ELEC 5564 Electric Power Generation Notes

Structure of Solar PV Systems

• Solar PV Array
• Power inverters in solar PV systems
• Power converters in solar PV systems
• State space analysis
• Maximum power point tracking control

Maximum Power Point (MPP)

• The power available from intermittent sources like hydro, wind, and solar varies with environmental conditions such as water flow rate, wind velocity, and temperature.
• For a specific solar isolation level, the potential power available for harvesting is at a maximum at a particular power point.
• Power system controllers are used to maximize energy yield and improve efficiency.

MPPT Control

• Automatically controls the output voltage of a PV panel so it operates continuously at the maximum power point (MPP), regardless of load variation, solar radiation, and temperature changes.
• MPPT utilizes DC-DC converters.

MPPT Control Using DC-DC Converters

• DC-DC converters include:
  • Step-Down (Buck)
  • Step-Up (Boost)
  • Buck-Boost
  • Cuk-converter
• P{PV} = V{PV} I{PV} = V{Out} I{Out} = P{Out}

Direct vs. Indirect MPPT Control

• Indirect MPPT Control
  • Requires prior knowledge of solar panel characteristics, usually obtained from online tests or simulations.
  • This information is stored in look-up tables and used by the controller as a reference to MPP.
• Direct MPPT Control
  • Independent of photovoltaic cell characteristics.
  • Uses algorithms based on system parameters like voltage, current, output power, and temperature.

Direct MPPT Tracking Methods

• Conventional Methods
  • Perturb and Observe (P&O)
  • Hill Climbing
  • Incremental Conductance
• Stochastic Methods
  • Particle Swarm Optimization
  • Fuzzy Logic
  • Artificial Neural Network
  • Genetic Algorithm

Perturb and Observe (P&O) Control

• Voltage and current are measured at the converter input/output, and power is calculated and stored.
• PO = IO \times V_O
• I_{ph} = k
• The algorithm perturbs the operating voltage and observes the change in output power. If power increases, the perturbation continues in the same direction; otherwise, it reverses.
• Repeated until the slope is zero.

Advantages of P&O

• Simple to implement (analogue and digital implementations are possible).
• Widely used due to its low complexity.

Disadvantages of P&O

• Oscillates around the MPP, especially under rapidly changing environmental conditions.
• Reduced efficiency under partial shading due to incorrect tracking during rapid fluctuations.
• Risk of drifting under rapidly varying weather conditions.

Limitations of P&O

• Drifting effect
  • Fix: Use Small steps, reduces responses, Tolerance to errors
• Power oscillations
• Stuck at local MPP
  Fix: P(3)- P(1) >0
  V(3)- V(1) >0
• Partial shading
• Decrease Duty cycle (D) to increase Voltage (V), moving away from real MPP

Improved Step Size in P&O

• Dynamically adjust the step size to balance tracking speed and stability.
• Improves performance under varying environmental conditions, such as rapid changes in solar irradiance.

Hill Climbing

• Focuses on perturbing the duty cycle of the power converter to find the MPP.
• The optimal point is continuously tracked and updated until dP/dV = 0
• If the current power is greater than the previous value, the slope is complemented.
• Basically the same as P&O, in that it regulates the PV voltage to follow the optimal MPP
• Duty ratio is decreased to increase the voltage
• Duty ratio is increased to decrease the voltage

Advantages of Hill Climbing

• Reduced implementation and computational complexity.
• Simpler to implement, as it requires measuring the power output and adjusting the operating voltage.
• Less Oscillation: generally exhibits less oscillation around the maximum power point (MPP) compared to P&O.

Disadvantages of Hill Climbing

• Slower Response to Rapid Irradiance Changes: slower than other algorithms like P&O or Incremental Conductance (IncCond).
• No Guaranteed Convergence in Shaded Conditions: susceptible to finding a local maximum rather than the global MPP in conditions of partial shading.

Hill Climbing vs. P&O

• Hill Climbing is simple and works well in steady conditions, with less oscillation around the MPP but struggles under rapid irradiance changes or partial shading, where it may converge to local maxima.
• P&O is faster and more responsive in changing conditions but suffers from oscillations around the MPP and requires fine-tuning to minimize oscillations.
• The choice depends on the specific application and environmental conditions. If simplicity and lower computational cost are critical, Hill Climbing is a good choice, whereas P&O may be better suited for more dynamic environments despite its oscillations.

