1. TO CURRENT TRANSFORMER PERFORMANCE ANALYSIS
INTRODUCTION TO
CURRENT TRANSFORMER
PERFORMANCE ANALYSIS
Hands on workshop developed for field relay techs practical approach

2. Yellow Brick Road INTRODUCTION DEFINITIONS PERFORMANCE CALCULATIONS
RATIO SELECTION CONSIDERATIONS
VARIOUS TOPICS
TEST

3. Z = V/I --- accurate value of I
DISTANCE ~ Z

4. INTRODUCTION
IEEE Standard Requirements for Instrument Transformers C57.13
IEEE Guide for the Application of Current Transformers Used for Protective Relaying Purposes C37.110

5. INTRODUCTION Bushing, internal to Breakers and Transformers
Free standing, used with live tank breakers.
Slipover, mounted externally on breaker/transformers bushings.
Window or Bar - single primary turn
Wound Primary
Optic

6. MAGNETO-OPTIC CT
Light polarization passing through an optically active material in the presence of a magnetic field .
Passive sensor at line voltage is connected to substation equipment by fiber cable.
Low energy output used for microprocessor relays
Eliminates heavy support for iron.

7. DEFINITIONS EXCITATION CURVE EXCITATION VOLTAGE EXCITATION CURRENT
EXCITATION IMPEDANCE

8. DEFINITIONS EQUIVALENT CIRCUIT/DIAGRAM POLARITY BURDEN
TERMINAL VOLTAGE
CLASSIFICATIONS T AND C

9. DEFINITIONS KNEE POINT RELAY ACCURACY CLASS MULTI-TAPS ACCURACY
SATURATION ERROR - RATIO/ANGLE

10. EXCITATION CURVE

11. Ve = EXCITATION VOLTAGE Vef Ie = CURRENT (read a few values)
EQUIVALENT DIAGRAM Ip Ie Ze Xp Rp e Rs
Sec g h c d
Pri Is f
Ve = EXCITATION VOLTAGE Vef
Ie = CURRENT (read a few values)
Ze = IMPEDANCE
Vt = TERMINAL VOLTAGE Vgh
POLARITY - next

12. TYPICAL EXCITATION BBC CURRENT vs VOLTAGE
V (volts) Ie(amps) Ze(ohms)

13. V (volts) Ie(amps) Ze(ohms)
CURRENT vs VOLTAGE
V (volts) Ie(amps) Ze(ohms)

14. Rsec
Zint I2 I1
Ie+I2 { Ie RB
EXTERNAL
BURDEN N1 N2 I1 Ze LB
POLARITY

15. DEFINITIONS EXCITATION CURVE EXCITATION VOLTAGE EXCITATION CURRENT
EXCITATION IMPEDANCE
EQUIVALENT CIRCUIT/DIAGRAM
BURDEN - NEXT

16. BURDEN The impedances of loads are called BURDEN
Individual devices or total connected load, including sec impedance of instrument transformer.
For devices burden expressed in VA at specified current or voltage, the burden impedance Zb is:
Zb = VA/IxI or VxV/VA

17. { EXTERNAL BURDEN RB BURDEN = VA / I² LB
Burden: A = …….. Ohms
A = …….. Ohms { RB
BURDEN =
VA / I² LB

18. RB QUIZ I2 CT winding resistance = 0.3 ohms
Lead length = 750 ft # 10 wire
Relay burden = 0.05 ohms

19. DEFINITIONS
CLASSIFICATIONS T AND C

20. ANSI/IEEE STANDARD FOR CLASSIFICATION T & C
CLASS T: CTs that have significant leakage flux within the transformer core - class T; wound CTs, with one or more primary-winding turns mechanically encircling the core. Performance determined by test.

21. CLASS C
CTs with very minimal leakage flux in the core, such as the through, bar, and bushing types. Performance can be calculated.
KNEE POINT

22. DEFINITIONS KNEE POINT IEEE IEC - effective saturation point
Quiz- read a few knee point voltages and also at 10 amps Ie.

23. QUIZ: READ THE KNEE POINT VOLTAGE
45° LINE
ANSI/IEEE
KNEE POINT
Excitation Volts
Knee Point Volts
QUIZ: READ THE KNEE POINT VOLTAGE

24. KNEE POINT OR EFFECTIVE POINT OF SATURATION
ANSI/IEEE: as the intersection of the curve with a 45 tangent line
IEC defines the knee point as the intersection of straight lines extended from non saturated and saturated parts of the excitation curve.
IEC knee is higher than ANSI - ANSI more conservative.

