There are a variety of protective relays using different measuring techniques to provide reliable and secure transformer protection. This includes electro-mechanical, solid state and numerical relays. Within each group, various algorithms exist. Advancements made to transformer technology and design over the past 3 decades, have changed the characteristics of the transformer inrush current, and have often introduced incorrect operations in the existing harmonic restraint relays during energization. The paper discusses:

• Comparison between old and newly developed more accurate calculations of peak values, magnitude of the second harmonic, and other parameters of Inrush current
• Design and system parameters which influence the magnitude and wave-shape of inrush current, e.g. winding design and connections, core material, core joint geometry, short-circuit capacity of network, etc.
• Impact of transformer design & performance parameters and new developments in the Transformer technology on magnitudes and nature of inrush current
• Current transformer influence on the speed, selectivity, and dependability of transformer protection

The paper addresses these topics, as well as provides solutions for selecting and setting transformer protection.