Photovoltaic Power Converter Students: Thomas Carley Luke Ketcham Brendan Zimmer Advisors: Dr. Woonki Na Dr. Brian Huggins Bradley University Department Of Electrical Engineering 5/1/12

Presentation Outline Project Summary Project Motivation Overall System Block Diagram Boost Converter Inverter Future Work

Project Summary Photovoltaic Array Supplies DC and AC Power Boost Converter to step up PV voltage Maximum Power Point Tracking DC-AC converter for 120Vrms 60Hz LC filter

Project Motivation Power Electronics Alternative Energy Sources Useful Applications Household grid-tie inverter Electric drives

System Block Diagram

BP350J PV Panel Pmax = 50W Voltage at Pmax = 17.5V Current at Pmax = 2.9A Nominal Voltage = 12V Isc = 3.2A Voc = 21.8V

DC Subsystem Requirements The boost converter shall accept a voltage from the photovoltaic cells. The input voltage shall be 48 Volts. The average output shall be 200 Volts +/- 25 Volts. The voltage ripple shall be less than 20 Volts The open-loop boost converter shall operate above 65% efficiency. The boost converter shall perform maximum power point tracking. The PWM of the boost converter shall be regulated based on current and voltage from the PV array. The efficiency of the MPPT system shall be above 80%.

Boost Converter Test Boost Converter 20V to 66V D = .3

Boost Converter Design

Hardware

Key Components MOSFET (IRFP4768PbF) Ultrafast Diode (HFA50PA60C) VDSS = 250V Id = 93A Ultrafast Diode (HFA50PA60C) VR = 600V If = 25A Trr = 50ns Inductors (3mH) Capacitors (6000uF)

Gate Driver IR2110

Boost Converter Simulations Output Voltage (V) 20V – 66V

Boost Converter Simulations Boost Converter Current (A) 20V-66V

Boost Converter Testing 10V to 16.5V 40% Duty Cycle Output Voltage Inductor Current

Eliminating Voltage Spikes Parasitic capacitance and inductance Diode forward recovery time Circuit Layout Add Gate Resistor to increase turn-on and turn-off time Add RC snubber

Increasing Turn off Time Turn off time increased from 92 ns to 312 ns

Determining RC snubber values

Reducing Voltage Spikes 20V to 66V 70% duty Without Gate Resistor And RC Snubber With Gate Resistor And RC Snubber

Boost Converter Current Efficiency = 60.7% Efficiency = 58.1% Without RC snubber and Gate Resistor With RC snubber and Gate Resistor

Future Work For Boost Converter Optimize inductor value Printed Circuit Board Layout Optimize RC snubber values Test with multiple solar panels

Maximum Power Point Tracking (MPPT) Every PV has a V-I and P-V curve for a given insolation and temperature The MPP is seen clearly from the P-V curve Anytime the system is not at the MPP, it is not at it’s most efficient point I V MPP P V

Perturb and Observe (P&O) Slight voltage perturbation Observation of: Change in PV power Change in boost converter duty cycle Make an increase or decrease in boost converter duty cycle based on observation

P&O ΔP ΔD D+ + -

MPPT Algorithm Comparison Perturb and Observe Pros: Very popular Simple to implement Con: Power loss from perturbation Incremental Conductance Pro: Tracks a rapidly changing MPP Cons: Increased complexity Increased susceptibility to noise

Implementing MPPT Spectrum Digital eZdsp F2812 Voltage Sensing Current Sensing Matlab Simulink Modeling with Code Composer Studio

eZdsp F2812 features Texas Instruments TMS320F2812 chip 32-bit DSP Core – 150 MIPS 18K + 64K RAM 128K Flash 30 MHz clock 12 PWM outputs 16 ADC 12 bit inputs 60 ns conversion time

Voltage Sensing Vpv is 0 to 24V VADC 0 to 3.3V

Current Sensing Ipv: 0 to 50A Vout: 0 to 4V

Simulink Model ADC measurement P&O Voltage and current every 100μs Mean value with running window of 1Hz

Simulink Model Soft Start

MPPT and Soft Start Results Soft start duty cycle control 0% to 30% 5% increase every 5 seconds Transition to MPPT after 40 seconds MPPT duty cycle control ADC measurements Voltage and current every 100μs Mean value with running window of 1Hz 1% increase/decrease every 1 second

Power Supplies 120Vrms 60Hz input from wall 15V, 5V, and 3.3V output Consists of Transformer, Diode Rectifier, 470uF capacitor, and voltage regulators Needed for Gate Drivers, Op Amps, Sensing ICs, and other logic devices

Power Supply Transformer (3FL20-125) Secondary Voltage of 10VAC Secondary Current of 0.25A RMS

Power Supply

AC Subsystem Overview

AC Subsystem Goals DC power to AC power AC power quality

AC Subsystem Requirements The AC side of the system shall invert the output of the boost converter. The output of the inverter shall be AC voltage. The output shall be 60Hz +/- 0.1Hz. The inverter output shall be filtered by a LC filter. The filter shall remove high switching frequency harmonics. Total harmonic distortion of the output shall be less than 15%.

