A method of measuring negative-phase-sequence currents in a three-phase system

Abstract

A method of measuring the negative-sequence current in an unbalanced three-phase circuit. A four-winding current transformer, in conjunction with a phase shift network and an alternating current ammeter, was used for obtaining a current proportional to, and in phase with, the negative-sequence current.

After calibration, readings were taken of the three line currents in a three-phase three-wire circuit and of the current in the negative-sequence ammeter. The negative-sequence current. was calculated from the line currents and compared with the ammeter reading. The difference between the two values, in most cases, was less than five per cent of the ammeter reading. It was found that. the circuit had to be calibrated for each type of ammeter used.

It was concluded that direct readings of negative-sequence currents can be made with accuracy consistent with commercial requirements.