Simulating NEC Voltage and Current Values: Page 2 of 3
Inside this Article
Table 1 provides system and component specifications required for calculating maximum voltage and current based on simulation program results. Figure 1 provides a high-level overview of the basic calculation process. As this flowchart illustrates, I start by entering project data into a system design model. I then select specific model data for post-processing to calculate maximum voltage and current values.
The calculations recommended in this example are appropriate for standard non-concentrating crystalline silicon PV modules, but may not be appropriate for all module technologies. The Code specifically requires that a licensed professional electrical engineer document and stamp these calculations. I recommend further that supervising engineers have training and experience relevant to PV power systems and hold an AHJ-accepted state license.
Generating capacity. The dc side of the example PV system integrates 380 Yingli polycrystalline PV modules, rated 330 W each, into 19-module source circuits; the 20 source circuits are split evenly across four 30 kW–rated 3-phase string inverters, with five source circuits per inverter. A new definition in Article 690 defines the generating capacity of a PV power system as “the sum of the parallel-connected inverter maximum continuous output power at 40°C in kilowatts.” The example PV system has a generating capacity of 120 kW (4 x 30 kW), which exceeds the ≥100 kW threshold in 690.7(A)(3) and 690.8(A)(1)(1). Therefore, the Code allows a licensed professional electrical engineer to calculate maximum voltage and current based on simulation program results.
Weather data. Reliable calculations require high-quality data, especially for weather. The example PV system connects directly to the local utility grid in Morrisville, North Carolina, home to the Raleigh-Durham (RDU) International Airport. The National Solar Radiation Data Base categorizes the typical meteorological year 3 (TMY3) data for RDU International (site number 723060) as a Class I dataset, which is the most certain weather data classification. While TMY3 weather data selection is outside the scope of this article, Class II datasets are relatively less certain, and Class III datasets are incomplete. (See “PV Performance Modeling: Assessing Variability, Uncertainty and Sensitivity,” SolarPro, September/October 2015.)
Simulation model. The PV Performance Modeling Collaborative (PVPMC) is an excellent resource for identifying industry-standard calculation methods that meet the new Code requirements in 690.7(A)(3) and 690.8(A)(1)(2). For example, the PVPMC website (pvpmc.sandia.gov) clarifies that Sandia utilizes a point-value model that defines five points on an I-V curve and uses these to predict performance as a function of environmental variables. The industry-standard method I used for this example is a single diode–equivalent circuit model known as the De Soto model or the five-parameter module model. This model can define the entire I-V curve as a continuous function of cell temperature and total absorbed irradiance. The PVsyst module model is another industry-standard single diode– equivalent model.
Modeling tool. PVsyst (pvsyst.com) is perhaps the industry’s best-known performance modeling tool , but it is a fee-based platform available only to licensed users. Therefore, I chose NREL’s free performance modeling tool for this example, specifically version 2017.9.5, 64 bits, revision 3. Interested parties can download SAM software via NREL’s website (sam.nrel.gov).