Titel: Analysis of High-Frequency Electrical Transients in Offshore Wind Parks
Författare: Tarik Abdulahovi
Utgivare: Chalmers
Årtal: 2011
Ämnesord: very fast transient, breaker, surge, surge protection, transient overvoltage, prestrikes, restrikes, voltage escalation
Doctoral thesis via Elforsk
Sammanfattning/Abstract: In this thesis, a study of high frequency electromagnetic transient phenomena is performed.Models of various components needed for simulations of high frequency transients, such as for transformers, cables and breakers are developed. Measurements were performed in Chalmers research laboratory as well as at ABB Corporate Research in Västerås, Sweden for the purpose of parameter estimation of models and for verification of simulations. Critical cases where the voltage surges of the magnitude and/or rise time above basic lightning impulse voltage level appear, are identified. Also, some transient protection schemes are analyzed and the performance of different transient mitigation devices is studied. Furthermore, the voltage distribution along the winding during very fast transients is studied in order to estimate turn-to-turn voltages and the critical voltage envelope.
In the work it was found that simulations of the high frequency transients are in a very
good agreement with the measurements obtained in the laboratory. Simulations predicted
accurately critical surges with the highest magnitude and matched with good accuracy surge
waveforms recorded during the measurements. The accuracy of the rise times is within
10%, while the magnitudes during the critical cases are within a 5% margin. During the testing and simulations, surges which exceed the basic lightning impulse voltage level of
dry-type transformers specifed by IEEE standards, both in magnitude and rise time are
observed at the transformers even when surge arresters are used to protect the transformers.
Furthermore, obtained voltage surges exceeded the proposed critical voltage envelope. It is
shown that the most critical voltage strikes are obtained with dry-type transformers, where
the rise time is five to ten times shorter compared to oil-insulated transformer with the same
rating. It was confirmed both in simulations and measurements that the use of additional
transient protections devices such as surge capacitors and RC protections, decreased the
magnitude of surges to be below the critical level. The analysis of the voltage during very
fast transients showed that the rise time of the transients directly in??uences the magnitude
of the turn-to-turn voltages. Furthermore, during the breaker closing transient, turn-to-turn
voltages, measured in delta connected dry-type transformers, were 2:5 times higher than the
voltages obtained during the same transient with a wye connected dry-type transformer, or
during the stress with a 4:4pu lightning impulse voltage.