Interaction between main components in wind farms

The purpose of the research documented in this thesis was to establish technical and scientific methods and tools for characterizing offshore wind farms’ electrical main components and their interaction in medium- and high-frequency, taking into account different electrical conditions and wind farms’ topology.

The research task formulated in this way encloses large variety of research areas and had to be limited. The focus was put on wide band modelling of power transformers, validating existing models of submarine cables, defining methods for developing black-box linear models of multiports as well as performing parametric variation studies.

Methods and tools were developed and shown to perform and estimate the severity of a potential mid- and high- frequency interaction between electric components in OWFs by robust sensitivity analysis in commercial EMT simulation tool. Performing interaction studies in electromagnetic transients programmes, as shown in this work, requires component models for medium and high frequency studies.

Although modern EMT programs contain accurate wide-band models of, e.g. cables, which can be used successfully in variety of transient studies, there is still a lack of wide-band models of transformers. Traditionally, black-box models made using dedicated, non-commercial measurement systems were used for that purpose.

This study shows how to develop wide-band, linear black-box model of a linear multiport system using commercial off-the-shelf sweep frequency response analyser. The method was validated on a 300 kVA power transformer with wye-wye winding connections giving accurate results. Large disproportions between magnitudes of eigenvalues of sub-matrices of admittance matrix are introduced when one of the windings of the transformer is ungrounded.

This results in poor fitting and the entire model being inaccurate. The accuracy of small eigenvalues was improved by performing additional measurements of zero-sequence admittance elements and approximating using vector fitting and constant-transformation matrix. The robustness of the measurement device and the simplicity of the measurement methods using the device makes it a good candidate for performing black-box modelling of multiports whenever such models are not available from the manufacturers.

Parametric variation method developed for EMT simulations in ATP-EMTP is a good tool for performing large sensitivity studies on almost any electrical circuit as the method allows changing any parameters of both, circuit and the simulation. The method is relatively time consuming and error-prone and therefore is recommended only when built-in ATP-EMTP methods cannot be used.

Employing the developed method with Frequency Domain Severity Factor proved to be a robust tool in assessing stresses on electric components arising from transient phenomena in offshore wind farms, including the voltage magnitude and frequency of oscillations. Quarter-wave resonance frequency is a good approximation of resonance frequency in collection grid cables due to travelling wave phenomena in a simple radial, where cable sections are made of the same cross-sectional area and are laid in series.

Oscillations of higher frequency are introduced into frequency spectrum of a voltage waveform in the radial when a point of discontinuity for the waveform is introduced in the string of turbines. This can be done either by using cables with conductors of different cross-sectional area or, especially, by line bifurcation, i.e. a situation when a cable in a string joins two or more other cables.

In both cases, the complex pattern of reflections and refractions can result in overvoltages of magnitude higher than 2 p.u at specific wind turbines. Comparison of results of energization simulations from ATP-EMTP and PSCAD with measurements showed comparable results. Thus, cable models with constant transformation matrix, which are available in ATP-EMTP can be used in such simulations, due to relatively short cable sections used to connect wind turbines and lack of cross-bonding.