A percentage differential protection scheme is to be designed for a Delta-Star connected, 20 MVA, 22/0.4 kV distribution transformer. The capacity of this transformer to handle the overload can be assumed in the range of 20-35%. The CTs to be used for sensing currents in the primary as well as secondary side of the transformer are available with 1A OR 5A rating. Choose appropriate ratios for the CTs to be used in the protection scheme. (ii) Determine pilot wire currents on primary as well as secondary side. (iii) Identify whether interposing CTs are necessary. If they are necessary, outline their specifications and exact location. (iv) Sketch the complete protection scheme showing primary/secondary windings for the transformer, CTs used on primary/secondary sides and exact position of interposing
do the iv) part with figure
A percentage differential protection scheme is to be designed for a Delta-Star connected, 20
MVA, 22/0.4 kV distribution transformer. The capacity of this transformer to handle the overload
can be assumed in the range of 20-35%. The CTs to be used for sensing currents in the primary
as well as secondary side of the transformer are available with 1A OR 5A rating.
Choose appropriate ratios for the CTs to be used in the protection scheme.
(ii) Determine pilot wire currents on primary as well as secondary side.
(iii) Identify whether interposing CTs are necessary. If they are necessary, outline their
specifications and exact location.
(iv) Sketch the complete protection scheme showing primary/secondary windings for the
transformer, CTs used on primary/secondary sides and exact position of interposing
CTs;
(v) For the differential relay of 40% setting, determine whether the relay will operate at 35% overload condition.
(vi) Investigate if the interposing CTs will be necessary if the transformer connections are changed to Star-Star instead of Delta-Star.
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