Center for Wind Power Drives
The Center for Wind Power Drives (CWD) coordinates the research work on wind energy of a consortium of seven RWTH Institutes. The CWD is about to commission a 4 MW ground level nacelle test bench for wind turbines. A 1 MW experimental setup, as a proof of concept, has already been fully set-up and tested. The scaled up 4 MW test bench has optimized multi-physics Power Hardware in the Loop, and offers more degrees of freedom in testing.
At the Center for Wind Power Drives (CWD) the nacelle under test is connected to two power ports, connected to mechanical and electrical interfaces respectively. The mechanical interface emulates the torque, forces and moments on the rotor hub. The electrical interface emulates the power grid, enforcing the voltage behavior. The whole system then constitutes a multi-physics Power Hardware in the Loop setup. The mechanical emulation is led by the real-time simulation of the wind field and rotor behavior in a dSPACE platform, the electrical emulation is led by the real-time simulation of the electrical power grid, computed by RTDS setup. The emulation of the external conditions guarantees the identical operating conditions of the nacelle from test to test, which is normally very difficult to achieve in a real in-field setup.
The 4 MW test bench is equipped with a direct drive motor and a load application system. In combination up to 3300 kNm of torque, forces of more than 3 MN and moments of more than 7 MNm can be applied to the rotor hub of the nacelle under test. A power converter provides the electrical point of common coupling (PCC) for the nacelle. Voltage and frequency at the PCC can be freely set up to 7.5 MVA and 65 Hz.
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One key factor in the successful deployment of wind turbines is their certification. Thanks to the fully controllable and repeatable operating conditions offered by the CWD’s wind turbine test bench, an accelerated wind turbine certification process is possible, resulting in short time-to-market. Every wind turbine, which is connected to the power grid, has to fulfill performance criteria, the so-called grid codes, defined by the grid operator. Compliance guarantees that the turbine does not negatively impact the grid, for instance with voltage flicker or with feed-in interruptions in the case of grid faults. The test bench supports a faster overall design process through specifically targeting certification procedures. Next to the establishment of a test plan for test bench based certification participation in committee work is done to support ground level testing in the relevant standards.
Using the 1 MW test bench, grid code compliance tests have been exemplarily performed on a Vestas V52 850 kW wind turbine nacelle. A broad range of test scenarios, derived from standard IEC 61400-21 was applied, allowing a detailed investigation on the grid code compatibility issues and set a focus on the behavior of the device under test (DUT) down to the component level. The test will be continued with the 4 MW set-up that allows for the testing of state-of-the-art power levels of onshore wind turbines.
An example of the results obtained with the 1 MW setup in the scenario of a grid fault is shown in figure 2. The DUT employs a doubly-fed-generator (DFG) whose stator is directly connected to the grid. This arrangement without Fault Ride Through (FRT) add-on of the generator system can make grid faults very destructive for electrical and mechanical components of the wind turbine, and therefore disconnects the wind turbine in case of severe grid faults – in contrast to the demand of current grid codes. The figure proves the expected incapability of the turbine to stay connected in case of FRT faults. It can be seen that the DUT disconnects from the grid after approximately 40 ms failure time.
The test bench also focusses on condition monitoring. If the in-field systems are enhance it can be applied to wind turbine drive train setups to increase the availability. Damages can be detected before the whole system fails. Hence, condition based maintenance of the turbine minimizes downtimes of the system. One common example for reduction in availability is thermal stress of the semiconductors in the power converter possibly leading to a failure. In supporting research work done at the institutes of the CWD, models are developed to calculate a detailed semiconductor temperature profile in reference to the load current. Such type of information can be used to predict the semiconductor’s lifetime and hence the probability of failure.
The CWD looks at alternative technologies for wind turbines. Examples are various drive train concepts power grid topologies for wind turbines. To improve the estimation of the efficiency for the variety of drive train systems, tools are developed to evaluate different power converter topologies in detail. The integration of wind turbines into future dc collector grids is evaluated as an alternative grid layout as the long-distance transmission of electrical energy is more efficient with dc and therefore is a promising technology for large wind parks.
Announcements
Anica Frehn and Robert Uhl attended the first German HiL workshop at KIT in Karlsruhe. They presented the real time simulation and Hardware-in-the-Loop capabilities of the ACS laboratory as well as the Center for Wind Power Drives (CWD).
Bettina Schäfer and Alexander Helmedag participated in the industry workshop on the Center for Wind Power Drives (CWD) held in Aachen on Campus.