DC leakage current detector and method of operation thereof for leakage current detection in DC power circuits
The DC leakage current detector for detecting leakage current in a DC bus includes a pair of transformers each comprising a magnetic core and excitation and detection windings would about the magnetic core, with the magnetic core positionable about a pair of conductors that create a magnetic field that is a sum of currents in the conductors. An excitation and biasing circuit is connected to the excitation winding in each transformer to inject a current signal thereto that creates a changing magnetic flux in the core of each transformer and a detector output connected to the detection winding in each transformer to receive a voltage therefrom generated responsive to the magnetic flux in the core of each transformer, wherein the voltage on the detection windings provides a net voltage at the detector output whose value is indicative of a presence of a leakage current on the DC bus.
1. A DC leakage current detector for detecting leakage current in a DC bus, the DC leakage current detector comprising:
a pair of transformers, each of the pair of transformers including:
a magnetic core comprising an opening positionable about a pair of conductors of the DC bus, the pair of conductors creating a magnetic field in the magnetic core that is a sum of currents in the conductors; and
a pair of windings wound about the magnetic core, the pair of windings including an excitation winding and a detection winding;
an excitation and biasing circuit connected to the excitation winding in each of the pair of transformers to inject a current signal thereto, the injected current signal creating a changing magnetic flux in the core of each of the transformers; and
a detector output connected to the detection winding in each of the pair of transformers to receive a voltage therefrom generated responsive to the magnetic flux in the core of each of the transformers, wherein the voltage on the detection windings provides a net voltage at the detector output whose value is indicative of a presence of a leakage current on the DC bus.
2. The DC leakage current detector of claim 1 wherein the pair of transformers comprise normally unsaturated transformers, with the injected current signal creating the changing magnetic flux to partially saturate the magnetic cores of the transformers.
3. The DC leakage current detector of claim 2 wherein, when no leakage current is present in the conductors, the magnetic flux in the pair of transformers are equal and opposite from one another, such that the net voltage at the detector output is zero.
4. The DC leakage current detector claim 2 wherein, when leakage current is present in the conductors, the magnetic flux in the pair of transformers are unequal from one another, such that the net voltage at the detector output has a non-zero value.
5. The DC leakage current detector of claim 4 wherein, when leakage current is present in the conductors, the magnetic core of one of the pair of transformer is partially saturated for periods of time and the magnetic core of the other of the pair of transformers is unsaturated.
6. The DC leakage current detector of claim 4 wherein, when no leakage current is present in the conductors, the pair of magnetic cores remains in saturation, such that no net voltage is present at the detector output.
7. The DC leakage current detector of claim 4 wherein, when leakage current is present in the conductors, the leakage current counteracts the DC bias in the injected current signal for one of the pair of transformers, so as to cause the one of the pair of transformers to be taken out of saturation, such that the net voltage at the detector output has a non-zero value.
8. The DC leakage current detector of claim 1 wherein the pair of transformers comprise normally saturated transformers, with the injected current signal including a DC bias therein that maintains the pair of magnetic cores in saturation.
9. The DC leakage current detector of claim 8 wherein a magnitude of the DC bias in the injected current signal is controlled based on a known magnitude of leakage current that might be present in the pair of conductors, with the magnitude of the DC bias in the injected current signal being controlled such that the leakage current approximately counteracts the DC bias.
10. The DC leakage current detector of claim 1 wherein the injected current signal comprises a current signal having a sawtooth waveform shape.
11. The DC leakage current detector of claim 1 wherein a number of turns in the excitation winding on one of the pair of magnetic cores differs from a number of turns in the excitation winding on the other of the pair of magnetic cores.
