Setup Inverse-Time Relay (IDMT) in ETAP

Описание: This is how to setup overcurrent relay in ETAP for inverse-time characteristic.

About the burden of relay, for simplicity, during simulation, I don't usually change it unless something is not working (e.g. if the tripping time is not accurate).

To calculate burden, here is a simple example:

Rating of 15VA/5P/10 means
VA_rated = 15VA
ALF_rated = 10 (accuracy limiting factor)
CT ratio of 200/5 means
I_1 = 200
I_2 = 5 = I_rated

Let's say FC=3kA, to accommodate this amount of fault current,
ALF_new = FC/I_1 = 3k/200=15

To achieve this higher ALF, we do so by manipulating z.
ALC_new = ALF_new * I_rated = 15 * 5 = 75 (accuracy limiting current)

V_knee = ALF_rated * V_rated
= ALF_rated * VA_rated/I_rated
= 10 * 15/5
= 30V

z_new = V_max/ALC = 30/75 = 0.4 ohm

Expressing this z in terms of VA (burden), we'll get
VA_new = I_rated^2 * z_new = 5 * 0.4 = 10VA