| 摘要 |
To protect electrical three-phase machines from damage on the appearance of an unbalanced load between the phases, so-called negative sequence current protection is used. Conventional measurement of the negative sequence current takes place in filters which contain frequency dependent LC components. Frequency variations in a three-phase supply, because of an unbalanced frequency response for harmonics in the three phase filters, may lead to an erroneous indication of negative sequence current. The invention provides a novel method for determining both the positive sequence and the negative sequence currents to a three-phase load. For grounded systems the zero-sequence current is eliminated. Measuring each phase current (IR, IS, IT) and the phase sequence, the positive sequence and/or negative sequence current are/is determined as the solution of an equation derived in advance from the electrical relationships between the phase currents. The invention also relates to a device for carrying out the method. |
