Doubly-fed generator and doubly-fed electric machine

摘要

The excitation overcurrent detection unit for the doubly-fed electric machine is provided with a function to determine an excitation current magnitude relationship among three phases. The firing pulse is held to on-state or off-state to cause the largest-current phase and the second-largest-current phase to charge the DC capacitor by the operation of diodes. The conduction ratio of the third-largest-current phase or minimum current phase is controlled according to the detected current value to protect against a possible short-circuit across the DC capacitor. When the voltage of the DC capacitor exceeds a preset value, the voltage is suppressed by operating active or passive power devices.

主权项

1. 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 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; 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.