Investigating converter control interactions in the transmission grid

  • Untersuchung von regelungsinduzierten Wechselwirkungen von Umrichtern im √úbertragungsnetz

Quester, Matthias Andreas; Moser, Albert (Thesis advisor); Monti, Antonello (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

The progressive integration of HVDC systems into the transmission grid has led to an increase in HVDC converters that impact system stability. It has been seen that the converters and their control system can interact with the connected AC system, causing high oscillations in the grid current and voltage. To prevent outages resulting from the oscillations, system operators need to identify interactions in the planning stage of HVDC systems. A promising method is to investigate the interactions in the frequency domain using impedance models. However, relying on full-system knowledge, often-applied analytical models cannot represent manufacturer converters, particularly their control system. Thus, this work develops a method that is also applicable for black-boxed systems. Frequency-dependent impedances of time-domain converter models and physical laboratory converters are derived without full system knowledge. Running on a real-time simulator, the method can also be applied to converter control replica systems. Based on an online perturbation approach, the method directly determines the impedance during the measurements, ensuring high accuracy while keeping data requirements to a minimum. Comparing the impedance of a small-scale laboratory converter to the impedance obtained from time-domain average value models shows that a simplified model can be used to represent the electrical structure of the converter when interactions are assessed that cause oscillations between 30 Hz and 3 kHz. The comparison further reveals that the converter transformer causes a deviating frequency behavior at sub-synchronous frequencies that cannot be seen in the time-domain model. Using the derived impedance models of the converter, several test cases demonstrate in which grid situations interactions might cause unstable systems. The results show that interactions can cause an unstable system for moderate short-circuit power ratios depending on the converter's control system and the power flow.

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