Electric power conversion system

An electric power conversion system comprising:

• a self-commutated power converter to perform power conversion between a three-phase alternating-current system and a direct-current system;
• a first transformer having a primary side connected to the alternating-current system and a secondary side connected to the self-commutated power converter, the secondary side having a Y-connection;
• and
• an impedance switching circuit electrically connected between a neutral point of the Y-connection of the first transformer and a ground, wherein
• when a single-line-to-ground fault occurs on three phase alternating-current lines connecting the first transformer and the self-commutated power converter, the impedance switching circuit is configured to have a reduced impedance, as compared to when the single-line-to-ground fault does not occur, wherein
• the impedance switching circuit includes a lightning arrestor connected between the neutral point and the ground, and
• a turn-on voltage for the lightning arrestor is lower than a voltage that develops at the neutral point upon occurrence of the single-line-to-ground fault, and higher than a voltage that develops at the neutral point when the single-line-to-ground fault does not occur.

An electric power conversion system comprising:

• a self-commutated power converter to perform power conversion between a three-phase alternating-current system and a direct-current system;
• a first transformer having a primary side connected to the alternating-current system and a secondary side connected to the self-commutated power converter, the secondary side having a Y-connection;
• and
• an impedance switching circuit electrically connected between a neutral point of the Y-connection of the first transformer and a ground, wherein
• when a single-line-to-ground fault occurs on three phase alternating-current lines connecting the first transformer and the self-commutated power converter, the impedance switching circuit is configured to have a reduced impedance, as compared to when the single-line-to-ground fault does not occur, wherein
• the electric power conversion system further comprises
• a detector to detect occurrence of the single-line-to-ground fault, wherein
• the impedance switching circuit includes a switch connected between the neutral point and the ground, and
• the switch is kept off in a time period in which occurrence of the single-line-to-ground fault is not detected by the detector, and turns on upon detection of occurrence of the single-line-to-ground fault by the detector.

An electric power conversion system comprising:

• a self-commutated power converter to perform power conversion between a three-phase alternating-current system and a direct-current system;
• a first transformer having a primary side connected to the alternating-current system and a secondary side connected to the self-commutated power converter, the secondary side having a Y-connection;
• and
• an impedance switching circuit electrically connected between a neutral point of the Y-connection of the first transformer and a ground, wherein
• when a single-line-to-ground fault occurs on three phase alternating-current lines connecting the first transformer and the self-commutated power converter, the impedance switching circuit is configured to have a reduced impedance, as compared to when the single-line-to-ground fault does not occur, wherein
• the impedance switching circuit includes an air gap formed between the neutral point and the ground, and
• the air gap is configured to cause flashover when applied a voltage developed at the neutral point upon occurrence of the single-line-to-ground fault, and cause no flashover when applied a voltage developed at the neutral point when the single-line-to-ground fault does not occur.

An electric power conversion system, comprising:

• a self-commutated power converter to perform power conversion between a three-phase alternating-current system and a direct-current system;
• a transformer connected between the alternating-current system and the self-commutated power converter;
• three phase alternating-current lines connecting between the transformer and the self-commutated power converter, and
• a filter circuit connected between the ground and each of a first direct-current transmission line and a second direct-current transmission line, the first direct-current transmission line and the second direct-current transmission line constituting the direct-current system, wherein
• the filter circuit is configured to pass a system frequency component of the alternating-current system.