Topology - Inverter Single-phase bridge inverter

Switching Logic Desire to control Output frequency Output magnitude  Sinusoidal PWM!

Theory of Sinusoidal (Bipolar) PWM The magnitude of a triangle carrier signal is compared to a sinusoidal reference If Vreference > Vcarrier PWM = high If Vreference < Vcarrier PWM = low A complementary signal drives opposite leg of H-bridge

Unipolar Sinusoidal PWM Two sinusoids compared to a triangle reference Each comparison drives one H-bridge leg respectively

Unipolar PWM in Action Two comparisons Each leg of H-bridge driven independently 3-level output Less harmonic distortion than bipolar PWM

Design Equations Modulation index, mi Frequency Modulation ratio, mf Fundamental Output Magnitude Output Frequency

Implications mi can be used to control output magnitude (voltage) Typically 0 < mi ≤ 1 Overmodulation if mi > 1 (non-linear operation) Useful for obtaining large output power, but harmonic distortion will be large

Implications Output Frequency Can select mf to remove even harmonics from output spectrum For Bipolar PWM, mf = odd integer For Unipolar PWM, mf = even integer Example (Unipolar): fcarrier = 60 Hz ftriangle = 2520 Hz mf = 42

Output Desire sinusoidal output Use a filter Output isn’t very sinusoidal Use a filter LC filter

LC Filter Goal: Smooth inverter output to smooth AC Second order LC filter transfer function: G(s) = 1/(L*C*s^2+1) fcarrier < cutoff frequency < fcarrier ∙ mf

Simulation PSIM, Circuit Simulation Software Proof of concept simulations Bipolar PWM vs. Unipolar PWM Effectiveness of LC filter with both schemes

PSIM Schematic (Bipolar PWM) mi = 0.8 mf = 11 Vd = 200 V

PSIM Schematic (Unipolar PWM) mi = 0.8 mf = 10 Vd = 200 V

Simulation Result – Vout (unfiltered) Bipolar PWM Unipolar PWM

Simulation Results (filtered) Bipolar PWM Unipolar PWM fout = 60 Hz (both cases)

Bipolar PWM – Frequency Domain Unfiltered Output Filtered Output mi = 0.8 mf = 81

Unipolar PWM – Frequency Domain Unfiltered Output Filtered Output mi = 0.8 mf = 80

Implementation Major Hardware Components IGBT Gate Drive LC Filter Spectrum Digital eZdsp F2812 Texas Instruments TMS320F2812 Simulink, Code Composer Studio

IGBT International Rectifier IRG4PC30UDPbF VCEmax = 600 V fswitching max = 40 kHz ICmax = 12 A Cost = $2-3 each

Gate Drive International Rectifier IR2110 Drives Two IGBTs/MOSFETs Cost $3 each

LC Filter L = 1 mH C = 100 μF fcutoff ≈ 500 Hz Cost of components = $6

Sinusoidal PWM – Simulink

Experimental Results Bipolar PWM Vd = 10V (DC) Vout = 13.6V (AC) mi=0.8 mf = 83

Future Work Closed-loop MPPT control with PV input Inverter voltage and current controller Tying the inverter to the grid Phase Locked Loop (PLL)

Special thanks to: Dr. In Soo Ahn Mr. Steve Gutschlag

References PV Module Simulink Models.” ECEN2060. University of Colorado Boulder. Rozenblat, Lazar. "A Grid Tie Inverter for Solar Systems." Grid Tie Inverter Schematic and Principles of Operation. 6 Oct. 2011. <http://solar.smps.us/grid-tie-inverter-schematic.html>. Tafticht, T., K. Agbossou, M. Doumbia, and A. Cheriti. "An Improved Maximum Power Point Tracking Method for Photovoltaic Systems." Renewable Energy 33.7 (2008): 1508-516. Tian, Yi. ANALYSIS, SIMULATION AND DSP BASED IMPLEMENTATION OF ASYMMETRIC THREE-LEVEL SINGLE-PHASE INVERTER IN SOLAR POWER SYSTEM. Thesis. Florida State University, 2007.  Zhou, Lining. EVALUATION AND DSP BASED IMPLEMENTATION OF PWM APPROACHES FOR SINGLE-PHASE DC-AC CONVERTERS. Thesis. Florida State University, 2005.

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