inductive VT

39. CVT Construction Details
There are 280 – 300 elements in
C1 & C2
C1 will be about 260 to 280 elements
C2 will be 15 to 20 elements
Ratio of C1/ C2 is 20 to 22
400/ 20 = 20kV (Tap Voltage)

40. Compensating Reactor
Compensating Reactor is provided to compensate for the phase displacement in Capacitor elements
wL = 1/w (c1+c2)
L = 1/ w2 (c1+c2)

41. Ferro Resonance
Ferro resonance in CVTs is due to the Capacitance in Voltage Divider in series with the inductance of the Transformer and series reactor. This circuit is brought to resonance by various disturbances in the network that may saturate the iron core of the transformer, over heat electro magnetic unit and lead to insulation breakdown.

42. Ferro Resonance Circuit
Ferro resonance circuit is provided in CVT Secondary to suppress Ferro resonance oscillations
There can be active or passive Ferro resonance circuits
It can be RLC circuit (ABB)
or RL circuit (CGL)
or Resistance (BHEL, WSI, AREVA)

43. CVT Secondary Voltage CVT Secondary Voltage v = k * V * C1/ (C1+C2)
V – Primary Voltage
k – Secondary Transformer Transformation ratio
Note: Puncturing of C1 – Secondary Voltage will increase
Puncturing of C2 – Secondary Voltage will decrease

44. CVT VA ratings
As per POWERGRID specifications, VA ratings for core-1, core-2 and core-3 are 50VA, 50VA and 50VA respectively. Earlier CVTs it was 200/ 200/ 50VA
CVT accuracies are guaranteed if connected burdens are within 25% to 100% of the rated burdens
In POWERGRID, with static meters and static/ numerical relays, connected burdens are 10 to 20 VA in each core which are very low as compared to earlier rated burdens.

45. PD Measurement CT PT CVT Test Voltage Before Revision IEC-185 IEC-186
Pre-stress Voltage-436kV
Test Voltage- 266kV(10pC)
After revision
IEC (1)
IEC (2)
Yet to be revised
Pre-stress Voltage–504kV
Test Voltage- 420kV(10pC)

46. REASONS OF FAILURES OF CVTs
WRINKLES ON ALUMINUM FOIL
POOR SOLDERING QUALITY
POOR QUALITY OF PAPER(LOCAL SOURCE)
PINHOLES IN BELLOWS
SNAPPING OF BELLOW CONNECTION
OVERHEATING OF DAMPING RESISTOR
SHORTING OF TRANSFORMER CORES
FAILURES OF FR CIRCUIT COMPONENTS
RUSTING OF COUPLING BOLTS (BETWEEN FLANGE AND EMU TANK)
RUSTING OF FLANGE

47. Reasons of CVT Failures
LOOSENESS OF CORE BOLTS
SNAPPING OF CONNECTION BETWEEN PRIMARY WINDING AND COMPENSATING REACTOR
FAILURE OF VARISTORS PROVIDED IN SECONDARY
ENTRY OF MOISTURE IN CAPACITOR STACKS
POOR GASKET QUALITY
ALMOST ALL COMPONENTS OF CVTs HAVE SHOWN FAILURE TREND

48. Rusting of EMU Tank

49. EMU Tank Transformer Winding shorted

50. Top Bellow with uneven surface and soldered material

51. Failure of Capacitor Element in

52. Rusting and shearing off of Coupling Bolts and also entry of Moisture in the stack

53. Mechanical shearing of the Bolt/ Stud

54. Matter of Concern
In POWERGRID, there have been many pre- mature failures of CVTs.
CVTs have given unreliable performance so far. Against life of about years, failures have occurred after 8-10 years of service.
About 900 CVTs of almost all makes CVTs needs to be refurbished/ repaired at manufacturers works.
This will need huge expenditure to be met from O&M budget affecting POWERGRID profitability.

55. Condition Monitoring of CVTs
Secondary Voltage Measurement – variation in voltages indicates shorting/ puncturing of capacitor elements.
Capacitance and Tan Delta Measurement
Precautions: Porcelain surfaces need to be thoroughly cleaned before measurements.

56. Secondary Voltage measurement
Periodic measurement to be carried out. In case of doubt, simultaneous measurement to be carried out with another feeder/ Bus CVT.
For 400kV CVTs puncturing of one Capacitor element in C1 side is likely to increase Secondary Voltage by about 0.35 – 0.45% (0.22 – 0.28V)
Failure of one Capacitor element in C2 side is likely to decrease Secondary Voltage by 5 – 6% (3.2 – 3.8V)

57. Capacitance and Tan delta measurement of stacks
Change in Capacitance value above 6%, CVT need to be replaced
Tan delta values more than from pre-commissioning value needs replacement

58. Low/ Negative values of Tan Delta
Lower values of Capacitance and Tan Delta in case of CVTs are due to parallel tapping at Intermediate point (20-22kV). At this point, there will be two parallel paths. Hence measured current flowing through C2, will be less than C1 and lower value of measured capacitance. Due to losses in Voltage Transformer Winding, Tan Delta value may also be less or even negative.
Hence, values should be compared w.r.t.pre- commissioning values only and not with factory values

59. Thank you
Randhir Singh