Over-current relay implementation assuring fast and secure operation in transient conditions

In order to reduce hazardous effects of over current caused by faults, faster operation of over-current protections is desirable which means maximum sensitivity of the over-current relays to the current and a minimum reaction time. This high sensitivity may lead to operation of relays in normal transient events, such as transformers energizing and induction motors starting. Therefore, proper methods must be used to prevent mal-trip of the relays, due to these transient currents. This paper presents a new method to discriminate over currents caused by fault from transformer energizing and induction motors starting. In this method, a criterion function is introduced in terms of variation of fundamental component amplitude of current signal over consecutive segments. The criterion function is then used in over-current protection, and faults are precisely discriminated from non-fault switching. The performance of this method is demonstrated by simulation of different faults and switching conditions on the IEEE 13 bus test system using Electromagnetic Transients Program (EMTP) and field measured data. Published by Elsevier B.V