2I2 = (IL1+alpha IL2+alpha IL3)/3× ×IECEQUATION2406 V1 EN (Equation 52)where:IL1, IL2 and IL3 are fundamental frequency phasors of three individual phase currents.alpha phasor with an angle of 120 degrees.The polarizing current is pre-processed by a discrete fourier filter. Thus the phasor ofthe fundamental frequency component of the polarizing current is derived. Thisphasor is then multiplied with pre-set equivalent zero-sequence source Impedance inorder to calculate equivalent polarizing voltage UIPol in accordance with thefollowing formula:0sUIPol Z IPol (RNPol j XNPol) IPol= × = + × ×EQUATION1877 V2 EN (Equation 53)which will be then used, together with the phasor of the operating directional current,in order to determine the direction to the earth fault (Forward/Reverse).In order to enable current polarizing the magnitude of polarizing current shall bebigger than a minimum level defined by setting parameter IPolMin.Dual polarizingWhen dual polarizing is selected the function will use the vectorial sum of the voltagebased and current based polarizing in accordance with the following formula:( )0sUTotPol=UUPol UIPol=UPol Z IPol UPol RNPol jXNPol Ipol+ + × = + + ×IECEQUATION2408 V1 EN (Equation 54)Upol and Ipol can be either zero sequence component or negative sequencecomponent depending upon the user selection.Then the phasor of the total polarizing voltage UTotPol will be used, together with thephasor of the operating current, to determine the direction of the earth fault (Forward/Reverse).8.2.7.3 External polarizing for earth-fault functionThe individual steps within the protection can be set as non-directional. When thissetting is selected it is then possible via function binary input BLKSTx(where xindicates the relevant step within the protection) to provide external directionalcontrol (that is, torque control) by for example using one of the following functions ifavailable in the IED:1. Distance protection directional function.2. Negative sequence based overcurrent function.1MRK 502 048-UEN A Section 8Current protection195Technical manual
