Plenz, Maik

In this work, a frequency-dependent transformer model is derived with the help of a measurement device that determines the frequency response of the grid impedance at an arbitrary point of connection to the power grid. In this case, the measurement device was connected to a 20 kV medium voltage grid of a wind farm. The disconnection and reconnection of the wind turbine’s transformers revealed the impact the open circuit input impedance of the transformers has on the positive sequence component of the grid impedance. First deriving a positive sequence grid model from the measurement without the transformers, later analyzing the impact the transformers have on the grid impedance, allowed the calculation of the positive sequence component of a frequency-dependent open circuit transformer model, which results in the same impact on the overall grid impedance as observed in the measurements.

Optimized grid integration of proton exchange membrane fuel cells in various possible applications requires a suitable protection system. For this reason, this paper examines the transient behavior of a fuel cell stack after an external electrical shortening. In order to show the influence of operating parameters on the short-circuit behavior, various experiments with changed anode and cathode humidity, cell temperature and anode and cathode stoichiometry are carried out. With this, manufacturers can estimate the short-circuit magnitude of their stacks and recommend a suitable plant protection system. It could be shown that the peak short-circuit current depends on the operating point as well as the operating conditions. For the steady-state short-circuit current, the gas stoichiometry has an impact on the deliverable current. For all other operating conditions the steady-state short-circuit current is approximately twice the recommended maximum operating current. Furthermore, a method to estimate the effective fuel cell stack capacity out of the transient short-circuit current is presented.