Novel Stepwise and Combined Stepwise- Piece-wise Linear Tripping Characteristics for Coordinating Numerical Overcurrent Relays

Technological developments in protective devices have established the basis for appearance of numerical overcurrent relays (OCRs), which enable the users to apply their desired time-current characteristics (TCCs). This study propounds novel stepwise and combined stepwise-piece-wise linear TCCs and presents clear procedure to utilise them for coordinating OCRs in large meshed power systems. The TCCs keep constant time interval between the main/backup relay pairs within entire fault current range. They are examined first by coordinating the OCRs in a 4-bus power system and by coordinating the ground fault OCRs in a practical 60 kV looped power system. Then, the OCRs of the IEEE 14-bus test system, which is a rather large meshed power system with 39 OCRs and 102 main/backup pairs, are coordinated by using the novel TCCs. The same relays in the aforesaid power systems are coordinated using some non-standard TCCs recently introduced in the literature. The comparison verifies that the lowest operating times are achieved by using the novel TCCs. The devised TCCs are simply formed as look-up tables to be exactly set on the existing industrial relays. This fact is verified by setting them on an industrial OCR, and then, by testing the relay through the current injection method.