Page:The Bell System Technical Journal, Volume 1, 1922.pdf/42

arc. Following its extinction, there is a further action of the resonant system consisting of the coil and the total capacity to ground which acts in such a way as to prevent any over-voltage and to restore the normal potentials to ground gradually, thus tending to prevent the arc from restriking. This action is as follows:

Fig. 2—Vector Diagram Showing Relations of Voltages and Currents at and Following Extinction of Arc

Referring to Fig. 2, the vectors E₁₀, E₂₀, E₃₀ represent the emfs. impressed between lines and neutral by the transformer bank. I_c, I_n, I_f are respectively the admittance current to the fault, the current supplied by the reactor to the fault, and the total fault current. The fault is assumed to be on phase 3. The arc will go out when I_n and I_c have nearly their maximum values. Their instantaneous values are, however, exactly equal and opposite in the sense indicated by the arrows in Fig. 1, i.e., regarded as currents fed to the fault by the two parallel circuits (1) coil-fault-faulty wire-E₃₀ and (2) admittance of sound phases-fault-faulty wire-transformer bank. These instantaneous currents are exactly equal in magnitude and are in the same direction in the single series circuit consisting of coil, transformers, admittance to ground of the three phases in parallel, and ground. Thus the condition in this series resonant circuit at the instant of extinction is that of an established free oscillation, the energy of the oscillation being at this instant wholly electromagnetic.

The voltage across the reactor due to the current I_n, in the direction of E₃₀ (Fig. 1) is represented in Fig. 2 by the vector E_0g. This is 180° out of phase with E₃₀ and initially of the same amplitude. At the instant the arc goes out, both are practically zero. As the oscillation progresses, E_0g dies away, due to damping, and the resultant