Filter system and X-capacitor protection method thereof

A filter system, electrically coupled to a three-phase three-wire power grid, wherein the filter system comprises a first line, a second line and a third line, and the filter system comprises:

• a first capacitor unit and a second capacitor unit, wherein each of the first capacitor unit and the second capacitor unit comprises a first capacitor, a second capacitor and a third capacitor, a terminal of the first capacitor, a terminal of the second capacitor and a terminal of the third capacitor are electrically connected to the first line, the second line and the third line respectively, the other terminals of the first capacitor, the second capacitor and the third capacitor in the first capacitor unit are electrically connected to a first electrical midpoint, and the other terminals of the first capacitor, the second capacitor and the third capacitor in the second capacitor unit are electrically connected to a second electrical midpoint;
• and
• a common mode choke, electrically connected in series between the first capacitor unit and the second capacitor unit, wherein the common mode choke comprises an auxiliary winding electrically connected between the first electrical midpoint and the second electrical midpoint,
• wherein a phase voltage between the first line and the first electrical midpoint and a phase voltage between the first line and the second electrical midpoint are equal and regarded as a first phase voltage, a phase voltage between the second line and the first electrical midpoint and a phase voltage between the second line and the second electrical midpoint are equal and regarded as a second phase voltage, and a phase voltage between the third line and the first electrical midpoint and a phase voltage between the third line and the second electrical midpoint are equal and regarded as a third phase voltage,
• wherein the filter system further comprises:
• a detection unit, configured for detecting one of the first phase voltage, the second phase voltage and the third phase voltage;
• and
• a control unit, configured for receiving the phase voltage detected by the detection unit, obtaining the other two of the first phase voltage, the second phase voltage and the third phase voltage according to the phase voltage detected by the detection unit and two line voltages, comparing the first phase voltage, the second phase voltage and the third phase voltage with a threshold, and outputting a protection signal when any one of the first phase voltage, the second phase voltage and the third phase voltage is higher than the threshold,
• wherein the common mode choke further comprises three main windings, and a size of the auxiliary winding is smaller than a size of any one of the three main windings.

An X-capacitor protection method of a filter system applicable for a three-phase three-wire power grid, wherein the filter system comprises a first line, a second line and a third line, and the X-capacitor protection method comprises steps of:

• providing a plurality of capacitor units, wherein each of the plurality of capacitor units comprises a first X capacitor, a second X capacitor and a third X capacitor, a terminal of the first X capacitor, a terminal of the second X capacitor and a terminal of the third X capacitor are electrically connected to the first line, the second line and the third line respectively, the other terminal of the first X capacitor, the other terminal of the second X capacitor and the other terminal of the third X capacitor are all electrically connected to a corresponding electrical midpoint;
• providing a plurality of common mode chokes, wherein each of the plurality of common mode chokes has an auxiliary winding;
• and
• electrically connecting the electrical midpoints corresponding to the adjacent capacitor units through the auxiliary winding,
• wherein the X-capacitor protection method further comprises steps of:
• detecting a phase voltage in a phase between one of the first line, the second line or the third line and one of the electrical midpoints;
• calculating two phase voltages in other two phases according to the detected phase voltage and two line voltages;
• comparing the three phase voltages with a first threshold;
• and
• outputting a protection signal when any one of the three phase voltages is higher than the first threshold,
• wherein each of the plurality of common mode chokes further comprises three main windings, and a size of the auxiliary winding is smaller than a size of any one of the main windings.