System and method for low speed control of polyphase ac machine

A power conversion system comprising:

• a first set of terminals configured to connect the power conversion system to a polyphase AC machine;
• a DC bus having a positive rail and a negative rail;
• a plurality of first switches configured to selectively connect the first set of terminals to the DC bus;
• a second set of terminals configured to connect to a utility grid;
• a plurality of second switches configured to selectively connect the DC bus to the second set of terminals;
• a memory device configured to store a series of instructions;
• and
• a controller configured to execute the instructions in a first operating mode and in a second operating mode, wherein:
• during the first operating mode, the controller generates a gating signal for each of the first and second switches to accelerate the AC machine to a predefined speed, and
• during the second operating mode, the controller generates the gating signal for each of the first and second switches to transfer energy generated by the AC machine to the utility grid.

A method of accelerating a polyphase AC machine for use in a wind turbine up to a predefined initial speed greater than a cut-in speed of the wind turbine, the method comprising the steps of:

• controlling a power converter in a first operating mode to execute a modulation module to generate a voltage for the AC machine, wherein the voltage has a variable magnitude and a variable frequency to control a rotational speed of the AC machine;
• disabling the modulation module at a periodic interval during execution in the first operating mode;
• determining a back-emf voltage present on the AC machine when the modulation module is disabled;
• determining the rotational speed of the AC machine as a function of the back-emf voltage;
• and
• controlling the power converter in a second operating mode when the rotational speed is greater than the predefined initial speed, wherein during the second operating mode, the power converter transfers energy from the AC machine to a DC bus in the power converter.

A power conversion system for transferring energy generated from an AC generation source to a utility grid, the power conversion system comprising:

• a set of terminals configured to connect the power conversion system to the AC generation source;
• a DC bus having a positive rail and a negative rail;
• a power converter connected between the set of terminals and the DC bus and configured for power transfer between the set of terminals and the DC bus;
• a memory device configured to store a series of instructions;
• and
• a controller configured to execute the series of instructions to:
• execute a modulation module above a predefined speed for continuous modulation of the power converter to transfer power generated by the AC generation source to the DC bus, and
• periodically insert a blanking time in coordination with the modulation module below the predefined speed for intermittent modulation of the power converter to transfer power generated by the AC generation source to the DC bus.