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Currently in the photovoltaic market, in the field of distributed commercial rooftops and ground power plants, large or small machines have caused great controversy in the industry.
According to the Professional PV Market Survey IHS published in February this year, "PV Inverter User Demand Survey Report" shows: "The report collected over 300 installers and distributors using PV inverters through online inverter user research. As well as the general contractor's preference and viewpoints on the purchase of PV inverters, provided detailed information on inverter manufacturers' better understanding of customer demand for the products. According to the survey, more than 200 people who had purchased string inverters Eighty percent of the customers of the changers said they might consider using string inverters in PV systems above 100kW, while nearly half of all those surveyed said they might consider using them in systems above 1MW This is a significant increase over the survey last year (2013), with only 17% of those surveyed last year saying they might consider using string inverters in 1MW and above systems "
The applications involved in the survey results are based on a 1MW cutoff and do not distinguish between ground applications or rooftop applications. So what is the big machine or small machine? How to choose the most suitable inverter for their own conditions? The following author through some comparison of data to do some analysis.
First, the main system components comparison
Centralized Inverter: Component DC Cable - Combiner Box - DC Cable - DC Bus Distribution - DC Cable - Inverter - Isolation Transformer - AC Distribution - Grid.
String inverters: components, DC cables, inverters, step-up transformers, AC power distribution, power grid.
The string inverter saves the confluence box, the DC cable less the exchange cable more, but the cost is higher than the centralized inverter 3 wool / W, the final cost is higher than about 0.25 Yuan / W, with 100MW ground photovoltaic power station For example, the initial investment increased by 25 million yuan.
Second, power generation contrast
There are many factors that affect the power generation. Although distributed inverters can theoretically save the loss of component mismatch, shading, consumption of auxiliary equipment such as combiner boxes and voltage difference of DC cables, etc., the centralized inverter In terms of conversion efficiency, cable loss and other advantages, while in high altitude, string inverters to be derated operation.
Of course, this is compared with the more advanced centralized inverters in China at present.
According to a PV company's internal data, some ground-based power plants produce about 0.3% of small-sized machines and some ground-based plants generate about 0.2% of their power.
Conclusion: new large ≈ small machine "old machine
Third, maintenance costs
Taking 20MW power station as an example, 40 sets of 500kW centralized inverters are used, while those of 30kW string type require 680 sets, and the failure rate under the same quality is increased by 17 times. However, it should be pointed out that although the failure of string inverters However, the loss of each failed machine to the power station is much smaller than the centralized inverter.
In addition, centralized inverter device level maintenance, string maintenance are generally machine replacement, warranty period outside the centralized inverter maintenance costs 20MW * 0.3 yuan / W * 5% = 300,000 yuan, string The formula is 20MW * 06 * 10% = 120 million.
Conclusion: From the current technology development, the mainstream of terrestrial PV power plants is still the big machine, suitable for large sunshine desert power plants, ground power plants and other large-scale power generation systems, the system has a large total power, usually above MW level.
String inverter: suitable for small and medium-sized roof photovoltaic power generation systems, small ground station.
But with the rapid development of inverter technology, do not rule out the possibility of small machines break through the technical barriers and cost bottlenecks.
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