25. EX: READ THE KNEE POINT VOLTAGE
IEC KNEE POINT
ANSI/IEE
KNEE POINT
EX: READ THE KNEE POINT VOLTAGE

26. DEFINITIONS EQUIVALENT CIRCUIT/DIAGRAM
EXCITATION VOLTAGE, CURRENT, IMPEDANCE
TERMINAL VOLTAGE
BURDEN
CLASSIFICATIONS T AND C
EXCITATION CURVE
KNEE POINT IEEE IEC
ACCURACY CLASS

27. CT ACCURACY CLASSIFICATION
The measure of a CT performance is its ability to reproduce accurately the primary current in secondary amperes both is wave shape and in magnitude. There are two parts:
Performance on symmetrical ac component.
Performance on offset dc component. Go over the paper

28. ANSI/IEEE ACCURACY CLASS
ANSI/IEEE CLASS DESIGNATION C200: INDICATES THE CT WILL DELIVER A SECONDARY TERMINAL VOLTAGE OF 200V
TO A STANDARD BURDEN B (2.0 ) AT 20 TIMES THE RATED SECONDARY CURRENT
WITHOUT EXCEEDING 10% RATIO CORRECTION ERROR. Pure sine wave
Standard defines max error, it does not specify the actual error.

29. ACCURACY CLASS C STANDARD BURDEN
ACCURACY CLASS: C100, C200, C400, & C800 AT POWER FACTOR OF 0.5.
STANDARD BURDEN B-1, B-2, B-4 AND B-8 THESE CORRESPOND TO 1, 2, 4 AND 8.
EXAMPLE STANDARD BURDEN FOR C100 IS 1 , FOR C200 IS 2 , FOR C400 IS 4  AND FOR C800 IS 8 .
ACCURACY CLASS APPLIES TO FULL WINDING, AND ARE REDUCED PROPORTIONALLY WITH LOWER TAPS.
EFFECTIVE ACCURACY =
TAP USED*C-CLASS/MAX RATIO

30. AN EXERCISE 2000/5 MR C800 tap used*c-class/max ratio
TAPS KNEE POINT EFFECTIVE ACCURACY
2000/5 ……………….. ……………...
1500/5 ……………….. ……………...
1100/5 ……………….. ……………...
500/5 ……………….. ……………...
300/5 ……………….. ……………...

31. AN EXERCISE
2000/5 MR C tap used*c-class/max ratio
TAPS KNEE POINT EFFECTIVE ACCURACY
2000/
1500/
1100/
500/
300/

32. AN EXERCISE 2000/5 MR C400 tap used*c-class/max ratio
TAPS KNEE POINT EFFECTIVE ACCURACY
2000/5 ……………….. ……………...
1500/5 ……………….. ……………...
1100/5 ……………….. ……………...
500/5 ……………….. ……………...
300/5 ……………….. ……………...

33. AN EXERCISE 2000/5 MR C400 tap used*c-class/max ratio
TAPS KNEE POINT EFFECTIVE ACCURACY
2000/
1500/
1100/
500/
300/

34. CT SELECTION ACCURACY CLASS
POINT OF SATURATION : KNEE POINT
IT IS DESIRABLE TO STAY BELOW OR VERY CLOSE TO KNEE POINT FOR THE AVAILABLE CURRENT.
Recap

35. ANSI/IEEE ACCURACY CLASS C400
STANDARD BURDEN FOR C400: (4.0 )
SECONDARY CURRENT RATING 5 A
20 TIMES SEC CURRENT: 100 AMPS
SEC. VOLTAGE DEVELOPED: 400V
MAXIMUM RATIO ERROR: 10%
IF BURDEN 2 , FOR 400V, IT CAN SUPPLY MORE THAN 100 AMPS SAY 200 AMPS WITHOUT EXCEECING 10% ERROR.

36. Rsec RB EXTERNAL BURDEN LB Zint Isec = 100 I1 Ie <10 N1 N2 I1 Ze
Ie+Isec
Ie <10 RB
EXTERNAL
BURDEN N1 N2 I1 Ze LB
ACCURACY ACLASS: C RATED SEC CURRENT = 5 A
EXTERNALBURDEN = STANDARD BURDEN = 2 .0 OHMS
Ve=200 V Isec = 100 A Ie <10 Amps.

38. PERFORMANCE CALCULATIONS

40. THE REST OF US “SHOW US THE DATA”
BUT
THE REST OF US
“SHOW US THE DATA”

41. PERFORMANCE CRITERIA
THE MEASURE OF A CT PERFORMANCE IS ITS ABILITY TO REPRODUCE ACCURATELY THE PRIMARY CURRRENT IN SECONDARY AMPERES - BOTH IN WAVE SHAPE AND MAGNITUDE …. CORRECT RATIO AND ANGLE.

