Patent ID: 8462476

Claim:
A method of protecting an electrical link in a medium, high, or very high voltage network, in which two differential protection devices, or relays, are disposed at the two ends of said link, said method including the following steps: apply a voltage to the relay situated at one of the two ends; measure the outward-and-return propagation time; phase shift the sinusoidal curves of the current received from the other end, on the basis of the ratio 0.5 outward/0.5 return; and first test to find out whether a determined time has elapsed; 1) If result of this first test is negative phase shift the sinusoidal curves of the current received from the other end, on the basis of the ratio 0.5 outward/0.5 return; obtain Fresnel diagrams for the current without servocontrolling the phases at the end; calculate the differential current and the retained current for each phase; second test to find out whether one of the differential currents is in a trip zone; provide the differential protection if result of this second test is positive; 2) If result of this first test is positive and if result of the second test is negative: Test to compare the phase difference ΔΦ between the locally measured current and the current measured remotely, for each phase, with at least one determined value, and optional servocontrol of the phase of each end current at 180° relative to each corresponding local current (step 30 ).