Sizing Knee point voltage - General question.
Sizing Knee point voltage - General question.
(OP)
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
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





RE: Sizing Knee point voltage - General question.
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.
RE: Sizing Knee point voltage - General question.
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