CT Dimensional factor Kx

[tsknan]

Can anybody explain what is dimensional factor in Current transformer?what is its significance?what is the effect on CT saturation with low/high Kxetc.

 

[Ohmly]

This might depend on the standard or document you are looking at, but it normally is a multiplication to allow for DC offset, this depends on the system X/R ratio

 

[tsknan]

Dear ohmly,

Thanks for your reply,
but can you put some more light on this issue?

tsknan

 

[tsknan]

please clarify in detail

 

[Ohmly]

It is also called asymmetrical current

This is a link with an explanation

http://www.skm.com/newsletters/winter99.pdf

- see page 2.

If you do some more googling you wil find further details

 

[Ohmly]

To add a little more, normally CT specification formulas usually talk about maximum secondary fault current but this is usually the steady state fault current with no offset.

The dimensioning factor normally caters for this. It's actual value depends on what equipment is connected to the CT.

If you need a more detailed response, I suggest a more detailed question !!

 

[tsknan]

With omicron CT analyser we tested 2 220KV cts,
one having Kx 26 & other having 75
both are having Knee point #voltage 1600 (Name plate),
actual 1800&2400respectively,
one is STR=26.1 other 40KA
during a fault of 34KA what will the difference in behaviour of CTs ,is there any chance of CT saturation?
here we came across with question of Kx and Ktd(Over dimensioning factor)can you explain?the Cts are used in Bus Differential protection,oneof the CT is suspected to have malfunctioned during a flashover fault.

 

[randyman]

The Kx is a dimensioning factor used in calculating the minimum knee point voltage for class PX (IEC 60044-1) and is only part of the calculation. In full, the min. Vk >= Kx * Is * (Rct + Rb). So the secondary winding resistance of the CT and the secondary burden come to play in this calculation. To say whether or not your CTs will saturate or not is impossible to answer without knowing the specific CT parameters, burden, and the primary circuit parameters.

The person specifying the CT in the first place must have taken all of this into consideration.

Are both CTs (per the N/P) supposed to be identical??