Increased reliability over current relay (ocr) as a transformer protection with non-cascade coordination patterns

Abstract

Protection using Over Current Relays (OCR) with cascade coordination patterns is commonly used in 20 kV primary distribution networks to protect transformers from phase to phase fault currents. To anticipate a large fault current, the OCR feeder is equipped with an instant time setting. The problem occurs when the OCR feeder or PMT feeder fails to work then the transformer will be at risk because it is burdened with the fault current for a relatively long time. This study will discuss the use of a non-cascade pattern for the coordination of OCR feeders and incoming OCR in a primary distribution system supplied by a 60 MVA transformer, 150/20 kV from the substation by the calculation method. OCR feeders use very inverse (VI) characteristics and OCR inverses use standard inverse (SI) characteristics. A large feeder fault current is obtained by determining the 3 phase fault current upstream of the feeder as 7 x the nominal current (In) of the transformer. The result is that when the PMT feeder fails to work, the incoming OCR cascade coordination pattern will work with a time of 1.05 seconds. Meanwhile, if we use a non-cascade pattern the incoming OCR will work in the time setting of 0.3 seconds. Work time difference of (1.05 - 0.3 = 0.75 s) shows an increase in protection reliability and reduces the risk of damage to the transformer.