# Switching transient in current transformer

Homework Helper
Gold Member
If a purely inductive circuit is excited with an AC source, a transient occurs before reaching the steady state 90 ophase shift i.e. the current takes some time to shift the phase depending on the switching instant. Is there any such transient in CT?? Suppose the primary circuit is initially off and I mounted a CT on the line. Then if I switched on the power supply, what will be the nature of this transient waveform of secondary current?? How does the primary current come to know how much of it should be the magnetizing current?? Or am I getting it wrong at all?? Thanks in advance..

## Answers and Replies

Hesch
Gold Member
If a purely inductive circuit is excited with an AC source, a transient occurs before reaching the steady state 90 ophase shift i.e. the current takes some time to shift the phase depending on the switching instant.

That's right. If you switch on power at θ = ±90°, there will be no transient ( contrary to θ = 0° ).

Is there any such transient in CT??

The CT must be loaded by some resistance (ohmic shunt). So the impedance, seen into the primary side of the CT, will also be ohmic. Thus there will not be a transient of importance, and you will see a phase shift ≈180° between primary and secondary currents (so that the flux in the CT-core ≈ 0).

• cnh1995
Homework Helper
Gold Member
That's right. If you switch on power at θ = ±90°, there will be no transient ( contrary to θ = 0° ).

The CT must be loaded by some resistance (ohmic shunt). So the impedance, seen into the primary side of the CT, will also be ohmic. Thus there will not be a transient of importance, and you will see a phase shift ≈180° between primary and secondary currents (so that the flux in the CT-core ≈ 0).
If in my original question, the CT were open initially and I somehow managed to turn on the primary and after exact one cycle delay, shorted the secondary at the zero crossing of the primary current. So, for the first cycle, a large emf must have been induced in the secondary (but not high enough to damage the CT). So my question is will the emf induced at previous zero crossing instant be equal to the emf induced at the current zero crossing instant?? Or shorting the secondary will affect it?? Because at the zero crossing instant, how will the CT know how much should be the magnetizing current?

Hesch
Gold Member
the CT were open initially and I somehow managed to turn on the primary and after exact one cycle delay, shorted the secondary at the zero crossing of the primary current.

It depends on at which voltage angle you switch on the primary. Say you switch on the primary at θ = 0 and the CT is ideal, then the current in the primary will never make a zero crossing, because the function af the primary current will be in the form:

I(t) = A*cos(ωt) + A.

This "+ A" is due to the transient (it's a dc-current), and the energy stored in the core = ½*L*I2 will never disappear as the CT is ideal ( where to move this energy away? ).

If you switch the primary on at θ = 90°, then the primary current will zero at θ = 450°, and at that instant the magnetic energy = 0. You may say that the CT is "reset" at this instant.

will the emf induced at previous zero crossing instant be equal to the emf induced at the current zero crossing instant?? Or shorting the secondary will affect it??

I don't quite understand: Previous zero crossing of what? ( voltage, current? ). There is no previous current zero crossing, and if voltage zero crossing is meant and the CT is ideal, there will be no current zero crossing at all.

Please make a sketch of what is meant.

Last edited:
Homework Helper
Gold Member
I don't understand the voltage part. If the primary current is sine wave and I closed the switch of secondary at zero crossing of primary current, what will be the emf induced in secondary?? Will it be high (like when the CT was open initially) or very low? I know the net mmf is very low but i don't understand what happens initailly..

Homework Helper
Gold Member
I don't understand the voltage part. If the primary current is sine wave and I closed the switch of secondary at zero crossing of primary current, what will be the emf induced in secondary?? Will it be high (like when the CT was open initially) or very low? I know the net mmf is very low but i don't understand what happens initailly..