System and method for controlling the operating area of an inverter coupled to an alternative energy source

Patent metadata

Field	Value
Publication number	US-9948211-B2
Application number	US-201615002556-A
Country	US
Kind code	B2
Filing date	Jan 21, 2016
Priority date	Jan 21, 2016
Publication date	Apr 17, 2018
Grant date	Apr 17, 2018

Abstract

A system includes a controller programmed to control an inverter to supply a power grid with a maximum amount AC electrical power while operating within an operating area that is pre-defined based on hardware limitations of the inverter. The system allows the inverter to operate with a variable low-DC voltage input into the inverter as opposed to a preset threshold DC voltage, thereby maximizing the amount of power the inverter supplies to a power grid. The inverter controller operates by prioritizing the reactive power output by the inverter over the active power output by the inverter so that the inverter is able to generate an output that meets the reactive power command from a utility and supplies the maximum amount of active power.

First claim

What is claimed is:

1. A system for controlling an inverter to supply power from a DC power source to a power grid, the system comprising:
a sensor system coupled to the power grid;
a voltage sensor coupled to an output of the DC power source; and
a controller coupled to the sensor system and the voltage sensor to receive signals therefrom, the controller programmed to:
calculate a maximum reactive power that the inverter can deliver to the power grid according to a reactive power algorithm and based on a reactive power command received from a utility, a power grid voltage received from the sensor system, and a voltage of the DC power source received from the voltage sensor;
calculate a maximum active power that the inverter can deliver to the power grid according to an active power algorithm and based on the maximum reactive power; and
control the inverter to deliver to the power grid the maximum reactive power and an active power equal to the smaller of the maximum active power and a maximum power point tracking active power command.

2. The control system of claim 1 wherein the reactive power algorithm calculates the maximum reactive power by:
calculating a power grid-requested inverter Q-axis current based on the reactive power command and the power grid voltage;
calculating a minimum allowable inverter Q-axis current based on the voltage of the power grid and a maximum peak AC voltage of the inverter;
calculating an inverter Q-axis current for delivery by the inverter based on the minimum allowable inverter Q-axis current and the power grid-requested inverter Q-axis current;
calculating a maximum power grid Q-axis current for delivery to the power grid based on the inverter Q-axis current for delivery to the power grid and an AC-side current limit of the inverter; and
calculating the maximum reactive power based on the maximum power grid Q-axis current.

3. The control system of claim 2 wherein the reactive power algorithm is configured to:
calculate a raw value for the maximum power grid Q-axis current using the inverter Q-axis current for delivery to the power grid;
set the maximum power grid Q-axis current equal to -kref·1.414·I ac _ lim , if the raw value for the maximum Q-axis current is less than -kref·1.414·I ac _ lim , where kref is a reference constant corresponding to the representation of D-axis and Q-axis currents and voltages and I ac _ lim is the AC-side current limit of the inverter;
set the maximum power grid Q-axis current to kref·1.414·I ac _ lim if the raw value for the maximum Q-axis current is greater than kref·1.414·I ac _ lim ; and
set the maximum power grid Q-axis current to the raw value for the maximum Q-axis current otherwise.

4. The control system of claim 2 wherein the reactive power algorithm is configured to:
set the inverter Q-axis current for delivery to the power grid equal to the minimum allowable inverter Q-axis current if the power grid-requested inverter Q-axis current is less than the minimum allowable inverter Q-axis current; and
set the inverter Q-axis current for delivery to the power grid equal to the power grid-requested inverter Q-axis current requested otherwise.

5. The control system of claim 2 wherein the reactive power algorithm calculates the maximum peak AC voltage of the inverter using a safety factor.

6. The control system of claim 1 wherein the active power algorithm calculates the maximum active power by:
calculating an inductor D-axis voltage drop corresponding to a maximum power grid Q-axis current the inverter can deliver to the power grid;
calculating an inverter Q-axis voltage based on the inverter D-axis voltage and a maximum peak AC voltage of the inverter;
calculating a maximum allowable inverter D-axis current based on the inverter Q-axis voltage;
calculating a commanded maximum inverter D-axis current based on the maximum allowable inverter D-axis current, the maximum power grid Q-axis current the inverter can deliver to the power grid, and an AC-side current limit of the inverter;
calculating a maximum allowable power grid D-axis current for delivery to the power grid based on the voltage of the power grid and a DC-side current limit of the inverter;
calculating a maximum power grid D-axis current the inverter can deliver to the power grid based on the commanded maximum inverter D-axis current and the maximum allowable power grid D-axis current; and
calculating the maximum active power based on the maximum power grid D-axis current.

7. The control system of claim 6 wherein the active power algorithm sets the maximum power grid D-axis current equal to the lesser of the commanded maximum inverter D-axis current and the maximum allowable power grid D-axis current.

8. The control system of claim 6 wherein the active power algorithm calculates the command maximum inverter D-axis current according to:
I L _ d _ cmd _ max =min(√{square root over ((( k ref·1.414·I ac _ lim ) 2 −I grid _ q _ delivered 2 ))},I L _ d _ max ),

wherein kref is a reference constant corresponding to the representation of D-axis and Q-axis currents and voltages, I ac _ lim is the AC current limit of the inverter, I grid _ q _ delivered delivered is the maximum power grid Q-axis current the inverter can deliver to the power grid, and I L _ d _ max is the maximum allowable power grid D-axis current for delivery to the grid.

9. A method for controlling an inverter comprising:
receiving a reactive power command from a utility;
sensing a voltage of the power grid and a direct current (DC) voltage of a power source providing power to the power grid;
calculating in a reactive power algorithm a maximum reactive power the inverter can deliver to the power grid based on the reactive power command, the voltage of the power grid, and the DC voltage of the power source;
calculating in an active power algorithm a maximum active power the inverter can deliver to the power grid based on the maximum reactive power the inverter can deliver to the power grid;
outputting the maximum reactive power to an inverter current control block; and
outputting control signals from the inverter current control block to switches of the inverter to control the inverter to output to the power grid the maximum reactive power.

10. The method of claim 9 further comprising:
receiving in the active power algorithm a maximum power point tracking active power command for the inverter;
calculating in the active power algorithm an active power for delivery to the power grid based on the maximum active power the inverter can deliver to the power grid and the maximum power point tracking active power command;
outputting the active power for delivery to the power grid to the inverter current control block; and
outputting control signals from the inverter current control block to switches of the inverter to control the inverter to output to the power grid the active power for delivery to the power grid.

11. The method of claim 9 further comprising:
converting the maximum reactive power the inverter can deliver to the power grid to a Q-axis reference current using the reactive power algorithm;
converting the maximum active power the inverter can deliver to the power grid to a D-axis reference current using the active power algorithm;
outputting the Q-axis and D-axis reference currents to the inverter current control block; and
outputting control signals from the inverter current control block to switches of the inverter to control the inverter based on the Q-axis and D-axis reference currents.

12. The method of claim 9 further comprisin

Assignees

• Eaton Corp

Inventors

• Baronian Armen
• Nirantare Prasanna S

Classifications

• H02J2101/25

  involving maximum power point tracking control for photovoltaic sources · CPC title

• H02J3/385

  Electricity · mapped topic

• H02M7/539 Primary

  with automatic control of output wave form or frequency (H02M7/5375 - H02M7/5387 take precedence) · CPC title

• H02J3/48 Primary

  Controlling the sharing of active power · CPC title

• H02J3/50

  Controlling the sharing of reactive power · CPC title