Variable speed generator/motor device

A variable speed generator/motor device comprising:

• a variable frequency power converter including a two-level, three-level, or five-level voltage type converter comprising m unit converters (where m is a natural number equal to or larger than one) each obtained by connecting six arm converters in a three-phase full wave bridge configuration and by providing two direct current side terminals and three intermediate terminals, the six arm converters each comprising k self-turn-off switching elements for pulse-width modulation (where k is a natural number equal to or larger than one), the m unit converters being connected in parallel through the two direct current side terminals, n energy storage elements (where n is a natural number equal to or larger than one) having voltage source characteristics being provided between the two direct current side terminals, first terminals of m current-limiting reactors being connected to the three intermediate terminals on a phase-to-phase basis, second terminals of the m current-limiting reactors being connected in a star configuration to serve as alternating current terminals;
• a direct current voltage device including a voltage type self-excited converter connected back-to-back to the two direct current side terminals of the variable frequency power converter;
• an automatic voltage adjuster configured to connect three alternating current terminals of the direct current voltage device to an alternating current system, connect three alternating current terminals of the variable frequency power converter to three alternating current terminals of a three-phase alternating current synchronous machine having an excitation winding, connect two excitation winding terminals of the three-phase alternating current synchronous machine to an excitation device, and adjust a manipulated variable for the excitation device so as to match an alternating current voltage of the three-phase alternating current synchronous machine with a command value;
• and
• a converter current adjuster configured to control the unit converters so as to match a current value from a direct current transformer with a converter current command value, wherein
• a first three-phase branch circuit is provided between the three alternating current terminals of the direct current voltage device and the alternating current system, a second three-phase branch circuit is provided between the three alternating current terminals of the variable frequency power converter and the three alternating current terminals of the three-phase alternating current synchronous machine, a first load switch for a bypass circuit is provided between the first three-phase branch circuit and the second three-phase branch circuit, a measurement current transformer is provided between three-phase alternating current terminals of the three-phase alternating current synchronous machine and the second three-phase branch circuit, and a power adjuster is provided that is configured to use a current value of the measurement current transformer to output the converter current command value to the variable frequency power converter, and
• a first mode switching device is provided that is configured to, when switching from a converter mode in which the variable frequency power converter drives the three-phase alternating current synchronous machine to generate power to a bypass mode in which the first load switch is closed to drive the three-phase alternating current synchronous machine to generate the power, switch the first load switch from an open state to a closed state, and subsequently stop a gate command to the unit converters, and a second mode switching device is provided that is configured to, when switching from the bypass mode to the converter mode, hold the current value of the measurement current transformer during the switching, calculate a current command value of the converter current adjuster from the current value and output the result, subsequently start the gate command to the unit converters, and subsequently open the first load switch.