Reassessing DC Voltage Drop Conventions
Inside this Article
While 2% voltage drop has become a de facto design standard for sizing PV system conductors, our data suggest that the acceptable voltage drop threshold is actually higher than current industry practice.
Ask almost anyone who works for a system integrator how to size PV system conductors with respect to voltage drop, and the nearly unanimous answer is “Keep voltage drop to less than 2%.” Typically, PV system designers hold a maximum 2% standard on both the dc and ac sides of the inverter. When pushed to explain why, nearly everyone (ourselves included) answers with some form of “That’s how it’s always been done.” As the industry continues to reduce system costs, however, we must reassess this rule of thumb to see if it still applies.
The industry, after all, is changing quickly. System costs and PPA costs are falling faster than anyone anticipated. System voltages are moving from 600 V to 1,000 V, and, in some applications, up to 1,500 V. System dc-to-ac ratios—the ratio of module power to inverter nameplate power—are increasing, from 1.2 to much larger numbers. These facts can have significant effects on optimal conductor sizing. In light of these changes, engineering analyses completed in 2012 may not be relevant in 2015.
We decided to dig a bit deeper into the voltage drop on the dc side of the inverter to see if the 2% threshold is still ideal. We started by surveying system integrators and learned that many are making conductor-sizing decisions based on the 2% voltage drop standard. However, some system integrators have developed extremely complicated in-house models to analyze in intense detail the cost and energy trade-offs associated with conductor sizing.
In this article, we attempt to split the difference. We put the 2% rule under the microscope to see if it pencils out economically. We also rigorously analyze the effects of changing system costs and design techniques. Rather than propose a single model that will automatically determine the ideal dc conductor sizes on any project, our objective is to help system designers develop a stronger and more nuanced understanding of the factors driving their decisions. For this article, we base all of our initial voltage drop calculations on standard test condition (STC) ratings, as is most common within the industry. We use production modeling and financial analysis tools to quantify and monetize the actual voltage drop and wire loss values. This helps us draw some realistic conclusions regarding optimal conductor sizing.
Calculating Voltage Drop
The voltage drop calculation itself is actually quite straightforward. The formula that we use as the basis for this article’s calculations is Equation 1:
VD% = (((2 x L x I x RC) ÷ 1,000) ÷ VSOURCE) x 100% (1)
where VD% is the voltage drop percentage, L is the one-way circuit length, I is the operating current, RCis the conductor resistance per 1,000 feet and VSOURCE is the voltage of the power source.
You can use several voltage drop calculators to derive the answer to Equation 1 simply by entering the required inputs. At first glance, the inputs to the equation seem simple enough. However, if you want to analyze the impacts of voltage drop, each input is critical and can drive the results in different directions. So what are the correct reference values for calculating voltage drop?
PV circuit parameters. If you consider a single PV source circuit, VSOURCE varies for nearly every hour of the day, not to mention from one day to the next. Early in the mornings, before the modules have heated up, the operating voltage is higher than in the middle of the afternoon. Average hourly cell temperature values—which directly influence operating voltage—vary from day to day, month to month and location to location. Operating current is also highly variable, based on system design detail and environmental conditions. However, operating current varies based on irradiance in the plane of the array rather than on cell temperature. Therefore, it is actually rather difficult to choose a single power-source voltage and operating-current value.
Some rebate programs require that designers estimate system voltage at a specific temperature for these types of calculations. Typically, these requirements specify an elevated design temperature based on summertime conditions. The concept is similar to sizing source circuits based on extreme cold temperatures: If you design for worst-case conditions, the array operates much better than calculated the majority of the time. This approach is valid for hard-stop design limits, such as maximum system voltage, intended to protect the equipment from damage. However, it does not provide as much value when applied to voltage drop, which is not a product-safety concern, because the worst-case design condition represents only a small number of hours per year.