Patent ID: 8736297

Claim:
A method of producing a fault signal that indicates a single-pole fault or a two-pole fault in a three-phase electrical power supply system, the fault having occurred during a present electrical oscillation in the electrical power supply system, the method which comprises: producing a balance signal during the oscillation, the balance signal indicating whether the oscillation is balanced or unbalanced; carrying out a check by checking the phases of the electrical power supply system for the presence of a fault, and thereby using the balance signal to carry out the check; if the balance signal indicates a balanced oscillation, determining the balanced components from phase currents detected in the power supply system, in the form of a positive phase-sequence system current, a negative phase-sequence system current and a zero phase-sequence system current, and carrying out the check for the presence of a fault by comparing these balanced components; and wherein the check comprises comparing a sum of the zero phase-sequence system current and the negative phase-sequence system current with the positive phase-sequence system current, and identifying a fault if a product of the sum and a factor exceeds the positive phase-sequence system current; and producing the fault signal if a fault has been identified during the check.