System Overview: The Components of the PV-Battery-Supercapacitor Model
The system consists of several key components:
PV Panels: A 2,000W PV system made up of eight 250W panels connected in series. This setup generates a total of 2,000W of power with each panel providing 30.7V.
Battery and Supercapacitor: A 300V battery and a 300V supercapacitor, both connected to a common DC bus via their respective boost converters. These storage devices manage fluctuations in power generation and consumption.
DC and AC Loads: The system powers both DC and AC loads, with the AC load being connected to the grid via an inverter.
Grid Connection: The inverter allows the system to feed excess power into the grid or draw power from it when necessary.
Maximizing Power from the PV Panel: MPPT with Boost Converter
To ensure that the PV panels operate at their optimal power point, Maximum Power Point Tracking (MPPT) is used. The MPPT algorithm adjusts the duty cycle of the boost converter, which controls the voltage of the PV panel. The goal is to continuously track and extract the maximum possible power from the panel.
The duty cycle is adjusted based on the current power and voltage values from the PV panel.
If there is a decrease in power, the duty cycle is adjusted (either increased or decreased) to maintain optimal power generation.
The algorithm checks for changes in power and voltage, adjusting the duty cycle accordingly, ensuring that the system operates at the maximum power point.
Energy Storage and Power Balancing with Battery and Supercapacitor
The battery and supercapacitor are integrated into the system to handle energy storage and manage power fluctuations. Both are connected to the DC bus via boost converters, and their operation is controlled by voltage and current control methods.
The system monitors the DC bus voltage and adjusts the reference current using a Proportional-Integral (PI) controller.
The difference between the filtered output and the unfiltered output helps determine the reference current for both the battery and supercapacitor.
These reference currents are compared to the actual currents from the battery and supercapacitor, and adjustments are made as needed to maintain power balance in the system.
This control method ensures that the battery and supercapacitor provide the required energy to the system when PV generation is insufficient.
Grid Integration and Inverter Control
The system includes an inverter to manage the connection between the DC side of the system (PV, battery, supercapacitor) and the AC grid. The inverter’s function is crucial for maintaining power balance between the renewable generation and the grid.
Inverter Control: The inverter is controlled using current control techniques, generating a reference current that adjusts based on the power available from the PV panels.
Grid Power Interaction: When the PV power exceeds 800W, the system supplies power to the grid from the combined sources of the PV, battery, and supercapacitor. However, if PV power drops below 800W, the system draws power from the grid to meet the load demand.
Phase-Locked Loop (PLL): The PLL ensures that the system synchronizes with the grid’s frequency, maintaining stability and preventing issues with power quality.
This synchronization is achieved by generating reference signals (sine and cosine) that are converted into DQ form for controlling the inverter. The inverter's current is then compared to the reference values, and any differences are corrected using a PI controller.
System Behavior During Changing Conditions
The system's response to changes in irradiance is a key feature of its flexibility. For instance, when irradiance drops (e.g., from 1800W/m² to 500W/m²), the power output of the PV panels decreases, leading to a reduction in system power.
Power Compensation: As PV power decreases, the battery and supercapacitor compensate by supplying additional power. If the PV output falls below 800W, the system draws power from the grid.
Supercapacitor Response: The supercapacitor can supply energy during rapid fluctuations in power generation, especially when the PV panel's output is not sufficient to meet the load.
This dynamic response ensures that the system can maintain stable operation regardless of environmental changes.
Implementing Maximum Power Point Tracking (MPPT) with Incremental Conductance
To further optimize the system’s efficiency, an Incremental Conductance (IncCond) MPPT algorithm is employed. This method adjusts the duty cycle based on changes in the voltage and current of the PV panel.
The algorithm compares the instantaneous changes in voltage and current to determine whether the system is operating at the maximum power point.
If the voltage or current is not at the optimal point, the duty cycle is adjusted (either increased or decreased) to move the system toward maximum power.
This incremental conductance method provides precise control and ensures that the PV system operates efficiently under varying conditions.
Modified Incremental Conductance: Improved Power Tracking
A modified version of the incremental conductance algorithm further improves performance by adding a parameter MMM, which adjusts the duty cycle more precisely.
This modification results in better tracking of the maximum power point compared to the standard incremental conductance method.
The algorithm uses the absolute value of the change in voltage divided by the change in current, multiplying it by a time parameter to adjust the duty cycle.
The modified version provides smoother transitions and better efficiency, especially when irradiance conditions fluctuate rapidly.
Comparing MPPT Methods: Incremental Conductance vs. Modified Incremental Conductance
In the final analysis, the performance of the standard incremental conductance algorithm is compared to the modified version. The results show that while both methods are effective in tracking the maximum power point, the modified approach offers superior performance in certain conditions.
Standard Incremental Conductance: Offers good tracking of the maximum power point but with less precision than the modified version.
Modified Incremental Conductance: Provides a more accurate and stable response, especially under rapidly changing irradiance conditions.
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