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In this follow-up to the article "Design Recommendations for 1500V String Inverters," we take a deeper look at the concept of "Free Voltage Drop" and how it plays a critical role in modern solar system design.
When value engineering large-scale solar projects, one often overlooked advantage is the potential for "free" voltage drop in systems with high DC-to-AC ratios. As PV modules have become more affordable over the past decade, and system voltages have increased from 600VDC to 1,500VDC, the typical DC-to-AC loading ratio has also risen—from around 1.15–1.25 to as high as 1.3–1.7. This shift has created new opportunities and challenges for engineers designing efficient and cost-effective solar arrays.
Voltage drop on the DC side is typically seen as a loss that reduces overall system performance. However, when inverters are operating at or near their maximum AC output (clipping), any additional DC power beyond what the inverter can handle is essentially wasted. In such cases, some of that lost DC power due to voltage drop doesn’t impact the final energy output—hence the term “free voltage drop.†This means that increasing conductor size to reduce voltage drop may not be as beneficial in high DC-to-AC ratio systems as it once was.
On the other hand, AC voltage drop remains a concern, especially in larger systems where long AC runs can lead to grid voltage issues and nuisance tripping. It’s crucial to keep AC voltage drop under control, even if DC voltage drop becomes less impactful due to clipping.
As PV systems move toward higher voltages like 1,500VDC, the number of parallel circuits decreases, which helps reduce overall system losses. Additionally, higher voltages mean lower current for the same power level, which significantly reduces I²R losses in the DC circuits.
To accurately assess voltage drop, engineers should use dynamic modeling tools like PVsyst, which simulate real-world conditions over time. These models account for variations in irradiance, temperature, and system performance throughout the year, providing a more accurate picture of actual voltage drop than static calculations.
It's also important to consider how voltage drop interacts with inverter clipping. During peak power periods, when clipping is most likely, the effective voltage drop can be much lower than theoretical values. This dynamic behavior highlights the need for a holistic approach to voltage drop analysis, especially in high DC-to-AC ratio systems.
By strategically placing inverters closer to the point of interconnection, you can minimize AC voltage drop and increase the amount of free DC voltage drop, improving both efficiency and return on investment.
If you're interested in optimizing your next commercial or utility-scale solar project, reach out to Pure Power Engineering for expert design and construction services tailored to your specific needs.
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