In bias differential protection for transformer, There are two basic settings required to setup the percentage biased differential protection. 1. Minimum pickup 2. The slope.
The minimum pick-up covers the transformer magnetizing current typically 1-4% of transformer rating. A pick up of 0.2 to 0.3 times the TAP is recommended for general cases.
The slope is defined by determining the potential sources of differential current.
Bus Type - Known Parameter - Unknown Parameter Load Bus -P, Q - V, phase angle Generator Bus - P, V (magnitude) - Q, Voltage phase angle Slack Bus Voltage - magnitude and phase angle - P, Q
There is one IEC guideline, number is IEC 599, Statement of guideline is that "If the ratio of carbon dioxide and Carbon monoxide in Dissolve gas analysis test result is more than 11, it is predict that the condition of paper insulation inside the transformer is poor." For healthy condition this ratio will be between 4 to 11.
Intern turn winding fault can be easily detect by differential protection relay. Also detect by buchholz relay (gas operated relay), cause buchholz relay is used to detect the internal fault of transformer.
EXAMIANS