A percentage differential protection scheme is to be designed for a 20 MVA, 22/0.4 kV distribution transformers. The primary windings are connected in delta while the secondary windings are connected in star. The transformer is designed to handle certain overloading. assume overloading 20% capacity for the transformer.

Consider the CTs 5 A secondary current rating;

1-Choose appropriate ratios for the CTs to be used in the protection scheme. Consider the CTs 5 A secondary current rating

2- Determine pilot wire currents on primary as well as secondary side

3- Identify whether interposing CTs are necessary. If they are necessary, outline their specifications and exact location.

4- Sketch the complete protection scheme showing primary/secondary windings for the transformer, CTs used on primary/secondary sides and exact position of interposing CTs;

For the differential relay of 35% setting, determine whether the relay will operate at 35% overload condition.