ormers. The first slope is usually set to accommodate CT
measurement errors, and the second slope for external fault stability with CT saturation. It
is a common practice to set 0.3 as the first slope, where Class 10P current transformers
are applied for a three winding transformer, and 0.7 to 1.5 for the second slope, to
provide sufficient restraint in order to stabilize the differential element during external fault
conditions.

This paper presents a more sensitive approach for the protection of transformer
windings. The ground differential protection, or restricted earth fault protection compares
the calculated
residual current from the phase CTs with the current measured from a residual
CT of a given winding to determine an internal fault. Detailed analysis and measures to stabilize
the ground differential element during transient saturation of external fault conditions will be
included, comparison with the
overall transformer differential protection during ground fault
conditions will also be presented, and finally an applications example will be given to
demonstrate that ground differential protection is indeed more sensitive, and will provide
better protection to the power transformers.



Contact Information

Dr. J C TAN
Applications Engineer
AREVA T&D Canada Inc.
Automation & Information Systems
3532 - 114 Avenue S.E.
Calgary, AB, T2Z 3V6
Tel.: (403) 236-6058
Fax.: (403) 236-6070
E-mail: jian-cheng.tan@areva-td.com