12. A method of sensing a leakage current fault in a DC power circuit, the method comprising:
positioning a DC leakage current detector about a pair of conductors of a DC bus in the DC power circuit, the DC leakage current detector comprising:
a pair of transformers, each of the pair of transformers including:
a magnetic core positioned about the pair of conductors; and
a pair of windings wound about the magnetic core, the pair of windings including an excitation winding and a detection winding;
an excitation and biasing circuit to inject a current signal onto the excitation winding of each transformer; and
a detector output coupled to the detection winding of each transformer;
injecting the current signal onto the excitation winding of each transformer via the excitation and biasing circuit, thereby creating a changing magnetic flux in the core of each of the transformers; and
measuring, via the DC leakage current detector, a voltage on each of the detection windings generated responsive to the magnetic flux in the core of each of the transformers, with the voltage on the detection windings providing a net voltage at the detector output whose value is indicative of a presence of a leakage current on the DC bus.
13. The method of claim 12 wherein the pair of transformers comprise normally unsaturated transformers, and wherein causing the excitation and biasing circuit to inject the current signal comprises injecting the current signal to create a changing magnetic flux that partially saturates the pair of magnetic cores of the transformers.
14. The method of claim 13 wherein, when no leakage current is present in the conductors, the magnetic flux in the pair of transformers are equal and opposite from one another, such that the net voltage at the detector output is zero; and
wherein, when leakage current is present in the conductors, the magnetic flux in the pair of transformers are unequal from one another, such that the net voltage at the detector output has a non-zero value.
15. The method of claim 12 wherein the pair of transformers comprise normally saturated transformers, and wherein causing the excitation and biasing circuit to inject the current signal comprises injecting a current signal that includes a DC bias therein that maintains the pair of magnetic cores in saturation.
16. The method of claim 15 wherein, when no leakage current is present in the conductors, the pair of magnetic cores remain in saturation, such that no net voltage is present at the detector output; and
wherein, when leakage current is present in the conductors, the leakage current counteracts the DC bias in the injected current signal for one of the pair of transformers, so as to cause the one of the pair of transformers to be taken out of saturation, such that the net voltage at the detector output has a non-zero value.
17. The method of claim 15 wherein injecting the current signal that includes the DC bias comprises controlling a magnitude of the DC bias in the injected current signal based on a known magnitude of leakage current that might be present in the pair of conductors, with the magnitude of the DC bias in the injected current signal being controlled such that the leakage current approximately counteracts the DC bias.
18. A DC leakage current detection system for detecting a leakage current fault in a DC bus of a DC power circuit, the DC leakage current detection system comprising:
a plurality of DC leakage current detectors for detecting leakage current in the DC bus at various locations in the DC power circuit, each of the DC leakage current detectors comprising:
a pair of transformers, each of the pair of transformers including:
a magnetic core comprising an opening positionable about a pair of conductors of the DC bus, the pair of conductors creating a magnetic field in the magnetic core that is a sum of currents in the conductors; and
a pair of windings wound about the magnetic core, the pair of windings including an excitation winding and a detection winding;
an excitation and biasing circuit connected to the excitation winding in each of the pair of transformers to inject a current signal thereto, the injected current signal creating a changing magnetic flux in the core of each of the transformers; and
a detector output connected to the detection winding in each of the pair of transformers to receive a voltage therefrom generated responsive to the magnetic flux in the core of each of the transformers, wherein the voltage on the detection windings provides a net voltage reading at the detector output whose value is indicative of a presence of a leakage current on the DC bus; and
a logic device operably connected with the plurality of DC leakage current detectors, the logic device configured to:
receive the net voltage reading from each of the plurality of DC leakage current detectors; and
locate the leakage current fault in the DC power circuit based on the net voltage readings received from the plurality of DC leakage current detectors.
19. The DC leakage current detection system of claim 18 wherein the pair of transformers in a respective DC leakage current detector comprise normally unsaturated transformers, with the injected current signal creating the changing magnetic flux to partially saturate the magnetic cores of the transformers; and
wherein the net voltage reading at the detector output is zero when no leakage current is present in the conductors and the net voltage reading at the detector output is a non-zero value when leakage current is present in the conductors.
20. The DC leakage current detection system of claim 18 wherein the pair of transformers comprise normally saturated transformers, with the injected current signal including a DC bias therein that maintains the pair of magnetic cores in saturation; and
wherein no net voltage reading is present at the detector output when no leakage current is present in the conductors and the net voltage reading at the detector output has a non-zero value when leakage current is present in the conductors.