This PPT contains basics of CT and PT in detailed manner. CT Earthing-Polarity & check-Current direction-Testing-Accuracy class-Burden-Errors

1. CURRENT TRANSFORMER(CT)

2. WHY CT S2 SHOULD EARTHED
• If secondary side of CT Open
Circuited(OC) the secondary
side current will be zero(I=0) and
the voltage will be very high
because of primary.
• It leads to Insulation damage
• To avoid the capacitive coupling
between primary and secondary
of CT.

3. CT POLARITY & I FLOW

4. NOTE:
• All CT’s are subtractive
• H1-should face primary current
Line facing direction
• H2-should face primary current
Load facing direction
• X1-secondary current
• Subtractive polarity-H1&x1 same
side of T/F

5. POLARITY CHECKING METHOD

6. PROCEDURE
• Connect analog voltmeter in secondary of CT(x1- +ve, x2- -ve)ire
• Run the piece of wire through the CT and consider as a primary then
connect the 9V battery (H1- “+ve”, H2- “-ve”) in primary wire using
switch.
• NOTE-avoid continuous contact because it will short circuit(SC) the
battery.
• If polarity is correct small deflection in +ve side in voltmeter if –ve the
polarity is reversed so x1&x2 to be reversed

7. ACCURACY
TYPES
METERING:
1. 0.2 or 0.2s class
Where,
0.2- 0.2% error
Class- 0.1,0.2,0.2s,0.5s,0.5,1&3
What is 0.2 & 0.2s?
0.2- 0.2% error in b/w 5% to 120% of rated current
0.2s- 0.2% error in b/w 1% to 120% of rated current

8. PROTECTION:
2.5P10
Where,
5-Accuracy of Ratio error (in %)
P-Protection class
10-ALF (Accuracy Limit Factor)
ALF for 5P10 – CT will give ratio error of 5% even if fault I
reaches 10 times the nominal value after CT will saturate.
AC-5P ,10P
ALF-5,10,15,20,30

9. 3.PS (Protection Special PX)
Use:
Differential Relay
Distance Relay
Where,
Vk-Knee point V
Ie or Im-Excitation I

10. CT BURDEN

11. INTERPOSING CT(ICT)

12. ICT

13. IN 3MW
• SEF – CTR 50/1A, 5P20, 10VA
• REF – CTR 50/1A, CBCT
• Rev. Power relay – CTR 250/1A, CLASS 0.2, 30VA

14. POTENTIAL TRANSFORMERS(PT)

15. ACCURACY CLASS
Metering-0.1, 0.2, 0.5, 1, 3
Protection-3P, 6P
The error % is guarantied when,
V= 80 to 120%
load=25 to 100% (for 3P,6P-5
to 100%)
Pf=0.8 lag
Application:
• Very acc. Metering - 0.1 to 0.2
• Tariff – 0.2 to 0.5
• Instruments and ctrl- 1
• Protection- 1,3P,6P (UV&OV,
UF&OF, 59D, etc)

16. POLARITY CHECK

17. Errors in PT & Burden
• RATIO ERROR-Error in secondary
voltage
• PHASE ANGLE ERROR-Phase different
b/w primary and secondary
IMPORTANT TO BE NOTE:
Protection-Ratio Error
Metering-Ratio error & Phase angle
error
IMPACT OF BURDEN:
If, Burden increase so,
Py&Sy I increase since,
Py V-const so,
Sy V-decrease so,
If Burden increase - Ratio error will
increase
Burden:
10,15,25,30,50,75,100,150,200,300,400,
500VA @ 0.8lag