Protection Engineering Basics and Schemes Protection Engineering | Page 273
This negative sequence current is very small in nature and only identified when rotor got heat, what actually happened in thermal over load relay is that the relay isolates negative sequence current with multiple of negative sequence current added into the positive sequence current.
I eq = √I 1 2 + K e I 2
2
Where I 1 is positive sequence and I 2 is negative sequence current, K e is negative sequence recognition factor, this factor is adjustable.
The setting of thermal overload must be set after considering over loading curves, these curves always provided by manufacturers.