Protection Engineering Basics and Schemes Protection Engineering | Page 280

Negative sequence current cause magnetic rotating field
but this field is in opposite direction to normal, this
opposite direction magnetic field induces double
frequency current in rotor. This double frequency current
causes excess heat in rotor and result in possible damage
to rotor.
Usually relays have provision to circuit from negative
sequence current by means of measuring negative
sequence current and then compare this negative
sequence current with preset negative sequence current,
other option is to monitor the difference between
maximum and minimum current, for example at
pg. 280