Driving Down the Cost of Testing 1,500 V Photovoltaic Systems

PV Inverter Testing

Driving Down the Cost of Testing 1,500 V Photovoltaic Systems

Demand for high-power solar strings has soared, driven by growth in e-mobility: hybrid electric vehicles, electric vehicles, and residential energy-management systems. The latest trend is a shift from 1,000 Vdc photovoltaic (PV) systems to 1,500 Vdc systems. This trend is especially strong in Europe and Asia as utility companies increase capital expenditures on the higher-voltage technology. Demand in the United States in starting to catch up as relevant industry standards and associated certification processes are resolved. According to Underwriters Laboratory (UL), raising the system voltage allows for longer strings, thereby cutting the number of combiners and the amount of wiring needed in the DC collection system. This also reduces labor costs. By decreasing wire losses and increasing efficiency at the power inverter, which converts DC to AC, higher-voltage systems also improve financial returns and improve the levelized cost of energy (LCOE), which is a key metric for large commercial and utility-scale systems. Crucial product differentiators—and therefore design goals—include the energy yields and long-term operational reliability of power inverters. The most widely deployed type is a string inverter, which collects the output of a solar-array string and is installed near a fuse box or electricity meter. New emerging technology is a smaller, more compact micro inverter that is installed at each solar panel.

The Challenge: More Testing in Less Time, with Less Cooling

Almost simultaneously, three prominent manufacturers of PV systems approached Keysight with similar challenges. All had been producing 600 and 1,000 Vdc PV inverters and were moving up to 1,500 Vdc designs. As part of their respective design and test processes, all were using DC power supplies to simulate the dynamic behavior of PV solar arrays. During testing, it’s seldom possible to use actual solar arrays to provide power. Two reasons are obvious: there is no direct sunlight in a test bay; and it isn’t practical to test outside. Two additional reasons are crucial to accurate testing: repeatability and controllability. These attributes make it necessary to simulate the effects of varying operating conditions—light intensity, temperature, shadow, eclipse—at multiple operating points. Specific to 1,500 Vdc inverters, all three manufacturers faced a common set of test challenges. First, all were tasked with testing more parameters in less time to achieve faster time-to-market (TTM). As a result, all needed fast, efficient and flexible testing that addressed the evaluation of both static and dynamic maximum power point tracking (MPPT) in a cost-effective manner. Second, they needed a way to reduce the escalating cooling costs in their test facilities due to the tremendous amounts of heat generated by higher-power inverters.

The Solution: Covering Mature and Emerging Technologies

After individual meetings with all three manufacturers, Keysight application specialists proposed solutions based on the Keysight N8957APV photovoltaic array simulator. Four capabilities addressed the major challenges: 1,500 Vdc output voltage; 1,000 Vdc isolation voltage; auto ranging; and ease of use. With support for both 1,000 and 1,500 Vdc, the engineers are now equipped to handle test requirements for mature and emerging solar technologies. For example, they can quickly create, visualize, and execute PV/solar I-V curves to quickly and comprehensively test inverter efficiency and MPPT algorithms (Figure 1). In addition to its auto ranging capabilities, the N8957APV also doubles as a power supply that can provide myriad combinations of higher voltage or higher current along a maximum power curve (Figure 2). Specifically, the Keysight solution provides continuous V/I combinations ranging from 1500 V/10 A to 500 V/30 A. This means manufacturers no longer need to invest in multiple power supplies, reducing equipment costs and saving bench space. The PV manufacturer engineers appreciated the ease of use that made these advanced capabilities so readily accessible. For example, they can simply input test parameters such as Pmp, Vmp, as so on, and then click the Start Test button. The solar array simulator (SAS) control software automatically creates the required reports in the specified formats, eliminating the time and tedium of data logging and report generation (see Figure 3).