High Performance Differential Protection , Analog versus Numerical

For bus zone protection applications, it is extremely important to have good security since an unwanted operation might have severe consequences. The unwanted operation of the bus differential relay will have the similar effect as simultaneous faults on all power system elements connected to the bus. On the other hand, the relay has to be dependable as well. Failure to operate or even slow operation of the differential relay, in case of an actual internal fault, can have fatal consequences. Human injuries, power system blackout, transient instability or considerable damage to the surrounding substation equipment and the close-by generators are some of the possible outcomes. These two main requirements are contradictory to each other in nature. To design the differential relay to satisfy these two requirements at the same time is not an easy task. The analog relays have been successfully used for these types of applications for years. Can their numerical counterparts offer the same performance?