Sizing Knee point voltage - General question.

[Sergejs]

Ok, in my quest of trying to understand knee point voltage I came to a dead end. I do understand that this is a saturation point, I do understand that to test CT for knee point voltage you apply voltage to its secondary until you see a higher change in current (that is where you get Vkp and Im), I know the formula Vkp= K * If/CTR * (RCT + RL + RR), but I do not understand how to size Class X CT. What I mean is:

Say a Utility needs to size Class X CT for Unit protection - do they apply Vkp= K * If/CTR * (RCT + RL + RR) formula? They know If but CTR, RCT, RL and RR are all variables that can be anything - how do we determine them? And what is the difference if we take RCT + RL + RR = X or RCT + RL + RR = Y and get Vkp = Y or Vkp = Z. How do we know that we calculated the right Vkp? I just don't understand.

Vkp - knee point voltage
If - fault current
K- koeficient (usually 2)
CTR - ratio
RCT - ct resistance
RL - wire resistance
RR - relay resistance

 

[davidbeach]

All you need is to write "C800" and you'll pretty much never have to worry about your CT again. ;-)

I've never really understood this IEC fascination with calculating knee points for every CT application.

The CTR is going to be one of a very few choices, pick one. RL and RR will be dependent on the installation. RR is fixed once you've selected the relay. RL is fixed once you have the conduit plan done and decided to use the CT circuit conductor size your standards say you should use. All that's left is RCT; find that value for the CT with the CTR previously selected. Done.

 

[oldfieldguy]

In field work, I've simply checked to make sure that the CT's knee point meets that established curve or David's 'C800' or whatever and that the actual burden produces a voltage at the terminals of less than half that number under 20 times rated current. You can determine this by calculating the component values, i.e., conductor lengths and impedances of lumped elements such as relay or metering components, OR you can pump rated current from the CT terminals through your installed circuit and measure the voltage at the terminals, then calculate that '20 times' value to see if you're in trouble. With modern CT's and relays, unless your conductors are very long, as in outdoor installations, you will seldom find difficulties.

In situations where I had mis-matched CT's, I made sure that with that system's available fault current, neither CT would approach saturation.

This has worked for me in the past.


old field guy