DC Ground fault detector wherein fault is sensed by noting imbalance of magnetic flux in a magnetic core

In a dc ground fault detector for connection to a dc system to detect any leakage or ground fault current in the system;

• a high permeability magnetic core having a strong monlinear characteristic;
• two windings wound about the magnetic core in a solenoidal configuration to form a coil and core combination which is a differential transformer, that is, in operation electric current flow in one winding of the two windings creates a magnetomotive force which is substantially equal and opposite to a magnetomotive force due to an equal electric current flow in the other winding of the two windings, any unbalance in the currents in the two windings acting to saturate the core;
• sensing means to note any differential between the magnetomotive force generated by said one winding and the magnetomotive force generated by said other winding due to any unbalance in electric current flow in the two windings and operable to relate said differential to the leakage or ground fault current in the system, the high permeability and strong nonlinear characteristic of the magnetic core serving to exaggerate the differential caused by any unbalance in the electric current flow in the two windings, which sensing means comprises a third winding in the form of a low current tickler coil wound about the magnetic core and which further includes means to apply an alternating voltage across the third winding to create alterating electric current flow therein, which means to apply includes means operable to sense threshold values of the alternating electric current flow in the third winding and operable to reverse the applied voltage whenever the electric current flow exceeds a predetermined value, saturation of the magnetic core acting to increase the frequency of the alternating voltage;
• and measuring means connected to receive as input thereto the alternating voltage and operable to relate the frequency thereof to leakage or ground fault.

A dc ground fault detector for connection to a dc system to detect any leakage or ground fault current in the system, that comprises:

• a high permeability magnetic core having a strong nonlinear characteristic;
• two windings wound about the magnetic core in a solenoidal configuration to form a coil and core combination which is a differential transformer, that is, in operation electric current flow in one winding of the two windings creates a magnetomotive force which is substantially equal and opposite to a magnetomotive force due to an equal electric current flow in the other winding of the two windings;
• and
• sensing means to note any differential between the magnetomotive force generated by said one winding and the magnetomotive force generated by said other winding and operable to relate said differential to the leakage or ground fault current in the system, which sensing means comprises a third winding wound about the magnetic core, a resistor and inductance in series therewith and a source of a variable-duty-cycle voltage square wave which is applied across the third winding, the resistor and inductance combination to effect electric current flow therein, said source comprising a comparator connected to sense the voltage across the resistor due to electric current flow therein and operable to reverse polarity of the square wave whenever a pre-set threshold value of current exists in the third winding, the frequency of the current in the third winding being affected by the state of saturation of the magnetic core, which state of saturation is affected by any said differential, so that the frequency of the square wave indicates the presence or absence of said differential, and means to relate said frequency to presence or absence of said differential, the presence of said differential indicating a ground fault.

In a ground fault detector for connection to a dc system to detect any leakage or ground fault current in the system,

• a high permeability magnetic core having a strong nonlinear characteristic;
• two windings having an equal number of turns wound about the magnetic core in a solenoidal configuration to form a coil and core combination which is a differential transformer, that is, in operation electric current flow in one winding of the two windings creates a magnetomotive force which is substantially equal and opposite to a magnetomotive force due to an equal in magnitude electric current flow in the other winding of the two windings, said two windings, in an operating system, being serially connected with the load to-be-monitored being connected therebetween so that the two windings ordinarily carry load current, but, during fault, one winding conducts a current that differs from the current in the other winding, any differential between the currents being conducted by the two windings having an exaggerated effect on the magnetic core because of its strong non-linear characteristic, which results in a net dc flux therein and saturation thereof;
• a tickler coil wound about the magnetic core to achieve magnetic coupling therewith;
• sensing means which includes means to apply an alternating current to the tickler coil, which current changes polarity every time the current in the tickler coil reaches a threshold amplitude, the period of the alternating current being a function of said differential;
• and
• means connected to detect the alternating current and operable to note the period thereof and to relate said period to leakage or ground fault current in the system.