6.4.6.1 Function calculation principlesTo make a differential protection as sensitive and stable as possible, the restraineddifferential characteristic is used. The protection must be provided with a proportionalbias, which makes the protection operate for a certain percentage differential currentrelated to the current through the generator stator winding. This stabilizes the protectionunder through fault conditions while still permitting the system to have good basicsensitivity. The following chapters explain how these quantities are calculated.The fundamental frequency phasors of the phase currents from both sides of the generator(the neutral side and the terminal side) are delivered to the differential protection functionby the pre-processing module of the IED.6.4.6.2 Fundamental frequency differential currentsThe fundamental frequency RMS differential current is a vectorial sum (that is, sum offundamental frequency phasors) of the individual phase currents from the two sides of theprotected generator. The magnitude of the fundamental frequency RMS differentialcurrent, in phase A, is as calculated in equation 30:2 2_ [(Re( )) (Im( )) ]Idiff A IAn IAt IAn IAt= + + +ANSIEQUATION2316 V2 EN (Equation 30)One common fundamental frequency bias current is used. The bias current is themagnitude of the highest measured current in the protected circuit. The bias current is notallowed to drop instantaneously, instead, it decays exponentially with a predefined timeconstant. These principles make the differential IED more secure, with less risk to operatefor external faults. The “maximum” principle brings as well more meaning to thebreakpoint settings of the operate-restrain characteristic.max( , , , , , )Ibias IAn IBn ICn IAt IBt ICt=EQUATION2049-ANSI V2 EN (Equation 31)1MRK 502 048-UUS A Section 6Differential protection137Technical manual
