Insulation monitoring system of a direct current power supply system

Claim 1

An insulator monitoring apparatus for detecting an insulation ground fault in an insulated DC power supply system having DC power supply means and an insulated conductor, comprising: an AC power supply having a supply line connected to said insulated conductor and a return line connected to ground so that no alternating current propagates in said conductor unless a leakage current to ground is present along said conductor creating a return path via ground, wherein an alternating current induced by said AC power supply on to said conductor has a magnitude of less than approximately 100 mA and greater than approximately 1 mA; Hall effect current sensor means having an air-gap toroid and said conductor as a primary winding for measuring a direction and a magnitude of a flux induced on to said conductor by said DC power supply means and producing a signal indicative thereof; direct current generating means connected to receive said indicative signal for producing, in response thereto, a direct current proportional to said magnitude of said flux; flux compensation means including a measurement toroid having said conductor as a primary winding and a compensating winding, connected to receive said proportional direct current, as a secondary winding for inducing on to said conductor a flux approximately equal in magnitude and opposite in direction to said flux induced on to said conductor by said DC power supply means so as to counteract and essentially nullify the latter; alternating current measuring means including said measurement toroid and a measurement winding as a secondary winding for measuring said alternating current propagating on said conductor and for indicating when said alternating current is present, the presence of said AC current indicating that said insulation ground fault has occurred; wherein said measurement toroid is comprised of magnetic material having a flat hysteresis cycle such that said toroid is provided with incremental permeability characteristic of magnetic material having said flat hysteresis cycle so that a flux created by a residual current that is equal to or less than about 1% of a rated current for said DC power supply means has only a negligible saturation effect on said toroid, and thereby does not impede the ability of said toroid to accurately measure said AC current.