Why CT Secondary Should Not Be Kept Open?
The electrical power
system load current always flows through current
transformer primary; irrespective of whether the current
transformer is open circuited or connected to burden at its secondary.
If CT secondary is open circuited, all the primary current
will behave as excitation current, which ultimately produce huge voltage. Every current
transformer
has its won non-linear magnetizing curve, because of which secondary open circuit voltage
should be limited by saturation of the core. If one can measure the rms voltage
across the secondary terminals, he or she will get the value which may not appear to be dangerous. As the CT primary current
is sinusoidal in nature, it zero 100 times per second.(As frequency of the current
is 50 Hz). The rate of change of flux at every current
zero is not limited by saturation and is high indeed. This develops extremely high peaks or pulses of voltage. This high peaks of voltage
may not be measured by conventional voltmeter
. But these high peaks of induced voltage
may breakdown the CT insulation, and may case accident to personnel. The actual open-circuit voltage
peak is difficult to measure accurately because of its very short peaks. That is why CT secondary should not be kept open.
This is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage
of rated frequency applied to the secondary terminals of current
transformer, with other winding being open circuited, which when increased by 10% cause the exiting current
to increase 50%. The CT core is made of CRGO steel. It has its won saturation level.
The EMF induced in the CT secondary windings is
E2 = 4.44φfT2
Where, f is the system frequency, φ is the maximum magnetic flux
in Wb. T2 is the number of turns of the secondary winding. The flux in the core, is produced by excitation current
Ie. We have a non-liner relationship between excitation current
and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B - H curve. Again from the equation above, it is found that, secondary voltage
of a current
transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage
and excitation current
as shown below.
It is clear from the curve that, linear relation between V and Ie is maintained from point A and K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.
