System and method for ground fault detection
Power conversion systems, disclosed examples include power conversion systems, ground fault detection apparatus and methods to detect and identify ground faults in a power conversion system using AC coupling to sense a system voltage to determine a leakage flux linkage, and to identify a faulted converter phase based on a phase shift angle of the leakage flux linkage.
1. An apparatus to sense a time-varying system voltage indicative of a leakage flux linkage to detect system ground faults in a power conversion system, comprising:
a first coupling capacitor, including a first terminal connected to a DC bus circuit of a power conversion system, and a second terminal;
a voltage divider circuit, including a plurality of resistors connected in series with one another between the second terminal of the first coupling capacitor and a reference node, and an output node that delivers a voltage signal corresponding to a voltage of the first terminal of the coupling capacitor, the coupling capacitor and the voltage divider circuit presenting a DC resistance to the DC bus circuit relative to a reference node;
a second coupling capacitor including a first terminal connected to the DC bus circuit, and a second terminal connected to the voltage divider circuit;
wherein the plurality of resistors are connected in series with one another between the second terminals of the first and second coupling capacitors and the reference node;
a signal conditioning circuit, including an input coupled to receive the voltage signal from the output node of the voltage divider circuit, and an output to provide an output voltage signal representing the voltage signal from the output node;
an analog-to-digital converter, including an input coupled with the output of the signal conditioning circuit, the analog-to-digital converter operative to provide digital values corresponding to the output voltage signal; and
a processor configured to determine a leakage flux linkage based on the digital values, and to detect ground faults in the power conversion system based at least partially on the leakage flux linkage.
2. The apparatus of claim 1 , wherein the apparatus presents a DC resistance greater than 100 MΩ to the DC bus circuit relative to the reference node.
3. The apparatus of claim 1 , wherein the coupling capacitor has a voltage rating greater than 2500 V.
4. The apparatus of claim 1 , wherein the coupling capacitor has a single digit nF capacitance value.
5. The apparatus of claim 1 , wherein the first terminal of the first coupling capacitor is connected to a first DC bus terminal, and wherein the first terminal of the second coupling capacitor is connected to a second DC bus terminal.
6. The apparatus of claim 5 , wherein the plurality of resistors provide a total series resistance of 1-2 MΩ.
7. The apparatus of claim 1 , wherein the plurality of resistors provide a total series resistance of 1-2 MΩ.
8. The apparatus of claim 1 , wherein the processor is configured to detect ground faults in the power conversion system based on an amplitude of the leakage flux linkage, a frequency of the leakage flux linkage, or a phase shift angle of the leakage flux linkage.