Titel: Cable System Transient Study Vindforsk V-110. Experiments with switching transients and their mitigation in a wind power collection grid scale model, Elforsk report 09:05
Författare: Muhamad Reza and Henrik Breder
Utgivare: Elforsk, Vindforsk
Årtal: 2009
Ämnesord: Infrastruktur
Sökord/keywords: ABB, wind park, grid
Rapport
Sammanfattning/Abstract: Large cable-based systems such as collection grids for wind parks emerge worldwide. Transient overvoltages (tov.), appear, e.g., at grid energizing, deenergizing and fault handling. IEC test standards do not consider cable characteristics or repetitive re-strikes. This needs a clarification of transients in wind park cable collection grids with respect to causes, prevention, and mitigation.
A laboratory environment has been created at ABB Corporate Research to enable measurements on typical apparatus within well-defined circuits and controlled parameters. Worst-case conditions can be tested with limited risks in terms of possible hazard and cost.
Results from experiments on characterizing transient overvoltages in a cable system are presented. This is made along with theory and the verifications and comparisons of commonly used surge protection methods, namely: surge arresters, surge capacitors and RC-protection.
- The experiments verify the consistent occurrence of repetitive prestrikes and re-ignitions in a cable system. The occurrence is determined by the electrical circuit and the point-on-wave of switching operations.
- Surge arresters always limit the maximum voltage magnitude but voltage steps with a magnitude twice the protective level could occur while the voltage can swing within a band in magnitude below the surge arrester protection level from negative to positive polarity.
- Surge capacitors and the RC-protection show mitigation effects although not removing the repetitive transients completely. The selection and application of such mitigation methods still requires engineering with a basic understanding of physical phenomena.
Mixing up different causes of transient overvoltages could lead to wrong decisions regarding the surge protection. Transient overvoltages caused by voltage escalation due to multiple re-ignitions, for instance, has been very little discussed today. Common knowledge of such is also limited.
• This investigation concludes and quantifies the occurrence of repetitive transients with high steepness compared with IEC standard impulse tests.
• The results also show that a few repetitive pre-strikes frequently occur during closing operations with inductive load, which are known to occur with all most commonly used types of breakers.
It is not within the scope of this work to determine what is a dangerous (respectively, a safe) level of steepness of tov. This is expected to be different for different components depending on type of insulation and electrical design. However, when considering:
• the fact that an insulation failure starts with a discharge avalanche due to local initial e-field stress.
• the fact that ac voltage magnitudes in high order of frequency content distribute locally different from the fundamental frequency case
these quantified transients must be considered in the dimensioning of the related components. It is therefore recommended to further investigate the insulation coordination of the components within large cable-based systems such as collection grids for wind parks.