Dc transmission system and dc/dc converter used in the same

A DC transmission system to transmit AC power generated by a generator to an AC distribution grid and a DC distribution grid using DC power, the DC transmission system comprising:

• an AC/DC converter to output DC power by converting AC power from the generator;
• a first DC/DC converter to output DC power having a second voltage by converting the DC power having a first voltage outputted from the AC/DC converter;
• a second DC/DC converter to output DC power having a third voltage by converting the DC power having the second voltage outputted from the first DC/DC converter;
• and
• a DC/AC converter to output, to the AC distribution grid, AC power by converting the DC power outputted from the second DC/DC converter,
• the DC power outputted from the second DC/DC converter also being transmitted to the DC distribution grid,
• when the third voltage changes,
• the second DC/DC converter being configured to control the third voltage in response to the change in the third voltage, and
• the first DC/DC converter being configured to control the first voltage in response to a change in the second voltage.

A DC/DC converter connected between an AC/DC converter and a DC/AC converter, the AC/DC converter being configured to output DC power by converting AC power from a generator, the DC/AC converter being configured to output, to an AC distribution grid, AC power by converting DC power, the DC/DC converter comprising:

• a first conversion unit including a plurality of semiconductor switching devices, the first conversion unit being configured to receive DC power having a first voltage outputted from the AC/DC converter and being configured to convert the received DC power into AC power,
• a transformer to transfer the AC power from the first conversion unit,
• a second conversion unit including a plurality of semiconductor switching devices, the second conversion unit being configured to receive the AC power transferred by the transformer and being configured to convert the received AC power into the DC power having a second voltage to output the DC power to the DC/AC converter;
• and
• a controller to control the first conversion unit and the second conversion unit using a pulse signal,
• the controller being configured to control a voltage ratio between the first voltage and the second voltage by adjusting a phase angle between a pulse signal of the second conversion unit and a gate pulse signal of the first conversion unit,
• in response to a change in the second voltage, the controller being configured to
• correct an instruction value of the first voltage, and control the phase angle so that the first voltage follows the corrected instruction value of the first voltage, or
• correct an instruction value of the second voltage in accordance with a difference between a detection value and an instruction value of the first voltage, and control the phase angle so that the second voltage follows the corrected instruction value of the second voltage.