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Isolated Single Phase Maximum Power Tracking Photovoltaic String Inverter

Isolated Single Phase Maximum Power Tracking Photovoltaic String Inverter. By Lloyd Caleb Breazeale Qualifying Examination Arizona State University March 10, 2010 . Discussion Topics . Isolated String Inverter Maximum Power Point Tracking Isolated Boost Control System

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Isolated Single Phase Maximum Power Tracking Photovoltaic String Inverter

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  1. Isolated Single Phase Maximum Power Tracking Photovoltaic String Inverter By Lloyd Caleb Breazeale Qualifying Examination Arizona State University March 10, 2010

  2. Discussion Topics • Isolated String Inverter • Maximum Power Point Tracking • Isolated Boost Control System • Inverter Control System • Anti-Islanding • Implementation and Test Results • Summary • Future Research

  3. Isolated String Inverter • Challenges • Maintain input/output power balance • Maximum power point tracking (MPPT) • Low distortion output current • Input/output isolation • Rapid shutdown when utility source is absent (anti-islanding)

  4. Topology Selection Full bridge isolated boost H-bridge inverter

  5. Maximum Power Point Tracking Typical photovoltaic power curve No change Increment voltage Decrement voltage • Adjustments are made through • control of the array voltage

  6. Incremental Conductance MPPT Equivalent conditions for maximum power • The power slope is determined from the incremental conductance • This method results in quick tracking without oscillations

  7. Isolated Boost Voltage Control System • Maximum power tracking necessitates control of the input voltage Controller Plant approximation Time delay Low pass filter

  8. Derivation of Small Signal Plant Model Basic relationships Combined Linearized

  9. Input Voltage Control System Open loop frequency response Step response Root-locus for increasing PV impedance

  10. Discrete Time Simulation of Isolated Boost

  11. Isolated Boost Simulation Results Inductor current (A) Input voltage (V) Reference step from 235 V to 600 V Load step from 500 W to 5000 W Reference step from 600 V to 235 V

  12. Inverter Control System • Power balance necessitates control of the DC link voltage • Harmonic compensators are needed in the AC current loop • to attenuate harmonic distortion • The phase locked loop creates the ideal sine wave reference

  13. Inverter Control System Models PLL filter Grid voltage filter DC link controller LCL filter DC link voltage filter LCL filter Grid current controller Grid current filter Harmonic compensators Link controller plant Time delay

  14. Inverter Current Control Open loop response Grid voltage disturbance attenuation Sensor noise attenuation Reference tracking Discrete time controller Root locus

  15. Phase Locked Loop Open loop system valid near zero phase error Discrete time open loop frequency response

  16. DC Link Voltage Control Small signal plant model Open loop response Basic relationships Discrete time controller Combined Linearized at constant power

  17. Inverter Discrete Time Simulation System interconnection Switchmode plant Phase locked loop Control system

  18. Inverter Simulation Results Grid current step response Link voltage step response Time (s) Time (s)

  19. Phase Locked Loop Simulation Time response of 60 degree phase step Normalized amplitude Time (s)

  20. Anti-Islanding • Islanding is a condition in which a portion of the electric grid stays energized after the section has been disconnected from the main utility. • Worst case test condition is when the grid is neither supplying nor absorbing real or reactive power at instant of disconnect: • The greater RLC load quality factor, the more difficult islanding is to detect:

  21. Anti-Islanding • Typical anti-island methods act to move the grid voltage beyond shutdown thresholds when the utility voltage source is absent • This is accomplished with a controlled instability of voltage, phase, or frequency • Common methods include Sandia Voltage Shift, Sandi Frequency Shift, and Slide Mode Shift

  22. Sandia Voltage Shift • Current reference is a function of the difference between • instantaneous and nominal grid voltage: Voltage and current after disconnect Voltage trajectory after disconnect

  23. Sandia Frequency Shift • Current wave shape is a function of the difference between • instantaneous and nominal grid frequency: where Positive chopping fraction Negative chopping fraction cf

  24. Sandia Frequency Shift Simulation Inverter current wave shape Frequency trajectory after island Theoretical non-detection zone

  25. Slide Mode Shift • Phase offset is a function of the instantaneous grid frequency: Slide mode shift phase characteristics

  26. Slide Mode Shift Simulation Phase and frequency trajectory after disconnect Phase (rad) Shutdown thresholds Frequency (rad/s) Time (s)

  27. Implementation Note: Maximum power tracking and anti-islanding were not included in this prototype. Also the input control was initially configured to regulate current not voltage as previously discussed

  28. Design and Simulation Script

  29. Hardware Power board Control board

  30. The LCL Filter Attenuation of inverter voltage to grid current Damped LCL Average inverter voltage Grid voltage

  31. Inverter Side Inductor High frequency flux density

  32. Firmware RUN mode State flow

  33. Switching Patterns Isolated boost Inverter

  34. IsolatedBoost Converter Clamp capacitor voltage (50 V/div) Startup voltage at inductor output (250 V/div) Inductor output voltage (50 V/div) Startup inductor current (20 A/div) 2 ms/div 50 us/div

  35. Inverter Output Without harmonic compensation With harmonic compensation Grid voltage (100 V/div) Grid current (5 A/div) 5 ms/div 100 Hz/div Generating VARs Absorbing VARs Grid voltage 25 V/div Grid current 1 A/div 5 ms/div

  36. CompleteClosedLoop System Output power: 300 W Output power: 170 W Grid current (5 A/div) Grid voltage (100 V/div) Input voltage (100 V/div) DC link voltage (100 V/div) 10 ms/div 10 ms/div

  37. Summary • A two stage power converter was analyzed, simulated, • and implemented • Other control aspects were explored: anti-island and MPPT • Complete functionality was verified at low power

  38. Why so much effort into a system that is not new ? • To create a control system test platform • To discover areas of improvement • To develop working circuits that can be applied to new power converters • Personal benefit of increasing capability level

  39. Future Research Control systems of power networks operating in a non traditional manner: • Energy storage • Intermittent distributed resources • Flexible power electronic circuits • Presence or absence of the greater electric grid • With communication interface

  40. Questions ?

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