Method and apparatus for increased energy harvest in a microgrid

1. A method for power converter current control, comprising:

controlling an instantaneous current generated by a power converter that is part of an AC battery such that the power converter appears, from the perspective of an AC line coupled to the power converter, as a virtual AC voltage source in series with a virtual impedance, wherein real and reactive phasor currents for the power converter are indirectly controlled by modifying amplitude and phase of a virtual AC voltage waveform that defines the virtual AC voltage source.

2. The method of claim 1 , wherein the instantaneous current is determined by:

determining a desired real phasor current value and a desired reactive phasor current value by a droop control technique;

generating a controlled real phasor current value based on a comparison between the desired real phasor current value and a real phasor current threshold;

generating a controlled reactive phasor current value based on (a) a first comparison between the desired reactive phasor current value and a reactive phasor current threshold and (b) a comparison between a charge threshold and an apparent current based on the controlled real phasor current value and a result of the first comparison;

computing the virtual AC voltage waveform based on the controlled real and reactive phasor current values; and

determining an instantaneous current value for driving the power converter to emulate the virtual impedance in the time domain while generating an amount of real and reactive current equal to the controlled real and reactive phasor current values.

3. The method of claim 2 , wherein the real and reactive phasor current thresholds are determined using one or more current limiting techniques, and wherein the apparent current is the magnitude of the vector sum of the controlled real phasor current value and the controlled reactive phasor current value.

4. The method of claim 3 , wherein the current limiting techniques comprise one or more of maximum power point tracking (MPPT), economic optimization, charge control, advanced grid functions, or demand response, and wherein the charge threshold is equal to a maximum allowable apparent current based on hardware limitations of the power converter.

5. The method of claim 2 , wherein the droop control technique is a phase-shifted virtual voltage-virtual impedance droop technique, wherein the virtual AC voltage waveform is a sinusoidal waveform having a constant amplitude and a phase shifted from the frequency of the AC line by an amount proportional to the difference between a measured frequency of the AC line and a nominal frequency of the AC line.

6. The method of claim 4 , wherein:

if the magnitude of the apparent current is greater than the charge threshold, the controlled reactive phasor current value is set equal to the maximum allowable apparent current;

if the desired real phasor current value is greater than the real phasor current threshold, the controlled real phasor current value is set equal to the real phasor current threshold;

if the desired real phasor current value is less than the negative of the real phasor current threshold, the controlled real phasor current value is set equal to the negative of the real phasor current threshold;

if the magnitude of the apparent current is not greater than the charge threshold and the desired reactive phasor current value is greater than the reactive phasor current threshold, the controlled reactive phasor current value is set equal to the reactive phasor current threshold; and

if the magnitude of the apparent current is not greater than the charge threshold and the desired reactive phasor current value is less than the negative of the reactive phasor current threshold, the controlled reactive phasor current value is set equal to the negative of the reactive phasor current threshold.

7. The method of claim 3 , wherein the instantaneous current is controlled such that the power converter appears as the virtual AC voltage source when the power converter is grid-connected and when the power converter is islanded to enable a seamless transition between operating in a grid-connected state and operating in an islanded state.

8. An apparatus for power converter current control, comprising:

a controller for controlling an instantaneous current generated by a power converter that is part of an AC battery such that the power converter appears, from the perspective of an AC line coupled to the power converter, as a virtual AC voltage source in series with a virtual impedance, wherein real and reactive phasor currents for the power converter are indirectly controlled by modifying amplitude and phase of a virtual AC voltage waveform that defines the virtual AC voltage source.

9. The apparatus of claim 8 , wherein the instantaneous current is determined by:

determining a desired real phasor current value and a desired reactive phasor current value by a droop control technique;

generating a controlled real phasor current value based on a comparison between the desired real phasor current value and a real phasor current threshold;

generating a controlled reactive phasor current value based on (a) a first comparison between the desired reactive phasor current value and a reactive phasor current threshold and (b) a comparison between a charge threshold and an apparent current based on the controlled real phasor current value and a result of the first comparison;

computing the virtual AC voltage waveform based on the controlled real and reactive phasor current values; and

determining an instantaneous current value for driving the power converter to emulate the virtual impedance in the time domain while generating an amount of real and reactive current equal to the controlled real and reactive phasor current values.

