[IEEE] Dual Buck and Boost Inverter: Solving the Partial Shading Puzzle in Transformerless PV Systems

Executive Summary: The Efficiency vs. Resilience Trade-off

TL;DR: Harvesting solar energy in mismatched environmental conditions (MEC)—like partial shading or uneven temperatures—usually forces a compromise between system complexity and power yield. This paper introduces a Dual Buck and Boost Inverter (DBBI) that splits a PV array into two independently optimized subarrays, achieving high efficiency (97.65% peak) and low leakage current while drastically reducing power loss during shading.

Positioning: This work is a significant advancement in Grid-Connected Transformerless (GCT) inverters. It moves beyond standard H-bridge or NPC topologies by integrating multi-input buck-boost capabilities directly into the inverter's front end, effectively "decoupling" subarray performance.

The Pain Point: The "Weakest Link" Problem

In traditional series-connected PV strings, the module with the lowest current (due to a passing cloud or a chimney shadow) limits the entire string's output. To reach the high voltages required for grid injection (e.g., >311V for a 220V AC grid), typical inverters force many modules into a single series chain.

• NPC/H-Bridge: Highly vulnerable to MEC; if one module fails or is shaded, the entire string suffers.
• Prior Buck/Boost attempts: Often used high component counts or suffered from high switching losses, leading to poor overall efficiency.

Methodology: Decoupled Optimization

The core innovation lies in the segmented DC-to-DC stage. The PV array is divided into PV1 and PV2. Each has its own converter (CONV1 and CONV2) that can operate in Buck Mode (if $V_{pv}>v_{co}$) or Boost Mode (if $V_{pv}<v_{co}$).

1. Architectural Innovation

By using two converters, the system only processes the power required to match the instantaneous grid voltage. The inverter stage then "unfolds" this DC power into a 50Hz AC sine wave.

Figure 1: The proposed DBBI schematic showing the two DC-DC segments (CONV1/2) and the unfolding inverter.

2. Leakage Current Suppression

In transformerless systems, parasitic capacitance between the PV panels and the ground can lead to dangerous leakage currents. The DBBI control strategy ensures that high-frequency components are absent from the common-mode voltage, keeping the leakage current at ~80mA, well below the 300mA safety standard (VDE 0126-1-1).

Experimental Results: Performance Under Pressure

The researchers tested a 1.5 kW prototype against several standard industry topologies. The results clearly demonstrate the DBBI's resilience.

SOTA Comparison: Power Yield

When one module experiences a 50% drop in solar intensity, the differences are stark:

• NPC Inverter: 31.5% Power Loss.
• H-Bridge: 18.3% Power Loss.
• DBBI (This Paper): Only 3.6% Power Loss.

Figure 2: Measured Efficiency curves showing a high European efficiency of 97.02%.

Grid Quality

Despite the complex dual-mode switching, the grid current THD remained at 4.61%, meeting the strict IEEE 1547 standards (which require <5%).

Critical Insight & Conclusion

Why it works

The DBBI succeeds because it reduces the "minimum module constraint." Because the inverter can boost voltage, you can have fewer modules in series per subarray. Fewer modules in series means a lower statistical probability that a single shaded module will cripple a large portion of the array's total potential.

Limitations & Future Work

While the DBBI is highly effective for two subarrays, scaling this to 4 or 8 segments for even higher MEC resilience might increase cost and control complexity. Future research could explore the use of Wide Bandgap (WBG) semiconductors like GaN to shrink the inductors ($L_1,L_2$) even further, potentially creating a highly compact "smart" micro-inverter for residential rooftops.

Final Takeaway: This is a robust, hardware-validated solution for the real-world problem of urban solar harvesting, where "perfect" sunlight is a rare luxury.