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Förslaget inkom 2011-05-17

Power quality analysis in the 130 KV connection of an 110 MW wind farm


Background, purpose: The increasing amount of wind power connected to different system voltage levels is affecting the connecting networks and the overall power system. In order to sustain a good enough voltage quality and system stability all equipment connected to the network need to fulfil a number of power quality requirements described in recommendations, standards, local network owner technical specifications and transmission system owner grid codes. Examples of power quality parameters are harmonics, interharmonics, resonances, flicker, voltage sags, transients and unbalance.
The 110 MW offshore wind farm Lillgrund is connected to a 130 kV onshore switchyard Bunkeflo. On the Lillgrund side of the circuit breaker in Bunkeflo, Vattenfall has installed measurements systems to supervise the power quality (PQ) during normal operation and during disturbances.

The purpose of the Master Thesis project is to analyse the impact on different power quality parameters from the interaction between the wind farm and the connecting network. This concerns specifically requirements on harmonics and interharmonics in the frequency interval 2.5 to 9 kHz but also requirements on measurement systems. If possible also general conclusions shall be formulated, based on Lillgrund as an example, and requirements in technical specifications be proposed in order to start a development towards more harmonised specifications.

Master Thesis project: The thesis is mainly divided into the following parts:

• Analysis of the measured data
The analysis will primarily be based on data delivered from the instruments. In the case of measurements in the frequency range above 2.5 kHz the spectral analysis may require MatLab, or other suitable analysis tool, programming. The data will be analysed in terms of impact from the connecting grid, e.g. varying short circuit power in Bunkeflo and other connected installations nearby PCC, and the impact from the wind farm, e.g. active and reactive power levels. The PQ parameter values shall be compared with values stated in the technical specifications and grid codes.

• Analysis of the measurement systems
Two set-ups of transducers measure voltage and current on each phase. The first set-up measures with conventional transducers with a limited bandwidth of around 1-2 kHz while the other set-up has a bandwidth of 400 kHz. Three different measurement instruments are connected to the transducers. The report from this part shall contain how the systems differ in measurements of PQ parameters and if they comply with requirements stated in international standards. The analysis shall also include discussion on where the measurement transducers should preferably be positioned.

• Information compilation, calculations
The literature study shall go through relevant documentation, recommendations and standards on measurement requirements and calculation procedures for PQ parameters. It is common praxis that the requirements for harmonics and interharmonics are stated as a fraction of the nominal current. The relation between voltage planning levels and emission of current as function of frequency shall be compared, and the background information found, with specifications where the current level is given directly in tables.

• Technical Specification proposal for future harmonisation
Based on the data analysis from the Lillgrund example and the information compilation an attempt shall be made to propose new ways to secure voltage quality in the connection point based on a method. The technical specification for Lillgrund has been received from EON. It should serve the project with access to similar documents from other network owners and discussions on underlying information.

Master Thesis Report and presentation: The report shall be written in English. Presentations will be given both at Vattenfall and at the home University. Revision of the report shall be based on comments during presentations.

Student education requirements: The student shall have fulfilled all courses on a relevant University program. Suitable programs are MSc in Power Technology or Physics. The student shall read, write and speak Swedish and English fluently. MatLab and other program experience is valuable.

Time schedule: The project shall preferably be started in mid August 2011. The project prolongs during the mandatory period of 20 weeks.

Supervision: Urban Axelsson, Vattenfall R&D, will supervise the project. The project will be conducted in the Vattenfall R&D office in Råcksta located west from Stockholm City.

Contact person:
Urban Axelsson, Vattenfall R&D +46 70 539 5876


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