10. The apparatus of claim 9 , wherein the real and reactive phasor current thresholds are determined using one or more current limiting techniques, and wherein the apparent current is the magnitude of the vector sum of the controlled real phasor current value and the controlled reactive phasor current value.

11. The apparatus of claim 10 , wherein the current limiting techniques comprise one or more of maximum power point tracking (MPPT), economic optimization, charge control, advanced grid functions, or demand response, and wherein the charge threshold is equal to a maximum allowable apparent current based on hardware limitations of the power converter.

12. The apparatus of claim 9 , wherein the droop control technique is a phase-shifted virtual voltage-virtual impedance droop technique, wherein the virtual AC voltage waveform is a sinusoidal waveform having a constant amplitude and a phase shifted from the frequency of the AC line by an amount proportional to the difference between a measured frequency of the AC line and a nominal frequency of the AC line.

13. The apparatus of claim 11 , wherein:

if the magnitude of the apparent current is greater than the charge threshold, the controlled reactive phasor current value is set equal to the maximum allowable apparent current;

if the desired real phasor current value is greater than the real phasor current threshold, the controlled real phasor current value is set equal to the real phasor current threshold;

if the desired real phasor current value is less than the negative of the real phasor current threshold, the controlled real phasor current value is set equal to the negative of the real phasor current threshold;

if the magnitude of the apparent current is not greater than the charge threshold and the desired reactive phasor current value is greater than the reactive phasor current threshold, the controlled reactive phasor current value is set equal to the reactive phasor current threshold; and

if the magnitude of the apparent current is not greater than the charge threshold and the desired reactive phasor current value is less than the negative of the reactive phasor current threshold, the controlled reactive phasor current value is set equal to the negative of the reactive phasor current threshold.

14. The apparatus of claim 10 , wherein the instantaneous current is controlled such that the power converter appears as the virtual AC voltage source when the power converter is grid-connected and when the power converter is islanded to enable a seamless transition between operating in a grid-connected state and operating in an islanded state.

15. A system for generating power, comprising:

an AC battery, comprising:

an energy storage device; and

a power converter that is bidirectional, coupled to an AC line and coupled to the energy storage device for storing energy when operating in a charge mode, wherein the power converter comprises a controller for controlling an instantaneous current generated by the power converter such that the power converter appears, from the perspective of the AC line coupled to the power converter, as a virtual AC voltage source in series with a virtual impedance, wherein real and reactive phasor currents for the power converter are indirectly controlled by modifying amplitude and phase of a virtual AC voltage waveform that defines the virtual AC voltage source.

16. The system of claim 15 , wherein the instantaneous current is determined by:

determining a desired real phasor current value and a desired reactive phasor current value by a droop control technique;

generating a controlled real phasor current value based on a comparison between the desired real phasor current value and a real phasor current threshold;

generating a controlled reactive phasor current value based on (a) a first comparison between the desired reactive phasor current value and a reactive phasor current threshold and (b) a comparison between a charge threshold and an apparent current based on the controlled real phasor current value and a result of the first comparison;

computing the virtual AC voltage waveform based on the controlled real and reactive phasor current values; and

determining an instantaneous current value for driving the power converter to emulate the virtual impedance in the time domain while generating an amount of real and reactive current equal to the controlled real and reactive phasor current values.

17. The system of claim 16 , wherein the real and reactive phasor current thresholds are determined using one or more current limiting techniques, and wherein the apparent current is the magnitude of the vector sum of the controlled real phasor current value and the controlled reactive phasor current value.

18. The system of claim 17 , wherein the current limiting techniques comprise one or more of maximum power point tracking (MPPT), economic optimization, charge control, advanced grid functions, or demand response, and wherein the charge threshold is equal to a maximum allowable apparent current based on hardware limitations of the power converter.

19. The system of claim 16 , wherein the droop control technique is a phase-shifted virtual voltage-virtual impedance droop technique, wherein the virtual AC voltage waveform is a sinusoidal waveform having a constant amplitude and a phase shifted from the frequency of the AC line by an amount proportional to the difference between a measured frequency of the AC line and a nominal frequency of the AC line.

20. The system of claim 15 , wherein the instantaneous current is controlled such that the power converter appears as the virtual AC voltage source when the power converter is grid-connected and when the power converter is islanded to enable a seamless transition between operating in a grid-connected state and operating in an islanded state.