System and method for low speed control of polyphase ac machine

A power converter comprising:

• an input configured to receive power from a multi-phase AC source;
• a DC bus having a positive and a negative rail;
• a plurality of positive switching devices, each positive switching device selectively connecting one phase of the AC source to the positive rail of the DC bus;
• a plurality of negative switching devices, each negative switching device selectively connecting one phase of the AC source to the negative rail of the DC bus;
• a memory device storing a series of instructions;
• and
• a controller configured to execute the series of instructions to:
• determine a magnitude of power generated by the AC source, and
• execute a modulation module to generate a positive control signal for each positive switching device and a negative control signal for each negative switching device, wherein:
• the control signals are generated in a first operating mode when the AC source is generating a magnitude of power greater than a first threshold,
• the control signals are generated in a second operating mode when the AC source is generating a magnitude of power less than the first threshold, and
• during the second operating mode, the controller periodically disables the control signals for a blanking time.

A method of converting power from a renewable energy source having variable power generation capability, the method comprising the steps of:

• monitoring a level of power generated by the renewable energy source;
• controlling a power converter in a first operating mode via pulse width modulation having a fixed modulation frequency and a fixed dead time compensation when the level of power generated is above a first predetermined threshold;
• and
• controlling the power converter in a second operating mode via pulse width modulation having a periodic blanking time, wherein the blanking time is repeated at a periodic interval during each cycle of a fundamental frequency of a voltage generated by the renewable energy source and wherein during the blanking time the pulse width modulation is disabled.

A power converter comprising:

• an input configured to receive power from an AC source;
• a DC bus having a positive rail and a negative rail;
• at least one positive switching device selectively connecting the input to the positive rail of the DC bus as a function of a corresponding positive gating signal;
• at least one negative switching device selectively connecting the input to the negative rail of the DC bus as a function of a corresponding negative gating signal;
• a memory device storing a series of instructions;
• and
• a controller configured to execute the series of instructions to:
• execute a modulation routine to generate each of the positive and negative gating signals;
• determine a magnitude of power generated by the AC source,
• generate the positive and negative gating signals for each of the positive and negative switching devices in a first operating mode when the magnitude of power generated by the DC source exceeds a first predefined threshold, and
• generate the positive and negative gating signals for each of the positive and negative switching devices in a second operating mode when the magnitude of power generated by the DC source is less than the first predefined threshold, wherein
• during the first operating mode, the controller periodically inserts a blanking time in the modulation routine, disabling the positive and negative gating signals during the blanking time, and
• during the second operating, mode each of the positive switching devices connects the input to the positive rail in tandem and each of the negative switching devices connects the input to the negative rail in tandem.