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dc.contributor.advisorUhlen, Kjetil
dc.contributor.authorØdegård, Jon Nerbø
dc.date.accessioned2015-12-17T08:02:37Z
dc.date.available2015-12-17T08:02:37Z
dc.date.created2015-06-10
dc.date.issued2015
dc.identifierntnudaim:13331
dc.identifier.urihttp://hdl.handle.net/11250/2368190
dc.description.abstractThe state of the art in wind power technology is offshore VSC-HVDC-connected (Voltage Source Converter-High Voltage Direct Current) offshore wind farm of Full Converter Wind Turbines (FCWT). This latest generation of wind power plants provide great opportunities with regard to efficiency and flexibility, but poses new challenges with regard to power system stability, as the asynchronous operation inertially decouples the generators from the residual power system. By utilising the flexibility of VSC-control, the goal is to improve the dynamic behaviour by coupling the system frequency and generator speed, and hence provide frequency support. The frequency support control strategies are initially designed by contextualising the wind power plants in classic power system stability theory. By the knowledge of lacking primary control in wind farms, it was predicted that an inertia response would compromise system damping as the turbines \textit{demand} power when returning to nominal speed, whereas classic power plants \textit{provide} power. The effectiveness of the design was evaluated by the results gathered from two experimental methods, simulation and laboratory. The simulation model was constructed in MatLab SimuLink and is a mathematical power flow model. Since the time scope of inertia response is in seconds, the microsecond switching of the converters is not modelled. The laboratory model was constructed by a 17 $kW$ grid and a 55 $kW$ wind farm equivalent connected by 60 and 20 kW VSC-converters. All converters are FPGA-controlled enabled by the graphical programming environment LabView and CAN-bus messages. The system load is pure resistive and the perturbation of the system is changing this resistance. The auxiliary control structure was designed with simplicity as the key feature. By frequency measurements, the deviation from nominal was scaled and mirrored on to the HVDC-voltage. This was denoted Scaled Deviation Mirroring, SDM. The same technique was used to couple the HVDC-voltage to the offshore frequency, and then couple the offshore frequency to the wind turbine speed. If neglecting any regulation delay, this would allow the wind turbine to swing in phase with the grid frequency. This novice control reduced the frequency drop by 35.3 \% and the overshoot by 12.5 \%, but the oscillations sustained twice as long. Using the prediction of compromised damping, a secondary controller was designed with the intention to damp the oscillations. Since the wind turbines lacks primary regulation (governors), regulation of the residual grid must ensure damping for an added inertial mass. A key characteristic of the full converter wind turbine has already been pointed out - asynchronous frequency - and this is utilised in the Wind Turbine Stabiliser control (WTS-control). Completely decoupled frequency allows the inertia response to be phase shifted without restriction, and it can be optimised to reduce frequency drop and improve damping. The results showed that a wind turbine frequency response phase lagging $15-20^{\circ}$ significantly improved damping, but on the expense of frequency reduction. The addition of WTS-control improved the frequency drop by 29.4 \%, the overshoot by 62.5 \%, while sustaining the oscillations for 25 \% longer compared to the reference. The fact that a lagged response improves the system performance pave way for two conclusions. Firstly, the regulation does not need to be stressed for optimal performance, as the response is nevertheless lagged on purpose. Component ratings can then be easier met, as high currents are avoided. Secondly, the lag is a proof of the greatest weakness of the wind turbines, no primary regulation. The auxiliary control must then be regarded as a method for improving system interaction, rather than local contribution. The lagging wind turbine inertia response facilitates an effective primary regulation of residual grid by providing inertia reserves at the right time, but also demanding accelerating power at the right time.
dc.languageeng
dc.publisherNTNU
dc.subjectEnergi og miljø, Elektrisk energiomforming
dc.titleLaboratory Demonstration of Frequency Support Provision from VSC-HVDC-connected Full Converter Wind Turbines
dc.typeMaster thesis
dc.source.pagenumber121


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