Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Phase comparison
Patent
1985-02-06
1987-02-03
Tokar, Michael J.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Phase comparison
324 83R, G01N 2500
Patent
active
046410880
ABSTRACT:
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 (I.sub.R, I.sub.S, I.sub.T) 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.
REFERENCES:
patent: 4210948 (1980-07-01), Waltz
patent: 4342062 (1982-07-01), Hodges
patent: 4447893 (1984-05-01), Massey
ASEA Aktiebolag
Tokar Michael J.
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