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Rotary-arc circuit breakers (CBs) rely on an externally imposed magnetic field to move the arc column in the arcing chamber, hence reducing contact erosion and enhancing the interruption capability. The three-dimensional computation of the magnetic field was carried out for a prototype 145-kV/40-kA rotary-arc CB and the influencing parameters were identified. It is shown that the presence of an arc column in the arcing chamber, whether at high or low currents, does not impose significant distortion to the magnetic field that has a dominant component in the axial direction and for design purposes, its effects can be neglected. Strong eddy current is induced in the ring-shaped copper arc runner distorting the local magnetic field. Reducing the electrical conductivity of the arc runner material by a factor of three, effectively suppresses the influence of this eddy current on the magnetic field. The assumption of a steadily alternating magnetic field lagging the exciting ac current by a fixed phase angle may not be safe in the design of such CBs since the phase angle varies remarkably in the arcing chamber and the transient effects of exciting current can significantly affect the magnetic field for a period of up to 5 ms.
Date of Publication: Jan. 2006