42. CT SELECTION AND PERFORMANCE EVALUATION FOR PHASE FAULTS
600/5 MR Accuracy class C100 is selected
Load Current= 90 A
Max 3 phase Fault Current= 2500 A
Min. Fault Current=350 A
STEPS:
CT Ratio selection
Relay Tap Selection
Determine Total Burden (Load)
CT Performance using ANSI/IEEE Standard
CT Performance using Excitation Curve

43. PERFORMANCE CALCULATION
STEPS:
CT Ratio selection
Relay Tap Selection
Determine Total Burden (Load)
CT Performance using ANSI/IEEE Standard
CT Performance using Excitation Curve
STEPS:
CT Ratio selection
- within short time and continuous current – thermal limits
- max load just under 5A
Load Current= 90 A
CT ratio selection : 100/5

44. PERFORMANCE CALCULATION
STEP: Relay Tap Selection
O/C taps – min pickup , higher than the max. load
167%, 150% of specified thermal loading.
Load Current= 90 A for 100/5 CT ratio = 4.5 A sec.
Select tap higher than max load say = 5.0
How much higher – relay characteristics, experience and judgment.
Fault current: min: 350/20 = 17.5
Multiple of PU = 17.5/5 = 3.5
Multiple of PU = 17.5/6 = 2.9

45. PERFORMANCE CALCULATION
STEP: Determine Total Burden (Load)
Relay: A and A
Lead: 0.4 Ohms
Total to CT terminals:
(2.64/5*5 = 0.106) = A
(580/100*100 = 0.058) = A

46. Determine Total Burden (Load) CT Performance using ANSI/IEEE Standard
PERFORMANCE CALCULATION
STEPS:
CT Ratio selection
Relay Tap Selection
Determine Total Burden (Load)
CT Performance using ANSI/IEEE Standard
CT Performance using Excitation Curve

47. PERFORMANCE CALCULATION
STEP: CT Performance using ANSI/IEEE Standard Ip Ie Ze Xp Rp e Rs
Sec g h c d
Pri Is
Determine max fault current CT must develop across its terminals gh

48. PERFORMANCE CALCULATION
STEP: Performance – ANSI/IEEE Standard
Vgh = 2500/20 * = 57.25
600/5 MR C100 CT used at tap 100/ effective accuracy class
(100/600) x 100 = ?
CT is capable of developing 16.6 volts.
Severe Saturation. Cannot be used.

49. PERFORMANCE CALCULATION
STEP: Performance – ANSI/IEEE Standard
For microprocessor based relay:
Burden will change from to o.4
Vgh = 2500/20 * 0.4 = 50.0
600/5 MR C100 CT used at tap 100/ effective accuracy class
(100/600) x 100 = ?
CT is capable of developing 16.6 volts.
Severe Saturation. Cannot be used.

50. PERFORMANCE CALCULATION
STEP: Performance – ANSI/IEEE Standard
Alternative: use 400/5 CT tap:
Max Load = 90 A
Relay Tap = 90/80 = Use: 1.5 relay tap.
Min Fault Multiples of PU=(350/80=4.38, /1.5= 2.9)
Relay burden at this tap = 1.56 ohms
Total burden at CT terminals = = 1.96
Vgh = 2500/80 * 1.96 = 61.25
600/5 MR C100 CT used at tap 400/5-- effective accuracy
class is = (400/600) x 100 = ?
CT is capable of developing 66.6 volts. Within CT capability

51. PERFORMANCE CALCULATION STEP: CT Performance using Excitation Curve
ANSI/IEEE ratings “ballpark”. Excitation curve method provides relatively exact method. Examine the curve
Burden = CT secondary resistance + lead resistance + relay burden
Burden = =
For load current 1.5 A:
Vgh = 1.5 * = 3.26 V Ie = 0.024
Ip = ( ) * 80 = 123 A
well below the min If = 350 A (350/123=2.84 multiple of pick up)

52. PERFORMANCE CALCULATION STEP: CT Performance using Excitation Curve
For max fault current
Burden = CT secondary resistance + lead resistance + relay burden
Burden = =
Fault current 2500/80 = A:
Vgh = * = V Ie = 0.16
Beyond the knee of curve, small amount 0.5% does not significantly decreases the fault current to the relay.

