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There is no natural zero crossing with DC currents so it is necessary to artificially generate such a crossing. The
proposed scheme is conceptually different from any of the presently utilized methods.
The invention uses an isolating transformer with a primary winding and mechanical switch in series with the DC load;
the only losses from such an arrangement are the DC resistances of the switch and of the winding itself, generally small.
The secondary winding is connected through a switch, either solid state or mechanical, to a pre-charged capacitor; the
secondary winding is connected so that current through the winding opposes the current through the primary winding.
A separate, high-impedance, power supply is used to maintain the voltage on the capacitor.
The device operates by causing a reverse current to flow in the primary winding when the switch is activated. With
proper selection of capacitor size the current is sufficient to cause the primary current to fall to below zero as the
beginning part of a ringing transient. At the instant the current crosses the zero point a mechanical switch is operated
to interrupt the main current in the primary.
The turns-ratio of the primary to secondary can be used to advantage to reduce the voltage requirement at the
secondary side. In this way a relatively low capacitor voltage can be utilized to interrupt a high level of DC voltage.
Energy stored in the capacitor must be at least equal to the energy stored in the magnetic field supported by the DC
current plus any saturation energy of the transformer core.
A third winding can be introduced into the transformer so that saturation effects can be eliminated or reduced. The
DC current through this winding is arranged to oppose the current flow in the primary. This current, typically small,
can be dynamically modulated to allow reduction of saturation effects over a wide range of primary current values.