Speeding up Time-to-Market with Testing for Grid Integration
The expectations placed on the performance of wind turbines have increased significantly during recent years. The increasing competitive pressure which prevails on the global market and the noticeable professionalization of the industry have increased these expectations: With new turbine designs, the expectation nowadays is that the first turbines of a new type already run with high reliability when they are first delivered. Investors demand proof of comprehensive operational experience before they will commit financing for projects. New developments – even modifications of existing products – therefore represent a significant economic risk as far as the manufacturers are concerned. The experimental validation of prototypes on large test benches reduces this risk, accelerates the certification, and improves the plannability.
The higher proportion of electricity from regenerative sources in the distribution and transmission grid structures at various voltage levels increases the demands being placed on the grid integration of wind turbines as power generating units (PGU) even further. These requirements are laid down in standards and guidelines which have to be taken into account in future developments. Turbine certificates are mandatory for new and modified turbine designs. They ensure that the PGU operation is compliant with the grid code and thus guarantee the grid connection in the long term and the continuation of the feed-in tariff.
Fraunhofer IWES assists turbine manufacturers by offering efficient test methods for the accelerated validation of the electrical properties of PGUs on test benches to meet the increasing requirements.
Status quo: field test and limited predictability for market launch
The testing of grid compatibility for the certification of the electrical characteristics of new wind turbines – or recertification when existing types of turbines are modified or improved – is currently undertaken almost exclusively with the aid of mobile test installations in the field. To determine the electrical characteristics, these field tests always include the following measurements, which are taken with the aid of the test installation, Fault-Ride-Through (FRT) containers and measuring systems.
The complete certification campaign usually covers a period of up to two years; this amount of time is a significant cost factor in turbine development and decisively determines the point in time at which the turbine is launched commercially. The demand for suitable locations for the prototype certification is high, as is the number of turbines to be certified. The site conditions largely determine the realizability of a certification campaign: alongside the need for good wind conditions, increasing turbine size means that higher demands are placed on the grid connection. Test for the certification of the electrical characteristics of power generating units affect the downstream supply network and therefore require close coordination in advance with the local network operator. In addition, field tests are practically irreproducible; it is extremely unlikely that two tests can be performed under precisely the same wind and grid conditions. The comparability of the results for verification is therefore limited. Moreover, there can sometimes be long delays before the requisite test conditions can be met. This therefore makes it much more difficult to plan the series production for the commercial launch.
Advantage of laboratory testing: reproducing critical load cases at a random number
In 2015, a first nacelle test rig for complete nacelles has been put into operation. Fraunhofer´s Dynamic Nacelle Testing Laboratory (DyNaLab) provides turbine manufacturers with a realistic testing environment in the multi-megawatt range to carry out tests under reproducible conditions. This combination of mechanical tests and a grid emulator to test wind turbines up to 10 MW offers unique possibilities for prototype validation. By using an artificial network with 44 MVA installed converter power, it is possible to reproduce typical grid faults such as voltage dips with a high repetition rate. In the Hardware-in-the-Loop (HiL) method, high-performance, real-time models and corresponding control algorithms are used to operate the test bench including the unit under test.
Compared with a field based certification campaign, DyNaLab shortens the certification process significantly: certain critical load cases which might appear during operation can be reproduced at a random number. A testing campaign for certification on the test bench can be scheduled precisely and defined so as to be manufacturer specific. In this manner, operation management and control can be improved and models can be validated. Therefore, reliability and availability of the turbine can be improved, and costs for maintenance and repair cut down.