The document discusses the importance of earthing the secondary side of current transformers (CTs) to prevent high voltage due to open circuits and potential insulation damage. It describes the correct polarity connections for CTs and provides details on accuracy classes for metering and protection devices, including measurement errors and effects of burden on performance. Additional notes on the application of potential transformers (PTs) and considerations for protection and metering accuracy are also included.

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