The Petersen earth coil

Accidental arcing grounds on transmission lines constitute the foremost problem to be solved in the transmission of electrical energy over great distances. There has come into use to a limited extent, arcing ground suppressors. This device consists in principle, of a switch in the station which is automatically closed in parallel with the accidental arc at any point out on the system. The parallel path through the switch shunts the current from the arc and thereby extinguishes the arc. This development is not yet completed. This paper gives the results of some experiments on an entirely different device for suppressing accidental grounds — a device that was first advocated by Prof. W. Petersen of Darmstadt, Germany. The essential part of this new apparatus is a suitable reactor connected between the neutral of the circuit and ground. This reactance is chosen of such a value as to neutralize the capacitance of the circuits when an accidental ground of one phase takes place. Under this accidental condition the reactor is electrically in parallel with the active capacitances and, by the well-known fundamental law, the only current that flows to the combination of the inductance and capacitance in parallel is the current necessary to supply the energy loss in the combination. The simplified equivalent conditions are shown in Fig. 4. This energy current can be made very small and it is this relatively small current that passes through the accidental arc to ground. If the ground is of the arcing type, the arc will, under favorable conditions be extinguished, as the energy flowing through the fault is only that necessary to supply the losses in the resonant circuit. If the losses are low, the energy flowing through the fault will be insufficient to support an arc and the voltage of the resonant system is gradually reduced to zero, while the voltage between the former faulty wire and ground gradually rises to normal value. In a comparison of the various methods of grounding and their effects on the operation of a power system, the solid and the low-resistance grounds assume first and second place in the order of desirability. The distinction however, between these two is slight and choice will be determined by local conditions. Either the Petersen earth coil or the critical-resistance ground will assume third place in the order of desirability as the relative advantages and disadvantages of these two are about equal. The advantages of the Petersen earth coil system are: first, the suppression of arcing grounds under favorable conditions; second, the reduction of insulator trouble; and third, small earth current when a fault occurs to ground. The disadvantages are: first high potentials between line and ground due to series resonance; second, maintenance of a series of arcs under unfavorable conditions, that is, resonance and high loss, or large dissonance and either high or low loss; third, difficulty in obtaining selection of the faulty line by means of relay protection; fourth, reduced lightning protection due to the necessity of high settings on arresters; and fifth, increased system insulation due to the shifting of the neutral with abnormals or transients.