Center for Wind Power Drives (CWD)

  CWD RWTH Aachen Illustration of the CWD 4MW wind turbine test-rig

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.

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 an 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.

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.

The test bench also focusses on condition monitoring. If the in-field systems are enhanced 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 this is a promising technology for large wind parks.