When a custom curve protection is selected from the list and added to Breaker unit, the user can configure its parameters. The custom curve is used when none of the protection fits user needs, or user has to add a dynamic protection curve, or user only has a protection curve on paper. The custom curve can be defined two ways – by defining a curve formula or by defining few points on the curve.
To define a curve by formula the user has to enter a correct formula containing variable “I” in it. The user has to fully define the formula with all parentheses and correct mathematical operators. In the following picture normal inverse time formula is defined with 600 A and 1s settings. The user also must define a starting value from which the curve starts and end value at which the curve ends (or leave it at zero and let the curve extend to short circuit current).
To define a curve by points, the user has to enter the base current and a few points in p.u. system from the manufacturer provided data as shown in the picture below. To enter points the user has to double-click on “Current, A” or “Time, s” column cell, enter a value and press “Enter”. There is an option to check “Interpolate” for each point. Use this option if you have a non-linear curve and you want it to be smoother. To get the best result user may change the polynomial power (best use 3-9 values).
