Modeling and evaluation of MPPT methods for a three-port converter used in PV applications A thesis submitted in partial fulfillment for the degree of Master of Science in. This thesis was written by Amani Saad Alzahrani under the direction of her thesis advisor and approved by her dissertation committee, has been presented to and accepted by the Dean of Graduate Studies and Research on May 22, 2022, as partial fulfillment of the requirements for the degree MASTER OF SCIENCE in Energy Engineering. It is cheaper and more efficient than using two separate power inverters for the PV and the battery.
By applying MPPT, PV arrays are guaranteed to operate at the maximum power they can deliver. The topological structure of the TPC is identified based on a comparison with available TPC topologies. Namely, the PV system installed on the roof of Effat library is simulated using MATLAB/SIMULINK with and without MPPT.
The second step compares the PV effect of a system with two-port converters and a system of TPC when the solar irradiance changes.
Introduction
Overview
- General Overview
- Solar PV Overview
- DC-DC Converter and Maximum Power Point Tracking (MPPT) Overview
- MPPT Techniques
- Three-Port Converter Overview
𝐼𝑀𝑃𝑃: Current measured at maximum power point (MPP) 𝑉𝑀𝑃𝑃: Voltage at maximum power point (MPP). First: the current flows out of the diode due to the energy gained from the sun. Second: the current entering the diode due to the positive polarity across the load.
Assuming that the solar current and the load voltage are independent, the current-voltage (I–V) characteristics could be plotted as shown in Figure 2.10 [6]. 𝑃𝑀𝑃 occurs when the differential power with respect to the voltage is zero, as shown in Figure 1.12 [24]. These techniques are applied to a DC-DC converter connected to a PV array as shown in the figure.
The general schematic of a PV array connected to a DC-DC converter is shown in Figure. The disadvantages of the two-port converter topology shown in Figure 1.26 (a) are low efficiency caused by the increase in conversion stages, high cost due to the switches required in each converter, and complications in control systems. These challenges paved the way to the use of a Three-Port Converter (TPC), as shown in Figure 1.26 (b), which does not face the mentioned disadvantages [47].
Research Aim and Objectives
Significant of the Study and Contribution
Outline of the Thesis
Litrature Review
- MPPT Techniques for PV Application: A Comparison
- Perturb and Observation (P&O) Method
- Incremental Conductance
- Fractional Open Circuit Voltage
- Particle Swarm Optimization
- Two Port DC-DC Converters
- MPPT in Three Port Converters (TPC)
- Three Port Converter (TPC)
It is based on the fact that the derivative of the power with respect to the voltage on the MPP is zero. Where ∆𝑉 and ∆𝐼 represent the increases in PV voltage and current, the working principle of IC is illustrated on the PV curve shown in Figure 2.2 [53]. The constant value 𝑘𝑜𝑐 is shown in Figure 3.3 and is close to the actual maximum voltage with a small difference.
The bees start their search by spreading randomly, as they have no prior knowledge of the area. Each bee sends information to the rest of the swarm about the place of highest density it has passed so far. Two thousand articles from 2011 to 2021 were extracted from Scopus and uploaded to biblometrix to visualize annual publication growth, shown in Figure 2.4.
Regarding the most used words in these publications, the articles were filtered to include 2017 to 2021. The word cloud shows the most frequent words/phrases in the larger font, as shown in Figure 2.5. The authors used Proteus software for modeling and simulation - The authors found that P&O.
As a result, the MPP tracking time was improved by 80% using the modified P&O algorithm. The converter control rule states that for a TPC, three power transmission lines and two control variables are required, as shown in the figure. As shown in Figure 2.8, the primary transmission path consists of only one path, while the secondary path consists of two paths.
Primary One-Stage, Secondary Two-Stage (P1S2) is the basic TPC topology based on these paths as shown in the figure. The selected TPC is partially isolated where there is insolation between the load and the rest of the circuit [75].
Methodology
- Comparing an ordinary PV system with MPPT and without MPPT
- Boost Converter Specifications
- Choosing the best TPC design
- Modeling the chosen TPC
- TPC overview
- Controller design
- Comparing two port converters and three-port converter
- Implementing MPPT techniques using MATLAB/SIMULINK
- Comparing the MPPT algorithms
The most appropriate design for TPC is shown in Figure 3.3 based on a literature review [75]. The choice of TPC is based on the presence of a partially isolated topology in which the load is isolated by a transformer. This topology ensures safer operation of the load, especially if the load is sensitive [75].
The other two ports are the PV, where MPPT is implemented to extract maximum power and a battery as a storage system. The PV port is unidirectional, while the battery port is bidirectional to allow the current to flow from or to the battery (change and discharge process). When the PV power is higher than the load demand, SB1 reaches the maximum value and allows the battery to charge.
The control of the TPC and the selection of an operating mode is accomplished by controlling the pulses generated to the four switches SA1, SA2, SB1 and SB2. In all the modes, SA1 and SA2 are compliments of each other, and SB1 and SB2 are compliments of each other. This step aims to compare the two-port and three-port converters with different weather conditions using two MPPT techniques.
The first simulated system is a traditional system where a boost converter connects the PV and the load and a bi-directional boost converter connects the battery and the load. MATLAB/SIMULINK software uses different MPPT techniques for a system consisting of PV, TPC and load. As already mentioned, this method is simple and does not require mathematical modeling of the system [65], [68].
If a PV array is operating at point 1 in the figure, the voltage must be increased by decreasing the duty cycle to increase the power. The output PV output is compared based on its response to solar irradiance and temperature changes.
Results and Discussion
The PV system without MPPT
The voltage and current waveforms of the PV system without MPPT are shown in Figure 4.3 and Figure 4.4. The above simulations show that MPPT techniques affect the PV system by increasing the output power closer to the maximum power. Therefore, the application of MPPT to the current PV system on the roof of the Effat library is recommended.
Comparing between two port converter and three port converter
We can conclude that the two-port converter handles the changes more effectively than the three-port converter. However, the three-port inverter has a more stable response than the two-port inverter when the temperature is kept constant for a certain time. Therefore, a two-port inverter is recommended if the PV system is installed in an area where the temperature changes rapidly.
Implementing the TPC in MATLAB/SIMULINK environment
The MPP of PV is affected by radiation; higher radiations result in higher MPP. In real life, the radiation is constantly changing, so the MPPT method must respond quickly to these changes with greater efficiency. The PV power of the system when using the P&O method and varying the radiation is shown in Figure 4.10.
The PV power of the system when applying the IC method and changing the irradiance is shown in Figure 4.11. However, in TPC, the difference between P&O and IC performance is less significant than the difference when these methods are applied to a standard two-port converter. This shows that TPC is more stable with respect to environmental changes than regular two-port converters.
In terms of changing the temperature, there is no difference in the MPP graph generated by both the P&O and IC methods, as shown in Figure 4.12 and Figure 4.13. However, with standard inverters, there is a difference between the two methods when the temperature changes.
Conclusion
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