Patent ID: 8340930

Claim:
An arrangement for protecting equipment of an AC electrical power system comprising: first means configured to detect electrical power parameters of said power system, second means configured to, while considering said electrical power system as an equivalent two machine system having at each of opposite ends thereof one theoretical electric machine with an electro-motive force and an interconnection therebetween, use values of said parameters detected to provide information relating to occurrence of a pole slip in the electric power system defined as a condition in which a mutual angle of the electro-motive forces of said two theoretical electrical machines goes past 180°, and third means configured to, based upon said information, carry out a control for protecting equipment of said electrical power system, characterized in that said first means is configured to measure the frequency of the current and voltage at at least one location in the electrical power system along said interconnection between said two theoretical electric machines, and that the arrangement comprises fourth means configured to use the values of the frequency of the current and voltage measured by said first means to determine whether a power swing has occurred in said electrical power system, a power swing being defined as a change of said angle between said electro-motive forces as a consequence of an occurrence of any disturbances of said electrical power system, and if an occurrence of a power swing has been determined determine whether said measuring location is located on a motor side or a generator side of a potential pole slip electrical centre along said interconnection, said centre being defined as the location where the voltage becomes zero during a said pole slip, and send this information further to said third means for use in said control for protecting the electrical power system, wherein said fourth means is configured to determine that a power swing occurs if said values of the frequency of the current and voltage delivered by said first means show that the measured current phasor start to rotate faster/slower than the measured voltage phasor and the relative angle thereof is continuously increasing/decreasing.