High Impedance Differential protection system versus Low Impedance Differential protection system

High Impedance Differential (HID) is a classical busbar protection system that has provided a high degree of reliability in worldwide use over the past six decades. During the past five years, however, there has been a growing movement to replace HID with state-of-the-art numerical Low Impedance Differential (LID) protection systems. This raises a question: Given HID's record of outstanding service, is it necessary or advisable to replace this remarkable classical protection system with the new numerical LID? Or put another way, is the LID as good as classical HID? This paper attempts to answer this question based on a detailed theoretical and practical application analysis of both HID and LID protection systems. HID's record of outstanding performance speaks for itself. But LID's advantage is its ability to easily adapt to any application. One of the main reasons for developing LID schemes was to overcome “applications shortcomings” of classical HID schemes, primarily the need for an equal CT ratio and direct switching of CT secondary currents. This alone argues for a transition to LID protection, though it must be noted that LID has its own very different problems. This paper presents performance comparison results and draws conclusions that can be used as a guide in making a final selection of either protection system.