| The Fraunhofer Institute for Solar Energy Systems ISE in Freiburg has taken its megawatt laboratory for PV inverters into operation. It is the first and currently the only laboratory in Europe where manufacturers can commission certification, in accordance with the medium-voltage grid codes that have applied since July 1st 2010, of inverters with a power rating of up to one megawatt. Manufacturers can have their products characterized and tested in full compliance with the certification specifications in the megawatt laboratory. "With a so-called ‘Low-Voltage Ride-Through’ test unit, we can investigate the performance of inverters with regard to voltage dips in the medium-voltage grid, as is required by the new regulations", stated Sönke Rogalla, the laboratory manager. The megawatt laboratory is equipped with three medium-voltage transformers, each with a power rating of 1.25 MW, which are connected to the 20 kV grid. One transformer supplies power to the solar array simulators, which supply the inverters to be tested with a voltage of up to 1000 V and a current of up to 1000 A. The other two transformers are used to feed the electricity into the medium-voltage grid. Their voltage can be adjusted between 255 and 790 V. This range covers all voltages that are commonly used internationally. "Power is drawn and fed in via two separate medium-voltage transformers", explained Prof. Bruno Burger, in charge of power electronics development at Fraunhofer ISE. "This prevents the solar simulators and inverters from mutually affecting each other, and avoids measurement errors." The control unit for the megawatt laboratory resembles the control room of a power station. The scientists can connect and disconnect 20 kV transformers by remote control and create voltage dips of varying depth and duration. In addition to the tests required by regulations, the spacious megawatt laboratory (with 250 m2 floor area) offers comprehensive analytical facilities to optimize inverters. For example, the Fraunhofer researchers measure not only the conversion efficiency but also the MPP tracking efficiency. This often neglected quantity quantifies the ability of a PV inverter to extract the maximum power from the connected solar cells at all times. |
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