Apparatus for a microinverter particularly suited for use in solar power installations

• US 8,422,249 B2
• Filed: 08/25/2011
• Issued: 04/16/2013
• Est. Priority Date: 08/25/2011
• Status: Expired due to Fees

• Current Assignee: Direct Power Technologies, Inc.
• Original Assignee: Direct Power Technologies, Inc.
• Inventors: Cooper, Frank G., Hoffman, Eric J., MacArthur, Richard
• Primary Examiner(s): Han, Jessica
• Application Number:
  US13/199,319
• Publication Number:
  US 20130051092A1
• Time in Patent Office: 600 Days
• Field of Search: 363/16, 363/17, 363/97, 363/98, 363/124, 363/131, 363/132
• US Class Current: 363/17
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/40 Synchronising a generator for connection to a network or to another generator
• CPC Class Codes: H02M 7/4807 having a high frequency intermediate AC stage
• CPC Class Codes: H02M 7/53871 with automatic control of output voltage or current
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

A microinverter is disclosed for use in a solar power installation. The microinverter incorporates a voltage-to-current control loop that initially converts output current produced by a photovoltaic panel into a pulse width modulated output synchronized and phase-locked to the utility grid voltage. The duty cycle of that modulated output is specified by output power internally requested from the microinverter. This modulated output is converted into a full-wave rectified unipolar waveform that is converted, through a Commutator, into a bipolar AC output that is also phase-locked and synchronized to the grid voltage. The commutator uses an H-bridge composed of four FETs, with each of two diagonally-oriented pairs of these FETs being advantageously switched on substantially at zero-crossing points in the grid voltage. Switching these FETs during times of zero current and voltage, and switching each pair on for substantially an entire half-cycle of grid voltage reduces switching loss and power dissipation of the FETs and ensures that these FETs remain substantially unaffected by transients which might appear in the grid voltage.

Claims

1. Apparatus for a microinverter which converts direct current (DC) input power into alternating current (AC) power for supply to an AC power grid, the apparatus comprising:

a DC-to-current converter, connected to a DC power source and responsive to an error control signal, for producing a full-wave rectified output synchronized to an AC grid voltage then appearing on the power grid, the error control signal being responsive to a difference between a current command and an actual amount of AC output current then being supplied by the microinverter to the power grid;

a commutator for converting the full-wave rectified output to the AC output current synchronized and phase-locked to the grid voltage, the commutator having switching components which are switched to a substantially on or off state substantially co-incident with zero crossings in the grid voltage, the commutator having a zero-crossing detector for detecting the zero crossings in the grid voltage so as to yield a zero crossing signal; and

a controller, responsive to the zero crossing signal, for generating the current command and for synchronizing and phase-locking the operation of the commutator to the grid voltage.

2. The apparatus recited in claim 1 wherein the DC power source is a photovoltaic (PV) panel.

3. The apparatus recited in claim 2 wherein the DC-to-current converter comprises:

a chopper circuit, responsive to the DC input power produced by the DC power source and controlled by a pulse width modulation control signal, for converting DC input current into a pulse width modulated current signal so as to form a chopped signal, wherein the amount of requested current determines a duty cycle of the chopped signal;

an isolation transformer, having a first winding carrying the chopped signal and a second winding providing a pulse width modulated output current to a rectifier/filter circuit; and

the rectifier/filter circuit for converting the pulse width modulated output current into full-wave rectified output current.

4. The apparatus recited in claim 3 wherein the apparatus further comprises:

a pulse width modulator, responsive to the error control signal, for producing the pulse width modulation control signal having a controlled duty cycle determined by the error control signal; and

the chopper circuit comprises;

a first plurality of four chopper field effect transistors (FETs) configured in an H-bridge arrangement for chopping the DC input power into the pulse width modulated current signal; and

a gate drive circuit, responsive to the pulse width modulation control signal, for providing gate drive signals to the FETs.

5. The apparatus recited in claim 4 wherein the commutator comprises:

a second plurality of four commutator field effect transistors configured in an H-bridge arrangement, wherein each of two diagonally oriented pairs of said commutator FETs is switched on during corresponding half-cycles of the grid voltage; and

a switch control circuit, responsive to the zero crossing detector and connected to each of the commutator FETs, for alternately switching each of the pairs of the commutator FETs on or off substantially co-incident with and phase-locked to corresponding zero crossings in the grid voltage.

6. The apparatus recited in claim 5 wherein the zero cross detector imparts a predefined amount of hysteresis to an output of the zero crossing detector provided to the switch control circuit such that the commutator FETs are switched off or on at a pre-defined period of time before or after each zero crossing in the grid voltage.