Mathematical Reconstruction of UVRT Tests for critical Standard Analysis

Inhalt:
In order to certificate wind turbines under voltage ride through tests must be performed. Within these tests symmetrical and unsymmetrical faults are tested. This paper shows analytically with an modeled fault, that unsymmetrical voltage dips must have a step-shaped course in the transient range of the positive sequence voltage. First the standard and the test bench at the Center for Wind Power drives (CWD) is introduced. Then measurements from the CWD are compared to the relevant standard, showing the general difference in the shape for symmetrical and unsymmetrical faults. The mathematical modeling of ideal versions of these faults allows the analytical calculation of the positive sequence voltage, that is used within the standard. Solving this problem analytically shows, that unsymmetrical faults must have a step-shaped course in the transient range of the positive sequence voltage. This is compared with symmetrical faults a general differnce that can cause issues in the tolerance band that is provided by the standard. Using this result it is concluded, that the tolerances for valid under voltage ride through tests are different for both types of faults.