Doubly-fed generator and doubly-fed electric machine

A doubly-fed motor/generator comprising:

• a wound-rotor induction machine having its stator side armature winding connected to an electric power grid;
• a first power converter having three-phase bridge connected arms of an IGBT as a self-turn-off power device and a free-wheeling diode or a MOSFET as a self-turn-off power device, each of the arms having its AC side terminals connected to a rotor side excitation winding of the wound-rotor induction machine, wherein the first power converter pulse-width-modulation-controls the self-turn-off power devices;
• a DC capacitor connected between DC side terminals of the first power converter;
• a second power converter having its DC side terminals connected to both terminals of the DC capacitor and its AC side terminals connected to the power grid;
• an excitation current sensor to detect a current flowing in an excitation winding of the wound-rotor induction machine;
• an excitation current controller to calculate an excitation current command value for a slip frequency equal to a difference between a grid frequency and a rotor frequency of the wound-rotor induction machine and to output a first pulse command that causes the detected excitation current value from the excitation current sensor to match the excitation current command value, the first firing pulse command being fed to the self-turn-off power devices of the first power converter;
• a pulse command controller having a function to identify the three phases as a largest-current phase, a second-largest-current phase and a third-largest-current phase in order of decreasing magnitude of absolute current value sent from the excitation current sensor, the pulse command controller being adapted to check a polarity of the detected current value of the largest-current phase, fix to a turn-on side the firing pulse to either a self-turn-off power device P1 between the largest-current phase and a positive pole or a self-turn-off power device N1 between the largest-current phase and a negative pole so as to charge the DC capacitor, fix to a turn-off side the firing pulse to the other self-turn-off power device, fix the firing pulse to a self-turn-off power device P2 between the second-largest-current phase and the positive pole to the same side as the firing pulse to the self-turn-off power device N1 and fix the firing pulse to the a self-turn-off power device N2 between the second-largest-current phase and the negative pole to the same side as the firing pulse to the self-turn-off power device P1, and output a firing pulse of a pulse modulation factor calculated from the detected current value for the third-largest-current phase as a second firing pulse command to a self-turn-off power device P3 between the third-largest-current phase and the positive pole and to a self-turn-off power device N3 between the third-largest-current phase and the negative pole;
• and
• a pulse switching controller to switch the firing pulse command between the first pulse command and the second pulse command bidirectionally for output to the first power converter, the pulse switching controller being adapted to switch the firing pulse command from the first pulse command to the second pulse command under conditions including one that any of the absolute values of the detected current values from the excitation current sensor exceeds an overcurrent preset level 1 and to switch from the second pulse command to the first pulse command under conditions including one that the absolute values of all the detected three-phase current values from the excitation current sensor fall to or below an overcurrent preset level 2.

A doubly-fed motor/generator comprising:

• a wound-rotor induction machine having its three-phase stator side windings connected to an electric power grid;
• a first power converter having semiconductor power devices connected at their AC side terminals to the three-phase rotor side windings of the wound-rotor induction machine;
• a DC capacitor connected between DC side terminals of the first power converter;
• a second power converter having its DC side terminals connected to both terminals of the DC capacitor and its AC side terminals connected to the power grid;
• an excitation current sensor to detect an excitation current of each phase supplied from the first power converter;
• and
• an excitation current controller to output a first firing pulse command to the semiconductor power devices of the first power converter to control the excitation current produced by the first power converter according to an operation condition of the wound-rotor induction machine;
• wherein the three phases are identified as a largest-current phase, a second-largest-current phase and a third-largest-current phase in order of decreasing magnitude of absolute current value sent from the excitation current sensor, the largest-current phase having an absolute current value of the excitation current that is a maximum among the excitation currents of the three phases, a polarity of a detected current value of the largest-current phase is checked, and
• wherein, when a detected absolute current value of a minimum current phase or the third-largest-current phase exceeds an overcurrent preset level, the firing pulse to the semiconductor power devices of each phase in the first power converter is held to an on-state or an off-state to charge the DC capacitor.

A doubly-fed variable-speed electric machine configured to operate as a doubly-fed generator and a variable-speed electric machine, the doubly-fed variable-speed electric machine comprising:

• a wound-rotor induction machine having its stator side armature winding connected to an electric power grid;
• a first power converter having three-phase bridge connected arms of an IGBT as a self-turn-off power device and a free-wheeling diode or a MOSFET as a self-turn-off power device, each of the arms having its AC side terminals connected to a rotor side excitation winding of the wound-rotor induction machine, wherein the first power converter pulse-width-modulation-controls the self-turn-off power devices;
• a DC capacitor connected between DC side terminals of the first power converter;
• a second power converter having its DC side terminals connected to both terminals of the DC capacitor and its AC side terminals connected to the power grid;
• an excitation current sensor to detect a current flowing in an excitation winding of the wound-rotor induction machine;
• an excitation current controller to calculate an excitation current command value for a slip frequency equal to a difference between a grid frequency and a rotor frequency of the wound-rotor induction machine and to output a first pulse command that causes the detected excitation current value from the excitation current sensor to match the excitation current command value, the first firing pulse command being fed to the self-turn-off power devices of the first power converter;
• a pulse command controller having a function to identify a phase with a minimum absolute current value sent from the excitation current sensor, the pulse command controller being adapted to give an on/off-fixed firing pulse to self-turn-off power devices between two phases excluding the minimum current phase and the positive and negative poles so as to charge the DC capacitor, give a pulse modulation factor firing pulse calculated from the detected current value of the minimum current phase to self-turn-off power devices between the minimum current phase and positive and negative poles, and output the pulse modulation factor firing pulse and the on/off-fixed firing pulse as a second pulse command;
• a pulse switching controller to switch the firing pulse command between the first pulse command and the second pulse command bidirectionally for output to the first power converter, the pulse switching controller being adapted to switch the firing pulse command from the first pulse command to the second pulse command under conditions including one that any of the absolute values of the detected current values from the excitation current sensor exceeds an overcurrent preset level 1 and to switch from the second pulse command to the first pulse command under conditions including one that the absolute values of all the detected three-phase current values from the excitation current sensor fall to or below an overcurrent preset level 2.