Direct current detection circuit

A direct current detection circuit comprising:

• a zero-phase current transformer having source lines inserted therethrough and being adapted to magnetically detect current differences among said source lines and to vary self-impedance according to changes in magnetic field generated by said current difference;
• a voltage divider resistor connected in series to said zero-phase current transformer;
• a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between said zero-phase current transformer and said voltage divider resistor according to a change in said self-impedance of said zero-phase current transformer;
• and
• a control circuit for detecting a present direct current value based on said comparison voltage value generated by said comparison voltage generating circuit;
• wherein said zero-phase current transformer has said source lines inserted therethrough a plural number of times so as to make it possible to detect on the basis of said comparison voltage value a direct current value in a range which is otherwise difficult to detect accurately on the basis of said comparison voltage value because of influence of hysteresis characteristic of said zero-phase current transformer;
• and
• wherein said control circuit is adapted to detect a current value on the basis of said comparison voltage value generated by said comparison voltage generating circuit and to calculate said present direct current value on the basis of said detected value and the value of a direct current value passing through said source lines passing through said zero-phase current transformer said plural number of times.

A direct grounding current detection circuit comprising:

• a zero-phase current transformer having source lines inserted therethrough and being adapted to magnetically detect current differences among said source lines and to vary self-impedance according to changes in magnetic field generated by said current difference;
• a voltage divider resistor connected in series to said zero-phase current transformer;
• a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between said zero-phase current transformer and said voltage divider resistor according to a change in said self-impedance of said zero-phase current transformer;
• a control circuit for detecting presence or absence of direct grounding current relative to said source lines based on said comparison voltage value generated by said comparison voltage generating circuit;
• an offset current line which is inserted through said zero-phase current transformer;
• an offset current generating circuit for generating an offset current through said offset current line so as to shift a range of current values where it is difficult to accurately detect a direct current value of said direct grounding current because of influence of hysteresis characteristic of said zero-phase current transformer into a detectable range where it is possible to detect the direct current value of said direct grounding current;
• and
• a threshold memory storing an upper threshold value and a lower threshold value to be used in determining presence and absence of said direct grounding current in said detectable range;
• wherein said control circuit is adapted to conclude that there is occurrence of said direct grounding current relative to said source lines if it is judged that said comparison voltage value generated by said comparison voltage generating circuit is equal to or above said upper threshold value or equal to or below said lower threshold value.

A direct grounding current detection circuit comprising:

• a zero-phase current transformer having source lines inserted therethrough and being adapted to magnetically detect current differences among said source lines and to vary self-impedance according to changes in magnetic field generated by said current difference;
• a voltage divider resistor connected in series to said zero-phase current transformer;
• a comparison voltage generating circuit for generating a comparison voltage value based on a divided voltage value obtained between said zero-phase current transformer and said voltage divider resistor according to a change in said self-impedance of said zero-phase current transformer;
• and
• a control circuit for detecting presence or absence of occurrence of a direct grounding current relative to said source lines based on said comparison voltage value generated by said comparison voltage generating circuit;
• wherein said zero-phase current transformer has said source lines inserted therethrough a plural number of times so as to shift a range of direct current values where it is difficult to accurately detect a direct current value of said direct grounding current because of influence of hysteresis characteristic of said zero-phase current transformer into a detectable range where it is possible to detect the direct current value of said direct grounding current based on said comparison voltage value.