Subsynchronous Resonance in Doubly Fed Induction Generator based Wind farms
Doktorsavhandling, 2018

The objective of this thesis is to investigate the risk for instabilities due to subsynchronous resonance (SSR) conditions in large wind farms connected to series-compensated transmission lines. In particular, the focus is on doubly-fed induction generator (DFIG) based wind farms. Analytical models of the system under investigation are derived in order to understand the root causes that can lead to instabilities. A frequency-dependent approach based on the generalized Nyquist criterion (GNC) has been applied to investigate the risk for SSR in DFIG based wind turbines. Through this approach, it is shown that the observed phenomenon is mainly due to an energy exchange between the power converter of the turbine and the series compensated grid. This phenomenon, here referred to as subsynchronous controller interaction (SSCI), is driven by the control system of the turbine’s converter, which results in a non-dissipative behavior of the DFIG system in the subsynchronous frequency range. The different factors that impact the frequency characteristic of the wind turbine, thereby making the system prone to SSCI interaction, are investigated. Through the analysis, it is shown that in a DFIG wind turbine, the current controller that regulates the rotor current plays a major role in the risk for SSCI, where an increased closed-loop bandwidth negatively impacts the system damping in the subsynchronous frequency range. The level of active power output from the wind farm also has an impact on the overall system stability; in particular, it is shown that the power-dissipation properties of the DFIG improves when the latter is operated in supersynchronous speed range (high-power output).

Methods for proper aggregation of the wind turbines in the farm are investigated. Time-domain studies are performed on the aggregated model connected to a series compensated transmission line to verify the analytical results obtained through the frequency domain analysis. Based on the theoretical analysis, mitigation strategy is proposed in order to shape the frequency behavior of the wind turbine. The effectiveness of the proposed mitigation strategy is evaluated both theoretically through frequency-domain analysis and using detailed time-domain simulations.

generalized Nyquist criterion (GNC).


doubly-fed induction generator (DFIG)

subsynchronous controller interaction (SSCI)

Wind power

Impedance-based analysis

induction generator effect (IGE)

subsynchronous resonance (SSR)

sal EA, Hörsalsvägen 11, Chalmers
Opponent: Prof. Pedro Rodriguez Cortes, Loyola University Andalucia (LUA), Seville, Spain.


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Renewable energy sources, such as wind, solar, wave, tidal and hydro, play a significant role in today’s electrical power systems and the share of electricity produced through this kind generation plants constantly increases. In particular, wind and solar installations are being favored by system utilities and power providers. As an example, the capacity of renewable generation in Sweden reached 51% of the total generation capacity in 2017, with a peak of about 11% of energy production coming from wind. Moreover, by the year 2040, the total energy generation capacity in Sweden is projected to go '100% renewable', with about 45% coming from wind and solar sources. 

In line with this trend, wind power has become one of the fastest growing sources of electricity globally over the past 15 years. As a result of this, various researches have gone underway to enhance the energy production, increase the lifetime of the wind turbines and their ability to safely connect to the power system. Modern wind turbines are interfaced with the power system through power-electronic converters to increase their flexibility and functionalities, allow to operate at variable speeds as well as to facilitate grid-code compliance. But an increased penetration level of wind power could compromise the stability of the power system due to various factors including: the intermittent nature of the produced power, reduced system inertia and interactions between power-electronic converters as well as between converters and other passive components installed in the power systems. In particular, the latter has been experienced in a number of actual installations, where the first recorded event occurred in southern Texas in 2009, when a wind farm became radially connected to a series-compensated transmission line.  The aim of this thesis is to provide the groundwork for a deep understanding of the root causes of this specific phenomenon. Through system modeling and frequency-domain analysis, the work focuses on the identification of the key parameters that affect the stability of the system, understand their impact as well as proposing effective countermeasures.




Annan elektroteknik och elektronik



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4395



sal EA, Hörsalsvägen 11, Chalmers

Opponent: Prof. Pedro Rodriguez Cortes, Loyola University Andalucia (LUA), Seville, Spain.

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