24-07-2012, 12:25 PM
Stand-Alone Solar Inverter with MPPT
Alone Solar Inverter.ppt (Size: 7.01 MB / Downloads: 413)
Objectives
To design a low cost solar inverter for areas without grid access or for backup power
Affordable
High efficiency
Battery charger stage for night time use
Maximum power point tracking for best use of photovoltaic panel
Boost Control: MPPT Continued
An even simpler method:
Constant Voltage (CV)
The point of maximum power transfer occurs at approximately 76% of the open circuit panel voltage
The ratio Vmpp/Voc is relatively constant throughout the operating range
Only voltage sensing is needed on the input
Transformer
Desired turns ratio of 14:1 (~180V:13V)
Approximately 40A current at 500W
Primary side requirement of 10 mm2 of copper
4-5 parallel strands of 14AWG wire
Secondary side requirement of 3 mm2 of copper
2 parallel strands of 16AWG wire
Multiple turns needed on primary for sufficient magnetic coupling
10 turns on primary corresponds to 140 turns on output!
Control Implementation: PIC18F2520
PIC18F2520
A/D converter
Internal 8 MHz oscillator
Internal voltage reference
2 Control, Capture, PWM modules for easy duty cycle adjustment
Control Implementation: Boost and Buck
Boost Control
- Inputs: 0-5V scaled boost input voltage feedback
- Outputs: Single 31.25kHz duty control signal (10-80% limited)
- Operation: Proportional control on boost input for CV MPPT
Buck Control
- Inputs: 0-5V scaled buck output voltage feedback
- Outputs: Single 31.25kHz duty control signal (10-80% limited)
- Operation: Proportional control on buck output for battery regulation
Open Loop Performance Summary
Boost stage performed as desired, with an average efficiency of about 92% up to 300W.
Buck stage experienced inductive voltage spikes that limited operation to about 150W (even with turn-off
snubbers) with an average efficiency of about 90%.
Half-Bridge performed as desired until the power was increased above approximately 100W.
H-Bridge produced desired waveforms, but was unable to function under high voltages or heavy loads.
Challenges Continued
Testing issues with the half-bridge
The half-bridge proved difficult to test because of the nature of its output waveform and the low resistance values needed to reach higher power levels
Grounding issues between the input and output prevented the electronic load from being used simultaneously with a high-power source.