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Stabilization of Floating Wind Turbines

Pedersen, Morten Dinhoff
Doctoral thesis
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URI
http://hdl.handle.net/11250/2450500
Date
2017
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  • Institutt for teknisk kybernetikk [4103]
Abstract
Wind turbines mounted on floating platforms are, under a specific set of circumstances,

prone to a phenomena called control-induced resonance. The phenomena

materializes as persistent oscillations in the platform’s surge and pitch

displacement and occurs when pitch-control algorithms without modifications

for floating operation are used.

This thesis examines the physical underpinnings of the resonance problem by a

novel modeling strategy. A systems approach is adopted where the floating wind

turbine is conceived as an interconnection of subsystems. New aerodynamic

models suitable for a systems approach are proposed, validated and examined.

Notably, a vortex theoretical approach is shown to be fruitful both in the steady

and unsteady loading regimes. A new type of dynamic inflow model based on

vortex transport and frequency domain identification is proposed and compared

favorably to existing strategies. The vortex lifting law is used to generate a

succinct model for loads that fits into the systems representation and replaces

the commonly used table-data approach. A simplified dynamic model suitable

for control analysis is derived and validated against a standard tool in windturbine

analysis.

The system model developed in the first part of the thesis is used for a smallsignal

stability analysis invoking passivity tools in the second part. It is shown

that floating wind turbines are stable under quite general conditions as long

as no pitch control is applied. If collective blade pitch is used for the sole

purpose of setpoint control of the angular velocity, destabilization is shown to

be inevitable. An examination of the energy flow within the wind turbine system

motivates a novel "energy shaping" control strategy. Existing control strategies

are also examined in the new paradigm. A case study of a representative 5MW

wind turbine is performed. The resonance phenomena is demonstrated under

realistic conditions. A number of control strategies are tested and compared.

The "energy shaping" control strategy is shown to perform well and eliminates

the control-induced resonance. Justification for the results are given based on

the theory developed in the thesis.
Publisher
NTNU
Series
Doctoral theses at NTNU;2017:132

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