DESC:FIELD OF THE INVENTION The present invention relates to a device and a method for power transformer differential protection based on EFT technique and more specifically to distinguish internal fault current from magnetizing inrush current and CT saturation and provide restraint for power transformer differential relay operation during magnetizing inrush and CT saturation due to external fault condition. BACKGROUND OF THE INVENTION The power transformer is one of the important equipment in a power system, since it handles large amount of power. When it undergoes maintenance due to outage, the economic impacts on power system is very high. Though many factors such as operating condition of power transformer, periodical maintenance, power transformer lifetime and maloperation of protection relay causes power transformer outages, the latter one is significant. Therefore, it is necessary to use an appropriate protection relay to ensure the reliability of power transformer protection. The biased restrained differential relay is widely used for power transformer protection. The operation of biased restrained differential relay operates based on the differential current (ID) and biasing current (IB), which are derived from the CT secondary current of HV and LV side of power transform using equation (1)& (2) after phasor correction with respect to transformer vector group. I_D=|I_1-I_2 | (1) I_B=|(I_1+I_2)/2| (2) Here, dual slopes (slope-1 and slope-2) are used in biased restraint characteristic (BRC),which alters the required current for relay operation (IDO) with respect to IB as shown in Figure 1. The initial setting (IIS) is a constant threshold value for low bias current operation up to IR1 (slope-1 takeover point), where it crosses the single end feed line on BRC plane. IIS also specifies the minimum current required for relay operation during the steady-state magnetizing current flow in the power transformer at no load. When bias current is more than IR1, the bias characteristic adopts the slope-1 which accounts power transformer ratio error due to tap-changer as well as CT errors. It should be noted that the slope-1 is followed till IR2 (slope-2 takeover point) up to which there are no possibilities of CT saturation. The slope-2 is effective in the infinite region beyond the knee point of the operating characteristic of the transformer. The IDO is calculated with respect to IB as per equations (3), (4) and (5). I_DO=I_IS if?I_B