Current transformers, or CTs, are devices that use to measure electric current. They are commonly used in electrical power systems to measure the current flowing in high-voltage transmission lines, in transformers and in other electrical equipment. CTs are important components in power distribution systems, as they provide a means to measure and monitor the electrical load on the system.
a. Rated Primary current:
The value of primary current on which the primary performance of the current transformer is specified by the manufacturer.
b. Rated short-time current (primary):
It is defined as the R.M.S. value of a.c. component which the CT can carry for rated time without damage due to thermal or electrodynamic stresses.
c. Rated secondary current:
The value of the secondary current is marked on the rating plate.
d. Rated existing current:
The R.M.S. value of current of taken by the secondary winding of a C.T. when the sinusoidal voltage of rated frequency is applied to the secondary with primary winding open-circuited.
e. Rated Burden:
The burden assigned by the manufacturer at which the C.T. performs with specified accuracy.
f. Current error or Ratio error:
The percentage error in the magnitude of the secondary current is defined in terms of current error.
g. Phase angle error:
The phase angle between the primary current vector and the reversed secondary current vector.
h. Composite error:
The r.m.s. value of the difference (Knits – Ip) integrated over
one cycle under steady conditions, given by
Where Kn = Rated transformation ration
IP = Primary current, r.m.s.
iP = Primary current, instantaneous
is = Secondary current, instantaneous
T = Time of one cycle, in sec.
i. Accuracy class:
The class is assigned to the current transformer with the specified limits of ratio error and phase angle error.
j. Over-current factor:
The ratio of rated short-time current to rated primary current.
k. Insulation level (primary):
The insulation level of the C.T. refers to the withstand values of
- Power frequency withstands voltage.
- Impulse withstand voltage.