ELEC 5564 Electric Power Generation Notes

ELEC 5564 Electric Power Generation by Renewable Sources

PV Power Generation System Structure

  • Lecturers: Dr. Salma Alarefi
  • Topic: PV Power Generation System Structure
  • Focus on Boost Converter

Outline

  • Structure of solar PV systems:
    1. Solar PV Array
    2. Power inverters in solar PV systems
    3. Power converters in solar PV systems
      • Buck Converter
      • Boost Converter
        • State space analysis

Step Up Converter (Boost Converter)

  • Switching period: T = T{on} + T{off}
  • Duty cycle: D = \frac{T{on}}{T} \therefore T{on} = DT
  • D is also represented as k. Therefore, k = \frac{T_{on}}{T}
  • Switch control signals
  • Circuit Diagram:
    • Components: Input voltage source (V{PV}, from PV panel), Inductor (L), Switch (S), Diode (VD), Output Capacitor (CO), Load Resistor (R_{in})
    • Variables: Input current (i{in}), Inductor current (iL), Switch current (i{SW}), Diode current (iD), Output voltage (V_O)
    • Relationship: VO = \frac{V{PV}}{(1-D)}

Step Up Converter Analysis

  • Inductor Voltage - Switch On:
    • VL = V{PV}
  • Inductor Voltage - Switching OFF mode:
    • VL = V{PV} - V_O
  • Steady State:
    • As current rise and fall values are equal, VO > VI

Ripple Current Analysis

  • Inductor is at the input side of the converter.
  • Consider voltage across L.
  • During ON Time.
  • During OFF Time.
  • Figure: Output current of a boost converter as a function of time.
    • Labels: T{ON}, T{OFF}, I{MAX}, I{MIN}
  • I{MAX}: The maximum input current at t = T{ON}
  • I{MIN}: The minimum input current at t = T{OFF}
  • \tau = \frac{L}{R_{in}}

Ripple Current Analysis (Continued)

  • Figure: Output current of a boost converter as a function of time.
    • Labels: T{OFF}, T{ON}, I{MAX}, I{MIN}
  • The peak-to-peak input current is \Delta I_{PP} (formula not provided in the transcript).
  • When k = 0.5 we have.
  • So the same formula for f and L evaluations applies.

Filter Inductor Design (Assignment)

  • In practical design, the procedures are as following:
    1. Evaluate I_{PP} for existing L and f by (formula using V, Rin, τ, and T).
      • V: voltage value at high side (for Step-down it is VI, Step-up it is VO).
      • R_{in}: should be the load resistance for step-down, input inductor resistance for step-up.
      • \tau: is time constant \frac{L}{R}. and T is switching period.
    2. To calculate new L or f, for the required ripple percentage %: work out the required I_{PP} = average input current times required ripple percentage %.
    3. Then calculate a, for fixed f.
    4. Calculate L by (formula).

Output Voltage Ripple and Capacitor Design (Assignment)

  • Diagram showing Io, I{o{ave}}, \Delta Io, Vo, \Delta Vo, T{on}, T{off}, V{o{ave}}, \Delta I, and \tau