It is difficult to construct ammeters and the current coils of watt meters,energy meters and relays to carry higher(>100A) alternating currents and if the voltage of the system exceeds 440V, it is dangerous to connect such instruments directly to the high voltage. These difficulties are overcome by using current transformers (C.T) and Potential transformer(P.T).
H1-H2 primary and X1-X2 secondary of CT
CT's are specified by current ratio, burden, window size, type of insulation and CT accuracy class.
Accuracy Classes depends on the application of the C.T i.e. Metering or Protection.
CT's are specified by current ratio, burden, window size, type of insulation and CT accuracy class.
Accuracy Classes depends on the application of the C.T i.e. Metering or Protection.
Burden:The load on an instrument transformer (PT or CT) is referred to as the burden.
Metering accuracy CT’s are used where a high degree of accuracy is required from low load up-to
full load values. An example of this application would be the current transformers utilized by utility
companies for revenue billing.
Maximum Ratio Error Classes of: +0.3% , +0.6% , +1.2%, +2.4%.
For Burdens (Loads) of: 0.1, 0.2, 0.5, 0.9, 1.8 ohms.
Which equals 2.5, 5.0, 12, 22-1/2, 45 volt amperes.
Since Power = I^2 x R, use 5 amp secondary for I, and burden value for R.
Which equals 2.5, 5.0, 12, 22-1/2, 45 volt amperes.
Since Power = I^2 x R, use 5 amp secondary for I, and burden value for R.
Metering CT can be designated by typical number shown below:
Relaying accuracy CT’s are used for supplying current to protective relays. In this application, the relays do not normally operate in the normal load range, but they must perform with a reasonable degree of accuracy at very high overload and fault current levels which may reach twenty times the full load amplitude.
Class C (C for Calculated) is low leakage reactance type -typically Window type units -Formerly Class L
( L for Low Leakage).
Class T (T for Tested) is high leakage reactance type -typically Bar type units -Formerly Class H ( H for High Leakage).
Protection CT can be designated by typical number shown below:
C100 CLASS:For example, a C100 CT class means that the CT is capable of passing 20 × 5 A (CT secondary) = 100 A through a standard 1ohm resistor with a current accuracy less than 10%. In more practical terms, it guarantees a 10% accuracy provided the voltage across the total burden does not exceed 100 V. If the burden is less than 1ohm, the CT is within 10% accuracy for more than 20 × I secondary. If the burden is larger then less current will exceeds the accuracy limit.
Class T (T for Tested) is high leakage reactance type -typically Bar type units -Formerly Class H ( H for High Leakage).
Protection CT can be designated by typical number shown below:
C100 CLASS:For example, a C100 CT class means that the CT is capable of passing 20 × 5 A (CT secondary) = 100 A through a standard 1ohm resistor with a current accuracy less than 10%. In more practical terms, it guarantees a 10% accuracy provided the voltage across the total burden does not exceed 100 V. If the burden is less than 1ohm, the CT is within 10% accuracy for more than 20 × I secondary. If the burden is larger then less current will exceeds the accuracy limit.
10P10 CLASS:
The 10P10 class is typically found in
European applications.It indicates 10% accuracy at 10 times rated
secondary current at rated burden. From the burden the rated impedance
can be calculated and consequently the maximum voltage for 10% accuracy.
Selection Guide of CT's ANSI accuracy class and dimensions:
**It is advisable not to kept secondary of CT open circuited i.e without burden while primary is energized, this lead to production of high voltage at CT secondary and in turn burning of CT.