A discussion on the characterization of grid emulators by apparent power rating and short-circuit power

Abstract

Wind turbines are a driving force in the change towards renewable energies today. Offshore wind turbines have reached power ratings around 15 MW, and the growth continues. Grid emulators have become inevitable for the test and validation of wind turbines and subsystems at full scale. Yet, large investments for new installations present major challenges.Therefore, the following discussion outlines basic design aspects for the grid emulator main components: converter, transformer, and filter. The aim is to clarify the implications of requirements on the design to exhaust ratings for minimum investments, and to satisfy testing against the most recent standards and grid codes. Appropriate ratings are derived based on steady-state and transient test needs. It is shown how much overrating is specifically needed and why the converter is usually rated a multiple compared to the device under test. Further dimensioning criteria such as power quality, switching frequency, and filter size are discussed in this context.Finally, the capacity of grid emulators is often characterized by the short-circuit power. Two essentially different variants of short-circuit power are introduced, either being dependent on the converter peak current or the passive components impedance. While the first one reveals the overload capability, the latter represents a proper measure for grid strength. However, the emulated grid strength can be adjusted in a limited bandwidth using impedance control. The concept might be confusing at first glance, but it effectively decouples the virtual short-circuit power from the ratings of the power hardware.