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Stand-Alone Solar Inverter with MPPT. Group 1 Andrew O’Connell & Jerry Klosek ECE 445 Senior Design April 25, 2008. 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

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stand alone solar inverter with mppt

Stand-Alone Solar Inverter with MPPT

Group 1

Andrew O’Connell & Jerry Klosek

ECE 445 Senior Design

April 25, 2008

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

Objectives
overview
Original Design Topology

Power Stages and Test Results

Control

Performance Summary

Cost Analysis

Analysis of Challenges

Recommendations

Overview
input stage boost converter
Input: 24-48Vdc, 0-500W

Nominal 36V solar panels in parallel

Output: 30-75Vdc, 0-500W

Maximum power point tracking…

Input Stage: Boost Converter
boost maximum power point tracking
Boost: Maximum Power Point Tracking

Getting the most out of the solar panels…

  • Match the perceived converter impedance to the solar panels for maximum power transfer
  • Several methods with varying degrees of difficulty…

Source: http://en.wikipedia.org/wiki/Image:Solar-Cell-IV-curve-with-MPP.png

boost control mppt continued
Boost Control: MPPT Continued
  • A Simple Method: Perturb and Observe (P&O)
    • “Climbing” the V-I Characteristic
    • Vary duty ratio based on past and present power input
    • Requires current and voltage sensing

Source: http://www.iet.aau.dk/~des/papers/PID253435-1_EPE-PEMC2006.pdf

boost control mppt continued9
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

Boost Control: MPPT Continued
boost performance continued
Boost Performance Continued

Input: 48V

Output: 60V

Power: 275W

Ch1: Vout, Ch2: Vgs, Ch3: Vds, Ch4: Inductor Current

buck converter
Input: 30-75Vdc from boost

Output: 26V for battery charging capability

Buck Converter
buck performance continued
Buck Performance Continued

Input: 50V

Output: 26V

Power: 125W

Note Vds Spikes—FETs only rated for 100V!

Ch1: Vout, Ch2: Vgs, Ch3: Vds, Ch4: Inductor Current

half bridge inverter
Input: 24-26Vdc from buck/battery

Output: 31.25kHz, ±13Vac square wave to transformer

Half-Bridge Inverter
half bridge performance continued
Half-Bridge Performance Continued

Input: 26V

Output: ±13V

Power: 67W

Ch1: Vout, Ch2: Vgs, Ch3: High-side Vds, Ch4: Inductor Current

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!

Transformer
h bridge performance
H-Bridge Performance

Ch1: Reference Sine Wave, Ch2: Low-side Vds,

Ch3: Output Waveform, Ch4: Inverted Output Current

Ch1: Vgs Left , Ch2: Vgs Right,

Ch4: Output Waveform

h bridge performance continued
Limitations

Up to 120Vdc (85VRMS) operation

Low load functionality only

Inability to adequately filter signal

H-Bridge Performance Continued
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: PIC18F2520
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

Control Implementation: Boost and Buck
control implementation half bridge and h bridge
Control Implementation: Half-Bridge and H-Bridge
  • Half-Bridge Control
  • - Inputs: None (open loop)
  • - Outputs: Single 31.25kHz, 50% duty cycle control signal
  • - Operation: Fixed duty cycles open loop control
  • H-Bridge Control
  • - Inputs: None (open loop)
  • - Outputs: Dual 20kHz PWM duty cycle control signals (80% limited)
  • - Operation: Varied duty cycles based on PWM sinusoid look-up table
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.

Open Loop Performance Summary
losses
Losses
  • Static Losses
    • Minimize on-state resistance and diode forward voltage
  • Dynamic Losses
    • Factor ‘a’ ranges from 1.5 to 2 with no correction
    • Lower switching frequency
    • Select low switching time
the bottom line
Total Cost (including transformer estimation): ~$225

Excludes packaging and economies of scale

Compare to ~$450 for a typical 500W inverter

Modified sine wave

The Bottom Line
challenges
Inductive Voltage Spikes

Even with wire path lengths reduced through use of PCBs, damage to transistors and diodes was consistently a problem

Parts were undersized

Converter FETs were not given enough voltage head room.

Buck was unable to be demonstrated due to slight errors in testing that resulted in FET failure.

Turn-off snubbers helped but did not solve the problem

Challenges
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

Challenges Continued
challenges continued35
PCB Troubleshooting

Breadboard tests gave expected results

PCB debugging required time and effort

High current problems

Lack of vias

Challenges Continued
recommendations
Additional PCB revision

Increased safety factor on part ratings for reliability

Need specialized testing equipment

Additional feedback filtering and interference reduction

Output filtering

I/O protection

Recommendations
acknowledgements
Special thanks to:

Grant Pitel

Professor Krein

Professor Swenson

Acknowledgements