Current transformer ratio test

Thread Starter

Ubaid Ansari

Joined Oct 10, 2022
7
I am Using a variac - loading transformer - current transformer.
what happens is when CT secondary kept open, variac and loading transformer is unable to inject the current to CT primary. Why it is so while CT secondary has no role impacting its primary circuit. Please help me to understand this phenomenon.

Variac is single phase auto transformer
Loading transformer rating I/p 230V ac 50 Hz and output 5v, 1000A
CT ratio under test - 200/5
 

crutschow

Joined Mar 14, 2008
34,047
what happens is when CT secondary kept open
Why are you opening the secondary of the CT?
The secondary of a CT should never be open as that can cause secondary voltages high enough to break down the winding insulation.
The secondary must always have a low impedance connected (or shorted).
 

Ian0

Joined Aug 7, 2020
9,503
A 200/5 CT (assumed to mean 200A primary 5A secondary) doesn't have a primary winding it just has a hole through the middle where the wire goes. So I don't quite follow what you mean by "unable to inject the current to CT primary".

I have also found with current transformers of that size, that operating them open-circuit doesn't result in high secondary voltages, because the magnetising inductance is present as a load impedance. On a 100A/1A CT I measured about 18V with the secondary open circuit.
 

Thread Starter

Ubaid Ansari

Joined Oct 10, 2022
7
Why are you opening the secondary of the CT?
The secondary of a CT should never be open as that can cause secondary voltages high enough to break down the winding insulation.
The secondary must always have a low impedance connected (or shorted).
Thank for the reply. It was not intentional. Actually i was performing that CT ratio test, able to measure the primary injected current using the clamp meter momentarily/intermittently and then primary injected current is reduced to zero even though the primary circuit is under energized . Upon investigation CT S2 wire was loosely terminated at CT terminal itself. So, when the S2 terminal contact is making momentarily, i was able to measure the current and when the contact got loosen/broken, current on the clamp meter reduced to zero immediately. My doubt is why the primary current gets zero at the CT secondary opens.
 

Thread Starter

Ubaid Ansari

Joined Oct 10, 2022
7
A 200/5 CT (assumed to mean 200A primary 5A secondary) doesn't have a primary winding it just has a hole through the middle where the wire goes. So I don't quite follow what you mean by "unable to inject the current to CT primary".

I have also found with current transformers of that size, that operating them open-circuit doesn't result in high secondary voltages, because the magnetising inductance is present as a load impedance. On a 100A/1A CT I measured about 18V with the secondary open circuit.
I meant, At CT open secondary, The equipment( Variac to loading transformer making closed circuit/loop with the bus bar/conductive wire inserted via CT aperture/window ) is unable to inject the current to closed loop ( CT primary). But the same equipment is able to inject the primary current to the closed loop at CT closed secondary. Why? Please peruse and revert. A lot thanks
 

Ian0

Joined Aug 7, 2020
9,503
I meant, At CT open secondary, The equipment( Variac to loading transformer making closed circuit/loop with the bus bar/conductive wire inserted via CT aperture/window ) is unable to inject the current to closed loop ( CT primary). But the same equipment is able to inject the primary current to the closed loop at CT closed secondary. Why? Please peruse and revert. A lot thanks
There is absolutely no way that a CT can affect the current flowing through its primary aperture by any meaningful amount. You must be connecting something wrongly or measuring it wrongly. Please post a circuit diagram and the spec for the current transformer.
 
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