Incremental Conductance (IC)

• Exploits the assumption that the rate of change in PV output conductance is equal to the negative instantaneous output conductance.
• P = V I
• At the MPP, \frac{dP}{dV} = 0
• Since \frac{dP}{dV} = \frac{d(VI)}{dV} = I + V \frac{d(I)}{dV} = 0
• The above equation could be written as \frac{d(I)}{dV} = - \frac{I}{V}
• The MPPT regulates the terminal voltage until the condition: \frac{d(I)}{dV} = - \frac{I}{V}
• Tracks the point where the derivative of the power with respect to voltage is zero (the MPP).
• It uses the comparison of the incremental conductance with the instantaneous conductance to determine the direction of voltage adjustment.

Advantages of Incremental Conductance

• More accurate than P&O, especially under rapidly changing environmental conditions.
• Better performance in partially shaded conditions because it can track the maximum power point more accurately.

Disadvantages of Incremental Conductance

• Computational complexity

Direct MPPT Control Comparison

• P&O: Simple and fast, but it may cause oscillations around the MPP and doesn't perform well under rapidly changing environmental conditions.
• Hill Climbing: It is simpler but has a slower response and may converge to a local maximum in complex shading conditions.
• IncCond: It is more accurate and performs better in dynamic conditions but is more complex to implement.

Indirect MPPT Control

• Fractional Open Circuit
• Fractional Short Circuit
• Temperature Control
• Curve Fitting
• Offline techniques that control the operation of the PV using characteristics of the cell itself or using a reference cell

Fractional Open Circuit (FOCV)

• Applies an approximately linear relationship between the V{mpp} and V{OC} under varying atmospheric condition (irradiation and temperature levels).
• The FOCV method estimates the V_{mpp} voltage by measuring the open circuit output voltage of PV panel/array and multiplying it by voltage factor.
• V{mpp} ≈ k \times V{OC}
• The measurement can be performed periodically by momentarily disconnecting the load and measuring the voltage of the whole panel/array.
• The V{OC} is measured by shedding the load of the PV array, allowing V{mpp} to be approximated.
• The main advantages of this method are low complexity and easy implementation (only one voltage sensor is needed). k = 70-80%

Disadvantages of FOCV

• Although simple in the implementation, is only an approximation
• Accuracy of the PV array operating at the MPP is not high.
• These temporary disconnections result in loss of power supply for the load
• The circuit operation might be interfered with by the periodic shedding of the load

Advantages of FOCV

• Simple to implement.
• Requires no complex calculations.
• Fast and easy to integrate.

Disadvantages of FOCV

• Less accurate in varying environmental conditions, as it assumes the fraction remains constant.
• Doesn't adapt to changes in temperature or irradiance, leading to less efficient tracking under dynamic conditions.

Fractional Short Circuit (FSCC)

• Another simplified off-line method, similar principles to FOCV
• Under any given environmental condition, the current at MPP (I{mpp}) has approximately a linear relation with short-circuit current (I{sc}).
• I{MPP} ≈ k \times I{SC}
• Where k is typically between 72-92% of Isc
• Similar to the V{OC} method in measuring ISC, load has to be shed. Then, I{MPP} can be determined
• This method is much more accurate compare to VOC but it requires higher cost of implementation
• It is cheap and easy to implement because it needs only one current sensor
• Value of k is unique for different PV modules.
• It approximates the MPP
• The tracking speed is fast with a reasonable efficiency up to 90%

Advantages of FSCC

• Simple to implement.
• Requires no complex calculations.
• Fast and easy to integrate.

Disadvantages of FSCC

• Not as accurate as more sophisticated methods.
• Like FOC, it doesn't adapt well to changes in environmental conditions.
• May not work well when the irradiance is low or fluctuating.

Temperature Measurement Method

• Uses a temperature sensor attached to the back of a solar module to measure its temperature.
• With the temperature coefficient of the open circuit voltage α= -2.4mV/C° to -5mV/C° for silicon solar cells, the duty ratio for a buck converter can be estimated with the MPP voltage V{MPP} at a reference temperature and known output voltage V2

Advatages of Temperature Measurement Method

• More accurate than simple fractional methods.
• Can better account for temperature-related shifts in the MPP.

Disadvantages of Temperature Measurement Method

• Requires temperature sensors.
• Slightly more complex than fractional methods.
  Still, it may not be as efficient in rapidly changing conditions compared to more dynamic methods like Perturb and Observe (P&O) or Incremental Conductance (INC).

MPP Tracking in Partial Shading Conditions

• Shading occurs over prolonged periods of time and can cause high losses.
• A good MPP tracker should be able to provide good results for irregular operating conditions in which multiple local maxima can occur in the P-V characteristic.

Current Development in PV MPP Tracking

• Hybrid MPPT
• AI based MPPT

Key Design Factors

• Design complexity and cost
• Ability to track the actual MPP
• Convergence speed
• Control accuracy (sensitivity to changing weather conditions)