Method and system to convert direct current (DC) to alternating current (AC) using a photovoltaic inverter

A method to convert direct current (DC) power generated by a plurality of photovoltaic (PV) arrays to alternating current (AC) power using a PV inverter having a DC boost circuit for each of the PV arrays and a DC to AC inverter, the method comprising:

• applying the DC power generated by each PV array to an input to the PV inverter corresponding to the PV array;
• operating each PV array at a maximum power voltage corresponding to a maximum power output of the array to produce DC power;
• determining a duty cycle for each PV array which regulates the PV array to output the maximum power voltage for the array and boosts the maximum power voltage to at least a minimum DC link voltage;
• applying the duty cycle to each boost circuit to boost the DC power from the array connected to the boost circuit from the maximum power voltage of the array to at least the minimum DC link voltage;
• converting the boosted DC power to AC power, and
• disabling the boost circuit connected to a one of the PV arrays having reached a maximum power voltage at least as great as the minimum DC link voltage.

A photovoltaic inverter comprising:

• a plurality of input connections each adapted to receive direct current (DC) power from a photovoltaic array;
• a separate DC boost circuit electrically connected to each of the input connections, wherein the boost circuit includes at least one switch operated in accordance with a duty cycle;
• a DC link circuit electrically connected to a DC output from each of the DC boost circuits, wherein the DC link circuit includes a DC to alternating current (AC) inverter and an AC power output adapted to deliver AC power from an AC power grid, and
• a controller executing processes for:
• determining a voltage at which each of the photovoltaic arrays connected to an input connection produces a predetermined power;
• generating the duty cycle for each boost circuit to control the at least one switch to cause the PV array to produce the predetermined power and the boost circuit to output a predetermined DC link voltage, and disabling one of the boost circuits coupled to a PV array generating a voltage greater than the predetermined DC link voltage while the voltage from the PV array generating the voltage greater than the predetermined DC link voltage is being converted to AC power.

A method to determine, on an ongoing basis, a maximum power point for a photovoltaic array, the method comprising:

• a. varying a voltage output of the photovoltaic array over an operating range of voltages for the array;
• b. measuring current output from the photovoltaic array as the voltage output is varied over the operating array;
• c. using the measured current and the voltage corresponding to the measured current to determine a power output of the array for each of the various voltages in the operating range;
• d. selecting a maximum power output from the power outputs in the operating range and designating a corresponding voltage as a voltage at maximum power point (Vmpp) for the array;
• e. periodically repeating steps (a) to (d) to update Vmpp;
• f. operating the photovoltaic array at the Vmpp;
• g. dithering the Vmpp by slightly shifting the voltage of the array away from the Vmpp, and
• h. if the power output of the array at the shifted voltage is greater than the power output of the array at Vmpp, designating the shifted voltage as the Vmpp and repeating steps (f) to (h).

A method comprising:

• applying DC power generated by a photovoltaic (PV) array to a PV inverter circuit;
• boosting the DC power to a predetermine DC link voltage, while a DC voltage output by the PV array is below a predetermined threshold level;
• converting the boosted DC power to AC power, and
• while the PV array outputs a DC voltage level at least as high as the threshold level, disabling the boost of the DC power and converting the DC power generated by the PV array to AC power while the boost is disabled.