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

Step Up Converter Analysis

• Inductor Voltage - Switch On:
  • $V<em>L = V</em>{PV}$
• Inductor Voltage - Switching OFF mode:
  • $V<em>L = V</em>{PV} - V_O$
• Steady State:
  • As current rise and fall values are equal, $V<em>O > V</em>I$

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: $T<em>{ON}$, $T</em>{OFF}$, $I<em>{MAX}$, $I</em>{MIN}$
• $I<em>{MAX}$: The maximum input current at $t = T</em>{ON}$
• $I<em>{MIN}$: The minimum input current at $t = T</em>{OFF}$
• $\tau = \frac{L}{R_{in}}$

Ripple Current Analysis (Continued)

• Figure: Output current of a boost converter as a function of time.
  • Labels: $T<em>{OFF}$, $T</em>{ON}$, $I<em>{MAX}$, $I</em>{MIN}$
• 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 $V<em>I$, Step-up it is $V</em>O$).
     • $R_{in}$: 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 $I<em>o$, $I</em>{o<em>{ave}}$, $\Delta I</em>o$, $V<em>o$, $\Delta V</em>o$, $T<em>{on}$, $T</em>{off}$, $V<em>{o</em>{ave}}$, $\Delta I$, and $\tau$