CT dimensioning factor definition 3

[TS Liew]

Hi buddies, I hope you guys can help me understand the definition of current transformer transient dimensionning factor Ktf. It has been defined as the ratio of theoretical max flux over the max instantaneous flux of the ac component. I find this definition more confusing that the term itself. If anyone could illustrate in graphical form, it would be of great help. There's another Kssc, short circuit factor, just as confusing.

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[scottf]

Kssc is easy...it's just the short-time current rating over the nominal current rating. So if the rated primary current is 2000A and the thermal current rating is 40,000A, then Kssc = 40,000 / 2000 = 20. In other words, the "over-current factor".

Think of Ktf as a way to express how quickly the flux density rises...meaning what will the flux density rise to at a specified time.

Also not Ktf is transient factor

Ktd is transient dimensioning factor

Frankly the manufacturer calculation of a TPY core gets pretty complicated. The calculation is all about translating all of the specified parameters into a secondary voltage that the CT has to be able to generate...which is equiv to the CT not saturating during the transient event given the parameters specified and the reclosing cycle specified. For TPY rated CTs, gaps are introduced in the core to reduce the remenance flux...all of this calculation is manufacture-specific, somewhat proprietary in nature, so you won't typically get the entire calculation method from the supplier.

 

[TS Liew]

Hi Scottf, thanks for your sharing,

Let's say a CT has a Ktf=10, what does it mean? If I were to perform a test on such a CT, How do I measure the Ktf of the CT? If the CT is good, should the Ktf be <10 or >10. Some literature define Ktd as Ktf with duty cycle, as such Ktd is derived from Ktf is it?

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[RRaghunath]

CT Dimensioning factor is dependent on the system X/R and is to necessary to ensure that the CT doesn't saturate during transient conditions, causing maloperation.
This is critical for primary (or main) protection that is expected to operate without any intended delay.
All manufacturers advise the factor to be considered while CT sizing, in their technical manuals.