A VTBBased Virtual Environment for Solar Array Maximum Power Point Tracker Design

1
A VTB-Based Virtual Environment for Solar Array
Maximum Power Point Tracker Design

• Zhenhua Jiang Roger Dougal
• Department of Electrical Engineering
• University of South Carolina
• Columbia, SC 29208, USA

2
Outline

• Introduction/Motivation
• VTB-Based Development Procedure
• Characteristics of Solar Array
• Review of Maximum Power Point Tracking Techniques
• Virtual-Prototyping of Solar Illumination
• Processor-in-the-Loop Simulation Method
• Hardware-in-the-Loop Testing Environment
• Conclusions

3
Introduction/Motivation

• Objective Develop a rapid prototyping
  environment for solar array maximum power point
  tracker design based on VTB
• Challenges
• It is necessary to test the maximum power point
  tracker under complex solar illumination
  conditions.
• It is difficult to obtain the same solar
  illumination condition during different steps of
  design process.
• Approach Apply a consistent design process in a
  rapid development environment to make incremental
  improvements before construction of final system.

4
Development Procedure
Develop and test MPPT algorithms under particular
illumination conditions
Processor-in-the-Loop Simulation
Identify Complex Illumination Conditions
Same Illumination Condition
Same Control Algorithm
Model the various cloud effects Find particular
illumination conditions where there are multiple
maxima
Hardware-in-the-Loop Testing
Validate MPPT algorithms on real hardware
5
Solar Array Equivalent Circuit
Rs
i


v
Ip
Rsh
_
Solar Cell Equivalent Circuit
_
Solar Array
6
VTB Model of Solar Array
T increases
Electrical () (Voltage, Current)
Illumination (Signal)
Thermal (Temp, Power)
Electrical (-) (Voltage, Current)
Model Icon
Model Validation
7
I-V P-V Curves of Ideal Solar Cells
Current I (A)
Power P(W)
Vmp, Imp
Isc
Pm
Max Power Point
Voc
Voltage V(V)
8
Review of MPPT Techniques

• Perturb and Observe Method
• By periodically perturbing the array voltage and
  comparing the output power with that at the
  previous perturbing cycle. But the operating
  point oscillates around the MPP since the system
  must be continuously perturbed
• Incremental Conductance Method
• By comparing incremental conductance with
  instantaneous conductance
• Short-Circuit Current Method
• The operating current of solar array at the MPP
  is approximately linearly proportional to its
  short-circuit current
• Open-Circuit Voltage Method
• The voltage of solar array at the MPP is
  approximately linearly proportional to its
  open-circuit voltage
• Nonlinear Optimization Method
• Artificial Intelligence (Fuzzy Logic, Neural
  Network, etc.)

9
Incremental Conductance Method
Instantaneous
Incremental
If the instantaneous conductance is greater than
the incremental conductance, the operating
voltage is below the voltage at MPP, and vice
versa. The MPPT algorithm is therefore to
search the voltage operating point at which the
instantaneous conductance is equal to the
incremental conductance.
10
Flowchart of Incremental Conductance Method
Read in voltage and current
Read V(k), I(k)
dV V(k) - V(k-1) dI I(k) - I(k-1)
Compute changes of voltage and current
Y
dV 0?
Make judgment
N
dI/dV - I/V?
Y
Y
dI 0?
N
N
Y
dI/dVgt -I/V?
Y
dI gt 0?
Modify references
N
N
Vref Vref deltaV
Vref Vref deltaV
Vref Vref deltaV
Vref Vref deltaV
V(k-1) V(k) I(k-1) I(k)
Keep voltage and current
Return
11
Development Procedure
Processor-in-the-Loop Simulation
Identify Complex Illumination Conditions
Same Illumination Condition
Same Control Algorithm
Hardware-in-the-Loop Testing
12
Mismatched Cells in a String
_
_

I
_

Dissipate Power
Solution Group the cells and add a bypass diode
within each group
I
IL
I-IL
Problem May introduce multiple maximum power
points
13
Effect on Total output of a Poor Cell with a
Bypass Diode
Current I (A)
Power P(W)
Good Cells
Bad Cell
Combination
Multiple Local Max Power Points
Voltage V(V)
14
Interrupt Scan Method

• One possible method to overcome multiple maxima
  problem is to interrupt the normal operation and
  then scan the entire control range to find the
  global MPP.
• This process is repeated every fixed duration
  (e.g., 10 minutes). Once the global MPP is found,
  the incremental conductance method is thereafter
  used to find the MPP within the fixed duration.
• This tracking method is best suited for digital
  control implementation.
• The disadvantage of this method is that the
  operation is interrupted and a small amount of
  power is lost when scanning the entire control
  range.

15
Cloud Model
Always Partial Shading
Partial Shading
Random Shading
Full Shading
16
Multiple Local MPPs
70
40
17
Virtual-Prototyping of Solar Illumination
Always Partial
Always Partial Shading
Full
Partial Shading
Full Shading
Partial
18
Development Procedure
Processor-in-the-Loop Simulation
Identify Complex Illumination Conditions
Same Illumination Condition
Same Control Algorithm
Hardware-in-the-Loop Testing
19
Processor-in-the-Loop Simulation Setup
Compare two MPPT methods Incremental
Conductance Interrupt Scan
Serial Comm. Line
Infineon C167CR-LM Microcontroller
Windows PC / VTB
20
Results Output Power
Interrupt Scan


Incremental Conductance
21
Results SA Diode Currents
Interrupt Scan

Incremental Conductance
Diode Currents
Interrupt Scan
D3
Partially shaded
D2
Incremental Conductance
22
Results Voltages
Incremental Conductance
Incremental Conductance

Interrupt Scan
Interrupt Scan
23
Development Procedure
Processor-in-the-Loop Simulation
Identify Complex Illumination Conditions
Same Illumination Condition
Same Control Algorithm
Hardware-in-the-Loop Testing
24
Solar Array Simulator
Current I (A)
Vmp, Imp
Isc
Load Line
Voc
Current Source
Voltage Source
Voltage V(V)
25
Hardware-in-the-Loop Testing Environment
LabVIEW program
VTB Environment
Programmable Power Supply
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Hardware-in-the-Loop Testing Approach

• Measure the output current and voltage of the
  solar array simulator and send them back to the
  VTB
• When the voltage is less than Vm, the VTB
  calculates the output current of the solar array
  by operating the solar array at a constant
  voltage LabVIEW program sends a current command
  to the power supply.
• When the voltage is greater than Vm, the VTB
  calculates the output voltage of the solar array
  by drawing a current from the solar array
  LabVIEW program sends a voltage command to the
  power supply.

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Advantages

• Complex solar illumination conditions are
  necessary to thoroughly test the maximum power
  point tracker. The described VTB-based virtual
  environment allows the user to virtual-prototype
  ANY possible weather conditions.
• It is possible to obtain the SAME solar
  illumination conditions during each step of the
  development process with the described VTB-based
  virtual environment.
• The HIL environment provides a convenient and
  cost-effective method to simulate the complex
  weather condition on the hardware.

28
Conclusions

• A consistent rapid prototyping process for solar
  array maximum power point tracker design in a
  VTB-based virtual environment was described.
• Solar array characteristics were analyzed and max
  power point tracker issues were addressed.
• The same solar illumination conditions can be
  obtained in PIL simulation and HIL testing steps.
  
• The described virtual environment provides a good
  tool to design and verify the solar array max
  power point tracker design.