A method in a three-terminal differential protection includes measuring a terminal phase current (IT1(y), IT2(y), IT3(y)) at each terminal (T1, T2, T3) of a protected zone of the differential protection in each phase (L1, L2, L3) to be protected, and determining a bias current (Ib(y)) of the phase on the basis of the terminal phase currents (IT1(y), IT2(y), IT3(y)). The bias current determination includes subtracting phasor values of the terminal phase currents of two terminals from the phasor value of the terminal phase current of one terminal. The terminal phase current, which is a minuend in the phasor difference computation, is selected on the basis of the phase angles or amplitudes of the terminal phase currents.