53. TEST RB Determine CT performance using Excitation Curve method: I2
CT winding resistance = 0.3 ohms
Lead length = 750 ft # 10 wire
Relay burden = 0.05 ohms as constant
Fault current = 12500A/18000A
CT CLASS = C400/C800
2000/5 MR current transformer
CT RATIO = 800/5

54. AN EXAMPLE – C400 CT RESISTANCE 0.3 OHMS LEAD RESISTANCE 1.5 OHMS
IMPEDANCE OF VARIOUS DEVICES 0.05 OHMS
FAULT CURRENT AMPS
CT RATIO 800/5
ACCURACY CLASS C400
supply curves C400/800

55. CALCULATIONS for A – C400
BURDEN = ( Z-LEAD + Z - CT SEC + D - DEVICES)
Ve = ( ) 12500/160
Ve = VOLTS Plot on curve
Plot on C400

56. CALCULATIONS for –C400
BURDEN = ( Z-LEAD + Z - CT SEC + D - DEVICES)
Ve = ( ) 18000/160
Ve = 209 VOLTS Plot on curve
Plot on C400

57. ANOTHER EXAMPLE C800 CT RESISTANCE 0.3 OHMS LEAD RESISTANCE 1.5 OHMS
IMPEDANCE OF VARIOUS DEVICES 0.05 OHMS
FAULT CURRENT AMPS
CT RATIO 800/5
ACCURACY CLASS C800
supply curves C400/800

58. CALCULATIONS for A – C800
BURDEN = ( Z-LEAD + Z - CT SEC + D - DEVICES)
Ve = ( ) 12500/160
Ve = VOLTS Plot on curve
Plot on C800

59. CALCULATIONS for A –C800
BURDEN = ( Z-LEAD + Z - CT SEC + D - DEVICES)
Ve = ( ) 18000/160
For 18,000 A (Ve =209 V) Plot on curve
Plot on C800

60. FAULT CURRENT MAGNITUDES
KA 8
KA 10
KA 46
KA 35
KA 35
<10 KA
REFER TO PAGE 6 OF PAPER

61. RED DELICIOUS
C400
ZONE1

62. Z = V/A
DISTANCE ~ Z

63. STANDARD DATA FROM MANUFACTURER
ACCURACY:
RELAY CLASS C200
METERING CLASS, USE 0.15%
0.3%, 0.6% & 1.2% AVAIALABLE BUT NOT RECOMMENDED
0.15% MEANS +/- 0.15% error at 100% rated current and 0.30% error at 10% of rated current ( double the error)

64. STANDARD DATA FROM MANUFACTURER
CONTINUOUS (Long Term) rating
Primary
Secondary, 5 Amp ( 1Amp)
Rating factor (RF) of 2.0 provides Twice Primary and Secondary rating continuous at 30degrees

65. STANDARD DATA FROM MANUFACTURER
SHORT TIME TERMINAL RATINGS
Transmission Voltage Applications
One Second Rating = 80% Imax Fault, based on IxIxT=K where T=36 cycles & I=Max fault current
Distribution Voltage Applications
One Second Rating = Maximum Fault Current level

66. RATIO CONSIDERATIONS
CURRENT SHOULD NOT EXCEED CONNECTED WIRING AND RELAY RATINGS AT MAXIMUM LOAD. NOTE DELTA CONNECTD CT’s PRODUCE CURRENTS IN CABLES AND RELAYS THAT ARE TIMES THE SECONDARY CURRENTS

67. RATIO CONSIDERATIONS
SELECT RATIO TO BE GREATER THAN THE MAXIMUM DESIGN CURRENT RATINGS OF THE ASSOCIATED BREAKERS AND TRANSFORMERS.

68. RATIO CONSIDERATIONS
RATIOS SHOULD NOT BE SO HIGH AS TO REDUCE RELAY SENSITIVITY, TAKING INTO ACCOUNT AVAILABLE RANGES.

69. RATIO CONSIDERATIONS
THE MAXIMUM SECONDARY CURRENT SHOULD NOT EXCEED 20 TIMES RATED CURRENT. (100 A FOR 5A RATED SECONDARY)

70. RATIO CONSIDERATIONS
HIGHEST CT RATIO PERMISSIBLE SHOULD BE USED TO MINIMIZE WIRING BURDEN AND TO OBTAIN THE HIGHEST CT CAPABILITY AND PERFORMANCE.

71. RATIO CONSIDERATIONS
FULL WINGING OF MULTI-RATIO CT’s SHOULD BE SELECTED WHENEVER POSSIBLE TO AVOID LOWERING OF THE EFFECTIVE ACCURACY CLASS.

72. TESTING Core Demagnetizing
The core should be demagnetized as the final test before the equipment is put in service. Using the Saturation test circuit, apply enough voltage to the secondary of the CT to saturate the core and produce a cecondary currrent of 3-5 amps. Slowly reduce the voltage to zero before turning off the variac.

73. TESTING
Saturation
The saturation point is reached when there is a rise in the test current but not the voltage.

74. TESTING Flashing Ratio Insulation test
This test checks the polarity of the CT
Ratio
Insulation test