Performance of Grid Connected Wind Turbines under Grid Faults

Amir Arasteh*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

Power systems face new challenges as the converter-based generation, such as wind and solar, becomes more prevalent. Compared to conventional power systems, networks with a high volume of power electronic converters lack features, like mechanical inertia, that are traditionally provided by synchronous generators. Therefore, the grid codes for converter-based generation units, such as Wind Turbines (WTs), are becoming more and more stringent. New grid codes, for instance, require connectivity of WTs during severe faults (even zero voltage) and are expanding toward specifying requirements for asymmetrical faults as well. Further, it is expected that wind turbines will be required to have grid forming capabilities i.e. create and maintain voltage and frequency. In other words, requirements (expressed in grid codes) and expectations from wind turbines are becoming more demanding in several areas including: severity of the faults, types of the faults, and grid forming capabilities (voltage/frequency creation).

Severe grid faults occurring nearby wind power plants can cause instability of the WTs, i.e. an inability to maintain synchronisation with the main grid, known as Loss of Synchronisation (LoS). Consequently, if WTs are mandated to remain connected during severe faults, it becomes imperative to implement a control scheme that can prevent LoS. In this thesis, LoS is analysed from a distinct perspective to gain a better understanding of the phenomenon. The presented analysis enlightens the root cause of the associated instability and clarifies the key role of PLL in LoS. Based on this analysis, a hybrid solution combining an Adaptive-PLL with impedance estimation is proposed. The work also proposes a logical circuit for a more reliable detection of LoS, allowing the controller to only act when necessary.

Asymmetrical faults are, basically, creating unbalanced three phase voltage (and consequently current) in the system, which leads to additional challenges such as over voltage, and power oscillations. Analysing performance of WTs under asymmetrical faults paves the way toward proposing control solutions for the problem. In this thesis, the behaviour of WTs under asymmetrical faults is investigated analytically. The performed analysis has the advantage of working with voltages and currents as they exist in the real power systems and enables better selection of control variables under asymmetrical faults. Accordingly, a novel solution for damping power oscillations is proposed. The solution damps the oscillations through adjusting the ratio and phase shift between positive and negative sequence currents. It is also shown that the solution is capable of seamless switching between different control modes, i.e. giving priority to power oscillation damping, grid code requirements, etc. Furthermore, the analytical equation for deriving the peak current value in all circumstance is derived which accommodates implementation of an effective current limiter.

Finally, it is widely expected that WTs will, in the future, have grid forming capabilities, hence creating the voltage and frequency. There are several grid forming control schemes (GFCSs) proposed in the literature; including: power synchronisation control (PSC), virtual synchronous machines (VSM), and distributed PLL based control (dPLL). These GFCSs are compared in this thesis with the aim of identifying the advantages and disadvantages of each control scheme under fault condition.
Original languageEnglish
Place of PublicationRisø, Roskilde, Denmark
PublisherDTU Wind and Energy Systems
Number of pages143
DOIs
Publication statusPublished - 2022
SeriesDTU Wind Energy PhD
NumberPhD-115

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