Impact of variable frequency drive and cyclic loading on multi-function motor protection relays

Multifunctional motor protection relays are associated with various challenges/problems when applied to the special purpose motor applications such as variable frequency drives (VFD) and cyclic loading. For the VFD motor applications, these challenges include distorted input signals; varying frequency; current and voltage measurements at load and source side of VFD, respectively; switching of the VFD bypass; etc. For cyclic load motor applications, variation in the current magnitude during a load cycle period can result in mal-operation of the thermal overload protection and less sensitivity to short circuit faults. This paper is reviewing the aforementioned challenges, and is presenting solutions in order to establish a reliable protection solution that is secure and sensitive. To fully analyze the impact of VFD and cyclic load on multifunctional motor protection, various normal and faulted system conditions are simulated in the transient simulator software (PSCAD) in order to validate proper performance of the proposed solutions.