High Gain Non-Isolated Switched Inductor SEPIC Converter for Renewable Energy Applications

Technical Summary: High Gain Non-Isolated Switched Inductor SEPIC Converter for Renewable Energy Applications

Problem Statement
Renewable energy systems, particularly photovoltaic (PV) generation, require efficient DC-DC converters to interface low-voltage sources with high-voltage loads or DC microgrids. Conventional high-gain converters often suffer from high voltage stress on semiconductor devices, low efficiency, and complex circuit architectures. Furthermore, standard topologies like the Buck converter lack step-up capability, while Buck-Boost and Ćuk converters produce inverted output voltages. Although the standard Single-Ended Primary-Inductor Converter (SEPIC) offers a non-inverted output and continuous energy transfer, it typically lacks the high voltage conversion ratios necessary for many renewable applications without increasing component stress. Additionally, Maximum Power Point Tracking (MPPT) under Partial Shading Conditions (PSCs) remains challenging, as traditional algorithms often struggle to distinguish between local and global maximum power points.

Methodology
The paper proposes a two-fold solution: a modified power converter topology and an enhanced control algorithm.

1. Converter Topology (SI-SEPIC):
   The authors introduce a High Gain Non-Isolated Switched Inductor (SI) SEPIC converter. This topology replaces the standard input and output inductors of a conventional SEPIC with a switched-inductor network comprising four inductors ($L_1$ to $L_4$) and multiple diodes.

   • Operation: The converter operates in Continuous Conduction Mode (CCM). During the switch ON state, all inductors charge simultaneously; during the OFF state, they discharge simultaneously.
   • Voltage Gain: By utilizing the switched-inductor network, the converter achieves a higher voltage conversion ratio compared to the standard SEPIC. The derived DC voltage transfer function is $M_{vdc} = V_o / V_{in} = 2D / (1-D)$, where $D$ is the duty cycle.
   • Design: The paper details the design methodology for a 75 W prototype, including calculations for inductor selection (targeting 20% ripple), output and coupling capacitor sizing, and component stress analysis for the MOSFET and diodes.

2. Control Algorithm (Adaptive PSO):
   To address MPPT challenges under PSCs, the study implements an Adaptive Particle Swarm Optimization (APSO) algorithm.

   • Enhancement: Unlike conventional PSO, the proposed APSO dynamically adjusts the inertia weight ($\omega$), cognitive coefficient ($c_1$), and social coefficient ($c_2$) over iterations.
   • Strategy: The inertia weight and $c_1$ are linearly decreased from high to low values to transition from exploration to exploitation, while $c_2$ is increased to accelerate convergence toward the Global MPP (GMPP).
   • Re-initialization: A mechanism is included to reinitialize particles if a significant change in power ($\Delta P$) is detected, ensuring the system tracks new MPPs when environmental conditions shift.

Key Contributions

• Topology Innovation: The development of a non-isolated SI-SEPIC converter that retains the non-inverted output and wide operating range of the standard SEPIC while significantly boosting voltage gain and reducing voltage stress on components.
• Algorithmic Improvement: The formulation of an APSO-based MPPT controller specifically tuned for PSCs, utilizing variable coefficients to balance tracking speed and accuracy.
• Experimental Validation: The construction and testing of a 75 W laboratory prototype using a Chroma 62150H-600S solar array simulator and an OPAL-RT OP5600 controller to validate both the converter's steady-state performance and the MPPT algorithm's dynamic response.

Results
The experimental results, obtained from the 75 W prototype, demonstrate the following:

• Converter Performance: Under an input of 18 V and a duty cycle of 0.40, the converter successfully produced a 24 V output with a power of 75 W. The voltage stress across the switch and diodes was measured at 42 V, confirming reduced stress compared to high-gain alternatives. The output voltage ripple was measured at 0.6 V (2% of 24 V).
• MPPT Efficiency: The APSO algorithm was tested under various shadow patterns and initial operating points (both left and right of the MPP).
  • In Shadow Pattern 1, the system achieved a tracking efficiency of 99.89% with an average output power of 42.43 W.
  • In Shadow Pattern 2, the system achieved a tracking efficiency of 99.88% (operating point far right) and 99.80% (operating point far left), with an average output power of 56.64 W.
  • The system demonstrated the ability to locate the GMPP quickly, with a tracking time of approximately 2.30 seconds in one test case.

Significance and Claims
The paper claims that the proposed SI-SEPIC converter is a suitable solution for interfacing low-voltage renewable sources (such as PV, fuel cells, and batteries) with high-voltage DC microgrids. The authors assert that the topology offers a "remarkable improvement in voltage boosting capability" while maintaining the simplicity and non-inverted output of the standard SEPIC. The combination of the high-gain converter and the APSO-MPPT algorithm is presented as a robust method for achieving stable operation, high voltage gain, and efficient power conversion under diverse loading and shading conditions. The study concludes that the system is well-suited for applications including electric vehicles, LED lighting, and other high step-up renewable energy systems, with future work planned to scale the topology to higher power ratings.