Method of measuring the A.C. current in a conductor in an A.C. power transmission network

A method of measuring an AC current in a conductor of an AC power transmission network, the method comprising the steps of:

• obtaining a measurement quantity proportional to the AC current using a current transformer;
• generating digital samples from the measurement quantity using a sampling device, the digital samples including first digital samples and second digital samples, the second digital samples being generated before the first digital samples by a quarter period of the AC current;
• forming first and second initial auxiliary measurement quantities as a function of the first digital samples using first and second FIR filters, the first and second initial auxiliary measurement quantities being normal to each other;
• forming third and fourth initial auxiliary measurement quantities as a function of the second digital samples, the third and fourth initial auxiliary measurement quantities being normal to each other;
• generating an absolute value signal as a function of an amplitude of the measurement quantity;
• generating an angle signal by complex division of a first vector formed from the first digital samples and a second vector formed from the second digital samples, the complex division being performed using the first, second, third and fourth initial auxiliary measurement quantities;
• and
• generating an evaluation output signal as a function of the absolute value signal and the angle signal when the absolute value signal and the angle signal exceed respective predetermined threshold values.