by Stefan Froböse, Sungrow
Historically, central inverters have been mainly used in utility-scale projects. Larger power units lead to lower system costs by making installation, operations & maintenance easier. By contrast, string inverters were mainly utilized in complex terrain due to smaller power units and more MPPTs (Maximum Power Point Tracking).
Many of the utility-scale project developers are now moving beyond their traditional understanding of string inverters. It is about making the case for marrying string inverters for large-scale PV plants with the centralized controls that are one of the key advantages of central inverters.
“It is sort of a paradigm shift because classic string inverters – over the last four or five years – have been from 25 or 30 kilowatts up to 60 kilowatts”, said Stefan Froböse, Technical Director, Sungrow EMEA. “We are working with our customers here in Europe and beyond to get them out of that paradigm and into the 1,500-volt world, given technical conditions comply. The new system requires a new process for system design incorporating more solar modules per string, fewer strings, fewer combiner boxes and longer runs.”
Sungrow’s central inverter concept brings together the company’s 1,500-volt, 125-kilowatt inverter to replace each 2.5-megawatt central inverter with twenty 125-kilowatt inverters. It then combines a collection of power modules with a centralized point of control and communication.
Good-bye to the concept of integrated combiner box?
The various features of Sungrow’s 125-kilowatt string inverter have important implications for designing the overall system. For example, each Sungrow string inverter has a single DC input and just one MPPT. It also does not have an integrated string combiner box, which most string inverters do. It’s another design element that often needs some explaining.
“Think of it like a mini central inverter. Architecturally, it makes more sense to not have the string combiner box integrated”, Froböse continues.
One reason is size and weight. If a combiner box is integrated into the inverter, it increases the total weight enough that two people can no longer install it. As a result, this increases the time, complexity, and -- most importantly -- the cost of installation and maintenance. At just 72 kg, Sungrow’s 125-kilowatt string inverter has the highest power output-to-weight ratio for any string inverter.
There are other reasons why Sungrow’s 125-kilowatt string inverter does not have an integrated combiner box. Traditionally, 1,000-volt string inverters with integrated combiner boxes have been located near the solar array in order to keep DC wiring runs short while utilizing the AC output for longer wire runs to the point of connection. Because of local regulations about voltage drop, this meant AC wiring had to be more extensive – and more expensive.
Other than traditional decentralized string inverter design, the 125-kilowatt string inverter is best located near the point of connection, rather than within the PV array.
Lowering costs, increasing flexibility
Sungrow’s 125-kilowatt inverter has an AC output voltage of 600, compared to 480 volts for most other string inverters. The AC output voltage also has important implications for choosing the correct wire size; for example, higher AC voltage allows for smaller and less expensive wire.
The virtual central inverter concept allows solar power plant designers to potentially mix traditional central inverters with the more customizable virtual central inverters by at the same time driving down total system costs for utility scale systems.
Outline of Virtual Central Solution ©Sungrow
About the author:
Stefan Froböse is Technical Director at Sungrow and responsible for the EMEA region. He has been with the Company since 2016.
Previously he was Director Product Management at Conergy in Hamburg, Germany, where he was responsible for the supplier selection from technical perspective.
Stefan possesses more than 17 years of management experience in the semiconductor and solar industry which includes various positions in sales and product management at e.g. Transwitch Inc., Solyndra Inc. and Monier.
From 2012-2015 Stefan was a member of the Board of Directors at PV Cycle in Brussels, Belgium.
Stefan has graduated in electrical engineering.