Solar arrays, particularly utility-scale and commercial and industrial (C&I) installations, collect a significant amount of data for efficient asset management and qualitative operational metrics such as performance ratio. A substantial amount of this data can be collected from inverters and substations with easy access to AC power and communications.
However, sites for other information, such as weather data, solar irradiance, back of module temperatures, combiner box monitoring, etc., are often in distributed locations throughout a solar array. Powering data acquisition hardware at these locations, often via AC sources and using trenching and/or conduit runs, requires upfront costs that must be borne at the time of array installation.
Typically, distributed PV data acquisition hardware only has a low demand for power. Within arrays, however, there is a significant amount of power at array voltages available at combiner boxes, recombiners and other locations.
Using these sources via array-powered DC/DC converters can be a cost-effective option by eliminating expenditures to supply power for distributed data acquisition systems. In addition, completely autonomous data systems are also possible using wireless technology.
Options for powering distributed PV data acquisition hardware
AC power distribution via trenching and conduit runs have been the most frequently used methods of supplying distributed PV data acquisition hardware. The main advantage of this method is that once installed, continuous power makes measurement possible 24 hours a day. The largest challenges are high upfront installation costs and limited flexibility in making changes post installation.
In addition, in scenarios where extensive intra-array measurements are desired for asset management purposes, the upfront costs for power distribution during installation must be balanced against additional benefits realized during operation — often a conflicting set of goals. For critical data, batteries or a UPS is often used to ensure operation during potential power interruptions.
While often seen in solar tracking, completely autonomous power systems utilizing independent solar panels and batteries are rarely used to power distributed PV data acquisition hardware. These systems, however, do address the need to reduce upfront installation costs, and a requirement for reserve power. They also allow part operation of the arrays during installations. Low equipment power demand, as well as the distributed nature intra-array measurements, make these systems most practical for the isolated equipment installations such as remote weather stations.
Array-powered DC/DC converters offer the advantages of the hard-wired power distribution systems, as well as the flexibility of the autonomous power system. By utilizing the distributed nature of DC interconnections within an array, cabling to power PV data acquisition hardware is minimized. In addition, 1000V and 1500V nominal input array voltage models allow installation with both C&I and utility scale installations. Low voltage 24V DC output is compatible with most data acquisition hardware. The wide input ranges of these array-powered DC/DC converters ensure stable operation over the complete input range of most inverters. Using a suitable 24V DC UPS, t is possible to operate on 24 hours a day basis and meet reserve power requirements.
Installation considerations for array-powered DC/DC converters
Array-powered DC/DC converters connect directly to the high-voltage DC side of a PV array. As a result, they must be tested to ensure safe operations at these voltages. In the U.S., standards most often applied are UL 1741 or the globally harmonized UL 62109.
Array-powered DC/DC converters are connected to the array using appropriately rated PV fuses. Depending on the model, current of 1 or 2 amps are typically used. Surge suppression, if not already deployed at the connection point, is recommended.
Typical C&I and utility-scale PV solar arrays have a large amount of power available and concerns about power used is minimal. As an example, with a 1500V array and worst-case operation at a minimum DC input voltage of 600V, the power required to supply 8A at 24V DC output is approximately 200 watts. This only around 4% of a typical 4 kWp string output and relatively insignificant when connected to the output of a combiner or recombiner. It is possible to install array-powered DC/DC converters in parallel for increased output current using a suitable blocking diode unit.
When a 24V DC UPS is used, these systems must be sized according to the output power required and the duration of the reserve power needed. Consult UPS manufacturers for appropriate equipment choices. For 24-hour continuous operation where UPS batteries are cycled daily, depending on temperature, Li-ion batteries can provide cycle life approaching 7,000 charging cycles.
Array-powered DC/DC converters
Array-powered DC/DC converters can offer an alternative to hardwired AC power sources for distributed PV data acquisition measurements and can reduce the capital costs to install these systems. Their autonomous nature offers flexible powering options and allows easy installation within arrays. When used with wireless communications, completely autonomous data systems are possible. Powering distributed PV data acquisition systems, array-powered DC/DC converters can assist in supporting efficient asset management of solar arrays.
Dan Sylawa is Business Development Manager – Renewable Energy for Phoenix Contact.
— Solar Builder magazine