CT Saturation
CT Saturation
(OP)
Could someone please explain exactley what happens when a CT goes into saturation, e.g. does the secondary current drop off as the primary current still increase or does it flatten out, does it deviate from a sine wave.
Thank you
Thank you






RE: CT Saturation
CT Saturation
DC Saturation
When a fault occurs the current usually contains a DC component. The total flux required to produce the offset drives the CT into saturation.
Flux reaches saturation during the positive cycle and the exciting reactance decreases shunting the primary current thus distorting the secondary.
During the negative cycle of the primary current, the core becomes unsaturated.
As the DC component decays the negative cycle of the primary current and flux become greater and the core eventually runs out of saturation during a complete cycle returning to steady state.
The DC component time constant is the X/R ratio of the primary circuit.
RE: CT Saturation
As was mentioned above, this will happen once per ac cycle if the saturation is induced by dc offset. This will happen twice per ac cycle if the saturation is simply due to high ac current magnitude and burden.
I posted an example waveform of ct saturation during induction motor start at the following link. You'll also see an analysis which shows that the distortion ("glitch") occurs at current zero crossing assuming a purely resistive burden.
Thread237-9886
RE: CT Saturation
Nitro is correct about saturation during a fault.
Of course you can also drive a CT into saturation by driving too much current through it under a steady state condition. ANSI/IEEE C57.13, defines saturation for protection cores to be where the error exceeds 10%. In practice, saturation is said to occur at the point that the secondary current versus primary current fails to be linear.
Normally, a CTs performance is evaluated by the excitation curve, which is a plot of secondary terminal voltage versus excitation current. Saturation is said to start at the "knee" region of the curve and it becomes that small increases in the secondary terminal voltage must be brought about by large increases in excitation current.
In "steady-state saturation", the waveform will be sinusoidal (in CTs).
Hope this helps...
RE: CT Saturation
RE: CT Saturation
I hope that once again you will permit me a respectful disagreement. I know you have a lot of experience with instrument transformers but one of your statements sounds incorrect to me.
I don't believe that the output waveform of a ct in saturation can be sinusoidal under any real-world circumstances. During different parts of a cycle the flux will sometimes be above the knee of the curve and sometimes below the knee of the curve. This gives a varying error current (magnetizing current) during different parts of the cycle. And the error current will not be a simple percentage of the input current. The output is therefore distorted and nonsinusoidal, even for a sinusoidal input.
RE: CT Saturation
RE: CT Saturation
RE: CT Saturation
Since you mention deep saturation, I think I know what you're driving at. In deep saturation the fraction of a cycle that the ct spends below saturation is very small. The majority of the waveform might look sinsusoidal except that small fraction of cycle.